The effects of matrix and interface on damage in GRP cross-ply laminates

A study has been made of the transverse cracking behavior of a series of cross-ply laminates with different matrices, fiber coatings (sizes) and fiber volume fractions. On the basis of unpublished results, a correlation was assumed between material effects on leakage pressure in pressure vessels of given stacking sequence and transverse cracking behavior in cross-ply laminates. Mechanisms for crack initiation and growth were studied by optical microscopy. The strain at the onset of transverse cracking, εTOS, increased and the slope, K, of the curve of crack density as a function of strain decreased as the GIC of the matrix increased. Improved fiber/matrix adhesion and lower fiber content had similar effects. The properties of polyester-based composites were usually inferior to those of vinylester composites as a consequence of pre-existing debonds and subcritical cracks resulting from microlevel curing stresses. From observations of failure mechanisms one may infer that proof testing of pipes or pressure vessels will increase the extent of subcritical damage in the material.