Cracking paths at the ply interface in a cross-ply laminate

It has been observed in both ceramic and polymer-matrix composites that the evolution of transverse matrix cracking shows two distinct stages. When tensile stress is applied in the fiber direction of the 0° ply, cracks are formed in the 90° ply at a load level much lower than the ultimate strength of the laminate. As the load increases, more transverse cracks are initiated. At some critical load, the number of cracks ceases to increase; instead, the existing cracks penetrate into the adjacent 0° layers and eventually fracture the 0° fibers. Catastrophic failure becomes imminent when the 0° fibers are broken. The purpose of this paper is to identify conditions under which transverse cracks are deflected into the ply interfaces at the second stage of damage evolution. To this end, the energy release rates for deflection and penetration are used as a criterion to predict the crack growth paths. It is found that the predicted crack path depends on the geometry, the material properties and the degree of damage. Based on this analysis, a crack path prediction diagram is obtained for crack growth at the ply interface.