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 ℃ to 600 ℃, mode I fracture toughness decreases from 1.286MP·m to 0.612MP·m; mode Ⅱ fracture toughness decreases from 0.830MP·m to 0.413MP·m; mixed-mode Ⅰ+Ⅱ fracture toughness decreases from 1.082 MP·m to 0.530MP·m at e = 0.705 and from 0.914 MP·m to 0.445 MP·m at e = 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.

中文翻译：

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液氮冷却热处理花岗岩I+II混合型断裂特性理论与实验研究

液氮压裂技术广泛应用于干热岩（HDR）地热资源的开发。有必要了解干热岩石在液氮冷却下的断裂特征。在这项研究中，研究了液氮冷却下热处理花岗岩的混合模式I + II断裂特征。结果表明，随着暴露温度的升高，花岗岩内部微裂纹数量增多，断裂能和断裂韧性降低。从25℃到600℃，I型断裂韧性从1.286MP·m下降到0.612MP·m； Ⅱ型断裂韧性由0.830MP·m下降到0.413MP·m；混合模式Ⅰ+Ⅱ断裂韧性在e = 0.705时分别从1.082 MP·m下降到0.530MP·m，在e = 0.427时从0.914 MP·m下降到0.445 MP·m。此外，晶间断裂行为随着温度升高而增加，导致断裂表面的不规则性增加。广义最大切向应力（GMTS）准则用于预测热损伤花岗岩的断裂特征（断裂起始角度和断裂韧性）。通过考虑临界损伤半径的变化，GMTS 准则在预测 II 型载荷占主导地位时的抗裂性方面的准确性得到了提高。