Numerical study on failure mechanism and acoustic emission characteristics of granite after thermal treatment

To investigate the strength characteristics and failure mechanism of granite after thermal treatment are critical for geothermal energy storage and development. Acoustic emission (AE) is widely used to deduce the process of rock crack generation, development and penetration in laboratory tests, thus revealing the mechanism of rock failure. However, previous investigations have shown that laboratory tests cannot directly observe the interaction of thermal cracks and thermal stress, and more than 90\(\%\) of AE tensile failure sources cannot be captured. This paper investigates the generation mechanism of thermal cracks and thermal stress distribution in thermally treated specimens using the discrete element method. After that, the evolution of AE failure sources is quantitatively analyzed by the moment tensor inversion results. The results showed that: (1) Thermal cracks destroy the internal structure of the specimen, thus weakening its mechanical properties. The number of thermal cracks increases with the temperature, further aggravating the damage to the mechanical properties of specimens; (2) as the temperature increases, the failure mode of the specimen changes from splitting failure to shear failure. Moment tensor inversion revealed that tensile failure dominated the final damage of samples. The shear and compaction failure sources increase with temperature, while tensile failure sources decrease; (3) the b value increased by 215\(\%\) from 25 \(^{\circ }\)C to 1000 \(^{\circ }\)C. As the number of microcracks in a single AE event increases, the AE frequency decays exponentially, and most AE events have 1–5 microcracks.