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The study of the evolution law of basic physical parameters and dynamic compression performance of deep granite under the environment of the heating-cooling cycle is of great significance for the stability evaluation of deep underground engineering and the development of deep resources. In this study, heating-cooling cycle tests and dynamic compression tests were conducted on a large number of fine-grained granite specimens with heating temperatures from 200 to 600 °C and times from one to twenty times using a box-type high-temperature muffle furnace and Hopkinson pressure bar (SHPB) test system, and the evolution law of basic physical parameters and dynamic compression mechanical properties of fine-grained granite were studied using theoretical and fitting analysis. The test results showed that: the changes of the basic physical parameters of granite have obvious temperature effect; 600 °C is a threshold value for the changes of each physical parameter of granite; the sensitivity of each physical parameter to the number of heating and cooling cycles is small before 600 °C; and the sensitivity of each physical parameter to the number of heating and cooling cycles significantly increases at 600 °C. The dynamic compressive strength and elastic modulus of granite decreased with the increase in heating and cooling cycles, and the maximum decrease rate was 89.1% and 85.9%, respectively, and the strain rate linearly increased with the increase in heating and cooling cycles, and the maximum strain rate was 123 s−1. The temperature, the number of heating and cooling cycles, and the impact air pressure, all had significant effects on the damage mode and crushing degree of granite. [ABSTRACT FROM AUTHOR]

Flame jet-assisted mechanical rock drilling is expected to solve the hard-rock crushing challenges in underground engineering such as mining, tunneling, and drilling. Therefore, it is important to investigate mechanical behavior of the rock after flame jet-water cooling treatment. In this paper, granite blocks were subjected to flame jet-water cooling treatment, which means the rapid heating of the granite using a flame with a temperature of 1564 °C, followed by water cooling. Then, physico-mechanical behaviors of samples at different distances (all the distances mentioned later mean the vertical distance between the sample axis and the flame jet-water cooling path) were measured. Results showed physico-mechanical parameters of the sample increased as the distance increased. The V p (P-wave velocity), UCS (uniaxial compressive strength) and BTS (Brazilian tensile strength) of the samples increased from 3171.7 m/s, 84.84 MPa and 5.01 MPa at 0 mm to 3619.3 m/s, 145.86 MPa and 7.30 MPa at 70 mm, respectively. Meanwhile, physico-mechanical parameters of samples at 70 mm all reached more than 95 % of those of untreated samples. As the distance increased, cumulative AE counts at peak stress gradually enhanced, and failure modes of uniaxial samples shifted from shear to tensile splitting failure. The SEM (scanning electron microscope) images illustrated that the closer to the flame jet-water cooling path, the more thermal cracks produced in the sample. Meanwhile, the damage factor can well characterize the thermal damage degree of granite. According to the variation of physico-mechanical parameters and SEM images of samples with distances, it can be assumed that the thermal damage width of granite after flame jet-water cooling was about 140 mm. It was much greater than the 50 mm width of spalling pits, which greatly increased the feasibility of thermal-assisted mechanical rock drilling. The tensile stress induced by the temperature gradient below the spalling pit is the fundamental cause of thermal damage to the rock, which is further aggravated by water cooling. The findings can provide some guidance for flame jet-assisted mechanical rock drilling. [ABSTRACT FROM AUTHOR]

Dynamic compression tests of granite after thermal shock were performed using the split Hopkinson pressure bar system, to determine the effects of strain rate and temperature on the dynamic mechanical parameters, energy dissipation features and failure modes of granite. The results indicate that the dynamic compressive strength increased exponentially with strain rate and decreased with increasing temperature. Temperature and incident energy can equivalently transform for the same dynamic compressive strength. Dynamic elastic modulus of granite decreased obviously with increasing temperature but did not have a clear correlation with strain rate. As the impact gas pressure increased, the stress-strain curves changed from Class II to Class I behavior, and the failure modes of specimens transformed from slightly split to completely pulverized. The critical temperature at which the stress-strain curves changed from Class II to Class I was determined to be 300 °C, when the impact gas pressure is 0.6 MPa. As the applied temperature increased, density, wave velocity and wave impedance all decreased, meanwhile, the degree of granite specimen crushing was aggravated. Under the same incident energy, as the temperature increased, the reflected energy increased notably and the absorbed energy increased slightly, but the transmitted energy decreased. For the same temperature, the reflected and absorbed energies increased linearly as the incident energy increased, whereas the transmitted energy increased logarithmically. The SEM images of the thermal crack distribution on the granite specimen surface at different temperatures can well explain the essence of mechanical parameters deterioration of granite after thermal shock. This work can provide guidance for impact crushing design of high temperature rocks during excavations. [ABSTRACT FROM AUTHOR]

Under certain extreme conditions in rock engineering works, fast change in temperature in the load-bearing rocks can happen. Known as thermal shock (TS), such process involves rapid temperature rise or drop, which causes fracturing in the rock material and thus can pose as a threat to the stability of the rock structures. To investigate the influence of thermal shock caused by fast cooling on the mechanical property of rock, laboratory tests are performed on heated granite which are cooled with different methods, with the highest cooling rate reaching 167.4 °C/min. The dynamic loading tests are performed on the heated granite specimens utilizing the split Hopkinson pressure bar (SHPB) system. The test results show that the dynamic compressive strength drops with the increase in heating level or cooling rate. This pattern is explained by the nuclear magnetic resonance (NMR) test data that the pores inside the heated granite increase both in size and quantity as heating level or cooling rate rises. Damage patterns of the tested granite specimen fragments are analyzed based on the observation with scanning electron microscope (SEM), and the mechanisms of thermal shock in granite are also discussed. [ABSTRACT FROM AUTHOR]

Variation in the heating rate due to different geothermal gradients is a cause of much concern in underground rock engineering such as deep sea and underground tunnels, nuclear waste disposal, and deep mining. By using a split Hopkinson pressure bar (SHPB) and variable-speed heating furnace, the dynamic compressive properties of granite were obtained after treatments at different heating rates and temperatures; these properties mainly included the dynamic compressive strength, peak strain, and dynamic elastic modulus. The mechanism of heating rate action on the granite was simultaneously analyzed, and the macroscopic physical properties were discussed. The microscopic morphological features were obtained by scanning electron microscopy (SEM), and the crack propagation was determined by high-speed video camera. The experimental results show that the dynamic compressive strength and elastic modulus both show an obvious trend of a decrease with the increasing heating rate and temperature; the opposite phenomenon is observed for the peak strain. The relationships among the dynamic compressive properties and temperature could be described by the quadratic function. The ductility of granite is enhanced, and the number and size of cracks increase gradually when the heating rate and temperature increase. The microstructure of rock is weakened by the increased thermal stress, which finally affects the dynamic compressive properties of rock. [ABSTRACT FROM AUTHOR]

The Earth's deep rock mass is subjected to various complex temperature and stress perturbations. To study the effect of temperature and dynamic disturbance on the damage mechanical properties of these rocks, a series of laboratory tests was carried out by means of a modified split Hopkinson pressure bar system. Specimens were heat treated from room temperature up to a maximum of 800 °C and then cooled to room temperature. During repeated loading tests, the dynamic incident energy was kept constant in each cycle. For samples under the same treated temperature, the dynamic strength and deformation capacity degraded gradually with increasing impact number. Furthermore, rock strength decreased with increasing treated temperature, with the number of impacts before failure reduced accordingly. Since damage can be initiated by thermal treatment and then aggravated by dynamic disturbance, according to the observed decrease in rock strength and increase in strain rate under the different temperature conditions, a temperature of 400 °C was found be a significant failure threshold. The maximum strain was employed to describe damage evolution during cyclic impact loading, indicating that fatigue damage is gradually accumulated and maintains a three-segment growth with an increase in repeated impacts. Under the coupled effect of temperature and cyclic impact loading, specimens exhibited two different failure modes: split tensile failure and unloading failure. The micro-properties of fracture morphology in granite after different temperature and repeated impact were also discussed in detail. [ABSTRACT FROM AUTHOR]

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek-Brown model is more accurate than the linear failure criterion based on the Mohr-Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing. [ABSTRACT FROM AUTHOR]

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi-Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process. [ABSTRACT FROM AUTHOR]

To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks. [ABSTRACT FROM AUTHOR]

To understand the effects of increasing temperature and loading rate on the flexural tensile strength of Laurentian granite, dynamic flexural tensile strength experiments were carried out by means of a semi-circular bend specimen with a modified split Hopkinson pressure bar system. The tests were performed at different loading rates, specimens were treated from room temperature up to 850 °C, and a high-speed camera was utilized to monitor the failure process of the specimen. For samples in the same temperature group, a loading rate dependence of the flexural tensile strength was observed; it increased consistently with the increase of loading rate. Temperature effects on rock mechanical properties were investigated from the microscopic viewpoint, and the dynamic flexural tensile strength decreased with the treatment temperature. A formula relating dynamic flexural tensile strength to loading rate and temperature is presented to quantify the results. It was found that the change regulation of the dynamic flexural tensile strength of rock is very similar to that of its crack growth along with the increase of loading rate, which indicates that the essence of rock failure is the initiation and propagation of the internal cracks. Compared with our earlier work on dynamic tensile tests using the Brazilian test, it was observed that the flexural tensile strength is higher than the tensile strength. Non-local failure theory can be adopted to explain this discrepancy at low temperature conditions, but it is no longer effective at high temperatures. Under high loading rates, rock failure is initiated at the centre of the half circular disc, and finally it is separated completely into two equal parts. [ABSTRACT FROM AUTHOR]

True triaxial tests have been carried out on granite, sandstone and cement mortar using cubic specimens with the process of unloading the minor principal stress. The strengths and failure modes of the three rock materials are studied in the processes of unloading σ and loading σ by the newly developed true triaxial test system under different σ, aiming to study the mechanical responses of the rock in underground excavation at depth. It shows that the rock strength increases with the raising of the intermediate principal stress σ when σ is unloaded to zero. The true triaxial strength criterion by the power-law relationship can be used to fit the testing data. The 'best-fitting' material parameters A and n ( A > 1.4 and n < 1.0) are almost located in the same range as expected by Al-Ajmi and Zimmerman (Int J Rock Mech Min Sci 563 42(3):431-439, 2005). It indicates that the end effect caused by the height-to-width ratio of the cubic specimens will not significantly affect the testing results under true triaxial tests. Both the strength and failure modes of cubic rock specimens under true triaxial unloading condition are affected by the intermediate principal stress. When σ increases to a critical value for the strong and hard rocks (R4, R5 and R6), the rock failure mode may change from shear to slabbing. However, for medium strong and weak rocks (R3 and R2), even with a relatively high intermediate principal stress, they tend to fail in shear after a large amount of plastic deformation. The maximum extension strain criterion Stacey (Int J Rock Mech Min Sci Geomech Abstr 651 18(6):469-474, 1981) can be used to explain the change of failure mode from shear to slabbing for strong and hard rocks under true triaxial unloading test condition. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on several physical properties and the tensile strength of Laurentian granite (LG) are measured in this study. Brazilian disc LG specimens are treated at temperatures of up to 850 °C. The physical properties such as grain density, relative volume change per degree, and P-wave velocity are investigated under the effect of heat treatment. The results indicate that both the density and the P-wave velocity decrease with the increase in heating temperature. However, the relative volume change per degree is not sensitive below 450 °C, while a remarkable increase appears from 450 to 850 °C. All cases are explained by the increase in both number and width of the thermally induced microcracks with the heating temperature. Brazilian tests are carried out statically with an MTS hydraulic servo-control testing system and dynamically with a modified split Hopkinson pressure bar (SHPB) system to measure both static and dynamic tensile strength of LG. The relationship between the tensile strength and treatment temperatures shows that static tensile strength decreases with temperature while the dynamic tensile strength first increases and then decreases with a linear increase in the loading rate. However, the increase in dynamic tensile strength with treatment temperatures from 25 to 100 °C is due to slight dilation of the grain boundaries as the initial thermal action, which leads to compaction of rock. When the treatment temperature rises above 450 °C, the quartz phase transition results in increased size of microcracks due to the differential expansion between the quartz grains and other minerals, which is the main cause of the sharp reduction in tensile strength. [ABSTRACT FROM AUTHOR]

Rocks in underground projects at great depth, which are under high static stresses, may be subjected to dynamic disturbance at the same time. In our previous work (Li et al. Int J Rock Mech Min Sci 45(5):739-748, ), the dynamic compressive behaviour of pre-stressed rocks was investigated using coupled-load equipment. The current work is devoted to the investigation of the dynamic tensile behaviour of granite rocks under coupled loads using the Brazilian disc (BD) method with the aid of a high-speed camera. Through wave analyses, stress measurements and crack photography, the fundamental problems of BD tests, such as stress equilibrium and crack initiation, were investigated by the consideration of different loading stresses with abruptly or slowly rising stress waves. The specially shaped striker method was used for the coupled-load test; this generates a slowly rising stress wave, which allows gradual stress accumulation in the specimen, whilst maintaining the load at both ends of the specimen in an equilibrium state. The test results showed that the tensile strength of the granite under coupled loads decreases with increases in the static pre-stresses, which might lead to modifications of the blasting design or support design in deep underground projects. Furthermore, the failure patterns of specimens under coupled loads have been investigated. [ABSTRACT FROM AUTHOR]

Rock mass is frequently subjected to rapid cooling in geothermal reservoir during water injection and reinjection. In this paper, to understand the effects of cooling treatments on heated granite, heat conduction tests, magnetic resonance imaging tests and numerical investigations were carried out to evaluate variations of thermal damage. The test results reveal that the heat flux and the heat transfer coefficient increases to a maximum within a few seconds and then gradually decreases. The maximum heat transfer coefficient of the samples treated with the initial temperature of 500, 400, 300, 200 and 100 °C is 2.3, 2.15, 1.9, 1.22 and 1.86 W·m−2K−1, respectively. The edge area with drastic temperature changes is accompanied by the densely distributed microcracks; in contrast, the internal cracks of the specimen with gentle temperature are relatively sparse. The thermal damage contributed by the heating cracks occurs at a continuous decrease, and the thermal damage contributed by cooling occurs at a continuous increase, with the increasing heating temperature. The damage caused by heating is the result of the uneven thermal expansion of the local particles, the propagation of cooling cracks is strongly affected by heating cracks, and stress concentration induced by thermal shock promotes the coalescence of the pre-existing heating cracks. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 °C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 °C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 °C and above 450 °C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 °C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges. [ABSTRACT FROM AUTHOR]

• Granite with higher temperature, cooling rate and cycles suffered more damage. • The increase of permeable pores and reduce the fractal dimension of pore structure. • Granite with coarser particles is more susceptible to the cyclic thermal treatment. • The existing cracks in granite can reduce further damage induced by thermal shock. High-temperature rocks in the geothermal reservoir are susceptible to damage under periodic extraction and reinjection conditions, and variation of the pore structure threatens the efficient production of geothermal energy. We mainly conducted research on the influence of cyclic heating and cooling on the pore structure of granite by nuclear magnetic resonance technology and magnetic resonance imaging (MRI). The results show the significant increase of porosity and the sharp drop of ultrasonic velocity after cyclic heating and cooling. The total porosity and movable porosity of the sample increase successively with the increasing heating temperature, cooling rate and thermal cycles, while the fractal dimension of the pore structure shows the opposite tendency, this can be attributed to the fact that the dominant permeable pores weaken the pore complexity. The pore structure of granite with lower heating temperature or finer particles can maintain stability for more thermal cycles. MRI observations verified the extended and deepened micro-cracks in granite during thermal cycles, which can be attributed to the comprehensive effects of uneven expansion and shrinkage of the particles, the transformation of the mineral crystal structure, fatigue damage and the cooling-induced thermal shock. [ABSTRACT FROM AUTHOR]

• The temperature of granite increased linearly as microwave heating time. • The increasing microwave power contributes to enhancing the heating rate. • Fracture toughness of granite decreased exponentially as the heating time. • The FPZ size is positively related to the ductility of microwave heated granite. Hard rock breaking is a great challenge for deep drilling and mining engineering, and the microwave-assisted rock breaking technique offers a promising solution. However, the lack of knowledge of the effects of microwave irradiation on hard rocks limits the further development of this technology. Laboratory tests have been conducted to investigate the variations of fracture properties of granite subjected to microwave heating. The semi-circular bend (SCB) specimens were prepared, and mode I fracture tests were conducted on the microwave treated granite specimens. The acoustic emission (AE) events was recorded and the strain field was calculated by the digital image correlation (DIC) technique. The test results demonstrated that the temperature of the sample surface increased linearly as the increasing heating time, and increasing microwave power contributes to enhancing the heating rate. According to the results of scanning electron microscope (SEM), intergranular cracks were the main form of cracks, and the transgranular cracks usually appeared in samples with high microwave power or longer heating time. The generation of micro-cracks can be attributed to the high temperature and the thermal shock. With the increasing heating time, the fracture toughness decreased exponentially, the AE active period prolonged, and the fractal dimension of the fracture surfaces (FSD) continuous increased. Compared to an untreated sample, the maximum principal strain and fracture process zone (FPZ) size of the microwave heated sample were larger at the same loading level, which indicated the enhancement of ductility. According to the experimental results, the relationships between FPZ size, fracture toughness and FSD were empirically regressed. Understanding the effects of microwave heating on the fracture properties is of significant to improve hard rock breaking efficiency. [ABSTRACT FROM AUTHOR]

• Thermal damage of water-cooled specimens is mainly concentrated on the surface of the specimens and is mainly controlled by temperature gradient. • Thermal damage of air-cooled specimens is randomly distributed inside the specimens and controlled by thermal expansion coefficient. • From the heat transfer point of view, the essential difference between the two cooling methods is the convective heat transfer coefficient. • Thermal damage of water-cooled specimen is much larger than that of air-cooled specimen, which is the essential reason for lower strength of water-cooled specimen. This work investigated the fundamental reason for the deterioration of mechanical properties of granite after two different heat treatments (slow heating followed by water or air cooling). The responses of mechanical and acoustic properties of the granite after two different heat treatments were obtained by a servo testing machine, a Digital Image Correlation technology (DIC), and an acoustic emission (AE) system. Meanwhile, the heat transfer laws and thermal damage distributions of granite under different cooling methods were discussed by using COMSOL software. The results show that the tensile strength of the water-cooled specimen is lower than that of the air-cooled. The AE counts of the water-cooled specimen are more intensive than that of the air-cooled during the compaction stage. With increasing initial temperature, the accumulated AE counts of the air-cooled specimen increase, but the accumulated AE counts of the water-cooled specimen first increase and then decrease. Meanwhile, the cooling methods have a great influence on the distribution of tensile stresses and thermal damage of the specimens. The distribution of tensile stress and thermal damage of air-cooled specimens is random, which is mainly controlled by the heterogeneity of the thermal expansion coefficient. However, the tensile stresses and thermal damage of water-cooled specimens are distributed on the surface of the specimen, which is mainly controlled by the temperature gradient. Meanwhile, water cooling has a more severe thermal damage on specimens than air cooling. The difference in the convective heat transfer coefficient between rocks and cooling mediums determine heat transfer process. Results indicate that the selection of work fluid with a higher convective heat transfer coefficient is of great significance for improving the efficiency of geothermal energy extraction. [ABSTRACT FROM AUTHOR]

Highlights • The dynamic fracture parameters of rocks under high temperature are measured. • A numerical procedure of rock dynamic fracture test is developed based on the laboratory tests. • Particle flow code can reproduce the mechanical behaviors of the high-temperature rock. • The simulated dynamic load test is critical for the acquisition of the critical time. • The loading rate effect and temperature dependence of dynamic fracture toughness are discussed. Abstract With the limitations of measuring techniques, limited investigation on dynamic fracture testing of rocks at high temperature is reported. In this paper, an effective measuring method of the dynamic mode I fracture toughness of high-temperature rocks, combined with laboratory tests with the Discrete Element Method (DEM), is proposed. Based on the laboratory experiments conducted, including uniaxial compressive tests under different temperature conditions, the Brazilian test and split Hopkinson pressure bar (SHPB) test, the micro-parameters of the numerical model are calibrated and the validity of the numerical model of specimens and the simulated dynamic load tests are examined. The results show that the DEM can truly reflect the mechanical properties of rocks at high temperatures, and the simulation test can capture the dynamic fracture parameters of rocks at elevated temperatures. The test data indicates that the fracture initiation toughness of specimens increases linearly with the loading rate, while the fracture initiation time detected by the measurement circle near the notch tip decreases. When the loading rate is higher than 130 GPa m1/2/s, the fracture initiation toughness has a significantly negative relationship with the temperature within the temperature range of 25–400 °C. [ABSTRACT FROM AUTHOR]

• A numerical fracture model of granite under indentation conditions is established. • Indentation energy of treated granite is linearly correlated with brittleness index. • Specific energy is more sensitive to temperature as increasing confining stress. • Poly function of specific energy, temperature and confining stress is established. Microwave-assisted rock breakage is a promising solution for drilling and crushing hard-rock formations. The fundamental mechanical and thermophysical properties of microwave-treated granite were obtained, and the factors affecting the fracture behaviour under indentation were investigated using experimental and numerical methods. The results show that the uniaxial compressive strength, tensile strength, brittleness index, thermal conductivity, and thermal diffusivity of microwave-heated granite are negatively correlated with temperature, whereas the specific heat capacity of treated granite is positively correlated with temperature. The fracture process of microwave-heated granite under indentation can be divided into three stages: elastic deformation, fluctuating penetration, and post-peak. The indentation damage zone near the indenter consists mainly of tensile cracks. Meanwhile, regression analysis shows that the peak indentation force, penetration depth, and consumed energy of the microwave-heated sample are linearly correlated with the brittleness index. In addition, microwave treatment can weaken the inhibitory effect of a high confining stress on the damage zone and is conducive to reducing the indentation force and penetration depth of rock fragmentation. The specific energy is more sensitive to temperature under high confining stress conditions, and the relationship between the specific energy, temperature, and confining stress can be described by a 2D poly function. [ABSTRACT FROM AUTHOR]

• Cyclic heating and cooling leads to significant deterioration of fracture toughness of granite. • Increasing temperature and cycles enhance the ductility of granite. • Enhancement of ductility results in rougher fracture surfaces and greater fractal dimensions in treated granite specimens. • Generation of micro-cracks induced by cyclic heating and cooling is the result of a combination of various factors. Rock in the geothermal reservoir is frequently subjected to cyclic heating and cooling, and fracture characteristics are closely related to the stable operation of Enhanced Geothermal Systems. To examine impacts of cyclic heating and cooling treatments on mode I fracture characteristics of granite, fracture tests of semi-circular bend specimens were conducted, acoustic emission (AE) events were monitored, and morphology characteristics of fracture surfaces were analyzed. Meanwhile, the open porosity was measured, and the scanning electron microscope (SEM) was applied to identify variations of microstructure in granite affected by cyclic treatments. Failure pattern and modifications of microstructure in treated granite specimens were discussed. Results indicated that cyclic treatments can lead to serious degradation in granite, manifested as the increase of open porosity, the decrease of fracture toughness and cumulative AE counts with both increasing temperature and cycles. The smoother post-peak stage in load-displacement curves and the prolonged active period of AE events of granite specimens after cyclic treatments implied the enhancement of ductility. Fractal dimensions of fracture surfaces exhibited evidently negative correlation with the brittleness index, which demonstrated that the enhanced ductility promotes the generation of rougher fracture surfaces. The failure pattern of treated granite specimens revealed that the propagation path of the main crack is strongly affected by the thermal cracks. SEM observations verified the increased, extended and deepened micro-cracks in granite after cyclic treatments, which can be attributed to the comprehensive effects of heating, rapid cooling, thermal cycling, and water-induced weakening. [ABSTRACT FROM AUTHOR]

Highlights • The dynamic fracture behaviour of granite was investigated with dynamic and static coupling test after heat treatment. • Dynamic fracture toughness of heat treated granite specimens under static preload was obtained using quasi-static formula. • The variation pattern of dynamic fracture toughness with loading rate and heat treatment temperature is discussed. • Observations have been made on the failure process and fracture pattern of the specimens was analysed. Abstract In order to study the effects of temperature and loading rate on the dynamic fracture behaviour of granite under a given preload, dynamic fracture tests of pure mode-I were carried out on cracked straight-through Brazilian disc (CSTBD) specimens with a dynamic and static coupling test device based on the split Hopkinson pressure bar (SHPB) system. The experiments were performed at different loading rates and specimens were heat-treated at different temperatures. The thermal damage of specimens increased with ascending treatment temperature in general except at 100 °C, and the samples almost lost their load-bearing capacity after heat treatment at 800 °C. Based on the one-dimensional stress wave theory, under the condition of dynamic stress balance, the dynamic fracture toughness of the granite under a given static preload was obtained by substituting the average force on both ends of the CSTBD specimen into the quasi-static formula of the stress intensity factor of the central cracked disk and combining the crack initiation time measured by the strain gauge method. The results showed that the dynamic fracture toughness increases linearly with the loading rate after the same heat treatment, and decreases with increasing treatment temperature in general under the same loading rate, although the fracture toughness values of heat treatment at 100–200 °C and 400–600 °C are very close to each other. Partial correlation analysis was used to prove that the effect of loading rate on the dynamic fracture toughness of granite is greater than that of temperature. A high-speed camera was utilized to record the failure process of specimens. Most of the specimens were separated into two roughly identical halves, while a small number of specimens were broken into several blocks due to multiple crack growth. In addition, triangular crushed zones of different sizes are observed at the loading ends of all specimens. The results obtained in this paper are of great value for the analysis of the fracture characteristics and induced cracking of deep rocks. [ABSTRACT FROM AUTHOR]

Triaxial compression tests were conducted on fine-to-medium–grained granite specimens, with initial confining pressures of 10, 20, 40, and 60 MPa, under various loading and unloading stress paths. The energy evolution characteristics of granite specimens from a quarry in Miluo city (China) were studied in the triaxial deformation and failure process of the rocks. The results show that the time history curves of the total strain energy, elastic strain energy, and dissipative strain energy exhibit significant stage features. In particular, the ratio of the dissipative strain energy to the total strain energy can be used to describe the deformation and degree of damage to rock specimens during the triaxial loading and unloading processes. Under the same initial confining pressure, the maximum values of the total strain energy, elastic strain energy, and dissipative strain energy occur in the conventional triaxial compressive testing of group I, and the minimum values occur in test group II with constant axial stress and decreasing confining pressure. The total strain energy, elastic strain energy, and circumferential strain energy all increase as the initial confining pressure increases, whereas the dissipative strain energy does not. During the process of unloading the confining pressure, the increase of the circumferential strain is considerably larger than that of the axial strain. Under unloading conditions, rock bursts may occur more easily for hard rocks than under conventional triaxial loading conditions, especially under the conditions of test group III with increasing axial stress and decreasing confining pressure. The micro-difference in the granite micro-cracks was identified using a scanning electron microscope (SEM) combined with an energy dispersive spectrometer (EDS). Shear failure characteristics were observed in a conventional triaxial test, and the combined tension and shear failure was identified through unloading confining pressure tests. The tensile failure characteristics of the granite in group III are more pronounced than those of group II. This indicates that the triaxial failure of rock results from the development of micro-extension cracks and volumetric expansion in the granite specimen under unloading confining pressure tests. [ABSTRACT FROM AUTHOR]