Residual strength and load redistribution in multi-bolted single lap joints with simulated environmentally assisted cracks

Environmentally assisted cracking (EAC) in bolted joints poses a serious threat to the structural integrity of aerospace components. Despite widespread industry awareness, the mechanical consequences of spanwise EAC on load sharing in multi-bolt configurations remain insufficiently characterised. This study presents the first systematic experimental investigation into the residual strength and internal load redistribution of three-bolt single-lap shear joints containing predefined, EAC-like cracks. To isolate geometric effects from material-specific corrosion behaviour, 1050 aluminium was employed as a model material, and artificial cracks ranging from 15 to 30 mm were introduced at the central fastener hole. Quasi-static tensile testing, supported by strain-gauge instrumentation, a validated three-dimensional finite element model, and a simplified analytical model, was used to evaluate joint performance. All cracked specimens retained peak load capacities comparable to uncracked controls, indicating significant structural redundancy. However, this apparent resilience masked a critical shift in internal force flow: the fastener adjacent to the crack experienced up to 60 % load reduction, with adjacent bolts compensating for the loss. Importantly, all joints failed by abrupt net-section fracture at the outermost bolt, with no evidence of progressive bearing failure. These results challenge conventional assumptions of damage tolerance by revealing that preserved load capacity can coexist with unpredictable and brittle failure modes. The findings provide experimentally validated benchmarks for stiffness degradation and load sharing in damaged joints, offering guidance for the design, analysis, and maintenance of EAC-prone aerospace structures.