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Deep rock masses exist in a complex environment with multi-field coupling; therefore, it is necessary to develop a true-triaxial static-dynamic-coupling loading test machine to explore their characteristics and mechanical response mechanism. To meet the test requirements of true-triaxial loading and strong disturbance, a wave-absorbing metal plate was selected as the boundary material between the granite and transmission end, and the modified SHPB was used to perform static-dynamic-coupling loading tests. In this study, two series of experiments on wave- absorbing metal plates were conducted, which were fixed aperture sizes with different thicknesses and fixed thicknesses with different aperture sizes. The static-dynamic-coupling loading tests on each aperture size and plate thickness were carried out under the condition of equal energy impact. The effects of the aperture size and plate thickness on the incident- and reflection-stress curves, reflectivity, energy consumption law, energy evolution, and other mechanical properties of the wave-absorbing metal plate materials were studied. The results show that the peak stress and reflectivity decrease with increasing aperture size and plate thickness, and the influence of the thickness is greater than that of the aperture size. The energy-absorption rate of the wave-absorbing metal plate increased with increasing thickness and aperture size and was maximized when the aperture size and thickness were 6–7 mm and 3–4 mm, respectively. The variation trend of the energy reflectance is opposite to that of the energy absorption and reaches a minimum when the aperture size is 6–7 mm and plate thickness is 3–4 mm. The energy transmittance of the wave-absorbing metal plate fluctuated in a stable range, but the variation range was less obvious compared to that of the energy-absorption rate. [ABSTRACT FROM AUTHOR]

The rock-shotcrete interface has become common in underground tunnels following the widespread application of shotcrete as a support element. The static and dynamic tensile properties of the rock-shotcrete interface were measured using Instron 1342 and split Hopkinson pressure bar (SHPB) apparatus. The effect of loading rate and interface geometry on interface tensile behavior was studied using four levels of loading rate and six types of interface geometries. The results demonstrate that the interface has typical loading rate dependency, which means that bearing capacity of the interface increases with an increase in the loading rate. The bearing capacity and failure modes of the interface are sensitive to the interface geometry. The measured peak load exhibits an early increase and later decreasing trend; there is a critical value (1.2 mm and 1.8 mm under static and dynamic load) in geometry parameter “h” as the dominant failure mode changes from adhesion to cohesion. An interface with greater “h” leads to low bearing capacity. Therefore, the rock surface should be a reasonable roughness during rock surface preparation, and an appropriate depth of sawteeth is crucial to obtain reliable support strength in tunneling engineering. [ABSTRACT FROM AUTHOR]

To investigate the static mechanical properties and failure characteristics of damaged rocks after impact under realistic in-situ stress, dynamic triaxial compression tests with different strain rates and confining pressures were conducted on diorite by a modified split Hopkinson pressure bar. The damage factor was calculated according to the change of the longitudinal wave velocity of rocks before and after the impact, and the residual strengths of the damaged diorite were obtained by uniaxial compression and Brazilian tensile tests. In addition, three brittleness indexes based on the residual strength were adopted to evaluate the brittleness of damaged specimens, and fractal dimension was introduced to study the fragmentation characteristics. The results show that the dynamic peak stress, strain, elastic modulus, damage factor and plastic deformation of diorite all increase with increasing strain rate, i.e., rate dependence, and the increasing rate decreases with increasing confining pressure. The increment in confining pressure leads to the increase of dynamic peak stress, elastic modulus, static residual strength and brittleness, and the increasing rate increases with increasing strain rate, while other physio-mechanical parameters decrease with confining pressure. The post-peak behavior of dynamic stress-strain curves is divided into two classes according to the strain rate and fracture state of rocks under dynamic loading. Upon the strain rate increases above 87 s−1, the brittleness of damaged specimens generally decreases to moderately brittle or even low brittle, and the corresponding drillability becomes much easier. The fractal dimension of damaged specimens increases with the increase of confining pressure and decreases linearly with the increase of strain rate. [ABSTRACT FROM AUTHOR]

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

Natural or artificially disturbed rocks containing massive fillings are frequently subjected to various sophisticated temperature and stress perturbations. To investigate the effects of temperature and filling angle on the dynamic mechanical characteristics and fracture patterns of red sandstone, thermal treatments at 25°C, 200°C, 400°C, 600°C, and 800°C were conducted on red sandstone samples containing single fracture filling angles at 0°, 30°, 60°, and 90°. Subsequently, the dynamic mechanical information and crack evolution of specimens after thermal treatment were tested and recorded utilizing a modified split Hopkinson pressure bar (SHPB) test system integrated with a high-speed camera, and the crack expansion modes were identified and analyzed. Meanwhile, X-ray diffraction (XRD), scanning electron microscopy (SEM), simulation and one-dimensional stress wave theory were employed to analyze and discuss the thermal damage and failure mechanism of the specimens in detail. The results demonstrate that temperature and filling angle have conspicuous effects on the elastic modulus, thermal expansion, mineral composition, dynamic compressive strength, strain rate effects, and crack expansion modes of the specimens. Tensile cracks dominate the fracture of the specimen during dynamic loading, while shear cracks tend to appear in thermal stress concentration area. Moreover, thermal damage, wave impedance and stress wave propagation path are the critical factors controlling the fracture evolution and dynamic mechanical properties of the specimens. • Effects of thermal treatment on the dynamic behavior of fissure-filled red sandstone. • Fracture patterns of the specimen after the effect of temperature and filling angle are exhibited. • Damage parameters and simulation are employed to characterize the thermal damage. • Wave impedance and stress wave path control the dynamic mechanical response of the specimen. [ABSTRACT FROM AUTHOR]

To explore the variation in rock acoustic emission (AE) characteristics with strain rate, uniaxial compression tests at different loading rates and impact loading tests were conducted on granite using a MTS322 rock mechanical test system and split Hopkinson pressure bar (SHPB) system, respectively. The effect of the strain rate on the AE characteristic parameters, rock fracture properties, and destruction evolution were systematically analyzed. The results demonstrated that with increasing strain rate, the cumulative AE count decreases as a power function, and the variation between the cumulative AE count and strain rate can be fitted log-linearly with a slope of 0.48. The peak frequencies of the AE signals are mostly distributed in the zones of 0–100 kHz, 175–250 kHz, and 400–550 kHz. The signal proportion in the 0–100 kHz zone gradually increases with strain rate, while the signal proportions in the 175–250 kHz and 400–550 kHz zone exhibit decreasing trends. A transition of a sudden increase in the RA -value and decrease in the AF -value occurs when the stress reaches a certain level, and the stress level corresponding to this transition will increase with strain rate. Meanwhile, the RA–AF distribution is mostly concentrated on the abscissa in the low strain rate tests, but gradually concentrates on the longitudinal axis as the strain rate increases. This indicates that tensile cracking becomes the dominant fracture mode with increasing strain rate. The b -value decreases with increasing strain rate in the uniaxial compression tests; however, the b -value in the impact loading tests is higher than that in the uniaxial compression tests. Furthermore, to distinguish the signals generated by stress wave propagation from the signals generated by rock fracturing in the impact loading tests, a four-parameter k -means algorithm is used to conduct a clustering analysis. The results indicate that the signals can be classified into four clusters: tensile fracturing signals (cluster A), mixed stress wave and shear fracture signals (cluster B), mixed stress wave and tensile fracture signals (cluster C), and stress wave signals (cluster D). [ABSTRACT FROM AUTHOR]

• Dynamic mixed mode I/II fracture properties of CSTBD diorite specimens were studied. • Mixed mode fracture toughness and crack initiation angle were predicted by fracture criteria. • Dynamic fracture process and CMOD were monitored by high-speed camera and DIC. • Fracture surface morphology features were analyzed by laser 3D scanner and SEM. To investigate the dynamic mixed mode I/II fracture properties of hard rock, split-Hopkinson pressure bar (SHPB) was utilized to conduct dynamic fracture tests on cracked straight through Brazilian disc (CSTBD) diorite specimens with different mode mixities. The dynamic fracture load and fracture toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) of CSTBD specimens were monitored by high-speed camera and digital image correlation (DIC) technique, and the fractal and morphology characteristics of fracture surfaces were further analyzed using laser 3D scanner and scanning electron microscope (SEM). The test results indicate that the dynamic CMOD behavior is interrelated closely to the force acting on the crack surface, and the allowable CMOD corresponding to the dynamic peak load decreases with increasing crack angle. In addition, the stress-based and strain-based fracture criteria were introduced to analyze the mixed mode fracture mechanism of diorite. It is found that the GMTS criterion with small fracture process zone (FPZ) size and the EMTSN criterion have the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results show that the macro fracture trajectories of primary cracks and secondary cracks are controlled by the prefabricated crack angle, while the variation in mode mixity has little impact on the meso -roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen under dynamic load exhibits the SEM morphology features of mixed tensile-shear cracks. [ABSTRACT FROM AUTHOR]

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

In this paper, a three-dimensional numerical model to study the static and dynamic behaviour of ultra-high performance steel fibre reinforced concrete is developed. Ultra-high performance steel fibre reinforced concrete is assumed to be a two-phase model consisting of concrete matrix and steel fibres. The concrete matrix is modelled with homogeneous material and the straight round steel fibres are assumed to be dispersed with random locations and orientations in the matrix. The interfacial transition zone (ITZ) effect is studied based on the single fibre pull-out tests, and parameters describing the fibre-matrix one dimensional bond-slip behaviour are obtained and discussed based on both experimental and theoretical results. After the three-dimensional model is validated with static split tensile tests, split Hopkinson pressure bar (SHPB) split tensile tests are numerically modelled and the stress-time history is interpreted in the mesoscale level. The proposed model qualitatively and quantitatively predicts the material static and dynamic behaviours, and also gives insights on the fibre reinforcement effect in the concrete matrix. [ABSTRACT FROM AUTHOR]