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

Abstract The gradual prominence of environmental problems has brought infinite possibilities for the development and utilization of geothermal resources. Due to the discontinues of rock mass widely distributed and the huge earth stress in deep underground, rock failures which induced by compression and shear occur widely in rock mass. The mechanical and deformation responses of hot dry rock (HDR) under the action of compression and shear in Enhanced Geothermal Systems (EGS) are significant to investigate. We carried out cyclic heating and water cooling treatment on granite samples at 4 levels of heating temperatures (T = 150 ℃, 300 ℃, 450 ℃, 600 ℃) and 4 levels of cycle times (N = 1, 5, 10, 15). The preset angle shear (PAS) tests were carried out with three preset shear angles (α = 45°, 55°, 65°) for the samples of various T and N. The evolution law of shear strength parameters of granite for various T and N was obtained. Acoustic emission (AE) characteristics, the evolution of strain field, and brittle-ductile transformation were obtained by using acoustic emission and digital image correlation (DIC) techniques. Then, the microscopic mechanism of thermal damage was revealed by scanning electron microscopy at a microscopic level. It turns out that with the increase of T and N, the thermal damage degree of granite increases, and the peak load shows an accelerating downward trend. The cohesion c of granite declines, and the internal friction angle φ increases gradually as T and N increasing. The energy consumed by the closure of thermal induced cracks and friction between particles leads to a decrease of elastic energy stored, which eventually results in a decrease of accumulative AE events. When N = 1, T increases from 150°C to 600 ℃, the accumulative AE events decreased by 69.3%. When T = 450 ℃, N increased from 1 to 15, the accumulative AE events decreased by 82.4%. AE activities are generally active throughout the loading process, and the granite shows a ductile failure characteristic obviously. Under the action of compression and shear, with the increase of T, the crack initiation area gradually shifts from the two ends of the sample to the middle area. When T = 600 ℃ (N = 1) and N = 15 (T = 450 ℃), it finally shows a complex fracture network distribution of turtle shape and the shearing failure of "Z” shape on the sample surface. The crack density of the final failure cracks on sample surface also increases with the increase of T and N, and the damage of granite is more serious, showing strong ductile failure characteristics. At the micro level, the intergranular and transgranular cracks connect with each other and form a complex crack network as T and N increases, causing blocky matrix to peel off from the crystal. The continuous heating with high temperature causes minerals to melt and deform, resulting in the decrease of bonding strength between particles. The mechanical properties deteriorate significantly, and the ductile failure characteristics are enhanced.