AbstractFatigue damage is a major threat to the safety and integrity of many steel structures. Steel hydraulic structures (SHS), in particular, experience fatigue loading during operation and are exposed to harsh environmental conditions that can further reduce fatigue life through mechanisms. The traditional inspection and repair process for SHS is time-consuming and leads to economic losses. Studies investigating the behavior and advantages of using bonded carbon fiber-reinforced polymer (CFRP) to repair fatigue cracks in SHS are lacking. The main objectives of this study are to increase the bonding of CFRP, investigate the effectiveness of different fiber-reinforced polymers, and test different retrofitting configurations for SHS. In this study, eight large-scale center-cracked panels were tested under constant amplitude mode I fatigue loading that utilized different surrounding environments, repair materials, and retrofitting configurations. Results indicated that the use of both CFRP and basalt fiber-reinforced polymer (BFRP) are both effective at extending fatigue life. Steel retrofitted with full patches of BFRP that cover the crack can have infinite fatigue life. The extent of fatigue life extension was still controlled by the quality of the fiber-reinforced polymers bond to steel; however, bond behavior was significantly improved in comparison to previous underwater applications.