MATRIX CRACK INDUCED STIFFNESS REDUCTIONS IN [(0(M) 90(N)/+THETA(P)/-THETA(Q))(S)](M) COMPOSITE LAMINATES/

Two- and three-dimensional linearly elastic glass/epoxy and carbon/epo xy laminates of the type [(0m/90n/ + theta(p)/ - theta(q))s]M containi ng periodically distributed matrix cracks have been analysed by aid of the finite element method. The presented finite element model enables modelling of several important thick and thin ply stacking sequences like cross-plies, angle plies and quasi-isotropic laminates. Due to pe riodicity it suffices to model a representative volume element of the laminate. The boundaries of this unit cell represent prospective crack surfaces. In this way varying crack configurations and crack densitie s could be simulated. By application of periodic boundary conditions t he stiffness tensors for laminates containing different crack configur ations were calculated. The results are presented in the form of reduc ed engineering stiffness parameters as functions of matrix crack densi ties for a thick quasi-isotropic [(0-degree/90-degrees/ + 45-degrees/ -45-degrees)s]M glass/epoxy laminate, a thick [(0-degree/90-degrees 55-degrees/ - 55-degrees)s]M carbon/epoxy laminate and a thin (0-degre e/ +45-degrees/ -45-degrees)s glass/epoxy laminate. Comparisons are ma de to an approximate analytic model developed previously. An excellent agreement between the analytic predictions and the finite element res ults was found for all cases under consideration.