Connection behavior and failure mechanisms of FRP sleeved joints with bonded inserts versus conventional bolted joints

A method using bonded metallic inserts to improve fiber-reinforced polymer (FRP) bolted joint strength has been proposed, however, the improvement mechanisms and effectiveness across various configurations remain unclear, especially for different FRP fiber architectures. This study investigates sleeved bolted joints with bonded inserts in unidirectional (UD) and multi-directional (MD) FRP profiles, considering all-FRP and hybrid FRP-steel configurations, to evaluate the effects of adhesives and insert materials. Results show that off-axis tie action in MD fiber architecture enables progressive bearing failure and enhances conventional joint strength by 63 % compared to UD counterparts. In the wide range of sleeved joints, aluminum inserts with high-strength, high-modulus adhesive improve joint strength by 17.1 % in UD and 25.4 % in MD joints for all-FRP configurations. For hybrid FRP-steel configurations, steel inserts with high-strength adhesive increase joint strength by 11.1 % in MD joints. These findings suggest an adhesive-insert synergy improvement mechanism: reducing bearing stress through adhesive dispersing and redistributing stress effects coupled with insert deformation controlling to prevent premature adhesive failure. Accordingly, high-strength, high-modulus adhesives strategy and insert-connection plate stiffness matching strategy are proposed for optimal reinforcement. Furthermore, MD fiber architecture amplify bonded insert benefits due to more uniform stress distribution under bearing failure, where longitudinal support from 0° and 45° fibers and lateral tie-action from ±45° and 90° fibers disperse bearing stresses.