Bond behavior of CFRP-to-steel interfaces under quasi-static cyclic loading at mild temperatures

Carbon fiber-reinforced polymer (CFRP)-strengthened steel structures are likely to endure mild service temperatures and cyclic loading throughout their service lives. The bonding interface between CFRP and steel contributes to the energy absorption and suffers from the damage accumulation. However, investigations into the quasi-static cyclic interfacial behavior between CFRP and steel at mild temperatures remain scarce, leading to insecure strengthening designs. This paper presents an experimental study involving 32 pull-out tests on CFRP-to-steel bonded joints under both static and quasi-static cyclic loading across temperatures ranging from 25 °C to 75 °C. A novel holding device was designed to achieve mode-II stress conditions at the bonding interface. Measurements were taken for load-displacement behaviors at the loaded end and CFRP strain distributions. The failure modes, structural performance, and interfacial bond behaviors were analyzed and compared. The results indicated that interfacial shear stiffness and peak shear stress decreased with increasing temperatures. The fracture energy increased by 66.2 % at 45 °C compared to 25 °C, but then dropped by 77.4 % at 75 °C. Additionally, the global energy dissipation capacity under quasi-static cyclic loading exhibited a 20-fold improvement at 45 °C, but deteriorated with further temperature increases. A direct correlation between the damage parameter and interfacial energy dissipation was observed. New formulas were proposed to describe the relationships between interfacial damage parameters, slip, and dissipated energy at mild temperatures.