Experimental study on the effect of cyclic thermal shock on the mixed mode I-II fracture characteristics of granite

Thermal shock cycling induces progressive strength degradation in rock masses, significantly compromising the stability of underground excavations. By using central cracked Brazilian disc (CCBD) specimens, the pure mode I, pure mode II and mixed mode I-II fracture characteristics of thermally treated granite under heating-cooling cycles were investigated. The study focused on the effects of thermal shock cycling on the fracture mechanical behavior of granite, particularly in terms of crack propagation morphology, load-displacement response, and the evolution of fracture toughness. Meanwhile, scanning electron microscope (SEM) technology was also used to analyze the microstructure changes of granite, exploring the damage mechanism behind the deterioration of macroscopic mechanical properties. The results show that with the increase of temperature and cycle times, both mode I and mode II fracture components (K_I, K_II) of granite specimens exhibit a decreasing trend under all loading conditions. Among them, the degradation effect of thermal shock cycling on K_I is greater than that on K_II. Thermal shock cycling induced damage in granite shows pronounced temperature sensitivity, with 300 °C conditions producing damage nearly triple those observed at 100 °C. The strength degradation in granite primarily results from microstructural alterations induced by the coupled effects of cyclic thermal stresses and hydrothermal interactions.