Micromechanics of interlaminar fracture in carbon fibre reinforced plastics at multidirectional ply interfaces under static and cyclic loading

Delaminations have been grown at 0 degrees/0 degrees and 0 degrees/90 degre es ply interfaces under mixed mode static and cyclic loading, using the mix ed mode bending test. Fractographic analysis of the failure surfaces by sca nning electron microscopy gave insights into the micromechanisms of delamin ation growth. Improved delamination resistance with an increasing mode II c omponent was attributed to an increase in the total surface area generated. There was a secondary cracking mechanism at 0 degrees/90 degrees ply inter faces which resulted in slightly higher mixed mode toughnesses. Fatigue in thermoset matrix composites is very similar to static failure since there i s no mechanism comparable to dislocation movement in metals by which damage can migrate and accumulate under low cyclic stresses. The relatively high scatter in composite tough ness was explained in terms of variations in the spatial arrangement of fibres. It was shown how the examination of sheer c usps may be used to identify crack growth directions in post-mortem failure investigations.