To investigate the strength characteristics and failure mechanism of granite after thermal treatment are critical for geothermal energy storage and development. Acoustic emission (AE) is widely used to deduce the process of rock crack generation, development and penetration in laboratory tests, thus revealing the mechanism of rock failure. However, previous investigations have shown that laboratory tests cannot directly observe the interaction of thermal cracks and thermal stress, and more than 90\(\%\) of AE tensile failure sources cannot be captured. This paper investigates the generation mechanism of thermal cracks and thermal stress distribution in thermally treated specimens using the discrete element method. After that, the evolution of AE failure sources is quantitatively analyzed by the moment tensor inversion results. The results showed that: (1) Thermal cracks destroy the internal structure of the specimen, thus weakening its mechanical properties. The number of thermal cracks increases with the temperature, further aggravating the damage to the mechanical properties of specimens; (2) as the temperature increases, the failure mode of the specimen changes from splitting failure to shear failure. Moment tensor inversion revealed that tensile failure dominated the final damage of samples. The shear and compaction failure sources increase with temperature, while tensile failure sources decrease; (3) the b value increased by 215\(\%\) from 25 \(^{\circ }\)C to 1000 \(^{\circ }\)C. As the number of microcracks in a single AE event increases, the AE frequency decays exponentially, and most AE events have 1–5 microcracks.

Understanding deep rocks' mechanical properties and failure evolution is crucial for efficient resource development. This study investigates the mechanical properties of tight sandstone and analyzes its acoustic emission (AE) characteristics using a combined discrete element model and moment tensor inversion. The AE activity during loading is categorized into three stages: crack initiation, stable crack propagation, and unstable crack propagation. Confining pressure loading suppresses AE activity during the crack initiation stage due to damage healing phenomenon. Moment tensor inversion reveals that tensile failure is the primary AE failure source, despite samples exhibiting splitting and shear failure modes. The proportion of AE failure types varies with stress levels and depends on the mechanical environment. Microcracks initiate at the ends of the sample and propagate inward along the loading direction, resulting in a blank area of AE events in the middle. This blank area can be utilized to predict specimen failure mode. The b value, representing the ratio of small to large magnitude events, decreases with increase of the confining pressure, indicating higher energy release during specimen failure under high confining pressure. The research results can provide a reference for predicting the failure of tight sandstone.

Tensile strength is one of the important mechanical parameters for controlling the development of cracks in the Loess Plateau. However, the tensile strength of soil is difficult to be precisely measured due to the lack of satisfying laboratory techniques. Unconfined penetration test (UPT) is commonly used as an indicator to predict soil tensile strength characteristics, while there has been less research study about test parameter values range of UPT and micromechanical characteristics at failure. This study finished 50 groups of UPTs of remolded loess with different disk diameters (12.36–27.81 mm) at various sample heights (37–87 mm) based on Q₂ loess in Lintong areas, and the crack development and failure essence in the different failure modes process were investigated by the discrete element modeling. The results showed that disk diameters and sample height were closely correlated with the failure mode. The failure of remolded loess in the UPT is not only tensile failure, which also can occur compression failure and tensile–compression coupling failure. Among them, when the ratio of disk diameters to sample height is about in the range of 0.189–0.309, the sample appears tensile failure. In addition, the ratio of compressive and tensile crack numbers in the sample is 0.16–0.22 in the tensile failure mode. The numerical simulation method provides a way for the observation of failure process and crack development, thereby improving the understanding of failure essence in UPT. The findings will provide a reference way to improve test theory and method of loess tensile strength.

Investigating thermally treated rocks' mechanical properties and failure processes is crucial for advancing geothermal energy storage and extraction technologies. Naturally occurring rocks typically exhibit inherent flaws and fractures. Nevertheless, existing studies on the impact of thermal treatment on rock damage predominantly utilize intact rock specimens, either through numerical simulations or experimental approaches. This research introduces the Grain-Based model (GBM) to represent the complex shapes of mineral grains accurately and incorporates the heterogeneity of grain sizes. By employing the Weibull distribution, the model captures the variability in rock internal strength due to microcracks, porosity, mineral composition, and other factors. Furthermore, it integrates the temperature dependency of rocks' physical and mechanical properties when subjected to elevated temperatures, the numerical simulation reproduced the phenomenon that the compaction stage of the stress-strain curve caused by thermal damage extends as the temperature rises. Overall, this study provides a complete understanding of the thermal effects on rock integrity. 1. INTRODUCTION: Rocks in nature commonly contain defects such as joints, fissures, and structural planes, which have a decisive impact on the mechanical behavior and stability of the rocks. Particularly under conditions of high temperature, the mechanical properties and failure mechanisms of rocks with such defects can vary significantly from those observed at room temperatures, thus impacting the safety and reliability of rock engineering projects. In geological engineering, nuclear waste storage, and geothermal energy development, rocks are routinely subjected to high-temperature environments. Temperature variations can intensify the expansion of internal defects in rocks, modify their mechanical attributes, and trigger new failure modes. As a quintessential crystalline rock, granite possesses a markedly heterogeneous internal structure. This heterogeneity leads to unique thermal stress responses and failure mechanisms under high temperature and stress conditions, which are significantly different from those observed in homogeneous materials. The non-uniform thermal expansion can result in substantial stress concentration within the rock, facilitating the initiation and propagation of cracks. This process, in turn, impacts engineering structures' long-term performance and reliability.

Natural rocks usually contain joints, fractures, etc. However, current research on rocks' mechanical properties after thermal treatment focuses on intact rocks. To make up for this deficiency, we investigated the influence of temperature on the strength, deformation, and crack propagation behavior of granite with single fissures at different angles. The results indicate that: (1) Compared to intact specimens, the strength of defective granite is reduced due to the presence of defects. As the crack angle increases from 0° to 90°, peak strength and elastic modulus decrease and then increase. (2) Between 150°C and 300°C, the peak strength gradually increases. Beyond 300°C, the peak strength decreases with rising temperature. The initiation, propagation, and peak stress of cracks are closely correlated with temperature. (3) At lower temperatures, cracks' propagation speed and direction will change when encountering minerals. For specimens treated with high temperatures, the temperature caused many thermally induced cracks to form in the sample. The cracks no longer sought the minimum resistance path during the propagation process, and the resistance encountered by crack propagation was greatly reduced, resulting in more direct and predictable paths of crack propagation. 1. INTRODUCTION: Granite, a crystalline rock commonly found in the Earth's crust, is crucial in understanding crustal movements, geothermal energy development, and deep geotechnical engineering construction. The evolution of its mechanical behavior and fracture mechanisms under high-temperature conditions is particularly significant. As human activities increasingly impact the Earth's environment, the demand for high-temperature geological engineering encompassing nuclear waste disposal and underground energy extraction has surged. Exploring the principles and mechanisms behind temperature's influence on the mechanical properties of granite is thus crucial. Understanding how temperature changes affect thermal expansion, crack development, and strength weakening in rocks holds significant theoretical and practical value. Such knowledge is vital for predicting and preventing geological disasters and optimizing rock engineering designs. Scholars have conducted many studies to study the effect of temperature on rock mechanical properties and damage, with some widely recognized conclusions. For example, the temperature increases the porosity (Li et al., 2020) and permeability (Jiang et al., 2018). Rock's P-wave velocity and density decrease with the increase of temperature (Guo et al., 2020). Pan et al. (Pan et al., 2023)studied granite's strength and deformation characteristics after high temperature and chemical solution treatment through uniaxial compression tests. This study introduces a new statistical damage constitutive model considering initial thermochemical and loading damage. This model is used to examine the effects of thermal and chemical treatments on the damage evolution of granite during uniaxial compression tests. It was found that, up to 150 °C, chemical treatment results in more significant damage than thermal treatment. However, after thermal treatment at 300 °C or higher, thermal damage progressively increases, and the disparity in chemical damage induced by various solutions diminishes. Zhang et al. (Zhang et al., 2016) investigated the thermal effects on the physical and mechanical properties of rocks by analyzing changes in mineral composition, internal structure, and water content. Based on the temperature range of adhesive water, bound water, and structural water escaping, they divided the temperature into three ranges: from room temperature to 100 °C, 100 °C to 300 °C, and 300 °C to 500 °C. In the second and third stages, the porosity, permeability, and acoustic emission signals of the rock significantly increase, while the P-wave velocity continues to decrease. The rapid increase in sample volume and number of microcracks within the temperature range of 400 °C to 500 °C leads to changes in rock mechanical properties. Yavuz et al. (Yavuz et al., 2010) analyzed the influence of thermal damage on the physical properties of five types of carbonate rocks. Microscopic observation shows that the damage degree of rocks at high temperatures varies depending on particle size, porosity, structure, and texture characteristics. Microscopic analysis and P-wave results indicate that the internal structure of the rock is compacted below 150 °C, and the expansion of calcite does not cause significant damage to the rock structure. The compaction of rock structure leads to an increase in P-wave velocity and a decrease in porosity. Most of the damage occurs within 24 hours of the heating time, and further heating treatment has a relatively small impact on the changes in physical properties.