Fracture instability and acoustic emission critical slowing down characteristics of rock with hole-shaped flaw under the coupling of high-temperature and cyclic load

In underground coal gasification (UCG) projects, the flow of high-temperature gas leads to the simultaneous exposure of surrounding rock in tunnels to high-temperature and cyclic load, posing risks of rock instability. To investigate the failure modes and precursors of rock instability under high-temperature and cyclic loading, uniaxial compression tests and cyclic loading–unloading tests were conducted, utilizing acoustic emission (AE) technology and discrete element method (DEM). In addition, the microstructural damage of red sandstone after heat treatment was analyzed utilizing scanning electron microscopy (SEM) images. The test results reveal that increasing temperature leads to a delayed appearance of change points in AE counts and b value and more pronounced AE signals. Fatigue loading cannot always cause severe damage to red sandstone at the first cycle of the load level, according to the 25 °C group, 400 °C group, and 600 °C group specimens. RA-AF data distribution reveals that shear crack proportion generated by uniaxial compression in red sandstone subjected to the same temperature treatment is significantly greater than that caused by fatigue loading. Furthermore, the shear crack proportion in red sandstone specimens drops and then climbs with increase in temperatures. Based on the critical slowing down (CSD) theory, this paper analyzes the variance curves and autocorrelation coefficient curves of multiple AE parameters. The sudden drop point of the b value was taken as the starting point of rapid crack propagation, and the mutation points of various AE parameters lagging behind this starting point were searched for. It was found that the change point of AE energy variance is closest to the specimen instability point, which can be employed as a precursor for specimen instability. Heat treatments cause the internal structure of the red sandstone to evolve from a pompon-like structure to a flocculated porous structure. In addition, thermally induced cracks gradually develop and connect.