Abstract: Liquid nitrogen cyclic fracturing is an environmentally friendly and efficient waterless fracturing technology that induces significant thermal stress in hot dry rock (HDR) through repeated liquid nitrogen cooling cycles, effectively enhancing fracturing and permeability in HDR reservoirs. To investigate how cyclic liquid nitrogen cooling affects the fracture characteristics and damage behavior of HDR, granite samples underwent high-temperature heating and liquid nitrogen cooling treatments with varying cycle numbers. The physical and mechanical properties, such as fracture toughness and behavior, were evaluated using three-point bending tests. Surface morphology and roughness of fracture surfaces generated during bending tests were quantitatively analyzed using three-dimensional laser scanning technology and fractal theory. Additionally, a numerical method was employed to reconstruct the heterogeneous granite matrix using a stochastic four-parameter generation approach. A thermo-mechanically coupled hybrid phase-field model was developed to simulate microcrack evolution and macroscopic fracture processes of high-temperature granite under different liquid nitrogen cycles, providing insight into the impact of cyclic cooling on macroscopic fracture behavior. Experimental and numerical results show that repeated high-temperature heating and liquid nitrogen cooling exacerbate granite damage, significantly reducing mechanical parameters such as fracture toughness. The fractal dimension of fracture surfaces and surface morphology parameters showed a significant positive correlation with cycle numbers. At higher cycle numbers, fracture surfaces displayed complex and tortuous crack propagation paths. The thermo-mechanical hybrid phase-field model accurately replicated the thermo-mechanical cracking behavior of granite under high-temperature heating and liquid nitrogen cooling. Thermal damage in granite was primarily concentrated in regions with the highest tensile strain energy. Microcracking observed in high-temperature granite during cyclic treatment was mainly driven by temperature gradients and differential thermal expansion between adjacent mineral particles. Ultimately, microcracks induced by liquid nitrogen cooling resulted in more intricate and convoluted mode I fracture propagation paths in granite.

Key words: liquid nitrogen, granite, fracture toughness, roughness of fracture surface, phase-field method