Experimental and analytical study for developing flexural-tensile fatigue performance criteria for civil construction CFRP reinforcements

This study conducted experimental and analytical research to establish fatigue performance criteria for CFRP reinforcements under flexural-tensile cyclic loading conditions. To investigate the influence of fiber-volume fraction on fatigue behavior, additional fatigue tests were performed using specimens with a 60% fiber-volume fraction. The results were incorporated into and calibrated within the fatigue limit curve proposed in previous studies, enhancing its reliability. The fatigue test results indicated that higher fiber content reduced fatigue resistance under high load conditions, while it improved fatigue resistance under low load conditions. This suggests that, under high load levels, the resin matrix primarily governs the response, and early interfacial debonding between fibers and resin significantly affects fatigue resistance. In contrast, under low load levels, the response remains within the elastic range of the resin, and the stiffness of the fibers becomes dominant. Based on these findings, it is recommended to select an appropriate fiber-volume fraction depending on the applied load conditions during the design phase. If selecting the optimal fiber-volume fraction is not feasible, adjusting it to approximately 50% is considered suitable to ensure the required fatigue performance. In the analytical part of this study, D'Amore's fatigue life prediction model was applied. However, the parameters proposed in the original study (α= 0.184, β= 0.200) resulted in an RMSE of approximately 3.14, indicating that they did not adequately reflect the experimental results obtained in this study. Therefore, following the parameter estimation procedure suggested in previous research, new parameters α= 08843, β = 0.2019 were derived. These proposed parameters resulted in a 75.48% reduction in prediction error compared to the original parameters, thereby improving the reliability of the constructed fatigue limit curve and enhancing its applicability in practical intends to address them through continued research in the future.