Fatigue behavior of 3D multi-layer angle interlock woven composite plates

This study was aimed at investigating the fatigue behavior of three-dimensional (3D) multi-layer angle interlock woven composite plates. Tensile cyclic stress was applied to the specimen along the weft yarn direction. Unnotched three-layer and five-layer plates were used in this work in order to compare their fatigue properties and damage processes. The effect of a center hole on the fatigue behavior was also investigated by using notched three-layer and five-layer plates. The experimental results showed that the three-layer specimen exhibited higher static ultimate tensile strength (UTS), but lower fatigue limit strength or higher fatigue sensitivity than the five-layer specimen for both the unnotched and notched plates. Normalized S/N curves of the unnotched and notched specimens, the maximum cyclic stress being divided by the appropriate UTS, were observed almost to coincide on a single line for both the three-layer and five-layer plates. This indicated that a center hole did not affect the fatigue sensitivity of these plates. Metallography was employed to study the microscopic fatigue damage process. According to the observations, the fatigue process could be divided into five stages. The fatigue damage mechanism primarily involved transverse cracks in the warp yarns, debonding between the warp and weft yarns, debonding extension and deflection into the matrix, and weft fiber breakage.