Fracture evolution of granite under cyclic thermal shocks: effects of liquid nitrogen cooling on strength, toughness, and acoustic emission characteristics

• Three different lab tests were conducted on granite subjected to different high-temperature heating and LN 2 cooling cycles. • Explored the evolution of microcracks and macroscopic fractures in granite under different test modes using AE technology. • After approximately nine LN 2 cycles, further increases in damage and cracking are negligible. • The damage induced by LN 2 cold shock can be attached to the microstructure of granite, resulting in higher peak frequencies of AE signals (>700 kHz). Liquid nitrogen (LN 2 )–assisted fracturing has emerged as a promising technique to enhance reservoir permeability and stimulate geothermal energy extraction. This study investigates the progressive degradation of granite under repeated thermal shocks, simulating geothermal conditions by subjecting samples to high-temperature heating followed by rapid LN 2 cooling, with cycles ranging from 0 to 20. Mechanical behavior was assessed by uniaxial compression (UCT), Brazilian splitting (BST), and three-point bending (TPBT) tests, with real-time acoustic emission (AE) monitoring employed to track fracture evolution. Results reveal substantial degradation in mechanical properties, with tensile strength and fracture toughness decreasing by up to 67.48 % and 65.51 %, respectively, after 20 thermal cycles. The extent of microstructural damage increases rapidly in the initial cycles, then plateaus after approximately 7–9 cycles, indicating a saturation point in damage development. AE analysis indicates a transition from brittle to more ductile behavior, manifested by increased AE activity, a rise in high-frequency components (>700 kHz), and the development of complex fracture networks. The average frequency (AF)–rise time/amplitude (RA) distribution indicates a growing prevalence of shear-dominated microcracking as cycling progresses. These findings offer new insights into the micro-mechanical mechanisms activated by LN 2 cycling and highlight its effectiveness as a thermal stimulation strategy for enhancing the permeability of low-porosity crystalline rocks in geothermal applications.