An integrated DIC and CPG investigation of the model-I fracture features for granites after cyclic heating-cooling treatments

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.