Fracture properties of heated granite in different water-immersion durations after liquid nitrogen cooling

Liquid nitrogen (LN 2 ) assisted fracturing has received considerable attention in hot dry rock reservoir reforming. In this paper, the cracked straight through Brazilian disc (CSTBD) specimen was used to investigate the effect of water-immersion duration's effect on fracture characteristics of heated granite cooled by LN 2 . The target temperature ranges from 25 °C to 600 °C. The heated granite was cooled in LN 2 to −196 °C and then immersed in water for different durations of time. The CSTBD specimens were subjected to static compression and dynamic impact by a split Hopkinson pressure bar (SHPB). The results show that in static compression, granite's mode-I fracture toughness decreases with increasing temperature, and static maximum principal strain ( ε 1s ) and static crack tip opening displacement (CTOD s ) increase with increasing temperature. When the heating temperature is the same, fracture toughness reaches the minimum and maximum when water-immersion durations are 0.5 min and 1 min, respectively. At 25 °C–600 °C, ε 1s and CTOD s are maximum when immersed in water for 0.5 min. At 400 °C, the longer the granite is immersed in water, the dynamic fracture toughness (DFT) increases and then decreases under dynamic impact. The dynamic initiation toughness is less than the DFT, reaching the maximum at 0.5 min. Under dynamic impact, dynamic crack tip opening displacement (CTOD D ) and the maximum principal strain decrease and then increase, reaching their maximum at 1.5 min. At 400 °C, the DFT and the CTOD D under dynamic impact are 239.06% and 263.15% larger than those at static compression when immersed in water for 1.5 min.