Effect of high temperature on mode-I fracture toughness of granite subjected to liquid nitrogen cooling

Liquid nitrogen (LN2) ultra-low temperature fracturing technology has broad application prospects in the storage and transformation of geothermal wells. Therefore, it is of great significance to investigate the fracture toughness of granite samples after elevated temperature treatment and LN2 cooling. In this paper, the Cracked Straight Through Brazilian Disc (CSTBD) granite specimen was heat-treated at different temperatures to study the mode-I fracture toughness of granite after rapid cooling by LN2. Meanwhile, acoustic emission (AE) and digital image correlation (DIC) techniques were used to monitor the deformation and failure process of granite. Our results show that when the heating temperature is more than 400 °C, after LN2 cooling, the internal thermal damage of granite is intensified, the plastic characteristics are gradually enhanced, and the fracture toughness decreases significantly. The DIC results show that, with increasing stress, cracks start to form at the two ends of the pre-crack and propagate to the contact point between the press and rock sample along the loading direction under mode-I loading. Near the peak load, the Von-Mises equivalent strain (Evm) gradually increases, and a high strain band appears along the loading direction. When the heating temperature exceeds 400 °C, the density and width of thermally induced cracks in granite gradually increase after LN2 cooling, the fracture track gradually deviates from the straight line, secondary cracks extend, and the plastic characteristics are enhanced. High temperatures cause thermal damage to the rock, and rapid cooling exacerbates this damage. It can be considered that 400 °C is the critical temperature threshold at which the physical–mechanical behavior and fracture characteristics of CSTBD granite change significantly under the condition of rapid LN2 cooling.