Dynamic properties of rocks are important in a variety of rock mechanics and rock engineering problems. Due to the transient nature of the loading, dynamic tests of rock materials are very different ...from and much more challenging than their static counterparts. Dynamic tests are usually conducted using the split Hopkinson bar or Kolsky bar systems, which include both split Hopkinson pressure bar (SHPB) and split Hopkinson tension bar (SHTB) systems. Significant progress has been made on the quantification of various rock dynamic properties, owing to the advances in the experimental techniques of SHPB system. This review aims to fully describe and critically assess the detailed procedures and principles of techniques for dynamic rock tests using split Hopkinson bars. The history and principles of SHPB are outlined, followed by the key loading techniques that are useful for dynamic rock tests with SHPB (i.e. pulse shaping, momentum-trap and multi-axial loading techniques). Various measurement techniques for rock tests in SHPB (i.e. X-ray micro computed tomography (CT), laser gap gauge (LGG), digital image correlation (DIC), Moiré method, caustics method, photoelastic coating method, dynamic infrared thermography) are then discussed. As the main objective of the review, various dynamic measurement techniques for rocks using SHPB are described, including dynamic rock strength measurements (i.e. dynamic compression, tension, bending and shear tests), dynamic fracture measurements (i.e. dynamic imitation and propagation fracture toughness, dynamic fracture energy and fracture velocity), and dynamic techniques for studying the influences of temperature and pore water.
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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.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Rocks are increasingly used in extreme environments characterised by high loading rates and high confining pressures. Thus the fracture properties of rocks under dynamic loading and confinements are ...critical in various rock mechanics and rock engineering problems. Due to the transient nature of dynamic loading, the dynamic fracture tests of rocks are much more challenging than their static counterparts. Understanding the dynamic fracture behaviour of geomaterials relies significantly on suitable and reliable dynamic fracture testing methods. One of such methods is the notched semi-circle bend (NSCB) test combined with the advanced split Hopkinson pressure bar (SHPB) system, which has been recommended by the International Society for Rock Mechanics and Rock Engineering (ISRM) as the standard method for the determination of dynamic fracture toughness. The dynamic NSCB-SHPB method can provide detailed insights into dynamic fracture properties including initiation fracture toughness, fracture energy, propagation fracture toughness and fracture velocity. This review aims to fully describe the detailed principles and state-of-the-art applications of dynamic NSCB-SHPB techniques. The history and principles of dynamic NSCB-SHPB tests for rocks are outlined, and then the applications of dynamic NSCB-SHPB method (including the measurements of initiation and propagation fracture toughnesses and the limiting fracture velocity, the size effect and the digital image correlation (DIC) experiments) are discussed. Further, other applications of dynamic NSCB-SHPB techniques (i.e. the thermal, moisture and anisotropy effects on the dynamic fracture properties of geomaterials, and dynamic fracture toughness of geomaterials under pre-loading and hydrostatic pressures) are presented.
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Dynamic tensile failure of rocks under static pre-tension Bangbiao Wu; Chen, Rong; Xia, Kaiwen
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
December 2015, 2015-12-00, Volume:
80
Journal Article
Peer reviewed
A modified split Hopkinson pressure bar (SHPB) system is utilized to load Brazilian disc (BD) samples statically, and then exert dynamic load to the sample generated by impact. The pulse shaper ...technique is used to generate a slowly rising stress wave to facilitate the dynamic force balance in dynamic tests. Five groups of Laurentian granite BD samples (with static tensile strength of 12.8MPa) under the pre-tension of 0MPa, 2MPa, 4MPa, 8MPa, and 10MPa were tested under different loading rates. The results show that the rock dynamic tensile strength decreases with the increase of the pre-tension. It is also observed that under the same pre-tension stress, the dynamic tensile strength increases with the loading rate. However, the total tensile strength of the rock is roughly independent of the pre-tension. The failure patterns of the samples also reveal the rate dependence of the dynamic tensile strength of rocks.
•SHPB is modified to measure the dynamic tensile strength of rocks under static pre-tension.•The dynamic tensile strength decreases with the increase of pre-tension.•The dynamic tensile strength increases with the loading rate.•The total tensile strength is roughly independent of the pre-tension.
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Liu et al. (2018) preliminarily proposed a bonded-particle model (BPM) for disk-based discontinuous deformation analysis (DDA). This BPM can resist relative rotation of contact pairs by introducing a ...rolling spring. In this technical note, the ability of disk-based DDA with this BPM to simulate the fracturing process of rock is explored. The detailed formulations of the BPM fitted to DDA are derived. A simple elastic-brittle constitutive model is employed as the bond failure criterion, with the help of which the fracture of rock can be explicitly represented by the progressive failure of the bonds. Sensitivity analysis is performed to investigate the influence of micro parameters on the macro parameters, such as the elastic parameters (Young's modulus E and Poisson's ratio v) and the strength parameters (uniaxial compression strength and indirect tensile strength). Based on the sensitivity analysis, a calibration process is suggested. Finally, several numerical examples are presented to simulate the rock failure process under both static and dynamic loading. The results show that disk-based DDA is a good candidate for rock failure simulation.
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The mode I fracture toughness of rocks plays an important role in the stability assessment of rock structures. Semi-circular bend (SCB) is one of the methods suggested by International Society for ...Rock Mechanics and Rock Engineering for determining the mode I fracture toughness of rocks. However, due to the limitation in machining precision and quasi-brittle nature of rocks, the notch geometry of the SCB specimen and the fracture process zone (FPZ) may have significant impacts on the measurement accuracy of the fracture toughness. In this study, the SCB specimens made of Fangshan marble with notches featuring different tip shapes and widths were used to quantify the notch geometrical influences on the measurement accuracy. Three types of notch-tip shapes were considered: flat, semicircle and sharp. For each type of notch-tip shape, three notch widths were studied: 1 mm, 2 mm and 3 mm. Digital image correlation method was adopted to measure the FPZ lengths of the SCB specimens. With the FPZ lengths,
K
-resistance (
K
R
) curves were obtained using the contour integral. Subsequently, the fracture parameters were estimated from the
K
R
curves. The experimental results show that the initial fracture toughness depends on the notch geometry, whereas as an inherent rock property, the unstable fracture toughness is geometry-independent. These results support the geometrical requirements of the SCB specimen by the ISRM suggested method, in which the width/tip of the notch should be as narrow/sharp as possible to achieve accurate fracture toughness value.
Highlights
Effect of the notch geometry of SCB specimen on the initial load, the peak load and the critical crack length is investigated.
Geometrical dependence of the initial fracture toughness and the unstable fracture toughness obtained from
K
R
curves on the notch geometry of SCB specimen is revealed.
Measurement error of the fracture toughness obtained using the ISRM suggested SCB method is analyzed.
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The microwave-assisted rock fragmentation has been proven to be a promising approach in reducing cutting tools wear and improving efficiency in rock crushing and excavation. Thus, understanding the ...influence of damage induced by microwave irradiation on rock fragmentation is necessary. In this context, cylindrical Fangshan granite (FG) specimens were exposed to microwave irradiation at a power of 6 kW for different durations up to 4.5 min. The damages of the specimens induced by irradiation were quantified by using both X-ray micro-CT scanning and ultrasonic wave measurement. The CT value and P-wave velocity decreased with increase of irradiation duration. The irradiated specimens were then tested using a split Hopkinson pressure bar (SHPB) system to simulate rock fragmentation. A momentum-trap technique was utilized to ensure single-pulse loading on the specimen in SHPB tests, enabling valid fragment size distribution (FSD) analysis. The dependence of dynamic uniaxial compressive strength (UCS) on the irradiation duration and loading rate was revealed. The dynamic UCS increased with increase of loading rate while decreased with increase of irradiation duration. Using the sieve analysis, three fragmentation types were proposed based on FSD, which were dictated by both loading rate and irradiation duration. In addition, an average fragment size was proposed to quantify FSD. The results showed that the average fragment size decreased with increase of loading rate. A loading rate range was identified, where a dramatic reduction of the average fragment size occurred. The dependence of fragmentation on the irradiation duration and loading rate was also discussed.
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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.
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Dominant crack algorithm (DCA) is a statistical micromechanics model for the dynamic tensile response of brittle materials. In the DCA model, both microscopic and macroscopic material parameters are ...included to construct the constitutive relation, and the damage evolution is described by the growth of the dominant crack. In this study, the sensitivity analysis of the microscopic parameters (i.e. the characteristic volume size a, the initial crack size c0, and the crack velocity related parameter m) in the DCA model is conducted. The results demonstrate that the dynamic tensile strength is sensitive to the value of c0/a. Compared with the influence of the c0/a on the dynamic tensile strength, the dynamic tensile strength is less sensitive to the variation of the characteristic volume size a or the variation of the crack velocity related parameter m. Furthermore, the DCA model is applied to predict the dynamic tensile response of Laurentian granite (LG). A nonlinear regression method - Particle Swarm Optimization (PSO) - is utilized to optimize the values of the microscopic parameters. The results indicate that the dynamic tensile response of LG predicted by the DCA modeling has a good agreement with that obtained from the experiments. Therefore, the DCA modeling is valid and applicable to describe the dynamic tensile response and predict the dynamic tensile strength of rock-like materials.
•The sensitivity of the microscopic parameters in the DCA model is analyzed.•The dynamic tensile strength predicted by the DCA model is sensitive to the value of c0/a.•The dynamic tensile strength predicted by the DCA model is less sensitive to the value of a and m.•The predicted and measured dynamic tensile responses of rocks are consistent.•The DCA modeling is valid to predict the dynamic tensile response of rocks.
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As a common physical process in mining and other rock engineering industries, rock fragmentation is mainly induced by mechanical loading. To optimize the rock fragmentation process, many methods have ...been proposed to pre-condition the rock material. Heat-treatment is one such rock conditioning method to facilitate rock fragmentation. It is therefore necessary to investigate the effect of heat-treatment on the characteristics of rock fragmentation. This study is concerned with the rock fragmentation in crushing, which is experimentally studied utilizing the dynamic ball compression test. The spherical specimens made of Laurentian granite (LG) are heat-treated under various temperatures (250, 450, 600 and 850 ℃). The resulting thermal damage is first examined using the X-ray Micro-computed tomography (CT) method. The spilt Hopkinson pressure bar (SHPB) system in combination with a high speed camera is then utilized to conduct the dynamic ball compression test. With the aid of the moment-trap technique in SHPB and the high speed camera, the energy dissipation in crushing is obtained at five fixed input energy levels. The results indicate that the dynamic indirect tensile strength increases with the loading rate while decreases with the increase of the treatment temperature. The failure mode of ball specimen under dynamic compression can be categorized as fragmentation and pulverization based on the particle size distribution of fragments. The influence of the treatment temperature on the crushing ratio of LG ball specimen is more significant for higher input energy. Moreover, the energy efficiency of ball specimens generally decreases with the increase of the loading rate and the treatment temperature.
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