Fracture toughness is used to characterize rock resistance to fracturing and it is important in theoretical research and engineering applications. Mode-I fracture toughness can be predicted on the basis of an empirical relationship between fracture toughness (K_IC) and tensile strength (σ_t). In underground engineering, rocks are often subjected to different temperatures. Therefore, this paper explores the effect of temperature on the relationship between mode-I fracture toughness and tensile strength. The results show that the change trends in the K_IC and σ_t values of rocks at temperatures from 20 °C to 600 °C are broadly consistent with each other. For rocks heat-treated to the same temperature, the K_IC of the rock increases with an increase in σ_t. This positive correlation between K_IC and σ_t is different in rocks heat-treated to different temperatures. Critical crack propagation radius (r_IC) is an important factor in the relationship between K_IC and σ_t and is related to the type of rock and the conditions under which it is tested. For the same rock, r_IC is quite different after it has been exposed to different temperatures. The positive correlation between K_IC and σ_t results from a similarity in the fracture morphology and properties of failure when rock is destroyed in fracture and tensile tests.

Energy extraction from nuclear materials produces high-level radioactive waste. In geological nuclear waste storage repositories, the decay of radioactive elements generates heat, exposing the reservoir rocks to high-temperature conditions for long periods. To explore the effects of these conditions, this study examines the ability of granite to resist fracturing after thermal treatment for 10 h, 10 days, 30 days and 60 days. The results show that the fracture toughness of the granite remained basically unchanged for up to 10 days of thermal treatment. After thermal treatment for 60 days, the mode I, mode II and mixed-mode (I + II) fracture toughness decreased by 15.39%, 18.09% and 15.17%, respectively, compared with samples heated for 10 h. The change trends of the ability of granite to resist tensile, shear and mixed (tensile + shear) failure with an increased thermal treatment duration were basically consistent. Moreover, there was little change in its brittle fracturing characteristics with an increase in heating duration. Changes caused to the internal microstructure of the granite by high temperature were ongoing even up to 60 days.

Thermally induced changes in mesocrack and the physical properties of fine-grained granite may influence their stability, transport characteristics, and performance related to various deep subsurface energy projects. In this study, granite was heat-treated at different temperatures (20°C, 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C). The propagation and evolution of different types of cracks and the physical properties of the granite were quantitatively investigated, using optical observations of petrographic thin sections, $P$-wave velocity measurements, and permeability tests. The results show that as the temperature increased, the number and length of cracks increased, and the cracks were randomly distributed in all directions. This led to an increase in rock damage (⁠ $λn$⁠) and an increase in permeability (⁠ $K$⁠). In particular, when the temperature was ≥400°C, the damage rate significantly increased, and the number and length of intragranular cracks significantly exceeded the number and length of intergranular cracks. This led to changes in the permeation path, causing it to mainly travel through the interior of mineral particles. Using the inverse of $P$-wave velocity (⁠ $VP$⁠), the dimensionless crack density (⁠ $ρ$⁠) of granite was found to increase as the temperature increased, and this result was similar to the change of optical crack density (⁠ $Pl$⁠). These analyses laid a reference for understanding the correlation between microcrack characteristics and macrophysical properties of granite.

Energy is the fundamental prerequisite for human survival and development, as well as the driving force behind the progress of human civilization