Behaviour of fibre/matrix interface cracks under biaxial stress state caused by edge effect in carbon/epoxy laminates

Abstract

A recent study about the ''edge effect'' phenomenon in cross-ply laminates made of ultrathin plies composites has shown the presence of a relevant stress component through the laminate thickness. This fact implies that there is a biaxial stress state in the 90° ply block, which is present both before and after the cyclic loading tests. 4 cross-ply laminates were analysed, only varying the 90° ply block thickness. In each case, the biaxial stress state was obtained, selecting the most detrimental one to analyse the Energy Release Rate (G) with respect to the fibre/matrix interface crack growth. This analysis is performed using a BEM model from a previous study of the authors. An exhaustive microscopic revision is performed both before (only sanded and polished after curing process) and after cyclic testing, corroborating the occurrence of different crack growth of the present longitudinal fibre/matrix interface debonds. In conclusion, the single fibre numerical model using BEM and the experimental microscopic observations shed light on the micromechanical behaviour of the interface cracks which are subjected to a biaxial stress state.