Effects of bolt preload relaxation on the mechanical performance of composite structures

The durability of bolted composite joints has long been a significant concern within the field. However, the specific influence of transverse vibration relaxation on bolted composite joints has not been extensively studied. This study aims to investigate the effects of transverse vibration relaxation on bolted composite joints. A series of transverse vibration experiments were conducted to investigate the effect of initial preload, displacement load, and lubrication position on bolt preload relaxation. Additionally, tensile tests were performed on composite joints after relaxation and without relaxation to evaluate mechanical properties quantitatively. A finite element model was established to reveal the mechanism of damage evolution. The results indicate that displacement load and thread lubrication have the most significant influence on bolt preload relaxation. The clamping force of the composite structure generated by the smaller preload force has a limited effect on damage suppression during the tensile process. The relaxation of bolt preload can be effectively reduced by increasing the initial preload properly. The tensile strength of composite laminated structures with 10%, 22%, and 32% relaxation (10.4 kN initial preload) decreased by 5%, 6%, and 11%, respectively. Transverse vibration relaxation affects the tensile strength of composite structures, which is caused by the decay of preload. In contrast, the damage to the hole wall of the connection domain caused by transverse vibration almost does not affect the bearing capacity of the composite joints. Overall, this research contributes to the understanding of bolted composite joints’ durability by uncovering the novel effects of transverse vibration relaxation and providing valuable insights for design and optimization strategies in composite joint applications.