Fatigue Damage and Residual Strength Evaluation of Resin Transfer Molded (RTM) Composites Under Reversed Cyclic Loading

The fatigue loading in composite materials causes cumulative damages and therefore degrades the stiffness of the materials before ultimate failure. A comprehensive study has been conducted to characterize the fatigue damage for two different material systems under reversed fatigue (R = −1). Hercules’ IM7 6K plain weave fabric reinforced composites with two different epoxy based resin systems namely, PR 500 and PR 520 with laminate sequence of [+45,-45,0,90]2s were chosen. Static compression tests have been performed with open-hole notch specimens to determine the strength, modulus and failure strain. Series of fatigue tests with a frequency of 2 Hz at various stress levels have been conducted, and the fatigue life distributions are shown using the conventional S-N diagram. In order to determine the residual strengths and stiffnesses, the specimens were subjected to fatigue loading at various predetermined number of cycles of its fatigue life at a particular stress level. The fatigued specimens were then tested to failure under compression. The edge replica technique has been used to investigate the progressive damage due to fatigue loading. Weibull analysis has been performed to determine the statistical information for the static, fatigue and residual tests. The progressive damage due to fatigue loading has also been studied through ultrasonic C-scan, optical microscope (OM) and scanning electron microscope (SEM), and the failure mechanisms under both static and cyclic loading are discussed in this paper.