Fatigue damage model of stone matrix asphalt with polymer modified binder based on tensile strain evolution and residual strength degradation using digital image correlation methods

In this paper, the residual strength degradation was termed as the damage variable to evaluate the fatigue damage process of asphalt mixtures. A non-notched semi-circular bending (SCB) residual strength test with varying pre-loading cycles was designed to capture the residual strength of one stone matrix asphalt (SMA) mixture at 15 °C. The digital image correlation (DIC) method was utilized to characterize the strain field of specimens in the semi-circular bending (SCB) fatigue test under varying loading levels at 15 °C. According to the results of this study, the residual strength decreases smoothly before faster and faster with the increase of loading cycles, which fits well with an exponential model. A non-linear damage evolution model is built by the residual strength degradation law. Based on the transverse distribution characteristics of tensile strain at the bottom of SCB specimens, a 5 cm cracking zone in the center of specimens is determined and the average tensile strain is captured by DIC systems. The curves of average tensile strain versus loading cycles could also fit well with an exponential model. By using the normalized loading cycles (N/Nf) as the intermediate variable, the relationship between tensile strain evolution and residual strength degradation could be deduced and the residual strength in fatigue damage evolution model could be substituted by tensile strain, which means the average tensile strain has the potential to be a material state that dictates accumulation of damage.