Analysis of Local Delaminations Caused by Angle Ply Matrix Cracks

Abstract

Two different families of graphite/epoxy laminates with similar layups but different stacking sequences, (0θ/-θ) s and (-θ/θ/0) s, laminates, were analyzed using three-dimensional finite element analysis for 0 = 15 and 30 degrees. Delaminations were modeled in the -θ/θ interface, bounded by a matrix crack and the stress free edge. The total strain energy release rate, G, along the delamination front was computed using three different techniques: the virtual crack closure technique (VCCI), the equivalent domain integral (EDI) technique, and a global energy balance technique. The opening fracture mode component of the strain energy release rate, G1, along the delamination front was also computed for various delamination lengths using VCCT. Although the finite element model did not have an orthogonal mesh, VCCT still yielded accurate results which were in agreement with the global energy balance and yielded similar G distributions across the delamination front as the EDI technique. For both layups analyzed, the matrix crack length influenced the magnitude of G for delamination. Furthermore, the opening mode, G., was greatest near the matrix crack and decreased near the free edge. The laminate stacking sequences with a matrix crack in the surface angle ply had a greater GI value than the laminate stacking sequences with an angle ply matrix crack in the interior of the specimen thickness. This is consistent with test results in the literature that show delamination occurs earlier in the fatigue life of laminates with matrix cracks in the surface plies than in the interior plies.