Evaluation of detailed transverse crack propagation toward thickness direction for CF/PEEK quasi-isotropic laminates under fatigue loading

Abstract

Carbon fiber (CF) reinforced polyether ether ketone (PEEK) laminates are applied to the members of transportation structures exposed to severe conditions, because carbon fibers and PEEK have excellent heat-resistant and chemical stability. Therefore, it is important to ensure the long-term reliability of CF/PEEK laminates and evaluate the propagation of transverse cracks, which are generally the initial damage incurred by multidirectional laminates under fatigue loading. This study evaluated the detailed mechanism of transverse crack propagation toward the thickness direction in CF/PEEK quasi-isotropic laminates under fatigue loading. The transverse crack growth was observed using the replica method, and the energy release rate associated with the transverse crack propagation toward the thickness direction was calculated using a three-dimensional virtual crack closure-integral method. Thus, it was found that the transverse crack propagated in a stable manner until it passed through the 90° layer toward the thickness direction. Additionally, it was found that the crack growth rate increased in the range wherein the crack length was short, and maintained an approximately constant growth rate in the middle stage of propagation. According to Paris’ law, when the crack depth was relatively shallow, the experimental results are approximately consistent with the analytical evaluation results for the energy release rate associated with the transverse crack propagation toward the thickness direction. Finally, it is also concluded that the high ductility of the PEEK resin and good fiber/matrix interfacial property of the CF/PEEK laminates are important influencing factors for the stable and characteristic crack propagation observed in the experiment.