A review is given of NDT methods for fibre-reinforced composite materials based on fibres such as carbon, boron, glass and Kevlar. The capabilities of present methods, such as ultrasonics, radiography, holography and thermography, are considered with respect to defect detection. Techniques which show promise of development as predictors of failure, eg vibration measurement and acoustic emission, are also discussed.

A review of non-destructive testing of composite materials

The mechanical properties of unidirectional flax fibre reinforced unsaturated polyester resin composites were studied with particular emphasis on their tensile deformation behaviour. These materials displayed characteristic non-linear behaviour when loaded parallel to the axis of the fibre, with a distinct knee preceding a drop in stiffness. Further deformation resulted in strain hardening behaviour. Load cycling and acoustic emissions analysis were used to investigate the nature of the knee and it was found that this corresponded with yielding behaviour in the composite. A well-defined yield point could be identified, which in composites of around 60% fibre volume fraction, occurred at a strain of some 0.12% and a tensile stress of 32 MPa. Varying the interfacial properties, through chemical modification of the fibre prior to lamination, was found to have a marked effect upon the onset of yielding and the yield point itself, as well as the deformation and fracture behaviour of the laminate. It is considered that this behaviour is intimately linked to the straining behaviour of the fibre as well as the fibre–matrix interaction and hypotheses to explain the observed behaviour are presented.

Deformation and fracture behaviour of flax fibre reinforced thermosetting polymer matrix composites

Micromechanical modelling of matrix degradation in randomly oriented discontinuous-fibre composites

The fracture behaviour of starch and gluten in the glassy state

Stress corrosion cracking of GRP pultruded rods has been investigated in 0.0001 to 5.0 N hydrochloric acid environments under bending and tensile loading modes. Crack initiation takes place at exposed glass fibres in the surface of the rod, and crack propagation is planar and at right angles to the rod axis. Leaching of calcium and aluminium from the fibres takes place during the cracking process, and time-to-failure is dependent on the acid concentration, the stress level and the ease of access of the acid to the glass fibre surface. Possible mechanisms of crack propagation through the glass fibres and resin are discussed.

Stress corrosion cracking of GRP pultruded rods in acid environments

A study has been made of the amplitude distributions of acoustic emissions from glass reinforced plastics of several different kinds during tensile loading of test coupons. It is shown that the structure of a GRP laminate exerts a dominating effect on the AE patterns observed, and that direct correlations between microfailure events in the composite and the amplitude distributions of the emissions cannot be made. Understanding of AE patterns from any given GRP material (or structure) may only be possible if there is already a substantial background knowledge of the behaviour of similar materials or structures under known conditions.

F. J. Guild

Papers

Acoustic emission studies of damage in GRP

Amplitude of distribution analysis of acoustic emission from composites: a new method of data presentation