Bond Behavior, Fatigue Degradation, and Environmental Durability of Externally Bonded FRP-to-Steel Systems: A Comprehensive Review

Externally bonded fiber-reinforced polymer systems are used to strengthen steel structures, but their long-term reliability depends on the integrity of the FRP–adhesive–steel interfaces. Existing studies have examined static bond behavior, fatigue performance, and environmental durability as separate topics, while an integrated interface-centered synthesis remains limited. This paper reviews the bond behavior, fatigue degradation, and environmental durability of externally bonded FRP-to-steel systems. It compares specimen configurations, test and measurement methods, failure modes, governing factors, bond–slip relationships, fatigue degradation characteristics, environmental degradation mechanisms, residual bond capacity, and predictive models. The evidence indicates that static interfacial performance is governed by material properties, bond geometry, steel surface condition, corrosion, and bond defects. Repeated loading leads to stiffness degradation, slip accumulation, strength loss, and debonding, whereas the effects of variable-amplitude loading and the transferability of existing fatigue models remain insufficiently understood. Environmental exposure alters adhesive properties, interfacial adhesion, steel surface condition, and failure mechanisms, while existing durability models are calibrated for specific material systems and exposure regimes. Overall, a unified framework linking static bond behavior with fatigue- and environment-induced degradation remains to be established. The findings provide a basis for the characterization, modeling, and performance assessment of externally bonded FRP-to-steel systems.