Damage Mechanisms and Accumulation in Graphite/Epoxy Laminates

Abstract

Damage mechanisms and accumulation, and associated stiffness and residual strength reductions, were studied in cross-ply graphite/epoxy laminates under cyclic tensile loading. Stress-life data were fitted by straight lines on a log-log scale. The fatigue sensitivity decreases with the number of contiguous 90-deg plies. Five different damage mechanisms were observed: transverse matrix cracking, dispersed longitudinal cracking, localized longitudinal cracking, delaminations along transverse cracks, and local delaminations at the intersections of longitudinal and transverse cracks. Variations of residual modulus and residual strength were measured as a function of cyclic stress level and number of cycles. The residual modulus shows a sharp reduction initially, followed by a more gradual decrease up to failure. The residual strength showed some characteristic features: a sharp decrease initially then a near plateau in the middle part of the fatigue life, and a rapid decrease in the last part of the fatigue life. A cumulative damage model is proposed based on residual strength and the concept of equal damage curves.