Microscopic examination of load-interrupted samples has established the local shear instability in fiber and matrix as the failure initiation mechanism in biaxially-loaded graphite epoxy cross-ply laminates under compression. The failure data for all material orientations (with respect to loading axes) and in-plane biaxiality ratios correlated remarkably well using a local pressure-dependent matrix and fiber shear failure criterion. Figures 1 and 2 show results for two separate composite systems. Most samples failed by delaminations parallel to the principal load axis. Examination revealed these delaminations to be the wings of angled shear cracks, which in turn, were nucleated due to local matrix and fiber failure under shear. Interestingly, Figs. 1 and 2 also correlate strengths of samples that failed locally due to fiber microbuckling. The proposed criterion is relatively simple and practical to implement in existing design methodologies.

Figure 1. Failure in AS4/3502

Figure 2. Failure in IM7/8551-7