Failure analysis of hybrid bonded/bolted joints under cyclic loading in CFRP

This study presents an experimental investigation into the effects of adhesive layers on the mechanical properties and failure mechanisms of carbon fiber composite joints. Monotonic and incremental cyclic tensile tests were conducted on bonded, bolted, and hybrid bonded/bolted (HBB) joints to evaluate their mechanical responses and failure modes. The results indicate that, compared with bolted joints, the incorporation of adhesive layers significantly reduces compressive joint failure. Additionally, HBB joints using high-modulus brittle adhesive exhibit lower strength and stiffness than purely bonded joints. However, the use of low-modulus, high-strength flexible adhesive can effectively reduce stress concentrations and optimize the performance of HBB joints. An enhancement stage in the plastic strain rate was also observed in both bonded and HBB joints during incremental cyclic loading. Acoustic emission (AE) signals were acquired throughout the experiments, while digital image correlation (DIC) techniques were employed to capture displacement and strain fields in the composite plate connection areas. The results further demonstrate that all joint types manifest a distinct Kaiser effect under incremental cyclic loading, and that flexible adhesives promote a more even load distribution across the entire bonding surface, thereby enabling early bolt participation in load sharing.