Size effect of failure mode of thermally damaged torus granite

Abstract

The rocks around wellbores in energy-storage areas are affected by high temperatures and thermal stresses during the extraction of geothermal resources. In this study, the mechanical properties of torus granite samples with different aperture ratios and sizes are experimentally investigated at high temperatures. Further, Acoustic Emission (AE) and Digital Image Correlation (DIC) technology are used to monitor the deformation and failure process of torus granites. The research results show that the radial peak load decreases exponentially with increases in the aperture ratio, temperature, and thermal damage. 400 °C is the critical threshold for the strength instability of the torus granite. According to the failure characteristics. When the aperture ratio λ < 0.3, linear tensile cracks appear in the granite, and the “double fan” failure mode is finally formed. When the heating temperature reaches 400 °C, torus granite successively formed linear cracks and horizontal tensile cracks, and finally a ‘four fan’ failure mode. When the heating temperature reaches 600 °C, the horizontal tensile cracks produced when the torus granite fails evolve from asymmetric to symmetrical. Based on the deformation and failure results of the torus granite under compression, the dynamic failure criterion of the inner and outer pore walls of the torus granite under radial compression is derived. Finally, the theoretical analysis and experimental results are compared, the scale and temperature effects of the failure mode of the torus granite are deeply analysed. This study results can provide theoretical reference for surrounding rock support and shaft diameter ratio design.