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Composite Engineer’s Viewpoint

23 December 2019

Part 15 – Sandwich Structure Repairability

Rik Heslehurst - composites engineering consultant and trainer

By Rik Heslehurst, PhD, MEng, BEng (Aero) FIEAust, FRAeS, FSAMPE, CPEng Composites Australia (Honorary Member)

In the last article we considered non-destructive inspection of sandwich structures. Now the damage has been identified we look at the most appropriate restoration approach for the sandwich structure.

Prior to the implementation of a repair scheme a sound set of repair design, development, fabrication and installation criteria must be developed. Thus, the basis of composite and sandwich structure repair design follows a logical set of repair parameters which are outlined below:

Static Strength & Stability: Any repair must be capable of supporting the design loads that are applied to the original structure through either a strength or stability restoration focus.

Repair Durability: Any repair scheme that is designed to restore the structure to operational conditions is generally expected to remain an integral part of the structure for the remaining service life. Considerations include fatigue loading, corrosion, and environmental degradation during the design/development phase of restitution.

Stiffness Requirements: With light weight structures stiffness is often more critical than strength. Consideration should therefore be given to the repair scheme maintaining the integrity of structural stiffness via deflection limitations, flutter and vibration, and load path changes.

Surface Smoothness: Surface smoothness is an important consideration when maximum speed or fuel efficiency is required. You should always ask yourself is a flush repair required?

Weight & Balance: The size of the repair and the local changes in weight can be insignificant to the total component weight, but in weight sensitive structures, such as aerodynamic or hydrodynamic control surfaces, the effect to the mass balance can be highly significant.

Operational Temperature: The operating temperature influences the selection of repair materials particularly adhesives and composite resins. A combination of extreme temperatures with environmental exposure (the hot/wet condition) is often the critical condition for which the repair must be designed. Materials that develop adequate strength within the required operational temperature range must be selected.

Environmental Effects: Composite and adhesive bondlines are prone to degradation when exposed to various environments, in particular fluids and thermal cycling. The influence of moisture absorption can affect the durability of the repair design and should be considered in the repair scheme design.

Costs & Scheduling: It is well established that it is cheaper to repair than replace, given availability of appropriate facilities and adequate personnel skilled. Repair and their design costs in terms of platform downtime and operating expenses are a consideration.

Operational Performance & Appearance: Consideration should be given ow whether the repair adversely affect operational performance, in particular radar cross section characteristics. The repair should not be visible.

Facility Capabilities: Does your facility have the appropriate workshop equipment and facilities to undertake the repair? This will include the occupational health and safety of repair personnel.

Repair Staff: Consideration should be given to whether repair technicians and engineering staff are trained in the repair of composite sandwich structures; including whether they have been trained in damage inspection, damage characterization, damage removal, repair scheme fabrication, repair scheme installation (particularly surface preparation) and post-repair quality assessment.

This is the final article on sandwich structures. In the next article we will discuss the differences between a nested and a stacked composite laminated structure. This article will not consider the details and specific types of repair and how to develop such repairs. For such detail the reader is directed to the following book on composite repair design: Engineered Repairs for Composite Structures, Rikard Benton Heslehurst, ISBN 9781498726269, 2019 CRC Press

This article first appeared in:

Connection Magazine

Issue 51: December, 2019

Author: Rik Heselhurst

For this and more stories, please download the latest copy of our Connection magazine.

Issue 51, December 2019