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Accurate measurement of dynamic behaviors of rocks under complex static stress conditions can provide valuable insight into the design of underground rock engineering. The end friction between the specimen and the loading device becomes more severe in the measurement of rocks under static stress conditions because the end friction effect may be amplified by the static stress on the specimen ends. However, there are few studies on quantifying the end friction effect on the dynamic test for deep rocks. Thus, three groups of Fangshan marble (FM) specimens were prepared in this study. Specimen S1 and S3 are cylinders, and specimen S2 is a tube. The diameter of specimen S1 is the same as the outer diameter of specimen S2, and the cross-sectional area of specimen S3 is the same as that of specimen S2. The dynamic and total uniaxial compressive strength (UCS) of three types of specimens under various preloads were measured using the modified SHPB system. The splitting failure can be observed for these three types of specimens, and the center portion of specimen S1 was relatively intact, which can be attributed to the end friction effect. The end friction inhibits crack propagation, and thus the total UCS of rocks continuously increases with the axial preload. The increment of total UCS of FM at the same loading rate in terms of the preload differs for these three types of specimens due to the end friction effect. The difference in the dynamic UCS between lubricated and dry conditions is slight when the preload is higher, but the lubrication effect is more severe when the preload is lower. A formula is proposed to quantitatively assess the end friction effect and predict the total UCS of rocks under various dynamic loading rates and static axial preloads. Appropriate specimen geometries are suggested to ensure the precision of measurement of the dynamic UCS of deep rocks. Highlights: The total UCS of rocks increases with the axial preload due to the inhibition of microcrack propagation. The effect of lubrication at different preloads is analyzed. A formula is proposed to quantitatively assess the end friction effect. The total UCS of rocks under various dynamic and static loadings can be predicted. Appropriate specimen geometry is recommended to minimize the end friction effect. [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]

Rocks containing flaws around rock structures are often subjected to complex temperatures and geological conditions. This study investigates the mechanical behavior of heat-treated rock samples with a flaw under ungrouted and grouted conditions subjected to dynamic loading. Rock samples were prepared from two different types of rocks, model rock and natural marble, with an existing flaw at a fixed orientation under three different infill conditions, i.e., ungrouted, cement grouted, and epoxy grouted. Samples were initially heat treated to a temperature between 30 and 650 °C for 0–15 h and then subjected to dynamic loading using Split Hopkinson Pressure Bar (SHPB). A high-speed camera was coupled with SHPB to record the progressive failure process of these samples, and Digital Image Correlation (DIC) captured the strain evolution. It was observed that the strength and stiffness of samples remained constant up to a certain temperature, namely transition temperature (i.e., 100–150 °C and 500–650 °C for model rock and marble, respectively) and then reduced significantly. The presence of grouts improved the mechanical behavior of samples significantly. However, their efficiency was dependent on the degree and duration of heating. Epoxy grout was observed to have superior efficiency compared to cement grout at lower degrees and lesser durations of heating which reverses for their higher magnitudes. Except for the epoxy-grouted samples at low temperatures, fracture initiation took place from the flaw tips in all other samples. Fracturing was dominated by tensile cracks at lower degrees and lesser duration of heating which changes to shear cracks-dominated mechanism, especially for porous model rock with the increasing degree and duration of heating. Highlights: Grouts were efficient in improving the dynamic response of jointed rocks with efficiency dependent upon the heat treatment of samples. Crack initiation and progression for grouted samples depend on grout strength, degree and duration of heating. Crack initiation and progression for ungrouted samples depend on the degree and duration of heating. Epoxy grout was more efficient for lower degree and duration of heating while cement grout was for higher ones. The transition from tensile-dominated fracturing to shear-dominated fracturing with increasing duration and degree of heating with more dominant effect on porous model rocks as compared to crystalline marble. [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]

In waterway regulation engineering, submerged reefs are quite susceptible to the repetitive oblique strike from drop hammer and impact drilling, leading to a repetitive compression–shear impacting. Understanding the dynamic responses and failure mechanism of saturated rocks subjected to repetitive compression–shear impacting is thus of great significance for the efficient removal of submerged reefs. In this study, using the split Hopkinson pressure bar (SHPB) apparatus, the repetitive compression–shear impacting tests were conducted on the saturated and dry sandstone specimen from the harbor excavation project in Guoyuan Port (Chongqing, China). Our experimental results indicate that the increasing loading rate or shear component in the dynamic loading has a negative influence on the bearing capacity of saturated sandstone subjected to repetitive impacting. The deformability of saturated rocks increases as the impacting number or the shear component in the dynamic loading. The energy absorption ability as well as the energy utilization of the saturated rocks is highly improved by implementing repetitive compression–shear impacting, and such improvement yields an ascension limit with the increasing shear component in the dynamic loading. As the shear component introduces into the repetitive impacting loading, the failure patterns of rocks change from the relative tensile mode with a large deformation band along the loading direction to the combined compression–shear mode with two parallel large deformation bands inclined to the loading direction, the water content between the mineral grains promotes the shear failure in rock dynamic testing. These results can provide a better understanding of the dynamic responses and failure properties of saturated rocks subjected to repetitive compression–shear impacting. Highlights: Illustrated influences of repetitive compression–shear impacting on the bearing capacity and deformability of saturated and dry rocks. Investigated energy dissipation of saturated and dry rocks under repetitive combined compression–shear impacting. Revealed failure patterns of saturated and dry rocks under repetitive combined compression–shear impacting. [ABSTRACT FROM AUTHOR]

The aim of this study is to understand the strength behaviour and fragment size of rocks during indirect, quasi-static and dynamic tensile tests. Four rocks with different lithological characteristics, namely: basalt, granite, sandstone, and marble were selected for this study. Brazilian disc experiments were performed over a range of strain rates from ~ 10–5 /s to 2.7 × 101 /s using a hydraulic loading frame and a split Hopkinson bar. Over the range of strain rates, our measurements of dynamic strength increase are in good agreement with the universal theoretical scaling relationship of (Kimberley et al., Acta Mater 61:3509–3521, 2013). Dynamic fragmentation during split tension mode failure has received little attention, and in the present study, we determine the fragment size distribution based on the experimentally fragmented specimens. The fragments fall into two distinct groups based on the nature of failure: coarser primary fragments, and finer secondary fragments. The degree of fragmentation is assessed in terms of characteristic strain rate and is compared with existing theoretical tensile fragmentation models. The average size of the secondary fragments has a strong strain rate dependency over the entire testing range, while the primary fragment size is less sensitive at lower strain rates. Marble and sandstone are found to generate more pulverised secondary debris when compared to basalt and granite. Furthermore, the mean fragment sizes of primary and secondary fragments are well described by a power-law function of strain rate. Highlights: Dynamic tensile strength of the rocks are experimentally observed to obey the universal theoretical scaling relationship proposed by Kimberley et al. Dilatancy occurred directly from the start of loading for saturated coal due to regional overpressure. Primary (coarse) and secondary (fine) fragments size generated from the split tensile tests are represented by a power-law function of strain rate. The mean fragment size of primary fragments is less sensitive to strain rate, while the secondary fragments have a strong strain rate dependency. [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 damage accumulation and strength deterioration within rock masses due to frequent disturbance loads is a significant concern in the fields of rock mass engineering and deep mining. This study employed the ultrasonic detection system and the Hopkinson pressure bar test device to conduct cyclic impact tests on sandstone, aiming to unveil the energy evolution patterns and strain rate effect in sandstone under cyclic impact. The investigation focused on studying the characteristics of the stress wave spectrum during cyclic impact, as well as revealing anomalies in the stress wave spectrum resulting from progressive load cyclic impact. Additionally, a cumulative damage constitutive model of cyclic impact based on ultrasonic wave velocity was established. The results indicate the PWR initially shows two forms: a slight increase followed by a subsequent monotonic decrease, and a monotonic decrease throughout. The ed shows two trends: a slight increase in the initial stage followed by a decrease oscillation, and a slight rise in the early stage followed by a subsequent monotonic or direct decrease. The spectrum of reflected wave and transmission wave exhibit an initial increase followed by a decrease, with the peak frequency typically ranging from 1000 to 3000 Hz. Under constant load cyclic impact, the number of impacts enhances the spectrum peak of the reflected wave while weakening that of the transmission wave. Under progressive load cyclic impact, both the reflected wave and transmission wave show a gradual upward trend for their respective spectrum peaks, but with different rates of increase. [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]

Research on the dynamic tensile behaviour of microwave-irradiated rock is of great significance to microwave-assisted tunneling and mining. In order to study the effect of microwave heating and rapid cooling on the mechanical properties of basalt, dynamic Brazilian splitting tests were conducted using the split Hopkinson pressure bar (SHPB) equipment. Brazilian specimens were treated at microwave radiation (with different microwave powers and exposure times) and water cooling (after microwave processing) before the dynamic split test. The influence of microwave irradiation power (1–4 kW) and exposure time (10–40 s) on the surface temperature, P-wave velocity, and dynamic tensile strength of the basalt specimens have been studied. Test results indicate that the treatment has negative effect on the rock P-wave velocity and dynamic tensile strength, and high power and low time radiation can cause more damage than the specimens treated at low power and long time radiation, though equivalent microwave energy is exerted on the basalt samples. For example, the average rock dynamic tensile strength for the specimen treated at 1 kW for 40 s, 2 kW for 20 s, and 4 kW for 10 s is 37.51 Mpa, 37.98 MPa, and 31.94 MPa, respectively, which decreases by 13%, 12%, and 26% compared with the untreated specimens. When the specimens were heated at 4 kW for 28 s and longer, they break into debris abruptly and accordingly lost their bearing capacity. The internal structural characteristics of the treated basalts were studied using scanning electron microscopic (SEM) technique. These photographs showed that micro-cracks appear on the rock surface after microwave radiation and water cooling treatment. It confirms the damage caused by microwaves to basalt specimens on the microscopic level and provides an internal reason for the reduction in dynamic tensile strength and P-wave velocity. The research can provide certain experimental support for microwave-assisted mechanical rock breaking. [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]

Granite is a common construction material that is widely used in the various types of structures, but its dynamic behavior is not clearly understood. To investigate the uniaxial compressive behavior of the granite under high strain rates compression, three groups of specimens with the same aspect ratio (0.5) but different diameters were tested with two large split Hopkinson pressure bar systems with respective diameters of 60 and 155 mm. The latter allowed the large specimen with diameter up to 150 mm to be employed in the compressive tests. Brittle fracture was the main failure pattern in the dynamic tests. The fragment size decreased with the increasing strain rate, ranging from 21 to 286 s−1. The percentage of the fragments with small sizes (< 5 mm) increased with the increasing strain rate, while the percentage with larger sizes (> 20 mm) decreased. The incomplete stress–strain curves corresponded to the incomplete fragmentation of the specimens. The representative stress–strain curve of each impact velocity was developed using a mathematical model. The dynamic strength of the 50-mm- and 60-mm-diameter specimens were identical, while that of the 150-mm specimens was larger at the same strain rate level. The dynamic increase factor was < 1.0 at the low strain rates (< 55 s−1 for the 150-mm-diameter specimens, and < 100 s−1 for the 50-mm- and 60-mm-diameter specimens), while it was > 1.0 at higher strain rates. The energy absorption density increased with the impact velocity, dynamic strength, and strain rate. Two groups of representative curves were developed to describe the relationships between the energy absorption density, strain rates, and dynamic strength. [ABSTRACT FROM AUTHOR]

To study the effect of freeze–thaw weathering damage deterioration on the strength of Huashan granite, this paper conducted a freeze–thaw (FT) cycle test on Huashan granite at − 20 to 20 °C and 0–100 cycles. Thus, the basic physical parameters change law of Huashan granite after freeze–thaw weathering are obtained. The SHPB (Split Hopkinson Pressure Bar) device was used to perform uniaxial and confining impact compression of granite, analyze the macro-mechanical test phenomena, and reveal the influence of freeze–thaw damage on rock strength. The results show that frost heave failure is the principal mechanism that causes the strength to decrease after freeze–thaw weathering. The static uniaxial compressive strength and maximum tensile strength of Huashan granite decreased by 38.4 and 21.5% after 100 FT cycles, respectively. The rate of reduction of peak stress does not necessarily grow with the increase of the number of cycles, and the average reduction rate in each phase ranges from 0.1 to 2.4%. It means that the damage accumulation of freeze–thaw weathering on rocks is not a uniformly increasing damage process. The radial restraint of confining pressure can effectively increase the peak stress of the material. When the strain rate is low, no failure occurs in the specimens due to the hoop restraint of confining pressure. [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]

The effects of low temperature and strain rate on the dynamic mechanical properties of dry and saturated siltstones under sub-zero temperatures were investigated and presented in this paper. The siltstone specimens were first frozen to different sub-zero temperatures (from − 10 to − 50 °C), and then were tested at those temperatures using a split Hopkinson pressure bar (SHPB) system. The results indicated that compared to the dry specimens, the saturated specimens exhibit a much shorter compaction phase in the dynamic stress–strain curve due to the presence of the pore water or ice at sub-zero temperatures. The dynamic elastic modulus (Ed) and dynamic uniaxial compressive strength (UCSd) monotonically increase with the increase in the strain rate for both the dry and saturated specimens, and the dry specimens are more sensitive to the strain rate effect with respect to the UCSd. Furthermore, for both the dry and saturated specimens, the Ed and UCSd first increase with the decrease in the temperature from 18 to − 30 °C, and then decrease with a further drop in the temperature from − 30 to − 50 °C. Manifold reasons are responsible for this phenomenon, including the shrinkage of mineral grains, enhancement of the ice strength and interaction of the water/ice mixture with rock as the temperature drops. Using the NMR technique, the mechanisms of the mixed water/ice weakening and strengthening effects on the dynamic mechanical properties of siltstones at sub-zero temperatures were discussed. [ABSTRACT FROM AUTHOR]

Keywords: Microstructure damage; F-T cycles; NMR; Porosity; SCB; SHPB EN Microstructure damage F-T cycles NMR Porosity SCB SHPB 3853 3858 6 07/31/20 20200801 NES 200801 Introduction In high altitude and cold regions, cyclic freezing and thawing often caused rock degradation, resulting in the instability of rock involved engineering structures such as rock slopes and tunnels. 1, cylindrical rods between samples and transmitted bar acted as supporting pins, HT ht is the span between two rods. The strength can be calculated according to the peak value ( HT ht ), as follows (Dai et al. [4]): 1 HT ht Graph Graph: Fig. The dynamic flexural strength, HT ht , is approximately proportional to HT ht . [Extracted from the article]

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]

Cohesion and internal friction angle are the two material parameters used in the Coulomb model to predict rock failure in many rock engineering applications. Although these two parameters have been extensively quantified under static conditions using the direct shear or the triaxial compression methods, the effect of dynamic loading on these parameters is not yet clear. A dynamic punch shear method was proposed by Huang et al. (Rev Sci Instrum 82:053901. 10.1063/1.3585983, 2011) to measure the dynamic cohesion of rocks, and the dependence of cohesion on the loading rate has been revealed. To further investigate the effect of dynamic loading on the internal friction angle and thus the complete dynamic shear response of rocks, this method is extended in this study to include the normal stress by applying lateral confinement to a disc specimen. The confinement is realized by enclosing the specimen assembly in a 1.5 inch diameter Hoek cell. The dynamic load is applied by a split Hopkinson pressure bar system, which is modified to ensure that the specimen assembly remains intact in the Hoek cell during pressurization by applying a static axial pre-stress. Three groups of green sandstone specimens under confinements of 0, 10 and 20 MPa are tested with different loading rates. The results show that the dynamic shear strength exhibits significant rate dependency and it thus increases with the loading rate and the normal stress. The dynamic cohesion increases with the loading rate, while the internal friction angle remains constant. [ABSTRACT FROM AUTHOR]

The dynamic mechanical properties and behavior of transversely isotropic rock are vital in engineering applications. In this study, the dynamic compressive properties of a low anisotropy slate are investigated for five sample groups (θ = 0°, 30°, 45°, 60°, and 90°) using a split Hopkinson pressure bar (SHPB) with three loading levels. A cone-shaped striker is used for achieving a dynamic stress equilibrium. The test results illustrate that the failure patterns of slate at a low loading level can be divided into four major types and that bedding plays an increasingly dominant role in the failure of samples as the dip angle increases. The tensile splitting effect gradually begins to have a prominent influence on the failure patterns as the loading level increases. Moreover, the variations in the failure patterns may lead to a decrease in the strain rate effect when the dip angle increases from θ = 30° to θ = 60°. At θ = 90°, the measured dynamic critical strength is less than the static strength, possibly due to the anisotropic effect of bedding directivity. In addition, the dynamic strength of the slate exhibits a higher anisotropy under dynamic loading, and the strength anisotropy index first decreases and then increases as the loading level increases. [ABSTRACT FROM AUTHOR]

A novel method is developed for characterizing the mechanical response and failure mechanism of brittle rocks under dynamic compression-shear loading: an inclined cylinder specimen using a modified split Hopkinson pressure bar (SHPB) system. With the specimen axis inclining to the loading direction of SHPB, a shear component can be introduced into the specimen. Both static and dynamic experiments are conducted on sandstone specimens. Given carefully pulse shaping, the dynamic equilibrium of the inclined specimens can be satisfied, and thus the quasi-static data reduction is employed. The normal and shear stress-strain relationships of specimens are subsequently established. The progressive failure process of the specimen illustrated via high-speed photographs manifests a mixed failure mode accommodating both the shear-dominated failure and the localized tensile damage. The elastic and shear moduli exhibit certain loading-path dependence under quasi-static loading but loading-path insensitivity under high loading rates. Loading rate dependence is evidently demonstrated through the failure characteristics involving fragmentation, compression and shear strength and failure surfaces based on Drucker-Prager criterion. Our proposed method is convenient and reliable to study the dynamic response and failure mechanism of rocks under combined compression-shear loading. [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]

Useable land is a finite space, and with a growing global population, countries have been exploring the use of underground space as a strategic resource to sustain the growth of their society and economy. However, the effects of impact loading on rocks that have been heated, and hence the integrity of the underground structure, are still not fully understood and has not been included in current design standards. Such scenarios include traffic accidents and explosions during an underground fire. This study aims to provide a better understanding of the dynamic load capacity of Carrara marble at elevated temperatures. Dynamic uniaxial compression tests are performed on Carrara marble held at various temperatures using a split-Hopkinson Pressure Bar (SHPB) setup with varying input force. A customized oven is included in the SHPB setup to allow for testing of the marble specimens in a heated state. After the loading test, a three-wave analysis is performed to obtain the dynamic stress-strain curve of the specimen under loading. The fragments of the failed specimens were also collected and dry-sieved to obtain the particle size distribution. The results reveal that the peak stress of specimens that have been heated is negatively correlated with the heating temperature. However, the energy absorbed by the specimens at peak stress at all temperatures is similar, indicating that a significant amount of energy is dissipated via plastic deformation. Generally, fragment size is also found to show a negative correlation with heating temperature and loading pressure. However, in some cases this relationship does not hold true, probably due to the occurrence of stress shadowing. Linear Elastic Fracture Mechanics has been found to be generally applicable to specimens tested at low temperatures; but at higher temperatures, Elastic-Plastic Fracture Mechanics will give a more accurate prediction. Another contribution of this study is to show that other than the peak stress of the rock failure type, the strain history experienced by the rock during impact and the post-impact fragment size distribution are also significant distinguishing features of damage caused by dynamic loading on heated rocks. [ABSTRACT FROM AUTHOR]

This paper presents a new experimental method for the characterization of the surface damage caused by a heat shock on a Brazilian disk test sample. Prior to mechanical testing with a Hopkinson Split Pressure bar device, the samples were subjected to heat shock by placing a flame torch at a fixed distance from the sample's surface for periods of 10, 30, and 60 s. The sample surfaces were studied before and after the heat shock using optical microscopy and profilometry, and the images were analyzed to quantify the damage caused by the heat shock. The complexity of the surface crack patterns was quantified using fractal dimension of the crack patterns, which were used to explain the results of the mechanical testing. Even though the heat shock also causes damage below the surface which cannot be quantified from the optical images, the presented surface crack pattern analysis can give a reasonable estimate on the drop rate of the tension strength of the rock. [ABSTRACT FROM AUTHOR]

As a material for famous historical underground rock caverns, Longyou sandstone (LS) may fail under the combination of high loading rate and high temperature. The thermal damage induced by various heat-treatment temperatures (150, 250, 350, 450, 600 and 850 °C) is first characterized by X-ray Micro-computed tomography (CT) method. The damage variable derived from the average CT value for heat-treated LS specimen and reference specimen without heat treatment was used to quantify the thermal damage. The dynamic tensile strengths of these LS samples under different dynamic loading rates (ranging from 24 to 540 GPa/s) were then obtained using the split Hopkinson pressure bar (SHPB) system. The dynamic tensile strength of LS increases with the loading rate at a given heat-treatment temperature, and the tensile strength at the same loading rate decreases with the heat-treatment temperature except for 450 °C. Based on the experimental data, an empirical equation was established to relate the dynamic tensile strength of LS to the loading rate and the heat-treatment temperature. [ABSTRACT FROM AUTHOR]

The article presents an investigation on the mechanical properties of the leptynite under repeated impact loading based on the analysis of the evolution of the fatigue damage in the rock samples. It analyzes the elastic modulus of the sample, variation of the peak strain, and the decrease in the P-wave velocity. Also, the usage of the split Hopkinson pressure bar (SHPB) system to quantify the fatigue properties of the rocks is mentioned.

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]

FEM-based numerical testing systems of the split Hopkinson pressure bar (SHPB) and the split Hopkinson tensile bar (SHTB) are established to study the characteristics of rock materials under dynamic compressive and tensile loadings. First of all, the accuracy and applicability of the numerical testing system are validated and calibrated through comparison between the laboratory measurements and the simulation results. Subsequently, the dynamic behavior of rock is analyzed in detail with the numerical testing system followed by the underlying physical mechanism. For the SHPB tests, the simulation results demonstrate that the incident waveform is determined by the striker length, the striker shape and the pulse shaper. The dynamic increase factor (DIF) of the rock specimen varies with different impact velocities, which is attributed to the strain rate effect. The rock specimen size and bar size also have effects on the DIF. In addition, the interfacial friction between the rock specimen and the bars cannot be ignored. For the SHTB tests, it is found that the incident waveform is dependent on the striker tube length and the striker tube thickness. In addition, similar to the SHPB tests, the impact velocity, rock specimen size and bar size all have strong effects on the rock dynamic tensile strength. [ABSTRACT FROM AUTHOR]

It is critical to understand the dynamic tensile failure of confined rocks in many rock engineering applications, such as underground blasting in mining projects. To simulate the in situ stress state of underground rocks, a modified split Hopkinson pressure bar system is utilized to load Brazilian disc (BD) samples hydrostatically, and then exert dynamic load to the sample by impacting the striker on the incident bar. The pulse shaper technique is used to generate a slowly rising stress wave to facilitate the dynamic force balance in the tests. Five groups of Laurentian granite BD samples (with static BD tensile strength of 12.8 MPa) under the hydrostatic confinement of 0, 5, 10, 15, and 20 MPa were tested with different loading rates. The result shows that the dynamic tensile strength increases with the hydrostatic confining pressure. It is also observed that under the same hydrostatic pressure, the dynamic tensile strength increases with the loading rate, revealing the so-called rate dependency for engineering materials. Furthermore, the increment of the tensile strength decreases with the hydrostatic confinement, which resembles the static tensile behavior of rock under confining pressure, as reported in the literature. The recovered samples are examined using X-ray micro-computed tomography method and the observed crack pattern is consistent with the experimental result. [ABSTRACT FROM AUTHOR]

A dynamic load superposed on a static pre-load is a key problem in deep underground rock engineering projects. Based on a modified split Hopkinson pressure bar test system, the notched semi-circular bend (NSCB) method is selected to investigate the fracture initiation toughness of rocks subjected to pre-load. In this study, a two-dimensional ANSYS finite element simulation model is developed to calculate the dimensionless stress intensity factor. Three groups of NSCB specimen are tested under a pre-load of 0, 37 and 74 % of the maximum static load and with the loading rate ranging from 0 to 60 GPa m s. The results show that under a given pre-load, the fracture initiation toughness of rock increases with the loading rate, resembling the typical rate dependence of materials. Furthermore, the dynamic rock fracture toughness decreases with the static pre-load at a given loading rate. The total fracture toughness, defined as the sum of the dynamic fracture toughness and initial stress intensity factor calculated from the pre-load, increases with the pre-load at a given loading rate. An empirical equation is used to represent the effect of loading rate and pre-load force, and the results show that this equation can depict the trend of the experimental data. [ABSTRACT FROM AUTHOR]

A modified split Hopkinson pressure bar (SHPB) numerical testing system is established to study the characteristics of rocks under simultaneous static and dynamic loading conditions following verification of the capability of the SHPB numerical system through comparison with laboratory measurements (Liao et al. in Rock Mech Rock Eng, 2016. doi:). Three different methods are employed in this numerical testing system to address the contact problem between a rock specimen and bars. The effects of stand-alone static axial pressure, stand-alone lateral confining pressures, and a combination of them are analyzed. It is determined that the rock total strength and the dynamic strength are greatly dependent on the static axial and confining pressures. Moreover, the friction along the interfaces between the rock specimen and bars cannot be ignored, particularly for high axial pressure conditions. Subsequently, the findings are applied to determine the dynamic behavior of rocks with in situ stresses. The effects of the magnitude of horizontal and vertical initial stresses at varied depths and their ratios are investigated. It is observed that the dynamic strength of deep rocks increases with increasing depth or the ratio of horizontal-to-vertical initial stresses ( K). The dynamic behavior of deeper rocks is more sensitive to K, and the rock dynamic strength increases faster with depth in areas with higher K. [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 effects of specimen size and geometry on the dynamic mechanical properties of Carrara marble including compressive strength, failure strain and elastic modulus are investigated in this research. Four different groups of specimens of different sizes and cross-sectional geometries are loaded under a wide range of strain rates by the split Hopkinson pressure bar setup. The experimental results indicate that all these mechanical properties are significantly influenced by the specimen size and geometry to different extent, hence highlighting the importance of taking into account of the specimen size and geometry in dynamic tests on rock materials. In addition, the transmission coefficient and the determination of strain rate under dynamic tests are discussed in detail. [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]

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]

A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio ( $${{\tau_{\text{c}} } \mathord{\left/ {\vphantom {{\tau_{\text{c}} } {\sigma_{\text{c}} }}} \right. \kern-0pt} {\sigma_{\text{c}} }}$$ ) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials. [ABSTRACT FROM AUTHOR]

The tensile strength and fracturing behavior of Carrara marble subjected to the dynamic Brazilian disc test using the split Hopkinson pressure bar technique are determined and compared with those obtained by the conventional quasistatic Brazilian disc test. Detailed observation of the cracking processes is aided by high-speed video footage captured at a frame rate of 100,000 frames per second. The dynamic increase factor is computed, revealing a strong strain rate dependence of the Carrara marble when subjected to strain rates above 1 s. Similar to the quasistatic loading tests, conspicuous white zones/patches commonly appear prior to the initiation of visible cracks in the dynamic loading tests. Identification of the white patch initiation and evolution is aided by image comparison software. Comparing the cracking and failure processes under quasistatic and dynamic loading, some distinct differences in the white patch geometry and initiation load are observed. In addition, the extent of the compressive failure zones around the contact points between the loading platens and specimens is found to increase with the strain rate. [ABSTRACT FROM AUTHOR]

Granite exhibits anisotropy due to pre-existing microcracks under tectonic loadings; and the mechanical property anisotropy such as flexural/tensile strength is vital to many rock engineering applications. In this paper, Barre Granite is studied to understand the flexural strength anisotropy under a wide range of loading rates using newly proposed semi-circular bend tests. Static tests are conducted with a MTS hydraulic servo-control testing machine and dynamic tests with a split Hopkinson pressure bar (SHPB) system. Six samples groups are fabricated with respect to the three principle directions of Barre granite. Pulse shaping technique is used in all dynamic SHPB tests to facilitate dynamic stress equilibrium. Finite element method is utilized to build up equations calculating the flexural tensile strength. For samples in the same orientation group, a loading rate dependence of the flexural tensile strength is observed. The measured flexural tensile strength is higher than the tensile strength measured using Brazilian disc method at given loading rate and this scenario has been rationalized using a non-local failure theory. The flexural tensile strength anisotropy features obvious dependence on the loading rates, the higher the loading rate, the less the anisotropy and this phenomenon may be explained considering the interaction of the preferentially oriented microcracks. [ABSTRACT FROM AUTHOR]

The dynamic compressive strength of rock materials increases with the strain rate. They are usually obtained by conducting laboratory tests such as split Hopkinson pressure bar (SHPB) test or drop-weight test. It is commonly agreed now that the dynamic increase factor (DIF) obtained from impact test is affected by lateral inertia confinement, friction confinement between the specimen and impact materials and the specimen sizes and geometries. Therefore, those derived directly from testing data do not necessarily reflect the true dynamic material properties. The influences of these parameters, however, are not straightforward to be quantified in laboratory tests. Therefore, the empirical DIF relations of rock materials obtained directly from impact tests consist of contributions from lateral inertia and end friction confinements, which need be eliminated to reflect the true dynamic material properties. Moreover, different rocks, such as granite, limestone and tuff have different material parameters, e.g., equation of state (EOS) and strength, which may also affect the DIF of materials but are not explicitly studied in the open literature. In the present study, numerical models of granite, limestone and tuff materials with different EOS and strength under impact loads are developed to simulate SHPB tests and to study the influences of EOS and strength, lateral inertia confinement and end friction confinement effects on their respective DIFs in the strain rate range between 1 and 1,000 s. The commercial software AUTODYN with user-provided subroutines is used to perform the numerical simulations of SHPB tests. Numerical simulation results indicate that the lateral inertia confinement, friction confinement and specimen aspect ( L/ D) ratio significantly influence DIF obtained from impact tests and the inertia confinement effect is different for different rocks. Based on the numerical results, quantifications on the relative contributions from the lateral inertia confinement and the material strain rate effect on DIF of granite, limestone and tuff material compressive strength are made. The effects of friction coefficient, L/ D ratio and rock type on DIF are discussed. Empirical relations of DIF with strain rate for the three rock materials representing the true material strain rate effect are also proposed. [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]

It is well-known that the strengths of sandstones measured under fully saturated conditions are smaller than those measured under nominally dry conditions. This strength softening phenomenon has profound implications to rock engineering. In this work we investigate the tensile strength softening of Longyou sandstone from China. Defining the strength softening factor as the ratio of the strength under nominally dry conditions over that under saturated conditions, the static compressive strength softening factor of Longyou sandstone is close to 2 and the static tensile strength softening factor is about 7.9. To further address the applications, where the load may be dynamic, we examine the rate dependence of the tensile strength softening of this sandstone. The dynamic tensile strength is measured using the split Hopkinson pressure bar system in combination with the Brazilian disc sample geometry. The results show that the tensile strength softening factor decreases with the loading rate. Because the saturated sample shows stronger loading rate sensitivity than the dry sample, the softening factor decreases with the loading rate. [ABSTRACT FROM AUTHOR]

large number of tests have recently been conducted with the Split Hopkinson pressure bar (SHPB) method to determine the characteristics of rock dynamics. However, it is still impossible to get test results at a perfect constant strain rate from this set-up owing to the rate dependency of rock materials. For instance in most cases, dynamic behavior of rock can only be described with an average strain rate. The results from these methods, including rich strain rate information, frequently tend to be inexplicable or self-contradictory. The obtained stress-strain curves can then never be directly treated as constitutive curves as in static tests. In this paper, the reasons behind the controversial stress-strain results with current methods are analyzed. In addition, the requirement for the rock specimen to deform at a constant strain rate is demonstrated after theoretical analysis of correlations among specimen, deforming stress, incident stress, reflected stress and transmitted stress. With test results from SHPB by pulse shaper and special shape striker methods, the requirement is verified. Finally, the method of 3D scattergram considering stress-strain-strain rate simultaneously is brought up to get constitutive relationships of rate-dependent rock. The new method gives reasonable predictions for constitutive relationships of rock at different strain rates. At the same time, the new method has fewer requirements and has a wider application scope for SHPB tests. [ABSTRACT FROM AUTHOR]

urate characterizations of rock strengths under higher loading rates are crucial in many rock engineering applications. The split Hopkinson pressure bar (SHPB) system has been used to quantify the dynamic compressive strength of rocks using the short cylindrical specimen and the dynamic tensile strength of rocks using the Brazilian disc (BD) specimen. However, SHPB is a standard tool that is suitable for metal testing; there are some fundamental issues that need to be carefully visited in applying SHPB to rock dynamic tests. This paper addresses several such critical issues, including the choice of slenderness ratio of the compressive specimen, the effect of friction between the sample and bars on the measured results of compressive strength, the necessity of dynamic force balance on the dynamic BD test, and the validity of using the standard BD equation in the data reduction. We show that with proper experimental designs that address these issues, the dynamic compressive strength and dynamic tensile strength of rocks measured using SHPB are valid and reliable. [ABSTRACT FROM AUTHOR]