Fracture characteristics and fatigue failure signals of thermal storage rock under stepwise pulsation pressurization

With the exploitation of deep-earth energy, the challenges posed by fatigue disturbance and high temperatures are becoming increasingly severe. This paper studies fracture characteristics and fatigue failure signals of thermal storage rock under stepwise pulsation pressurization (SPP). First, the evolution of macroscopic mechanical parameters such as strength-deformation, fracture toughness, and fracture process zone (FPZ) size of the sample were analyzed. Then, the sample's fracture characteristics were studied based on acoustic emission and optical-electrical analysis techniques, and the fracture mechanism of the sample was explained using the cohesive zone model. Finally, based on the G-R relationship, time series analysis and critical slowing-down theory, three different fatigue failure signals were defined, and the early warning effects of the different signals were discussed. Laboratory work has shown that, compared with static monotonic loading, the K_Ic of the sample under SPP is reduced by about 9.3 % on average. The crack evolution in the sample under SPP can be divided into three stages, and the distribution of ringing counts and peak frequency signals has stage characteristics. The peak frequency signal is concentrated in the 100 kHz ∼ 400 kHz, and multi-band coexistence will occur before fatigue failure. In addition, with the increase of temperature, the K_Ic of the sample decreases nonlinearly and the FPZ size increases nonlinearly. Crack activity is enhanced in the early loading stage, and low-stress-induced cracks increase. The maximum strain at the crack tip decreases, and the relative rate of crack growth slows down. The strain level required for sample fracture is reduced, and the subcritical crack growth stage is prolonged. Compared with the b value, using the function F and RA/AF as fatigue failure signals can simplify the calculation and improve the early warning effect.