Thermal effect on the fracture behavior of granite using acoustic emission and digital image correlation: An experimental investigation

The irreversible deterioration of bearing rocks via the thermal effect is a severe challenge faced by underground engineering. A comprehensive understanding of the fracture behavior of rocks subjected to high-temperature treatments can contribute significantly to engineering infrastructure design and safety. We conducted three-point bending tests on semi-circular bend granite samples exposed to high-temperature treatments. Comple-mentary acoustic emission (AE) monitoring and digital image correlation analyses were used to examine the following: (i) AE timing response characteristics associated with microcracks initiation and propagation; (ii) acquired AE waveform frequency and shape parameters, which are closely related to the microcracking scale and mode; and (iii) fracture parameters, including the fracture process zone (FPZ) and crack opening displacement. The in-depth reasons for changes in the fracture behavior were then investigated. According to the results, the decreasing trend of multifractal index Delta alpha indicates that high-temperature treatments reduce the distribution difference in the value of AE counts and make microcracks with relatively low energy more prone to be generated. Above heating temperature of 600 degrees C, microcracking activities tend to be low frequency and high amplitude and shear fracturing gradually becomes dominant. Observations show that the cohesive stress in granite is remarkably reduced with the elevated thermal damage, and the accumulated elastic energy becomes easier to dissipate. This promotes FPZ development and makes granite more prone to cracking and opening. Moreover, the evolution of crack-opening displacement of granites before the peak load is highly correlated with cumulative AE hits, with correlation coefficients ranging from 0.922 to 0.997. Finally, it is found that the equivalent crack length should be adopted in the evaluation of fracture toughness, and the fracture toughness of lower-heat-treated granites is more closely linked to the FPZ length.