The first ply failure in composite laminates are studied by 3-D fracture mechanic approach. The fracture model is based on a crack embedded in one of the inner layers. Numeriil results for the case of "angle ply" laminates are based on a general 3-D finite difference progtam.
Stress intensi~ factors and fracture energy are ev~uated by the J-integral 2-D and 3-D methods as well as by the stresses and energy balance. It was found that the J(Z-D) method can be applied to 3-D problems while the 1(3-D) method is not suitable for the numerical compu~tion.
The influence of geometrical properties of the crack and the layers on the various stress intensity factors is presented and discussed. Change in failure mechanism of the weakest ply was proved by experiments to fit with numerical results, It was found that the weakest layer is strengthened by the others, a fact which has been verified experimentally and hasn't been explained by other approaches. NOTA~~N a, b half crack length and width c specimen width Ca stiffness matrix g Young's modulus: P-equivalent for orthotropic layer, EL, &-longitudinal and transverse G fracture energy G shear modulus: G$~quivalent for orthotropic layer, Gr, Go-lon~tudinal and transverse h layer's thickness i,j indices K stress concentration factor: Krrr-for two dimensional crack, K line i = 0, K/'-for the line i = 0 t,rfor the three dimensional case, K,"-for the I specimen's length L, T indices for longitudinal and transverse properties r radius from the crack tip t half specimen thickness u. external displacement ul. u2 elastic energy for positions 1 and 2 ui displacement vector U, V, W displacements in the x, y, z direction X, y, z coordinate system for the specimen f 9, i coordinate system for the crack vicinity fi angle between the crack and x direction Ejj strain tensor oa stress tensor fl< average external stress at failure v Poisson's ratio: vLR v,-longitudinal, VT-transverse