Hygrothermal aging effect on interfacial bonding performance of CFRP-to-steel joints with polyurethane-modified epoxy adhesive

Epoxy adhesives are widely used in CFRP-strengthened steel structures, yet their inherent brittleness and susceptibility to hygrothermal degradation remain critical concerns for long-term durability. Polyurethane (PU)-modified epoxy adhesives can effectively enhance the strength and toughness of carbon fiber reinforced polymer (CFRP)-to-steel joints. In this work, the hygrothermal aging effect on the interfacial bonding performance of CFRP-to-Steel joints with 0 wt%, 10 wt%, and 20 wt% PU-modified epoxy adhesives (designated as P0, P1, and P2) was investigated to evaluate the durability of PU-modified epoxy adhesive. Two hygrothermal aging environments were simulated by water bath immersion at 23°C and 50°C. The dynamic mechanical properties and tensile properties of the adhesive under the hygrothermal aging environments were investigated through dynamic mechanical analysis (DMA) and tensile mechanical tests. The microstructural morphology of the fracture surfaces before and after aging was observed using scanning electron microscopy (SEM). Tensile shear tests were conducted on the CFRP-steel double-lap joints to determine their ultimate load, failure mode, and interfacial shear stress distribution. Furthermore, the interfacial bond-slip constitutive relationship was established based on strain data. The results indicated that the moisture absorption behavior of the adhesives followed Fick’s law of diffusion. The incorporation of 10 wt% PU can improve the elongation at break and interfacial toughness of the adhesive while suppressing moisture diffusion. The glass transition temperature (Tg) of P1 and P2 continuously decreased with aging time due to the plasticization effect. The tensile strength of P1 slightly increased at the early stage and then gradually decreased, and the strength of P2 continuously deteriorated. For the samples of CFRP-steel joints subjected to hygrothermal aging, the failure mode transitioned from cohesive failure to interfacial debonding. All the bond-slip curves became bilinear, and the interfacial fracture energy significantly decreased. Specifically, the P1 joint exhibited relatively superior performance retention, with an ultimate load of 28.04 kN, which was significantly higher than that of P0 and P2. This study reveals the hygrothermal aging effect on the performance of PU-modified epoxy adhesives, and provides an optimum addition amount of PU for CFRP-to-steel joints using the modified epoxy adhesives in hygrothermal conditions.