Quasi-static tensile evolution process of CFRP-aluminium alloy bolted joints by preload torque

The joining of dissimilar materials, represented by composites and aluminum alloys, is one of the pathways to achieve structural lightweighting. Preload torque affects the mechanical performance of the structure, making the study of failure evolution behavior in quasi-static tensile loading of carbon fiber reinforced polymer (CFRP) and aluminum alloy bolted connections crucial. This study investigates the effect of preload torque (4 N·m, 8 N·m, and 12 N·m) on the quasi-static tensile evolution of single-lap CFRP-6082-T6 aluminum alloy bolted joints. Through quasi-static tensile tests combined with digital image correlation (DIC) and scanning electron microscopy (SEM), the joint performance was systematically analyzed. Results show that increasing preload torque enhances material stiffness: peak load rises from 10.94 kN to 11.51 kN, and initial damage load increases from 5.17 kN to 8.86 kN. DIC analysis reveals that lower preload torque exacerbates axial compressive strain concentration below the bolt hole and transverse strain concentration at the hole edge, increasing out-of-plane displacement. Higher preload torque mitigates these effects by improving static friction and expanding bolt-hole contact area. Failure modes consistently exhibit net tension failure in CFRP laminates, with cracks initiating at the bolt hole edge and propagating perpendicular to the tensile direction. This study confirms that appropriate preload torque enhances load-bearing capacity and delays initial damage, providing theoretical and experimental support for optimal design of heterogeneous material bolted joints.