Mixed-mode I/II fracture and crack propagation characteristics of granite after thermal shock cycles: An integrated DIC and CPG investigation

The fracture behaviors of reservoir rocks following thermal shock cycles are critical to the stability and heat extraction efficiency of enhanced geothermal system. In this study, notched semi-circular bend (NSCB) granite specimens with varying prefabricated crack angles were subjected to multiple thermal shock cycles. Subsequently, three-point bending tests were conducted, employing digital image correlation (DIC) technique and crack propagation gauge (CPG) measurement. The variations in mixed-mode I/II fracture toughness, fracture process zone (FPZ) length and crack propagation velocity with respect to the number of thermal shock cycles were deeply studied. The experimental results reveal that both the fracture toughness and maximum FPZ length decrease with an increasing number of thermal shock cycles and larger prefabricated crack angle. A strong positive linear correlation is observed between the maximum FPZ length and mixed-mode fracture toughness. Additionally, the average crack propagation velocity exhibits a marked exponential decay with an increasing number of thermal shock cycles. Detailed fracture surface observations indicate that the deviation between the actual crack propagation path and the theoretical trajectory increases significantly as thermal shock cycles accumulate. Furthermore, a larger prefabricated crack angle corresponds to a decrease in average crack propagation velocity. The deterioration of granite upon thermal shock cycles is attributed to the combined effects of repeated thermal-induced damage, water quenching and water-induced weakening mechanisms.