Study on the durability of CFRP-to-steel joints under neutral salt spray exposure

This study investigated the durability of CFRP-to-steel bonded joints. First, durability tests were conducted on adhesive, CFRP sheets, and CFRP-to-steel double-lap joints in a neutral salt spray environment. Three types of influencing factors were considered in the joints, including steel surface treatment methods, number of CFRP layers, and types of CFRP sheets. The salt spray exposure durations were set at 500 h, 1000 h, 1500 h, and 2000 h. Subsequently, tensile tests were conducted on adhesive, CFRP sheets, and CFRP-to-steel joints. The effects of neutral salt spray on appearance, failure modes, and mechanical properties of adhesive and CFRP sheets were discussed. The influence of the steel plate surface treatment methods, the number of CFRP layers, and the types of CFRP sheets on shear strength, stiffness, and fracture energy of CFRP-to-steel joints, as well as their degradation mechanisms, were analyzed. Finally, aging duration-dependent equations were proposed by regression analysis to calculate the normalized mechanical performance of the adhesive, CFRP sheets, and CFRP-to-steel joints. The results indicate that the degradation of the adhesive's mechanical properties was much greater than that of the CFRP sheets over the same aging duration. The deterioration in the bonding performance of CFRP-to-steel joints was related to the degradation in the mechanical properties of adhesive-steel interface and adhesive. The surface treatment of the steel plate was crucial to the mechanical performance and durability of the joints. A rough steel plate surface, dense CFRP sheets, and multiple layers of CFRP could enhance the durability of CFRP-to-steel joints.