深部储层岩石的热力学特性，尤其是循环热冷作用下损伤破坏特性，对于增强型地热系统井壁稳定性分析及地热开采效率评估具有重要意义。针对中心直裂纹半圆盘(NSCB)花岗岩试样，首先进行不同热冷循环处理(加热温度400 ℃，最高循环次数13次)，然后开展3点弯I型断裂韧度特性试验。基于裂纹扩展计(CPG)和数字图像相关(DIC)测试技术，研究了热冷循环作用下花岗岩I型断裂韧度、断裂过程区(FPZ)、裂纹扩展速率及断裂轮廓特征的影响规律。试验结果表明：当热冷循环次数达到10次以上，花岗岩试样脆性明显减弱，而峰前软化特性和峰后延性增强；花岗岩的断裂过程区由裂缝尖端开始逐步孕育，断裂过程区长度随荷载增大呈先增大后减小的趋势，I型断裂韧度、最大断裂过程区长度及裂纹平均扩展速度随热冷循环次数增大而指数减小，I型断裂面随热冷循环次数增大越来越不平整。最后，基于X-ray衍射(XRD)和扫描电镜(SEM)等微观测试技术，研究了热冷循环作用对花岗岩矿物成分及微观结构的影响规律，结果表明花岗岩4种矿物成分的峰值衍射强度及矿物质量分数均随热冷循环次数增大而降低，而微裂纹大小及数量随热冷循环次数增大而增大。热冷循环作用下花岗岩的损伤劣化机理包含了多次高温热损伤、水冷冲击及水弱化等3方面的联合作用结果。

The thermo-mechanical properties of deep reservoir rocks, especially the damage and failure characteristics under heating-cooling cycles, are of great importance to the stability analysis of well wall and the evaluation of geothermal extraction efficiency for the enhanced geothermal systems. In this study, the notched semi-circular bend (NSCB) granite samples were first subjected to varying cycle heating-cooling treatments, and then were submitted to three-point bending tests so as to obtain the model I fracture features. Based on the crack propagation gauge (CPG) measurement and digital image correlation (DIC) technique, the model I fracture toughness, fracture process zone (FPZ), crack propagation velocity and fracture surface topographic features of the samples after varying cyclic heating-cooling treatments were studied. The experimental results indicate that when the number of heating-cooling cycles reaches 10 or more, the brittleness of the granite specimens is obviously weakened, whereas the pre-peak softening and post-peak ductile responses are enhanced. The fracture zone of granite is gradually developed from the crack tip. The FPZ length tends to first increase and then decrease with increasing load, and the mode I fracture toughness, maximum length of FPZ and average crack propagation velocity decrease exponentially with increasing number of heating-cooling cycles. The fracture surface becomes more uneven with the increase in the number of the heating-cooling cycles. Finally, the mineral composition and microstructure of granite samples after different cycle heating-cooling treatments were investigated based on the X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests. The results show that the peak intensity of XRD and mineral content of the four minerals decrease with the increase in the number of heating-cooling cycles, while the size and number of the microcracks increase with the increase in the number of heating-cooling cycles. The deterioration mechanism of the granite upon the heating-cooling cycles includes the combined effects of thermal-induced damage, water quenching damage and water weakening effects.