Effect of adhesive thickness on mechanical properties of CFRP/Al single-lap adhesive-rivet hybrid joints

To investigate the effect of adhesive thickness on the mechanical properties of carbon fiber reinforced polymer (CFRP) and aluminum alloy single-lap adhesive-rivet hybrid joints, single-rivet and double-rivet joint specimens with adhesive thicknesses of 0.2 mm, 0.3 mm, and 0.4 mm were fabricated. Through quasi-static tensile tests combined with digital image correlation (DIC) technology, the load-bearing characteristics, failure displacement, and failure modes of the joints were systematically analyzed. The results indicate that the ultimate load of the double-rivet joints (10 kN) is twice that of the single-rivet joints (5 kN). The increase in adhesive thickness has a nonlinear effect on the failure load of the adhesive layer. When the thickness increases from 0.2 mm to 0.3 mm, the adhesive failure loads of the single- and double-rivet joints are improved by 12% and 14.6%, respectively, but when the thickness increases to 0.4 mm, the load tends to be saturated due to the increased eccentric bending moment or deteriorated stress distribution. In terms of failure modes, the single-rivet joints exhibit progressive failure dominated by CFRP delamination, showing a slow process and large displacement. In contrast, the double-rivet joints show sudden failure due to the instantaneous brittle fracture of CFRP induced by stress concentration. The increase in adhesive thickness significantly improves the failure displacement of the adhesive layer (both single- and double-rivet joints are improved by more than 40%). The effect of adhesive thickness on the mechanical properties and failure modes of different types of adhesive-rivet hybrid joints is revealed, providing guidance for the design of lightweight hybrid connections in fields such as rail transportation.