Stress state characterization of delamination cracks in [0/90] symmetric laminates by BEM

The presence in a laminate of plies oriented at 90 with respect to the preferred direction of load generates almost immediately the appearance in these plies of cracks transverse to the load (parallel to the fibres in the lamina). These cracks reach the interface with the neighbouring ply, which in this paper will be considered oriented 0 with respect to the direction of the load. This may cause the bifurcation of the crack, which now appears propagating as a delamination crack between the two plies. The objective of this study is to characterize the stress state at the tip of both, the transverse crack in the 90 ply reaching the interface with the 0 ply, and the delamination crack for different lengths of the debonding. The analysis is performed by means of the Boundary Element Method allowing contact, without or with friction, to take place between the faces of the crack. The plies are considered as equivalent homogeneous bodies under a generalized plane strain state. The results are compared with those predicted by the open and contact models of Interfacial Fracture Mechanics. Accurate knowledge of the stress state at the neighbourhood of the tips of the cracks studied is necessary to generate failure criteria based on Fracture Mechanics parameters to predict the appearance and growth of the type of damage described.