Experimental Investigation of the Damage Characteristics of High-Temperature Granite Subjected to Liquid Nitrogen Treatment

To reveal the damage characteristics of high-temperature granite caused by liquid nitrogen (LN) treatment, ultrasonic, Brazilian splitting, and acoustic emission (AE) tests were performed on high-temperature granite before and after LN treatment. Changes in ultrasonic velocity, tensile strength, energy evolution laws, and AE parameters induced by heating, LN cooling, and LN-induced cryogenic conditions were evaluated and compared. The results showed that LN cooling can create additional cryogenic damage to high-temperature granite, thereby increasing the damage degree. With heating temperature rising, the preliminary damage of high-temperature granite increased significantly. For heating temperatures of 300 °C, the decreasing amplitudes of ultrasonic velocity, tensile strength, and ultimate elastic and absorbed energies induced by LN cooling decreased with increased heating temperature and were much lower than those induced by heating. Especially, when heating temperature was > 300 °C, the deformation resistance and total shrinkage of high-temperature granite were greatly improved by LN-induced cryogenic conditions. In this case, the tensile strength, ultimate absorbed and elastic energies, and the proportion of elastic energy of LN-frozen samples were larger than the heated samples. This study provides theoretical guidance for hot dry rock fracturing with LN.