Estimation and warning of fatigue damage of FRP stay cables based on acoustic emission techniques and fractal theory

A damage assessment and warning method for stay cables based on the acoustic emission (AE) technique and fractal theory was developed. First, the AE signal features of Higuchi's fractal dimension (FD) were analyzed at various scales. The analytical results indicated that the FD was associated with the frequency response. Meanwhile, it was found that the curve length of the original signal reflected the fluctuation of the AE signal in the time domain. Both the FD and the curve length of the original signal were related to damage evolution. Based on the above analysis, a damage index, namely, the FD-based damage assessment index, was defined by the fractal features of AE signals generated by damaged structures, including the curve length of the original signal and its FD. Fatigue tests of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) cables with AE sensors were performed to validate the proposed approach. The time-history responses and frequency responses of the AE signals and the corresponding damage modes were analyzed during the entire cyclic loading process. The FD-based damage indices for all the FRP cables were obtained through analysis of the AE signals. The relationships of both the time-history responses and the frequency responses with the FD-based damage index were investigated. The results indicated that the FD-based damage index increased little with the number of loading cycles at the early loading stage but increased dramatically at the final stage of the fatigue test. The results of this article demonstrate that an FD-based damage index can quantify the evolutionary process of fatigue damage in FRP stay cables.