Damage evolution behaviour of CFRP-Ti hybrid bonded/bolted joints under quasi-static tensile loading based on acoustic emission and digital image correlation

CFRP-Ti hybrid bonded/bolted (HBB) joints are widely used in aerospace structures, yet their damage evolution under quasi-static tensile loading remains insufficiently understood, especially for different bolt configurations. In this study, a damage characterization framework is established by correlating acoustic emission (AE) and digital image correlation (DIC). The damage process is divided into four stages based on AE parameters and mechanical response, from elastic deformation to final failure. AE signals are classified by k-means clustering into four damage-related categories: matrix cracking/metal fracture, adhesive failure/cohesive failure, debonding/delamination, and fiber breakage. The occurrence sequence and signal characteristics of these damage modes are identified from AE energy evolution and characteristic parameters. AE-DIC correlation analysis links AE activity with DIC strain evolution and mechanical response at representative stages, and a quantitative indicator is proposed to evaluate the relative sensitivity of AE and DIC to damage development. Finally, mechanical and stress responses are compared to reveal the effect of bolt configuration on load transfer. The results show that bolt number, spacing, edge distance, and overlap length govern bearing capacity. Compared with the single-bolt joint, multi-bolt joints with larger overlap regions show higher ultimate loads, more dispersed AE activity, and more gradual damage development.