Damage development and property degradation in composite materials

Damage mechanisms and accumulation and associated stiffness reductions were investigated in crossply graphite/epoxy laminates subjected to tensile cyclic loading. Damage development consists of transverse matrix cracking, followed by longitudinal matrix cracking, local delaminations at crack intersections, and ultimately longitudinal fiber fractures of the primary load carrying plies. The reduction in axial modulus, even in normalized form, is a function of the cyclic stress level. The degradation of in-plane shear modulus was also monitored and found to be highly sensitive to longitudinal matrix cracking. The relationship between stiffness degradation and residual fatigue life was discussed.