ANALYSIS OF LOCAL DELAMINATIONS CAUSED BY ANGLE-PLY MATRIX CRACKS

Two different families of graphite/epoxy laminates with similar layups but different stacking sequences, (0/theta/-theta)(s) and (-theta/the ta/0)(s) laminates, were analyzed using three-dimensional finite eleme nt analysis for theta = 15 and 30 degrees. Delaminations were modeled in the -theta/theta interface, bounded by a matrix crack and the stres s free edge. The total strain energy release rate, G, along the delami nation front was computed using three different techniques: the virtua l crack closure technique (VCCT), the equivalent domain integral (EDI) technique, and a global energy balance technique. The opening fractur e mode component of the strain energy release rate, G(I), along the de lamination front was also computed for various delamination lengths us ing VCCT. Although the finite element model did not have an orthogonal mesh, VCCT still yielded accurate results which were in agreement wit h 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 delaminati on. Furthermore, the opening mode, G(I), was greatest near the matrix crack and decreased near the free edge. The laminate stacking sequence s with a matrix crack in the surface angle ply had a greater G(I) valu e than the laminate stacking sequences with an angle ply matrix crack in the interior of the specimen thickness. This is consistent with tes t results in the literature that show delamination occurs earlier in t he fatigue life of laminates with matrix cracks in the surface plies t han in the interior plies.