The effect of fracture growth rate on the fracture process zone of salt rock after heat treatment

The importance of rate on the mechanical properties of rocks has been widely acknowledged by numerous scholars. Currently, most researchers mainly focus on investigating the influence of loading rate, while there is limited research on the effects of fracture growth rate on mechanical properties such as fracture process zone (FPZ) and fracture toughness of rocks. In order to explore the impact of fracture growth rate on the mechanical properties of rocks, this study conducted three-point bending pure I-mode fracture experiments on salt rocks at temperatures ranging from ambient to 700 °C using different quasi-static loading rates (0.02–2 mm/min). Meanwhile, digital image correlation (DIC) technology was employed to monitor displacement and strain variations, identify and calculate the fracture growth rate and FPZ size, and subsequently compute the corrected fracture toughness. The study discussed the effects of temperature and fracture growth rate on the FPZ and fracture toughness of rocks. The following main conclusions are drawn based on the analyzed results: (1) Temperature and loading rate have a great influence on the onset of FPZ in salt rocks. The higher the temperature and loading rate, the earlier the onset of FPZ. (2) With the increase of the average fracture growth rate, the FPZ length at the peak load of rock salt and the modified fracture toughness increased continuously, and was divided into two stages, in the stage of the crack extension rate generated by the loading rate of 0.02 mm/min-1 mm/min grew faster, and the growth of the crack extension rate generated by the loading rate of more than 1 mm/min became slower. (3) The effect of temperature on the fracture parameters of rock salt is significant, the peak load moment FPZ length and fracture toughness first increase and then slowly decrease with increasing temperature from room temperature to 700℃. The inflection point is at 500℃, where the fracture toughness and FPZ length reach the maximum value. (4) As the loading rate increases the proportion of transgranular fracture in the rock salt fracture surface gradually rises, and the crack extension path becomes smoother and smoother.