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During the operation of an enhanced geothermal system (EGS), the reservoir rock undergoes different cooling treatments based on the distance to the surface rock. The tensile capacity of reservoir rocks is a key parameter in estimating the parameters of the hydraulic fracturing process. Therefore, the gain‐based model 3D (GBM3D) was constructed to explore the micro behavior of granite under the heating/cooling process and the Brazilian splitting test. The numerical results demonstrate the ability of GBM3D to reproduce the grain interlocking and rotational resistance behavior, making it applicable to simulate the mechanical behavior of granite under uniaxial compression and the Brazilian splitting test. Temperature and cooling rates have significant effects on the stress‐strain curves. With the increase in temperature, the curves after peak strength translate from brittle to ductile, and the turning point of water‐cooled granite is lower than that of air‐cooled granite. When more flaws are present in the specimen, micro‐cracks tend to occur along the loading axial near the center of the specimen. This result can be used to explain the defect in numerical simulation, where cracks are easy to develop near the loading position under the Brazilian splitting test. This work can provide recommendations for hydraulic fracturing. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of the mechanical behavior of thermally damaged rock is crucial for the design, prediction, and stability control of rock engineering. However, the strength and fracture characteristics of rock containing pre‐existing flaws have not been studied systematically. Therefore, in this study, uniaxial compression tests were conducted on preholed granite specimens using MTS816 rock mechanics testing and acoustic emission (AE) monitoring systems. The mechanical properties, AE characteristics, and failure patterns of thermally damaged granite specimens were analyzed, and the alteration of the mechanical behavior was explained by the P‐wave velocity, effective porosity, mineral components, and thermal cracking. The results show that the peak strength and elastic modulus of preholed granite specimens first increased and then decreased with the increasingly high temperature. Cracks were always initiated from the left and right perimeters of the pre‐existing hole, and the crack coalescence modes between the two pre‐existing holes were independent of the high temperature. After 150°C treatment, thermal cracks were observed, and the complexity and interconnectivity of the fracture surface increased with temperature. The variations of the mineral component and crystal structure induced by increasing the high temperature contributed to the thermal damage of the granite. The experimental results are expected to enhance the understanding of the macromechanical and mesomechanical behaviors of rock with respect to high temperature. Highlights: The effect of high temperature on the mechanical properties of granite specimens containing pre‐existing holes was investigated.The macro and meso failure characteristics of thermally damaged granite were explored.The mechanism of thermal treatment in changing mechanical behavior of granite was discussed. [ABSTRACT FROM AUTHOR]

Understanding thermo-mechanical properties of granites with different grain sizes is of great significance for the site selection of high-level radioactive waste disposal. In this paper, a cluster model was used to investigate the uniaxial compressive behaviors of granite specimens with different grain sizes. It turns out the results of simulation match experimental results. Grain size has a significant effect on the mechanical behaviors of granite. Macro-cracks can be easily observed in the coarse granite specimens after high-temperature treatment. However, the number of inter-cracks in the granite specimens decreased with increasing the grain sizes. When T ≥ 600 °C, main cracks were not observed in the compressed specimens, and the ultimate failure modes were mostly dominated by thermally induced cracks. The isolated grains can be observed in the coarse granite specimens. The reduction in the uniaxial compressive strength of the coarse-grained granite specimens was lower than that for the fine-grained granite specimens. [ABSTRACT FROM AUTHOR]

To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite. [ABSTRACT FROM AUTHOR]

The pre‐existence of openings, which play an important role in the mechanical properties and cracking behaviours of rock, is prevalent in rock mass. The interaction among pre‐existing openings (or holes) complicates the instability problems when rock contains multiple holes. Studying the strength failure behaviour of rock that contains multiple pre‐existing holes contributes to the fundamental knowledge of the excavation and stability of underground rock engineering. In this study, first, a series of uniaxial compression tests were performed on granite specimens that contain multiple small holes to investigate the effect of the geometry of pre‐existing holes on the strength and fracture behaviours of rock. The crack initiation, propagation and coalescence process, and acoustic emission (AE) characteristics were investigated using photographic and AE monitoring. Three failure modes were identified, ie, splitting failure, stepped path failure, and planar failure modes. Second, a set of micromechanical parameters in the PFC3D model were calibrated by comparison with the experimental results of an intact granite specimen. The numerically simulated peak strength, peak strain, and failure mode of preholed specimens were consistent with the experimental results. In accordance with the numerical results, the failure modes of the preholed specimens were dependent on the bridge angle and number of holes. Last, the internal fracture characteristics of numerical specimens were revealed by analyzing the horizontal and vertical cross sections at different positions. [ABSTRACT FROM AUTHOR]

Abstract Granite, an excellent medium for deep geological disposal projects, geothermal systems and geological carbon storage, may be affected by its grain size and pre-existing flaws. Thus, three types of granite specimens with different grain sizes containing two non-coplanar fissures under uniaxial compression were investigated experimentally and numerically. Notably, the ligament angle had more effect on the peak strength than the grain size. With an increasing ligament angle, the angle between the crack and the principal stress did not show an obvious trend for β ≤ 60°, whereas the angle between the crack and the principal stress increased when β ≥ 60°. This trend is similar to that for the peak strength. For the same ligament angle, the angle between the crack and the principal stress increased as the grain size decreased. Based on the micro-structure of the crack surface in the ligament (β = 60°), the hypothesis that a shear crack is more difficult to initiate in coarse granite specimens than in fine granite specimens was presented and verified by experiments and numerical simulations. [ABSTRACT FROM AUTHOR]

Granite material, as an excellent medium for deep geological disposal rock projects may be affected by high temperature and macro-porosity. However, there are limited experiments and numerical simulations that have been adopted to investigate the failure mechanism of granite specimens that contain pre-existing holes after high temperature treatment. As such, the cluster model in PFC 2D was used to explore the meso -mechanics of granite specimens containing pre-existing holes with different ligament angles (the angle between the line connecting the centers of two holes and the horizontal direction, and set as β = 0, 45 and 90°) after different temperature treatments (T = 25, 150, 300, 450, 600, 750 and 900 °C). The different mineral grains in the granite specimen were simulated by the cluster model with different linear thermal expansion coefficients. The phase transition is treated as a radius expansion with 1.0046 of quartz cluster. The results show that the numerical simulation method is reasonable and the numerical results show good consistency with experimental results. The mechanical property curves can be divided into three phases, where the distribution of micro-cracks in a specimen has more scatter and fail more seriously with increasing temperature. The ligament angle has a significant effect on the crack evolution of a specimen. It was observed that more new micro-cracks with a scattered distribution existed in the high temperature treated specimen because the tensile force is concentrated at the temperature induced cracks. The ligament concentrates with compression in the H-model ( β = 0°) specimen, and concentrates with shear stress in the D-model ( β = 45°) specimen, while the ligament has almost no force concentration in the V-model ( β = 90°) specimen. The first crack coalesced with holes as a shear crack regardless of the ligament angle, and the maximum values of shear stress decrease with increasing temperature. [ABSTRACT FROM AUTHOR]

To understand the high temperature effects on the mechanical and failure behaviors of rock, uniaxial compression tests were carried out on granite specimens containing three pre-existing holes using a rock testing system. Based on the experimental results, the influences of testing temperature on the physical and mechanical parameters of granite were analyzed in detail. An obvious color change of tested granite occurs from gray at room temperature to reddish after 450 °C and to red-brown after 900 °C high temperature treatment. The granite volume increases, mass decreases and density decreases with increasing testing temperature. As the temperature increases, the peak strength first increases and then decreases, while the elastic modulus decreases. However, the peak strain changes slightly before 450 °C, increases dramatically up 450 °C. As the bridge angle increases, the mechanical parameters of granite specimens first decrease and then increase. And then, the crack initiation, propagation and coalescence behavior of granite specimens after high temperature exposure was investigated using an acoustic emission (AE) and photography monitoring technique. The cracking process shows that the propagation of crack from the surface of holes leads to the coalescence between adjacent holes. A large AE count and a stress drop are observed during the crack initiation and propagation. The failure modes can be generally classified into three categories: splitting mode, shear mode and mixed mode and they are closely related to heat treatment temperature and bridge angle. Finally, the mechanism causing the differences in the mechanical parameters observed with increasing temperature was discussed based on the SEM observations. [ABSTRACT FROM AUTHOR]

The evolution of permeability and microcrack characteristics of granite at high temperature has significant effect on the safe and stable operation of high-level nuclear waste disposal repositories. We measure the permeability, porosity and formation factors of granite specimens following thermal treatment including the use of NMR to certify observed response. The results indicate that the initial and residual permeability and porosity change little when T ≤ 300 °C, but increase rapidly when 300 °C ≤ T ≤ 600 °C before entering a stable phase when 600 °C ≤ T ≤ 750 °C. At T = 150 °C microcrack apertures and radius only slight increase while microcrack density and fraction of connectedness all slight decrease, but net causing porosity, the peak strength and elastic modulus to increase. The strength and elastic modulus decrease exponentially with fractional connectivity of microcrack while the initial compressibility of microcracks scale linearly with initial aperture. The formation factor for the granite increases near-linearly with effective stress with the rate of increase generally decreasing with increasing temperature. Thermal conductivity first increases rapidly before remaining constant with increasing effective stress. With increasing temperature, the thermal conductivity decreases and becomes more sensitive to the effective stress. [ABSTRACT FROM AUTHOR]

The evolution of the permeability of granite at high temperatures has a significant effect on geothermal exploitation. In this study, the fluid-solid coupling algorithm is improved based on the different intra-granular, inter-granular and crack seepage characteristics of granite, and the permeability characteristics of thermally treated granite specimens are simulated under different confining pressures. The results show that the permeability of granite non-linearly decreases with confining pressure in a concave type and its variation with temperature is similar to the experimental results. When T ≤ 300 °C, temperature has a slight effect on the variation of permeability. Because the temperature has a slight effect on the distribution characteristics of the flow rate and pore pressure, the flow rate mainly dependent on the inter-granular contact, which is parallel to the gradient of pore pressure. However, when T ≥ 450 °C, the number of thermally induced cracks increases and forms a coalesced channel; thus, the permeability increases with temperature. The temperature has a significant effect on the distribution characteristics of the flow rate and pore pressure. Confining pressure can decrease the permeability, whereas it has a slight effect on the distribution of flow rate and pore pressure in the specimen. [ABSTRACT FROM AUTHOR]

This study developed a fully coupled thermo-mechanical model within the framework of ordinary state-based peridynamics to investigate the thermal-mechanical properties and fracture characteristics of granite materials under real-time temperature (RT) and post-temperature (PT) treatments. Moreover, a modified multi-layer computational method was proposed to eliminate the effect of thermal gradient-induced cracks. In this method, the size of the thermal layer was set to be larger than that of the tested sample, with the additional area used to withstand the thermal shock. During the loading process, the thermal damage information was transplanted to the corresponding position of the mechanical layer with the data of the additional area cut off. It was verified that thermal cycling cracks and thermal gradient-induced cracks could be modelled and distinguished, and the stress-strain and cracking behaviours of both RT and PT samples could be properly simulated using the proposed method. First, two benchmark examples involving heat conduction in a square plate and uniaxial compression of a granite specimen were simulated to investigate the numerical convergence and calibrate the simulation parameters, respectively. Then, two numerical examples were used to investigate and compare the stress-strain behaviours, cracking patterns, and temperature evolutions of the RT and PT samples. A systematic comparison with the experimental results makes it possible to discuss and summarise the failure mechanism of granite specimens subjected to PT and RT tests. [ABSTRACT FROM AUTHOR]

A detailed understanding of the thermal damage and failure mechanical behavior of granite at elevated temperatures is a key concern in nuclear waste disposal engineering, underground coal gasification, and heat mining in enhanced geothermal energy. In this research, uniaxial compression tests were first carried out to evaluate the effect of high temperature treatments (200, 300, 400, 500, 600, 700 and 800 °C) on the crack damage, strength and deformation failure behavior of a granite. The results demonstrated that, in all cases, the crack damage threshold, the strength and static elastic modulus of granite were increased at 300 °C, before decreasing up to our maximum temperature of 800 °C. However, the static Poisson’s ratio of granite first decreased at 600 °C, and then increased rapidly with the temperature. The crack damage and peak axial strain always showed an increase when the temperature was increased. However, the dynamic elastic modulus decreased with the temperature, whereas the dynamic Poisson’s ratio did not depend on the temperature. The gradual increase of temperature results in a more ductile failure of granite. Next, the thermal damage mechanism of uncompressed granite was analyzed by optical microscopic observation. At T = 25–300 °C, the mechanisms were favored by the thermal expansion of mineral grains but no microcracks were observed; at T = 400–600 °C, the mechanisms were contributed by boundary cracks and transgranular cracks in feldspar and quartz grains; and at T = 700–800 °C, the mechanisms were associated with the coalescence of boundary cracks and transgranular cracks. The internal crack evolution process was then monitored during deformation using acoustic emission (AE) monitoring. The results showed that the cracking process of granite depended on the heat treatment temperature. Finally, the deformation mechanism of failed granite at various temperatures was analyzed using X-ray micro CT. During loading, the uniaxial compression stress direction dominated the more brittle fracture process of granite at T = 25–600 °C, which led to splitting tensile main cracks induced along the axial stress, and thermal damage determined the larger ductile fracture process of granite at T = 700–800 °C, which resulted in a more ductile deformation after the peak strength. [ABSTRACT FROM AUTHOR]

• Analyze the effect of fissure inclination and confining pressure on mechanical behavior of fissured granite. • Investigate triaxial damage process of fissured granite by the evolution of microcracks. • Characterize three-dimensional crack characteristics by displacement field and X-ray CT images. In order to investigate the mechanical behavior and failure mechanism of fissured granite under triaxial compression, a three-dimensional grain-based model (GBM3D) was established in particle flow code (PFC) for simulating the triaxial compression of granite specimens with various fissure inclinations (α = 0°, 30°, 45°, 60° and 90°) under different confining pressures (0, 5, 10 and 20 MPa). First, a group of micro-parameters used in GBM3D was calibrated based on the experimental triaxial compression results of intact specimens, the simulation results of fissured specimens based on these parameters are in good agreement with the experimental results. Compared with intact granite, the fissured granite has smaller peak strength, crack damage threshold, elastic modulus and axial peak strain. Confining pressure can improve the mechanical properties of granite at the same fissure angle, and under the same confining pressure, the mechanical properties increase with the increase of fracture angle. With the increase of axial stress, the cracks first emerge from the crack apices and propagate toward the ends of the specimen along the direction of the principal stress, and the cracks penetrate each other and eventually lead to the failure of the specimens. The displacement field shows that the main cracks are all generated at the interfacial locations where large relative displacements occur between the particles. The three-dimensional spatial crack distribution characteristics of the damaged specimens are not only influenced by the confining pressure, but also related to the fissure angle. The confining pressure not only complicates the crack distribution characteristics, but also increases the total count of cracks. As the fissure angle increases, anti-tensile wing crack is more likely to appear in the specimen. [ABSTRACT FROM AUTHOR]