Experimental study on the fracturing process of thermally treated granite under mixed mode I-II loading

In rock mechanics and rock engineering, the fracture characteristics of rock under complex load and temperature conditions are critical. Through the three-point bending method of half disk granite samples in mixed I-II mode, the influence of thermal effect on fracture process and rock fracture parameters was investigated. The results suggested that high temperatures weaken both the physical and fracture behaviors of granite specimens, containing fracture toughness, fracture load, as well as P-wave velocity. The rock fracture toughness under pure mode I and mode II loading declines by 88.3% and 86.7%, respectively, under a temperature of 25-800 degrees C. At room temperature, the fracture toughness gradually decreases by 61.1% as the angle beta changes from 0 degrees to54 degrees. Based on the analysis of point energy dispersive spectrum, a feasible method was proposed to distinguish the transgranular and intergranular cracks. For thermally treated granite specimens, the generalized maximum tangential stress criterion can effectively display the characteristics of mixed mode fracture. In addition, the fracturing process of granite was analyzed in four stages based on the acoustic and optical information. The variations in the specimen surface's strain field were displayed and the maximum principal strain of the crack tip was quantitatively analyzed. Under loading, the mutation points on the curves of maximum principal strain at the region of crack tip can be found when the temperature is not over 600 degrees C, whereas they disappear at temperature of higher than 600 degrees C.