Experimental study on acoustic emission characteristics of high temperature thermal shock directional fracturing of granite with different heating conditions

Abstract

High temperature thermal shock directional fracturing of hard rock has tremendous promise in the mining and tunneling industries, but the utilization mechanism is unclear and must be resolved before taking it to the field application stage. In this study, the granite specimens were subjected to bidirectional horizontal loads using the real triaxial pressure testing machine, and the thermal shock fracturing tests were conducted utilizing the homemade thermal shock system. The AE (acoustic emission) system tracked the whole granite fracture process throughout the test in real time. The results showed that the granite‘s fracture time under the high temperature thermal shock was negatively related to the peak heating temperature and heating rate. The spectral analysis of the AE was employed to consider a large number of 20–40 kHz AE signals as a sign of thermal shock damage. By spatial localization of AE events, it was determined that the thermal shock cracks sprouted at the borehole edge in the specimen’s center. In the horizontal direction, the cracks extended from the center to the specimen edge; in the vertical direction, the cracks extended from the specimen surface to the deeper portion of the specimen. The RA-AF value analysis indicated that the granite thermal shock damage mode was a mixed tensile-shear damage mode dominated by tensile damage. In addition, the higher the heating rate, the higher the percentage of tensile cracks. Increasing the heating temperature promoted the formation of shear damage inside the specimen when the heating rate was equal. Analyzing the b-value and βt-value indicated that at low heating rates (10 and 60 °C/min), many micro-cracks formed inside the granite before merging into macro-cracks. However, with the higher heating rate (110 °C/min), the formation of micro-cracks and the macro-cracks within the granite proceeded almost simultaneously. The research results provided some experimental foundation and theoretical guidance for high temperature thermal shock directional fracturing of hard rocks.