Compressive fatigue behavior of low‐velocity impacted thermoplastic composite laminate

In this paper, residual compressive strength and compression–compression fatigue tests of two kinds of AS4/PEEK laminates with low‐velocity impact damage were carried out to study the damage propagation characteristics. It was found that local buckling occurred around the impact dent and extended perpendicularly to the loading direction under fatigue compression loading. The expansion process of local buckling could be divided into three stages, and the damage mechanism of each stage was discussed. The experimental results showed that the impact damage and residual fatigue life both were quite dispersed under the same impact energy. A laminate with more severe impact damage had shorter residual compressive fatigue life, and the depth of impact dent could be used as an effective index to characterize the severity of impact damage and the residual fatigue life. HIGHLIGHTS The low‐velocity impact damage of the thermoplastic laminates with two different stacking sequences was identified by visual inspection and ultrasonic technology. The impact damage exhibits significant dispersity under the same impact energy, it illustrates that the impact resistance varies from one laminate to another even with same lay‐up. A microscopic CCD camera was used to assess the damage propagation of the impacted laminates. Multiple fatigue damage growth was found during compression–compression fatigue loading, which including local buckling around impact dent, fiber fracture, and matrix splitting. The propagation of transverse dimension of local buckling can be divided into three stages, and it can be concluded that the impacted AS4/PEEK laminates will not fail suddenly under cyclic loading before the visually recognized local buckling region extends to the edge of impact damage region. The conclusion is beneficial to the transition from “No‐Growth” evaluation to “Slow Growth” evaluation and “Arrested Growth” evaluation in the damage tolerant fail‐safe evaluation, as mentioned in CMH‐17‐3G. The depth of impact dent was selected as the index to represent the severity of impact damage for a specific laminate. And a relationship between the dent depth and residual compressive life was established based on an equal rank assumption.