Experimental Study on the True-Triaxial Mechanical Properties and Fracture Mechanisms of Granite Subjected to Cyclic Thermal Shock

During reservoir stimulation and long-term operation of Enhanced Geothermal Systems (EGSs), repeated injection of cold fluids induces cyclic thermal shock in the surrounding rock mass, leading to progressive modification of mechanical properties and fracture behavior. However, the combined effects of cyclic thermal shock and true-triaxial stress conditions on granite strength and failure characteristics remain inadequately quantified. In this study, a series of true-triaxial compression tests were conducted on granite specimens subjected to cyclic thermal shock at 400 °C. Thermal shock cycles of 0, 1, 5, 10, and 15 were considered in conjunction with intermediate principal stress levels of 5, 20, 30, and 50 MPa to systematically evaluate their coupled influence on characteristic stresses and macroscopic failure behavior. The results show that the peak intensity increases with the rise of the intermediate principal stress, but with the increase in the number of thermal shocks, it first increases and then decreases. Macroscopic failure is dominated by asymmetric V-shaped fracture surfaces, roughly oriented along the σ2 direction. As the intermediate principal stress increases, the failure mode transitions from tensile–shear mixed failure to shear-dominated failure, whereas thermal cycling promotes the persistence of tensile–shear cracking even under relatively high σ2 conditions. Based on these observations, a modified Mogi–Coulomb strength criterion that accounts for thermal shock-induced damage is proposed to describe granite strength under true-triaxial stress conditions. The research results can provide a theoretical basis for optimizing the design of hydraulic fracturing in hot dry rock and evaluating reservoir stability.