Effect of strain rate on the dynamic splitting behavior and fracture surface morphology of granite subjected to high-temperature treatment

In Enhanced Geothermal Systems (EGS) for Hot Dry Rock (HDR) exploitation, the dynamic splitting behavior and fracture surface morphology of thermal storage rock after high-temperature cooling play a crucial role in determining thermal energy extraction efficiency. The rock mass in the open-hole section of the injection well becomes highly susceptible to instability under dynamic loads such as earthquakes and blasting after undergoing high-temperature cooling. Therefore, investigating the dynamic mechanical properties of rocks after high-temperature cooling is of significant engineering importance. Granite is one of the most common rocks in geothermal reservoirs, and this study focuses on granite from a region in South China with abundant HDR resources. Systematically analyzing the splitting behavior, fracture morphology, dynamic strength, deformation characteristics, crack evolution, and failure model of granite under various thermal conditions and impact loading conditions. The results show that thermal clay minerals in granite undergo significant phase changes and structural transformations under high temperatures, while the main framework minerals remain stable. The granite specimens exhibit a clear loading rate strengthening effect at all temperatures, and the overall splitting strength decreases significantly with increasing temperature, with the highest strength observed at 25 °C, and a nearly 80 % decrease at 600 °C. As the temperature and loading rate increase, the crack morphology of the Brazilian disc specimens evolves from a regular linear extension to severe fragmentation, with the fractal dimension of cracks increasing significantly. The failure mode of the granite Brazilian disc transitions from a typical tensile failure to a tension-shear composite failure, dominated by oblique cracks. These results provide important data support for optimizing HDR exploitation efficiency and assessing the stability of injection wellbore systems under high-temperature conditions.