Online fatigue performance evaluation method for smart CFRP-strengthened steel beam structures incorporating FBG monitoring information

Steel beams are susceptible to fatigue damage under long-term cyclic loading. Carbon fiber reinforced polymer (CFRP) strengthening technology has been widely adopted to improve the fatigue performance of steel beams due to its high strength-to-weight ratio, corrosion resistance, and ease of construction. However, existing fatigue performance analysis methods for CFRP-strengthened steel beams still face challenges in damage-state quantification, experience-based life evaluation, and dynamic updating based on in-service monitoring information. Previous studies have generally focused on individual fatigue mechanisms or FBG-based strain and damage monitoring, while the structural response evolution associated with the combined deterioration of the steel beam, CFRP plate, and CFRP–steel interface has received limited attention in fatigue performance evaluation. To address these issues, this study focuses on smart CFRP-strengthened steel beam structures integrated with surface-attached and embedded fiber Bragg grating (FBG) sensors, and proposes a fatigue performance evaluation method considering fatigue damage of the steel beam, CFRP material degradation, and interfacial damage evolution. The proposed method was validated through three-point bending fatigue tests. Good agreement has been observed between the theoretical predictions and experimental measurements in terms of strain evolution trends, and the quantitative comparison yields Pearson correlation coefficients ranging from 0.669 to 0.986 and RMSE values ranging from 9.732 με to 85.808 με, indicating that the proposed method can reasonably characterize the coupled load-carrying degradation process of smart CFRP-strengthened steel beams under fatigue loading. The experimental results show that the surface-attached and embedded FBG sensing systems can effectively capture the strain responses of the steel beam and CFRP plate under static and fatigue loading conditions. Local discrepancies in strain magnitude at some measuring points are mainly attributed to experimental implementation uncertainties, local interfacial defects, and strain-transfer errors of FBG sensors. These findings demonstrate that the proposed method is suitable for monitoring-information-incorporated fatigue performance evaluation of smart CFRP-strengthened steel beam structures and can support online service-life assessment by linking structural degradation state evolution with FBG monitoring information.