A-BASIS – The “A” mechanical property value is the value above which at least 99% of the population of values is expected to fall, with a confidence of 95%.  Also called A-allowable.  See also B-basis, S-basis, and typical basis.

ABLATION – The degradation, decomposition, and erosion of a material caused by high temperature, pressure, time, percent oxidizing species, and velocity of gas flow.  A controlled loss of material to protect the underlying structure.

ABLATIVE PLASTIC – A material that absorbs heat (with low material loss and char rate) through a decomposition process (pyrolysis) that takes place at or near the surface exposed to the heat.  This mechanism essentially provides thermal protection (insulation) of the subsurface materials and components by sacrificing the surface layer.  Ablation is an exothermic process.

ABRASION – The wearing a way of a portion of the surface by either natural (rain, wind, etc.), mechanical (misfit, etc.), or man-made (over-sanding, etc.), means.

ABRASIVES – Special hard mineral ingredients employed to impart abrasive power to rubber articles used for abrading, grinding, or polishing such as rubber erasers or hard or soft rubber grinding wheels.  Also used with paper or fabric backings and as abrasive discs or flapper wheels.  Pumice, Silica, Tripoli sand, Carborundum, Silicon Carbide, Cerium Oxide, and Diamond powder may be used as abrasives.  The surface finish is micro inches can be related directly to the grit size of the abrasive.

ABS – Acrylonitrile-Butadiene-Styrene

ABSORBENCY – The ability of a fibre or fabric to absorb moisture.

ACCELERATOR – A material that, when mixed with a catalyst or a resin, will speed up the chemical reaction between the catalyst and the resin either in polymerizing of resins or vulcanization of rubbers.  Also called promoter.  See ISO 472.

ACETONE – Commonly used wipe solvent.  Also known as 2-propanone and Di-methylketone.  Used for cleaning composite surfaces prior to bonding and also metal surfaces prior to other treatments.  Can also be used to remove uncured epoxy resin from tools and other items.  Classed as “Seriously Flammable” with a flashpoint of -4° F (-20° C).  Has a high evaporation rate.  May be procured to U.S. Federal Specification O-A-51H or ASTM D 329.

ACID – A chemical compound containing one or more hydrogen atoms available for reaction with active metals or alkaline solutions.

ACOUSTIC EMISSION – A measure of integrity of a material, as determined by sound emission when a material is stressed.  Ideally, emissions can be correlated with defects and/or incipient failure.

ACRYLIC PLASTIC – Any of a family of synthetic resins made by the polymerization of esters of acrylic acid and its derivatives.  See also polymethyl methacrylate.  See ISO 472.

ACRYLIC-SOFT – A washable, colourfast, synthetic fibres derived from polyacrylonitrile. Used in base-layer and insulating fabrics.

ACTIVATION – The (usually) chemical process of making a surface more receptive to bonding to a coating or an encapsulating material.

ACTIVATOR – An additive used to promote the curing of matrix resins and reduce curing time.  See accelerator.  See ISO 472.

ACTUAL END COUNT – The number of bundles or splits that are actually counted in one doff of roving. (This is less than the theoretical end count due to splitting efficiencies of less than 100%).

ADDITION POLYMERIZATION – A chemical reaction in which simple molecules (monomers) are added to each other to form long-chain molecules (polymers) without forming by-products.  The small molecules, monomers, and together to form the polymer molecule.  The polymer is usually linear.

ADDITIVE – Any substance added to another substance, usually to improve properties, such as plasticizers, initiators, light stabilizers, and flame retardants.  See also filler.

ADHERE – To cause two surfaces to be held together by adhesion.

ADHEREND – A body that is held to another body, usually by an adhesive.  A detail or part prepared for bonding.

ADHESION – The state in which two surfaces are held together at an interface by mechanical or chemical forces or interlocking action or both.

ADHESION PROMOTER – A coating applied to a substrate before it is coated with an adhesive, to improve the adhesion of the plastic.  Also called primer.  A primer may and should improve the durability of a joint in the long-term, but does not always increase joint strength.

ADHESION, MECHANICAL – Adhesion due to the physical interlocking of the adhesive with the substrate irregularities.

ADHESION, SPECIFIC – Adhesion due to valence forces at the adhesive-substrate surface interface.  Such valence forces are of the same type that give rise to cohesion.

ADHESIVE – A substance capable of holding two materials together by surface attachment.  Adhesive can be in film, liquid, or paste form.  In this context, the term is used to denote structural adhesives, i.e., those which create attachments capable of transmitting significant structural loads.

ADHESIVE BONDING – A materials joining process in which an adhesive, placed between facing surfaces, solidifies to bond the surfaces together.

ADHESIVE FAILURE – Rupture of an adhesive bond, such that the separation appears to be at the adhesive-adherend interface.

ADHESIVE FILM – A synthetic resin adhesive, with or without a film carrier fabric, usually of the thermosetting type, in the form of a thin film of resin, used under heat and pressure as an interleaf in the production of bonded structures.

ADHESIVE FLASH – The cured adhesive squeezed out around the edges of a doubler, at butt splices, and at the end of the assembly.

ADHESIVE JOINT – The location at which two adherends or substrates are held together with a layer of adhesive.  The general area of contact for a bonded structure.

ADHESIVE, SUPPORTED – An adhesive film which has a woven or nonwoven carrier cloth.

ADSORPTION – The adhesion of the molecules of gases, dissolved substances, or liquids in more or less concentrated form, to the surfaces of solids or liquids with which they are in contact.  A concentration of a substance at a surface or interface of another substance.

ADVANCED COMPOSITES – Composite materials applicable to aerospace construction and made by imbedding high-strength and/or high-modulus fibres within an essentially homogenous matrix.  See filamentary composites.

ADVANCED FILAMENTS – Continuous filaments made from high-strength, high-modulus materials for use as constituents of advanced composites.

AFRP (OR ARP) – Aramid Fibre Reinforced Plastic.

AIR VENT – Small outlet to prevent entrapment of gases in a moulding or tooling fixture.

ALCOHOL – A hydrocarbon derivative in which one or more hydroxyl (OH) groups have replaced a corresponding number of hydrogen atoms.  Some are produced by fermentation and others synthetically.  Ethyl grain is the best known and is described as “alcohol”.  Commercial alcohol generally contains a de-naturant to render it unfit for human consumption and exempt from taxation.  This solvent group is relatively expensive and is usually considered to be among the more harmless industrial solvents.

ALDEHYDE(A) Acetaldehyde CH3-CHO reacts with aniline to give an accelerator.  One of the first know antioxidants.  (B) Aldehydes are volatile liquids with sharp, penetrating odors that are slightly less soluble in water than are corresponding alcohols.  (C) A broad class of organic compounds having the generic formula RCHO and characterized by an unsaturated carbonyl group (C=O).  They are formed from alcohols by either dehydrogenation or oxidation, and thus occupy an intermediate position between primary alcohols and the acids obtained from them by further oxidation.

ALIPHATIC – Organic compounds (hydrocarbons) in which carbon atoms are arranged in an open or straight chain.  More commonly known as napthas, they are prepared by straight-run, overhead distillation of petroleum.  Familiar examples include gasoline, kerosene, paraffin, and natural gas.  Of the common solvents, they are the lowest in price and the least toxic

ALKALI – Substance that neutralizes acids to form a salt and water.  Yields hydroxyl (OH) ions in water solution.  Proton acceptor.  Turns litmus paper blue.

ALKALINITY – The condition of having or containing hydroxyl (OH) ions.  Containing alkaline substances.  The opposite of acidic.  The property of turning red litmus paper blue and of neutralizing acids to form salts.

ALKYD PLASTIC – Thermoset plastic based on resins composed principally of polymeric esters, in which the recurring ester groups are an integral part of the main polymer chain, and in which ester groups occur in most cross-links that may be present between chains.

ALLOY – In plastics, a blend of polymers or copolymers with other polymers or elastomers under selected conditions:  for example, styrene-acrylonitrile.  Also called polymer blend.  In metals, a substance having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

ALLYL PLASTIC – A thermoset plastic based on resins made by addition polymerization of monomers containing allyl groups:  for example, diallyl phthalate (DAP).

A-MATRIX – see Extensorial Stiffness Matrix

AMBIENT – The surrounding environmental conditions, such as pressure, temperature, or relative humidity.

AMINE RESIN – A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.  (See amino.)

AMINO-SILANE FINISH – Applied to glass fibres to give a good bond to epoxide, phenolic and melamine resins.  Must be used in glass-fibre for use with wet lay-up phenolic resins.

AMORPHOUS PLASTIC (AMORPHOUS PHASE) – A plastic that has no crystalline component.  There is no order or pattern to the distribution of the molecules.

ANAEROBIC ADHESIVE – An adhesive that cures only in the absence of air after being confined between assembled parts (absence of oxygen).

ANGLE WARP – Tape fabric wrapped on a starter dam mandrel at an angle to the centreline.

ANGLE-INTERLOCK 3D WEAVE – Three-dimensional, orthogonal woven fibre preforms are woven structures containing a set of yarn or tows laying in an X-axis, Y-axis, and Z-axis. The orthogonal woven structure affects the physical properties in three planar directions as compared to a traditional woven composite form with yarns laying in only the X-axis and Y-axis. Three Dimensional woven structures can create composite materials with fibre volume fractions around 50% in both 3D unit cell and 3D orthogonal structures.

ANGLE-PLY LAMINATE – A laminate having fibres if adjacent plies oriented at alternating angles.  The laminae are at any angle other than a right angle.

ANHYDRIDE – Usually an acid from which water has been removed, practically or theoretically, (e.g. acetic anhydride).

ANHYDROUS – Term used to describe a compound or mixture which has lost all its water, in particular, water of crystallization or absorbed water.

ANISOTROPIC – Not isotropic.  Exhibiting different properties along axes in different directions. See also anisotropy of laminates.

ANISOTROPY OF LAMINATES – The difference of the properties along the directions parallel to the length or width of the lamination planes and perpendicular to the lamination.

ANNEALING – In plastics, heating to a temperature at which the molecules have significant mobility, permitting them to reorient to a configuration having less residual stress.

ANODE – The positive anode in a solution of electrolytes (or in a vacuum tube) at which electrons leave the solution, and at which oxidation occurs.  It is also the positive pole of a battery.

ANODIZING – The application of a protective oxide film on aluminium, magnesium and other light metals by passing an electric current through an acid bath in which the metal is suspended.  The metal serves as the anode.  The most common acids used are sulfuric, chromic and phosphoric.

ANTIOXIDANT – A substance that, when added in small quantities to the resin during mixing, prevents its oxidative degradation and contributes to the maintenance of its properties.  See ISO 472.

ANTISTATIC AGENTS – Agents that, when added to a moulding material or applied to the surface of the moulded object, hinder the fixation of dust or the buildup of electrical charge.

ANTISYMMETRIC LAMINATE(A) A special laminate type that is balanced, but unsymmetric i.e. (+0,-0,+0,-0) and A16 = A26 = D16 = D26 = 0 and Bij = 0.  (B) A laminate is said to be antisymmetric when for a given ply configuration (that is a ply with certain elastic properties and thickness) in the lower half of a laminate there is an identical ply configuration in the upper half, but with an alternating ply angle.  Note that for an antisymmetric laminate, the corresponding plies in the lower and upper half of the laminate do not have to be placed at their corresponding equal distances from the laminate midplane.  Antisymmetric laminates, by definition, are possible only with orthotropic plies.  It is not possible to have an antisymmetric laminate made from isotropic plies as there are no directional properties in the isotropic plies. 

API’S – Addition-reaction polyimides

AQUEOUS – Water-containing or water-based.

ARAMID – A type of highly oriented organic material derived from polyamide (nylon) but incorporating aromatic ring structure.  Used primarily as a high-strength, high-modulus fibre.  Kevlar® and Nomex are examples of aramids.

AREAL DENSITY – Areal density, or weight per unit area (usually just called weight), of the dry fabric. The most common unit of measure is ounces per square yard, often simply abbreviated as ounces. Thus, a fabric with a weight of 5.4 oz really has an areal density of 5.4 oz/sq yd.

AREAL WEIGHT – The weight of fibre per unit area (width X length) of tape or fabric.

AROMATIC – Unsaturated hydrocarbon with one or more benzene ring structures in the molecule.

A-SCAN – A scope presentation in which time or distance is related to amplitude.  A nondestructive inspection technique for finding voids, delaminations, and defects in laminates.

ASSEMBLY – A group of materials or parts, including adhesives, which has been placed together for bonding or which has been bonded in place.

A-STAGE – The initial state of the resin as produced by the manufacturer.  An early stage in the polymerization reaction of certain thermosetting resins (especially phenolics) in which the material is still linear in structure, soluble in some liquids and fusible.  The A-stage is usually considered to be a point where little or no reaction has occurred.  Prepreg in an A-stage condition would be extremely sticky, lumpy, and have little integrity.  Also called resole.  See also B-stage and C-stage.  See ASTM D 907 and ISO 472.

ASTM – American Society for Testing and Materials.

ATLAS (acronym) – Automated tape lay-up systems.

ATTENUATION – The diminution of vibrations or energy over time or distance.  The process of making thin and slender, as applied to the formation of fibre from molten glass. 

AUTOCLAVE – A closed vessel for producing an environment of fluid pressure, with or without heat, to an enclosed object while it undergoes a chemical reaction or other process.

AUTOCLAVE MOULDING – A process in which, after lay-up, winding, or wrapping, and entire assembly is placed in a heated autoclave, usually at 340 kPa to 1380 kPa (50 psi to 200 psi).  Additional pressure permits higher density and improved removal of volatiles from the resin.  Lay-up is usually vacuum bagged with a bleeder and release cloth.  The vacuum bag is normally vented to atmosphere.


BACK DRAFT – An area of interference in an otherwise smooth-drafted encasement; an obstruction in the taper which would interfere with the withdrawal of the model from the mould.

BACK PRESSURE(A) Resistance of a material because of its viscosity, to continued flow when mould is closing.  (B) In autoclave moulding, the pressure under the vacuum bag after the bag has been vented to atmosphere and the autoclave is at operating pressure.

BADGE – Bisphenol “A” diglycidylether.  See DGEBA, another way of saying the same thing.

BAG SIDE – The side of the part that is cured against the vacuum bag.

BAGGING – Applying an impermeable layer of film over an uncured part and sealing edges so that a vacuum can be drawn.

BALANCED CONSTRUCTION – Equal parts of warp and fill in fibre fabric.  Construction in which reactions to tension and compression loads result in extension or compression deformations only, and in which flexural loads produce pure bending of equal magnitude in axial and lateral directions.

BALANCED DESIGN – In filament-would reinforced plastics, a winding pattern so designed that the stresses in all filaments are equal.

BALANCED LAMINATE:  A composite laminate in which all laminae at angles other than 0°and 90° occur only in ± pairs (not necessarily adjacent) and are symmetrical around the centreline.  This type of laminate will have the least tendency to bow after cure.  See also symmetrical laminate.  A laminate may be balanced but not symmetrical, or symmetrical but not balanced.  See textbooks on composite laminate design.

BALANCED TWIST – An arrangement of twists in a combination of two or more strands that does not cause kinking or twisting on themselves when the yarn produced is held in the form of an open loop.

BALLISTICS CLOTH – A thick nylon weave used in clothing, packs, and luggage for reinforcement. Named for its similarity, in terms of durability, to bulletproof fabric.

BARRIER FILM – The layer of film used to permit removal of air and volatiles from a composite lay-up during cure while minimizing resin loss.

BARRIER PLASTICS – A general term applied to a group of lightweight, transparent impact-resistant plastics, usually rigid co-polymers of high acrylonitrile content.  Barrier plastics are generally characterized by gas, aroma, and flavour barrier characteristics approaching those of metal and glass.

BASKET WEAVE – A variation of the plain weave in which two or more threads weave alike in both warp and weft, joined in the regular order of the plain weave. Named for the basket-like pattern of the weave.

BATCH(A) In general, a quantity of material formed during the same process or in one continuous process and having identical characteristics throughout.  Also called a lot.  A discrete quantity of material with a total commonality of raw materials and process history.  (B) (Resins, adhesives, fabrics.) Resins or adhesives that are made in one mixer load and fabric woven from one warp loom setup.  A batch maybe a large or small amount of resin, adhesive, or woven fabric.  See ASTM D 907.  (C) (Prepreg material.)  A production run of prepreg material that is preimpregnated using one batch of resin and fabric, all in one continuous operation.  A batch of prepreg may consist of one or more rolls of material.

BATT or BATTING – Sheets or rolls of carded cotton or wool or other fibre or mixtures thereof which is used for woollen spinning or for stuffing, padding, quilting, and felting.

B-BASIS – The “B” mechanical property value is the value above which at least 90% of the population of values is expected to fall, with a confidence of 95%.  See also A-basis, S-basis, and typical-basis.

BEAD YARN – A yarn upon which is fastened either an actual bead or (commercially) a lump of hardened gelatine of a bead-like form.

BEAM – A spool on which parallel ends of single or plied yarns are wound for use in weaving or similar processing operations.

BEAMING – Operation in which many ends of yarn from a creel are combined on a section beam.

BEARING LOAD – A compressive load on an interface.

BEARING STRAIN – The ratio of the deformation of the bearing hole, in the direction of the applied force, to the pin diameter.  Also the strength or deformation strain for a sample under bearing load.

BEARING STRESS – The applied load divided by the bearing area.  Maximum bearing stress is the maximum load sustained by the specimen during the test, divided by the original bearing area.

BEARING YIELD STRENGTH – The bearing stress at which a material exhibits a specified limiting deviation from the proportionality of bearing stress to bearing strain.   Usually obtained using an offset.  CMH-17 (WAS MIL-HDBK-17).

BED – The mat of chopped glass fibres deposited over a layer of resin mix on carrier film following a chopping operation.

BEETLE – A large wooden mallet used to help soften cellulose fibres. Often used with linen and ramie.

BEETLING – The process of striking woven linen or ramie fabric with rollers to flatten the fibres. This leaves you with a more lustrous fabric.

BIAS FABRIC – Fabric in which warp and fill fibres are at an angle to the length, i.e. 60°, 45° etc. not 0° or 90°.

BIAXIAL FABRIC – Fabric with two non-interwoven layers – a unidirectional warp (0°) layer and a unidirectional weft (90°) layer – which are stitched or held together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also triaxial fabric, quadraxial fabric.)

BI-DIRECTIONAL LAMINATE (Cross Laminate)-A reinforced plastic laminate whose fibres are oriented in two directions in its plane. A reinforced plastic laminate with the fibres oriented in two direction in its plane.  A cross laminate.  See also unidirectional laminate. Laminate with fibres oriented in more than one direction on the same plane.

BINDER – The resin or cementing constituent (of a plastic compound) that holds the other components together.  The agent applied to fibre mat or preforms to bond fibres before laminating or moulding.  See ASTM D 907 and ISO 472.

BINDING THREADS – Threads used to unite two or more ply into one firm (stable) structure.

BIRDNEST – A large, tangled up collection of continuous glass bundles unable to run through the guide eye into roving creel. In the field, it can also be a large tangled collection of roving which does not run through the tube or guide eyes to the chopper.

BISMALEIMIDE (BMI) – A type of polyimide that cures by an addition rather than a condensation reaction, thus avoiding problems with volatiles formation, and which is produced by a vinyl-type polymerization of a pre-polymer terminated with two maleimide groups.  Intermediate in temperature capability between epoxy and polyimide.

BISPHENOL ‘A’ – A condensation product formed by the reaction of two molecule of phenol with acetone.  This polyhydric phenol is the standard intermediate resin that is reacted with epichlorohydrin in the production of epoxy resins.

BLANKET – Fibre or fabric plies that have been laid up in a complete assembly and placed on or in the mould all at one time (flexible bag process). Also, the form of bag in which the edges are sealed against the mould.

BLEEDER CLOTH – A nonstructural layer of material used in the manufacture or repair of composite parts to allow the escape of excess gas and resin during the cure.  The bleeder cloth is removed after curing and is not part of the final composite.

BLEEDING – The removal of excess resin from a laminate during cure.  The diffusion of colour out of a plastic part into the surrounding surface or part.

BLIND FASTENER – A fastener that is installed with access from one side only.

BLISTER – Debond of paint or other coating from part surface.  Undesirable rounded elevation of the surface of a plastic with boundaries that are more or less sharply defined, resembling in shape a blister on the human skin.  The blister may burst and become flattened.  See ASTM D 907 and ISO 472.

B-MATRIX – see Coupling Stiffness Matrix

BMI – See Bismaleimide.

BOBBIN – The spool or shipping package on which textile yarns are wound.

BOCULÈ – A compound yarn comprising a twisted core with an effect-yarn wrapped around so as to produce loops on the surface.

BODY – A term applied to wool when the staple has a good “hand” (full and with bounce). It can also refer to the fullness of a fabric. This is a subjective quality and has to do with a lack of limpness and/or stiffness. A fabric is said to have a good body when it has a full, rich, and supple hand.

BOLTED TYPE REPAIR – A repair attached with mechanical fasteners.  The fasteners may be rivets or bolts of various types, and the repair plate and/or angled sections may be either of precured composite construction or made from sheet metal or extrusions.  The parts used in the repair may additionally be adhesively bonded.

BOND(A) The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.  The sticking together of two or more surfaces with or without glue. See ASTM D 907.  (B) To join together with an adhesive and/or by fusing the resins of preimpregnated materials.  (BDS 1330)

BOND LINE – The layer of adhesive that attached two adherends.  Synonym for glue line.  See ASTM D 907.

BOND STRENGTH – The unit load applied in tension, compression, flexure, peel, impact or shear required to break an adhesively bonded assembly with the failure occurring either within the adhesive or at the adhesive/adherend interface.  See also adhesion and bond.

BONDED PART(A) See bonded structure.  (B) See composite part.

BONDED STRUCTURE – The structure resulting when a combination of parts is assembled and intimately attached to each other by applying a structural adhesive to the faying surfaces, followed by curing of the adhesives by pressure, heat, or both.

BORON FIBRE – A fibre produced by vapour deposition of elemental boron, usually on to a tungsten filament core, to impart strength and stiffness.

BRADFORD COUNT or BRADFORD SYSTEM – The British standard is based on the Bradford Spinning Count System. This originated in the 19th century and is based on the number of 560-yard worsted skeins that can be produced from one pound of clean wool. The clean wool is then thoroughly oiled which aids in producing a smooth, lustrous yarn for suiting. With this system the larger number will be a finer wool. Please see wool grades.

BRAID – A fabric structure made by interlacing textile yarns in such a manner that all yarns lie at an angle other then 0° or 90° to the length direction of the fabric.  See ISO 472.

BRAID/BRAIDER – A narrow tubular or flat fabric produced by intertwining a single set of yarns according to a definite pattern.

BRAIDING – Weaving of fibres into a tubular shape instead of a flat fabric, as for graphite fibre reinforced golf club shafts.

BRAIDS – Braiding is defined by the Textile Institute [1] as “the process of interlacing three or more threads in such a way that they cross one another and are laid in a diagonal formation.  Flat tubular or solid constructions may be formed in this way”.  A typical braiding machine operates using a “maypole” action whereby half of the yarn carriers rotate on a clockwise path whilst weaving in and out of the remaining 50% of the yarn carriers which are following a counter-clockwise path.  In the tubular braided reinforcement, each fibre follows a helical path around the principal axis of the braid.

BREAKOUT – Fibre separation or break on surface plies at drilled or machined edges.

BREATHER (BREATHER CLOTH) – A loosely woven material such glass fabric or Osnaburg cloth that will serve as a continuous vacuum path over a part or the repair area, but is not in direct contact with the part or the repair area.

BREATHING(A) The opening and closing of a mould to allow gas to escape early in the moulding cycle.  Also called degassing; sometimes called bumping in phenolic moulding.  (B) Permeability to air of plastic sheeting.  (C) The removal of air or gases from an assembly during autoclave moulding by use of a breather.

BREECH or BRITCH WOOL – Wool from the thigh and rear region of the sheep. It is the coarsest and poorest wool on the entire fleece. It is usually manure-encrusted and urine-stained fibre. It should be “skirted” and removed from a fleece for a hand spinner.

BRIDGING(A) Condition in which fibres do not move into or conform to radii and corners during moulding, resulting in voids and dimensional control problems.  (B) A condition where one or more plies of prepreg span a radius step of the fluted core of a radome without full contact.

BROAD STRANDsee Wides or Matchsticks

BROADCLOTH – A fine, closely woven, lustrous cotton or cotton/poly blend made in plain weave with a fine rib in the weft. The filling yarn is heavier and has less twist than the warp. The cloth is usually mercerized, and has a soft, firm finish.

BROADGOODS(A) Fibre woven to form fabric up to 1270 mm (50 in) wide.  It may or may not be impregnated with resin and is usually furnished in rolls of 25 to 140 kg (50 to 300 lb.).  (B) A term loosely applied to prepreg material greater than about 12 inches in width usually furnished by suppliers in continuous rolls.  The term is currently used to designate both collimated uniaxial tape and woven fabric prepregs.

BROCADE – Brocade is a jacquard weave with an embossed effect and contrasting surfaces. Can also be woven with synthetic or man-made fibres.

B-STAGE – An intermediate stage in the reaction of certain thermosetting resins in which the material softens when heated, and is plastic and fusible, but may not entirely dissolve or fuse.  Helps facilitate handling and processing.  Also called Resitol.  The resin in an uncured prepreg is usually in this stage.  See also A-stage and C-stage. See ISO 472.

BUBBLE – A spherical internal void; globule of air or other gas trapped in a plastic.

BUCKLING – Crimping of fibres in a composite material, often occurring in glass-reinforced thermoset due to resin shrinkage during cure.

BUCKLING (COMPOSITE) – A mode of failure generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.  In advanced composites, buckling may take the form not only of conventional general instability and local instability, but also a micro instability of individual fibres.

BUILDUP – An area within a laminate made thicker by addition of more plies or layers of material.

BULK MOULDING COMPOUND (BMC) – Thermosetting resin mixed with strand reinforcement, fillers, and so on, into a viscous compound for compression or injection moulding.  See also premix and sheeting moulding compound.

BUMP – A cylinder of coiled, prepared fibres ready for spinning. This is how commercially prepared fibres are delivered. Rather like if you had access to a really big ball winder and used it to wind the top you had just hand combed.

BUNDLE – A discrete collection of many parallel glass filaments. A collection of individual filaments; a sub-strand.

BUTT JOINT – A type of edge joint in which the edge faces of the two adherends are at right angles to the other faces of the adherends.

CABLED YARN – Two or more plied yarns twisted together. One or more part of a cabled yarn can be a single. So if you took two 2-ply and plied them again, you would have a cabled yarn. It is important to remember that you reverse the twist for each step. So if you spun your singles Z, the 2-ply would be spun S, and the cabled yarn would be produced by plying Z. You will need to have extra twist in the singles and the first ply to produce a “balanced yarn“. Please check Mabel Ross’ book “Essentials of Yarn Design for Hand spinners” for detailed directions.
A 3-2 cable refers to three 2-ply.

CALENDER – To produce a smooth finish and a desired dimensional thickness for sheet material by passing it between sets of pressure rollers.

CAMBRELLE – A nylon fabric, often used as a boot liner that resists abrasion, odor, and pilling.

CAMEL’S HAIR – Hair from the two-humped Bactrian camel, which is softer, lighter in weight and more fragile than wool. It provides warmth without weight, it never wrinkles and is water repellent.

CANVAS – A general classification of strong, firm, closely woven fabrics usually made with cotton. A heavier, open weave comprised of plied yarns. Characteristics: very hard-wearing, generally water-resistant.

CARBON – The element that provides the backbone for all organic polymers.  Graphite is a more ordered form of carbon.  Diamond is the densest crystalline form of carbon.

CARBON FIBRE – Fibre produced by the pyrolysis of organic precursor fibres, such as rayon, polyacrylonitrile (PAN), and pitch in an inert environment.  The term is often used interchangeably with the term graphite; however, carbon fibres and graphite fibres differ.  The basic differences lie in the temperature at which the fibres are made and heat treated, and in the amount of elemental carbon produced.  Carbon fibres typically are carbonized in the region of 1315°C (2400°F) and assay at 93 to 95% carbon, while graphite fibres are graphitized at 1900 to 2480°C (3450 to 4500°F) and assay at more than 99% elemental carbon.  See also pyrolysis (of fibres).

CARBON-CARBON – A composite material consisting of carbon or graphite fibres in a carbon or graphite matrix.

CARBONIZATION – The process of pyrolysation in an inert atmosphere at temperatures ranging from 800 to 1600°C (1470 to 2910°F) and higher, usually at about 1315°C (2400°F).  Range is influenced by precursor, individual manufacturer’s process, and properties desired.

CAST – To form material into a certain shape by pouring it into a mould and letting it harden without applying external pressure.

CASTING RESIN – A resin in liquid form that can be poured or otherwise introduced into a mould and shaped without pressure into solid articles.   See ISO 472.

CATALYST – A substance that changes the rate of a chemical reaction without itself undergoing permanent change in composition or becoming a part of the molecular structure of the product.  A substance that markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts of primary reactants.  See also accelerator, curing agent, hardener, inhibitor, and promoter.  See ASTM D 907 and ISO 472.

CAUL PLATES(A) Smooth metal plates, free of surface defects, the same size and shape as a composite lay-up, used immediately in contact with the lay-up during the curing process to transmit normal pressure and temperature, and to provide a smooth surface on the finished laminate.  (B) A caul is used to protect either the faces of the assembly or the press platens, or both, against marring and staining to prevent sticking; to facilitate press loading; to impart a desired surface texture or finish; and to provide a uniform pressure distribution.  (C) A caul may be of any suitable material such as aluminium, stainless steel, hardboard, fiberboard, or plastic, the length and width dimensions generally being the same as those of the plates of the press where it is used.  Sometimes plates the same size as the part are used.  See ASTM D 907.

CAVITY – The space inside a mould in which a resin or moulding compound is poured or injected.  The female portion of mould.  That portion of the mould that encloses the moulded article (often referred to as the die).  Depending on the number of such depressions, moulds are designated as single cavity or multiple cavity.

CELL(A) In honeycomb core, a cell is a single honeycomb unit, usually in a hexagonal shape.  (B) (In cellular plastics.)  A single small cavity surrounded partially or completely by its walls.  See ISO 472.

CELL SIZE – The diameter of an inscribed circle within a cell of honeycomb core.

CELLULAR PLASTIC – (Expanded plastic, foamed plastic) A plastic whose density is reduced by the presence of numerous small cavities (cells), interconnecting or not, dispersed throughout the mass.  See ISO 472.

CERAMIC – A rigid, frequently brittle material made from clay and other inorganic, nonmetallic substances and fabricated into articles by forming, followed by sintering or densification of the article at high temperature.

CERMET – Composite materials consisting of two constituents, one being either an oxide, carbide, boride, or similar inorganic compound, and the other a metallic binder.

CFRP – Carbon Fibre Reinforced Plastic

C-GLASS – A glass with a soda-lime borosilicate composition that is used for its chemical stability in corrosive environments.

CHAMBRAY – A general class of plain weave, usually cotton, made with a coloured warp and white weft.

CHEMICALLY TREATED FABRICS – Base-layer fabrics the outer surfaces of which have been treated with a hydrophilic (water-loving) chemical that wicks moisture through the fabric to the outer surface.

CHOPPED STRAND MAT – A mat formed of strands cut to a short length, randomly distributed, without intentional orientation, and held together by a binder.  See ISO 472.

CHOPPED STRANDS – Short strands cut from continuous filament strands, not held together by any means.  See ISO 472.

CHROME FINISH – (Volan ‘A’) Applied to glass fibres to give good bonding to polyester and epoxy resins.

CLEAN ROOM – A special room where composite or bonded metal components are assembled prior to bonding in a clinical atmosphere.  The room is sealed as far as possible, kept scrupulously clean at all times, temperature and humidity controlled, and has a positive air pressure inside so that air flow is always outwards to minimize contamination. Operators wear clean overalls, clean gloves, and caps, and visitors are generally discouraged.  When allowed into a clean room, they also have to wear clean outer garments to prevent contamination.  No cutting, drilling or sanding operations are permitted in a clean room, it is for the assembly of clean, dry components and materials only, immediately prior to bonding.

CLIMA-GUARD – A Swiss-made polyester microfibre with super-strength durable water repellent treatment that’s used in light shells and sleeping bags.

CLIMBING DRUM PEEL TEST – For details see ASTM D 1781 and Reference 2.15.  This test is used where the bonded members are not flexible enough to be peeled in a more usual manner.  The less rigid of the two members is peeled by winding it around a rigid cylinder – the “climbing drum”.  This is a practical test used to assess the quality of adhesive bonds between thin skins and honeycomb or other core materials for lightweight sandwich panels, and also for testing adhesives by peeling a thin sheet from a much thicker one.

CLOSED CELL – A cell totally enclosed by its walls and hence non-interconnecting with other cells.  See ISO 472.

CLOTH – A woven product made from continuous yarns or tows of fibre. “Cloth” and “fabric” are usually used interchangeably.

CLOUD YARN – A term given to yarns of irregular twist obtained by alternately holding one of the component threads while the other, being delivered quickly, is twisted around it, and then reversing the position of the two threads; thus producing alternate clouds of the two colours.

CLUMP – A group of chopped bundles of glass fibres that has collected on the SMC machine and then fallen into the bed of glass. The clump produces areas of high glass content which may not wet-through.

COBONDING – The curing together of two or more elements, of which at least one has already been fully cured and at least one is uncured.  This requires film adhesive between any precured and uncured interface.

CO-CONSOLIDATED – A processing step where two or more thermoplastic preformed parts are joined by properly locating in a fixture or tool and reheating to melt under pressure.

CO-CURING – The act of curing a composite laminate and simultaneously bonding it to some other uncured structure, or to a core material such as honeycomb or foam.  See also secondary bonding.

COEFFICIENT OF THERMAL EXPANSION (CTE) – The change in length or volume per unit length or volume produced by a 1° rise in temperature.  See ISO 472.

CO-FAB – Fabrication process where close-outs and inserts are bonded into the panel at the same time as the facings are bonded to the core.  ASTM C 274.

COHESION – The propensity of a single substance to adhere to itself. The internal attraction of molecular particles toward each other.  The ability to resist partition of itself.  The force holding a single substance together.  See ASTM D 907.

COHESIVE FAILURE – Failure of an adhesive joint occurring primarily in an adhesive layer.  See ASTM D 907.

COLLAPSE YARN – Collapse yarn is (usually) an over spun single, dried under tension (see “blocking“) that is then knit or woven. When the item is moistened, the yarn returns to its original elastic state. See “balanced yarn“.

COLLET – A spool on which the gathered strands from the bushing are wound for further processing.

COMFORTREL – A chemically treated polyester base-layer fabric made by Wellman Fibre Industries.

COMPOSITE MATERIAL – A combination of two or more materials (reinforcing elements, fillers, and composite matrix binder), differing in form or composition on a macro scale.  The constituents retain their identities; that is, they do not dissolve or merge completely into one another although they act in concert.  Normally, the components can be physically identified and exhibit an interface between one another.  See ISO 472.

COMPOSITE PART – An individual part that is either of the following:  (A)  An inseparable assembly of composite materials cured, consolidated, cocured, cobonded or secondary bonded together, alone, or in combination with other composite or non-composite parts.  (B)  An uncured assembly of composite materials that have been stacked up and compacted together alone or in combination with other composite or non-composite parts.  (BDS 1330)

COMPOUND – The intimate admixture of a polymer with other ingredients, such as fillers, softeners, plasticizers, reinforcement, catalysts, pigments, or dyes.  A thermoset compound usually contains all the ingredients necessary for the finished product, while a thermoplastic compound may require subsequent addition of pigments, blowing agents, etc.  See ISO 472.

COMPRESSION MOULD – A mould that is open when the material is introduced and that shapes the material by the pressure of closing and by heat.

COMPRESSION TEST (COMPOSITES) – See ASTM D 3410.  Compressive properties of unidirectional or cross-ply fibre-resin composites.  This gives two methods, one using a specially designed fixture to give true compressive failure and the other using a sandwich beam specimen.

COMPRESSIVE STRENGTH – The ability of a material to resist a force that tends to crush or buckle. The maximum compressive load sustained by a specimen divided by the original cross-sectional area of the specimen.

COMPRESSIVE STRESS – The normal stress caused by forces directed toward the plane on which they act.  The compressive load per unit area of original cross section carried by the specimen during the compression tests.

CONDENSATION POLYMERIZATION – A chemical reaction in which two or more molecules combine, with the separation of water or some other simple substance.  If a polymer is formed, the process is called poly-condensation.  See also polymerization.

CONDENSATION REACTION – A chemical reaction in which two different molecules react to form a new compound of greater complexity, with the formation of water, alcohol, ammonia, etc., as a by-product.

CONFORMABILITY – A mat’s ability to conform to difficult shapes without wrinkling or leaving excessively resin-rich or glass-rich radii, which may craze.

CONSISTENCY – That property of a liquid adhesive by virtue of which it tends to resist deformation.  Consistency is not a fundamental property, but is composed of viscosity, plasticity, and other phenomena.  See also viscosity.   See ASTM D 907.

CONSOLIDATION(A) In metal matrix or thermoplastic composites, a processing step in which fibre and matrix are compressed by one of several methods to reduce voids and achieve desired density.  (B) The joining together under heat and pressure of multiple plies of thermoplastic composite materials, which may be resoftened without undergoing a chemical change when heated.  This is a reversible process with some limitations on the number of reprocessing cycles possible, depending on the thermoplastic. (C) A process that fuses each ply together by tacking and flowing the matrix between the plies.  Usually involves heat and pressure.

CONSTITUENT – In general, an element of a larger grouping.  In advanced composites, the principal constituents are the fibres and the matrix.

CONTACT ADHESIVE – An adhesive that is apparently dry to the touch and which will adhere to itself simultaneously upon contact.  An adhesive applied to both adherends and allowed to become dry, which develops a bond when the adherends are brought together without sustained pressure.

CONTACT ANGLE – When a drop of liquid is placed on a surface it will either retract into a ball, like a drop of mercury, or spread out like water on a clean, high energy surface.  The contact angle is the angle made between the surface and a tangent to the surface of the drop at the point of contact with the surface.  Low contact angles mean good “wetting” of a surface and high contact angles mean poor “wetting”.  A major purpose of surface treatments prior to adhesive bonding is to ensure good “wetting” of the bonding faces when the adhesive is applied.

CONTAMINANT – An impurity or foreign substance present in a material or environment that affects one or more properties of the material, particularly adhesion.

CONTINUOUS FILAMENT(A) An individual flexible rod of small diameter of great or indefinite length.  (B) A yarn or strand in which the individual filaments are substantially the same length as the strand.  CMH-17 (WAS MIL-HDBK-17).

CONTINUOUS ROVING – Parallel filaments coated with a sizing (finish), gathered together into single or multiple strands.  It may be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high strength moulding compounds, or it may be used chopped.

CONTINUOUS STRAND – Fibreglass mat of very long individual fibres that have a regular crossed pattern and are loosely held together with a binder.

CONTINUOUS-FILAMENT YARN – Yarn formed by twisting two or more continuous filaments into a single, continuous strand.

COOLING FIXTURE – A fixture used to maintain the shape or dimensional accuracy of a moulding or casting after it is removed from the mould and until the material is cool enough to hold its own shape.

COOLMAX – DuPont’s hydrophobic (water-hating) polyester with fibre cross sections that produce a strong wicking action; often used in outerwear linings and light layering garments.

COPOLYMER – A long-chain molecule formed by the reaction of two or more dissimilar monomers.  See ISO 472.

COPOLYMERIZATION – Polymerization in which a copolymer is formed.  See ISO 472.

CORDURA – DuPont’s abrasion-resistant, texturized nylon fabric used mostly in backpacks. In clothing, it’s sometimes used to reinforce such high-wear areas as knees, shoulders, and elbows.

CORDURA PLUS – DuPont’s fine-filament Cordura; it has a soft texture and is used for outerwear.

CORDUROY – A strong, durable fabric with cotton ground and vertical cut-pile stripes (wales) formed by an extra system of filling yarns. The foundation of the fabric can be either a plain or twill weave.

CORE(A) The central member, usually foam or honeycomb, of a sandwich construction to which the faces of the sandwich are attached or bonded. (B) The central member of a plywood assembly.  (C) A channel in a mould for circulation of heat transfer media.  (D) Part of a complex mould that forms undercut parts.  (E) A device on which prepreg is wound.

CORE CRUSH – A collapse, distortion, or compression of the core.

CORE DEPRESSION – A localized indentation or gouge in the core.

CORE NODES – The points at which honeycomb cells are bonded to each other.

CORE SPLICING – The joining of segments of a core by bonding, or by overlapping each segment and then driving them together.

CORE STABILIZATION – A process to rigidize honeycomb core materials to prevent distortion during machining.

CORONIZING – Continuous heat cleaning and weave setting.

CORROSION – The deterioration of a metal by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals or other agents or media.  See electrolytic corrosion, galvanic corrosion.

CORROSION FATIGUE – The formation of fatigue cracks due to the conjoint action of alternating mechanical loading and corrosion.  The presence of corrosion reduces the stress required to cause cracking.

CORROSION RESISTANCE – The ability of a material to withstand contact with ambient natural factors or those of a particular artificially created atmosphere, without degradation or change in properties.  For metals this could be pitting, general surface corrosion, or in the case of iron and steel, rusting.

COTTON – A cellulose fibre collected from the perennial shrub from the genus Gossypium; predominantly G. hirsutum (upland or long-staple cotton), but also some G. barbadense (Pima or extra-long-staple cotton). A vegetable fibre consisting of unicellular hairs attached to the seed of the cotton plant. Most cotton is coloured a light to dark cream, and its chemical composition is almost pure cellulose. Coloured cottons in shades of tan, greens, blue, and rust are also less commonly available. A distinct feature of the mature fibre is its spirality or twist.

COTTON COUNT – The cotton count expresses the number of hanks required to make a pound of yarn. A hank of cotton is equal to 840 yards. So 1 cc = 840 yards of cotton, the coarsest cotton yarn. A 3 cc yarn would then be one third as course and would be expressed as 3/1 cc show that it is a single strand. Likewise plies are designated by two numbers separated by a slash such as 4/2 cc. This equals 3360 yards (4 x 840) of two-ply yarn. This yields 1680 yards of yarn per pound (3360/2). An 8/4 cc yarns would yield the same number of yards per pound, but would be a 4 plies of finer yarn. So a Number 8 four-ply yarn is the same diameter as a Number 4 two ply yarn.

COTTON DUCK – A heavy canvas treated to make it water-resistant.

COUNT – For fabric, number of warp and filling yarns per inch in woven cloth.  For yarn, the size based on relation of length and weight.  Basic unit is a “tex”.The number given to a yarn of any material, usually indicating the number of hanks per pound of that yarn. May also refer to the fineness to which a fleece may be spun. There are at least three definitions. In raw wool, a number used to indicate fineness, see “micron count“. In worsted yarn, the number of 560-yard skeins weighing one pound (Bradford method). In woolen yarn, the number of 256-yard skeins weighing one pound (Yorkshire method).

COUPLANT – See liquid couplant.

COUPLING AGENT – Any chemical substance designed to react with both the reinforcement and matrix phases of a composite material to form or promote a stronger bond at the interface.  A bonding link.  See ISO 472.

COUPLING STIFFNESS MATRIX – The [B] matrix is developed in composite laminated structures where there is a lack of symmetry of the stacked plies through the thickness about the mid-plane of the laminate.  The [B] matrix terms cause an out-of-plane deformation under in-plane loads, or this out-of-plane twist and bending result in an induced axial extension.  The [B] matrix is also termed the extensiorial-flexural coupling matrix.

COUPON – Usually a specimen for a specific test, as a tensile coupon.

CO-WOVEN FABRIC – A reinforcement fabric woven with two different types of fibres in separate yarns; for example, thermoplastic fibres woven side by side with carbon fibres.

CRABBING – A term used in the textile industry. Crabbing sets the cloth and yarn twist by rotating the fabric over cylinders through a hot-water bath, or through a series of progressively hotter baths, followed by a cold-water bath. Crabbing is done to stabilize the fabric before dyeing and finishing and is necessary only for worsted fabrics.

CRACK – Fractures in either matrix or both matrix and fibres.  An actual separation of material.  Does not necessarily extend through the thickness of the composite, but can be stopped by differently oriented plies.

CRACK GROWTH – Rate of propagation of a crack through a material due to a static or dynamic applied load.

CRAZING – Region of ultrafine cracks, which may extend in a network on or under the surface of a resin or plastic material.  May appear as a white band.  Often found in a filament-wound pressure vessel or bottle.  In plastics, crazing occurs where a crack is bridged by fibrils that still carry significant load.  Crazing becomes cracking when the fibrils break and no load is transmitted.  Crazing appears as cracking to the naked eye.  A craze is a narrow zone of highly deformed and voided polymer resembling a true crack.  Crazing occurs at a critical strain, or stress, and these can be greatly reduced in the presence of active environments (notably organic solvents in the case of polymeric glasses).  See ASTM D 907 and ISO 472.

CREEL – Glass Fibre Manufacturing – A framework used to hold forming cakes so they can be wound or roved into roving doffs. Creels generally hold 10 to 33 forming cakes that are replaced randomly when they run out or as doffs are roved. Composite Fabrication – The area in which pallets of roving are placed and “threaded up” through metal tubes or guide eyes to a chopper.

CREEP – The change in dimension of a material under load over a period of time, not including the initial instantaneous elastic deformation.  (Creep at room temperature is called cold flow.)  The time-dependent part of strain resulting from an applied stress.  See ISO 472.

CRIMP[A] Crimp is defined [1] as “the waviness of a fibre” and is normally expressed numerically as either “the number of waves or crimps per unit length” or “the difference in distance between points on the fibre as it lies in a crimped condition and the same two points when the fibre is straightened under suitable tension”. The mechanical properties of composites reinforced with these fabrics increase as the crimp decreases, but drape and in-plane permeability normally decrease as the crimp decreases. [B] The wave effect in the wool fibre. Usually the finer wools show the most crimp. Uniformity of desired crimp generally indicates superior wool.  Distinct refers to crimps that are sharp and clear — fine wools have more crimps per inch bold larger crimp refers to spaces widely apart — coarser wools have fewer crimps per inch. In yarns, crimp relates to the distortion of a yarn due to its interlacing in a fabric.

CRIMP RECOVERY – The ability of a yarn or fibre to return to its original crimped state after being released from a tensile force.

CRINKLE – The waviness of each individual fibre when separated from a lock. It is responsible for elasticity and is usually irregular.

CRITICAL LENGTH – The minimum fibre length required for shear loading to its ultimate strength by the matrix.

CRITICAL STRAIN – The strain at the yield point.

CROPPING – The process of cutting the pile on a fabric to uniform height or cutting loose fibres from the surface of a cloth after weaving.

CROSS LAMINATED – Material laminated so that some of the layers are oriented at various angles to the other layers with respect to the laminate reference axis.  A cross-ply laminate usually has plies oriented only at 0 and 90. Laminated with some of the layers oriented at one or more angles to the other layers with respect to the principal laminate axis. (See cross-ply laminate and fibre architecture.)

CROSS LINKING – Applied to polymer molecules, the setting-up of chemical links between the molecular chains.  When extensive, as in most thermosetting resins, cross-linking makes one infusible super molecule of all the chains.  See ISO 472.

CROSSLINK (verb) – To form multiple intermolecular covalent or ionic bonds between polymer chains.  See ISO 472.

CROSS-PLY – Any filamentary laminate which is not uniaxial. In some references, the term cross-ply is used to designate only those laminates in which the laminae are at right angles to one another while the term angle-ply is used for all other layup combinations. In this manual, the terms cross-ply and angle-ply are used synonymously. A laminate with plies usually oriented at 0 and 90 only.  See ISO 472. The laminae are at right angles to each other.

CROSS-PLY LAMINATE – A laminate with plies usually oriented at 0° and 90° only.  See ISO 472.

CROSSWISE DIRECTION – Crosswise refers to the cutting of specimens and to the application of load.  For rods and tubes, crosswise is any direction perpendicular to the long axis.  For other shapes or materials that are stronger in one direction than in another, crosswise is the direction that is weaker.  For materials that are equally strong in both directions, crosswise is an arbitrarily designated direction at right angles to the lengthwise direction.  See ISO 472.

CRUSH SPLICING – The joining of segments of core by overlapping each segment two to four cells and then driving them together.

CRYSTALLINITY – In polymers, a microstructure in which the linear molecular chains are arranged in an orderly fashion.  Branched or network polymers are not crystalline, but have pockets of order within their bulk. They are, therefore, said to be “semi-crystalline”.

C-SCAN – A nondestructive inspection technique for finding voids, delaminations, defects in fibre distribution, etc., using ultrasonics.  In a basic C-Scan system, the search unit is moved over the surface of the test piece in a search pattern. The part must be scanned in “through-transmission mode” to produce a picture of the damage.  C-Scan results are easier to interpret than A-Scan signals, but C-Scan is usually limited to flat or mildly curved parts which will fit into the tank.  See also A-Scan and NDI.

C-STAGE – The final stage in the reaction of certain thermosetting resins in which the material is practically insoluble and infusible.  Sometimes referred to as Resite.  The resin in a fully cured thermoset moulding is in this stage.  See also A-stage and B-Stage.  See ASTM D 907 and ISO 472.

CURE – To irreversibly change the properties of a thermosetting resin by chemical reaction, that is condensation, ring opening, or addition.  Cure may be accomplished by addition of curing (cross-linking) agents, with or without heat and pressure.  To permanently change the state of an epoxy material from the B-stage to the C-stage by the controlled action of heat and pressure.  See ASTM D 907.

CURE CYCLE – The time/temperature/pressure cycle used to cure a thermosetting resin system or prepreg.

CURE MONITORING, ELECTRICAL – Use of electrical techniques to detect changes in the electrical properties and/or mobility of the resin molecules during cure.  A measuring of resin cure.

CURE STRESS – A residual internal stress produced during the curing cycle of composite structures.  Normally, these stresses originate when different components of a lay-up have different thermal coefficients of expansion.

CURING AGENT – A catalytic or reactive agent that, when added to a resin, causes polymerization.  Also called hardener.  See ISO 472.

CURING TEMPERATURE – Temperature at which a cast, moulded, or extruded product, a resin-impregnated reinforcement, an adhesive, etc., is subjected to curing.  The temperature at which the chemical reaction required to cure a particular material will be activated and run to completion.  In some cases, the temperature needs to be fairly closely controlled.  See data sheet for each material.

CURING TIME – The length of time necessary for a part to be subjected to heat or pressure (or both) in order to fully cure a resin.  The interval of time between the instant at which relative movement between the moving parts of mould ceases and the instant pressure is released.  Further cure may take place after removal of the assembly from the conditions of heat and pressure.

CURL YARN – A type of yarn which presents curls or loops of various sizes all along its surface. It is usually produced as follows: Two threads, a thick and a thin are twisted together, the thin being held tightly and the thick thread slackly twisted around it. This two-fold yarn is then twisted in the reverse direction with another thin thread, this untwisting throwing up the thick thread as a loop, the two fine threads holding the loops firmly.

CUTS AND SCRATCHES – Cuts and scratches can be treated as surface damage.   The severity of surface scratches and notches depends on their width, depth and orientation to the fibres or loading direction.   A high reduction in the static strength is possible, but with the current design allowable strain they are not critical.

CYCLE – One full sequence in a moulding operation, from a point in the process to the same point in the next sequence.  The complete, repeating sequence of operations in a process or part of a process.


DAM – Boundary support or ridge used to prevent excessive edge bleeding or resin run-out of a laminate and to prevent crowning of the bag during cure.

DAMAGE TOLERANCE(A) A design measure of crack growth rate.  Cracks in damage tolerant designed structures are not permitted to grow to critical size during expected service life.  (B) The ability of a structure to withstand damage, as by impact, and still perform acceptably.

DAMAGED FILAMENTS – Broken filaments, knots, splices, split tow, fibre separation, hollow fibres or interrupted fibres all come under the heading of damaged filaments.   Such damage will reduce the filament strength and fibre/matrix interface strength, and thus could degrade the lamina stiffness.   Filament damage is a result of poor prefabrication control and handling.

DAMPING – The decay with time of the amplitude of free vibrations of a specimen.  See also hysteresis and attenuation.

DDM – Diaminodiphenyl Methane.  An aromatic amine curing agent for epoxy resins.  Requires elevated temperature cure.

DDS: – Diaminodiphenyl Sulphone.  An aromatic amine curing agent for epoxy resins.  Requires elevated temperature cure.

DEBOND – A separation of a bonded joint or interface, sometimes deliberately created for repair or rework purposes.  Also an unbonded or non-adhered region; a separation at the fibre-matrix interface due to strain incompatibility.  The term often refers to accidental damage.  Also see disbond and delamination.

DEBULKING – Compacting of a thick laminate under moderate heat and pressure, i.e., non-curing conditions and/or vacuum to remove most of the air, to ensure seating on the tool and to prevent wrinkles.

DECITEX – A unit of weight indicating the fineness of yarns and equal to a yarn weighing one gram per each 10,000 meters. The abbreviation for this is “d’tex”.

DECOMPOSITION – A chemical and physical material breakdown due to an excess exposure to heat and oxidation, or due to the effects of bacterial contamination of the adhesive.

DEFLASHING – A finishing technique used to remove the flash (excess, unwanted material) on a plastic moulding.

DEFLECTION TEMPERATURE UNDER LOAD – The temperature at which a simple cantilever beam deflects a given amount under load.  Formerly called heat distortion temperature.

DEGASSING – Removal of air or gases from an adhesive usually accomplished by subjecting the material to a vacuum.

DEGRADATION – A deleterious change in the chemical structure, physical properties, or appearance of a plastic adhesive or any other material.

DEGREASE – To remove oil and grease from adherend surfaces.

DEHYDRATION:  Removal of water as such from a substance or after formation from a hydrogen and hydroxyl group in a compound, by heat or a dehydrating substance.

DELAMINATE – To split a laminated plastic material along the plane of its layers.

DELAMINATION – Separation of the layers of material in a laminate, either local or covering a wide area.  Can occur in the cure or subsequent life.  See ASTM D 907 and ISO 472. 

DENIER – A textile term for the weight, in grams, of 9000 meters of fibre tow. Numbering system for continuous yarn and continuous filaments in which the yarn number is equal to the weight in grams per 9,000 meters of yarn; the finer the yarn, the lower the denier. (Engineering term)A yarn and filament numbering system in which the yarn number is numerically equal to the weight in grams of 9000 meters of yarn.  Used for continuous filaments.  The lower the denier, the finer the yarn.  Most commonly seen in aramid and polyethylene fibres. A direct numbering system for expressing linear density, equal to mass in grams per 9000m of yarn, filament, fibre, or other textile strand. A unit of weight expressing the size or coarseness of a natural or synthetic fibre or yarn. The weight in grams of 9000 meters of a fibre or yarn; the lower the denier, the finer the yarn. 1 denier = wt. in g/9000 m = 1/9 tex. A unit of measure expressing the thickness of a thread or yarn. A high number identifies a large fibre and strong material, and a low number applies to a fine fibre and less yield. One denier equals 1 gram per 9,000 meters (5.5 miles). In packs, 500- to 1000-denier fabrics are commonly used. In clothing, a 70-denier fabric is strong, heavy, and often used in tough shell garments; a 30-denier fabric is lightweight and usually found in insulated garments like down jackets. A unit of weight indicating the fineness of fibre filaments and yarns, both silk and synthetic, and equal to a yarn weighting one gram per each 9000 meters; used especially in indicating the fineness of women’s hosiery. The abbreviation for this is “d” or “D”. Another term used to describe yarns is denier. This term isn’t used very often for composites, and is most often applied to Kevlar. Denier is equal to the weight in grams of 9000 meters of yarn. The lower the number, the finer the yarn. For example, a 380 denier Kevlar 49 yarn has 267 filaments, and an 1140 denier yarn has 768 filaments.

DENIM – A well-known basic cotton or blended fabric in a right- or left-hand woven twill. Generally, the warp is dyed blue with a weft.

DENSITY – Weight per unit of volume.  Commonly expressed in grams per cubic centimeter, pounds per cubic inch, and pounds per cubic foot or kilograms per cubic meter.  Pounds per cubic foot and kilograms per cubic meter are usually used for sandwich filler materials, foams and honeycomb core, while the other units are used more generally.

DEPOLYMERIZATION – Separation of a more complex molecule into two or more simpler molecules chemically similar to and having the same empirical composition as the original.  Reverse of polymerization.

DEPOSITION – The process of applying a material to a base by means of vacuum, electrical, chemical, screening, or vapour methods, often with the assistance of a temperature and pressure container.

DESICCANT – Substance which can be used for drying purposes because of its affinity for water.

DESIGN ALLOWABLES – Material property allowable strengths, usually referring to stress or strain, for design purposes based on a sufficient number of tests to be statistically significant, and to give values with specified levels of confidence.  See also A-basis, B-basis, S-basis, and typical basis.  A limiting value for a material property that can be used to design a structural or mechanical system to a specified level of success.

DESIZING – The process of eliminating sizing, which is generally starch, from gray (also Griege) goods before applying special finishes or bleaches (for yarn such as glass or cotton).  Also, removing lubricant size following weaving of a cloth.

DESTRUCTIVE TESTING – Actual destruction of a bonded assembly or test pieces for the purpose of evaluating the bond properties.  Destructive tests are usually made initially for qualification of materials and tools and periodically for tool, process and material control.

DETA – Diethylene Triamine.  A curing agent for room temperature curing epoxies.

DETERIORATION – A permanent change in the physical properties of a plastic evidenced by impairment of these properties.  See ISO 472.

DEVIATION – Variation from a specified dimension or requirement, usually defining the upper and lower limits.  CMH-17 (WAS MIL-HDBK-17).

DEW POINT – The temperature to which water vapour must be reduced to obtain saturation vapour pressure, that is, 100% relative humidity.  As air is cooled, the amount of water vapour it can hold decreases.  If air is cooled sufficiently, the actual water vapour pressure becomes equal to the saturation water vapour pressure, and any further cooling normally results in the condensation of moisture.

DGEBA – Di-glycidylether of Bisphenol “A”, the basic epoxy resin.

D-GLASS – A high boron content glass made especially for laminates requiring a precisely controlled dielectric constant.

DICY – Dicyandiamide.  A curing agent for epoxy resins.  Used with 180°C curing systems and with an accelerator is also used with 120°C curing systems.

DIELECTRIC – A nonconductor of electricity.  The ability of a material to resist the flow of an electrical current.

DIELECTRIC MONITORING – A means of tracking the cure of thermosets by changes in their electrical properties during material processing.

DIFFERENTIAL SCANNING CALORIMETRY (DSC) – Measurement of the energy absorbed (endotherm) or produced (exotherm) as a resin system is cured.  Also detects loss of solvents and other volatiles.  May be applied to melting, crystallization, resin curing, loss of solvents, and other processes involving an energy change.  May also be applied to processes involving change in heat capacity such as the glass transition.  See ISO 472.

DIFFERENTIAL THERMAL ANALYSIS (DTA) – An experimental analysis technique in which a specimen and a control are heated simultaneously and the difference in their temperature is monitored.  The difference in temperature provides information on relative heat capacities, presence of solvents, changes in structure (that is, phase changes, such as melting of one component in a resin system), and chemical reactions.  See also differential scanning calorimetry.  See ASTM D 907.

DIFFUSION – The movement of a material such as gas or a liquid, in the body of a plastic.  If the gas or liquid is absorbed on one side of a piece of plastic and given off on the other side, the phenomenon is called permeability.  Diffusion and permeability are not due to holes or pores in the plastic, but are caused and controlled by chemical mechanisms.

DILUENT – An ingredient usually added to an adhesive to reduce the concentration of bonding materials.  A diluting agent, any liquid or solid which, when added to another liquid or solid, reduces the quantity per unit volume of the base material in the total volume.  A reactive diluent may also take part in the curing action and is primarily used to reduce viscosity.

DIMENSIONAL STABILITY – Ability of a plastic part to retain the precise shape to which it was moulded, cast, or otherwise fabricated.

DIRECTION OF TWIST – (S twist or Z twist) to determine twist, hold yarn in a vertical position and examine the angle of the spiral. The angle of the S twist will correspond to the centre portion of the S. The angle of the Z twist will correspond to the centre portion of the Z. When spinning, the wheel should rotate counter clockwise for an S twist and rotate clockwise for a Z twist.

DISBOND – An area within a bonded interface between two adherends in which an adhesion failure or separation has occurred.  It may occur at any time during the life of the structure, and may arise from a wide variety of causes.  Also, colloquially, an area of separation between two laminae in the finished laminate (in this case, the term delamination is normally preferred.)  See also debond.

DISTORTION – In fabric, the displacement of fill fibre from the 90° angle (right angle) relative to the warp fibre.  In a laminate, the displacement of the fibres (especially at radii), relative to their idealized location, due to motion during lay-up and cure.

D-MATRIX – see Flexural Stiffness Matrix

DOBBY – A general term for a fabric woven on a special dobby loom, which allows the weaving of small, geometric figures. A dobby weave can often be distinguished from a plain weave by the patterns are beyond the range of simple looms.

DOUBLE CANTILEVER BEAM TEST (DCB) – See ASTM D 3433 for both the parallel and tapered adherend versions.  Used to obtain more accurate values of Mode I adhesive fracture energy than those obtained from wedge tests.  The tapered version can be made by tapering depth or width.  The latter is favored for composite specimens.  Both tapered versions are designed to give a constant value of fracture energy with increasing crack length.

DOUBLER(A) Localized area of extra layers of reinforcement, usually to provide stiffness or strength for fastening or other abrupt load transfers.  See tabs.  (B) An extra piece of facing attached to strengthen or stiffen the panel or to distribute that load more widely into the core.  See ASTM C 274.

DOUBLING – The process of combing by twisting together two single yarns to form a double yarn.

DRAFT – The taper or slope of the vertical surfaces of a mould designed to facilitate removal of moulded parts.

DRAFT ANGLE – The angle of a taper on a mandrel or mould that facilitates removal of the finished part.  The angle between the tangent to the surface at that point, and the direction of ejection.

DRAPE – The ability of pre-impregnated broad goods to conform to an irregular shape; textile conformity. The way a fabric hangs. Drape is affected by yarns, weave structure, and finish. The ability of a fabric or prepreg to conform to a contoured surface. If the resin becomes hard because of loss of solvent or staging, the prepreg becomes stiff and loses its drape characteristics.

DRAW – To shape plastic by stretching or deforming through dies.

DRY(A) To remove standing water from a part that has been in service.  (B) To remove absorbed moisture from a part that has been in service using moderate heat or heat and vacuum.  Composite parts require drying before repair.

DRY FIBRE AREA – Area of fibre not totally encapsulated by resin.  CMH-17 (WAS MIL-HDBK-17).

DRY LAMINATE – A laminate containing insufficient resin for complete bonding of the reinforcement.  See also resin-starved area.

DRYING TEMPERATURE – The temperature to which an adhesive on an adherend or in any assembly, or the assembly itself, is subjected to dry the adhesive.

DRYING TIME(A) The time required to dry a component with absorbed moisture to a specified level that can be measured.  (B) The time specified for drying a component, which is considered sufficient to achieve an adequate condition for bonding, when measurement methods are not available.

DRYING TOWER – Used in the production of prepreg via a solvent process, a conveyor belt carries the prepreg through a drying section which uses heated air to remove excess solvent from the prepreg.  Usually this heated section is vertical, due to space limitations.

DRYLINE – A hydrophilic nylon/polyester push-pull fabric made by Milliken. Most often used to line waterproof/breathable outerwear.

DRYLOFT – A 2-ply laminated shell fabric from W.L. Gore designed specifically for insulating parkas and sleeping bags. It is windproof and roughly twice as breathable as Gore-Tex, but only a third as waterproof.

DRYSKIN – Schoeller Textile’s fast-drying synthetic that is stretchy and breathable as well as water, wind, and abrasion resistant. Its most common use is for outerwear.

DSC – See differential scanning calorimetry.

DTA – See differential thermal analysis.

DUCTILITY – The amount of plastic strain that a material can withstand before fracture.  Also, the ability of a material to deform plastically before fracturing.

DULL – A yarn or fibre surface lacking in lustre.

DUPLEX FILM – An adhesive film (Type II, Boeing) that has two different adhesives separated by a scrim cloth that is manufactured into one film.  BMS – 5- 70 is a duplex film with epoxy on one side and nitrile-phenolic on the other.

DURAPEL – Burlington’s water-repellent finish for use on fabrics.

DWELL – A pause in the application of pressure or temperature to a mould, made just before it is completely closed, to allow the escape of gas from the moulding material.  In filament winding, the time that the traverse mechanism is stationary while the mandrel continues to rotate to the appropriate point for the traverse to begin a new pass.  In a standard autoclave cure cycle, an intermediate step in which the resin matrix is held at a temperature below the cure temperature for a specified period of time sufficient to produce a desired degree of staging used primarily to control resin flow.

DWR – Durable Water Repellency. Generally, a silicone- or fluorocarbon-based treatment is applied to outerwear fabrics to help keep them from becoming saturated with rainwater. DWR fabrics require periodic touching up by machine drying on medium heat, careful ironing, or applying wash-in or spray-on treatments available at outdoors stores.


E.C.O. FLEECE – A synthetic fleece from Dyersburg Fabrics; 89 percent of the fabric is recycled plastic soda bottles. Various manufacturers use it under different proprietary names.

ECOPILE – A synthetic fleece from Draper Knitting Mills that is 100 percent postconsumer recycled plastic soda bottles. Various manufacturers use it under different names.

EDGE BLEED – Removal of volatiles and excess resin through the edge of the laminate, as in matched die moulding of a laminate.  In autoclaved parts, edge bleeding is discouraged, since excess resin will only be removed from the area near an edge, resulting in uneven resin distribution.  May also be called “Squeeze Out” or “Horizontal Bleed”. 

EDGE CLOSE-OUTS – Members placed around the panel sides to protect the sandwich from damage or to attach the panel to a support or other panel.

EDGE DELAMINATION – A separation of the detail parts along an edge after the assembly has been cured.

EDGE JOINT – A joint made by bonding the edge faces of two adherends.

EDGEWISE – Refers to cutting specimens and to the application of load.  The load is applied edgewise when it is applied to the edge of the original sheet or specimen.  For compression-moulded specimens of square cross-section, the edge is the surface parallel to the direction of motion of the moulding plunger.  For injection moulded specimens of square cross section, this surface is selected arbitrarily; for laminates, the edge is the surface perpendicular to the laminae. See flatwise.

E-GLASS – A family of glasses with a calcium aluminoborosilicate composition and a maximum alkali content of 2.0%.  A general purpose fibre that is most often used in reinforced plastics, and is suitable for electrical laminates because of its high resistivity.  Also called electric glass.

EJECTION – Removal of the moulded part from the mould by mechanical means or with compressed air.

ELASTIC DEFORMATION – The part of the total strain in a stressed body that disappears upon removal of the stress.

ELASTIC LIMIT – The greatest stress a material is capable of sustaining without permanent strain remaining after the complete release of the stress.  A material is said to have passed its elastic limit when the load is sufficient to initiate plastic, or non-recoverable deformation.  See ISO 472.

ELASTIC RECOVERY(A) The fraction of a given deformation that behaves elastically.  (B) A perfectly elastic material has an elastic recovery of 1; a perfectly plastic material has an elastic recovery of 0.  Elastic Recovery = elastic extension divided by total extension.

ELASTICITY – That property of materials by virtue of which they tend to recover their original size and shape after removal of a force causing deformation.  See also viscoelasticity.  See ISO 472.

ELASTOMER – A material that substantially recovers its original shape and size at room temperature after removal of a deforming force.  See ASTM D 907 and ISO 472.

ELASTOMERIC TOOLING – A tooling system that uses the pressure from the thermal expansion of rubber materials to form composite parts during cure.

ELECTROFORMED MOULDS – A mould made by electroplating metal on the reverse pattern of the cavity.  Molten steel may then be sprayed on the back of the mould to increase its strength.  Nickel is sometimes used for this purpose.

ELONGATION – Deformation caused by stretching. The fractional increase in length of a material stressed in tension.  (When expressed as percentage of the original gauge length, it is called percentage elongation.)  See ISO 472.

EMI/RFI – Electromagnetic/radio frequency interference.

ENCAPSULATION – The enclosure of an item in plastic, or other material.  Sometimes used specifically in reference to the enclosure of capacitors or circuit board modules.  See ISO 472.

END – A strand of roving consisting of a given number of filaments gathered together.  The group of filaments is considered an “end” or strand before twisting and a “yarn” after twist has been applied.  An individual warp yarn, thread, fibre, or roving. A single bundle of filaments. A warp yarn.

END AND PICK COUNT – If the end and pick counts are roughly equal, the fabric is considered bidirectional, often called BID. If the pick count is very small, most of the yarns run in the warp direction, and the fabric is nearly unidirectional. I have seen some unidirectional cloths which have no fill yarns; instead, the warp yarns are held together by a thin stream of glue (picture squeezing a bottle of glue over the fabric and moving it around randomly). An exact number of ends supplied on a ball of roving. A single fibre, strand, roving or yarn incorporated into a product. An end may be an individual wrap yarn or cord in a woven fabric. In referring to aramid and glass fibres, an end is usually an untwisted bundle of continuous filaments.

ENDOTHERMIC – A chemical reaction which absorbs heat energy is said to be endothermic.  A compound, the formation of which absorbs heat, is an endothermic compound.  Such compounds are less stable than exothermic compounds, many of them being explosive.

ENVIRONMENT – The aggregate of all conditions (such as contamination, temperature, humidity, radiation, magnetic, and electric fields, shock, and vibration) that externally influence the performance of an item.

ENVIRONMENTAL STRESS CRACKING (ESC) – The susceptibility of a thermoplastic resin to crack or craze when in the presence of surface active agents or other environments.

EPICHLOROHYDRIN – The basic epoxidizing resin intermediate in the production of epoxy resins.  It contains an epoxy group and is highly reactive with polyhydric phenols such a Bisphenol A.

EPOXIDE – Compound containing the oxirane structure, a three-member ring containing two carbon atoms and one oxygen atom.  The most important members are ethylene oxide and propylene oxide.

EPOXIDE EQUIVALENT – The weight of a resin in grams which contains 1 gram equivalent of epoxy.

EPOXY RESIN – A polymerizable thermoset polymer containing one or more epoxide groups and curable by reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans.   An important matrix resin in composites and structural adhesives.  See ISO 472.

EROSION – Destruction of metal or other material by the abrasive action of liquid or gas.  Usually accelerated by the presence of solid particles of matter in suspension and sometimes by corrosion.

ESD – Electrostatic discharge – A large electrical potential (4000 V or more) moving from one surface or substance to another.  ED is also an abbreviation for electrostatic dissipation.

ESTER – The reaction product of alcohol and an acid.

EUTECTIC MIXTURE – A mixture of two or more substances in such a ratio that it has the lowest melting point of any combination.

EXCESSIVE PLY OVERLAP – Excessive ply overlap will occur when the ply is not correctly trimmed during assembly.   This can result in laminate dimensional tolerance errors.   These errors could cause warping during, or induce high peel stresses.

EXFOLIATION – A surface defect on composite parts where the resin appears scaled or flaky.

EXOTHERM – The liberation or evolution of heat during the curing of a plastic product or during any chemical reaction.

EXOTHERMIC REACTION – A reaction obtained when mixing certain substances together characterized by the evolution of heat.  A reaction that gives off heat when certain substances are mixed together.  The mixing of the two parts of an epoxy resin system produces an exothermic reaction.  Exotherm can exceed curing temperature if uncontrolled.

EXPANDABLE TOOLING – Use of a hollow rubber mandrel which can be pressurized to form composite hardware during cure.

EXTENSORIAL STIFFNESS MATRIX – The [A] matrix is the in-plane stiffness matrix parameters of a composite laminated structure that defines the in-plane axial and shear deformations based on individual ply orientations.  The extensiorial stiffness matrix is independent of stacking sequence of the laminated plies.


FABRIC – A material constructed of interlaced yarns, fibres or filaments. Used interchangeably with “cloth”.  A material made of woven fibres or filaments.

FABRIC COUNT – refers to the number of warp yarns (ends) and fill yarns (picks) per inch. For example, a 24 x 22 fabric has 24 ends in every inch of fill direction and 22 picks in every inch of warp direction Note that you count warp yarns in the fill direction and fill yarns in the warp direction.

FABRIC FILL FACE – That side of the woven fabric where the majority of the exposed yarns are perpendicular to the selvage edge.

FABRIC WARP FACE – That side of the fabric where the majority of the exposed yarns are parallel to the selvage edge.

FABRIC, NONWOVEN – Planar textile constructed by bonding or interlocking but not interlacing fibres or yarns by mechanical, chemical, thermal or solvent means.

FABRIC, WOVEN – Planar textile constructed by interlacing fibres or yarns, using a weaving process.

FABRICATING (FABRICATION) – The manufacture of products from moulded parts, rods, tubes, sheeting, extrusions, or other forms by appropriate operations, such as punching, cutting, drilling, and tapping.  Fabrication includes fastening parts together or to other parts by mechanical devices, adhesives, heat sealing, welding, or other means.

FACE DIMPLING – Buckling of the compressive facing into a honeycomb cell.  ASTM C 274.

FACE WRINKLING – Buckling of the compressive facing into or away from the core.  ASTM C 274.

FACINGS(A) Skins and doublers in any layup.  (B) The outermost layer or composite component of a sandwich construction, generally thin and of high density, which resists most of the edgewise loads and flatwise bending moments, synonymous with face, skin, and face sheet.  ASTM C 274.

FAILURE – An event which occurs when a material, device, equipment, or system ceases to perform its intended function acceptably.

FAIRING – A member or structure, the primary function of which is to streamline the flow of a fluid by producing a smooth outline and to reduce drag, as in aircraft frames and boat hulls.

FAN OR CURTAIN – Chopped bundles that fall or are thrown off the chopper and cot.

FATIGUE(A) The failure or decay of mechanical properties after repeated applications of stress.  Fatigue tests give information on the ability of a material to resist the development of cracks, which eventually bring about failure as a result of a large number of cycles.  The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material.  Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.  (B) In composites, the effect of cyclic damage (fatigue) is manifested in different ways to metals, e.g., splitting and delamination, matrix cracking, and eventually fibre breakage.  Although the failure modes are different, the failure of composites under cyclic loading fits the basic definition of fatigue given in the first sentence of paragraph (A).  See ISO 472.

FATIGUE LIFE – The number of cycles of deformation required to bring about failures of the test specimen under a given set of oscillating conditions (stresses or strains).  See ISO 472.

FATIGUE LIMIT – The stress level below which a material can be stresses cyclically for an infinite number of times without failure.  See ISO 472.

FATIGUE RATIO – The ratio of fatigue strength to tensile strength. Mean stress and alternating stress must be stated.

FATIGUE STRENGTH – The maximum cyclical stress a material can withstand for a given number of cycles before failure occurs.  The residual strength after being subjected to fatigue.

FATTY ACID – An organic acid obtained by the hydrolysis (saponification) of natural fats and oils, for example stearic and palmitic acids.

FAYING SURFACE – The surfaces of materials in contact with each other and joined or about to be joined together by mechanical fastening.

FEATHERING – The tapering of an adherend on one side to form a wedge section, as used in a scarf joint.

FELT – A fibrous material made up of interlocked fibres by mechanical or chemical action, moisture, or heat.  Made from fibres such as aramid, cotton, glass, etc. 

FELT OR FELTING – Non-woven fabric made by layering thin sheets of carded wool fibres, then applying heat, moisture and pressure to shrink and compress the fibres into a thick matted cloth that will not ravel or fray. A fibrous material made up of interlocked fibres by mechanical or chemical action, moisture, or heat.  Made from fibres such as aramid, cotton, glass, etc.

FELTABILITY – The degree to which fibres will consolidate by felting.

FELTED – Describes a fabric that’s been brushed, sanded, or texturized.

FELTING – The matting together of fibres during processing or use.

FELTING PROPERTY – The property of wool and some other fibres to interlock with each other to create felt. Felting is caused by the directional friction effect of scales on the fibre surfaces. The factors involved in felting are the fibre structure, the crimp of the fibres, the ease of deformation of the fibre and the fibre’s power of recovery from deformation.

FIBRE – A general term used to refer to filamentary materials.  Often fibre is used synonymously with filament.  It is a general term for a filament with a finite length that is at least 100 times its diameter, which is typically 0.10 to 0.13 mm (0.004 to 0.005 in).  In most cases, it is prepared by drawing from a molten bath, spinning, or deposition on a substrate.   See whisker.  Fibres can be continuous or specific short lengths (discontinuous), normally no less than 3.2 mm (0.125 in).   See ISO 472.

FIBRE ARCHITECTURE – The design of a fibrous preform or part in which the fibres are arranged in a particular way to achieve a desired result. Mats and braided, stitched and woven fabrics are common forms of fibre architecture.

FIBRE CONTENT:  The amount of fibre present in a composite.  This is usually expressed as a percentage volume fraction or weight fraction of the composite.

FIBRE COUNT – The number of fibres per unit width of ply present in a specified section of a composite.

FIBRE DIAMETER – The measurement (expressed in hundred thousandths) of the diameter of individual filaments.

FIBRE DIRECTION – The orientation or alignment of the longitudinal axis of the fibre with respect to a stated reference axis. 

FIBRE DISTRIBUTION VARIANCE – Unevenness of fibre distribution or improper yarn spacing could change the laminate properties to the extent that the laminate load response will be different from design requirements.   The effects will depend on the degree of variance of the fibre distribution.   Fibre distribution is controlled by the curing process, thus this is a manufacture type defect.  

FIBRE KINKS – Fibre kinks are sharp edge buckling of fibres within the matrix.   Previous studies have concluded that kinking is a direct consequence of microbuckling.   Excessive kinking of the fibres will eventually lead to fibre fracture.

FIBRE-MATRIX DEBOND – Fibre/matrix debonding is separation at the fibre/matrix interface.   This will result in loss of shear transfer and degradation of the overall strength of the laminate. Fibre/matrix interface debonding results from excessive local shear transfer stresses, particularly where short fibres are present.  This type of defect is matrix cracking at the microscopic level.

FIBRE MISALIGNMENT – Fibre misalignment is when there is either misorientation of the ply, deviation from predetermined winding patterns, or washout of fibre from excessive resin flow.   Fibre misalignment is a fabrication control error.   The effect is a localised change in the load response of the laminate.   Local misalignment such as fibre kinks can cause fibre damage leading to loss of tensile strength or, under compressive loading, can precipitate fibre buckling and premature failure.  

FIBRE ORIENTATION – Alignment of warp fibres in accordance with an engineering drawing.  Can also mean fibre alignment in a nonwoven or mat laminate where the majority of fibres are in the same direction, resulting in a higher strength in that direction.

FIBRE PATTERN – Visible fibres on the surface of laminates or moulding.  The thread size and weave of glass cloth.

FIBRE SYSTEM – The type and arrangement of fibrous material which comprises the fibre constituent of an advanced composite.  Examples of fibre systems are collimated filaments or filament yarns, woven fabric, randomly oriented short-fibre ribbons, random fibre mats, whiskers, etc.  CMH-17 (WAS MIL-HDBK-17).

FIBRE TOW – A loose, untwisted bundle of continuous fibres. In composite technology, “tow” is often used interchangeably with “yarn”, the twisted version.

FIBRE WASH – Dislocation or displacement of reinforcing fibres placed within a mould caused by the force of the resin flow, resulting in unintended fibre distribution within the finished part. Splaying out of woven or nonwoven fibres from the general reinforcement direction.  Fibres are carried along with bleeding resin during cure.

FIBRE-COMPOSITE MATERIAL – A material consisting of two or more discrete physical phases, in which a fibrous phase is dispersed in a continuous matrix phase.  The fibrous phase may be macro, micro, or submicroscopic, but is must retain its physical identity so that it could conceivably be removed from the matrix intact.

FIBREGLASS – An individual filament made by drawing molten glass.  A continuous filament is a glass fibre of great or indefinite length.  A staple fibre is a glass fibre of relatively short length, generally less than 430 mm (17 in), and the length related to the forming or spinning process used.  A general term having the same meaning as fibreglass reinforcement below.

FIBREGLASS REINFORCEMENT – Material used to reinforce a resin matrix using continuous or discontinuous glass fibres.  Available as mat, roving, fabric, etc., it is incorporated into both thermosets and thermoplastics.

FIBRE-MATRIX INTERFACE – The region separating the fibre and matrix phases, which differs from them chemically, physically, and mechanically.  In most composite materials, the interface has a finite thickness (nanometers to thousands of nanometers) because of diffusion or chemical reactions between the fibre and matrix.  Thus, the interface can be more properly described by the terms “interphase” or “interfacial zone”.  When coatings are applied to the fibres or several chemical phases have well defined microscopic thicknesses, the interfacial zone may consist of several interfaces.

FIBRE REINFORCED PLASTIC (FRP) – A general term for a composite that consists of a resin reinforced with cloth, tape, mat, or strands of any fibre form and using any type of fibre.

FILAMENT COUNT – Number of filaments in the cross-section of a fibre bundle.

FILAMENT WINDING – A process for fabrication a composite structure in which continuous reinforcements (filament, wire, yarn, tape, or other), either previously impregnated with a matrix material or impregnated during the winding, are placed over a rotating and removable form or mandrel in a prescribed way to meet certain stress conditions.  Generally the shape is a surface of revolution, and may or may not include end closures.  When the required number of layers is applied, the wound form is cured and the mandrel removed.  See ISO 472.

FILAMENT WOUND – Pertaining to an object created by the filament winding method of fabrication.

FILAMENT YARN – A yarn composed of continuous filaments assembled with or without twist.

FILAMENT:  The smallest unit of a fibrous material.  The basic units formed during drawing and spinning, which are gathered into strands of fibre for use in composites.  Filaments usually are of extreme length and very small diameter, usually less than 25 microns (1 mil). Normally, filaments are not used individually.  Some textile filaments can function as a yarn when they are of sufficient strength and flexibility.  See ISO 472.

FILAMENTARY COMPOSITES – A major form of advanced composites in which the fibre constituent consists of continuous filaments.  Filamentary composites are defined here as composite materials composed of laminae in which the continuous filaments are in nonwoven, parallel, uniaxial arrays.  Individual uniaxial laminae are combined into specifically oriented multi-axial laminates for application to specific envelopes of strength and stiffness requirements.

FILL – Yarn oriented at right angles to the warp in a woven fabric.  Also called “weft” or “woof”.

FILL YARNS – The short yarns which run crosswise to the roll direction are called the fill yarns.

FILLER – A relatively inert substance added to a material to alter its physical, mechanical, thermal, electrical, and other properties, or to lower cost or density. Sometimes the term is used specifically to mean particulate additives.  See also inert filler.  See ASTM D 907 and ISO 472.

FILLER PLY – Partial plies of a lay-up, usually located on honeycomb sandwich edge bands, which do not extend on to any portion of the honeycomb surface.  Partial plies of a lay-up which run up to, but not on to a honeycomb sandwich assembly.  Used to thicken the edge for countersunk screws or bolts.  A partial ply used to stabilize or fill a local area.

FILLET – A rounded filling of adhesive that fills the corner or angle where two adherends are joined.

FILM ADHESIVE – A synthetic resin adhesive, usually of the thermosetting type, in the form of a thin, dry film of resin with or without a paper, glass, or other carrier or scrim cloth.

FINES – Bundles that have been split apart into smaller bundles composed of only a few or single filaments. Fuzz is usually made of fines.

FINISH – Chemical finish applied to glass fibres to facilitate resin wetting, resin bonding and good environmental performance of a cured laminate.  Also improves fabric handling. A dilute epoxy finish is sometimes used on carbon fibre before producing prepreg material.  A number of finishes are used on glass-fibre fabrics and tapes depending on the resin system to be used.  In order to ensure a good, durable bond, it is absolutely essential that the glass to be used has been treated with a finish compatible with the chosen resin system, i.e., polyester, epoxy, phenolic, or other resin.  Tables of resins and suitable glass finishes are supplied by glass-fibre manufacturers.

FINISHING – This refers to additional steps used after the yarn is removed from the bobbin..

FIRST PLY FAILURE – In a composite structure that is made from laminated plies the first ply that shows signs of irreversible fracture is considered the first ply to fail.  Typically in well designed and fabricated laminates first ply failure is a matrix crack in that ply.  Rarely is fibre fracture a failure mode in first ply failure.  Also, interfacial failure between fibre and matrix can occur as the failure mode in first ply failure – this is also an undesirable failure mode.

FIXTURES – See tooling.

FLAME RESISTANCE – Ability of a material to extinguish flame once the source of heat is removed.  See also self-extinguishing resin.

FLAME RETARDANTS – Certain chemicals that are used to reduce or eliminate the tendency of a resin to burn.

FLAME RETARDED RESIN – A resin compounded with certain chemicals to reduce or eliminate its tendency to burn.

FLAME SPRAYING – Method of applying an aluminium coating to non-conducting composite panels to provide lightning protection or electromagnetic shielding to electronic components mounted behind the part.  A method of applying a plastic coating in which finely powdered fragments of a plastic, together with suitable fluxes, are projected through a cone of flame on to a surface.

FLANNEL – Traditionally, an all-wool fabric of plain or twill weave with a soft handle.

FLASH – That portion of the charge which flows from or is extruded from the mould cavity during the moulding.  Extra plastic attached to a moulding along the parting line, which must be removed before the part is considered finished.

FLASH POINT – The temperature to which a liquid must be heated before its vapours will flash or burn momentarily when a small flame is applied.  This ignition will not take place unless there is also a spark or open flame.  There are several standard methods of determining flash point, most of which are classified as “open cup” or “closed cup”.

FLASHBREAKER TAPE – This is a pressure-sensitive adhesive tape used to remove excess or overflow adhesive or resin after cure, and for a myriad of purposes in vacuum bagging, such as securing thermocouple wires, bagging materials, etc.  It is capable of withstanding high temperatures, and is then easily removed after the cure.

FLATWISE – Refers to cutting specimens and the application of load.  The load is applied flatwise when it is applied to the face of the original sheet or specimen.  See ISO 472.

FLAX – A slender, erect, annual plant (genus Linum having narrow, lance-shaped leaves and blue flowers, cultivated for its fibre and seeds. The fibre of this plant, manufactured into linen yarn for thread in woven fabrics.

FLEECE – The generic term for synthetic pile fabrics like Malden’s Polartec, Draper’s EcoPile, and Dyersburg’s E.C.O. Fleece.

FLEXIBILIZER – An additive that makes a finished plastic more flexible.  See also plasticizer.

FLEXURAL MODULUS – The ratio, within the elastic limit, of the applied stress on a test specimen in flexure to the corresponding strain in the outermost fibres of the specimen.

FLEXURAL STIFFNESS MATRIX – The [D] matrix is the stiffness parameters that define the out-of-plane deformation of a laminate under bending and/or twist moments.  The flexural stiffness matrix is defined by both ply orientation and the position of individual plies in the stack of the laminate.

FLEXURAL STRENGTH – The maximum stress that can be borne by the surface fibres in a beam in bending.  The flexural strength is the unit resistance to the maximum load before failure by bending, usually expressed in force per unit area.

FLOW – enhancement tows to improve the in-plane permeability of twill or satin fabrics, flow-enhancement tows (FET) may be introduced as in, for example, Hexcel Injectex fabrics.

FLOW – The movement of resin under pressure, allowing it to fill all parts of a mould.  The gradual, but continuous distortion of a material under continued load, usually at high temperatures.  Also called creep.  A qualitative description of the fluidity of an adhesive material during the process of bonding, before the adhesive is set.

FLUOROPLASTICS – Polyolefin polymers in which fluorine, fluorinated alkyl groups or other halogens replace hydrogen atoms in the carbon chain.  This structure has outstanding electrical properties, excellent resistance to chemical attack, low coefficient of friction, excellent fire resistance, exceptionally good performance at high and low temperatures, low moisture absorption and outstanding weatherability.  Fluoroplastics include PTFE, FEP, PFA, CTFE, ECTFE, ETFE, and PVDF.  Strength is low to moderate.  Cost is high.

FLUTED CORE – An integrally woven reinforcement material consisting of ribs between two skins in a unitized sandwich construction.  Fluted core radomes consist of two fibreglass, or other non-conducting skins, separated by square fibreglass tubes.  The tubes are impregnated with resin and laid side by side as an alternative to honeycomb core.  The shape of the tubes is maintained by a wax mandrel which is melted out after the resin has cured.  The advantage of this method is that any water penetrating the skin is automatically drained away.  Other varieties of fluted core are also available.

FOAM – See cellular plastic.

FOAMING ADHESIVE – An adhesive film used to join honeycomb core in bonded assemblies.  Contains a foaming agent that produces an expansion ratio, usually between 2 and 3, during cure.

FOD – Foreign object damage.

FOLDED YARNS – Another term for plied yarns.

FOREIGN OBJECT – Any object that causes damage to an aircraft, e.g., stones or other materials from a runway, tools left in an engine intake or other material sucked in by the airflow.  Hail impact and bird strikes are usually considered separately.

FPF – see First Ply Failure

FP FIBRE – Polycrystalline alumina fibre (AL203).  A ceramic fibre useful for high-temperature (1370 to 1650°C, or 2500 to 3000°F) composites.

FRACTURE – The separation of a body.  Defined both as rupture of the surface without complete separation of laminate and as complete separation of a body because of external or internal forces.

FRACTURE DUCTILITY – The true plastic strain at fracture.  CMH-17 (WAS MIL-HDBK-17).

FRACTURE STRESS – The true, normal stress on the minimum cross-sectional area at the beginning of fracture.

FRACTURE TOUGHNESS – A measure of the damage tolerance of a material containing initial flaws or cracks.  Used in aircraft structural design and analysis.

FRAT – Fibre-reinforced advanced titanium.

FREE RADICAL – Compound which contains at least one unpaired electron in place of the more normal and more stable bond consisting of shared electrons.

FREE WALL – The portion of a honeycomb cell wall that is not connected to another cell.

FREE-RADICAL POLYMERIZATION – A type of polymerization in which the propagating species is a long-chain free radical initiated by the introduction of free radicals from thermal or photochemical decomposition.

FRP – See fibre-reinforced plastic.

FUMED SILICA – Silica powder used as a filler in epoxy resins. One trade name is CAB-O-SIL.

FURANE PLASTIC – Dark coloured thermosetting resins obtained primarily by the condensation polymerization of furfuryl alcohol in the presence of strong acids, sometimes in combination with formaldehyde or furfuryl aldehyde.  The term also includes resin made by condensing phenol with furfuryl alcohol or furfuryl and furfuryl ketone polymers.  The resins are available as liquids ranging from low viscosity fluids to thick, heavy syrups which cure to highly cross-linked, brittle substances.  Also spelled “Furan”.  See ISO 472.

FUSIBLE – Capable of being melted and formed into a continuous adhesive film.  The property of adhesive melting in combination with substrate melting to form a homogeneous mass at the interface.

FUZZ – Accumulation of short broken filaments after passing glass strands, yarns, or rovings over a contact point.  Often weighted and used as an inverse measure of abrasion resistance. In glass fibre manufacturing, the broken filaments found around and on a roving creel. Chopper Fuzz – In Composite Fabrication, the broken filaments found around the glasscutter or chopper. see Fines. In the field, the broken filaments found around a roving pallet.

FUZZ BALL – Loose or frayed fibres that have formed into a ball, and are entwined either with the fabric or on the surface.  A minor fuzz ball is loose or frayed fibres which are not raised above the fabric surface.  See pills.


GALVANIC CELL – A cell made up of two dissimilar conductors in contact with an electrolyte or two similar conductors in contact with dissimilar electrolytes.  More generally a galvanic cell converts energy liberated by a spontaneous chemical reaction directly into electrical energy.

GALVANIC CORROSION – Corrosion associated with the current of a galvanic cell made up of dissimilar electrodes.

GAP – In filament winding, the space between successive windings, which windings are usually intended to lay next to each other.  Separations between fibres within a filament winding band.  The distance between adjacent plies in a lay-up of unidirectional tape material.

GAP-FILLING ADHESIVE – An adhesive subject to low shrinkage in setting, used as sealant.  An adhesive with sufficient viscosity to fill a gap and not run out of it.

GARNETTED YARN – A yarn that has little bits (“garnets”) of other fibres carded in. Usually the garnets are of a different colour — but they can also be from a different fibre.

GASSED YARNS – Spun cellulose yarns passed over a heat source (or through a flame) to remove unwanted fibres on the surface. This gives a smoother surface but is not recommended at home. (Cellulose fibres are quite flammable.)

GATE – (In injection and transfer moulding), the channel or orifice through which material is injected from the sprue (or runner in a multi-cavity mould) into a mould cavity.  See ISO 472.

GAUGE LENGTH – Length over which deformation is measured for a tensile or compressive test specimen.  The deformation over the gauge length divided by the gauge length determines the strain.  See ISO 472.

GEL – The initial jelly-like solid phase that develops during the formation of a resin from a liquid.  A semisolid system consisting of a network of solid aggregates in which liquid is held.  See ISO 472.

GEL COAT – A quick setting resin applied to the surface of a mould and gelled before lay-up.  The gel coat becomes an integral part of the finished laminate, and is usually used to improve surface appearance and bonding.  See ISO 472.

GEL POINT – The stage at which a liquid begins to exhibit pseudo-elastic properties.  This stage may be conveniently observed from the inflection point on a viscosity time plot.  The point in a cure beyond which the material will no longer flow without breaking down the matrix network formed to that point.  The point at which the matrix transition from a fluid to a solid state takes place.  See ISO 472.

GELATION – The point in a resin cure when the resin viscosity has increased to a point such that it barely moves when probed with a sharp instrument.

GELATION TIME – That interval of time, in connection with the use of synthetic thermosetting resins, extending from the introduction of a catalyst into a liquid adhesive system until the start of gel formation.  Also, the time under application of a specified temperature for a resin to reach a solid state.  See ISO 472.

GERBER CUTTER – A trade name for a type of computer controlled reciprocating knife process for cutting, kitting, and labeling prepreg fabric and tape plies.

GFRP (or GRP) – Glass Fibre Reinforced Plastic.

GLASS – An inorganic product of fusion that has cooled to a rigid condition without crystallizing.  Glass is typically hard and relatively brittle, and has a conchoidal fracture.  And rigid non-crystalline solid, applied more commonly to non-crystalline inorganic oxides than to non-crystalline polymers.

GLASS CLOTH – Conventionally woven glass fibre material.  See also scrim.

GLASS FIBRE – A fibre spun from an inorganic product of fusion that has cooled to a rigid condition without crystallizing.  A glass filament that has been cut to a measurable length.  Staple fibres of relatively short length are suitable for spinning into yarn.

GLASS FILAMENT – A form of glass that has been drawn to a smaller diameter and extreme length.  Most filaments are less than 0.15 mm (0.005 in) in diameter.

GLASS FINISH – A material applied to the surface of a glass reinforcement to improve the bond between the glass and the plastic resin matrix.

GLASS TRANSITION TEMPERATURE (Tg) – The approximate midpoint of the temperature range over which the glass transition takes place:  glass and silica fibre exhibit a phase change at approximately 955°C (1750°F) and carbon/graphite fibres at 2205 to 2760°C (4000 to 5000°F).   The temperature at which increased molecular mobility results in significant changes in the properties of a cured matrix resin system or plastic fibre.  It is the point at which rigid behavior changes to a rubbery behavior.  Both fibres and resin matrices have glass transition points, but they are usually fairly widely separated.  However, in the case of Polyethylene and Polypropylene fibres, their Tg values are below 0° C, and they require resin matrices that cure well below their melting points.  Also, the inflection point on a plot of modulus versus temperature.  The measured value of Tg depends to some extend on the method of test.  See ISO 472.

GORE-TEX – W.L. Gore’s micro porous membrane that, when laminated to an outer fabric, keeps rain out while allowing perspiration vapour to escape. Garments of “three-layer” construction look like a single layer of fabric, but are really a sandwich of Gore-Tex membrane laminated to one of a variety of tough outer fabrics and backed by a protective tricot inner face. “Two-layer” construction mates the Gore-Tex membrane and an outer fabric with a free- hanging liner.

GORE-TEX PACLITE – A Gore-Tex fabric that replaces the inner-lining fabric with abrasion-resistant dots on the Gore-Tex membrane, resulting in a thinner and lighter cloth. As a result of having one less layer, the fabric’s breathability is increased and the garment’s weight is substantially reduced.

GORE-TEX XCR – A version of the classic Gore-Tex that retains the original membrane’s waterproofness, but offers, on average, 25 percent more breathability than classic Gore-Tex. Like the classic membrane, Gore-Tex XCR comes in two- and three-ply fabrics.

GRADE – A measurement used in knitted garments that reflects the size of the needles used to knit the garment. The larger the gauge, the smaller the needle the finer the knit.

GRAPHITE – The crystalline allotropic form of carbon.

GRAPHITE FIBRE – A fibre made from a precursor by oxidation, carbonization, and graphitization process (which provides a graphitic structure).  See also carbon fibre.

GRAPHITIZATION – The process of pyrolyzation in an inert atmosphere at temperatures in excess of 1925°C (3500°F), usually as high as 2480°C (4500°F), and sometimes as high as 2700°C (4890°F), converting carbon to its crystalline allotropic form.  Temperature depends on precursor and properties desired.

GREIGE, GRAY GOODS – Any fabric before finishing, as well as any yarn or fibre before bleaching or dyeing; therefore, fabric with no finish or size.

GREEN STRENGTH – Green strength is when a polymer has cured and is in a handleable state, can be machined and undertake other post cure fabrication steps, but is unable to carry any significant load.  The cross-linking of the polymer or complete polymerization is not completed.

GRIST – The yards (or meters) per pound (YPP). So if you had a finished yarn that came up 890 YPP, one pound of yarn would equal 890 yards. The grist (or “count“) may range from 300 yds/lb to 3,000,000 yds/lb for a single filament of silk (theoretically).


HAND – The softness of a piece of fabric, as determined by the touch (individual judgment). Fabric softness as determined by touch (individual judgment). The drape or feel of a fabric, as in “this fabric has a soft hand.”

HAND LAY-UP – The process of placing (and working) successive plies of reinforcing material or resin-impregnated reinforcement in position on a mould by hand.

HANDLING LIFE – The out-of-refrigeration time over which a material retains its handleability.

HANDLING STRENGTH – A low level of strength initially obtained by an adhesive that allows specimens to be handled, moved, or unclamped without causing disruption of the curing process or affecting bond strength.  See ASTM D 1144.

HANG PICK – (Hung filling yarn) A pick (fill yarn) caught on a warp yarn knot producing a triangular shaped hole in the fabric.

HANK – A package of yarn from a reel, hopefully with the yardage and fibre content noted on a label. This may refer to a specified yardage, as in a hank of worsted yarn contains 560 yards, cotton and silk is 840 yards, and linen is 300 yards.

HARD TWIST – A yarn with increased twist.

HARDENER – A substance or mixture added to a plastic composition to promote or control the curing action by taking part in it.  The term is also used to designate a substance added to control the degree of hardness of the cured film.  See also catalyst.  See ASTM D 907 and ISO 472.

HARDNESS – The resistance to surface indentation, usually measured by the depth of penetration (or arbitrary units related to the depth of penetration) of a blunt point under a given load using a particular instrument according to a prescribed procedure. 

HARNESS SATIN – Describes a set of weaving patterns which produce a fabric having a satin appearance. “8HS” describes a harness satin weave where the warp fibre tows go over seven fill tows and then under one fill tow, for a repeating total of 8. By itself, “8HS” is not a complete description, because there are many possible patterns of where the crossover points of adjacent tows are located.

HEAT CLEANED – A condition in which glass or other fibres are exposed to elevated temperatures to remove preliminary sizings or binders not compatible with the resin system to be applied.

HEAT DISTORTION POINT – The temperature at which a standard test bar deflects a specified amount under a stated load.  Now called deflection temperature.

HEAT SINK – A contrivance for the absorption or transfer of heat away from a critical element or part.  Bulk graphite is often used as a heat sink.  Also a metal part in a composite structure that will act as a heat sink.

HEAT-DEFLECTION TEMPERATURE – The temperature at which a standard test bar deflects a specific amount under a stated load.  Now called deflection temperature under load (DTUL).

HEATER BLANKET – Rubber blanket, often silicone rubber for higher temperatures, containing electrical heating elements.  Used with a “hot bonder” control unit to heat up repair areas to cure film adhesives and prepregs.

HELICAL WINDING – In filament-wound items, a winding in which a filament band advances along a helical path, not necessarily at a constant angle except in the case of a cylinder.

HEMP – The fibre from the plant Cannabis sativa.

HERMETICALLY SEALED – Air tight closure by fusion etc.

HERRINGBONE TWILL-A broken twill weave composed of vertical sections which are alternately right hand and left hand in direction. The twill changes direction perfectly where the weave breaks, balancing the overall pattern of the fabric.

HETEROGENEOUS – Descriptive term for a material consisting of dissimilar constituents separately identifiable.  A medium consisting of regions of unlike properties separated by internal boundaries.  Note that not all nonhomogeneous materials are necessarily heterogeneous.

HEXA – Shortened form of hexamethylenetetramine, a source of reactive methylene for curing novolacs.

HIGH PRESSURE LAMINATING – A term usually reserved for matched die moulding, typically done under high pressures in a press.  High pressure laminating is not as common as autoclave moulding.

HIGH PRESSURE MOULDING – A moulding process in which the pressure used is greater than 6.9 MPa (1,000 psi).

HIGH-PRESSURE LAMINATES – Laminates moulded and cured at pressures not lower than 6.9 MPa (1,000 psi), and more commonly in the range of 8.3 to 13.8 MPa (1,200 to 2,000 psi).

HOLOGRAM – Three-dimensional photograph or image produced by interference between two sets of coherent light waves.

HOMOGENEOUS – Descriptive term for a material of uniform composition throughout.  A medium that has no internal physical boundaries.  A material whose properties are constant at every point, that is, constant with respect to spatial coordinates (but not necessarily with respect to directional coordinates).

HOMOPOLYMERIZED – A condition whereby a monomeric material is polymerized only with itself.

HONEYCOMB – Manufactured product of resin-impregnated sheet material (paper, glass fabric, and so on) or metal foil, formed into hexagonal-shaped cells.  Other cell shapes are produced.  Used as a core material in sandwich construction.  See also sandwich constructions.

HONEYCOMB SANDWICH ASSEMBLY – A structural composition consisting of relatively dense, high-strength facings (skin) bonded to a lightweight, cellular honeycomb core.  See also sandwich construction.

HOT HEAD TAPE LAYER – A computer controlled automated tape placement process for thermoplastic prepregs utilizing a gantry mounted hot shoe to partially consolidate each consecutive ply in a programmed orientation and laminate size.

HOT ISOSTATIC PRESSING (HIP) – A process for fabricating certain metal matrix composites.  A preform is consolidated under fluid pressure (usually an inert gas) at high temperature and pressure in a pressure vessel.  This method can also be used to consolidate ceramic materials which have no metal matrix.

HOT/WET PROPERTIES – The mechanical properties required of a composite or bonded metal assembly under prescribed conditions of temperature, time and relative humidity or water immersion.  Testing is usually carried out at a specified temperature after exposure to the required environment for a specified period of time.

HOT-MELT ADHESIVE – An adhesive that is applied in a molten state and forms a bond after cooling to a solid state.  A bonding agent that achieves a solid state and resultant strength by cooling, as contrasted with other adhesives, which achieve the solid state through evaporation of solvents or chemical cure.  A thermoplastic resin that functions as an adhesive when melted between substrates and cooled.  See ASTM D 907.

HYBRID – A composite laminate consisting of laminae of two or more composite material systems.  A combination of two or more different fibres, such as carbon and glass or carbon and aramid, into a structure.  Tapes, fabrics, and other forms may be combined; [usually only the fibres differ.  See also interply hybrid and intraply hybrid. A composite laminate comprised of laminae of two or more composite material systems. Or, a combination of two or more different fibres such as carbon and glass or carbon and aramid into a structure (tapes, fabrics and other forms may be combined).

HYBRID COMPOSITE – Composite containing at least two distinct types of matrix or reinforcement. The matrix or reinforcement types can be distinguished by their physical properties, mechanical properties, material form and/or chemical composition.

HYDROBURST – Test in which a fluid is used to load a pressure vessel to failure.

HYDROCLAVE – An autoclave that uses water or steam as its pressurizing medium.

HYDROFIL – A hydrophilic nylon from Allied Fibres that transports moisture outward; commonly found in outerwear liners as the “pull” component in push-pull fabrics and in moisture-managing mesh.

HYDROGEN BONDING – Hydrogen bonding is a very important mechanism for intermolecular attraction and therefore adhesion.  Hydrogen bonding is due to the strong interaction of hydrogen attached to one atom (such as oxygen, nitrogen, or carbon) by a polar covalent bond with an adjacent atom of high electronegativity (such as oxygen, nitrogen, or one of the halogens).

HYDROLYSIS – Chemical decomposition of a substance involving the addition of water.  Reaction between ions of a salt and ions of water forming a solution which is either acidic or alkaline.

HYDROPHILIC – Having an attraction of water.  Capable of absorbing or absorbing water.  Easily wetted by water.

HYDROPHOBIC – Capable of repelling water.  Poorly wetted by water.  The opposite of hydrophilic.

HYGROSCOPIC – Capable of attracting, absorbing, and retaining atmospheric moisture.

HYGROTHERMAL EFFECT – Change in properties due to moisture adsorption and temperature change.



IMPACT DAMAGE – Damage from foreign object (other than ballistic). Impact damage is the principle cause of penetration and the amount of damage depends on the energy level of the projectile involved.   The amount of damage is also dependent on material properties, geometry, and velocity of imparter and superimposed static loads.  Damage propagation resulting from impact loads depends on loading type and strain levels, where the resulting strength losses can be conservatively approximated on the basis of an “equivalent” round hole

IMPACT STRENGTH – The ability of a material to withstand shock loading.  The work done in fracturing a test specimen in a specified manner under shock loading.

IMPREGNATE – In reinforced plastics, to saturate the reinforcement with a resin.

IMPREGNATED FABRIC – A fabric impregnated with a synthetic resin.  See also prepreg.

IMPREGNATOR – A mechanical device for wetting or impregnating fabrics with resin.  Generally consists of a trough through which the fabric is drawn and a set of adjustable scraper bars to remove excess resin.

INCLUSION – A physical and mechanical discontinuity occurring within a material or part, usually consisting of solid, encapsulated foreign material.  Inclusions are often capable of transmitting some structural stresses and energy fields, but in a noticeably different degree from the parent material.  Visible foreign material such as particles, chips, and films.  See also voids.

INERT ATMOSPHERE – The use of a gas (usually nitrogen) instead of air to pressurize an autoclave, primarily for fire prevention.

INERT FILLER – A material added to a plastic to alter the end-item properties through physical rather than chemical means.

INFRARED – Part of the electromagnetic spectrum between the visible light range and the radar range.  Radiant heat is in this range, and infrared heaters are frequently used in the thermoforming and curing of plastics and composites.  Infrared analysis is used for identification of polymer constituents.

INHIBITOR – A substance that retards a chemical reaction.  A material added to a resin to slow down curing.  Also used in certain types of monomers and resins to prolong storage life.  Synonym for retarder.  See ASTM D 907 and ISO 472.

INHOMOGENEOUS – Consisting of more than one phase, e.g., discrete regions of different materials.

INITIATOR – Sources of free radicals, often peroxides or Azo compounds.  They are used in free-radical polymerizations, for curing thermosetting resins, as cross-linking agents for elastomers and polyethylene, and for polymer modification.  See ISO 472.

INJECTION MOULDING – Method of forming a plastic to the desired shape by forcing the heat-softened plastic into a relatively cool cavity under pressure. The production of a composite component by the injection of resin or a fibre/resin mix into a closed mould.

INNER SKIN – That side of the part which is cured against the vacuum bag.

INORGANIC – Designating or pertaining to the chemistry of all elements and compounds not classified as organic.  Matter other than animal or vegetable, such as earthy or mineral matter.  Applies to the chemistry of all elements and compounds not classified as organic.

INSERT(A) An integral part of a plastic moulding consisting of metal or other material that may be moulded or pressed into position after the moulding is completed.  (B) Apparatus placed into the sandwich for attaching items; synonymous with hard points.  ASTM C 274.  (C) In the case of floor panels and others where high shear loads have to be carried through the panel to surrounding structure, special inserts are used to ensure that the required loads can be transmitted.

INSULATOR – A material of such low electrical conductivity, that the flow of current through it can usually be neglected.  Similarly, a material of low thermal conductivity, such as that used to insulate structural shells.

INTEGRAL COMPOSITE STRUCTURE – Composite structure in which several structural elements, which would conventionally be assembled together by bonding or mechanical fasteners after separate fabrication, are instead laid up and cured as a single, complex, continuous structure, for example, spars, ribs, and one stiffened cover of a wing box fabricated as a single integral part.  The term is sometimes applied more loosely to any composite structure not assembled by mechanical fasteners.  All or some parts of the assembly may be co-cured.

INTEGRALLY HEATED – A term referring to tooling that is self-heating, through use of electrical heaters such as cal rods. Most hydroclave tooling is integrally heated.  Some autoclave tooling is integrally heating to compensate for thick sections, to provide high heat-up rates, or to permit processing at a higher temperature than is otherwise possible with the autoclave.

INTERFACE – The boundary or surface between two different, physically distinguishable media.  On fibres, the contact area between fibres and sizing or finish.  In a laminate, the contact area between the reinforcement and the laminating resin.

INTERFERENCE FITS – A joint or mating of two parts in which the fasteners have external dimensions larger than the internal dimensions of the fastener holes.  Distension of the fastener holes by the fastener creates a stress, which supplies the bonding force for the joint, and reduces fastener “tipping” in the holes under joint loading.

INTERLAMINAR – Descriptive term pertaining to an object (voids, etc.), event (fracture, etc.), or potential field (shear stress, etc.) referenced as existing or occurring between two or more adjacent laminae.

INTERLAMINAR SHEAR – Shearing force tending to produce a relative displacement between two laminae in a laminate along the plane of their interface.

INTERLAMINAR SHEAR STRENGTH (ILSS) – The maximum shear stress between layers that a laminated material can resist.  See ASTM D 2344.

INTERNAL ADHESIVE STRESS – Stress created within an adhesive layer by the movement of the adherends at differential rates or by the contraction or expansion of the adhesive layer.

INTERPHASE – The boundary region between a bulk resin or polymer and an adherend in which the polymer has a high degree of orientation to the adherend on a molecular basis.  It plays a major role in the load transfer process between the bulk of the adhesive and the adherend or the fibre and the laminate matrix resin.

INTERPLY HYBRID – A composite in which adjacent laminae are composed of different materials.

INTRALAMINAR – Descriptive term pertaining to an object (voids, etc.), event (fracture, etc.), or potential field (temperature gradient, etc.), existing entirely within a single lamina without reference to any adjacent laminae.

INTRAPLY HYBRID – A composite in which different materials are used within a specific layer or band.

IPA – Isopropylalcohol (or isopropanol, or 2-propanol).  A wipe solvent used as a less hazardous replacement for MEK, or Acetone.  It is rates as “Flammable” with a flash point of 53° F (11.7° C) and is toxic by inhalation and ingestion.

IRRADIATION – As applied to plastics, the bombardment with a variety of subatomic particles, usually alpha-, beta-, or gamma-rays.  Used to initiate polymerization and copolymerization of plastics, and in some cases to bring about changes in the physical properties of a plastic.

IRREVERSIBLE – Not capable of redissolving or remelting.  Descriptive of chemical reactions which proceed in a single direction and are not capable of reversal (as applied to thermosetting resins).

ISOCYANATE PLASTICS – Plastics based on resins made by the condensation of organic isocyanates with other compounds.  Generally reacted with polyols on a polyester or polyether backbone molecule, with reactants being joined through the formation of the urethane linkage.  See also polyurethane and urethane plastics.  See ISO 472.

ISOTROPIC – Having uniform properties in all directions.  The measured properties of anisotropic material are independent of the axis of testing.

ISOTROPIC LAMINATE – A laminate in which the strength properties are the same in all directions.  Difficult to achieve.  See quasi-isotropic.

IZOD IMPACT TEST – A test for shock loading in which a notched specimen bar is held at one end and broken by striking, and the energy absorbed is measured.


JOGGLE – The projecting or retreating surface of a formed part, or the section of a tool that forms a joggle.  An offset formed in a part or tool to step over or jog over another part.

JOINT, BONDED – That part of a structure at which two adherends are held together with a layer of adhesive.  The point in a structure at which two parts are bonded together.

JUTE – A vegetable bast fibre often used for basketry and course weavings.


K FACTOR – The coefficient of thermal conductivity.  The amount of heat that passes through a unit cube of material in a given time when the difference in temperature of two opposite faces is 1°.

KAPTON – Du Pont Trade Name for Polyimide Film (HPF) – Low smoke and flame.  Lightweight, resists hydraulic fluids.  Heat resistant.  Good low temperature performance.

KEMP – A white, straight, opaque, coarse, non-felting, in-elastic fibre having a thick central medulla with hollow inter spaces. It will not take a dye; hence, its presence in wool is most objectionable. Often found around the head and legs.

KERF – The width of a cut made by a saw blade, torch, water jet, laser beam, etc.

KEVLAR® – Du Pont company name for an aramid fibre.  An organic polymer composed of aromatic polyamides having a para-type orientation (parallel chain extending bonds from each aromatic nucleus).  Has good impact resistance, low density, high strength, and low ratio frequency attenuation.  Absorbs some moisture.  See aramid.

KINK – A yarn that has doubled back on itself to form a loop in the yarn.

KNIT – Textile process that interlocks, in a specific pattern, loops of yarn by means of stitching process, using needles or wires.

KNITTED FABRIC – A textile structure produced by interlooping one or more ends of yarn or comparable material.

KNOP – A “bunch” of fibres appearing along the length of yarn, giving a spot effect.

KNOT – The means of joining strands of two doffs of roving. The knot is generally a reduced triple loop surgeon’s knot, square knot or overhand knot.


LACQUER – Solution of natural or synthetic resins in readily evaporating solvents, used as a protective coating.

LAMINA – A single ply or layer in a laminate made up of a series of layers (organic composite).  A flat or curved surface containing unidirectional fibres or woven fibres embedded in a matrix (metal matrix composite).

LAMINAE – Plural of lamina.

LAMINATE (noun) – A product made by bonding together two or more layers (plies) of material.  See also bi-directional laminate and unidirectional laminate, quasi-isotropic and isotropic laminate.  See ASTM D 907 and ISO 472.

LAMINATE (verb) – To unite laminae with a bonding material, usually with pressure and heat (normally used with reference to flat sheets, but also rods and tubes).  See ASTM D 907.

LAMINATE ORIENTATION – The configuration of a cross-plied composite laminate with regard to the angles of cross-plying, the number of laminae at each angle, and the exact sequence of the lamina lay-up.

LAMINATE PLY – One fabric-resin or fibre-resin layer of product that is bonded to adjacent layers in the curing process.

LAMINATION – The process of preparing a laminate.  Also any layer in a laminate.  See ASTM D 907.

LAMINATION SEQUENCE – Composite lay-up usually begins with the ply nearest the tool surface. Each successive ply is then stacked or nested in sequence.  Plies are laid up from the tool surface out.

LAP – In filament winding, the amount of overlay between successive windings, usually intended to minimize gapping.  In bonding, the distance one adherend covers another adherend.

LAP JOINT – A joint made by placing one adherend partly over another and bonding the overlapped portions.  See ASTM D 907.

LAP SHEAR TEST – This is carried out to ASTM D 1002 and Reference 2.15.  Aluminium alloy adherends of a standard thickness are normally used, but the test can be adapted for use with other materials including composites.  In the standard test 1 in wide (25.4 mm) sheets are overlapped by 1/2 in (12.7mm) and pulled at a specified crosshead speed. 

LAST PLY FAILURE – The last ply to failure of a composite laminate is also known as ultimate failure.  The last ply to failure will involved fibre fracture.

LATENT CURING AGENT – A curing agent that produces long-term stability at room temperature, but rapid cure at elevated temperatures.

LATEX – A generic term describing any of many stable dispersions of insoluble resin particles in a water system.  A milky juice, other than sap, secreted by certain plants in special cells usually present in all parts of the plant.  Rubber latexes from different plants vary widely in properties. Hevea latex, of greatest interest, is thin and white like milk and consists of a dispersion of rubber in an aqueous serum containing other substances.  See ISO 472.

LAY – In glass fibre the spacing of the roving bands on the roving package expressed in the number of bands per inch. In glass fibre, the spacing of the roving bands on the roving package expressed in the number of bands per inch; in filament winding, the orientation of the ribbon with some reference, usually the axis of rotation.

LAYERED LAMINATE – When two or more plies, either of the same or different materials, are bonded and stacked one on top of the other to act as a single structural layered element, then this structural element is called a layered laminate.

LAY-UP(A) The reinforcing material placed in position in the mould.  (B) The process of placing the reinforcing material in position in the mould.  (C) The resin-impregnated reinforcement.  (D) A description of the component materials, geometry, etc., of a laminate.  (E) A stack up of composite materials that forms either a cured or uncured part.  (BDS 1330)  (F) A process of fabrication involving the assembly of successive layers of resin impregnated material.

L-DIRECTION:  The ribbon direction, that is, the direction of the continuous sheets of honeycomb.

LEA – A form of measuring linen yarns in 300-yard increments and weighing one pound. A 4-lea skein would also weigh one pound but would be 1200 yards long.

LEAS TIES – Also known as lees ties and lease ties. This is such an interesting term that pops in and out of textiles. They are the short threads tied around hanks of yarn to help keep them from tangling while being washed, dyed, and stored. They are also the short threads tied around a warp to allow you carry it from the warping board/mill to the loom. They serve the same process of keeping the threads in order. They are tied by running a thread at right angles to the warp/hank and interweaving through and coming back at opposite angles.

LENGTHWISE DIRECTION – Refers to cutting specimens and the application of loads.  For rods and tubes, lengthwise is the direction of the long axis.  For other shapes of materials that are stronger in one direction that in the other lengthwise is the direction that is stronger.  For materials that are equally strong in both directions lengthwise is an arbitrarily designated direction that may be with the grain, direction of flow in manufacture, longer direction, etc.

LIMIT LOAD – Limit Loads are the maximum loads anticipated on an aircraft during its service life.  The aircraft structure shall be capable of supporting the limit loads without suffering any detrimental permanent deformation.  For all loads up to the “limit” loads the deformation shall be such as not to interfere with the safe operation of the aircraft.

LINEAR EXPANSION – The increase of a given dimension, measured by the expansion or contraction of a specimen or component subject to a thermal gradient or changing temperature.  See also coefficient of thermal expansion.

LINEN – Linen is the term used for fabric made from flax. Linen is generally favoured for its fine, strong, cool-wearing properties. It is commonly cursed for wrinkling. In knitwear, linen is combined with other natural or synthetic fibres for improved strength and resiliency.

LIQUID COUPLANT – Liquid interface between a transducer and the subject of a nondestructive inspection.

LIQUID CRYSTAL POLYMER – A newer thermoplastic polymer that is melt processable and develops high orientation in moulding, with resultant tensile strength and high-temperature capability that is notably improved.  First commercial availability was as an aromatic polyester.  With or without fibre reinforcement.

LIQUID RESIN – An organic, polymeric liquid that becomes a solid when converted to its final state for use.

LIQUID SHIM – Material used to position components in an assembly where dimensional alignment is critical.  For example, epoxy adhesive is introduced into gaps after the assembly is placed in the desired configuration.

LOAD – The force applied to the specimen at any given time. See ASTM D 4027.

LOFTING:  The drawing of lines from the basic geometry drawing for the aircraft (full scale) on metal to make a master layout for use by tooling departments.  A loft will show sections through the aircraft giving profiles at particular points.  It will show butt lines, water lines and reference planes and provides the master profiles from which tools and parts are made.

LONGITUDINAL(A) Along the length of material. (B) In the 0° direction.

LONGOS – Low angle helical or longitudinal windings.

LOOM – A mechanical device that interlaces fibres at right angles with varying degrees of weave construction (weight, thickness and design). More modern looms are air jet but rapier and more traditional shuttle equipment is still in use. A machine for producing cloth by the interlacing of two sets of threads substantially at right angles to each other.

LOOM BEAM – A large, flanged cylinder onto which all warp yarns are wound and from which yarns enter the loom.

LOOM STATE – Most reinforcement fibres as supplied by the manufacturer have a small amount of SIZE on their surface, designed to reduce filament damage and facilitate handling during weaving.  Fabrics woven from these fibres and supplied untreated are described as “loom state”.

LOOP – Small open place in the strands due to excessive length of one or more strands.

LOOSE PICK – (Loose filling yarn) A filling yarn which is not flush with the surrounding fabric usually caused by insufficient tension.

LOT(A) A specific amount of material produced at one time using the same process and the same conditions of manufacture, and offered for sale as a unit quantity. (B) (Prepreg material) one batch of prepreg material or a portion of one batch that is shipped to a purchaser for acceptance at one time.  A lot may be one or more rolls of material, but cannot exceed the amount of material produced in the batch the rolls are taken from is the designated batch of materials used to produce a product. This allows us full traceability of our products based on the materials used to make them

LOW PRESSURE LAMINATES – In general, laminates moulded and cured in the range of pressures from 2760 kPa (400 psi) down to and including pressure obtained by the mere contact of the plies.

LOW TEMPERATURE REPAIR – A repair using a resin system that can be cured at temperatures not to exceed 70°C (160°F).

LPF – see Last Ply Failure

LUBRICANT – A material added to most sizings to improve the handling and processing properties of textile strands, especially during weaving.


MACERATE – To chop or shred fabric for use as a filler for a moulding resin.  The moulding compound obtained when so filled.

MACRO – In relation to composites, denotes the gross properties of a composite as a structural element but does not consider the individual properties or identity of the constituents.

MACROMECHANICS – Is the study of a composite material as a whole, and does not differentiate between the constituents (fibres and resin).

MACROSCOPY – Interpretation using only the naked eye, or magnification no greater than 10x.

MANDREL – The core tool around which resin-impregnated paper, fabric, of fibre is wound to form pipes, tubes, or structural shell shapes, usually by the filament winding process.   See ISO 472.  

MAN-MADE FIBRE – A man-made fibre, e.g., viscose, rayon. Also known as “manufactured fibre”.

MARCELLED FIBRES – Marcelling is waviness of the fibres.   Waves in the fibres will degrade the lamina compression strength due to a decrease in microbuckling resilience.   Marcelled fibres are a result of poor lay-up control.

MARKOFF(A) Visual evidence of interior details on the outer surface of a bonded assembly.  (B) Visual impression in a repair laminate caused by thermocouples or other items.

MARL YARN – A yarn consisting of two or more single ends of different colours plied together.

MASKING(A) A tape or sheet material used to prevent the masked area from chemical treatment, grit blasting or paint spray on a temporary basis.  It is easily removable after the treatment has been completed.  (B) Masking tape used to protect acrylic and other plastic sheeting must use an adhesive which does not itself damage the plastic in either short or long-term storage.  (C) Some special masking tapes are supplied, which can be used to protect treated surfaces between treatment and adhesive bonding or painting.  Such tapes must leave no residue when removed that could reduce adhesion.

MASTER, Model (M.M.) – A pattern whose contours are the absolute and final contour definition of a part or assembly.  Tooling masters are most generally made from tooling plaster, but can also be made of composite material.

MAT – A fibrous material consisting of randomly oriented chopped or swirled filaments loosely held together with a binder.  An unwoven textile fabric made of fibrous reinforcing material, such as chopped filaments (to produce chopped strand mat) or swirled filaments (to produce continuous strand mat) with a binder applied to maintain form. Available in blankets of various widths, weights, thicknesses and lengths. May be oriented. A fibrous material for reinforced plastic consisting of randomly oriented chopped filaments, short fibres (with or without a carrier fabric), or swirled filaments loosely held together with a binder.  Available in blankets of various widths, weights, and lengths.  Also, a sheet formed by filament winding a single-hoop ply of fibre on a mandrel, cutting across its width and laying out a flat sheet.

MAT BINDER – Resin applied to glass fibre and cured during the manufacture of mat, used to hold the fibres in place and maintain the shape of the mat.

MAT STRENGTH – The mat’s ability to resist being pulled apart under tension during impregnation and moulding.

MATCHED METAL MOULDING – A reinforced plastics manufacturing process in which matching male and female metal moulds are used (similar to compression moulding) to form the part, with time, pressure, and heat.

MATCHSTICKS – Strand-to-Strand adhesion. A matchstick is a wide bundle that has 3 to 4 times as many filaments than the majority of bundles in the bed.

MATERIAL ACCEPTANCE – The testing of incoming material to ensure that it meets requirements.

MATERIAL QUALIFICATION – The procedures used to accept a material by a company or organization for production use.

MATERIAL SYSTEM – A specific composite material made from specifically identified constituents in specific geometric proportions and arrangements, and possessed of numerically defined properties.

MATERIAL VARIABILITY – Any source of variability due to variations in the spatial, consistency, mechanical or physical properties, chemical content or processing aspects of a material.  CMH-17 (WAS MIL-HDBK-17).  

MATRIX – The essentially homogeneous resin or polymer material in which the fibre system of a composite is imbedded.  Both thermoplastic and thermoset resins may be used, as well as metals, ceramics, carbon and glasses.

MATRIX CRAZING – Matrix crazing is multiple cracks in all directions within the resin.   Non-structural matrix materials are more prone to this type of defect.   Crazing is usually caused by over aged material, or exposure to excessive heat and ultra-violet radiation.   Matrix crazing is not applicable to epoxy resin composites.

MATRIX DOMINATED PROPERTIES – Those mechanical properties that depend heavily on the strength and stiffness of the matrix, its compatibility or bondability to the fibres, and its ability to support the fibres so that they can continue to carry load.  Short beam shear and compression strength fall into this category.  Matrix dominated properties are also very sensitive to laminate quality in terms of void content, resin and fibre distribution.

MECHANICAL ADHESION – Adhesion between surfaces in which the adhesive holds the parts together by interlocking action.

MECHANICAL PRESSURE – A pressure applied by other than fluid means.  Mechanical pressure may be applied by deadweight, press, jacks, clamps etc.

MECHANICAL PROPERTIES – The properties of a material that are associated with elastic and inelastic reaction when force is applied, or the properties involving the relationship between stress and strain.  CMH-17 (WAS MIL-HDBK-17).

MEK – Methylethylketone.  Also known as 2-butanone.  A wipe solvent used for cleaning composite surfaces prior to bonding.  Also used for cleaning metal surfaces prior to other treatments.  Used as a diluent in some sprayable epoxy adhesives and primers.  Some evidence of toxicity in animals.  Classed as “Seriously Flammable” with a flash point of 20° F (-7° C).  May be procured to ASTM D 740.

MEKP – Methyl ethyl ketone peroxide, a catalyst for polyester resins.  Highly corrosive to the skin, and can cause almost immediate blindness if splashed in the eyes.

MELT(A) A charge of molten metal.  (B) A charge of molten plastic.  See also liquid metal infiltration.

MELTING RANGE – Thermoplastics whose makeup includes a distribution of molecular weights which will not have a well defined melting point, but have a melting point.

MESOPHASE – An intermediate phase in the formation of carbon from a pitch precursor.  This is a liquid crystal phase in the form of microspheres, which upon prolonged heating above 400°C (750°F) coalesce, solidify, and form regions of extended order.  Heating to above 2000 °C (3630 °F) leads to the formation of graphite structure.

METALLIC FIBRE – Manufactured fibre composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal.

METHACRYLATO-SILANE FINISH – Applied to glass fibres to give the highest performance of laminates using polyester resins.

M-GLASS – A high Beryllia (BeO2) content glass designed especially for high modulus of elasticity.

MIBK – Methylisobutylketone.  A less commonly used solvent similar to MEK but with a high flash point 64° F (18° C) and a lower evaporation rate.  May be procured to ASTM D 1153.

MICRO – In relation to composites, denotes the properties of the constituents, that is, matrix, reinforcement, and interface only, and their effects of the composite properties.

microballoons – Also called microspheres.  Small, hollow glass spheres used as fillers in epoxy and polyester compounds to reduce density.  Can also be made from phenolic resins and ceramics.

MICROBEADS – Small spheres manufactured to very close tolerances in diameter.  Used for bondline thickness control; much more expensive than microballoons.

MICROBERBER – Glenoit’s acrylic-polyester blend that’s both soft and durable. Used in outerwear, mittens, and linings.

MICROCRACKING – Cracks formed in composites when thermal or mechanical stresses locally exceed the strength of the matrix.  Since most microcracks do not penetrate the reinforcing fibres, microcracks in a cross-plied tape laminate or in a laminate made from cloth prepreg are usually limited to the thickness of a single ply.

MICROFIBRE – An extremely fine, tightly woven fibre that combines natural breathability with wind and water resistance. Microfibre fabrics are used in performance outerwear and often are laminated to waterproof/breathable fabrics or treated with waterproof/breathable coatings.

MICRON – A unit of length replaced by the micrometer = 10-6 m = 10-3 mm = 0.00003937 in. = 39.4 min.

MICROSTRAIN(A) The strain over a gauge length comparable to the material’s interatomic distance.  CMH-17 (WAS MIL-HDBK-17).  (B)  In design the limit is often expressed as 5000 microstrain or some other figure.  In this case, it means micro-inches per inch.  One micro-inch = one millionth of an inch.

MICROSTRUCTURE – A structure with heterogeneities that can be seen through a microscope.

MIL – The American term for one thousandth of an inch.  A common unit used in measuring the diameter of glass fibre strands, wire, etc.  (1 mil = 0.001in. = 25.4 mm).

MILLED FIBRE – Continuous glass strands hammer milled into very short glass fibres.  Useful as inexpensive filler or anti-crazing reinforcing fillers for adhesives.   See ISO 472.

MISCOLLINATION – Miscollination is a lack of straightness of the fibres and somewhat similar to fibre Marcelling.   Although not the same in principle as fibre misalignment, the effects are the same.

MISMATCHED PARTS – Mismatched parts are a tolerance error.   They will affect the load response of the laminate and may cause fitting errors.

MISPICK – A fill yarn not properly interlaced causing a break in the weaving pattern.

MISSING PICK – (Missing filling yarn) A filling yarn missing from all or a portion of the width of the fabric.

MISSING PLIES – An incorrect stacking sequence is the consequence of missing plies.   An asymmetric laminate may be produced, from which out-of-plane and bending stresses are induced.   Also, the component design stress distribution will be incorrect.

MMC – Metal matrix composite

MODIFIER – Any chemically inert ingredient added to an adhesive formulation that changes its properties.  See also filler, plasticizer and extender.

MODULUS – A measure of the ratio of the applied load to the resultant deformation of the material.  The stiffness of a material.  See modulus of elasticity.

MODULUS OF ELASTICITY – The ratio of the stress applied to the strain of deformation produced in a material that is elastically deformed.  If a tensile strength of 13.8 MPa (2.0 ksi) results in an elongation of 1%, the modulus of elasticity is 13.8 MPa (2.0 ksi) divided by 0.01, or 1380 MPa (200 ksi).  Also called Young’s modulus.  See also offset modulus and secant modulus.

MOHS HARDNESS – A measure of the scratch resistance of a material.  The higher the number, the greater the scratch resistance (No. 10 being termed diamond).

MOISTURE ABSORPTION – The pickup of water vapour from air by a material.  It relates only to vapour withdrawn from the air by a material and must be distinguished from water absorption, which is the gain in weight due to the take-up of water by immersion.

MOISTURE CONTENT – The amount of moisture in a material determined under prescribed conditions and expressed as a percentage of the mass of the moist specimen, that is, the mass of the dry substance plus the moisture present.

MOISTURE METER – An instrument that indicates moisture content in a material.  Originally developed for paper and wood.  Usually based on measurement of radio frequency signal loss attributed to the moisture content of the component or material.  Such instruments may be somewhat useful on fibreglass and aramids, but cannot be used on electrically conducting fibres such as carbon-fibre. 

MOLECULAR WEIGHT – The sum of the atomic weights of all the atoms in a molecule.  A measure of the chain length for the molecules that make up the polymer.

MONOFILAMENT – A single fibre or filament of indefinite length, strong enough to function as a yarn in commercial textile operations.  

MONOLAYER – The basic laminate unit from which cross-plied or other laminate types are constructed.  Also, a “single” layer of atoms or molecules absorbed on or applied to a surface.

MONOLITHIC – Originally derived from monolith, meaning a single block of stone.  In composite terminology it means made from fibre and resin only, i.e. solid composite as opposed to thin composite skins in a sandwich panel, which are bonded to a lightweight core of honeycomb or other material.  See solid laminate.

MONOMER – A single molecule that can react with like or unlike molecules to form a polymer.  The smallest repeating structure of a polymer (mer).  For additional polymers, this represents the original unpolymerized compound.   See ISO 472.

MORPHOLOGY – The overall form of a polymer structure, that is, crystallinity, branching, molecular weight, etc.

MOULD – The cavity or matrix into or on which the plastic composition is placed and from which it takes form.  To shape plastic parts of finished articles by heat and pressure.  The assembly of all the parts that function collectively in the moulding process.   See ISO 472.

MOULD SURFACE – The side of a laminate that faced the mould (tool) during cure; often called the tooled surface.

MOULDED EDGE – An edge that is not physically altered after moulding for use in final form, and particularly one that does not have fibre ends along its length.

MOULDING – The forming of a polymer or composite into a solid mass of prescribed shape and size by the application of pressure and heat for given times.  Sometimes used to denote the finished part.   See ISO 472.

MOULDING CYCLE – The period of time required for the complete sequence of operations on a moulding press to produce one set of moulds.  The operations necessary to produce a set of moulds without reference to the total time taken.

MOULD-RELEASE AGENT – A lubricant, liquid, or powder (often silicone oils and waxes), used to prevent sticking of moulded articles in the cavity.

MULTIDIRECTIONAL – Having multiple ply orientations in a laminate.

MULTIFILAMENT – Yarn or tow consisting of many continuous filaments (also see yarn and tow).

MULTIFILAMENT YARN – A large number (500 to 2000) of fine, continuous filaments (often 5 to 100 individual filaments) usually with some twist in the yarn to facilitate handling.   See ISO 472.

MULTI-ORIENTED PLY LAMINATE (MOPL) – A laminate made from multi-oriented plies, i.e., plies with fibres in more than one direction.

MULTIPLE LAYER ADHESIVE – A film adhesive, usually supported, with a different adhesive composition on each side; designed to bond dissimilar materials such as the core-to-face bond of a sandwich composite.  See also Duplex Film.

MYLAR – Du Pont Trade Name for polyester film.  Excellent moisture and oxygen barrier.  Used as a release sheet in adhesive and composite bonding.  Also used as food packaging.


NAP – Soft, fuzzy surface produced on a fabric by brushing it to raise the fibre.

NAPPING – The process of raising fibres from the base structure of a fabric or felt.

NAVAJO PLY – Basically a hand-crocheted loop used to create a thee-ply yarn.

NEAT RESIN – Resin to which nothing (additives, reinforcements, etc.) has been added.

NECKING – The localized reduction in cross section that may occur in a material under tensile stress.

NEEDLED MAT – A mat formed of strands cut to a short length, then felted together in a needle loom, with or without a carrier.

NESTED LAMINATE – In reinforced plastics, the placing of plies of fabric so that the yarns of one ply lie in the valleys between the yarns of the adjacent ply (nested cloth).

NESTING – In reinforced plastics, placing of plies of fabric so that the yarns of one ply lie in the valleys between the yarns of the adjacent ply (nested cloth).

NIDDY-NODDY – A traditional, low-tech way to wind a skein and measure yarn.

NOBLE COMB – Used commercially in producing worsted yarn.

NODE – The connected portion of adjacent ribbons of honeycomb.

NODE BONDS – That area of the honeycomb core where the cell walls are adhesively bonded.

NOILS – The short fibres that are removed from the fibre in the combing or top-making process. Wool noil is satisfactory for the manufacture of felts and woollens. Silk noil is sportier in appearance and created by short fibres, often from the innermost part of the cocoon.

NOMEX – Aramid fibre or paper.  The paper form is used to make honeycomb.  Low smoke and flame.

NOMINAL SPECIMEN THICKNESS – The nominal ply thickness multiplied by the number of plies.   CMH-17 (WAS MIL-HDBK-17).

NONDESTRUCTIVE EVALUATION (NDE) – Broadly considered synonymous with nondestructive inspection (NDI).  More specifically, the analysis of NDI findings to determine whether the material will be acceptable for its function.

NONDESTRUCTIVE INSPECTION (NDI) – A process or procedure, such as ultrasonic or radiographic inspection, for determining the quality or characteristics of a material, part, or assembly, without permanently altering the subject or its properties.  Used to find internal anomalies in a structure without degrading its properties.  A term preferred to NDT because it describes more accurately the task being performed.

NONDESTRUCTIVE TESTING (NDT) – Broadly considered synonymous with nondestructive inspection (NDI).

NON-UNIFORM AGGLOMERATION OF HARDENER AGENTS – Non-uniform agglomeration of hardener agents will be like a foreign body or resin starved area in the matrix.   There will be a degradation of the local matrix properties.

NONWOVEN FABRIC – A planar textile structure produced by loosely compressing together fibres, yarns, rovings, etc., with or without a scrim cloth carrier.  Accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof. A textile structure produced by bonding or interlocking fibres, or both, accomplished by mechanical, chemical, thermal and/or solvent means.

NOTCH FACTOR – Ratio of the resilience determined on a plain specimen to the resilience determined on a notched specimen.

NOTCH SENSITIVITY – The extent to which the sensitivity of a material to fracture is increased by the presence of a surface non-homogeneity, such as a notch, a sudden change in cross section, a crack, or a scratch.  Low notch sensitivity is usually associated with ductile materials, and high notch sensitivity is usually associated with brittle materials.

NOTCHED SPECIMEN – A test specimen that has been deliberately cut or notched, usually in a V-shape, to induce and locate point of failure.

NOVELTY YARNS – Yarns with an unusual character. Rather like marketing types referring to software bugs as “features”, many spinners call distinctive yarns “novelty yarns”. (The bias of the editor is showing.)

NOVOLAC – A linear thermoplastic B-staged phenolic resin, which, in the presence of methylene or other cross-linking groups, reacts to form a thermoset phenolic.

NYLON – The generic name for all synthetic polyamides.

NYLON PLASTICS – Plastics based on a resin composed principally of a long-chain synthetic polymeric amide that has recurring amide groups as an integral part of the main polymer chain.  Numerical designations (nylon 6, nylon 66, and so on) refer to the monomeric amides of which they are made.  Characterized by great toughness and elasticity, low coefficient of friction and excellent electrical properties, chemical resistance and wear resistance.  Resins are hygroscopic and dimensional stability is poorer than with most other engineering plastics.


OLEFIN – A group of unsaturated hydrocarbons of the general formula CnH2n named after the corresponding paraffin by the addition of “ene” or “ylene” to the root, for example, ethylene, propylene, and pentene.

OLIGOMER – A polymer consisting of only a few monomer units such as a dimer, trimmer etc., or their mixtures.  CMH-17 (WAS MIL-HDBK-17).

ORGANIC – Matter originating in plant or animal life or composed of chemicals of hydrocarbon origin, either natural or synthetic.

ORIENTATION(A) The alignment of the crystalline structure in polymeric materials in order to produce a highly aligned structure.  Orientation can be accomplished by cold drawing or stretching in fabrication.  (B) Direction of alignment of tape or warp direction of fabrics in a composite.

ORIENTED MATERIALS – Materials, particularly amorphous polymers and composites, whose molecules and/or macro constituents are aligned in a specific way.  Oriented materials are anisotropic.  Orientation can generally be divided into two classes, uniaxial and biaxial.

ORTHOTROPIC – Having three mutually perpendicular planes of elastic symmetry.

OSNABURG CLOTH OR FABRIC – see breather cloth.

OUT LIFE – The period of time that a prepreg or film adhesive material remains in a handleable form and with properties intact outside of the specified storage environment and at room temperature.

OUT TIME – The time a prepreg is exposed to ambient temperature, namely the total amount of time the prepreg is out of the freezer.  The primary effects of out time are a decrease in the drape and tack of the prepreg and the absorption of moisture from the air.  Also called “open time”.

OUTER SKIN – That side of the part which is cured against the mould.

OVEN – An unpressurized vessel used to provide controlled and uniform heating.

OVEN DRY – The condition of a material that has been heated under prescribed conditions of temperature and humidity until there is no further significant change in its mass.

OVER AGED PREPREG – Over aged prepreg is where the B-staged prepreg resin has aged or partially set to a point where the final cure will not provide adequate fibre/matrix adhesion and volatile evacuation.   Reduction in the strength or stiffness of the laminate will result.   Also, voids and porosity are common in over aged prepreg, because the volatiles remain trapped within the resin.

OVERCURING – The beginning of thermal decomposition resulting from too high a temperature or too long a moulding time.

OVERLAP – A simple adhesive joint, in which the surface of one adherend extends past the leading edge of another.

OVERLAY SHEET – Non-woven fibrous mat (of glass, synthetic fibre or other material) used as the top layer in a cloth or mat lay-up to provide a smoother finish, minimize the appearance of a fibrous pattern, or permit machining or grinding to a precise dimension. Also called surfacing mat.

OXFORD NYLON – Super heavy-duty (usually 200-denier) basket-weave nylon cloth commonly used in rainwear and tent floors.

OXIDATION – In carbon/graphite fibre processing, the step of reacting the precursor polymer (rayon, PAN, or pitch) with oxygen, resulting in stabilization of the structure for the hot stretching operation.  In general usage, oxidation refers to any chemical reaction in which electrons are transferred.


PACKCLOTH – Typically, a nylon fabric of a medium-density weave with a urethane coating on one side to give it some water repellency.

PAN – See Polyacrylonitrile.

PARALLEL LAMINATE – A laminate of woven fabric in which the plies are aligned in the same position as originally aligned in the fabric roll.  A series of flat or curved cloth-resin layers stacked uniformly on top or each other.   See ASTM D 907 and ISO 472.

PARAMETER – An arbitrary constant, as distinguished from a fixed or absolute constant.  Any desired numerical value maybe given as a parameter.

PARTICULATE COMPOSITE – Material consisting of one or more constituents suspended in a matrix of another material.  These particles are either metallic or nonmetallic.

PARTING AGENT – A material, liquid or solid film used on the tool surface to ease removal of the assembly.  See mould release agent.

PARTING LINE – A mark on a moulded piece where the sections of a mould have met in closing.

PASCAL SECOND – The measure of the specific viscosity of a fluid.  The older unit is the poise.  To convert poise to Pa.s divide by 10.

PASTE(A) An adhesive composition having a characteristic plastic-type consistency, which is a high order of yield value, such as that prepared by heating a mixture of starch and water and subsequently cooling the hydrolyzed product.  See ASTM D 907.  (B) An adhesive that flows very little without being forcibly spread.  Having a similar consistency to toothpaste as it leaves the tube.

PEEK – See polyether Etherketone.

PEEL PLY – A layer of open-weave material, usually fibreglass, polyester, or heat-set nylon, applied directly to the surface of a prepreg lay-up.  The peel ply is removed from the cured laminate immediately before bonding operations, leaving a clean resin-rich surface that may need no further preparation for bonding, other than application of a primer where one is required.  However, some types of peel ply may contaminate a surface with release agents.

PEEL STRENGTH – The adhesive bond strength obtained in the peeling mode selected.  For details of specific tests see ASTM D 903, ASTM D 1876, and for sandwich panels, ASTM D 1781.

PEGGING – The joining of two pieces of core by crush splicing them together with a third piece of core.

PENETRATION(A) A surface discontinuity which penetrates one skin and core or both skins and core, and whose width is the same order of magnitude as its length, i.e., a hole, ballistic damage.  (B)  The entering of an adhesive into an adherend.

PERFORATED SKIN – An outer skin of an acoustical panel that has a pattern of small holes punched or drilled to allow air passage in and out of the panel, often used for noise abatement.

PERMANENCE – The property of a plastic that describes its resistance to appreciable changes in characteristics with time and environment.   See ASTM D 907 and ISO 472.

PERMANENT PATTERN (P.P.) – A secondary pattern or master having all of the necessary coordination data and reference points on its surface from which a tool can be directly made with repeated accuracy.

PERMANENT SET – The deformation remaining after a specimen has been stressed a prescribed amount in tension, compression, or shear for a specified time period and released for a specified time period.  For creep tests, the residual unrecoverable deformation after the load causing the creep has been removed for a substantial and specified period of time.  Also, the increase in length, expressed as a percentage of the original length, by which an elastic material fails to return to original length after being stressed for a standard period of time.

PERMEABILITY – The passage or diffusion (or rate of passage) of a gas, vapour, liquid, or solid through a barrier without physically or chemically affecting it.

PERTEX – A particularly soft microfibre, rip-stop nylon used for windbreakers and sleeping bag shells; highly water resistant, but not waterproof.

PES – Polyether Sulfone

PET – Polyethylene terephthalate

pH – The measure of the acidity or alkalinity of a substance, neutrality being at pH 7.  Acid solutions are less than 7, alkaline solutions are more than 7.

PHENOLIC (PHENOLIC RESIN) – A thermosetting resin produced by the condensation of an aromatic alcohol with an aldehyde, particularly of phenol with formaldehyde.  Used in high-temperature applications with various fillers and reinforcements.  Used for aircraft interior components because in fire it gives off less smoke and toxic fumes than epoxy or polyester resins.   See ISO 472.

PHENYLSILANE RESINS – Thermosetting copolymers of silicone and phenolic resins.  Furnished in solution form.

PI – See Polyimide.

PICK – An individual filling yarn, running the width of a woven fabric at right angles to the warp, also called fill, woof and weft.  In weaving, a weft yarn. In spinning, the process of opening out the fibre to help in the cleaning and processing. This process (picking) allows a lot of vegetable matter to drop out of fleece.

PICK COUNT – The number of filling yarns per inch of woven fabric.

PILE – A generic name for synthetic fleece; often used for single-sided fleeces that are thicker and furrier than the typical two-sided fabrics.

PILED YARN – Yarn made by collecting two or more single yarns.  Normally, the yarns are twisted together, though sometimes they are collected without twist.

PILLS OR FUZZ BALLS – Pills or fuzz balls are prepreg deviations, they are also known as `furring’ of the fibres (Figure 2.A30).   When contained within a laminate it can represent a contamination, where the area could also become susceptible to localised high peel stresses.

PIN HOLES – Small cavities that penetrate the surface of a cured part.

PIT(A) A small, regular, or irregular crater in the surface of a plastic material, usually of a width approximately the same order of magnitude as its depth.  (B) A form of corrosion that occurs under particular conditions which result in small pits rather than general surface corrosion.

PITCH – A high molecular weight material left as a residue from the destructive distillation of coal and petroleum products.  Pitches are used as base materials for the manufacture of certain high-modulus carbon fibres and as matrix precursors for carbon-carbon composites.

PITCH FIBRES – Reinforcement fibre derived from petroleum or coal tar pitch.

PLAIN WEAVE – A weaving pattern in which the warp and fill fibres alternate; that is, the repeat pattern is warp/fill/warp/fill, etc.  Both faces of a plain weave are identical.  Properties are significantly reduced relative to a weaving pattern with fewer crossovers.

PLANAR – Lying essentially in a single plane.

PLANAR HELIX WINDING – A winding in which the filament path on each dome lies on a plane that intersects the dome, while a helical path over the cylindrical section is connected to the dome paths.

PLANAR WINDING – A winding in which the filament path lies on a plane that intersects the winding surface.  See also polar winding.

PLASTIC – A material that contains as an essential ingredient, an organic polymer of large molecular weight, hardeners, fillers, reinforcements, etc; is solid in its finished state; and at some stage in its manufacture or it’s processing into finished articles, can be shaped by flow.  Made of plastic.  A plastic may be either thermoplastic or thermoset.   See ISO 472.

PLASTIC DEFORMATION – Change in dimensions of an object under load that is not recovered when the load is removed, as opposed to elastic deformation.

PLASTIC TOOLING – Tools constructed of plastics, generally laminates or casting materials.  A term employed for structures composed of plastics, usually reinforced thermosets, which are used as tools in the fabrication of metals or other materials including plastics.

PLASTICIZE – To make a material mouldable by softening it with heat or a plasticizer.

PLASTICIZER – A material incorporated in a plastic to increase its workability and flexibility.  Normally used in thermoplastics.  A lower molecular weight material added to an epoxy to reduce stiffness and brittleness, but also resulting in a lower glass transition temperature for the polymer.   See ASTM D 907 and ISO 472.

PLATE SHEAR STRESS – A type of honeycomb shear strength test in which the honeycomb is bonded between two thick steel plates which are displaced relative to each other to place the specimen in shear.  Displacement is accomplished by loading either in tension or compression.  The plate shear represents the best currently known method for obtaining true shear data on honeycomb material.

PLATENS – The mounting plates of a press, to which the entire mould assembly is bolted.

PLIED YARN – Yarn formed by twisting two or more single yarns together in one operation. (Synonyms: folded yarn, formed yarn). Yarn made by collecting two or more single yarns.  Normally, the yarns are twisted together, though sometimes they are collected without twist.

PLY – (A) In general, fabrics or felts consisting of one or more layers.  (B) The layers that make up a stack.  (C) Yarn resulting from twisting operations (three ply yarn, etc.).  (D) A single layer of prepreg.  (E) A single pass in filament winding (two plies forming one layer).  (F) A sheet or layer that is considered to be one discrete piece of manufactured material such as fabric, tape or adhesive film etc.:  A discrete piece may consist of just one piece or of adjoining pieces of the same material. (BDS 1330). A single unit of yarn. A 2-ply yarn would involve taking two singles and then plying them in the opposite direction they were originally spun. See “S-twist” and “Z-twist“. The number of single yarns twisted together to form a plied yarn; also the number of plied yarns twisted together to form a cord. The individual yarn in a plied yarn or in a cord. One of several layers of fabric.

PLY GROUPING – Uncured or unconsolidated plies that are only part of a cured part and grouped together for drawing clarity or for manufacturing engineering purposes.  (BDS 1330)

PLY ORIENTATION – The tow filament orientation of uni-axial or bi-axial material with respect to the tool or part reference axis.  Usually expressed as 0 o, +45 o, -45o, 90 o, or a similar variation.

PLY UNDER LAP OR GAP – A ply under lap occurs when the ply size is too short.   This will cause localised inadequate strength and stiffness load response of the component as opposed to the design requirements.

PMR POLYIMIDES – A novel class of high temperature resistant polymers.  PMR represents in situ polymerization of monomer reactants.

POISE – The measure of the specific viscosity of a fluid.  The c.g.s. unit of viscosity.  1 poise  = 100 centipoise.  The term poise is derived from the name of the man who discovered the laws of flow – Poiseuille.  A more recent term is now the Pascal second (Pa.s).  Pa.s x 10 = poise.

POISSON’S RATIO – The ratio of the change in lateral width per unit width to change in axial length per unit length caused by the axial stretching or stressing of a material.  The ratio of transverse strain to the corresponding axial strain below the proportional limit.  

POLAR – In an unsymmetrical molecule such as water or sulfur dioxide, the mean centre of all the electronic charges does not coincide with the mean electrical centre of the nuclei.  Such a molecule is termed polar; it may be regarded as an electric doublet or dipole, analogous to a tiny magnet.  Polar molecules have an electric moment which is equal to the distance between the two electrical centres multiplied by the total electric charge of either sign in the molecule.  Liquid polar molecules in an electric field tend to orient themselves so that their potential energy is reduced to a minimum.  When the electrical centres of a molecule coincide, the molecule has no electric moment and is said to be nonpolar.

POLAR WINDING – A winding in which the filament path passes tangent to the polar opening at one end of the chamber and tangent to the opposite side of the polar opening at the other end.  A one-circuit pattern is inherent in the system.

POLARGUARD 3D – A newer version of Polarguard made of finer fibres that create a much lighter and more packable insulation; used in parkas and sleeping bags.

POLARGUARD DELTA – The newest refinement of Polarguard, Polarguard Delta has a larger fibre, a larger air space within the fibre, and thinner walls, resulting in a 10% increase in thermal efficiency.

POLARGUARD/POLARGUARD HV – Hoechst Celanese’s high-loft, synthetic insulation of 100 percent polyester continuous-filament fibres. Polarguard is one of the original and most durable synthetic fills, but is somewhat bulky when stuffed. The HV version uses hollowed fibres, is about 25 percent more stuffable, and retains the original Polarguard’s longevity.

POLARITY – Refers to the relative surface charge of a material resulting from the molecular structure of the adherend surface.

POLARTEC – Family name for polyester fleece fabrics of various weights made by Malden Mills and used by a variety of manufacturers. Some Polartecs feature BiPolar construction, which uses different weaving techniques, yarn types, and chemical treatments on each side of the fabric. This creates a two-faced polyester fabric that reacts to the conditions on each side of the surface.

POLARTEC 100 SERIES – Polartec 100 lightweight fleece insulation fabrics are base layers with stretch for freedom of movement and good wicking ability.

POLARTEC 200 BIPOLAR SERIES – Two hundred-weight fleeces made with BiPolar construction to have two distinct surfaces: a durable, weather-shedding external layer and a lofty internal insulating layer.

POLARTEC 200 SERIES – Mid-weight, double-sided fleeces suitable for general-purpose layering; used as a middle or outer layer.

POLARTEC 200 SERIES WITH DWR – Fleeces of 200 weight with durable water repellency to shed snow and water.

POLARTEC 300 BIPOLAR SERIES – Fleeces of 300 weight made with BiPolar construction to have two distinct surfaces: a durable, weather-shedding external layer and a lofty internal insulating layer.

POLARTEC 300 SERIES – A group of heavyweight, double-sided fleeces for use as a middle or outer layer in cold-weather apparel.

POLARTEC POWER DRY SERIES – Fabrics that combine, through BiPolar construction, a highly breathable, next-to-the-skin, wicking layer, which is more of a cloth than a fleece, with an outer layer that spreads moisture for quick evaporation. It comes in light, medium, and heavy weights.

POLARTEC POWER STRETCH SERIES – These body-hugging, four-way stretch fleeces can be the base or only layer depending on conditions and the weight of the fabric. The light, medium, and heavy versions all have a durable and a soft surface.

POLARTEC WINDBLOC – Combines a windproof/breathable laminate sandwiched between two layers of 100-weight Polartec; suitable for use as a cool- or cold-weather outer layer.

POLYACRYLONITRILE (PAN) – Used as a base material or precursor in the manufacture of certain carbon fibres.

POLYAMIDE – A thermoplastic polymer in which the structural units are linked by amide or thio-amide groupings (repeated nitrogen and hydrogen groupings).  Many polyamides are fibre forming.

POLYAMIDEIMIDE – A polymer containing both amide (nylon) and imide (as in polyamide) groups; properties combine the benefits and disadvantages of both.

POLYARYLSULFONE (PAS) – A range of high temperature resistant thermoplastics with Tg values ranging from 190 to 260o C (374 to 500oF).  The term is also occasionally used to describe the family of resins which includes polysulfone and polyether sulfone.

POLYBENZIMIDAZOLE (PBI) – A condensation polymer of diphenyl isophthalate and 3,3’ – di-aminobenzidine.  Extremely high temperature resistance.  Available as adhesive and fibre.

POLYCARBONATE RESIN – A thermoplastic polymer derived from the direct reaction between aromatic and aliphatic dihydroxy compounds with phosgene or by the ester exchange reaction with appropriate phosgene-derived precursors.  Highest impact resistance of any transparent plastic.  Impact resistance rapidly lost when in contact with certain solvents.  

POLYCONDENSATION – See condensation polymerization.

POLYESTER RESINS – Family of resins produced by the reaction of dibasic acids with dihydric alcohols.  Polyethylene terephthalate (PET) is a thermoplastic which may be extruded, injection moulded or blow moulded.  Unsaturated polyesters are thermosets and are used in the reinforced plastics industry for applications such as boats, auto parts, etc.  Modifications with multifunctional acids and bases and some unsaturated reactants permit crosslinking to thermosetting resins.  Polyesters modified with fatty acids are called alkyds.

POLYETHER ETHERKETONE (PEEK) – A linear aromatic crystalline thermoplastic.  A composite with a PEEK matrix may have a continuous-use temperature as high as 250o C (480o F).

POLYETHERIMIDE (PEI) – An amorphous polymer with good thermal properties for a thermoplastic.  Reported Tg of 215o C (419o F) and continuous-use temperature of about 170o C (338o F).

POLYETHERSULFONE (PES) – A thermoplastic material with a Tg of 230° C, (446° F).  One Trade Name is “Victrex”.

POLYIMIDE (PI) – A polymer produced by reacting an aromatic dianhydride with an aromatic diamine.  It is a highly heat-resistant resin.  315o C (600o F).  Similar to a polyamide, differing only in the number of hydrogen molecules contained in the groupings.  Suitable for use as a binder or adhesive.  May be either thermoplastic or thermoset.

POLYMER – A high molecular weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, the mer, for example, polyethylene, rubber, and cellulose.  Synthetic polymers are formed by addition or condensation polymerization of monomers.  Some polymers are elastomers, some are plastics, and some are fibres.  When two or more dissimilar monomers are involved, the product is called a copolymer.  The chain lengths of commercial thermoplastics vary from near a thousand to over one hundred thousand repeating units.  Thermosetting polymers approach infinity after curing, but their resin precursors, often called prepolymers, may be relatively short – 6 to 100 repeating units – before curing.  The lengths of polymer chains, usually measured by molecular weight, have very significant effects on the performance properties of plastics and profound effects on processability.  See ISO 472 and ASTM D 907.  

POLYMER MATRIX – The resin portion of a reinforced or filled plastic.

POLYMERIZATION – A chemical reaction in which the molecules of a monomer are linked together to form large molecules whose molecular weight is a multiple of that of the original substance.  When two or more monomers are involved, the process is called copolymerization.  See ASTM D 907.

POLYMETHYL METHACRYLATE – A thermoplastic polymer synthesized from methyl methacrylate.  It is a transparent solid with exceptional optical properties: available in the form of sheets, granules, solutions, and emulsions.  Certain versions are commonly used for aircraft passenger and cockpit windows.  See ISO 472

POLYPHENYLENE SULPHIDE (PPS) – A high-temperature thermoplastic useful primarily as a moulding compound.  Optimum properties depend on slightly cross-linking the resin.  Known for chemical resistance.

POLYPROPYLENE – A tough, lightweight, thermoplastic made by the polymerization of high-purity propylene gas in the presence of an organometallic catalyst at relatively low pressures and temperatures.

POLYSULFONE – A high temperature resistant thermoplastic polymer with the sulfone linkage, with a Tg of 190­­­­­­­o C (375o F).

POLYSULPHIDE – A synthetic polymer containing Sulphur and carbon linkages, produced from organic dihalides and sodium polysulphide.  Material is elastomeric in nature, resistant to light, oil, and solvents, and impermeable to gases.

POLYURETHANE – A thermosetting resin prepared by the reaction of diisocyanates with polyols, polyamides, alkyd polymers, and polyether polymers.  See also isocyanate plastics and urethane plastics.   See ISO 472.

POROSITY – A condition of trapped pockets of air, gas, or vacuum, within a solid material.  Usually expressed as a percentage of the total nonsolid volume to the total volume (solid plus nonsolid) of a unit quantity of material.  See void content.

POSITIVE PRESSURE(A) Pressure that is above normal atmospheric pressure as differentiated from vacuum pressure, which is below normal atmospheric pressure.  (B) A term also applied to air pressure in a clean room where lay-up takes place prior to bonding.  In this case, the pressure in the room is slightly higher than outside to cause an outward flow of air to prevent dust and other contaminants entering the clean area.

POSTCURE – Additional elevated-temperature cure, usually without pressure to improve final properties and/or complete the cure, or decrease the percentage of volatiles in the compound.  In certain resins, complete cure and ultimate mechanical properties are attained only by exposure of the cured resin to higher temperatures than those of curing.  See ASTM D 907.

POST-FAB – Fabrication process where close-outs and inserts are attached or put into the panel after the facings are bonded to the core.   ASTM C 274.

POSTFORMING – The forming, bending, or shaping of C-staged thermoset laminates that have been heated to make them flexible.  On cooling, the formed laminate retains the contours and shape of the mould over which it has been formed.   See ISO 472.

POT – To embed a component or assembly in liquid resin, using a shell, can or case that remains an integral part of the product after the resin is cured.

POT LIFE – The length of time at some specified temperature that a catalyzed thermosetting resin system retains a viscosity low enough to be used in processing.  Also called working life.

POTTING – Similar to encapsulating except that steps are taken to ensure complete penetration of all the voids in the object before the resin polymerizes.

POTTING COMPOUND – A resin which has been thickened by use of filler (e.g., milled fibres or fine metal shavings).  Also a resin reduced in density by the addition of hollow glass or phenolic microspheres.  Used for joining honeycomb, and edge fill of sandwich panels.

POURCOAT – A liquid honeycomb sealant material (Type III – Boeing) of high solids content that is used to stabilize honeycomb core after crushing or as a seal against moisture entry by coating the cell walls.  This pourcoat is applied by pouring it through the honeycomb cells.

PPS – See Polyphenylene Sulphide.

PREBOND TREATMENT – Synonym for surface preparation.   See ASTM D 907.

PRECURE(A) The full or partial setting of a synthetic resin or adhesive in a joint before the clamping operation is complete or before pressure is applied.  (B) To cure a part prior to joining it with other parts to form a bonded part.  (BDS 1330)  

PRECURSOR – For carbon or graphite fibre, the rayon, PAN, or pitch fibres from which carbon and graphite fibres are derived.

PREFIT – A process for checking the fit of mating detail parts in an assembly prior to adhesive bonding, to ensure proper bond lines.  Mechanically fastened structures are sometimes prefitted to establish shimming requirements.

PREFORM – A preshaped fibrous reinforcement formed by distribution of chopped fibres or cloth by air, water flotation, or vacuum over the surface of a perforated screen to the approximate contour and thickness desired in the finished part.  Also, a preshaped fibrous reinforcement of mat or cloth formed to the desired shape on a mandrel or mock-up before being placed in a mould press.   See ISO 472.

PREFORM BINDER – A resin applied to the chopped strands of a preform, usually during its formation, and cured so that the preform will retain its shape and can be handled.

PREGEL – An unintentional, extra layer of cured resin on part of the surface of a reinforced plastic.  Not related to gel coat.

PREHEATING – The heating of a compound before moulding or casting, to facilitate the operation or reduce the moulding cycle.

PREIMPREGNATION – The practice of mixing resin and reinforcement and effecting partial cure before use of shipment to the user.  See also prepreg.

PREMIX – A moulding compound prepared prior to and apart from the moulding operations and containing all components required for moulding: resin, reinforcement, fillers, catalysts, release agents, and other ingredients.   See ISO 472.

PREMOULDING – The lay-up and partial cure at an intermediate cure temperature of a laminated or chopped-fibre detail part to stabilize its configuration for handling an assembly with other parts for final cure.

PREPLIED PLIES – Multiple plies of preimpregnated material that have been stacked up and compacted together to form a layup and packaged or stored prior to being cured individually or in combination with other parts. 

PREPLIED TAPE – Tape received from the manufacturer with two or more plies laid into specific orientation.

PREPLY – A composite material lamina in the raw-material stage, ready to be fabricated into a finished laminate.  The lamina is usually combined with other raw laminae before fabrication.  A preply includes a fibre system that is placed in position relative to all or part of the required matrix material to constitute the finished lamina.  An organic matrix preply is called a prepreg.  Metal matrix preplies include green tape, flame-sprayed tape, and consolidated monolayers.

PREPOLYMER – A chemical intermediate whose molecular weight is between that of the monomer or monomers and the final polymer or resin.

PREPREG – Either ready-to-mould material in sheet form or ready-to-wind material in roving form, which may be cloth, mat, unidirectional fibre, or paper impregnated with resin and stored for use. The resin is partially cured to a B-stage and supplied to the fabricator, who lays up the finished shape and completes the cure with heat and pressure.  The two distinct types of prepreg available are:  (1) commercial prepregs, where the roving is coated with a hot melt or solvent system to produce a specific product to meet specific customer requirements; and (2) wet prepreg, where the basic resin is installed without solvents or preservatives but has limited room-temperature shelf life.   See ISO 472.

PREPREG BATCH – Prepreg containing reinforcement material from one batch, impregnated with one batch of resin in one continuous operation.

PREPREG LOT – Prepreg from one batch submitted for acceptance at one time.

PRESS CLAVE – A simulated autoclave made by using the platens of a press to seal the ends of an open chamber, providing both the force required to prevent loss of the pressurizing medium and the heat required to cure the laminate inside.

PRESSURE – Force measured per unit area.  Absolute pressure is measured with respect to zero.  Gauge pressure (or relative pressure) is measured with respect to atmospheric pressure.

PRESSURE INTENSIFIER – A layer of flexible material (usually a high-temperature rubber) used to ensure the application of sufficient pressure to a location, such as a radius, in a lay-up being cured.

PRESSURE-IMPREGNATION-CARBONIZATION (PIC) – A densification process for carbon-carbon composites involving pitch impregnation and carbonization under high temperature and isostatic pressure conditions.  This process is carried out in hot isostatic press (HIP) equipment.

PRESSURE-SENSITIVE ADHESIVE – A viscoelastic material that, in solvent-free form, remains permanently tacky.  Such material will adhere instantaneously to most solid surfaces with the application of very light pressure.

PRIMADOWN – A Primaloft refinement that mixes Primaloft and down insulations to produce down-like warmth-to-weight ratios. Primadown is used in outerwear and sleeping bags.

PRIMALOFT/PRIMALOFT 2/PRIMALOFT LITE-HIGH – loft synthetic insulations from Albany International; large- and small-diameter polyester fibres are intermingled to create a down-like feel and spring. Because it’s remarkably water-resistant, it’s a good choice for boaters and wet-weather hikers. Primaloft 2 is a newer, softer, and loftier version. Primaloft Lite features an advanced rib construction that increases loft. All three types are used in insulated clothing and sleeping bags.

PRIMARY STRUCTURE – One critical to structural safety. 

PRIMER – A coating applied to a surface, before the application of an adhesive, lacquer, enamel, etc., to improve the adhesion performance or load-carrying ability of the bond.  Some primers contain a corrosion inhibitor.   See ASTM D 907.

PROCESSING WINDOW – The range of processing conditions, such as stock (melt) temperature, pressure, shear rate, etc., within which a particular grade of plastic can be fabricated with optimum or acceptable properties by a particular fabrication process, such as extrusion, injection moulding, sheet moulding, etc.  The processing window for a particular plastic can vary significantly with design of the part and the mould, with the fabricating machinery used, and with the severity of the end-use stresses.

PROMOTER – A chemical, itself a feeble catalyst, that greatly increases the activity of a given catalyst.  See also accelerator.

PROOF – To test a component or system at its peak operating load or pressure.

PROOF PRESSURE – The test pressure that pressurized components shall sustain without detrimental deformation or damage.  The proof pressure test is used to give evidence of satisfactory workmanship and material quality.

PROTOTYPE – A model suitable for use in complete evaluation of form, design, performance, and material processing.

PS – Polysulfone

psi – Pounds per square inch.

PUCKERS – Areas on prepreg materials where material has locally blistered from the separator film or release paper.

PULLED IN FILLING – An extra filling yarn dragged into the fabric along with a regular filling yarn and extending across a portion of the fabric.

PULTRUSION – A continuous process for manufacturing composites that have a constant cross-sectional shape.  The process consists of pulling a fibre-reinforcing material through a resin impregnation bath and through a shaping die, where the resin is subsequently cured.   See ISO 472.

PUSH-PULL FABRIC – A two-layer fabric made of a nonabsorbent hydrophobic knit next to the skin and an absorbent hydrophilic knit on the outside.

PVF – Polyvinylfluoride.  See “Tedlar”.

PYROLYSIS – With respect to fibres, the thermal process by which organic precursor fibre materials, such as rayon, polyacrylonitrile (PAN), and pitch, are chemically changed into carbon fibre by the action of heat in an inert atmosphere.  Pyrolysis temperatures can range from 800 to 2800o C (1470 to 5070o F), depending on the precursor.  Higher processing graphitization temperatures of 1900 to 3000o C (3450 to 5430o F) generally lead to higher modulus carbon fibres, usually referred to as graphite fibres.  During the pyrolysis process, molecules containing oxygen, hydrogen, and nitrogen are driven from the precursor fibre, leaving continuous chains of carbon.  Also means material decomposition from heat.


QUADRAX® BIAXIAL TAPE – Trade name of unidirectional thermoplastic prepreg tape ribbons interlaced in fabric weaves that have the benefits of fabric formability and the strength of unidirectional tapes in both longitudinal and transverse directions.

QUADRAXIAL FABRIC – Fabric with four non-interwoven layers +45°, -45°, 0° and 90° – which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also biaxial fabric, triaxial fabric.)

QUALIFICATION – Demonstrated skill, training, knowledge, and experience required for personnel to properly perform the duties of a specific job.

QUALIFICATION TEST – A series of tests conducted by the procuring activity, or an agent thereof, to determine conformance of material, or materials system, to the requirements of a specification which normally results in a qualified products list under the specification.  NOTE:  Generally, qualification under a specification requires a conformance to all tests in the specification, or it may be limited to conformance to a specific type or class, or both, under the specification.  See ASTM D 907. 

QUALIFIED PRODUCTS LIST (QPL) – A list of commercial products that have been pretested and found to meet the requirements of a specification, especially a government specification.

QUALITY ASSURANCE – The function of evaluating product quality and the procedures taken to ensure that the final product conforms to the specification requirements.  It has for its purpose the continuing assurance of the customer that the product he receives is of or better than the quality level he expects.

QUALLOFIL – DuPont’s high-loft, seven-channel polyester insulation with a soft, down-like feel; used primarily in sleeping bags, but also in insulating outerwear and accessories.

QUARANTINED – To be located in a separate place awaiting some further action, i.e., re-qualification test, inspection, repair information, spare parts or disposal.

QUASI-ISOTROPIC LAMINATE – A laminate laid up symmetrically with an equal number of plies at each 0°, ±45°, and 90° angle.  The properties in the plane of such a laminate will be nearly the same in all directions.  Approximates isotropy by orientation of plies in multiple directions. +45°, -45°, 0° and 90°


RADICAL – A very reactive chemical intermediate.

RAMPING – A gradual programmed increase/decrease in temperature or pressure to control cure or cooling.

RANDOM PATTERN – A winding with no fixed pattern.  If a large number of circuits is required for the pattern to repeat, a random pattern is approached.  A winding in which the filaments do not lie in an even pattern.

REACTION INJECTION MOULDING (RIM) – A process for moulding polyurethane, epoxy, and other liquid chemical systems.  Mixing of two to four components in the proper chemical ratio is accomplished by a high-pressure impingement-type mixing head, from which the mixed material is delivered into the mould at low pressure, where it reacts (cures).

REACTIVE DILUENT – As used in epoxy formulations, a compound containing one or more epoxy groups that functions mainly to reduce the viscosity of the mixture.

REFRACTORY – Refractories are special materials of construction capable of withstanding high temperatures in various industrial processes and operations.  The main bulk of commercial refractories are complex solid bodies consisting of high melting oxides or a combination of oxides of elements such as silicon, aluminium, magnesium, calcium, and zirconium, with small amounts of other elements present as impurities.  Refractoriness is the ability of a material to withstand the action of heat without appreciable deformation or softening under particular service conditions.  These materials are used for furnace linings, crucibles for melting metals, rocket nozzles, etc.

REINFORCED PLASTICS – Moulded, formed, filament-wound, tape-wrapped, or shaped plastic parts consisting of resins to which reinforcing fibres, mats, fabrics, etc., have been added before the forming operation to provide some strength properties greatly superior to those of the base resin.  See ISO 472.

REINFORCED REACTION INJECTION MOULDING (RRIM) – A reaction injection moulding with a reinforcement added.  See also reaction injection moulding.

REINFORCEMENT – A strong material bonded into a matrix to improve its mechanical properties.  Reinforcements are usually long fibres, chopped fibres, whiskers, particulates, etc.  The term should not be used synonymously with filler.  A material used to reinforce, strengthen or give dimensional stability to a part.

RELATIVE HUMIDITY – The ratio of the actual pressure of existing water vapour to the maximum possible (saturation) pressure of water vapour in the atmosphere at the same temperature, expressed as a percentage.

RELEASE AGENT – A material that is applied in a thin film to the surface of a mould to keep the resin from bonding to the mould.  Also called parting agent.  See also mould release agent.   See ISO 472.

RELEASE FILM – An impermeable layer of film that does not bond to the resin being cured.  See also separator.

RELEASE PAPER – A sheet, serving as a protectant or carrier, or both, for an adhesive film or mass, which is easily removed from the film or mass prior to use.   See ASTM D 907.

RESILIENCE – The ratio of energy returned, on recovery from deformation to the work input required to produce the deformation (usually expressed as a percentage).  The ability to regain an original shape quickly after being strained or distorted.

RESIN – A solid or pseudo solid organic material, usually of high molecular weight, that exhibits a tendency to flow when subjected to stress.  It usually has a softening or melting range, and fractures conchoidally.  Most resins are polymers.  In reinforced plastics, the material used to bind together the reinforcement material; the matrix.  See also polymer.  See ISO 472 and ASTM D 907.

RESIN BATCH – Resin mixed in one mixer in one operation or resins blended together in one homogeneous mix with traceability to the individual component lots.

RESIN CONTENT – The amount of resin in a laminate expressed as either a percentage of total weight or total volume.

RESIN POCKET – An apparent accumulation of excess resin in a small, localized section visible on cut edges of moulded surfaces, or internal to the structure and nonvisible.  See also resin-rich area.  See ISO 472.

RESIN RIDGE – A sharp buildup on the surface of a part consisting of only resin.

RESIN SYSTEM – A mixture of resin and ingredients such as catalyst, initiator, diluents, etc., required for the intended processing and final product.

RESIN TRANSFER MOULDING (RTM) – A process whereby catalyzed thermosetting resin is transferred or injected into an enclosed mould in which the fibre reinforcement has been placed.  Cure is normally accomplished without external heat.  RTM combines relatively low tooling and equipment costs with the ability to mould large structural parts.  In general, thermoplastics are too viscous to be used in RTM even if heat is applied.

RESIN, LIQUID – An organic polymeric liquid which becomes a solid when converted into its final state for use.  Various curing agents may be used.  Some require heat for a specified time, some cause curing at room temperature.  See ASTM D 907.

RESIN/FIBRE DUST – Nuisance dust composed of a mixture of resin and fibre formed from solid material by crushing, grinding, drilling, etc., of nonmetallic composites.

RESINOGRAPHY – The science of the morphology, structure, and related descriptive characteristics as correlated with the composition or conditions and with the properties or behavior of resins, polymers, plastics, and their products.

RESINOID – Any of the class of thermosetting synthetic resins, either in their initial temporarily fusible state or in their final infusible state. See ASTM D 907.  See also novolak and thermosetting.

RESIN-RICH AREA – Localized area filled with resin and lacking reinforcing material.  See also resin pocket.

RESIN-STARVED AREA – Localized area of insufficient resin, usually identified by low gloss, dry spots, or fibre showing on the surface.

RESITE – An alternative term for ‘C’ stage.   See ASTM D 907 and ISO 472.

RESITOL – An alternative term for ‘B’ stage.   See ASTM D 907 and ISO 472.

RESOLE – An alternative term for ‘A’ stage.   See ASTM D 907 and ISO 472.

RETARDER – See inhibitor.   See ISO 472.

RETICULATION – The term applied to the process needed before using a film adhesive to bond the skin to the core on large cell honeycomb materials, usually for acoustic panels.  The film adhesive is laid on the honeycomb and made to shrink back onto the cell ends by careful heating with hot air using a temperature controlled hot air gun, so that the perforations in the skin for acoustic attenuation are not blocked during skin to core bonding in an autoclave.  This process requires film adhesive without any carrier fabric so that under the careful application of hot air the adhesive can shrink back onto the cell ends.  The heat must be sufficient to cause shrink back but not too much so as to cause cure.  The adhesive on the cell ends must melt again and flow to give good fillets between the honeycomb and the skin during the final curing process.  Also used with very lightweight spacecraft structures, as thinner, lighter adhesive films can be used.

REVERSE HELICAL WINDING – In filament winding, as the fibre delivery arm traverses one circuit, a continuous helix is laid down, reversing direction at the polar ends.  In contrast to biaxial, compact, or sequential winding.  The fibres cross each other at definite equators, the number depending on the helix.  The minimum region of crossover is three.

REVERSE IMPACT TEST – A test in which one side of a sheet of a material is struck by a pendulum or falling object and the reverse side is inspected for damage.

R-GLASS – American and European version of S-Glass.

RHEOLOGY – The study of the flow of materials, particularly plastic flow of solids and the flow of non-Newtonian liquids.  The science treating the deformation and flow of matter.

RIB – A reinforcing member designed into a plastic part to provide lateral, horizontal, hoop, or other structural support.

RIBBON – A fibre having essentially a rectangular cross-section, where the width-to-thickness ratio is at least 4:1. The propensity of glass bundles to “stick” together and act as a strand or end.

RIBBON DIRECTION – In honeycomb, the direction of the node bonds.

RIGID PLASTICS – For purposes of general classification, a plastic that has a modulus of elasticity either in flexure or in tension greater than 690 MPa (100 ksi) at 23o C (70o F) and 50% relative humidity.   See ISO 472.

RIGID RESIN – A resin having a modulus high enough to be of practical importance, for example, 690 MPa (100 ksi) or greater.

RIGIDITY – The property of bodies by which they can resist an instantaneous change of shape.  The reciprocal of elasticity.

RIM – See reaction injection moulding.

RIPSTOP – A fabric, usually nylon or polyester, woven with double threads at regular intervals to prevent tears in the fabric from spreading; used for outerwear, sleeping bags, packs, and tents.

RISE TIME – In urethane foam moulding, the time between the pouring of the urethane mix and the completion of foaming.

ROCKWELL HARDNESS – A value derived from the increase in depth of an impression as the load on an indenter is increased from a fixed minimum value to a higher value and then returned to the minimum value.  Indenters for the Rockwell test include steel balls of several specific diameters and a diamond cone penetrator having an included angle of 120o with a spherical tip having a radius of 0.2 mm (0.0070 in.).  Rockwell hardness numbers are always quoted with a prefix representing the Rockwell scale corresponding to a given combination of load and indenter, for example, HRC 30.

ROOM TEMPERATURE – A temperature in the range of 20 to 30o C (68 to 86o F).  The term room temperature is usually applied to an atmosphere of unspecified relative humidity.

ROOM-TEMPERATURE CURING ADHESIVE – An adhesive that sets (to handling strength) within an hour at temperatures from 20 to 30o C (68 to 86o F) and later reaches full strength without heating.

ROOM-TEMPERATURE VULCANIZING (RTV) – Vulcanization or curing at room temperature by chemical reaction; usually applies to silicones and other rubbers.

ROOM-TEMPERATURE-SETTING ADHESIVE – An adhesive that sets in the temperature range from 20 to 30o C (68 to 86o F), in accordance with the standard limits for room temperature specified in ASTM D 618 and ASTM D 907.  Compare to cold-setting adhesive, hot-setting adhesive, and intermediate-temperature-setting adhesive.

ROSIN – A resin obtained as a residue in the distillation of crude turpentine from the sap of the pine tree (gum resin) or from an extract of the stumps and other parts of the tree (wood resin).

ROVING – A number of yarns, strands, tows, or ends collected into a parallel bundle with little or no twist.  This term is applied most commonly to glass and Kevlar®.   See ISO 472. A collection of untwisted strands wound together into a doff (ball). Also, another name for the fabrication process step. If the filaments are all parallel to each other, the end is called a roving (graphite rovings are also referred to as tows). Rovings are usually denoted by the number of ends they contain; tows by the number of filaments. The most common graphite tows are 3K, 6K, and 12K. The soft strand of carded fibre that has been twisted, attenuated, and freed of foreign matter preparatory to spinning.

ROVING BALL – The supply package offered to the winder, consisting of a number of ends or strands wound to a given outside diameter onto a length of cardboard tube.  Usually designated by either fibre weight or length in yards. A term used to describe the supply package offered to the winder. It consists of a number of ends or strands wound to a given outside diameter onto a length of cardboard tube.

ROVING CLOTH – A textile fabric, coarse in nature, woven from rovings.

ROVING DOFF OR “DOFF” – The final product sold to the customer. Made by roving or pulling together a group of forming cakes. (The number of which depends upon the product being made)

ROW NUCLEATION – The mechanism by which stress-induced crystallization is initiated, usually during fibre spinning or hot drawing.

RRIM – See reinforced reaction injection moulding.

RTM – See resin transfer moulding.

RTV – See room-temperature vulcanizing.

RUBBER – Cross-linked polymers with glass transition temperature below room temperature, which exhibit highly elastic deformation and have high elongation.   See ISO 472.

RULE OF MIXTURES – When two materials are mixed together, it is normally the case that the properties of the mixture are an average of the properties of the constituents according to the proportion of each in the mixture.  This applies, for example, to particulate reinforced composites and fillers in resins.

RUNNER(A) The secondary feed channel in an injection or transfer mould that runs from the inner end of the sprue to the cavity gate.  (B) The moulding material in this secondary feed channel.  ISO 472.

RUPTURE – A cleavage or break resulting from physical stress.  Work of rupture.  The integral of the stress-strain curve between the origin and the point of rupture.


SAGGING – Run-off or flow-off of adhesive from an adherend surface due to application of excess or low viscosity material.

SAMPLE(A) A small portion of a material or product intended to be representative of the whole.  (B) Statistically, a sample is the collection of measurements taken from a specified population.   CMH-17 (WAS MIL-HDBK-17).

SANDING – The use of abrasive papers to either prepare a surface by light abrasion using a fine grit paper; or remove material and/or shape a part with a coarse grit paper.

SANDWICH CONSTRUCTIONS – Panels composed of a lightweight core material, such as honeycomb, foamed plastic, etc., to which two relatively thin, dense, high-strength or high-stiffness faces or skins are adhered.

SANDWICH PANEL – A panel consisting of two thin face sheets bonded to a thick, lightweight honeycomb or foam core.

SATIN – A type of finish having a satin or velvety appearance, specified for plastics or composites.

SATIN WEAVE – See harness satin.

SATURATION – An equilibrium condition in which the net rate of absorption under prescribed conditions falls essentially to zero.  CMH-17 (WAS MIL-HDBK-17).

S-BASIS – The S-basis property allowable is the minimum value specified by the appropriate federal, military, Society of Automotive Engineers, American Society for Testing and Materials, and other recognized approved specifications for the materials.

SCARF ANGLE – The angle of taper of a scarf joint, i.e., the taper ratio of length to thickness.

SCARF JOINT:  A joint made by cutting away similar angular segments on two adherends and bonding the adherends with the cut areas fitted together.  See also lap joint.  See ASTM D 907.

SCORING – A type of wear in which the working face acquires grooves, axial or circumferential, according to whether the motion is reciprocating or rotary.  Also applied to a similar effect on the rigid, nonmoving member.  A groove which is smooth and has significant width compared to depth.  A blunt scratch.

SCOURING – Cleaning process to remove size (steam, acid wash) followed by drying under controlled conditions.

SCRATCH – An elongated surface discontinuity which is infinitely small in width compared to length.  Shallow mark, groove, furrow or channel normally caused by improper handling or storage.

SCRIM – A low-cost reinforcing fabric made from continuous filament yarn in an open-mesh construction.  Used in the processing of tape or other B-stage material to facilitate handling.  Also used as a carrier of adhesive, to be used in secondary bonding.

SEALANT – A material applied to a joint in paste or liquid form that hardens or cures in place, forming a seal against gas or liquid entry.

SECONDARY BONDING – The joining together, by the process of adhesive bonding, of two or more already cured composite parts, during which the only chemical or thermal reaction occurring is the curing of the adhesive itself.

SECONDARY STRUCTURE – In aircraft and aerospace applications, a structure that is not as critical to flight safety as is primary structure.

SELF-EXTINGUISHING RESIN – A resin formulation that will burn in the presence of a flame, but will extinguish itself within a specified time after the flame is removed.

SELF-IGNITION TEMPERATURE – The temperature of a material at which spontaneous combustion takes place when the temperature rises slowly.

SELF-SKINNING FOAM – A urethane foam that produces a tough outer surface over a foam core upon curing.

SELVAGE – The woven-edge portion of a fabric parallel to the warp, finished off so as to prevent the yarns from unraveling.  Modern “Jet” looms (since 1980s) leave cut ends in the weft but locking threads provided to stop edge unraveling.  Selvage is always cut off.  It is never incorporated into the work.

SEMICRYSTALLINE – In plastics, materials that exhibit localized crystallinity.  See also crystalline plastic.

SEPARATOR – A permeable layer that also acts as a release film.  Porous Teflon-coated fibreglass is an example.  Often placed between lay-up and bleeder to facilitate bleeder system removal from laminate after cure.  Also called “separator cloth”.

SEPTUM – Adhesive and prepreg cured between two pieces of core.

SERVICE CONDITIONS – The heat, cold, flexing, shock, impact, vibration, etc., that an adhesive or composite will be subjected to in service.

SERVING – Wrapping of yarn around a product in one or more layers to form a protective covering.

SET(A) The irrecoverable or permanent deformation or creep after complete release of the force producing the deformation.  (B) To convert an adhesive into a fixed or hardened state by chemical or physical action, such as condensation, polymerization, oxidation, vulcanization, gelation, hydration, or evaporation of volatile constituents.  See ASTM D 907.  See also cure and dry.

SET UP – To harden, as in curing of a polymer resin.

SETT – A term used to define the weft or warp density of a woven fabric, usually in terms of a number of threads per inch.

SETTING TEMPERATURE – The temperature to which an adhesive or assembly is subjected to set the adhesive.  See cure temperature.

SETTING TIME – The period of time during which an assembly is subjected to heat or pressure, or both, to set the adhesive.

SEWINGThread-Flexible small diameter yarn or strand, usually treated with a surface coating and/or lubricant, used to stitch one or more pieces of material together or stitch an object to a material.

S-GLASS – (Silica/Alumina/Magnesia) Structural Glass, used as fibre reinforcement, designed to give high tensile strength.  More expensive than E-Glass.

SHEAR CRIMPING – Buckling of the compressive facing due to low core shear modulus.  Usually causes the core to fail in shear at the crimp.   ASTM C 274.

SHEAR FRACTURE (FOR CRYSTALLINE TYPE MATERIALS) – A mode of fracture resulting from translation along slip planes, which are preferentially oriented in the direction of the shearing stress.   CMH-17 (WAS MIL-HDBK-17).

SHEAR MODULUS – The ratio of shearing stress to shearing strain within the proportional limit of the material.

SHEAR RIGIDITY – The sandwich property which resists shear distortions; synonymous with shear stiffness.   SEE ASTM C 274.

SHEAR STRAIN – The tangent of the angular change, caused by a force between two lines originally perpendicular to each other through a point in a body.  Also called angular strain.

SHEAR STRENGTH – The maximum shear stress that a material is capable of sustaining.  Shear strength is calculated from the maximum load during a shear or torsion test and is based on the original cross-sectional area of the specimen.

SHEAR STRESS – The component of stress tangent to the plane on which the forces act.  See ISO 472.

SHEAR:  An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact.  In interlaminar shear, the plane of contact is composed primarily of resin.  See also shear strength and shear stress.

SHEAROGRAPHY – Was developed for strain measurements.  This process now provides a full-field video strain gauge, in real time, over large areas.  It is an enhanced form of holography, which requires that part to be under stress.  A laser is used to illuminate the part while under stress.  The output takes the form of an image processed video display.

SHEET MOULDING COMPOUND (SMC) – A composite of fibres, usually a polyester resin, and pigments, fillers and other additives that have been compounded and processed into sheet form to facilitate handling in the moulding operation.

SHEET, SHEETING – A thin, generally plane product in which the thickness is small in proportion to length and width.   See ISO 472.

SHELF LIFE – The length of time a material, substance, product, or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended function.  Synonym for storage life.

SHELL TOOLING – A mould or bonding fixture consisting of a contoured surface shell supported by a substructure to provide dimensional stability.

SHOE:  A device for gathering filaments into a strand, in glass fibre forming.

SHORE HARDNESS – A measure of the resistance of material to indentation by a spring-loaded indenter.  The higher the number, the greater the resistance.  Normally used for rubber materials.  See ASTM D 314.

SHORT BEAM SHEAR (SBS) – A flexural test of a specimen having a low test span-to-thickness ratio (for example, 4:1), such that failure is primarily in shear.

SHORT BEAM SHEAR STRENGTH – The interlaminar shear strength of a parallel-fibre-reinforced plastic material as determined by three-point flexural loading of a short segment cut from a ring specimen.  Four-point loading may also be used.   See ASTM D 2344.

SHORTS – Short pieces or locks of fibre that are dropped out while fibres are being sorted.

SHRINKAGE(A) The relative change in dimension from the length measured on the mould when it is cold to the length of the moulded object 24 hours after it has been taken out of the mould.  (B) (Of cellular plastics) Inadvertent dimensional decrease of cellular plastics without breakdown of cell structure.   See ISO 472.

SILICA GEL – A form of colloidal silica which has the appearance of coarse sand and has many fine pores.  It is extremely absorbent and is used as a catalytic material.

SILICON CARBIDE – Reinforcement, in whisker, particulate, and fine or large fibre, that has application as metal matrix reinforcement because of its high strength and modulus, density equal to that of aluminium and comparatively low cost.  As a whisker or particulate, it gives the composite isotropic properties and is easily machined.

SILICONE BAG – A permanent vacuum bag used in curing of composite lay-ups.  It is made of silicone rubber sheet and has an inner locking seal or rope-type seal.

SILICONE PLASTICS – Plastics based on resins in which the main polymer chain consists of alternating silicon and oxygen atoms, with carbon-containing side groups.  Derived from silica (sand) and methyl chlorides and furnished in different molecular weights, including liquids, solid resins, and elastomers.   See ISO 472.

SILICONE RUBBER – A rubber prepared by the action of moisture on dichloro-dimethyl silicone.  These rubbers withstand temperatures from -60 to 250o C and are vulcanized with benzoyl peroxide.

SILICONES – Resinous materials derived from organosiloxane polymers, furnished in different molecular weights including liquids, solid resins and elastomers.  Silicones exhibit outstanding heat resistance (from -100 to 500o F or -73 to 260o C), electrical properties and compatibility with body tissues.  They cure by a variety of mechanisms, are relatively expensive and are available in many forms – namely, laminating resins, moulding resins, coatings, casting or potting resins and sealants.

SILK – The product of the silkworm. The cocoon thread is of indefinite length but exceedingly fine and lustrous. Continuous protein filament produced by the larvae of various insects, especially the caterpillar when constructing its cocoon. The chief portion of commercial silk is produced in Japan and China. Characteristics: resiliency, aesthetics, elasticity and strength, warm in winter, cool in summer. See Reeled Silk and Doupioni.

SILMOND – A satiny polyester microfibre shell fabric that is inherently wind and water resistant; it often is enhanced with a water-repellent finish.

SINGLE LAP SPECIMEN – In adhesive testing, a specimen made by bonding the overlapped edges of two sheets or strips of material, or by grooving a laminated assembly, as shown in ASTM D 2339 and ASTM D 3165.  In testing, a single lap specimen is usually loaded in tension at the ends.  See ASTM D 4896.  For metal bonds the most common method is ASTM D 1002.

SINGLE-CIRCUIT WINDING – A winding in which the filament path makes a complete traverse of the chamber, after which the following traverse lies immediately adjacent to the previous one.

SINGLES – The individual unit of yarn. Referring to a “single ply” this is almost guaranteed to make experienced spinners cringe.

SINGLES YARN – One or more strands with applied twist.   See ISO 472. The simplest strand of textile material suitable for weaving, knitting and other operations.

SINGLY ORIENTED PLY LAMINATE (SOPL) – A laminate made from singly oriented plies, i.e., plies with fibres in only one direction.

SINTERING(A) The bonding of powders by solid-state diffusion, resulting in the absence of a separate bonding phase.  The process is generally accompanied by an increase in strength, ductility, and density.  (B) A process by which fine particles, in direct contact with each other, form a solid body when heated to a suitable temperature.

SINTERING TEMPERATURE – The temperature at which a given powdered compact will densify to a certain desired density, say 90% of the theoretical density during a certain heating period.

SIZE – Any treatment consisting of starch, gelatin, oil, wax, or other suitable ingredient applied to yarn or fibres at the time of formation to protect the surface and aid the process of handling and fabrication or to control the fibre characteristics.  The treatment contains ingredients that provide surface lubricity and binding action but, unlike a finish, contains no coupling agent.  Before final fabrication into a composite, the size is usually removed by heat cleaning, and a finish is applied.  Sometimes (incorrectly) used to mean “finish”.

SIZING(A) The process of applying a size.  See “size”.  (B) Applying a material on a surface in order to fill pores and thus reduce the absorption of the subsequently applied coating. This term is sometimes incorrectly used when a finish to improve adhesion is being described.

SIZING CONTENT – The percent of the total strand weight made up by the sizing; usually determined by burning off or dissolving the organic sizing; known as loss on ignition.   See ASTM D 907.

SKEIN – A continuous filament, strand, yarn, or roving, wound up to some measurable length and usually used to measure various physical properties.

SKEWING – A condition where the warp and fill yarns are not at right angles to each other.

SKIN – The relatively dense material that may form the surface of a cellular plastic or of a sandwich.

SLIP – The relative co-linear displacement of the adherends on either side of the adhesive layer in the direction of the applied load.   See ASTM D 4027.

SLIP ANGLE – The angle at which a tensioned fibre will slide off a filament-wound dome.  If the difference between the winding angles is less than the slip angle, fibre will not slide off the dome.  Slip angles for different fibre-resin systems very and must be determined experimentally.

SLIPPAGE – Undesired movement of the adherends with respect to one another during the bonding process.  Slipping of the surfaces of parts to be bonded during the bonding process.

SLIVER – A number of staple or continuous-filament fibres aligned in a continuous strand without twist.  Pronounced “slyver”.  See also strand.

SLUB – An abruptly thickened place in a yarn.

SMASH – A place in the fabric where a number of warp or filling yarns have been broken.

SMC – See sheet moulding compound.

S-N DIAGRAM – A plot of stress (S) against the number of cycles to failure (N) in fatigue testing.  A log scale is normally used for N.  For S, a linear scale is often used, but sometimes a log scale is used here, too.  Also a representation of the number of alternating stress cycles a material can sustain without failure at various maxim stresses.

SOLID LAMINATE – A structurally reinforced resin impregnated composite cured to a solid state containing no sandwich layers of honeycomb, plastic foam or other material.  See monolithic.

SOLIDS CONTENT – The percentage by weight of nonvolatile matter in an adhesive.  See ASTM D 907.

SOLUBILITY – The degree to which a substance will dissolve in a particular solvent, usually expressed as grams dissolved in 100 grams of solvent.

SOLVATION – The process of swelling, gelling, or dissolving a resin by a solvent or plasticizer.

SOLVENT – A substance (usually a liquid) used for dissolving and/or cleaning materials during reinforced plastics operations.  Often flammable or toxic.  Should be handled in accordance with safety instructions.

SOLVENT ACTIVATED ADHESIVE – A dry-film adhesive that is rendered tacky by the application of a solvent just prior to use.  

SOLVENT ADHESIVE – An adhesive having a volatile organic liquid as a vehicle.  This term excludes water-based adhesives.   See ASTM D 907.

SOLVENT WELDING – Also known as solvent cementing, is used for the mass production of strong and reliable joints for thermoplastic materials.  After cleaning with a solvent that has no effect on the polymer to be joined, a suitable solvent is selected and applied only to the mating faces, masking being used to protect other areas.  See Handbook of Adhesion (Reference 2.15).

SPANDEX – The generic name for a synthetic fibre having great stretch, good strength and abrasion resistance, and long-term resistance to body acids; always used in combination with another fibre, such as cotton, polyester, or nylon. DuPont’s Lycra is the best-known brand of spandex.

SPANDURA – Cordura filaments spun around a stretchy Lycra core; used in rugged trail and climbing clothes.

SPECIFIC ADHESION – Adhesion between surfaces that are held together by valence forces of the same type as those that give rise to cohesion.  See ASTM D 907.

SPECIFIC GRAVITY – The density (mass per unit volume) of any material divided by that of water at a standard temperature.

SPECIFIC HEAT – The quantity of heat required to raise the temperature of a unit mass of a substance by 1o C under specified conditions.

SPECIFIC PROPERTIES – Material properties divided by the material density.

SPECIFICATION – A detailed description of the characteristics of a product and of the criteria which must be used to determine whether the product is in conformity with the description.

SPECIMEN(A) A piece or portion of a sample or other material taken to be tested.  Specimens normally are prepared to conform to the applicable test method.  CMH-17 (WAS MIL-HDBK-17).  (B) In the case of adhesives a specimen (or test piece) is made up to a ASTM or other standard using metal or composite adherends bonded together in order to test the adhesive and not the adherends.  Some tests such as compression tests on short bars machined from cast adhesive samples, may be carried out using a sample of adhesive alone.

SPECTRA® – Allied Fibres and Plastics Trade Name for ultra high molecular weight Polyethylene fibres.

SPF – See superplastic forming.

SPLASH – A “splash” is an intermediate tool made using a fibre filled synthetic plaster material.

SPLAY – A fan like surface defect near the gate on a part.

SPLICE – The joining of two ends of glass fibre yarn or strand, usually by means of an air-drying adhesive.

SPLICE FABRIC – A portion of the fabric which has been cut and rejoined.

SPLINTERING – A combination of cracking and delamination of the outer skin.

SPLIT CORE – Core cell walls ruptured or split.

SPRAYED METAL MOULDS – Moulds made by spraying molten metal onto a master until a shell of predetermined thickness is achieved. The shell is then removed and backed up with plaster, cement, casting resin, or other suitable material.  Used primarily as a mould in the sheet forming process.

SPRAY-UP – Technique in which a spray gun is used as an application tool.  In reinforced plastics, for example, fibrous glass and resin can be simultaneously deposited in a mould.  In essence, roving is fed through a chopper and ejected into a resin stream that is directed at the mould by either of two spray systems.  In foamed plastics, fast-reacting urethane foams or epoxy foams are fed in liquid streams to the gun and sprayed on the surface.  On contact, the liquid starts to foam.

SPREAD – The quantity of adhesive per unit joint area applied to an adherend, usually expressed in pounds of adhesive per thousand square feet of joint area.  See ASTM D 907. 

SPRUE – A single hole through which moulding compounds are injected directly into the mould cavity.   See ISO 472.

SPUN ROVING – A heavy, low-cost glass or aramid fibre strand consisting of filaments that are continuous but doubled back on themselves.

SQUEEZE OUT – See Edge Bleed.

SRM – Structural Repair Manual.  Provided by component manufacturers.

STABILITY – A property which allows a compound to be stored under specific conditions without loss of its original properties.

STABILIZATION – In carbon fibre forming, the process used to render the carbon fibre precursor infusible prior to carbonization.

STABILIZED CORE – Honeycomb cores in which the cells have been filled with a specified reinforcing material for the purpose of supporting the cell walls during machining.  Honeycomb core in which the cell walls have been reinforced with a specified reinforcing material.

STABILIZER – A substance used in the formulation of some plastics to assist in maintaining the properties of the material at or near their initial values during processing and service life.   See ISO 472.

STACKING – The lamination sequence in which the warp surface of one ply is laid against the fill surface of the preceding ply.  The sequence of lay-up repeats warp-fill, warp-fill, warp-fill, etc., until the lay-up is completed.  The lamination sequence in which the plies are continuously laid-up warp surface to fill surface until the lay-up is completed.

STACKING SEQUENCE – A description of a laminate that details the ply orientations and their sequence in the laminate.

STAGING – Heating a premixed resin system, such as in a prepreg, until the chemical reaction (curing) starts, but stopping the reaction before the gel point is reached.  Staging is often used to reduce resin flow in subsequent press moulding operations. 

STAPLE FIBRES – Fibres of spinnable length manufactured directly or by cutting continuous filaments to short lengths (usually 12.7 to 50 mm or 1/2 to 2” long; 1 to 5 denier).  See ISO 472.

STARVED AREA – An area in a plastic part that has an insufficient amount of resin to wet out the reinforcement completely.  This condition may be due to improper wetting, impregnation, or resin flow; excessive moulding pressure; or improper bleeder cloth thickness.

STARVED JOINT – An adhesive joint that has been deprived of the proper film thickness of adhesive due to insufficient adhesive spreading or to the application of excessive pressure during the lamination process.  A joint which has an insufficient amount of adhesive to produce a satisfactory bond.

STATION LINE – Reference lines on loft and print used for locating and dimensioning purposes.  They are in planes perpendicular to the horizontal centreline in the fuselage.  They are in planes perpendicular to the wing reference plane in the wing.  Stations are called out by number.  This number is in inches (or millimetres) from Station 0.  Fuselage Station 0 is often located at a point well ahead of the nose of the aircraft.

STEP SAND – A sanding process that removes each layer of tape or fabric in turn by an amount required to give the specified overlap for each repair layer and so generates a series of steps around the repair area.  Care is required to ensure that only one layer is removed, and no damage is done to the next layer.   

STEP-GROWTH-POLYMERIZATION – A chemical reaction in which polymers are formed by the stepwise intermolecular addition of molecules through reactive groups.  Any two molecular species present can react.  Monomers disappear early in the reaction and polymer molecular weight rises steadily throughout the reaction.  CMH-17 (WAS MIL-HDBK-17).

STIFFNESS – A measure of modulus.  The relationship of load and deformation.  The ratio between the applied stress and resulting strain.  A term often used when the relationship of stress to strain does not conform to the definition of Young’s modulus.  See also stress-strain.

STITCHED FABRICS – Stitched fabrics use a lightweight fibre as a loop (left-hand figure below) sewn or knitted around the reinforcement tow to create the fabric (photograph of a real reinforcement at centre figure below).   The fabrics may be just a single layer, or multiple layers as either biaxial (0°/90° or ±45° cross-plied) or multi-axial (e.g. 0°/45°/-45°/90°).   The reinforcement tow remains aligned in the plane without crimp (so these reinforcements are commonly referred to as non-crimp fabrics (NCF)).   In consequence, they have the potential for a higher fibre orientation distribution factor than for a woven fabric and each layer can pack as a unidirectional with the possibility of higher fibre volume fractions (see photograph of a cross-section at right-hand figure below).  It is important that full consideration is given to selection of the stitching fibre as poor resin penetration or a poor bond to this thread could be a precursor to laminate failure.

STOPS – Metal pieces inserted between die halves.  Used to control the thickness of a press-moulded part.  Not a recommended practice, because the resin will receive less pressure, which can result in voids.

STORAGE LIFE – The period of time during which a liquid resin, packaged adhesive, or prepreg can be stored under specified temperature conditions and remain suitable for use.  Also called shelf life.

STRAIN – Elastic deformation due to stress.  Measured as the change in length per unit of length in a given direction, and expressed in percentage or mm/mm (in/in).

STRAIN GAUGE – Devise to measure strain in a stressed material based on the change in electrical resistance.

STRAIN RELAXATION – Reduction in internal strain over time.  Similar molecular processes occur as in creep, except that the body is constrained.

STRAND – Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tow, ends, yarn, etc. Sometimes a single fibre or filament is called a strand.

STRAND – Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tows, ends, yarn, etc.  Sometimes a single fibre or filament is called a strand.  In the roving process or shop, a primary group of bundles gathered together in a creel. A strand is that which is pulled out of a doff; also a plurality of drawn and elongated individual filaments combined together to form an individual strand. Strands are held together and protected by sizing.

STRAND COUNT – The number of strands in a plied yarn.  The number of strands in a roving. According to the U.S. Yardage System, the length in hundreds of yards of a single strand having a mass of one pound. In the European TEX System, the mass in grams of a strand 1000 metres in length.

STRAND INTEGRITY – The size’s ability to keep all filaments in a bundle stuck together during chopping. Good strand integrity is required for good flow in or wet-through and wet-out on the mould.

STRENGTH – The maximum stress which a material is capable of sustaining.  CMH-17 (WAS MIL-HDBK-17).

STRENGTH, GREEN – See Green Strength.

STRENGTH, WET – The strength of an adhesive joint or composite determined immediately after removal from a liquid in which it has been immersed under specified conditions of time, temperature, and pressure. NOTE:  The term is commonly used alone to designate strength after immersion in water.

STRESS – The internal force per unit area that resists a change in size or shape of a body.  Expressed in force per unit area.  See ISO 472.

STRESS CONCENTRATION – On a macromechanical level, the magnification of the level of an applied stress in the region of a notch, void, hole, or inclusion.

STRESS CORROSION – Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

STRESS CRACK – External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength, frequently accelerated by the environment to which the plastic is exposed.  The stresses that cause cracking may be present internally or externally or may be combinations of these stresses.  See also crazing.

STRESS CRACKING – The failure of a material by cracking or crazing sometime after it has been placed under load.  Time-to-failure may range from minutes to years.  Causes include moulded-in stresses, post fabrication shrinkage or warpage, and hostile environment.

STRESS RELAXATION – The decrease in stress under sustained, constant strain.  Also called stress decay.

STRESS-STRAIN – Stiffness at a given strain.

STRESS-STRAIN CURVE – Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissae, respectively, to obtain a stress-strain diagram.  See ISO 472.

STRUCTURAL ADHESIVE – Adhesive used for transferring required loads between adherends exposed to service environments typical for the structure involved.  See ASTM D 907.

STRUCTURAL BOND – A bond that joins basic load-bearing parts of an assembly.  The load may be either static or dynamic.  See ASTM D 907.

STRUCTURAL GLASS – (²S² glass).  A magnesia/alumina/silicate glass reinforcement providing high strength.

STRUCTURAL SANDWICH CONSTRUCTION – A laminar construction comprising a combination of alternating dissimilar simple or composite materials assembled and intimately fixed in relation to each other so as to use the properties of each to attain specific structural advantages for the whole assembly.  See ASTM C 274.

STRUCTURALLY ALTERED FABRIC – A base-layer fabric, such as Thermax, CoolMax, and ThermaStat, that absorbs water and wicks moisture because it is woven from fibres having a modified shape.

S-TWIST – Spinning clockwise. Traditionally, this is the direction “singles” are spun. If your singles have been spun S-twist, you would ply Z-twist, and then cable S-twist.

STYLE – Finally, fabrics are often referred to with a style number. This number is a brief way of referring to a specific weight (or denier), count, and weave. For example, the three most common Kevlar styles are: 120: 1.8 oz, 34 x 34, plain , 281: 5.0 oz, 17 x 17, plain, 285: 5.0 oz, 17 x 17, crow-foot satin The most common glass styles include 7533, 7520, 7781, and 7500. Graphite fabrics, although they also have a style, are usually referred to by their weave and tow size (or ply thickness).

SUBSTRATE – A material upon the surface of which as adhesive or resin is spread for any purpose such as bonding or coating.  A broader term than adherend.  See ASTM D 907.

SUPERFORM – A patented double metallic diaphragm process utilizing the unique superplastic deformation properties of the aluminium cauls to form and consolidate thermoplastic composite parts.

SUPERPLASTIC FORMING (SPF) – A strain rate sensitive metal forming process that uses characteristics of materials exhibiting high elongation-to-failure.

SUPPLEX – DuPont’s texturized nylon known for its durability and soft hand; often used in outerwear.

SUPPORTED ADHESIVE FILM – An adhesive supplied in a sheet or in a film form with an incorporated carrier that remains in the bond when the adhesive is supplied and used.  Both mat and woven fabrics are used for this purpose.

SURFACE ACTIVATION – The (usually) chemical process of making a surface more receptive to bonding to a coating or an encapsulating material.

SURFACE DAMAGE – Notches or any other surface irregularity resulting from mishandling or poor release procedures are termed as surface damage.

SURFACE OXIDATION – Surface oxidation can result from lightning strikes, local overheat or battle damage (laser).   The effect on structural integrity will be a degradation of the matrix properties.   Surface oxidation is similar to other surface type damage.

SURFACE PREPARATION – Physical and/or chemical preparation of an adherend to make it suitable for adhesive bonding.  Synonym for prebond treatment.  See ASTM D 907.

SURFACE RESISTIVITY (ELECTRICAL) – The surface resistivity of a material is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of surface.  Surface resistivity is numerically equal to the surface resistance between opposite sides of a square of any size when the current flow is uniform.

SURFACE SWELLING – Blisters caused by the use of undesirable solvents on the outer ply are examples of surface swelling.   A localised break down of the matrix occurs, where a loss of fibre/matrix shear transfer and stiffness will result.

SURFACE TENSION – The contractive force in the surface film of a liquid which tends to make the liquid occupy the least possible volume.  It is due to the tendency of the body of liquid to attract the unbalanced surface molecules towards the interior.  It is expressed in dynes per centimetre and varies for different liquids, being very high for mercury and very low for ether.  It decreases with increasing temperature.  Lyophilic colloids in sol form, such as soap and gelatin solution, lower the surface tension of the medium appreciably, while lyophobic colloids have practically no effect.

SURFACE TREATMENT(A) A material (size or finish) applied to fibrous material during the forming operation or in a subsequent process.  For carbon-fibre surface treatment, the process used to enhance the bonding capability of fibre to resin.  (B) The term is also used for the preparation of surfaces to be bonded together.

SURFACER – Material applied on the tool side surface of a part to fill in resin starvation, porosity or roughness to maintain contour or aerodynamic smoothness.

SURFACING MAT – A thin mat of fine fibres used primarily to produce a smooth surface on an organic matrix composite.  CMH-17 (WAS MIL-HDBK-17). Very thin mat, usually 180 to 510 mm (7 to 20 mil) thick, used primarily to produce a smooth, resin-rich surface on a reinforced plastic laminate, or for precise machining or grinding. See Veil

SURFACTANT – A compound that affects interfacial tensions between two liquids.  It usually reduces surface tension.

SUSPENSION – A dispersion of a solid in a liquid.  See ISO 472.

SYMMETRICAL LAMINATE(A) A composite laminate in which the sequence of plies below the laminate midplane is a mirror image of the stacking sequence above the midplane.  (B) A laminate is said to be symmetric when the plies in the upper half of the laminate (for ordinate z = 0) are identical, in terms of ply properties, ply angle, ply thickness and ply position relative to the mid-plane, to the plies in the lower half of the laminate (for ordinate z = 0).  Symmetric laminates can be constructed from individual plies whose fibres are either singly oriented or multi-oriented.  The plies can be all of one composite material or a hybrid configuration can be constructed where more than one composite material is used.

SYMPATEX – A non-microporous (solid) waterproof/breathable membrane through which perspiration vapour is pushed to the outside by body heat. Available as a direct laminate (applied to the back of an outer shell fabric), a three-ply laminate (a Sympatex layer sandwiched between the outer fabric and lining), or an insert laminate (a free-hanging layer between a separate outer shell and lining). Made by Akzo, Sympatex, which competes with Gore-Tex, is used in outerwear.

SYNTACTIC FOAMS – Compounds made by mixing hollow microspheres of glass, epoxy, phenolic, etc., into fluid resins (with additives and curing agents) to form a mouldable, curable lightweight, fluid mass; as opposed to foamed plastic, in which the cells are formed by gas bubbles released in the liquid plastic by either chemical or mechanical action.  See ISO 472.

SYNTHETIC RESIN – A complex, substantially amorphous, organic semi-solid or solid material (usually a mixture) built up by chemical reaction of comparatively simple compounds, approximating the natural resins in luster, fracture, comparative brittleness, insolubility in water, fusibility or plasticity, and some degree of rubber-like extensibility, but commonly deviating widely from natural resins in chemical constitution and behavior with reagents.

SYNTHETIC RUBBER – An elastomer manufactured by a chemical process as distinguished from natural rubber obtained from trees.


TABS – Extra lengths of composite or other material at the ends of a tensile specimen to promote failure away from the grips.

TACK(A) Stickiness of an adhesive or filament reinforced resin prepreg material.  (B) The property of an adhesive that enables it to form a bond of measurable strength immediately after the adhesive and the adherend are brought into contact under low pressure.  (C) To locally join thermoplastic plies during lay-up by spot welding in order to maintain orientation and hold the laminate together for handling and transfer operations before consolidation.

TACK-FREE – A condition in which a plastic material can be dented with an inert object without sticking to it.  It is indicative of a definite stage of hardening.

TACKING – To locally join thermoplastic plies by spot welding in order to maintain orientation and hold the laminate together for handling and transfer operations.

TAFFETA – Any fabric with a plain weave that gives it a fine, smooth look.

TAP TEST – Using a coin to tap a laminate in different spots, listening for a change in sound, which would indicate the presence of a defect.  A surprisingly accurate test in the hands of experienced personnel.  Also called Coin Test.

TAPE – Unidirectional prepreg fabricated in various widths.  Dry cloth tapes available in various fibre fabrics are usually less than 100 mm (4 in) in width.  Dry cloth tapes also have woven selvages which are usually left in the work, i.e., NOT removed.  Unidirectional material is called tape regardless of width. A narrow fabric whose mass per unit area is less than 0.5 kg/m 2 (0.1 LB/ft2) for each 25.4 mm (1 in.) of width; used primarily for utilitarian purposes.

TAPE LAYING – A fabrication process in which prepreg tape is laid side by side or overlapped to form a structure.

TAPE WRAPPED – Fabric tape is heated and wrapped onto a rotating mandrel and subsequently cooled to form the surface for the next tape layer application.

TAPED SEAMS – Seams subjected to a seam-sealing process to prevent leakage through needle holes in waterproof fabrics. The manufacturer applies seam tape during fabrication.

TAPER PLIES – Plies which taper off in specific increments or a blend of plies used as reinforcements.

TAPER SAND – See also “Scarf Joint”.  A sanding process to produce a uniform taper in a skin for the production of a scarf joint.  The taper must be accurate and uniform and must produce the taper angle specified for the repair.  Typically these are 1 in 30 to 1 in 80 for skin joints at the centre of a panel and 1 in 20 around panel edges where the skin is thickened to receive countersunk bolt or rivet holes.  The angle specified for each particular repair must be used.

TASLAN – An air jet-textured nylon yarn used for fabrics recognizable by their cottony feel, light weight, durability, and abrasion resistance; similar to Supplex, but coarser.

TCE – Thermal Coefficient of Expansion.

TEDLAR – Du Pont Trade Name for Polyvinylfluoride film (PVF).  Used as a waterproof film on some composites and also for interior decor.  Low smoke and flame. Easy to clean.

TELEGRAPHING – The dimpling of the prepreg plies into the core cells.

TEMPERATURE RATE RISE – Temperature change in an individual thermocouple divided by the elapsed time between the two measurements.

TEMPLATE – A pattern used as a guide for cutting and laying plies.

TENACITY – The term generally used in yarn manufacture and textile engineering to denote the strength of a yarn or of a filament of a given size.  Numerically, it is the grams of breaking force per denier unit of yarn or filament size.  Grams per denier is expressed as gpd.

TENCEL – A new fibre created from the wood pulp of specially selected trees, processed in a non-chemical, environmentally-safe way. Tencel was introduced to the world of apparel in 1992 and is the first new fibre introduction in over thirty years. The characteristics of the fibre are a subtle lustre, high-wash stability, extremely low shrinkage, and good tear resistance.

TENSILE BAR – A compression or injection moulded specimen of specified dimensions used to determine the tensile properties of a material.

TENSILE MODULUS – See Young’s modulus.

TENSILE STRENGTH – The maximum load or force per unit cross-sectional area, within the gauge length, of the specimen.  The pulling stress required to break a given specimen.

TENSILE STRESS – The normal stress caused by forces directed away from the plane on which they act.

TENSILE TEST – A test in which specimens are subjected to an increasing pull until they fracture.

TENSION TEST (COMPOSITES) – See ASTM D 3039.  Tensile properties of fibre-resin composites.

TENTERING – Tentering is used to straighten the fabric and dry it. The fabric is stretched taut while steaming or drying and held in place with clips or pins, called tenterhooks. (Giving rise to that slang expression of suspense, “on tenterhooks.”)

TEPA – Tetraethylenepentamine.  A curing agent for room temperature curing epoxies.

TETA – Triethylenetetramine.  A curing agent for room temperature curing epoxies.

TEX – A unit for expressing linear density equal to the mass or weight in grams of 1000 metres of filament, fibre, yarn, or other textile strand.

TEXTILE FIBRES – Fibres or filaments that can be processed into yarn or made into a fabric by interlacing in a variety of methods, including weaving, knitting, and braiding.

TEXTURE – The surface effect of cloth or fibre as dull, lustrous, woolly, stiff, soft, fine, coarse, open or closely woven. Also known as “hand” or “feel”.

TEXTURIZED GLASS YARN – Yarn processed from continuous filaments that have been disoriented, adding bulk.

TG – See Glass Transition Temperature.

TGA – See Thermogravimetric Analysis.

THEORETICAL END COUNT – The maximum number of bundles in a roving doff; for example, a roving doff made with 18 forming cakes in the creel that were “split out” 4 ways in forming will have 64 theoretical ends.

THERMAL CONDUCTIVITY – The quantity of heat conducted per unit time through unit area of a slab of unit thickness having unit temperature difference between its faces.  The ability of a material to conduct heat.

THERMAL ENDURANCE – The time at a selected temperature for a material or system of materials to deteriorate to some predetermined level of electrical, mechanical, or chemical performance under prescribed conditions of test.

THERMAL EXPANSION (COEFFICIENT OF) – The fractional change in length of a material for a unit change in temperature.  Values for plastics range from 0.01 to 0.2 in/in/°C.  Some fibre reinforcements have a negative longitudinal thermal coefficient of expansion.

THERMAL EXPANSION MOULDING – A process in which elastomeric tooling details are constrained within a rigid frame to generate consolidation pressure by thermal expansion during the curing cycle of the autoclave moulding process.

THERMAL RESIDUAL STRESSES – Following a high temperature cure of a laminate the cool-down phase will induce thermal strains between fibre and matrix and between plies of different orientation.  If a laminate is not symmetrically about the mid-plane then theses thermal strains will cause the panel to warp.  If a warped panel is held flat the thermal strains become thermal residual stresses. 

THERMAL STRESS(A) Stress caused by the different expansion in different directions of plies in a laminate at difference orientations.  (B) In metals or other materials the stress that is developed if materials having different coefficients of thermal expansion are bolted or otherwise joined together and heated.  (C) The stress developed if a block of material is restrained from expanding while heated or from contracting when cooled.  The stress is E x alpha x delta theta. E = Young’s Modulus, Alpha = Thermal expansion coefficient, Delta Theta = temperature change.

THERMAL STRESS CRACKING – Crazing and cracking of some thermoplastic resins, resulting from overexposure to elevated temperature.  See also stress cracking.

THERMOCOUPLE – A device which used a circuit of two wires of dissimilar metals or alloys, the two junctions of which are a different temperatures.  A net electromotive force (emf) occurs as a result of this temperature difference.  The minute electromotive force, or current, is sufficient to drive a galvanometer or potentiometer.  The most common thermocouples used in composite fabrication are “J-type” (Iron – Constantan).

THERMOELASTICITY – Rubber-like elasticity resulting from an increase in temperature.  See ISO 472.

THERMOFORMING – Forming a thermoplastic material after heating it to the point where it is soft enough to be formed without cracking or breaking reinforcing fibres.

THERMOGRAVIMETRIC ANALYSIS (TGA) – The study of the mass of a material under various conditions of temperature and pressure.  Usually the measurement of weight loss over a period of time at a constant temperature.

THERMOLOFT – DuPont’s medium-loft synthetic insulation that combines solid-core polyester fibres with hollow Quallofil fibres; used most often in insulated outerwear, where high-loft fills are too bulky and low-loft fills aren’t warm enough.

THERMOPLASTIC – Capable of being repeatedly softened by an increase of temperature and hardened by a decrease in temperature.  Applicable to those materials whose change upon heating is substantially physical rather than chemical, and that in the softened stage can be shaped by flow into articles by moulding or extrusion – See ISO 472 and ASTM D 907.

THERMOSET – A plastic that when cured by application of heat or chemical means, changes into a substantially infusible and insoluble material.  Once cured, a thermoset cannot be returned to the uncured state.  See ISO 472.

THERMOSETTING POLYESTERS – A class of resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl-type active monomer such as styrene, methyl styrene, or diallyl phthalate.  Cure is effected through vinyl polymerization using peroxide catalysts and promoters or heat to accelerate the reaction.  The two important commercial types are:  (1) liquid resins that are cross-linked with styrene and used either as impregnates for glass  reinforcements in laminates, filament would structures, and other built-up constructions, or as binders for chopped-fibre reinforcements in moulding compounds, such as sheet moulding compound (SMC), bulk moulding compound (BMC), and thick moulding compound (TMC); and (2) liquid or solid resins cross-linked with other esters in chopped fibre and mineral-filled moulding compounds, for example, alkyd and diallyl phthalate.

THERMOSETTING RESIN – A resin which undergoes cross-linking and becomes infusible when heated.

THICKNESS – is primarily a function of weight and fibre type, but also depends on the weave. For example, Aircraft Spruce lists two glass fabrics of similar weight (5.79 oz and 5.85 oz); the heavier cloth has a thickness of 0.009 in, the lighter cloth a slightly larger thickness of 0.010 in.

THIGH-SPUN – A yarn produced by aboriginal people.

THINNER – A volatile liquid added to an adhesive to modify the consistency or other properties.  (See also diluent and extender).  See ASTM D 907.

THINSULATE – 3M’s 35 percent polyester/65 percent olefin insulation spun into a low-loft construction. It is an efficient insulator for its minimal thickness, and is most often used in outerwear, footwear, and gloves because of its low bulk.

THINSULATE LITE LOFT – A fine-denier version of Thinsulate that provides a high loft-to-weight ratio; used in outerwear and sleeping bags.

THINSULATE ULTRA INSULATIONS – Three soft and highly compressible fabrics. Ultra Extreme Performance fabric is a medium-loft insulation used for gear made for harsh conditions. Ultra Active Performance fabric is a slimmer insulation used in milder-weather gear, and Ultra Performance fabric is a thin, durable footwear insulation.

THIXOTROPIC (THIXOTROPHY) – Concerning materials that are gel-like at rest, but fluid when agitated.  Having high static shear strength and low dynamic shear strength at the same time.  To lose viscosity under stress.  See ASTM D 907.

THREAD – A fine cord of fibrous material spun out to considerable length, especially when composed of two or more plies twisted together.

THREAD COUNT – The number of yarns (threads) per inch or centimetre in either the lengthwise (warp) or crosswise (fill or weft) direction of woven fabrics.

THREE BAR KNIT – A synthetic wicking material often used to cover boots. Inner layers of insulation or waterproof/breathable fabrics.

TOLERANCE – The guaranteed maximum deviation from the specified nominal value of a component characteristic at standard or stated environmental conditions.

TOLERANCE LIMIT – A lower (upper) confidence limit on a specified percentile of a distribution.  For example, the B-basis value is a 95% lower confidence limit on the tenth percentile of a distribution.  CMH-17 (WAS MIL-HDBK-17).

TOOL SIDE – The side of the part that is cured against the tool (mould or mandrel).

TOOLING – The moulds and fixtures used to produce a composite article.  Tooling may be made of any suitable material, including composites.

TOOLING CLOTH – A composite broad goods fabric (fibreglass, Kevlar® or carbon/graphite) that is thicker than the material used to fabricate parts and less costly due to the fewer quality control checks required as compared to aircraft grade fabric.

TOOLING RESINS – Resins that have applications as tooling aids, core boxes, prototypes, hammer forms, stretch forms, foundry patterns, etc.  Epoxy and silicone are common examples.

TORSION – Twisting stress.

TORSIONAL MODULUS – The ratio of the torsion stress to the strain in the material over the range for which this value is constant.

TORSIONAL STRESS – The shear stress on a transverse cross section caused by a twisting action.

TOUGHNESS – A property of a material for absorbing work.  The actual work per unit volume or unit mass of material that is required to rupture it.  Toughness is proportional to the area under the load-elongation curve from the origin to the breaking point.  See ISO 472.

TOW – An untwisted bundle of continuous filaments.  Commonly used in referring to man-made fibres, particularly carbon and graphite, but also glass and aramid.  A tow designated as 140K has 140,000 filaments.  Equivalent meaning to “roving” in glass. The shorter flax fibres removed by hackling.

TOW LINEN – These are the shorter, less desirable flax fibres separated from bast line fibres in hackling. Tow linen is usually carded and spun into a woollen-style yarn. A wonderful use for tow linen is to knit bath mitts — a situation where you want all of the rough, scratch nature of this yarn.

TOXICITY – A term referring to the physiological effect of absorbing a poisonous substance into the human system by ingestion or through the skin, mucous membranes or respiratory system.  See the Material Safety Data Sheet (MSDS) for each material before use and use all recommended precautions.

T-PEEL STRENGTH – The average load per unit width of bond line required to produce progressive separation of two bonded, flexible adherends, under conditions designated in ASTM D 1876.

T-PEEL TEST – See ASTM D 1876. This test requires flexible adherends whether they be metal, plastic, rubber, or fabric.  The materials must bend through 90° and are therefore usually thin.  The top of the “T” consists of the two loaded arms and the vertical of the ²T² the remaining bonded length.  Usually used with elastomeric or rubbery adhesives.  Only fairly tough adhesives can be tested by this method.

TPI – Twists per Inch (or Turns per Inch).

TRACER – A fibre, tow, or yarn added to a prepreg for verifying fibre alignment and, in the case of woven materials, for distinguishing warp fibres from fill fibres. A visually different or distinctive fibre, tow, or yarn added to a prepreg to verify fibre alignment or to distinguish warp fibres from fill fibres.

TRANSDUCER – A force measuring device.  It has the characteristics of providing an output, usually electrical, which serves as the measurement of load, force, compression, pressure, etc., when placed along the sensitive axis of the core cell.

TRANSFER MOULDING – Method of moulding thermosetting materials in which the plastic is first softened by heat and pressure in a transfer chamber and then forced by high pressure through suitable sprues, runners, and gates into the closed mould for final shaping and curing.

TRANSITION TEMPERATURE – The temperature at which the properties of a material change.  Depending on the material, the transition change may or may not be reversible.

TRANSITION, FIRST ORDER – A change of state associated with crystallization or melting in a polymer.  CMH-17 (WAS MIL-HDBK-17).

TRANSLAMINAR CRACKS – Through-the-thickness cracks where fibres are broken are translaminar cracks.   Translaminar cracks are a direct result of extreme overload or impact damage, and have the same effect as a hole stress concentrator.

TRANSVERSE(A) Perpendicular to longitudinal.  (B) In a unidirectional composite or prepreg tape, in the material plane and perpendicular to the fibres.

TRANSVERSE LOADS – Loads which are not perpendicular to the facings; synonymous with flatwise loads.  ASTM C 274.

TRANSVERSE STRAIN – The linear strain in a plane perpendicular to the loading axis of a specimen.

TRAPPED END – A loop embedded into a doff during the roving process that gets stuck during run-out with such tenacity that it prevents the entire strand from running freely to the chopper.

TRIAXIAL FABRIC – Fabric with three non-interwoven layers – oriented at +45°, – 45° and either 0° or 90° – which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also biaxial fabric, quadraxial fabric.)

TRIAXIAL WEAVES – Whilst the vast majority of woven reinforcements are in the form of two-dimensional planar fabrics with orthogonal fibres, it is also possible to produce triaxial weaves (fibres at 0, ±60°).  There is increasing interest in 3-D textile reinforcements which find application primarily as preforms for Liquid Composite Moulding processes.  A review of three-dimensional reinforcements in which they make a clear distinction between 3D interlock weave (either layer-by-layer or angle-interlock, with binder fibres crossing more than one tow) and 3D orthogonal non-crimp weave (with binder fibres running at 90° to the two other sets of tows).

TURNAROUND – The portion of the roving doff where the roving changes direction when it is pulled out of the doff.

TURNS PER INCH (TPI) – A measure of the amount of twist produced in a yarn, tow, or roving during its processing history.  Also, the lead rate of a hoop layer at a specified bandwidth.  See also twist.

TWEED – A fabric made from woollen-spun yarn in a variety of colour and weave effects.

TWILL – A fabric woven by alternatively passing weft threads over one or more and under two or more warp threads to produce diagonal lines or ribs.

TWILL WEAVETwill weave is a type of textile weave with a pattern of diagonal parallel ribs (in contrast with a satin and plain weave).

TWIST – Measure of the number of turns per unit length that a fibre bundle makes around its axis. “Z”-twist denotes a right-handed twist, while “S”-twist denotes a left-handed twist. “U” is often used to represent no twist and “N” means never twisted. The spiral turns about its axis per unit of length in a yarn or other textile strand.  Twist may be expressed as turns per inch (TPI).  S and Z refer to direction of twist, in reference to whether the twist direction conforms to the middle-section slope of the particular letter.  In pultruded parts, this term describes a condition of longitudinal, progressive rotation that can be easily detected for a noncircular cross section by placing the pultrusion on a plane surface, holding one end flat with the surface, and observing whether one edge or side of the other end does not lie parallel with that surface. In spinning, this refers to the turns inserted into a yarn to bind its fibres together and thereby add more strength. It is usually indicated as turns per inch or TPS. Greater twist would be caused by additional revolutions of the flyer on a set amount of fibre.

TWIST AND PLY FRAMES – Machines used to twist and ply glass yarns.

TWIST, DIRECTION OF – The direction of twist in yarns and other textile strands is indicated by the capital letters S and Z.  Yarn has S twist if, when held in a vertical position, the visible spirals or helices around its central axis are in the direction of slope of the central portion of the letter S, and Z twist is in the other direction.  CMH-17 (WAS MIL-HDBK-17).

TWO STAGE BONDING – Bonds that are cured in two primary stages, with manufacturing operations accomplished in between; namely trailing edge structure that bonds the lower skin to the honeycomb core and spar in the first stage, and the upper skin to the assembly after it has been machined to contour.

TWO-COMPONENT ADHESIVE – An adhesive supplied in two parts that are mixed before application.  Such adhesives usually cure at room temperature.

TYPICAL BASIS – A typical property value is a sample mean.  Note that the typical value is defined as the simple arithmetic mean has a statistical connotation of 50% reliability, with a 50% confidence.  CMH-17 (WAS MIL-HDBK-17).


UHM – Ultra high modulus

ULTIMATE ELONGATION – The elongation at rupture.

ULTIMATE LOAD – Limit Load x Factor of Safety – In aircraft, the ultimate load factor of safety is usually 1.5.  The requirements also specify that these ultimate loads be carried by the structure without failure.

ULTIMATE TENSILE STRENGTH – The ultimate or final (highest) stress sustained by a specimen in a tension test.  Rupture and ultimate stress may or may not be the same.

ULTRASONIC TESTING – A nondestructive test applied to materials for the purpose of locating internal flaws or structural discontinuities by the use of high-frequency reflection or attenuation (ultrasonic beam).

ULTRASONIC WELDING – A method of joining thermoplastic composites utilizing vibrations at a fixed frequency into a properly designed set of parts.  The rapid agitation of the joint area under pressure creates frictional heat, melting the plastic in a fraction of a second.

ULTRAVIOLET (UV) – Zone of invisible radiation beyond the violet end of the spectrum of visible radiation.  Since UV wavelengths are shorter than visible wavelengths, their photons have more energy, enough to initiate some chemical reactions and to degrade most plastics, particularly aramids.

ULTRAVIOLET (UV) STABILIZER – Any chemical compound that, when admixed with a resin, selectively absorbs UV rays.

UNBOND – An area within a bonded interface between two adherends in which the intended bonding action failed to take place, or where two layers of prepreg in a cured component do not adhere to each other.  Also used to denote specific areas deliberately prevented from bonding in order to simulate a defective bond, such as in the generation of quality standards specimens.

UNDERCURE – A condition of the moulded article resulting from the allowance of too little time and/or temperature or pressure for adequate hardening of the moulding.

UNDERCUT – A protuberance or indentation that impedes the withdrawal of a moulded part from a two-piece, rigid mould.  Any such protuberance or indentation, depending on the design of the mould.

UNFINISHED WORSTEDS – A woven fabric made from worsted yarn that has then been brushed. This produces a firm fabric with a tight weave hidden beneath a soft nap.

UNIAXIAL LOAD – A condition whereby a material is stressed in only one direction along the axis or centreline of component parts.

UNIDIRECTIONAL LAMINATE – A reinforced plastic laminate in which substantially all of the fibres are oriented in the same direction.

UNSATURATED COMPOUNDS – Any compound having more than one bond between two adjacent atoms, usually carbon atoms, and capable of adding other atoms at that point to reduce it to a single bond.

UNSYMMETRIC LAMINATE – A laminate type which is neither symmetric nor antisymmetric is classified as an unsymmetrical laminate.  Unsymmetrical laminates are made from multi-oriented plies.  An unsymmetrical laminate can be made from isotropic plies, using at least two different isotropic materials.  Orthotropic plies in the form of specially and/or generally orthotropic plies can be used to form an unsymmetrical laminate, the plies do not have to be of the same thickness.

UP TWIST – Using the same direction as the preceding spinning. So if you spun your singles “S” and then plied them “S” you would have used up twist. You would also have a yarn that could be used in collapse fabrics.

UREA-FORMALDEHYDE RESIN – A synthetic resin derived from the condensation of urea with formaldehyde or its polymers.

URETHANE PLASTICS – Plastics based on resins made by condensation of organic isocyanates with compounds or resins that contain hydroxyl groups.  The resin is furnished as two component liquid monomers or prepolymers that are mixed in the field immediately before application.  A great variety of materials are available, depending upon the monomers used in the prepolymers, polyols, and the type of disocyanate employed.  Extremely abrasion and impact resistant.  See also isocyanate plastics and polyurethane.  See ISO 472.

USABLE LIFE – The maximum number of hours allowed to elapse between the time an adhesive is sampled for quality assurance evaluation testing and the time the adhesive is cured.

UV – See ultraviolet.


VACUUM BAG – The plastic or rubber layer used to cover the part so that a vacuum can be drawn.

VACUUM BAG MOULDING – A process in which a sheet of flexible transparent material plus bleeder cloth and release film are placed under the layup on the mould and sealed at the edges.  A vacuum is applied between the sheet and the lay-up.  The entrapped air is mechanically worked out of the lay-up and removed by the vacuum, and the part is cured with temperature, pressure, and time.  Also called bag moulding.

VACUUM BAGGING – A process in which the lay-up is cured under pressure generated by drawing a vacuum in the space between the lay-up and a flexible sheet placed over it and sealed at the edges.

VACUUM BAG SEALERS – The sealing tape or putty used on the periphery of the tool to seal the bag to the tool.

VACUUM HOT PRESSING (VHP) – A method of processing materials (especially powders) at elevated temperatures and consolidation pressures, and low atmospheric pressures.

VACUUM MOULDING – Low cost entry method into RTM which uses a rigid cavity mould half and a semi-rigid upper mould half, both made of FRP. Capable of producing 3-4 times as many mouldings as that produced through open mould, with acceptable repeatability (but not equal to that of RTM.)

VACUUM PRESSURE – The pressure under the vacuum bag.  A suitable gauge should be used on the opposite side of the work to the vacuum outlet.

VACUUM-ASSISTED RESIN TRANSFER MOULDING (VARTM) – An infusion process by which a vacuum draws resin into a one-sided mould; a cover, either rigid or flexible, is placed over the laminate and taped or otherwise fixed to form a vacuum-tight seal. (See previous entry.)

VAPOUR – The gaseous form of substances that are normally in the solid or liquid state, and that can be changed to these states either by increasing the pressure or decreasing the temperature.

VAPOUR DEGREASING – A cleaning process that used the hot vapours of a chlorinated solvent to remove soils, especially oil, grease, waxes, fingerprints, etc.

VARIATION IN DENSITY – Variation in the density of the laminate is associated with resin inconsistencies, voids or porosity.   Their effect on the laminate is similar to that stated in Prepreg Variability.

VBL – Vapour barrier layer or liner. Generic name for a thin, plastic like layer, most often a sleeping bag or sock liner that traps Vapour, and thus heat, in extremely cold conditions.

VEIL – An ultra thin mat similar to a surface mat, often composed of organic fibres, as well as glass fibres.

VENT – A small hole or shallow channel in a mould that allows air or gas to exit as the moulding material enters.

VENT CLOTH – A layer or layers of open-weave cloth used to provide a path for vacuum to “reach” the area over a laminate being cured, such that volatiles and air can be removed.  Also causes the pressure differential that results in application of pressure to the part being cured.  Also called breather cloth.

VENTING – In autoclave curing of a part or assembly, venting refers to turning off the vacuum source and venting the pressure difference between the pressure in the clave and atmospheric pressure. Venting is usually used to prevent the resin boiling that can occur when a resin is heated and simultaneously subjected to reduced pressure (vacuum).

VERSATECH – A microfibre fabric from Burlington used for sleeping bag covers and outerwear. Its tight weave resists wind and water, yet retains breathability.

VERTICAL BLEED – Removal of volatiles and excess resin through a perforated release sheet into a bleeder cloth over the whole are of a part.

VERTICAL BOND VOIDS – Absence of a bond or bonding material between the vertical edges of two core sections or core and inserts.

VINYL ESTERS – A class of thermosetting resins containing esters of acrylic and/or methacrylic acids, many of which have been made from epoxy resin.  Cure is accomplished as with unsaturated polyesters by copolymerization with other vinyl monomers, such as styrene. Thermosetting resins containing esters of acrylic and/or methacrylic acids, many of which have been made from epoxy resin. Cure is accomplished as with unsaturated polyesters by CO-polymerization with other vinyl monomers, such as styrene.

VINYL-COATED GLASSYarn-Continuous glass filament yarn coated with pigment and plasticized vinyl chloride resin.

VISCOELASTICITY – A property involving a combination of elastic and viscous behavior in the application of which a material is considered to combine the features of a perfectly elastic solid and a perfect fluid.  Phenomenon of time-dependent, in addition to elastic, deformation (or recovery) in response to load.

VISCOSITY – The property of resistance to flow exhibited within the body of a material, expressed in terms of relationship between applied shearing stress and resulting rate of strain in shear.  Viscosity is usually taken to mean Newtonian viscosity, in which case the ratio of shearing stress to the rate of shearing strain is constant.  In non-Newtonian behavior, which is the usual case with plastic, the ratio varies with the shearing stress.  Such ratios are often called the apparent viscosities at the corresponding shearing stresses.  Viscosity is measured in terms of flow in Pa.s (P), with water as the base standard (value of 1.0).  The higher the number, the less flow.  See ASTM D 907. Measure of a liquid’s resistance to flow.

VM – See Vacuum Moulding.

VOID – Any pocket of enclosed gas or air within a composite.

VOID CONTENT – Volume percentage of voids, usually less than 1% in a properly cured composite.  The experimental determination is indirect, that is, calculated from the measured density of a cured laminate and the “theoretical” density of the starting material.

VOIDS – Air or gas that has been trapped and cured into a laminate.  Porosity is an aggregation of microvoids.  Voids are essentially incapable of transmitting structural stresses or non-radiation energy fields.  Associated with bridging and resin starved areas.

VOLATILE CONTENT – The percentage of volatiles that are present in a plastic or an impregnated reinforcement.  Drying a specimen under specified conditions to determine the amount driven off is a common prepreg physical property test.

VOLATILE LOSS – Weight loss by evaporation.

VOLATILE ORGANIC COMPOUNDS (VOCs) – Chemical substances, such as solvents, that readily evaporate or volatilize into the air. Many VOCs also are considered hazardous air pollutants (HAPs) because of potential health

VOLATILES – Refers to gaseous materials leaving a laminate that is being cured, and which were liquids or solids before the cure cycle started. Volatiles produced usually include residual solvents and absorbed or adsorbed water. Many materials also produce volatiles as by-products of the curing reactions. Materials, such as water and alcohol, in a sizing or a resin formulation, that are capable of being driven off as a vapour at room temperature or at a slightly elevated temperature.

VULCANIZATION – A chemical reaction in which a rubber is cured by reaction with sulfur or other suitable agents.  See ASTM D 907.


WARP – The yarn running lengthwise in a woven fabric.  A group of yarns in long lengths and approximately parallel.  Fabrics are tensioned in the warp direction during weaving.  The weft is not tensioned.  A change in dimension of a cured laminate from its original moulded shape. The yarns that run the length of the loom. The warp yarns are pulled through the loom as the weft or filling yarns are woven across the warp to make the fabric.

WARP CLOCK – A composite fabrication and engineering symbol used as reference for aligning the warp yarns or tows in the desired direction.

WARP DIRECTION – The direction of the warp yarns or tows in a fabric or tape.  In a woven cloth, the warp direction is parallel to the selvage edge.

WARP SIZE – Chemicals applied to the warp yarn to improve strand integrity, strength and smoothness in order to withstand rigors of weaving.

WARP SURFACE – The surface of a fabric that has a majority of warp fibres woven above the fill fibres.  Warp surfaces exist in harness satin weaves and crowfoot weaves, but not in plain, basket, or twill weaves.  The opposite side of the cloth is known as the “Fill Surface”.

WARP YARNS – are usually called ends and fill yarns picks, but sometimes the fill yarns are also called ends. (The terms apply equally to rovings, but yarn will be used in the rest of the discussion for simplicity.) If you look at a roll of fabric, some of the yarns run in the direction of the roll and are continuous for the entire length of the roll. These are the warp yarns.

WARPING BOARD OR FRAME – A rectangular frame with strong pegs inserted into its sides used to wind a warp.

WARPING REEL – A rotating frame that can be mounted horizontally or vertically. It’s used to wind a warp and help keep the threads in order.

WASTAGE – -In weaving, this refers to the part of the warp that cannot be used, often about 24-36 inches of the total warp. In spinning, this refers to the loss between the weight of fibre acquired and the weight of fibre that can be used. In spinning, the amount of dirt, tag ends, and unusable fibre are all part of the wastage.

WATER ABSORPTION – Ratio of the weight of water absorbed by a material to the weight of the dry material.

WATER BREAK TEST – In this test, water is applied to the prepared surface and should remain in a continuous film over the whole area for at least 30 seconds.  If the water separates out into droplets, or the film is not continuous, then the cleaning operation has to be repeated until this requirement is met.  See ASTM D 3932.

WATER JET – Water emitted from a nozzle under high pressure (70 to 410 MPa, or 10 to 60 ksi or higher).  Useful for cutting organic composites.

WEATHERING – Exposure of plastics to the outdoor environment.

WEAVE – The particular manner in which a fabric is formed by interlacing yarns.  Usually assigned a style number.  Particular manner in which a fabric is formed by interlacing yarns; usually assigned a style number. The last important fabric term is the weave. The weave describes how the warp and fill yarns are interlaced. The most popular weaves are plain, twill, harness satin, and crow-foot satin. Weave determines drapeability and isotropy of strength (some weaves are biased to the warp or fill direction). Because it’s difficult to describe a weave without showing a picture of it, I’ll be discussing weaves in more detail in a future column.

WEB BEAM – Another term for the cloth beam.

WEDGE TEST – Originally known as the Boeing wedge test where it was developed.  For details see ASTM D 3762 and Reference 2.15.  Two thin adherend beams are bonded together with a short unbonded length at one end into which a wedge is driven.  It was developed as a simple method of evaluating metal surface pretreatment particularly with reference to predicting the durability in service of bonded aerospace structures.  Composite adherends may be used.  The initial crack length is measured between the shoulder of the wedge and the crack tip.  The specimen is then exposed to a chosen environment and the crack growth followed with time.  A formula is given in Reference 2.15 from which the adhesive or resin fracture energy can be calculated from the crack length, specimen dimensions, and material properties.  Because fracture energy declines rapidly with increasing crack length, it is important to plot fracture energy versus time and not just crack growth.  Bondline thickness will affect the results obtained.  It is a simplified version of the double cantilever beam test.

WEEPING – Slow leakage manifested by the appearance of a fluid on a surface.

WEFT – The transverse threads or fibres in a woven fabric.  Those fibres running perpendicular to the warp.  Also called fill, filling yarn, or woof. The set of threads which crosses the warp at right angles. Depending on the use, there may be more than one weft used: ground weft, pattern weft)

WEFT-FACE FABRICS – Any fabric in which the warp is completely covered with weft.

WEFT-INSERTION WARP-KNIT OR WARP-INSERTION WEFT-KNIT (WIWK) – Bellara [11] states that “yarn in a knitted fabric follows a meandering path forming symmetric loops”.  This produces a fabric with more elasticity (more easily stretched) than woven counterparts.  The fabric is considered to consist of courses (the horizontal row of loops) and wales (the vertical lines).  In warp knitting each stitch within a row has a separate thread, while in weft knitting there is one yarn per row.  Warp knitting is harder to unravel, while weft knitting produces more elastic fabrics.

WEIGHTING – A process by which the weight of a fabric is increased by impregnating it with mineral salts, starch, or other materials. This process was often used on yardage that was sold by the pound (e.g. silk).

WELD LINE – The mark visible on a finished part made by the meeting of two flow fronts of plastic material during moulding.  Also called weld mark or flow line.

WET FLEXURAL STRENGTH (WFS) – The flexural strength after water immersion, usually after boiling the specimen for 2 hours in water.

WET INSTALLATION – A bolted joint in which sealant is applied to the head and shank of the fastener so that after assembly, a seal is provided between the fastener and the elements being joined.

WET LAY-UP – A method of making or repairing a reinforced product by applying the resin system as a liquid when the reinforcement is put in place.

WET STRENGTH – The strength of an adhesive joint determined immediately after removal from a liquid in which it has been immersed under specified conditions of time, temperature, and pressure.

WET WINDING – In filament winding, the process of winding glass on a mandrel in which the strand in impregnated with resin just before contact with the mandrel.  See also dry winding.

WET-OUT – The condition of an impregnated roving or yarn in which substantially all voids between the sized strands and filaments are filled with resin.

WET-OUT RATE – The time required for a resin to fill the interstices of a reinforcement material and wet the surface of the reinforcement fibres, usually determined by optical or light-transmission means.

WETTING – The spreading, and sometimes absorption, of a fluid on or into a surface.

WHISKER – A short single crystal fibre or filament used as reinforcement in a matrix.  Whisker diameters range from 1 to 25 mm (40 to 980 min), with aspect ratios between 100 and 15,000.  See ISO 472.

WICKING – The flow of a liquid along a surface into a narrow space.  This capillary action is caused by the attraction of the liquid molecules to each other and to the surface. A fabric that pulls moisture away from the skin and disperses it throughout the material; this is a critical attribute in high-performance base layers.

WIDES – Term describing bundles of roving that are wider than most of the other bundles in a bed of chopped glass fibres. They usually contain 3 to 4 times more filaments than most of the other bundles in the roving. (See Matchstick).

WIND ANGLE – The angular measure in degrees between the direction parallel to the filaments and an established reference.  In filament-wound structures, it is the convention to measure the wind angle with reference to the centreline through the polar bosses, that is, the axis of rotation.

WINDING PATTERN – The total number of individual circuits required for a winding path to begin repeating by laying down immediately adjacent to the initial circuit.  A regularly recurring pattern of the filament path after a certain number of mandrel revolutions, leading eventually to the complete coverage of the mandrel.

WINDING TENSION – In filament winding or tape wrapping, the amount of tension on the reinforcement as it makes contact with the mandrel.

WOOF – See weft. An old term for weft. Same as fill.

WOOL – The fleece of sheep, goats and other like animals, which retains some of its insulating qualities when damp; requires line drying to prevent heat shrinkage in a dryer.

WORK HARDENING – Increase in resistance to further deformation with continuing distortion.  Hardening and strengthening of a metal or alloy caused by the strain energy absorbed from prior deformation.

WORKING LIFE – The period of time during which a liquid resin or adhesive, after mixing with catalyst, solvent, or other compounding ingredients, remains usable.  See also gelation time and pot life.

WORSTED – Worsted refers to two different processes which are combined to produce a smooth, clean yarn. Originally it referred to a woollen yarn manufactured in Worstead, Norfolk, England. It now refers to yarn (and fabric) made of long fibres, combed, and tightly twisted in spinning. Fabrics made from worsted yarns are smooth and cool to wear. Fibre Preparation: Yarns spun from wool where the wool fibres are markedly ‘parallelized’ as distinct from woollen yarns in which anything but a parallel position is noticeable. In commercial yarns, almost without exception, worsted yarns are combed yarns. One of my reference texts states: “…but it is quite conceivable that wool fibres might be so parallelized by careful drawing and spinning that practically a yarn of worsted characteristics might be produced without combing.” Traditionally, worsted yarns were from fibres 3+ inches in length, but this is no longer true as now many shorter wools are also worsted spun. Spinning: The spinning process where the twist is not allowed into the drafting triangle. They are usually plied yarns, and are finer and more tightly twisted than woollen yarns.

WORSTED COUNT – The worsted count also expresses the number of hanks required to make a pound of yarn. A hank of worsted wool is equal to 560 yards. So 1 wc = 560 yards of cotton, the coarsest worsted yarn. Worsted sizes are expressed the reverse of cotton sizes. A two-ply number 6 worsted yarn would be expressed as 2/6 wc and would yield 1680 yards per pound. You can covert worsted count to cotton count by multiplying the cc by 1.5, or wc = cc x 1.5. See “Bradford Count“.

WOVEN FABRIC – A material (usually a planar structure) constructed by interlacing yarns, fibres, or filaments, to form such fabric patterns as plain, harness satin, or twill weaves.

WOVEN FABRIC COMPOSITE – A major form of advanced composites in which the fibre constituent consists of woven fabric.  A woven fabric composite normally is a laminate comprising a number of laminae, each of which consists of one layer of fabric embedded in the selected matrix material.  Individual fabric laminate are directionally oriented and combined into specific multi-axial laminates for application to specific envelopes of strength and stiffness requirements.  CMH-17 (WAS MIL-HDBK-17). A major form of advanced composites in which the fibre constituent consists of woven fabric. A woven fabric composite normally is a laminate comprised of a number of laminae, each of which consists of one layer of fabric embedded in the selected matrix material. Individual fabric laminae are directionally oriented and combined into specific multi-axial laminates for application to specific envelopes of strength and stiffness requirements.  A material (usually a planar structure) constructed by interlacing yarns, fibres, or filaments, to form such fabric patterns as plain, harness satin, or twill weaves.

WOVEN ROVING – A heavy glass fibre fabric made by weaving roving or yarn bundles.  See ISO 472. If roving is used, the fabric is called woven roving; if yarn is used the fabric is called cloth. Many suppliers use the term cloth for woven roving, but refer to the weave as flat, not twisted. Cloth composites can have a slightly lower resin content than woven rovings because the yarn bundles are more compact than rovings, but the twist weakens the fibres somewhat. In the example given above from the Aircraft Spruce catalog, the thicker but lighter fabric is a woven roving, and the other is a cloth.

WOVEN ROVING FABRIC – Heavy fabrics woven from continuous filament in roving form. Usually in weights between 18-30 oz. per square yard.

WRINKLE – A surface imperfection in laminated plastics that has the appearance of a crease or fold in one or more outer sheets of the paper, fabric, or other base, which has been pressed in.  Also occurs in vacuum bag moulding when the bag is improperly placed, causing a crease.  See ISO 472.


X-AXIS – In composite laminates, an axis in the plane of the laminate which is used as the 0o reference for designating the angle of a lamina. Usually, the axis in the plane of the laminate used as 0° reference. Typically, the y-axis is the axis in the plane of the laminate perpendicular to the x-axis, and the z-axis is the reference axis normal to the laminate plane in the composite laminate. (See also laminate coordinate axes, off-axis laminate and principal axis.)

X-RAY NDI – Non-destructive inspection using X-rays, i.e., radiation in the 1016 to 1022 Hertz frequency band.

XY-PLANE – In composite laminates, the reference plane parallel to the plane of the laminate.

XZ-PLANE – In composite laminates, the reference plane parallel to the X-axis of the laminate and through the thickness.


YARDAGE – Similar to Yield, but used to describe the linear density of “bare glass” or an unsized product. Yardage specifies the number of yards of glass required to weigh one pound, measured in hundreds. For example, K18 is a K fibre diameter that has 180yards in one pound of glass. Any fabric made and sold by the yard.

YARN – An assemblage of twisted filaments, fibres, or strands, either natural or manufactured, to form a continuous length that is suitable for use in weaving or interweaving into textile materials.  Generic term for a continuous strand of textile fibres, filaments or material in a form suitable for knitting, weaving or intertwining to form a textile fabric. A continuous strand of textile fibres that may be composed of endless filaments or shorter fibres twisted or otherwise held together. It may be made up woof vegetable (linen, hemp, jute, sisal, ramie, cotton), animal (wool, mohair, silk), or artificial fibres (gold, silver and other metals rayon, nylon, Orlon). Yarns are utilized in making fabric. Yarn is characterised by its composition, its thickness (or grist or count), number of strands (or plies), direction and degree of twist, and the colour.

YARN BEAM – The same as a “Warp Beam”.

YARN BUNDLE – See bundle.

YARN SPLICE – A yarn which has been severed or broken and subsequently rejoined.

YARN, PLIED – Yarns made by collecting two or more single yarns together.  Normally, the yarns are twisted together though sometimes they are collected without twist.  CMH-17 (WAS MIL-HDBK-17).

Y-AXIS – In composite laminates, the axis in the plane of the laminate that is perpendicular to the x-axis.  Contrast with x-axis.

YIELD POINT – The first stress in a material, less than the maximum attainable stress, at which the strain increases at a higher rate than the stress.  The point at which permanent deformation of a stressed specimen begins to take place.  Only materials that exhibit yielding have a yield point.  See ISO 472.

YIELD STRENGTH – The stress at the yield point.  The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.  The lowest stress at which a material undergoes plastic deformation.  Below this stress, the material is elastic; above it, the material is viscous.  Often defined as the stress needed to produce a specified amount of plastic deformation (usually a 0.2% change in length).

YOUNG’S MODULUS – The ratio of normal stress to corresponding strain for tensile or compressive stresses less than the proportional limit of the material.  See also modulus of elasticity.  See ISO 472.

YZ-PLANE – In composite laminates, the reference plane parallel to the Y-axis of the laminate and through the thickness.


Z-AXIS – In composite laminates, the reference axis normal to the plane of the laminate. The axis perpendicular to the plane formed by the x and y axes. In a sheet laminate, if the x and y axes are parallel to the length and width, respectively, the z-axis would indicate sheet thickness. (See x-axis and y-axis.)

ZERO BLEED – A laminate fabrication procedure that does not allow loss of resin during cure.  Also describes prepreg made with the amount of resin desired in the final part, such that no resin has to be removed during cure.


Z-TWIST – Spinning counterclockwise. Traditionally, the direction “plied” yarns are spun.


The terminology above has been primarily collated by Abaris Training Resources Inc. www.abaris.com from a number of industry standard sources.  However, much of this document has been provided courtesy of Dr. Keith Armstrong, and the training task group of the IATA/ATA/SAE Commercial Aircraft Composite Repair Committee. 

Original references include:

ISO 472:2013 defines terms used in the plastics industry, including terms and definitions appearing in plastics standards (of ISO/TC 61) and general terms and definitions of polymer science used in all aspects of plastics technology.

BDS 1330 Boeing Drafting Standard Composite Material Drawings

MIL-HDBK-17-1F Military Handbook: Composite Materials Handbook, Volume 1 – Polymer Matrix Composites Guidelines for Characterization of Structural Materials – now Composite Materials Handbook 17 (CMH-17). https://www.cmh17.or

MIL-HDBK-23A STRUCTURAL SANDWICH COMPOSITES Materials – now Composite Materials Handbook 17 Volume 6 (CMH-17-V6). https://www.cmh17.org/

ASTM C 274 Standard Terminology of Structural Sandwich Constructions

ASTM D 314 Method of Test for Hardness of Rubber (Withdrawn 1969), REPLACED BY ASTM D 1415

ASTM D 329 Standard Specification for Acetone

ASTM D 618 Standard Practice for Conditioning Plastics for Testing

ASTM D 740 Standard Specification for Methyl Ethyl Ketone

ASTM D 1002 Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)

ASTM D 1144 Standard Practice for Determining Strength Development of Adhesive Bonds

ASTM D 1153 Standard Specification for Methyl Isobutyl Ketone

ASTM D 1415 Standard Test Method for Rubber Property—International Hardness

ASTM D 1781 Test Method for Climbing Drum Peel for Adhesives

ASTM D 1876 Standard Test Method for Peel Resistance of Adhesives (T-Peel Test)

ASTM D 2344 Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates

ASTM D 2339 Standard Test Method for Strength Properties of Adhesives in Two-Ply Wood Construction in Shear by Tension Lading

ASTM D 3039 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials

ASTM D 3165 Standard Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies

ASTM D 3410 Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading

ASTM D 3433 Standard Test Method for Fracture Strength in Cleavage of Adhesives in Bonded Metal Joints

ASTM D 3762 Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test)

ASTM D 3932 Practice for Control of the Application of Structural Fasteners When Attached by Hot Melt Adhesive (Withdrawn 1991)

ASTM D 4027 Standard Test Method for Measuring Shear Properties of Structural Adhesives by the Modified-Rail Test

ASTM D 4896 Standard Guide for Use of Adhesive-Bonded Single Lap-Joint Specimen Test Resul

ASTM D 907 Standard Terminology of Adhesives

Dictionary of Composite Materials Technology, Stuart M. Lee CRC Press, Nov 26, 1995

Glossary of Composite Materials Terminology

Glossary of Textile Terms

Glossary of Weaving Terms

Glossary of Fabrics

Glossary of Polymers

Glossary of composites, adhesives