Evolution of acoustic emission and macro-microscopic fracture characteristics in thermally treated and cryogenically cooled granite

Liquid nitrogen (LN2) fracturing is an emerging anhydrous technology with significant potential for reservoir stimulation in Hot Dry Rock (HDR) geothermal exploitation. Understanding the fracture mechanics of rock subjected to LN2 cooling under high-temperature conditions is essential for optimizing stimulation results. In this study, granite samples heated to various temperatures were subjected to LN2 thermal shock. Acoustic emission (AE) technology and a mechanical testing system were employed to investigate the amplitude distribution, spatial signal evolution, and both macroscopic and microscopic fracture behaviors. The results indicate that AE activity and signal strength intensify significantly with increasing heating temperature. While low-amplitude signals predominate, the spatial distribution shifts from localized main fractures below 300 °C to widespread micro-cracking above 300 °C. Surface morphology analysis reveals a surge in roughness, with median roughness height increasing by 75% between 200 and 300 °C, indicating a critical threshold for thermal damage. The maximum crack displacement distance increased by ∼55.2% from 25 to 400 °C, accompanied by increased tortuosity. Correspondingly, the fractal dimension rose by 19.6% to 1.778. Microscopic observations show that at 300 °C and above, fracture surfaces exhibit layered and step-like matrix failure. This is attributed to high thermal stress weakening the structural bonding, facilitating crack propagation along weak zones to form complex fracture networks. These findings demonstrate that sequential high-temperature heating and LN2 cooling significantly enhance fracture network complexity and density while reducing mechanical strength, providing theoretical and practical support for LN2-based HDR stimulation.