Experimental and numerical research on the failure pattern and mechanisms of granite disc with a pre-crack after heating and liquid nitrogen cooling

The use of ultra-low temperature liquid nitrogen to induce fractures in granite is considered advantageous for deep geothermal engineering. To investigate the fracturing characteristics and damage mechanisms of granite following liquid nitrogen cooling, pre-cracked granite discs with high temperatures (100–600 °C) were promptly placed in a liquid nitrogen container for rapid cooling. A series of Brazilian splitting tests were conducted on the damaged discs to investigate their fracturing characteristics. During loading, acoustic emission and digital image correlation were utilized to capture deformation and failure processes. The experimental findings reveal that, within the temperature range of room temperature to 600 °C, the tensile strength decreases from approximately 4.9 MPa to around 2.2 MPa, marking a 55 % reduction. Acoustic emission energy analysis indicates that brittle failure predominates at low temperatures, while plastic failure gradually becomes dominant with increasing heating temperature. In the absence of heating and liquid nitrogen cooling, the failure cracks exhibit relatively straight patterns. However, after liquid nitrogen cooling, the failure cracks become crooked, with the observation of wing cracks and other forms. The failure pattern is categorized into four groups based on heating temperature: room temperature, 100–200 °C, 300–400 °C, and 500–600 °C. Image-based computational models derived from actual disc photos and an inhomogeneous damage model were introduced into the 2D distinct lattice spring model. Numerical simulations successfully reproduced the deformation and failure processes of granite discs. By combining experimental and numerical results, the revealed failure mechanisms suggest that the granite specimen's failure, treated with heating and liquid nitrogen cooling, is influenced by mechanical factors, the presence of cracks, damage extent/distribution, and geometry. As the heating temperature increases, the impacts of damage extent/distribution and geometry become increasingly dominant. These findings provide valuable insights to guide the implementation of real liquid nitrogen-based fracturing technology in deep geothermal engineering.