Fatigue behavior of CFRP bars at different stress ratios and long-term sensing stability of embedded FBG sensors

This study investigates the tension–tension fatigue performance of pultruded carbon fiber-reinforced polymer (CFRP) bars, employing expansion cement grout as a load transfer medium (LTM) for bonded-type anchorages. The transverse shear strength, apparent interlaminar shear strength, and ultimate tensile strength (UTS) of CFRP bars were first quantified. Subsequently, tension–tension fatigue tests under three stress ratios (R = 0.3, 0.5, and 0.65) were conducted at 25 °C, respectively, generating corresponding stress–life (S-N) and probabilistic stress–life (P-S-N) curves through Basquin’s model and Weibull distribution fitting. Notably, the fatigue limit boundary was determined based on the Goodman relation, with safe stress amplitudes identified as 385 MPa (R = 0.3), 300 MPa (R = 0.5), and 245 MPa (R = 0.65). Throughout the fatigue testing, the bonded-type anchorage demonstrated exceptional performance, exhibiting cumulative residual displacements within 0.15 mm after 2 million load cycles. Furthermore, the fatigue damage accumulation in CFRP bars manifests a highly nonlinear trend: minimal damage occurs during the initial 90 % of fatigue life, followed by rapid degradation in the final 10 % of fatigue life prior to fracture. A model has been proposed which can effectively describe the nonlinear damage evolution. Additionally, the long-term sensing performance of carbon fiber reinforced polymer-optical fiber Bragg grating (CFRP-OFBG) bars was evaluated. No apparent Bragg wavelength drift was observed over 2 million fatigue cycles, confirming their sensing stability.