Experimental Study of Adhesively Bonded Single Lap Joint Behaviour in CFRP-to-CFRP, Al-to-Al, and CFRP-to-Al Configurations

This study evaluates the tensile performance of single-lap joints with three configurations: CFRP-to-CFRP, Al-to-Al, and CFRP-to-Al. The effect of CFRP and aluminum adherends on joint strength was analyzed based on surface characteristics and mechanical properties. CFRP substrates were fabricated using vacuum-assisted resin infusion with vinyl ester resin and unidirectional carbon fibers, while aluminum plates were cut to ASTM D5868 standards. Epoxy adhesive was employed, and surface preparation included 800-grit sanding to enhance bond quality. Surface roughness, wettability, and morphology were characterized using a surface roughness tester, contact angle, and atomic force microscopy measurements. Sanding significantly reduced surface roughness (Ra) for CFRP from 6.48 ± 0.24 µm to 1.49 ± 0.19 µm, improving surface wettability. Aluminum exhibited lower contact angles than CFRP, reflecting its distinct surface properties. Tensile tests of the epoxy adhesive revealed an ultimate tensile strength of 54.30 ± 2.20 MPa and a tensile modulus of 2.21 ± 0.08 GPa, confirming its suitability for SLJs. Lap shear tests demonstrated that CFRP-to-CFRP joints achieved the highest average shear strength (5.05 ± 0.64 MPa), followed by Al-to-Al (3.03 ± 0.57 MPa) and CFRP-to-Al (3.52 ± 0.75 MPa). The CFRP-to-Al configuration exhibited the highest variability due to material incompatibilities. Failure analysis indicated mixed adhesive and cohesive failure modes across all configurations, with CFRP-to-CFRP joints showing superior bond consistency. These findings highlight the critical role of material compatibility and surface treatment in optimizing SLJ performance for composite and dissimilar material joints.