Fatigue Damage in Laminated Composites

Abstract

The present work investigates the damage development under fatigue and static loading in graphite/epoxy laminates. The tensile strength, modulus and tensiontension fatigue data for [0/0/90] 3s laminate were generated at three stress levels (70%, 80% and 90% of ultimate strength). The residual strength and modulus of the specimens were determined at 70% and 90% of the ultimate strength. The characteristic lives of cycles to failure ( N f ) as a function of applied stress were determined using Weibull analysis. The specimens were subjected to cyclic loading a determined percentage of the characteristic life at a given stress level, and the progressive damage determined as a function of applied number of cycles was analyzed using ultrasonic non-destructive technique.The results indicate that there is insignificant degradation in residual strength and stiffness as a result of static tension loading (90% of S ult ). The Weibull analysis shows a significant change in characteristic lives of the number of cycles to failure at stress levels 70, 80 and 90% of ultimate strength. The change of residual strength and modulus is negligible within 50% of N f at both 70% and 90% of S ult . The ultrasonic C-scan in such case shows resin cracks with no delamination and fiber breakage. When the specimens were fatigued for cycles less than half of the total cycles required for failure ( N f ) at 70 % and 90% of the ultimate strength, the change in both residual strength and modulus was observed to be negligible. The ultrasonic C-scans of the fatigue specimens, in these cases, showed micro and macro cracks in the matrix but no delamination and fiber breakage. On the other hand, there was significant reduction in the strength and modulus when the material was fatigued for cycles higher than 50% of N f . The non-destructive evaluation of the fatigued specimens in these cases revealed cracks in the matrix as well as debonding and breakage of fibers and a significant amount of delamination.