Granite subjected to high-cycle thermal shock (CTS) undergoes progressive degradation in microstructure and fracture resistance, with implications for the stability of enhanced geothermal systems (EGS). This study investigates granite exposed to 1–50 CTS cycles at 300 °C and 450 °C with pre-existing crack angles of 0° (Mode I) and 30° (mixed mode). Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) reveal pore enlargement and coalescence from micropores/mesopores into macropores, accompanied by increased porosity. Semi-circular bend (SCB) tests combined with digital image correlation (DIC) quantify critical load F cr , fracture toughness ( K IC , K IIC ), and strain-field evolution. Both F cr and fracture toughness decrease nonlinearly with cycle count, with over 30% of the total 50-cycle reduction occurring after the first cycle; degradation is greater at 450 °C. Crack paths evolve from linear to tortuous and bifurcated, and post-peak behavior transitions toward ductile characteristics with increasing cycles at 450 °C. A thermo-mechanical damage (TM(D)) model with Weibull heterogeneity reproduces the biphasic degradation trend, the rise in heterogeneity (decreasing Weibull parameter), and curvature/bifurcation of fracture paths, in agreement with experiments. The results illuminate temperature-dominant damage acceleration and mixed-mode frictional mitigation, supporting reliability assessments and fracture management in EGS.
Liu et al. (Thu,) studied this question.