To evaluate the long-term interfacial performance of carbon fiber-reinforced polymer (CFRP) repaired steel structures in coastal environments, it is essential to incorporate the coupled effects of fatigue loading and marine atmospheric exposure into the interfacial bond-slip model. In this study, interfacial fatigue damage (FD) was introduced via 300 cycles of cyclic loading with high stress amplitude, and the marine atmospheric environment was simulated using 90-day wet-dry cycling (WDC) in a 3.5% NaCl solution. Accordingly, the individual and combined influences of FD and WDC on the interfacial performance of CFRP-steel interfaces were investigated. Particularly, the evolution of interfacial bond strength and failure modes under different conditions was analyzed in detail. On this basis, interfacial bond-slip constitutive models were developed to account for the coupling effects of FD and WDC, with model parameters derived from CFRP strain distribution characteristics under various scenarios. Furthermore, the model was applied to numerically simulate the intermediate crack debonding process in prestressed CFRP-strengthened beams under identical working conditions. By comparing the simulated and experimental values of interfacial debonding load and corresponding deflection, with deviations within approximately 10%, the accuracy of the models was verified. The proposed bond–slip models provide practical references for the durability assessment and maintenance-oriented design of CFRP-strengthened steel structures exposed to coupled fatigue loading and marine environments.
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