The effect of cyclic heating and cooling on mechanical and deformation responses of granites under preset angle shearing

In the process of geothermal energy exploitation, granites undergo cyclic heating and water cooling due to the heat conduction from distant rock mass to the cooled rock. The mechanical response, deformation and acoustic emission (AE) characteristics of granite treated by cyclic heating (150–300 °C) and water cooling (1–15) under the coupling action of compressive and shear stress were analyzed by preset angle shear tests in three preset angles (45°, 55°, 65°) at macroscopic level. Given that the peak strength of granite shows an accelerating downward trend with the increase of heating temperature and cycle times. The cohesion of granite declines, and the internal friction angle increases in general. AE activities are active throughout the loading process, which shows ductile failure characteristics of granite obviously. The accumulative AE events decrease by 69.3% with increasing heating temperature (150–600 °C), and 82.4% with increasing cycle times (1–15). The crack initiation area gradually shifts from the two ends of sample to the middle area with the increasing thermal damage. Shear behaviors inside the granite start earlier and last for a longer period, which results in the complex fracture network distribution of turtle shape and the shearing failure of “Z” shape on the surface of 600-1 sample. Scanning electron microscopy (SEM) was conducted to reveal the microscopic mechanism of thermal damage. Given that the intergranular and transgranular cracks induced by the uneven expansion of particles connect with each other and form a complex crack network as the thermal damage increasing. The continuous heating with high temperature causes minerals to melt and deform, resulting in the decrease of bonding strength between particles. All above results can provide references for the analysis and prediction of rock stability of granite in Enhanced Geothermal Systems (EGS) project.