The physical and mechanical properties of rocks change significantly after being subjected to high temperatures, which poses safety hazards to underground projects such as coal underground gasification. In order to investigate the effect of temperature on the macroscopic and microscopic properties of rocks, this paper has taken sandstone as the research object and conducted uniaxial compression tests on sandstone specimens at different temperatures (20–1000 °C) and different heating rates (5–30 °C/min). At the same time, the acoustic emission (AE) test system was used to observe the acoustic emission characteristics of the rock damage process, and the microstructural changes after high temperature were analyzed with the help of a scanning electron microscope (SEM). The test results show that the effect of temperature on sandstone is mainly divided into three stages: Stage I (20–500 °C) is the strengthening zone, the evaporation of water and the contraction of primary fissures, and sandstone densification is enhanced. In particular, the compressive strength and elastic modulus increase, the macroscopic damage mode is dominated by shear damage, and the fracture micromorphology is mainly brittle fracture. Stage II (500–600 °C) is the transition zone, 500 °C is the threshold temperature for the compressive strength and modulus of elasticity, and the damage mode changes from shear to cleavage damage, and the sandstone undergoes brittle–ductile transition in this temperature interval. Stage III is the physicochemical deterioration stage. The changes in the physical and chemical properties make the sandstone compressive strength and modulus of elasticity continue to decline, the macroscopic damage mode is mainly dominated by cleavage damage, and the fracture microscopic morphology is of a more toughness fracture. The effect of different heating rates on the mechanical properties of sandstone was further studied, and it was found that the mechanical properties of the rock further deteriorated under higher heating rates.

The deformation control of surrounding rock in the combustion air zone is crucial for the safety and efficiency of underground coal gasification (UCG) projects. Coal-bearing sandstone, a common surrounding rock in UCG chambers, features a brittle structure composed mainly of quartz, feldspar, and clay minerals. Its mechanical behavior under high-temperature and dynamic loading is complex and significantly affects rock stability. To investigate the deformation and failure mechanisms under thermal–dynamic coupling, this study conducted uniaxial impact compression tests using a high-temperature split Hopkinson pressure bar (HT-SHPB) system. The focus was on analyzing mechanical response, energy dissipation, and fragmentation characteristics under varying temperature and strain rate conditions. The results show that the dynamic elastic modulus, compressive strength, fractal dimension of fragments, energy dissipation density, and energy consumption rate all increase initially with temperature and then decrease, with inflection points observed at 400 °C. Conversely, dynamic peak strain first decreases and then increases with rising temperature, also showing a turning point at 400 °C. This indicates a shift in the deformation and failure mode of the material. The findings provide critical insights into the thermo-mechanical behavior of coal-bearing sandstone under extreme conditions and offer a theoretical basis for designing effective deformation control strategies in underground coal gasification projects.

In this study, a split Hopkinson pressure bar (SHPB) test system with real-time temperature control was developed, and dynamic tests on limestone taken from deep coal mines within real-time temperatures of 25 to 800 °C were carried out. Additionally, the scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersion spectrum (EDS) tests were conducted to analyze the fracture mechanism of limestone at real-time temperatures. The results reveal that the dynamic compressive strength of limestone linearly declines with increasing temperatures; due to not being affected by thermal shock damage, its strength degradation is not significant after cooling to room temperature, whereas the dynamic elastic modulus exhibits a negative exponential nonlinear decrease with the increase in temperatures. The average strain rate has a positive correlation with the dynamic compressive strength of limestone, while the dynamic elastic modulus exhibits variations in accordance with the Boltzmann function and its relationship with the strain rate. The combined influence of strain rate and temperature on the dynamic compressive strength of limestone can be accurately described by a binary quadratic function. The mechanism of real-time action on limestone can be divided into three stages: when the temperature is between 25 and 200 °C, crystal micro-expansion leads to the densification of micropores, which leads to the increase in limestone strength. When the temperature is between 200 °C and 600 °C, the formation of microcracks induced by thermal stress and intergranular expansion results in a reduction in limestone strength. When the temperature is between 600 and 800 °C, in addition to the continued expansion of the intergranular resulting in the increase in the number of micro-cracks, the decomposition of dolomite at high temperatures leads to chemical deterioration and further reduction in the strength of limestone.

The stability control of slope rock masses is a fundamental prerequisite for the safe and efficient extraction of coal resources in cold regions. However, the damage behavior of slope rock masses subjected to low-temperature freezing and high-intensity blasting poses significant challenges to stability control in cold-region open-pit mining. Therefore, investigating the dynamic failure mechanical properties of frozen rock and elucidating the coupled damage mechanisms under freezing conditions represent essential scientific issues that must be addressed. In this study, a Split Hopkinson tension bar (SHTB) system was employed to examine the macroscopic tensile failure behavior of water-saturated frozen sandstone. The experimental results demonstrate that, under the combined influence of low temperature and high strain rate, the sandstone exhibits increased strength and brittleness. At lower strain rates, the stress–strain curves display features of ductile deformation, whereas at higher strain rates, they exhibit typical brittle failure behavior. The dynamic tensile strength of coal-bearing sandstone increases progressively with decreasing freezing temperature and increasing strain rate. Similarly, the dynamic elastic modulus rises with decreasing temperature. In contrast, the dynamic peak strain decreases with either decreasing temperature or lower strain rate. High-speed camera imaging and scanning electron microscopy (SEM) analysis further reveal that decreasing temperature or increasing strain rate leads to more complex macroscopic failure patterns and rougher, more irregular fracture surfaces. The strain rate controls the development rate of the internal stress–strain state within the sandstone, directly influencing the dynamic fracture process. Furthermore, the micro-damage evolution is closely linked to freezing temperature, being affected by internal moisture conditions, the material’s brittle–ductile transition behavior, and its microstructural characteristics.

The stability of open-pit mines under low-temperature conditions is critical for safe and efficient coal extraction. However, the mechanisms of rock damage and fracture under combined temperature and stress effects remain unclear, particularly regarding the evolution of mechanical properties under repeated freeze–thaw cycles and varying peripheral pressures. This study investigates the damage and rupture behavior of coal-bearing sandstone in cold-region open-pit mines through experimental testing and theoretical modeling. The research was conducted in three stages: (1) freeze–thaw and peripheral pressure experiments to evaluate mechanical property evolution; (2) acoustic emission monitoring to analyze internal fracture initiation, propagation, and coalescence under temperature–stress coupling; (3) development of a local deterioration model to quantify post-damage strength decay considering low-temperature erosion and freeze–thaw effects. Results show that increasing freeze–thaw cycles leads to a transition from brittle to ductile behavior, while higher peripheral pressures significantly enhance ductility. Mechanical parameters are highly sensitive to peripheral pressure but largely independent of freeze–thaw cycle count. Acoustic emission signals respond strongly to temperature, and temperature–stress coupling governs the three-stage evolution of fracture germination, extension, and penetration. The local deterioration model effectively captures post-peak residual strength and damage evolution. These findings indicate that in regions with higher microcrack density, fault propagation is driven by rapid coalescence under stress concentration, whereas in lower-density regions, it is dominated by gradual fracture growth and temperature-induced expansion. The results provide theoretical guidance for stability assessment and support design in open-pit coal mines in cold environments.

The physical and mechanical properties of rocks will change significantly after being subjected to high temperatures, which poses safety hazards to underground projects such as coal underground gasification. In order to investigate the effect of temperature on the macroscopic and microscopic properties of rocks, this paper has taken sandstone as the research object and conducted uniaxial compression tests on sandstone specimens at different temperatures (20-1000 ℃) and different heating rates (5-30 ℃/min) at 1200 ℃. At the same time, the acoustic emission (AE) test system was used to observe the acoustic emission characteristics of the rock damage process, and the microstructural changes after high temperature were analyzed with the help of a scanning electron microscope (SEM). The test results show that the effect of temperature on sandstone is mainly divided into three stages: Ⅰ (20-500 ℃) stage is the strengthening zone, the evaporation of water and the contraction of primary fissures, sandstone densification is enhanced. Specifically, the compressive strength and elastic modulus increase, the macroscopic damage mode is dominated by shear damage, and the fracture micro-morphology is mainly brittle fracture. Stage II (500-600°C) is the transition zone, 500°C is the threshold temperature for compressive strength and modulus of elasticity, and the damage mode changes from shear to cleavage damage, and the sandstone undergoes brittle-ductile transition in this temperature interval. Stage III is the physicochemical deterioration stage. The changes in physical and chemical properties make the sandstone compressive strength and modulus of elasticity continue to decline, the macroscopic damage mode is mainly dominated by cleavage damage, and the fracture microscopic morphology is more toughness fracture. The effect of different heating rates on the mechanical properties of sandstone was further studied, and it was found that the mechanical properties of the rock were more deteriorated under higher heating rates.