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The objective of this study is to investigate the dynamic mechanical properties of coal and rock under deep water conditions. The research employs an enhanced Split Hopkinson Pressure Bar (SHPB) testing system. Five sets of dynamic impact experiments were conducted on coal samples under varying loading conditions to analyse the changes in dynamic strength, energy dissipation, fractal dimension and other characteristics of coal samples under different water content states were analyzed. The experimental results demonstrate that: (1) Under specific strain rate conditions, the dynamic strength of saturated coal samples is lower than that of natural coal samples. As the strain rate gradually increases, the bonding force generated by free water and the Stefan effect jointly act, and the peak strength of saturated coal samples under high strain rate loading conditions is higher than that of natural coal samples. (2) Under certain strain rate conditions, the absorption energy of saturated coal samples is approximately 10% to30% lower than that of natural coal samples, and deformation hysteresis phenomenon occurs in natural coal samples, thereby improving the dynamic strength of natural coal samples relative to saturated coal samples; (3) The fractal dimension of saturated coal samples with a specific strain rate under three-dimensional dynamic static combination loading is higher than that of natural coal samples, and the percentage of small particle coal samples with debris is higher than that of natural coal samples; Finally, based on the HJC model, some coal samples were selected to simulate the coal rock failure characteristics during the triaxial loading process using ANSYS/LS-DYNA, and their stress–strain curves and failure morphology diagrams were obtained. The discrepancy between the numerical simulation and the experimental results was less than 10%, thereby further elucidating and corroborating the coal failure process and dynamic mechanical characteristics. [ABSTRACT FROM AUTHOR]

The surrounding rock of a tunnel is subjected to cyclic impacts from different directions in tunnel engineering construction, resulting in directional damage to the rock mass. To investigate the impact direction effects on damage, a split Hopkinson pressure bar (SHPB) system was utilized to conduct cyclic impact tests on marble. The dynamic mechanical response of the marble was analyzed under different impact directions and loadings. Additionally, the P-wave velocity and pore distribution of damaged marble under different modes were obtained. The results showed that the P- wave velocity of marble decreased with the increasing impact times and loadings, while the porosity increased. The P-wave velocity of marble in Mode I was generally lower than that in Mode II, while the porosity was higher than that in Mode II. At an impact loading of 98 MPa, the porosity of marble subjected to three impacts was 3.21% and 1.98% under Modes I and II, respectively. The micropores showed a decreasing trend with impact times in Mode I, and the proportion of medium pores increased in both modes. In terms of dynamic properties, the dynamic peak strength and elastic modulus of marble declined with increasing damage. The degradation effect of Mode I on marble was significantly greater than that of Mode II, and it intensified with the increasing number of impacts and loadings. The difference in mechanical parameters and absorbed energy of marble under different impact modes increased with impact times and loadings. The energy absorption ratio of marble under Mode I was superior to that under Mode II. Additionally, a correction function was incorporated into the damage constitutive model to portray the compaction phase of marble. The improved model can accurately depict the mechanical response of marble under different modes. Finally, the damage anisotropy ratio (DAR) was defined based on P-wave velocity to characterize damage anisotropy, and the mechanism of damage anisotropy of marble under different impact modes was discussed. Highlights: Dynamic compression tests were conducted on marble under different impact directions and loadings ultilizing an SHPB system. The directional effects of the P-wave velocity and pore characteristics of marble under different impact modes were obtained. The dynamic mechanical response of the marble and the differences in response under different impact modes were analyzed in depth. The damage anisotropic characteristics of marble under different impact modes were investigated, and the directional effect on the damage mechanism was discussed. [ABSTRACT FROM AUTHOR]

As the mileage of long tunnels and the number of underground engineering projects continue to increase, the issue of rock mass stability after fire and explosion becomes increasingly prominent. Therefore, studying high strain rates mechanical properties of rocks under different heat treatment methods is of great significance to the design of geological engineering. In this study, Caledonian granite of Sejila Mountain is selected as the research object. Impact compression tests are conducted after high-temperature natural cooling and real-time high-temperature treatment to analyze the mechanical properties and crushing energy consumption characteristics after exposure to 400 ℃. The results show that the temperature threshold of Sejila Mountain granite is 800 ℃. On average, the peak stress after natural cooling at 400 ℃, 500 ℃, 600 ℃, and 700 ℃ decreases by 6.96%, 8.77%, 16.11%, and 31.51% compared to room temperature. Under real-time high-temperature, the peak stress decreases by 8.36%, 14.65%, 27.83%, and 46.55% on average compared to room temperature. The specimen fragments under real-time high-temperature are more rounded than those after natural cooling, and the fractal dimension is larger. The specific energy absorption (SEA) has a sudden change critical temperature, which is 500 ℃ for real-time high-temperature and 600 ℃ for natural cooling. Highlights: The damage and deterioration laws of granite mechanical properties after natural-cooling and real-time hightemperature are compared through impact compression mechanical test. It is analyzed the mechanical properties and crushing energy consumption characteristics of Caledonian granite in Sejila Mountain at 400 ℃-800 ℃. The defects of fractal dimension calculation are discussed. [ABSTRACT FROM AUTHOR]

Different fractures exist widely in rock mass and play a significant role in their deformation and strength properties. Crack rocks are often subjected to dynamic disturbances, which exist in many fields of geotechnical engineering practices. In this study, dynamic compression tests were carried out on rock specimens with parallel cracks using a split hopkinson pressure bar apparatus. Tests determined the effects of strain rate and crack intensity on dynamic responses, including progressive failure behavior, rock fragmentation characteristics, and energy dissipation. Based on the crack classification method, tensile–shear mixed cracking dominates the failure of rock specimens under the action of impact loading. Increasing the flaw inclination angle from 0°–90° changes the predominant cracking mechanism from tensile cracking to mixed tensile–shear cracking. The larger the loading rate, the more obvious the cracking mechanism, which indicates that the loading rate can promote the cracking failure of rock specimens. The fragmentation analysis shows that rock samples are significantly broken at higher loading rates, and higher loading rates lead to smaller average fragment sizes; therefore, the larger the fractal dimension is, the more uniform the broken fragments of smaller sizes are. Energy utilization efficiency decreases while energy dissipation density increases with increasing strain rate. For a given loading rate, the energy absorption density and energy utilization efficiency first decrease and then increase with increasing flaw inclination, while the rockburst tendency of rock decreases initially and then increases. We also find that the elastic–plastic strain energy density increases linearly with the total input energy density, confirming that the linear energy property of granite has not been altered by the loading rate. According to this inherent property, the peak elastic strain energy of the crack specimen can be calculated accurately. On this basis, the rockburst proneness of granite can be determined quantitatively using the residual elastic energy index, and the result is consistent with the intensity of actual rockburst for the specimens. [ABSTRACT FROM AUTHOR]

The validity of calculating the dynamic tensile strength of rock materials based on dynamic Brazilian tests is problematic. In order to gain a deeper understanding of the effects of three typical loading methods on the damage mechanism of rock specimens in the dynamic Brazilian tests, five different rocks were selected for the study. In the constant incident energy dynamic Brazilian test, the loading modes had a significant effect on the loading rate and dynamic tensile strength of the specimen, with the highest loading rate and tensile strength of the specimens under mode-III loading, followed by mode-I loading and mode-II loading. A high-speed camera and the digital image correlation (DIC) technique were used to successfully capture the rupture process of the Brazilian disc during impact loading. The evolution of the displacement and strain fields of the specimen was obtained by DIC technique, and four typical failure patterns and two rupture characteristics in the dynamic Brazilian test were summarized. The loading mode determined the crack initiation position of the specimen in the dynamic Brazilian test. The results showed that the mode-III loading is the most consistent with the Brazilian test theory, while the mode-II loading violates the test principle. [ABSTRACT FROM AUTHOR]

In order to study the rock energy absorption and fractal characteristics of crushing under impact load, the dynamic impact tests of granite with aspect ratios of 0. 6, 0. 8, 1. 0, 1. 2, and 1. 4 were carried out by split Hopkinson bar» The effects of strain rate effect and size effect on the crushing energy consumption and failure mode of granite specimens were analyzed. On the basis of considering the time factor, a new energy time density index is proposed to evaluate the energy dissipation. Combined with fractal dimension calculation and energy time density analysis» the energy time density and fractal characteristics of rocks during the impact process are studied. The results showed that the strain rate and energy-time density of the granite specimens with an aspect ratio of 0. 6 to 1. 4 are in line with the power relationship, and the energy-time density of the specimens with the same aspect ratio increases with the increase of the strain rate. In the strain rate range of 48. 8 〜124. 2 s_1, the fractal dimension increases significantly with the increase of the strain rate, the energy-time density and fractal dimension of the granite specimen under dynamic load conform to a power relationship, and the more energy absorbed by the rock per unit time, the more obvious the fractal characteristics. The energy-time density combined with the fractal dimension calculation of rock fragmentation can quantitatively study the energy absorption law of rock per unit time. [ABSTRACT FROM AUTHOR]

Understanding accumulated damage effects is essential when undertaking deep underground rock engineering, as complex in situ environments and intense engineering disturbances realistically affect the physical and mechanical properties of rocks. Accumulated damage mainly causes the extension of micro-cracks and the sprouting of specific defects in the rocks, altering the microstructural parameters. In this investigation, loading and unloading tests were used to simulate the damage states of the deep underground coal measures sandstone. The accumulated damage factor was formed by combining the P-wave and energy damage variables. The effect of accumulated damage on the bearing capacity and deformation behavior of sandstone was particularly pronounced after experiencing impact loading. The experimental results demonstrate that the accumulated damage factor can depict the initial damage state of sandstone as well as the subsequent dynamic and progressive damage. There is a mutually governing effect between accumulated damage and strain rate. In contrast, accumulated damage significantly extends the range of strain rates, which is fed back into the dynamic uniaxial compressive strength of the sandstone. There is a negative correlation between dynamic fracture energy and accumulated damage, which strongly agrees with the sandstone's deformation mechanism. The combination of accumulated damage and impact loads can be used to assess the long-term safety of deep underground rock engineering. [ABSTRACT FROM AUTHOR]

The frequency and intensity of coal-rock dynamic incidents in underground coal mining, such as coal bumps and outbursts of coal and gas, tend to increase with mining depth. These dynamic incidents are closely related to the dynamic mechanical behavior of coal. In this experimental study, the dynamic mechanical behavior of coal was investigated with an active triaxial split Hopkinson pressure bar (SHPB) test system. In the test, the in-situ stress field for coal with an overburden depth of 100 m to 600 m was simulated and the dynamic loading tests of coal were undertaken under low, medium, and high loading rates. The results of the study show that the dynamic compression strength of coal increases with loading rate and axial and confining stress, and the effect of confining stress is more profound than that of axial stress. The results also reveal that the energy consumption and energy density per unit volume of coal are positively correlated with the depth and loading rate. This study may help gain insights into the occurrence mechanism of coal-rock dynamic incidents in underground coal mining. [ABSTRACT FROM AUTHOR]

Rock is the main construction material of rock engineering, such as the engineering of mines and tunnels; in addition, its mechanical properties and failure laws are of great significance to the stability evaluation of rock engineering, especially under the conditions of coupled static–static stresses. In this study, granite specimens were manufactured with artificial flaws. Coupled static and dynamic loads tests were carried out with a modified split Hopkinson pressure bar (SHPB) apparatus; and six typical levels of axial pre-stresses and three crack inclination angles were designed. Three-dimensional digital image correlation (3D-DIC) was also applied to record and analyze the fracturing process and damage evolution of the specimens. The test results show that there was no compaction stage in the stress–strain curve under combined dynamic and static loading. The dynamic strength of the specimens increased first and then decreased with the increase in the static pressure; moreover, the specimens reached the maximum dynamic strength when the static pressure was 10% UCS. The dynamic strength decreased first and then increased with the increase in the crack inclination angle; and the lowest strength appeared when the inclination angle was 45°. The change in axial compression had a significant influence on the failure mode, and the failure mode gradually transformed from shear–tensile failure to shear failure with the increase in the pre-stress. The tensile strain was usually generated at the end of the fractures or near the rock bridge. When the axial pressure was small, the tensile strain zone parallel to the loading direction was easily generated; and when the axial pressure was large, a shear strain zone developed, extending along the diagonal direction. The research results can provide a theoretical reference for the correct understanding of the failure mechanisms of granite and its engineering stability under actual conditions. [ABSTRACT FROM AUTHOR]

The study of the dynamic fracture properties of granite after heating and water-cooling treatments is of great significance for analyzing the sustainability and stability of geothermal reservoir rocks. This work investigated the effects of high temperature and water-cooling treatments on the mode I dynamic fracture characteristics of granites. Notched semicircle bending (NSCB) specimens were exposed to five temperatures of 200, 400, 600, 800, and 1000 °C. A Separate Hopkinson Pressure Bar (SHPB) system was used to conduct dynamic fracture tests on NSCB specimens with four impact pressures (0.10, 0.20, 0.30, and 0.40 MPa). The granite microstructure characteristics were obtained through mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests. A 3D scanner was applied to examine the fracture surface of the NSCB specimens. The fractal dimension of the fracture surface was calculated with the cube covering method. The results show that the damage degree of the granite specimens could be divided into two stages with a critical temperature of 400 °C. The porosity was less than 2.20% when the heating temperature was below 400 °C, and the porosity increased rapidly for heating temperatures above 400 °C. The fracture toughness, fracture energy, and kinetic energy of the NSCB specimens increased with loading rate, while the fractal dimensions of the fracture surfaces decreased with loading rate. Both dynamic fracture toughness and fracture energy showed a power function decline trend with an increase in the fractal dimensions of the fracture surface. Finally, the effects of impact pressure and heating temperature on the fracture behavior of granite specimens were discussed. Highlights: The dynamic fracture properties of rocks after heating and water-cooling treatments were studied. The influence of the microstructure on the fracture properties of granite was analyzed. The roughness of the fracture surface was accurately calculated using the self-written MATLAB program based on the cube covering method. The influence of the loading rate and the heating and water-cooling treatments on the roughness of the fracture surface was discussed. [ABSTRACT FROM AUTHOR]

The drilling-hammer method is adopted for the breaking over submerged reefs in waterway regulation engineering. However, rocks with pre-drilled holes are highly susceptible to the repetitive impacts from drop hammers or drilling equipment. Exploring the dynamic responses and failure behaviors of drilled-hole rocks under repetitive impacting is important for promoting implementation of this technique. In this study, the repetitive impacting tests were conducted on the drilled rocks by the split Hopkinson pressure bar to investigate the rock bearing capacity, deformation property, failure behavior, and energy evolution. The results indicate that the presence of pre-drilled holes and the decreasing loading rates attenuate the bearing capacity of rocks subjected to repetitive impacting. In contrast, the rock deformation capacity is impacted subtly by the impact loading rate and increases with the number of impacts and borehole size. The drilled rock failure occurred with the prevalence of intergranular fractures; subsequently, the proportion of transgranular fractures increased, contributing to the loading rate-dependent bearing capacity. The inverted S-shaped damage evolution curve was assessed using the inverted logistic growth model. The specimen absorbed few incident energies for fracture generation and propagation, and the rock with a larger drill size displays higher energy utilization and larger fragments. • Illustrated the influences of repetitive impacting on the bearing capacity and deformability of pre-cracked hole rocks. • Revealed the failure behavior and fracture mechanism of pre-cracked hole rocks under repetitive impacting. • Depicted the damage evolution of pre-cracked hole rocks under repetitive impacting by the inverted logistic growth model. • Investigated the dynamic energy dissipation of pre-cracked hole rocks under repetitive impacting. [ABSTRACT FROM AUTHOR]

A deep understanding of how flawed rock cracks under high temperatures and dynamic impacts is crucial for safe deep rock engineering design and operation. Dynamic impact tests were conducted on sandstones with various flaw inclinations using a Split Hopkinson Press bar (SHPB) system at room temperature and after high-temperature treatment at 600 °C. The results show that the dynamic peak stress and energy consumption of rock were reduced by thermal treatment. The study identified the circular tensile- and compressive-tangential stress zones around the flaw boundary using elastic theory. The strain accumulation zones gradually shift from the tensile- to the compressive-tangential stress zones as the flaw inclination increases, resulting in a larger crack initiation angle and opening displacement, while also constraining the propagation path. Additionally, the high-temperature treatment strengthened the deformation in the circular tensile-tangential stress zone. In contrast to the severe failure of specimens at room temperature, the clear failure pattern of the specimens after high-temperature treatment merely consists of wing cracks. Finally, this study presents a detailed theoretical explanation of the cracking behavior under various conditions, which is consistent with the experimental results. This research enhances the understanding of dynamic crack propagation in deep high-temperature rocks and provides scientific references for engineering safety design and protection. [ABSTRACT FROM AUTHOR]

High-speed digital image correlation (DIC) offers a powerful tool for full-field measurement of transient deformation of rock. One of the critical components controlling the measurement accuracy in DIC is the processing parameter, but neither manufacturers of DIC supplier nor the literature provides guidelines for optimal selection of processing parameter in high-speed DIC for rock testing. In this work, effect of six processing parameters on the accuracy of DIC-measured strain is evaluated through a dynamic compression test on sandstone. The processing parameters, including area of interest (AOI) size, correlation criterion, subset size, step size, filtering and incremental correlation are evaluated by comparing the error between DIC-measured and theoretical overall strain history. The quality of strain field obtained with different processing parameters is also assessed. Results show that the overall strain averaged from the virtual strain gauge (VSG) of AOI covering more than 43% of the specimen length is reliable. To characterize reasonable strain field, the VSG of AOI should cover the whole length of the desired strain direction. The selection of commonly used correlation criteria is of little concern in high-speed DIC processing on rock dynamic tests. The optimum subset size to achieve a good balance between accurate overall strain and high-resolution strain field with less noise is found close to three times of the largest speckle size. Coupled pre- and post-filtering are recommended to apply to correct the bias of overall strain at initial stage of loading and reducing the noise in strain field. A step size of 1 or 2 pixels is suggested for better identification of the strain localisation on rock surface. DIC-measured overall strain is valid until 50% residual peak stress at unloading stage with incremental correlation utilised at the peak stress. Highlights: VSG of AOI covering at least 43% length of specimen aligning with the desired strain orientation is reliable to extract the overall strain of rock. The subset size close to three times of the largest speckle size integrated with step size of 1 or 2 pixels is optimum in high-speed DIC measurement of rock strain. Coupled pre- and post-filtering in high-speed DIC is beneficial to achieve either accurate overall strain or good-quality strain field of rock. The overall strain of rock measured by high-speed DIC is valid to unloading stage at 50% residual peak stress with the incremental correlation. [ABSTRACT FROM AUTHOR]

Rock is an easily weathered material. Especially in alpine regions, freeze–thaw (F-T) cycles have a significant effect on the mechanical behavior of rock. A dynamic disturbance load further complicates the problem. Rock is also a multi-scale material; it may be more suitable to investigate the change in the macroscopic behavior from a microscopic perspective. Since rock fractures exhibit self-similarity, the material responses can be analyzed using fractal theory to determine the chaotic morphology of macroscopic fractures. We use a Φ 100 mm split Hopkinson pressure bar (SHPB) to perform dynamic tests on red sandstone after F-T cycles at three impact strength levels to obtain the dynamic fracture morphology. The fractal dimension of the dynamic fracture is calculated. The macroscopic fracture degree is quantitatively characterized. The relationship between the fractal dimension and the F-T cycles and impact strength are established. A scanning electron microscope (SEM) is used to obtain microscopic images of the dynamic fracture, which are processed to extract quantitative information, including the number, area, length, and width of the pores. The relationship between the fractal dimension and the pore information is investigated to reflect the macro-microscopic characteristics of rock under the coupling environment of F-T cycles and impact loads. [ABSTRACT FROM AUTHOR]

The complex response of underground geomaterials subjected to dynamic disturbance arises from the microstructure redistribution under high in- situ stress and the resulting fracture behaviors at multiaxial stress states. Inclined specimens were employed in an axially constrained split Hopkinson pressure bar (SHPB) system to achieve a combination of compression-shear stress states and static-dynamic loads. The loading rate under investigation ranged from 500 to 4,000 GPa/s, along with the axial prestress of 7, 21, 35, 49, and 63 MPa on specimens with an inclination of 0°, 3°, 5°, and 7°. The modified SHPB experimentation and discrete-element method modeling were implemented to unravel the combined effects of the loading rate, preload, and stress path on the failure mechanism of sandstone specimens involving the failure strength and envelope, fracturing pattern, fragmentation, and microcracking process. The positive rate dependence of the failure strength and Drucker–Prager envelope was observed. The preload showed double effects on the failure strength, indicated by an upper bound of the failure envelope as it expanded with the increasing preload. The microdamage accumulated during preloading and the global stress field collectively influenced the failure pattern of the inclined specimen, altering from a shear fracturing mode under dynamic loading or high-preload static-dynamic loading to an axial splitting mode near the specimen surface under low-preload static-dynamic loading. [ABSTRACT FROM AUTHOR]

Understanding the dynamic cracking behaviors and energy evolution of flawed rocks is highly relevant to underground rock engineering. In this study, multi-flawed rock specimens are tested under coupled static–dynamic compression using a modified SHPB system combined with high-speed photography and DIC monitoring. We systematically investigated the influences of pre-stress ratio, flaw inclination angle and strain rate on the dynamic progressive cracking mechanism and energy evolution of multi-flawed rocks. Experimental results show that the dynamic/total strength generally increases with increasing strain rate, featuring evident rate-dependence. With increasing flaw inclination angle from 15° to 60°, the dynamic/total strength initially decreases and subsequently increases with the minimum achieved around 45°. With the pre-stress ratio increasing from 0.2 to 0.8, the dynamic strength persistently decreases while the total strength initially increases and subsequently decreases with the maximum achieved at 0.6. Furthermore, based on the displacement trend lines method, a novel crack classification method is developed to analyze the progressive cracking mechanism of multi-flawed rocks using high-speed photography and DIC technique. Generally, mixed cracking dominates the failure of multi-flawed rocks under coupled static–dynamic compression. With increasing flaw inclination angle form 15°–60°, the predominant cracking mechanism changes from mixed tensile-shear cracking to mixed compression-shear cracking. The increasing pre-stress ratio promotes shear cracking under lower flaw inclination angles while facilitates tensile cracking under higher flaw inclination angles. In addition, the energy evolution for coupled static–dynamic SHPB tests is re-evaluated and a new energy calculation formula is proposed. The results show that the increasing strain rate reduces the energy utilization while promotes the energy dissipation density. Both the energy utilization and energy dissipation density increase with increasing pre-stress ratios. [ABSTRACT FROM AUTHOR]

• The dynamic mechanical response of defective specimens with different rock bridge lengths under dry, natural and water-saturated conditions were investigated. • The relationship between the dynamic damage process and the water content state of the rock and the length of the rock bridge is analyzed. • The dynamic damage ontology model of defective rocks is established and validated based on the improved ZWT model. In rock slope engineering, the dynamic mechanical behavior of rocks is significantly influenced by the length of rock bridges and their moisture content. This study explored the dynamic characteristics of defective rocks under impact loads through dynamic impact tests on specimens varying in rock bridge lengths and moisture states, conducted using a Split Hopkinson Pressure Bar (SHPB). The experimental results revealed that both moisture state and rock bridge length markedly affect the dynamic compressive strength and elastic modulus. Additionally, there is a significant correlation between the energy dissipation density and the specimen's moisture state and rock bridge length. The failure process, captured via a high-speed camera, exhibited a transition in fracture mode from predominantly tensile to tensile-shear failure with increasing rock bridge length. Moreover, an enhanced ZWT model was developed and validated for the dynamic damage constitutive model of defective rocks. This model, incorporating four parameters, precisely characterizes the stress–strain relationship of defective rocks under high strain rates and effectively demonstrates the dynamic response of rocks with varying rock bridge lengths and moisture states. The findings of this study are vital for advancing the understanding of rock dynamics in varying conditions and offer significant insights for geological and civil engineering applications, particularly in the design and assessment of rock structure stability in slopes. [ABSTRACT FROM AUTHOR]

• The elastic modulus which is more consistent with the stress–strain relationship is defined. • There are stress thresholds for dividing different stages of ultimate strain and dynamic strength. • The linear variation law of strain rate and fractal dimension is obtained. • The law of energy evolution shows that the threshold is the best position. To investigate the impact of unloading on the dynamic mechanical properties of surrounding rock in roadway excavation, sandstone specimens were subjected to loading and unloading at six levels ranging from 0% to 80% of the uniaxial compressive strength (UCS). Subsequently, an improved SHPB was employed to perform dynamic compression tests, aiming to analyze the mechanical characteristics of the sandstone at various degrees of loading and unloading damage, investigate fractal attributes, and examine the dynamic failure process. The study uncovered that the stress–strain curve exhibits three distinct stages. Correspondingly, a more suitable set of dynamic elastic modulus denoted as E d 1 , E d 2 , and E d 3 , were defined. Using 45% of the UCS as the threshold for unloading stress (75 MPa), both the dynamic elastic modulus and dynamic strength displayed an initial upward trend, succeeded by a subsequent decline. In contrast, the defined damage peak strain ratio η ε p and damage ultimate strain ratio η ε m demonstrated the opposite trend, amounting to 45.64% and 51.03% at the threshold. In addition, increasing unloading damage stress (UDS) decreases the sandstone strain rate( ε ̇ max and ε ̇ ave) gradually. Simultaneously, the fractal dimension of the cracks exhibited a linear increase, ranging from 1.159 to 1.481. Similarly, the rock exhibited the lowest proportion of reflected energy and absorbed energy at the threshold. This holds noteworthy implications for examining the dynamic load response behavior of surrounding rock in roadway unloading, ensuring the dependability of blasting unloading techniques. [ABSTRACT FROM AUTHOR]

Dynamic fragmentation is one of the primary fracture types for rock mass in rock engineering. The dynamic fragmentation process and dynamic fragment size characteristics are important for the fragmentation optimization in field. Most researchers studied the fragmentation forming process via dynamic compression test using the traditional split Hopkinson pressure bar (SHPB) system, in which the multiple dynamic loading is generally applied on the rock specimen. However, the effect of the multiple loading on the dynamic rock fragmentation characteristics was rarely attempted. In this study, the momentum-trap system is used in SHPB to achieve the single dynamic compressive loading on the rock specimen. Westerly granite and Fangshan marble were chosen to investigate the effect of multiple dynamic compressive loading pulses on the fragmentation characteristics. During the dynamic loading, the fragmentation process was captured using the high-speed camera. The results show that under the specific loading rate, the multiple loading has no influence on measuring the dynamic compressive strength of rocks, but it was observed in the high-speed images that more fragments were produced in the multiple loading. Moreover, the failure mode of tested specimens under distinct compressive loading rates was examined. The results indicate that the fragmentized degree of rocks with single dynamic loading is less than that with multiple dynamic loading at a given loading rate. This trend is relatively obvious when the loading rate is lower. In addition, the D50 values of rocks with multiple dynamic loading are smaller than those with single dynamic loading. The fragment size distribution also indicates that the multiple loading pulses dominantly influence the produce and size distribution of the fine-grain fragment. Furthermore, the mean fragment size derived from Weibull distribution decreases as the loading rate increases. The multiple loading pulses could decrease the mean fragment size and generate more fine grains when comparing to the fragment size of rocks derived from the single loading pulse. Therefore, the multiple compressive loading may result in an inaccurate fragmentation size analysis, and the single dynamic compressive loading should be applied when evaluating the dynamic fragmentation efficiency of rocks. [ABSTRACT FROM AUTHOR]

Rocks in the deep underground are likely subjected to both hydrostatic confining pressure and dynamic compression–shear load. Thus, accurately characterizing the dynamic properties and failure mechanism of hydrostatically confined rocks under combined compression–shear impacting is crucial for the stability assessment of deep underground rock structures. In this study, on the basis of an improved split Hopkinson pressure bar (SHPB) apparatus, the combined dynamic compression–shear tests are performed on inclined cylindrical sandstone specimens with hydrostatic confining pressures. During the test, the dynamic force balance of rock specimens can be well satisfied using the pulse shaping technique. Our results show that the hydrostatic confining pressure and dynamic loading rate help strengthen the load-carrying capacity of rocks. In contrast, the shear component in the dynamic load limits the dynamic peak stress of rocks. As hydrostatic confining pressure increases, the failure surface based on the Drucker–Prager criterion gradually expands outward. Under dynamic loading, the compressive deformation modulus of rocks decreases with increasing shear component in the dynamic load, contrary to its response to hydrostatic confining pressure. Fragmentation analysis indicates that the hydrostatic confining pressure and the shear component of dynamic loading restrict the fracture behavior of rocks. Besides, as the specimen inclination angle and the hydrostatic confining pressure increase, the failure pattern of rock specimens changes from the tensile-dominated failure with a truncated conical surface to the shear-dominated failure with a single shear plane along its short diagonal. [ABSTRACT FROM AUTHOR]

Cracked rocks are quite susceptible to dynamic loading from drilling, blasting, and impacting events. Understanding the dynamic response of cracked rocks under dynamic loadings is crucial for the assessment of rock structure stability. In this study, dynamic compression tests were carried out on rock specimens with multiple parallel cracks using a split Hopkinson pressure bar apparatus. Effects of strain rate and crack intensity on dynamic responses, including strength and deformation properties, progressive failure behavior, rock fragmentation characteristics, and energy dissipation of cracked rock specimens, were systematically investigated. Stress–strain curves migrate from class I curves into class II with increasing strain rate. Dynamic strength shows clear rate dependence while dynamic elastic modulus is independent of strain rate. Progressive failure behavior of cracked rock specimens under high loading rates was analyzed using high-speed photography and digital image correlation technique. Results show that the X-shaped shear failure mode is the final failure modes of all specimens, regardless of crack intensity. Fragmentation analysis indicates that increasing strain rate intensifies fragmentation, decreases mean fragment size, and increases fractal dimension of rock fragments. At low strain rates, specimens remain unbroken or slightly spilt; at high strain rates, specimens are systematically pulverized. Energy utilization efficiency decreases while energy dissipation density increases with increasing strain rate. For a given strain rate, crack intensity has no significant influences on energy dissipation and fragmentation characteristics of cracked rock specimens under dynamic loading. [ABSTRACT FROM AUTHOR]

The dynamic shear failure of rocks with confinements is important in underground rock engineering and it is thus necessary to quantify the dynamic Mode II fracture toughness KIIC of rocks under confining conditions. A punch-through shear (PTS) method is introduced to measure the KIIC of rocks under different confining pressures. The dynamic PTS specimen is a cylinder with two 10-mm-deep circular notches at two ends. A modified split Hopkinson pressure bar system with a Hoek cell is employed to apply the dynamic loading and confining pressure to the rock specimen. The equation to calculate the dynamic KIIC value is determined for the new dynamic PTS specimen. The validity and applicability of the dynamic PTS method with confinement are demonstrated via the measurement of the dynamic KIIC of Fangshan marble (FM) under five confining pressures. The PTS specimens are sheared into a short bar and a hollow cylinder and are soft recovered for post-mortem examination. The dynamic shear fracture pattern of the recovered FM specimen is examined through X-ray micro-CT technique. The results demonstrate that the rock PTS specimen with confinement fails with the shear dominant fracture and the values of KIIC of FM at a certain confining pressure increase as the loading rate. Another observation is that the values of KIIC of FM at a given loading rate rise with the lateral confinement. [ABSTRACT FROM AUTHOR]

• Total fracture surface area and specific fracture energy increases with strain rate under dynamic compression. • Total fracture surface area of broken fragments increases with dissipated energy. • Fracture surface roughness and macro-crack abundance generally increase with strain rate. Effects of strain rate on specific fracture energy and micro-fracture surface properties of rock specimen were investigated at strain rates of ∼ 60–115.0 s - 1. To achieve this, rock breakage was conducted under dynamic uniaxial compression using the split Hopkinson's pressure bar. The fracture surface area of fine fragments (<8* 10 - 3 m) were measured using gas absorption method while the fracture surface area of large fragments (>8* 10 - 3 m) were measured using close-range photogrammetry. Results reveal that fracture surface area and specific fracture energy generally increase with strain rate during rock breakage under dynamic uniaxial compression. Similarly, the total fracture surface area of broken fragments also increased with dissipated energy. Results from scanning electron microscopy showed that fracture surface roughness and macro-crack abundance also increased with strain rate. Following energy requirement for rock breakage into size specifications for engineering applications, strain rate provides a reliable parameter for estimating specific fracture energy and fracture surface area at the forementioned strain rates. Accordingly, optimizing rock breakage processes using strain rate could reduce ore loss and energy wastage associated with production of unwanted fracture surfaces during rock breakage. [ABSTRACT FROM AUTHOR]

In western China, red sandstone is widely distributed. This type of rock is susceptible to generate cracks after being disturbed, and thus becomes a communication channel for groundwater, which poses a great hidden danger in Engineering, such as shaft and tunnel construction. To solve this problem, artificial freezing method is applied to underground engineering. This article focuses on the dynamic tensile strength of red sandstone (RS) at negative temperatures. According to the actual freezing temperature in the site, the temperature range was set to − 5, − 10, − 20 °C in the test, and the rock at normal temperature was set as a control group. The results show that the tensile strength of RS at temperatures below zero is significantly greater than the tensile strength of rock at normal temperature, and − 10 °C is a turning point of rock strength. In order to reveal the mechanism of this change, the scanning electron microscopic (SEM) technique was used to observe the rock fragments after the rock rupture. It is found that the rock fracture patterns are closely related to the rock cement property and its environmental temperature. [ABSTRACT FROM AUTHOR]

In the process of deep underground resource development and utilisation, the response of the surrounding rock mass is affected by high temperatures and the dynamic loadings that can trigger strain bursts. Therefore, strain burst failure is highly related to the rock mass's dynamic fracture toughness and energy absorption capacities. Understanding the dynamic fracturing behaviour of rock mass is crucial for designing stable underground structures and controlling the hazard associated with strain bursts. In this paper, dynamic Mode I fracture toughness tests using a split Hopkinson pressure bar (SHPB) were conducted on Cracked Chevron Notched Semicircular Bend (CCNSCB) granite specimens to investigate the relationship between the strain burst mechanism and its dynamic fracture propagation. The tests were performed on thermally treated granite specimens (from 25 to 250 °C) under a wide range of impact velocities. The dynamic fracturing characteristics and crack propagation speeds were measured by a high-speed camera (HSC). The dynamic fracturing process and the effects of thermal damage on the dynamic fracture modes were identified by detailed image analysis. Energy partition characteristics in dynamic fracture of granite samples were quantified. The results revealed that dynamic fracture initiation toughness and energy partitions highly depend on loading rate and temperature. As the loading rate increased, the failure modes changed from axial splitting to pulverisation. Under the same dynamic loading, an increase in the temperature can exacerbate the fragmentation degree of granite. Under the same dynamic loading, the dissipated and released energies increased with increasing temperature. When the loading rate was high, the loading rate strengthening effect became remarkable, and the dynamic fracture toughness of granite increased under all temperatures. Finally, the dynamic energy mechanism of strain burst with the increased loading rate and heat-treatment temperature was further discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

• Cyclic impact tests were conducted to investigate the damage accumulation of marble specimens with multiple impacts. • The extent of damage was analysed from an energy perspective using acoustic emission techniques. • The influence of cycle impact loading on the mode II fracture toughness are analyzed. • The 3D surface topography and fractal dimension value of fracture surface was obtained. Rock masses in underground engineering projects, such as roadways or pillars, are inevitably subjected to cyclic dynamic loading, and the damage behaviour of jointed rock masses is often influenced by the different characteristics of the preexisting rock damage. To investigate the effect of cyclic impact on the fracture characteristics of rocks, fracture tests were conducted using SCC (short core in compression) specimens with dissimilar impact damage. The experimental results showed that cyclic impact led to mesoscale fracture and strength deterioration in rock specimens. As the number of impacts increased, the dynamic peak stress of marble decreased, and the maximum strain and the energy absorbed by marble increased. At the same time, cyclic impacts generated continuous damage to the interior of the specimen, and the white spot inside the specimen with higher impact times was more obvious. Based on the fracture test results, the mode II fracture toughness was calculated, and it decreased by 6.9, 20.9, 43.3, 66.8, and 87.0% for specimens that underwent 1–5 cyclic impacts, respectively. After fracture, the three-dimensional topography characteristics of the fracture surface were obtained by a 3D optical scanner. Overall, the fracture surface became increasingly tortuous with increasing impact number. For the fractal factor D, the variation in the mean squared deviation of the asperity height and slope angle exhibited an increasing tendency with an increasing number of impacts. [ABSTRACT FROM AUTHOR]

• The strength, deformation, failure and energy evolution of parallel double fractured sandstone are studied. • The crack propagation process of parallel double fractured sandstone is studied quantitatively. • From the perspective of experiment to engineering, we propose a new method of rock burst prevention. To obtain the mechanical characteristics and energy evolution law of sandstone with double cracks under dynamic load, the test is carried out by using the SHPB test system, and the failure process of the sample is recorded by the high-speed camera system. The stress-strain curves under dynamic load and the failure characteristics of the sample are obtained, and the effects of crack angle and impact pressure on the mechanical characteristics of sandstone with double cracks are analyzed. The results show that the compressive strength of the specimen decreases initially and increases afterwards with the increase of the crack angle under dynamic load, and the compressive strength of the specimen increases gradually with the increase of the impact pressure. At the pressure of 0.03 MPa, the peak strain increases initially, followed by decreases with the increase of crack angle, and shows the opposite law with the increase of impact pressure. Under dynamic loading, the crack tip first starts to crack outwards along the axial direction, and the penetration of the rock bridge lags. The macroscopic cracks of 0° and 90° sandstone are mostly tensile cracks, while those of 30°, 45°, and 60° are mostly shear cracks. With the increase of crack angle, the failure mode changes from tensile failure to shear failure and then to tensile failure. The fractal dimension increases with the increase of impact pressure, and increases and then decreases with the increase of crack angle, and the fractal dimension increases with the increase of energy dissipation per unit volume. In addition, in the process of rock burst prevention and control, the size of the dynamic load and crack angle should be evaluated in advance. When the dynamic load in this area is large, the surrounding rock pressure relief should be carried out in advance to increase the dissipation energy. When there are 60° and 45° crack angles in the rock mass, the stress propagation path should be changed by structural regulation based on grouting filling micro-fracture modification. [ABSTRACT FROM AUTHOR]

• Dynamic peak load, strain field distribution, fracture process of sandstone containing parallel pre-cracks were investigated by dynamic Brazilian splitting tests. • Six fracture modes of sandstone containing parallel pre-cracks under impact load were identified. • The damage and stress distribution in the fracture process of sandstone containing parallel pre-cracks were numerically studied by ANSYS/LS-DYNA. Parallel cracks have a significant influence on the dynamic tensile failure behavior of rock. The systematic investigation of dynamic tensile mechanical properties and fracture evolution characteristics of fractured rocks are significant for guiding the safe excavation and stability sustaining of rock engineering. Two groups of dynamic Brazilian splitting tests were conducted for the sandstone specimens containing parallel pre-cracks. The influences of crack inclination angle (Group A) and rock bridge length (Group B) on the tensile mechanical properties and fracture evolution characteristics were analyzed. Results showed: the dynamic peak load of pre-cracked specimens is significantly lower than that of intact specimens. The peak load presents a V-shaped evolution trend with the growth of crack inclination angle. Differently, the dynamic peak load increased monotonically with the increasing of rock bridge length. In addition, the crack initiation points move from crack tips to crack walls when the crack inclination angle is increasing. When rock bridge length is large enough, the crack propagation paths are observed near the central axis of specimens. By utilizing the DIC method, six different failure modes are identified to explain the crack initiation and propagation mechanisms under dynamic loading. The tensile-shear fracture modes are mainly presented for specimens with different crack inclinations. Differently, the ultimate failure modes change from tensile-shear mode to tensile mode when the rock bridge length is increasing. In addition, numerical results (ANSYS/LS-DYNA) are in good agreement with the experimental results as a whole, which can verify and explain the experimental results. This study is expected to provide guidance for controlling fractured rock masses. [ABSTRACT FROM AUTHOR]

The cyclic wetting-drying phenomenon, which is a part of weathering processes, plays a vitally important role in affecting the properties of rock materials. To investigate the effect of wetting and drying cycles on the physical and dynamic compressive properties of rocks, some essential physical properties of sandstone specimens including density, water absorption, porosity, P-wave velocity and slake durability index (SDI) were measured after every 10 cycles (for a total of 50 cycles). Dynamic compressive tests were conducted using a modified split Hopkinson pressure bar (SHPB) technique for rock specimens. Laboratory tests results showed that, with the increase of wetting and drying cycles, the porosity and water absorption of rock increases while the density, P-wave velocity, SDI, dynamic compressive strength and elastic modulus decrease. In addition, the surface microscopic morphological characteristics of specimens were examined by scanning electron microscope (SEM). It was observed that the micro-cracks grow and expand in rock after cyclic wetting-drying treatments, which is the main cause of the reduction in dynamic compressive strength. Based on experimental results, an empirical equation was established to describe the effect of strain rate and number of wetting and drying cycles on the dynamic compressive strength of rock materials. [ABSTRACT FROM AUTHOR]

Many projects such as the underground gasification of coal seams and coal-bed methane mining (exploitation) widely involve the dynamic problems of coal measures sandstone achieved via thermal treatment. This study examines the dynamic mechanical properties of coal measures sandstone after thermal treatment by means of an MTS653 high-temperature furnace and Split Hopkinson pressure bar test system. Experimental results indicate that 500 °C is a transition point for the dynamic mechanical parameters of coal measures sandstone. The dynamic elastic modulus and peak strength increase linearly from 25 to 500 °C while the dynamic peak strain decreases linearly over the same temperature range. The dynamic elastic modulus and peak strength drop quickly from 500 to 800 °C, with a significant increase in the dynamic peak strain over the same temperature range. The rock mechanics are closely linked to material composition and mesoscopic structure. Analysis by X-ray diffraction and scanning electron microscopy indicate that the molecules inside the sandstone increase in density due to the thermal expansion of the material particles, which effectively improves the deformation resistance and carrying capacity of the sandstone and reduces the likelihood of axial deformation. With heat treatment that exceeds 500 °C, the dynamic mechanical properties rapidly weaken due to the decomposition of kaolinite; additionally, hot cracking of the mineral particles within the materials arises from coal sandstone internal porosity, and other defects gradually appear. [ABSTRACT FROM AUTHOR]

The International Society for Rock Mechanics (ISRM) has suggested a notched semi-circular bend technique in split Hopkinson pressure bar (SHPB) testing to determine the dynamic mode I fracture toughness of rock. Due to the transient nature of dynamic loading and limited experimental techniques, the dynamic fracture process associated with energy partitions remains far from being fully understood. In this study, the dynamic fracturing of the notched semi-circular bend rock specimen in SHPB testing is numerically simulated for the first time by the discrete element method (DEM) and evaluated in both microlevel and energy points of view. The results confirm the validity of this DEM model to reproduce the dynamic fracturing and the feasibility to simultaneously measure key dynamic rock fracture parameters, including initiation fracture toughness, fracture energy, and propagation fracture toughness. In particular, the force equilibrium of the specimen can be effectively achieved by virtue of a ramped incident pulse, and the fracture onset in the vicinity of the crack tip is found to synchronize with the peak force, both of which guarantee the quasistatic data reduction method employed to determine the dynamic fracture toughness. Moreover, the energy partition analysis indicates that simplifications, including friction energy neglect, can cause an overestimation of the propagation fracture toughness, especially under a higher loading rate. [ABSTRACT FROM AUTHOR]

• PTS tests are employed as the approaches for determining static and dynamic K II C. • The fracture mode is verified to be the dominant Mode II by SEM observations of fracture surfaces. • Static K II C increases with the treatment temperature within 200 °C and decreases substantially from 200 °C to 600 °C. • Dynamic K II C appears obvious loading rate effect and its strengthening effect is almost unchanged below 400 °C. • Mesoscopic characteristics are in accord with the variations of physico-mechanical parameters. Most studies of thermal effects on rock fracture have focused on Mode I, while the temperature-dependence and loading rate effect on Mode II fracture are rarely involved. We present the static and dynamic punch-through shear (PTS) tests, implemented on the INSTRON 1342 and the modified split Hopkinson pressure bar (SHPB) system respectively, as the approaches for determining the Mode II fracture toughness (K II C) of PTS specimens at a series of thermal treatments and loading rates. As any macroscopic characteristic of rock is related to its microscopic or mesoscopic structures and properties, the SEM technique is thus applied to observe the mesoscopic morphology of heated and tested specimens. The fracture mode in the static and dynamic PTS tests is verified to be the dominant Mode II by fracture surface SEM observations. The results show that static K II C (K I I C S) increases with the thermal treatment temperature while static Mode I fracture toughness (K I C) is almost unchanged within 200 °C, and they all decrease substantially from 200 °C to 600 °C. Moreover, the dynamic K II C (K I I C D) at each thermal treatment temperature appears obvious loading rate effect. The strengthening effect of high loading rate on K I I C D is almost unchanged below 400 °C, while the thermal effect on K I I C D / K I I C S becomes apparent when the temperature reaches 600 °C. The SEM observation displays that almost no cracks initiating within 200 °C, and the growth of obvious intergranular and transgranular cracks commence at about 400 °C and 600 °C, respectively. Besides, reticular cracks start to generate at 400 °C. As such, the mesoscopic characteristics at different thermal treatment temperatures are in accord with the variations of physical and mechanical parameters. [ABSTRACT FROM AUTHOR]

The Split Hopkinson Pressure Bar (SHPB) is an apparatus for testing the dynamic stress-strain response of the cement mortar specimen with pre-set joints at different angles to explore the influence of joint attitudes of underground rock engineering on the failure characteristics of rock mass structure. The nuclear magnetic resonance (NMR) has also been used to measure the pore distribution and internal cracks of the specimen before and after the testing. In combination with numerical analysis, the paper systematically discusses the influence of joint angles on the failure mode of rock-like materials from three aspects of energy dissipation, microscopic damage, and stress field characteristics. The result indicates that the impact energy structure of the SHPB is greatly affected by the pre-set joint angle of the specimen. With the joint angle increasing, the proportion of reflected energy moves in fluctuation, while the ratio of transmitted energy to dissipated energy varies from one to the other. NMR analysis reveals the structural variation of the pores in those cement specimens before and after the impact. Crack propagation direction is correlated with pre-set joint angles of the specimens. With the increase of the pre-set joint angles, the crack initiation angle decreases gradually. When the joint angles are around 30°–75°, the specimens develop obvious cracks. The crushing process of the specimens is simulated by LS-DYNA software. It is concluded that the stresses at the crack initiation time are concentrated between 20 and 40 MPa. The instantaneous stress curve first increases and then decreases with crack propagation, peaking at different times under various joint angles; but most of them occur when the crack penetration ratio reaches 80–90%. With the increment of joint angles in specimens through the simulation software, the changing trend of peak stress is consistent with the test results. [ABSTRACT FROM AUTHOR]

Within the framework of combined finite-discrete element method (FDEM), we propose a novel and efficient semi-adaptive contact activation approach (semi-ACAA). The semi-ACAA adaptively activates contact calculations for continuum solid elements around the cohesive element which has just been subjected to shear softening while its softening function has just satisfied a prescribed threshold. The semi-ACAA is implemented in a three-dimensional (3D) FDEM code parallelized based on general-purpose graphic processing units (GPGPU) to model dynamic fracture processes of marbles in dynamic Brazilian tensile strength (BTS) and uniaxial compressive strength (UCS) tests. The semi-ACAA not only overcomes spurious fracturing mode associated with FDEM simulations using ACAA but also is more physically sound and computationally efficient compared with brute-force contact activation approach (BCAA), which has been prevalent in FDEMs with intrinsic cohesive zone model (ICZM)-based cohesive elements. Furthermore, it is proven that the spurious fracturing mode is caused by unphysical movements of mesh due to missing contacts during the shear softening of cohesive elements instead of the sudden activation of contact force calculations as indicated in some literature. Finally, a series of simulations of the dynamic BTS and UCS tests conducted by split Hopkinson pressure bar (SHPB) facility reveals that, if the threshold is set to around unity, not only the spurious fracturing mode can be overcome but also the modelling precision of stress waves in intact rocks can be improved. The speed-up times of semi-ACAA compared with BCAA are between 1.23 and 13.5 depending on the intensity of shear cracking in the simulations including mixed-mode cracking. Since the proposed semi-ACAA is simple and can be easily implemented into any FDEM codes with ICZM-based cohesive elements, it is expected to accelerate future developments and applications of FDEM with ICZM-based cohesive elements in rock dynamics such as drilling and blasting. [ABSTRACT FROM AUTHOR]

Beishan area in Gansu province of China has been listed as a major candidate for underground constructions of high-level radioactive waste repository. Therefore, to explore the dynamic responses of Beishan granite is important to the related excavation engineering. This paper investigates the rate dependent effect on the dynamic fracture behaviors of the Beishan granite. The notched semi-circular bend (NSCB) and cracked chevron notched semi-circular bend (CCNSCB) specimens are prepared with speckles uniformly distributed on surface for digital image correlation (DIC) image processing. An ultra-high-speed camera is employed for the modified split Hopkinson pressure bar (SHPB) system to capture the complete cracking process of the rock specimen. The dimensionless stress intensity factors (SIFs) of NSCB and CCNSCB specimens are calculated numerically by the J-integral method in ABAQUS simulation software. To systematically reveal the dynamic fracture mechanisms of Beishan granite specimens, the key dynamic fracture parameters including the dynamic fracture toughness, the evolution of dynamic stress intensity factor (DSIF) during fracturing and the dynamic fracture energy are obtained for further investigations and comparisons. The results show that the dynamic fracture toughness of NSCB and CCNSCB specimens exhibits a distinct rate dependence-the fracture toughness increases with the loading rate rises. Furthermore, the toughness value of the CCNSCB specimen are mostly higher than that of the NSCB specimen due to a more severe stress concentration level near the notch tip. The evolutions of DSIFs of Beishan NSCB and CCNSCB specimens are quite similar to each other, and the corresponding variation curves are all subjected to three typical developing phases until specimens are totally fractured. The dynamic fracture energy of NSCB and CCNSCB specimens is relatively consistent and shows a positive relationship with the loading rate. However, the fracture energy absorption rate (defined as the ratio between the fracture energy and the energy generated in the incident bar) gradually decreases with the loading rate rising, indicating a lower loading rate may be more suitable to improve rock fragmentation efficiency. • The mode I DSIFs are extracted from DIC analysis and the cracking process of Beishan NSCB and CCNSCB specimens is obtained. • The dimensionless SIFs of NSCB and CCNSCB specimens are determined using J-integral method in ABAQUS simulation software. • Key dynamic fracture parameters of Beishan granite specimens are obtained to study rate effects on dynamic mechanisms. [ABSTRACT FROM AUTHOR]

The strain rates can significantly change the mechanical properties of rock materials. In this study, the static and dynamic compressive behaviors of marble under different strain rates were investigated by a series of compressive tests. The highest strain rate reached to1230s−1 by using a split Hopkinson pressure bar (SHPB). A high-speed camera was employed to capture the fracture and failure progress of rock specimens during SHPB tests. The compressive stress-strain response of the tested rock material and the strain rate effects were analyzed. The experimental results showed that the fracture behavior and mechanical properties of marble have a significant correlation with the strain rate. Under quasi-static loading, the failure process of marble was dominated by one main crack. However, under dynamic loadings, there were more micro-cracks were generated within the specimens with the increase of strain rate. Moreover, the predominant failure modes of rock specimens change from intergranular fracture to transgranular fracture. The ultimate compressive strength increased with the increase of strain rate, and the grain size of the rock fragment decreased with the increasing strain rate. [ABSTRACT FROM AUTHOR]

• An oblique impact method using inclined cylinder specimens is employed in a split Hopkinson pressure bar (SHPB) apparatus modified with an axial pressure chamber to achieve the compression-shear loading in a combined static-dynamic mode. • Numerical simulations using both the finite element method and the discrete element method are conducted to assess the validity of this compression-shear specimen and to establish a new data processing method. • Experimental results highlight the combined effects of the loading rate and the axial confinement on the failure strength, fracturing mode and deformation characteristics of sandstone. The compression-shear behavior and failure mechanism of rocks under combined static-dynamic loading are of essential importance for mechanical characterization and construction safety in deep underground engineering applications. An inclined cylinder specimen is designed using a split Hopkinson pressure bar (SHPB) apparatus modified with an axial pressure chamber for laboratory studies. Specific issues on the validity of this compression-shear specimen that were not addressed previously have been numerically assessed involving the dynamic equilibrium of force and moment, stress uniformity, and interfacial friction effects. Experiments are conducted at different loading rates ranging from 500 to 4000 GPa/s and under axial pressures of 7, 21, 35, 49, and 63 MPa. Experimental results verify the dynamic equilibrium of inclined specimens in the axially constrained SHPB tests using pulse shaping techniques, which validates the data processing method newly proposed. Both the dynamic strength and total strength linearly increase with the loading rate. With the increasing pre-load ratio (ratio of the axial confining load to the unconfined compression strength of the tested material), the dynamic strength decreases, while the total strength increases and then keeps constant, where the turning point of the pre-load ratio approximates 0.7. High-speed photographs at a high loading rate illustrate that the increasing confinement can result in less shear but more tensile cracks. Particularly at a high confining condition, the inclined specimen fails by a feather-like shear fracture band where tensile secondary cracks extensively develop. The equivalent stress-strain relationships indicate the reinforcing effect of the confinement on the deformation and energy dissipation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The mode-Ι fracture parameters were measured using a SHPB system with the laser gap gauge technique. • Both of the CT value and the P-wave velocity after microwave irradiation significantly decrease. • Microwave irradiation has a significant weakening effect on dynamic initiation fracture toughness. • The fracture energy and propagation fracture toughness are sensitive to the irradiation duration and the loading rate. Microwave irradiation has been considered as one of the most promising approaches to assisting mechanical rock breakage in rock engineering. Understanding the influence of microwave-induced damage on rock dynamic fracture properties is thus desirable. Using the notched semi-circular bend (NSCB) method, dynamic fracture tests were carried out on rock specimens irradiated by microwaves with different durations. Prior to the dynamic tests, X-ray micro-CT scanning and P-wave velocity measurement were employed to quantify the microwave-induced damage. Both the CT value and the P-wave velocity decrease with the increase of the irradiation duration. During the dynamic tests with loads exerted by a split Hopkinson pressure bar (SHPB) system, a laser gap gauge (LGG) technique was used to monitor the crack surface opening distance (CSOD). The results show that the dynamic initiation fracture toughness increases linearly with the loading rate, and the microwave irradiation has a significant weakening effect. Moreover, the propagation fracture toughness and fracture energy are also sensitive to the irradiation duration and the loading rate, which both increasing with the loading rate while decreasing with the increase of the irradiation duration. In addition, the dynamic fracture velocity increases with the loading rate, whereas the limiting fracture velocity and the fracture arrest toughness are significantly reduced by the microwave irradiation. [ABSTRACT FROM AUTHOR]

• Conducted the dynamic compression-shear impacting tests on hydrostatically pressurized rocks using SHPB modified with triaxial confining system. • Investigated the influences of dynamic loading rate and hydrostatic confining pressure on the dynamic compression-shear mechanical response of rocks. • Revealed the progressive failure behavior and stress evolutions of hydrostatic pressurized rocks under combined dynamic compression-shear loading. Accurately characterizing the mechanical responses and failure mechanism of hydrostatically pressurized rocks under combined compression-shear impacting are of great significance for construction safety in deep underground engineering. Using a split Hopkinson pressure bar (SHPB) modified with axial and radial confining systems, oblique cylindrical specimens with different hydrostatic confining pressures of 7, 14, 21 and 28 MPa are tested under loading rates varying from 1200 to 3200 GPa/s. The dynamic force equilibrium of the oblique specimen in the triaxial SHPB tests can be satisfied by using the pulse shaping techniques. A complementary discrete element method (DEM) simulation is conducted to describe the failure behaviours of hydrostatically confined rocks under combined compression-shear loading. The experimental and numerical results show the effects of confining pressure and loading rate on the compression-shear impact responses and failure mechanism of rocks. Under different hydrostatic confinements, both the dynamic compressive and shear strength of rocks increase linearly with equivalent stress rate, while the rate of increase slows as confinement increases. Post-mortem examination of recovered specimens and the DEM simulation show that the failure pattern of hydrostatically pressurized rocks under compression-shear impacting changes from two parallel shear bands with small inclination to a single shear band along the short diagonal of oblique specimen as confinement increases. Fragmentation analysis shows that the rocks are more fragmented under a higher loading rate and that confinement enlarges the average fragment size and its distribution range. Image, graphical abstract [ABSTRACT FROM AUTHOR]