Abstract
Failure mechanisms in composite laminates are quite different from those in homogeneous materials. While homogeneous materials under fatigue fail as a result of initiation and growth of a single dominant crack, composite laminates can sustain many cracks in the weak phase before ultimate failure. In unidirectional composites under longitudinal fatigue the subcritical failure can take the form of interfiber matrix cracking normal to the fibers. The matrix cracking is enhanced by the biaxial state of stress between fibers especially when applied stress is outside linear elastic range of the resin. The dominant modes of subcritical failure in multidirectional laminates are ply cracking and delamination. Ply cracking leads to a stress relaxation in the cracked ply and hence slows down with fatigue cycles. Even of the same stacking sequence a laminate with thicker plies shows earlier crack initiation but slower crack multiplication than a laminate with thinner plies. Delamination between plies can start not only at free edges but also from ply cracks. Both ply cracking and delamination reduce stored energy and hence stiffness. The energy release rates associated with ply cracking and delamination are independent of crack size, unlike the crack growth in homogeneous materials.
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