Enhancing tensile performance of CFRP–steel single-lap joints: Effects of spew fillets, surface grooving, and adhesive layer type and thickness

Surface modification methods, such as grooving or notching, have shown promise in improving the performance of adhesively bonded single-lap joints (SLJs). However, most studies on bonded metal joints have focused on thin adhesive layers and shallow surface modifications, which are more relevant to lightweight applications in industries such as aerospace and automotive. Conversely, civil engineering applications often require thicker steel substrates and adhesives, allowing for deeper grooves without compromising the adherend’s strength. Although spew fillets have been widely studied to enhance joint strength, their combination with surface modifications remains underexplored. This study investigated the effects of spew fillets, groove configurations, and adhesive layer type and thickness on the tensile performance of CFRP-steel SLJs. Grooves of 1 mm depth were created on the steel adherent’s bond surface in two configurations: edge grooves only and a combination of edge and interior grooves. The effects of these factors on edge bending moment, damage initiation, post-initiation behavior, and ultimate load were evaluated. Finite-element analysis was used to examine adhesive stress distributions and concentrations at the overlap ends. Experimental and analytical results showed that grooves altered adhesive stress distributions, reducing shear stress concentrations at the adhesive–adherend interface, while spew fillets mitigated peel stress, particularly in stiff adhesives. A combination of spew fillets and grooves resulted in superior load capacity compared to either method applied individually. The ultimate load of squared-end specimens without grooves increased by 24.6 %–29.7 % and 55.0 %–59.3 % for stiff and relatively ductile adhesives, respectively, under the combined method.