The thermo-mechanical properties of deep reservoir rocks,especially the damage and failure characteristics un-der 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 velo-city 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 de-crease with increasing load,and the mode I fracture toughness,maximum length of FPZ and average crack propagation ve-locity decrease exponentially with increasing number of heating-cooling cycles.The fracture surface becomes more un-even 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 micro-cracks 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.
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