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.

The stress–strain curves of red sandstone are obtained by performing conventional triaxial compression tests after freeze–thaw cycling. The loss rates of red sandstone under the action of freeze–thaw cycles are expressed by the extreme stress and elastic modulus. The logistic curve is an S-shaped curve, which is consistent with the damage development trend of rock subjected to freeze–thaw cycling during the loading process. Therefore, the logistic equation is adopted and modified to establish a damage constitutive model of rock under freeze–thaw and load conditions. The results show that with the increase in the number of freeze–thaw cycles, the damage to the rock is aggravated, the loss rates obtained from the extreme stress and elastic modulus gradually increase, the compressive strength decreases, and the plastic characteristics are enhanced. The confining pressure suppresses the damage of the rock, which macroscopically manifests as the enhancement of the compressive strength and plastic characteristics. The constitutive model derived using the modified logistic equation can effectively reflect the deformation and failure process of rock under freeze–thaw and load conditions. The model can be simply established, and the expressions of the model parameters can be obtained through the test parameters. Hence, the model can be widely used.