Influence of joint configuration and bolt size on the load-bearing capacity and failure mechanisms of CFRP bolted joints

The load-bearing performance of bolted joints in open-hole CFRP laminates is critical to the structural integrity of composite assemblies. However, existing studies typically investigate the independent contribution of laminate and bolt, neglecting their coupled interaction and resulting in conservative predictions. This study aims to systematically investigate the influence of joint type and bolt size on the coupled load-bearing mechanical properties and failure modes of CFRP bolted joints. Single-lap and double-shear bolted joint configurations with various bolt sizes were examined through quasi-static tensile tests and nonlinear finite element analysis to obtain load-displacement curves, characterize mechanical properties and identify damage evolution as well as failure modes. The results indicate that the double-shear bolted joint exhibits higher ultimate load and failure displacement than the single-lap one. A transition in failure mechanism is observed for the M3 bolted joint. The failure mode shifts from brittle bolt fracture in single-lap configurations to bolt bending accompanied by fiber extrusion in double-shear configurations, effectively delaying bolt failure. Mechanical properties show a non-monotonic dependence on bolt size, initially increasing and then decreasing. The optimal bolt size differs between configurations. M6 maximizes ultimate load, initial stiffness, and bearing stiffness in single-lap joints, whereas in double-shear joints, the ultimate load peaks at M5 and failure displacement at M6. Matrix damage dominates in all cases, and the failure location progressively shifts from the hole edge to the laminate sides with increasing bolt size, a trend accelerated in double-shear configurations.