Theoretical and experimental study on mixed-mode I plus II fracture characteristics of thermally treated granite under liquid nitrogen cooling

Liquid nitrogen fracturing technology is widely used in the exploitation of hot dry rock (HDR) geothermal resources. It is necessary to understand the fracture characteristics of hot dry rocks under liquid nitrogen cooling. In this study, the mixed-mode I + II fracture characteristics of the thermally treated granite under liquid nitrogen cooling were investigated. The results indicate that as the exposed temperature increases, the number of internal microcracks increases, while the fracture energy and fracture toughness of the granites decrease. From 25 degrees C to 600 degrees C, mode I fracture toughness decreases from 1.286MP & sdot;m1/2 to 0.612MP & sdot;m1/2; mode II fracture toughness decreases from 0.830MP & sdot;m1/2 to 0.413MP & sdot;m1/2; mixed-mode I+II fracture toughness decreases from 1.082 MP & sdot;m1/2 to 0.530MP & sdot;m1/2 at Me = 0.705 and from 0.914 MP & sdot;m1/2 to 0.445 MP & sdot;m1/2 at Me = 0.427, respectively. Additionally, the intergranular fracture behavior increases as temperature increases, resulting in an increase in irregularity of fracture surfaces. The generalized maximum tangential stress (GMTS) criterion is used for predicting the fracture characteristics (fracture initiation angle and fracture toughness) of thermal damaged granites. The accuracy of the GMTS criterion in predicting crack resistance when mode II loading dominates has been improved by considering the variation of critical damage radius.