Experimental investigation into influences of Z-pin and deltoid on structural properties and damage tolerance of CFRP T-joints

Composite T-joints bear low-velocity impact in service. The characteristics of their impact response and residual tensile performance are crucial to structural safety but have not been researched in sufficient detail. This study applies the Z-pin pre-hole insertion process in concert with two fillet radii to manufacture four types of carbon fiber reinforced plastics (CFRP) T-joints. The joints were then characterized by using robust low-velocity impact tests at different energies as well as post-impact quasi-static tensile tests. The results show that increasing the size of the deltoid improved the impact resistance of the joints but the deltoid itself was more vulnerable to damage, which reduced its influence on the residual tensile performance of the joints with increasing impact energy. The Z-pin reinforcement in the non-impact region had a limited influence on the low-velocity impact resistance, while its influence on the residual tensile performance of the T-joints was positively correlated with the degree of damage to them after impact. Moreover, the difference in influence between the deltoid and the Z-pin on the structural properties and damage tolerance of the T-joints was closely related to their respective reinforcement mechanisms, and can contribute to the lightweight design of the T-joint. We also used infrared thermography to observe the evolution of damage to the T-joints during the loading process. The significant fracture of the joint was predicted successfully by comparing different infrared characteristics.