Steel-FRP composite bar (SFCB) is used in structural engineering owing to low weight, high strength, corrosion resistance, and high ductility. However, under combined environmental and mechanical loading, SFCBs may still sustain damage. Although optical fiber-embedded FRP bars have achieved strain sensing, in-situ damage detection for SFCB remains underdeveloped. To address this gap, an optical fiber-steel-FRP composite bar (OF-SFCB) integrating an all-fiber acoustic sensing (AFAS) system was developed. Two optical fibers were embedded within the OF-SFCB. Ultrasonic-detected fiber was inscribed with a fiber Bragg grating (FBG) to monitor strain and acoustic emission (AE). Ultrasonic-excited fiber incorporated photoacoustic transducers to transmit ultrasonic waves. Together, these fibers constitute the AFAS system for ultrasonic testing. Specimens fabricated by vacuum bagging exhibited tensile strengths exceeding 800 MPa and secondary-stiffness behavior. From the FBG strain-sensing characteristic curve, strains of OF-SFCB from 0–13,200 με were monitored. Using sideband filtering of FBG, AE generated by damage of outer FRP during the tensile process of OF-SFCB was continuously monitored. Cumulative AE hit and count curves clearly tracked FRP damage evolution. Feature-based clustering partitioned AE events into three classes over loading time, enabling pattern recognition of FRP damage. The AFAS system also enabled in-situ ultrasonic testing of the steel reinforcement in OF-SFCB under accelerated corrosion. Both the peak-to-peak value and dominant frequency of the ultrasonic signal decreased with corrosion progression. The determination coefficient for the linear fit between peak-to-peak value and mass loss rate is 0.9878. This study provides a novel approach to condition monitoring and damage detection of SFCB.
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