Numerical analysis of the influence of quartz crystal anisotropy on the thermal–mechanical coupling behavior of monomineral quartzite

Studying crystal anisotropy is of great importance for understanding the thermal–mechanical coupling behavior of crystalline rocks in deep underground engineering. In this study, a microscopic parameter calibration method incorporating the size effect is proposed. Subsequently, a thermal–mechanical coupling model accounting for the quartz crystal anisotropy is established to investigate the thermal–mechanical coupling behavior of monomineral quartzite. The results show that thermal-induced microcracks are exclusively distributed along crystal boundaries, and initiate preferentially from crystal boundaries with a larger average linear thermal expansion coefficient, eventually leading to the formation of a crack network. With the increase in temperature, the peak strength of monomineral quartzite increases slightly at first and then decreases rapidly, and the transition threshold temperature is 200 °C. Both elastic modulus and Poisson’s ratio show a monotonic pattern, with abrupt changes occurring at 200 and 300 °C, respectively. The monomineral quartzite exhibits a significant compaction stage under uniaxial compression, and the ductile strengthening critical temperature for monomineral quartzite are between 400 and 500 °C. The quartz crystal anisotropy leads to an anisotropic distribution of inclination angles for tensile microcracks under high temperatures while having no obvious effect on the shear microcracks. In addition, the average size of fragments generated under uniaxial compression is influenced by thermal cracking, demonstrating an initial decrease followed by an increase, and the distribution of fragment sizes is solely correlated with the temperature, which is more concentrated with the increase in temperature.