An in-depth understanding of the effect of real-time high temperature and loading rate on the fracture toughness of rocks is highly important for understanding the fracture mechanism of Hot Dry Rock (HDR). Three-point bending tests on notched semi-circular bending (NSCB) samples at the real-time temperatures (25, 100, 200, 300, 400 and 500 ℃) and different loading rates (0.1, 0.01 and 0.001 mm/min) were performed to characterize the temperature and rate dependence of the mode I fracture toughness. Besides, the characteristic of the fracture surface morphology was investigated by scanning electron microscope (SEM) and crack deviation distance analysis. Results show that the temperature has a significant effect on the development of intergranular and transgranular cracks. The fracture toughness and peak load are similarly influenced by temperature (i.e., they both decrease with increasing temperature). At the loading rates of 0.1 mm/min and 0.01 mm/min, from 25 to 400 °C, the fracture toughness decreases slightly with decreasing loading rates. However, at a loading rate of 0.001 mm/min, the fracture toughness values above 200 °C are very similar, and the fracture toughness does not strictly follow the law of decreasing with decreasing loading rate. Especially at 500 °C, fracture toughness and loading rate are negatively correlated. Our study also indicates that the effect of loading rate on macroscopic crack propagation path at real-time high temperature is not obvious. This study could provide an important basis for evaluating the safety and stability of geothermal engineering.

Knowledge of deformation and strength behavior of rocks under high in situ temperature is highly important for the control of geological disasters in exploration of hot dry rock and mining in deep formation. In this study, uniaxial compression tests were carried out on granite under different real-time high-temperature conditions (25, 200, 300, 400, 500, 600, and 700°C) and loading rates (0.01, 0.1, and 0.5 mm/min). The effects of real-time high temperature and loading rate on the uniaxial compressive strength and elastic modulus of granite were studied, and the microscopic morphology of the fracture surface was analyzed. The results show that the uniaxial compressive strength and elastic modulus of granite increase first and then decrease with the increase of temperature. The uniaxial compressive strength clearly increases at 200°C and decreases gradually when the temperature exceeds 300°C. Under the same temperature conditions, the uniaxial compressive strength of granite decreases and the elastic modulus increases with increasing loading rate. When the temperature reaches 600°C, the effect of the loading rate on the uniaxial compressive strength and elastic modulus of granite decreases significantly. The test results are compared with the results of work performed on quenched granite. Under real-time high-temperature conditions, the thermal crack effect has a significant influence on the uniaxial compressive strength and elastic modulus of granite, without the thermal hardening effect of quenched granite. During hydraulic fracturing, the rock skeleton near the injection well is cooled and shrunk, as is the thermal hardening effect caused by high-temperature quenching. The formation of thermal equilibrium leads to the large-scale extension of fracture cracks along the weak plane structure, such as the effect of thermal cracks on granite under real-time high-temperature conditions.

A low temperature and the degree of saturation have important influence on the mechanical properties of rock surrounding cave-type underground liquefied natural gas storage. Uniaxial compression tests were carried out on dry and saturated granite under different real-time loadings at designated low-temperature conditions (25, 0, −20, −40, −50 and −60°C). The porosity and microstructure were then observed by nuclear magnetic resonance spectroscopy and scanning electron microscopy. The results show that with the decrease in temperature, the uniaxial compressive strength (UCS) and porosity of both dry and saturated granite monotonically increase, while the elastic modulus undergoes an increase–decrease transition and finally stabilises. At the same temperature, the UCS and elastic modulus of dry granite are generally greater than those of saturated granite. The reinforcement in the UCS and elastic modulus is caused by cold shrinkage, while the degradation in the elastic modulus, particularly for saturated granite, is attributed to freezing damage below 0°C. The results of this study can provide technical support for the long-term stability analysis and evaluation of underground cavernous liquefied-natural-gas-storage reservoirs.

Concrete is widely used in underground engineering and bears three‐dimensional stress transmitted by overlying load. When a fire occurs, the thermal expansion of concrete structure under such stress state is different from that under stress‐free state. For this purpose, a self‐developed real‐time high‐temperature true triaxial test system was applied to investigate the thermal expansion behavior of concrete under three‐dimensional stress state. The thermal expansion strain of concrete under the three‐dimensional stress undergoes strain increasing and strain stabilizing stages. At 600°C, the maximum thermal expansion strain of concrete under the three‐dimensional stress is 0.75%. The average coefficient of thermal expansion of concrete under three‐dimensional stress condition was then calculated, and its value reaches the minimum of 8.68 × 10⁻⁶/°C at 200°C and the maximum of 13.41 × 10⁻⁶/°C at 500°C. Comparing the coefficient of thermal expansion of concrete under stress‐free condition given by Eurocode, it is found that the three‐dimensional stress has an obvious restraint on the thermal expansion of concrete. The research results can provide theoretical basis for the stability analysis of underground engineering concrete structures under high‐temperature environment.

In the development of geothermal resources in hot dry rocks, deep underground rock masses are typically subjected to real-time high-temperature environments. High temperatures alter the physical and mechanical properties of the rocks, directly affecting the safe and efficient utilization of hot dry rock resources. Therefore, a grain-based model (GBM) of particle flow code (PFC) was constructed based on uniaxial compression tests, and the model was verified according to macroscopic mechanical parameters and damage modes, in order to carry out the simulation study of the uniaxial compression of granite and explore the meso-failure mechanism of granite under real-time high temperature. The relationships between stress–strain curves and crack derivation, the evolution of microcracks, and the characteristics of acoustic emission activity and energy changes at different temperatures were investigated in conjunction with the results of laboratory tests. The results show that crack development, acoustic emission activity, and energy evolution during uniaxial compression include four main stages: initial compression, elasticity, plastic strengthening, and post-peak damage. The failure of granite is primarily controlled by mica and feldspar. During loading, intergranular tensile cracks first emerge within the granite, followed by intragranular tensile cracks, with shear cracks appearing last. As the temperature increases, the total number of microcracks continuously rises, the frequency of acoustic emission events increases, and both dissipated energy and boundary energy gradually decrease, showing an upward trend in the energy dissipation ratio, indicating an increase in thermal damage due to high temperatures. At 400 °C, the rate of microcrack formation increases significantly, with intergranular and intragranular cracks starting to coalesce into macroscopic cracks that extend outward. In the post-peak stage, the phenomenon of multiple peaks in acoustic emission events begins to appear. At 600 °C, the rate of microcrack formation reaches its maximum, with cracks extending throughout the sample to form a network of fractures, resulting in the granite exhibiting ductile failure characteristics.