INITIATION AND PROPAGATION CONTROLLED INTRALAMINAR CRACKING IN CROSS-PLY LAMINATES

Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive intralaminar cracking. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation and propagation; the former is governed by the local strength whereas the latter is controlled by the local fracture toughness. A simple statistical model is derived for the transverse ply fragmentation in a cross-ply laminate subjected to tension. The crack spacing distributions and crack density-applied load relations are obtained for two distinct fragmentation stages: the initial stage and the advanced fragmentation stage. The initial fragmentation phase is characterized by widely spaced, non-interacting cracks while later on crack evolution is marked by low damage rate due to closely spaced, interacting cracks. Based on the probabilistic cracking model, simple relations for crack density mastercurves are derived for vanishing scatter of strength and toughness. The mastercurve approach is applied to progressive cracking in glass/epoxy laminates with brittle and toughened epoxy matrices. The accuracy of estimation of laminate stiffness reduction by using crack density mastercurves is evaluated. Natural fiber cross-ply laminates with more disperse damage than in synthetic fiber composites are considered and modeling options are briefly discussed.