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Composite Sandwich Structures | Composite Engineer’s Viewpoint

1 July 2019

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

Part 14- Non-Destructive inspection of Sandwich Structures

Following sandwich structure fabrication or as a result of inservice damage, non-destructive inspection (NDI) can be used to characterise a defect/damage. Such NDI tests range from very simple visual inspection to the more extensive through-the- thickness non-destructive interrogation methods. The requirements of NDI are to determine the presence of any nonconformity, in particular debonds and delaminations, porosity, damaged core and internal foreign objects (i.e. fluids). A summary of NDI methods applicable to the composite facings, facing-to-core bondline and core integrity for sandwich structures are listed below.

Visual Inspection

A simple visual inspection can identify gross nonconformities such as bulges, warping and surface ply direction. Needless to say visual inspection is only useful for the outer surface of the sandwich structure and not core issues.

Acoustic Resonance

The simplest acoustic resonance method is the ‘coin tap test’. This method has been used for many years and provides qualitative information of near sub-surface debonds or delaminations. The tap test method is most useful on thin-skinned sandwich structures.

Ultrasonic Inspection

The implementation of ultrasonic inspection is probably the most extensively used of the NDI methods currently available. The ultrasonic inspection methods provide details of depth and size of the non-conformity, or full details of the topography of sub-surface defects. However, for thin skinned sandwich structures, the identification of laminate and core-to-skin anomalies can be challenging.

Thermography and Optical Methods

Three relatively new methods of full-field NDI are shearography, holography and thermography. All three methods provide a visual, quantitative measure of the effects of skin sub-surface and core defects on the component under load. Whereas most other NDI methods indicate the size and shape of the non-conformity, these optical and thermal methods also show the component’s response to load. Therefore, the criticality of the defect can be more easily assessed. Due to the poor response of core materials to other NDI techniques, the optical and thermographic methods have significant improvements in subsurface defect assessment.

Radiography

Particularly x-ray radiography (and with the more user-friendly digital x-ray systems) interrogation of foam and honeycomb cores provides detailed imagery and assessment of the core conditions.

Dynamic Response

Vibration frequencies and damping characteristics have recently shown favourable outcomes in detecting localised water in hollow core sandwich structures.

The next and final article on composite sandwich structures will cover Sandwich Structure Repairability and how to restore functionality.

Rik welcomes questions, comments and your point of view by email to rikheslehurst@gmail.com.

This article first appeared in:

Connection Magazine

Issue 50: July, 2019

Author: Rik Heslehurst

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

Issue 50: July, 2019