Increasing Adhesive Layer Thickness Reduces Ultimate Load Capacity in Double-Lap CFRP-to-Steel Joints

Carbon fiber-reinforced polymer (CFRP) strengthening relies on effective load transfer across the bonded interface, yet the influence of adhesive layer thickness remains incompletely quantified. Twelve double-lap CFRP-to-steel joints with adhesive layer thicknesses of 0.5, 1.0, 1.5 and 2.0 mm were tested under displacement-controlled tension. CFRP strain measurements were used to determine interfacial shear stress distributions and local bond–slip responses. All specimens failed abruptly by mixed debonding at the CFRP–adhesive and steel–adhesive interfaces. Increasing the adhesive layer thickness from 0.5 to 2.0 mm reduced the mean ultimate load capacity from 126.24 to 79.42 kN. The mean nominal interfacial shear stress decreased from 8.42 to 5.29 MPa, and the fitted interfacial stiffness decreased by 45.4%. At higher load levels, shear stress concentrated near both bond ends and remained greater at the loaded end. The measured bond–slip curves showed no descending branch, consistent with the brittle response of the low-ductility adhesive. A thickness-dependent analytical model reduced the group-level mean absolute percentage error from 19.0% for the Xia & Teng model to 6.9%. It also reduced the root-mean-square error from 18.7 to 7.3 kN. Within the tested material system and thickness range, thinner adhesive layers therefore provided stiffer load transfer and higher ultimate load capacity.