Spatiotemporal damage evolution in thermo-mechanically coupled rock using acoustic emission

Deep geological engineering presents substantial challenges in understanding the mechanical mechanisms driving anisotropic damage evolution in reservoir rocks under thermo-mechanical coupling, which directly impact engineering safety and economic efficiency. This study investigates the mechanical behavior and damage evolution of granite under real-time thermo-mechanical coupling at 120 °C during uniaxial compression, using active-passive acoustic emission (AE) tomography to examine the spatiotemporal damage mechanisms. The results indicate that thermal expansion causes the closure of primary cracks at temperatures up to 67 °C in localized regions, thereby enhancing wave velocity, while simultaneously inducing thermal cracking in other areas, which results in material weakening. The thermal crack network prolongs the stress redistribution phase and facilitates the formation of three-dimensional fracture surfaces, thereby significantly reducing the risk of rock failure. To address the limited sensitivity of traditional AE parameters to high-temperature damage, this study introduces the Ring-down to Energy Ratio (RER). The RER reveals distinct precursory failure signals, including a continuous decrease in the high-temperature specimen and abrupt increases in the room temperature specimen, thereby demonstrating its potential as a reliable indicator for thermal-mechanical rock failure. Spatiotemporal velocity field analysis further reveals that thermal crack propagation preferentially occurs in areas of low porosity. It also identifies a three-stage damage evolution pattern: dominant growth rate during crack closure, a significant increase in the attenuation rate during the elastic deformation stage, and secondary velocity growth during crack propagation. These findings elucidate the damage mechanism of deep rock masses under thermo-mechanical coupling and provide a dynamic monitoring approach.