The influence of void and fractured fiber defects on the ultrafine Z-pin and Mode I fracture toughness of CFRP laminates

To minimize the damage caused by Z-pin insertion to the in-plane properties of the laminate, researchers have initiated investigations into the utilization of fine Z-pins and reduced insertion densities. However, the manufacturing process of fine Z-pins introduces voids and fiber fracture defects, leading to a decline in Z-pin quality and consequently affecting the efficacy of interlaminar toughening. In this research, defects in the Z-pin preparation process were identified through extensive experiments, and pre-stressing is applied during the curing of the Z-pin, leading to the successful development of the high-quality 80 μm and 90 μm diameter Z-pins. Furthermore, this research extensively examines the impact of these two defects on the quality of Z-pins and provides a model to determine the optimal Z-pin size corresponding to various insertion depths under the existing processing conditions. For example, according to the model, the optimal Z-pin size for laminates with a thickness of 4 mm is 90 μm. This ensures that the Z-pin’s failure mode remains at the critical transition between pull-out and fracture, thereby maximizing its tensile strength utilization. The research findings are validated through experimental verification, thus providing insights for the application of ultrafine Z-pins.