Linear Elastic Stage Research Articles

Experimental verification and data analysis of Wet joint of pile slab

In order to realize the reasonable design of the new-type pile-slab subgrade, this study relied on the reconstruction and expansion project of the G5011 Wuhe Expressway from Lintou to Longxi Interchange to construct a pile-slab subgrade model with 15 holes and 6m each. The displacement of the wet joint of the pile-slab subgrade was obtained through the full-scale model. Based on the separate modeling method, the load-displacement curve and the load-stress curve are in good agreement with the actual test results. Linear analysis. Combined with the full-scale component static performance test, it is verified that the wet joints of the constructed pile-slab subgrade are basically in the linear elastic working stage under the design load frequency combination condition, which can be applied to engineering.

Study on Backfill Acoustic Emission Characteristics and Source Location under Uniaxial Compressive

This study investigates the acoustic emission (AE) characteristic of cemented tailings backfill (CTB) under uniaxial compressive loading. A classified tailings material source from Wushan Copper Mine was used to prepare cylindrical CTB samples. This study used an SLB10 loading machine for the uniaxial compressive test. A PCI‐2 acoustic wave system was attached to the CTB specimen to measure the acoustic emission during loading. Overall, the backfill specimen’s uniaxial compressive strength (UCS) is 3.58 MPa. The failure modes of the CTB sample could be divided into five stages, including the pore microfracture compacting stage (stage‐a), linear elastic deformation stage (stage‐b), yield deformation stage (stage‐c), failure stage (stage‐d), and residual stage (stage‐e). At each stage, the AE characteristic parameters such as the hits per second (HPS), the cumulative number of impacts, energy rate, and total energy vary a lot, which indicates the AE parameter reflects the internal failure evolution of the CTB sample. In stage‐a, no apparent AE behavior was measured. In Stage‐b, there is no evident macroscopic change in the specimen, while the AE parameters steadily and continuously increase. During stage‐c, the number of AE impacts grows sharply, the energy rate increases significantly, and changes suddenly. The cumulative number of AE impacts is 31.3% in stage‐d, and the cumulative energy rate is 49.4%, which is the most active stage of acoustic emission activities. The specimen has prominent crack propagation and bifurcation phenomena. In stage‐d, the CTB sample was destroyed entirely, and the characteristic parameters of acoustic emission were significantly reduced. Finally, multiple regression analysis methods are introduced into the time difference positioning method to locate the AE source. The positioning results show that the technique can better invert the close relationship between the damage and destruction of the CTB and the generation and penetration of the internal cracks and describe it to a certain extent. The failure surface of the filler specimen has an excellent guiding value for the analysis of the interior failure characteristics of the sample.

Mechanical Properties of the Shear Failure of Rock Masses with Prefabricated Sinusoidal Fractures

A large number of sinusoidal fractures exist in the open‐pit slopes of mines, and the mechanical properties of shear failure are of great significance to the stability of the open‐pit slope. Sandstone specimens containing sinusoidal fractures with different undulated heights were prefabricated by the CNC sand‐wire‐electrode cutting technology to explore the effect of sinusoidal fractures on the mechanical properties of shear failure of rock masses. The TFD‐20H/50J rock shear testing machine was used for shear tests on sinusoidal‐fracture sandstone with different undulated heights. In the shear loading process, the prefabricated‐fracture sandstone specimens with different undulated heights have the fracture‐compacting stage, linear‐elastic changing stage, bottom‐up stage, and residual strength stage. Before peak strength, the sinusoidal prefabricated‐fracture rock masses with different undulated heights have the precursory characteristics of decreased stress, which can provide an early warning for the instability and failure of rock masses. The undulated height of sinusoidal fractures significantly affects the fracture initiation and propagation of specimens. Fractures occur and develop from the prefabricated fractures at low undulated height (≤10 mm) to multiple ways under the high undulated height (>10 mm); that is, fractures occur at the prefabricated fractures and the end of specimens simultaneously. With the increased undulated height, the fractures expand from parallel to the slope of the prefabricated fractures to perpendicular to the prefabricated fractures. The fracture propagation direction of sinusoidal‐fracture rock masses with different undulated heights is mainly from the middle of the slope surface of prefabricated fractures to the end. It is the main direction of the shear failure of sinusoidal prefabricated fractures, and the monitoring of weak planes should be strengthened in the actual slope engineering.

Dynamic behaviour and failure mechanism of coal subjected to coupled water-static-dynamic loads

The stability of a coal pillar dam in a coal mine underground reservoir is affected by water pressure, static geo-stress, and coal mine earthquakes. Thus, it is essential to accurately evaluate the dynamic behaviour of coal under coupled water-static-dynamic loads to reasonably design a coal pillar dam. In this study, an innovative test system was developed based on a split Hopkinson pressure bar (SHPB) device, and a series of impact tests of coal under the coupled action of impact load, static axial preloading, and water pressure were performed with this system. The effects of coupled water-static-dynamic loads on mechanical behaviours, including strength and deformation characteristics, failure mode, and energy dissipation of the coal samples were systematically investigated. Results showed that the dynamic deformation process of the coal samples can be divided into three stages: linear elastic deformation stage, crack unstable propagation stage, and post-peak failure stage. Impact and higher axial static loads were the two negative factors that made the coal samples more prone to damage. In contrast, the presence of water pressure positively affected the sample, which improved the dynamic strength and reduced the failure strain of the sample. Energy dissipation characteristics showed that water pressure enhanced the energy utilisation efficiency of the sample. Furthermore, the microcosmic mechanism of free water protecting coal from the fractures was also examined.

Compressive behavior of rectangular concrete-filled fiber-reinforced polymer and steel composite tube columns with stress-release grooves

To investigate the behavior of rectangular concrete-filled fiber-reinforced polymer (FRP) and steel composite tube (CFCT) columns with stress-release grooves under axial compression, sixty specimens were fabricated and divided into five groups for axial compression testing. The main variables were the corner radius, the number of FRP layers, the steel tube thickness and the presence of stress-release grooves. The test results indicate that steel tubes and FRPs can complement each other and discontinuous steel tubes can provide an effective confinement force on in-filled concrete. The stress–strain responses of CFCT columns with stress-release grooves consist of a linear elastic stage, a nonlinear transition stage, a linear hardening stage and a residual stage. The ultimate strength of a CFCT column is in direct proportion to the number of FRP layers, the thickness of the steel tube and the value of the corner radius. Stress-release grooves only play a positive role in specimens with relatively large values of the corner radii (r ≥ 30 mm). A new model was suggested for calculating the ultimate stress of CFCT columns with stress-release grooves. The model is applicable for CFCT and concrete-filled steel tube (CFT) specimens with stress-release grooves; the predicted results agree well with the experimental statistics.

Experimental Study on Seepage and Fracture Characteristics of Cemented Rock Samples under Triaxial Stress

Abstract To study the seepage and fracture characteristics of cemented rock strata, a series of triaxial seepage tests on cemented rock samples under different confining pressures and water pressures were carried out in this study. The triaxial strength, elastic modulus, volume strain, and the permeability of the cemented rock samples were analyzed by the seepage unit connection probability model and Kozeny-Carman model. Based on test results, the stress state of cemented rock samples was divided into four stages: nonlinear compaction stage, linear elastic stage, stress yield stage, and failure and postfailure stage. The triaxial strength of the cemented rock samples gradually increased with the increase of confining pressure but decreased with the increase of water pressure. The elastic modulus of the cemented rock sample increased with the increase of confining pressure but decreased with the increase of water pressure. Besides, the volume strain of the cemented rock sample was analyzed, and the volume strain change of the cemented rock sample was also classified into three stages: the increasing stage of crack volume strain, the stable stage of crack volume strain, and the decreasing stage of crack volume strain. Based on the results of triaxial seepage tests, the evolution of permeability was divided into the declining stage, increasing stage, and redescend stage. Through the seepage unit connection probability model and Kozeny-Carman model, the evolution of crack volume was obtained, and the evolution of crack volume with axial strain was also classified into three stages: the original pore closure stage, crack network expansion stage, and crack network closure stage. The permeability evolution and the crack volume evolution were also compared. The comparison results suggest that three stages of crack volume evolution are all ahead of three stages of permeability evolution, verifying that the crack propagation induces the formation of seepage channels in cemented rock samples. This research will provide a valuable reference for the study of instability and water inrush mechanism in cemented rock strata.

Study on characteristic stress and energy damage evolution mechanism of cemented tailings backfill under uniaxial compression

This study attempted to explore, at curing age, the characteristics stress and energy evolution of the cemented tailings backfill (CTB) under uniaxial compression. Based on the energy consumption theory, the energy parameters at the characteristic stress varing with the curing age was analyzed. Furthermore, the evolution law of energy distribution and the energy damage mechanism of the CTB were revealed, and the failure criterion of the CTB was established based on the elastic energy consumption ratio. The results showed that the characteristic stress of the CTB follow an exponential function increasing law with the increase of curing age and the characteristic stress of the CTB with larger cement-to-tailings (c/t) ratio increases more rapidly when the curing age is 3d ~ 14d. The total energy, elastic strain energy and consumption energy at the peak stress point of the CTB all follow the exponential function increasing law with the increase of curing age, and the increase of cement-to-tailings (c/t) ratio and curing age can significantly improve the energy storage limit of the CTB. The deformation and failure of the CTB mainly go through five stages: initial crack compaction stage, linear elastic deformation stage, steady development stage of microcrack, accelerated crack expansion stage and post-peak failure stage. According to the damage value D of the CTB and the growth law of dissipated energy, elastic strain energy and axial strain, the loading damage of cemented CTB can be divided as follow: initial damage stage, damage stationary stage, damage stable stage, damage rapid stage and damage failure stage. The slow growth and abrupt change of the elastic energy consumption ratio K in the post-peak stage can be used as the criterion for strength instability of the CTB.

Large Deformation and Energy Absorption Behaviour of Perforated Hollow Sphere Structures under Quasi-Static Compression.

Hollow sphere structures with perforations (PHSSs) in three different arrangements (simple cubic (SC), body-centred cubic (BCC), and face-centred cubic (FCC)) were fabricated through three-dimensional (3D) printing, and the mechanical behaviours of these PHSSs under quasi-static compression were investigated experimentally and numerically. The results indicated that under uniaxial compression, the PHSSs mainly undergo three stages, i.e., a linear elastic stage, a large deformation or plateau stage, and a densification stage. During the stage of large deformation, the SC and BCC PHSSs experience a preliminary compaction sub-stage after layer-by-layer buckling, while for the FCC PHSS, layer-by-layer collapse and compaction are the dominant deformation behaviours. A numerical simulation was employed to study the mechanical properties of PHSSs with different geometric parameters under quasi-static compression and to explore the effect of the wall thickness, hole diameter, and sphere arrangement on the first peak stress, plateau stress, and specific energy absorption (SEA) of the PHSSs. The results reveal that the geometric parameters have a significant impact on the large deformation behaviour and energy absorption capacity of PHSSs. The presented PHSS is also proven to be much lighter than traditional metallic hollow sphere structure (MHSS) and has higher specific strength and SEA.

Investigation on Dynamical Mechanics, Energy Dissipation, and Microstructural Characteristics of Cemented Tailings Backfill under SHPB Tests

As mining depth increases, the backfill mining method is more and more widely used in underground mines. The dynamic load generated by the blasting can affect the stability of the cemented tailings backfill (CTB). The CTB samples were prepared to conduct a test of the split Hopkinson pressure bar (SHPB) to investigate the dynamic disturbance of CTB. The present paper discusses dynamical mechanics, energy dissipation, and microstructure analysis of CTB. Micro-computer tomography (micro-CT) scanning of CTB samples after the SHPB test was performed to analyze the evolution of internal cracks. The experimental results showed that when the average strain rate (ASR) increased from 30 to 98 s−1, the dynamic uniaxial compression strength (DUCS) of the CTB showed a trend of first increasing and decreasing with the increase in ASR. The dynamic stress–strain pre-peak curve of CTB directly enters the linear elastic stage. As ASR increases, the absorbed energy of the CTB shows a trend of first increasing and then decreasing. Moreover, according to the micro-CT scanning results, the crack area of CTB accounts for about 16% of the sample near the incident bar and about 1% near the transmitted bar. The crack area ratio is exponentially related to the specimen height. These findings can provide reasonable dynamical CTB strength data selection for underground pillar mining.

Numerical analysis of the strain distribution in skin domes formed upon the application of hypobaric pressure.

Suction cups are widely used in applications such as in measurement of mechanical properties of skin in vivo, in drug delivery devices or in acupuncture treatment. Understanding mechanical response of skin under hypobaric pressure is of great importance for users of suction cups. The aim of this work is to predict the hypobaric pressure induced 3D stretching of the skin. Experimental skin tensile tests were carried out for mechanical property characterization. Both linear elasticity and hyperelasticity parameters were determined and implemented in Finite Element modelling. Skin suction tests were performed in both experiments and FEM simulations for model validation. 3D skin stretching is then visualized in detail in FEM simulations. The simulations showed that the skin was compressed consistently along the thickness direction, leading to reduced thickness. At the center of the dome, the radial and angular strain decreases from the top surface to the bottom surface, although always in tension. Hyperelasticity modelling showed superiority over linear elasticity modelling while predicting the strain distribution because the stretch ratio reaches values exceeding the initial linear elastic stage of the stress-strain curve for skin. Hyperelasticity modelling is an effective approach to predict the 3D strain distribution, which paves a way to accurately design safe commercial products that interface with the skin.

Experimental Study on Mechanical Properties of Briquette Coal Samples with Different Moisture Content

Coal seam water injection is an important technical method to prevent and control coal and gas outburst and other disasters. Water can soften coal and change its mechanical properties. In order to study the mechanical properties of coal samples with different moisture content, briquette coal samples with five moistures content (4%, 6%, 8%, 10%, and 12%) were selected to carry out triaxial compression tests under different confining pressures (0.1, 0.2, 0.4, 0.8, and 1.2 MPa). Then, the mechanical response mechanism of the water-bearing briquette coal was analyzed. The results show that the slope of the linear elastic stage of the stress-strain curve gradually decreases with the increase of moisture content. Water-bearing coal exhibits strain strengthening characteristics under high confining pressure, which transforms the water-bearing coal from brittle to ductile state. The peak stress under different moisture content conditions shows a linear relationship with the confining pressure. The internal friction angle decreases linearly with the increase of moisture content. The cohesion varies parabolically with the increase of moisture content and reaches the maximum value when the moisture content is 8%. The coal body with moisture content between 7% and 9% has a high bonding force, which is beneficial to the consolidation of the coal body. Therefore, ensuring a reasonable moisture content of coal through coal seam injection can provide a basis for preventing coal and gas outburst.

Compressive properties and energy absorption behavior of Mg17Al12/Al ordered structure composites

In this study, Mg17Al12/Al ordered structure composites were prepared by infiltrating the ordered porous aluminum with the intermetallic compound Mg17Al12 under gravity condition. Their compressive properties and energy absorption behavior were investigated by quasi-static compression tests. The results showed that Mg17Al12/Al ordered structure composites exhibited distinctly different compressive stress-strain curves with a prominent linear elastic stage and had an advantage over ordered porous aluminum and Mg17Al12/Al composites with irregular structures on mechanical properties. Moreover, Mg17Al12/Al composites with strengthened truss structure showed better compressive properties than Mg17Al12/Al composites with basic truss structure. The ordered porous aluminum that acted as matrix skeleton showed great plasticity and ductility while the intermetallic compound Mg17Al12 as filler exhibited high compressive strength and obvious brittleness. The improvement of mechanical properties of Mg17Al12/Al ordered structure composites was attributed to the ordered composite structure and the distinct mechanical property of co-continuous intermetallic compound Mg17Al12 and pure Al.

Research on Damage and Acoustic Emission Properties of Rock Under Uniaxial Compression

In this paper, the YSSZ-500A rock biaxial rheological tester was used to investigate the acoustic emission characteristics of coal seam roof rock under uniaxial compression, and analyzed the acoustic emission characteristics, fracture mode and energy damage mechanism in the progressive failure process, and study the damage characteristics of rock samples from the perspective of damage mechanics. The results are as follows: (1) The whole stress–strain process of uniaxial compression can be divided into three stages: compaction stage, linear elastic stage, yield stage and failure stage. (2) In the middle of the period, the AE of the sample increases slowly and rapidly in the process of compression. For AE, the logarithm of the corresponding AE cumulative ring count can be divided into rapid increase period, slow increase period and sharp increase period. (3) The damage constitutive model of specimen during uniaxial compression is established, and the relationship between damage variable and axial strain is analyzed. (4) The energy absorbed by the specimen before the peak is mainly in the form of releasable elastic strain energy, and part of the energy is dissipated by the specimen damage propagation, and the energy dissipation is mainly concentrated in the yield section. A large amount of elastic strain energy is released after the peak, and most of the released energy is transformed into the energy dissipated by the propagation of fracture surface through the surface, which leads to the instability of rock samples.

Experimental study on mechanical properties of shale rock and its strength criterion

The mechanical properties of shale rock have a great significance on drilling engineering and the establishment of safety protection facilities in the process of shale gas exploitation. Hence, in this article, the deformation characteristics and the strength criterion of the Longmaxi formation shale rock are examined through uniaxial direct tension tests, uniaxial compression tests, and triaxial compression tests. The results show that shale rock experienced linear elastic stage, yield stage, and failure stage, under the direct tensile state and triaxial compression state. The shale rock experienced initial compaction stage, linear elastic stage, yield stage, and failure stage under uniaxial compression. The fracture surface of shale rock is smooth horizontal section under uniaxial tension. Under uniaxial compression, it is mainly manifested as splitting failure with shear failure in part. Under low confining pressure of triaxial compression, the shale rock show shear-dilatancy failure, and under high confining pressure, the shale rock show local shear failure along the primary fracture. The six strength criteria are used to describe the strength characteristics of shale rock. The results show that the Mohr-Coulomb criterion and Hoek-Brown strength criterion can well describe the shale rock under relative low confining pressure. It is found that both the generalized Hoke-Brown criterion and the power function strength criterion can better describe the strength characteristics of shale rock under different confining pressures than the other four strength criteria. The research results can provide experimental basis and mechanism understanding for the analysis of mechanical properties and engineering application of shale rock.

Experimental and numerical investigations on dynamic mechanical responses and failure process of gas-bearing coal under impact load

With mining depth increasing, the mining structure becomes more and more complex. It is easy to cause dynamic disaster during coal mining. To reveal the failure process of gas-bearing coal at complex mining environment, the dynamics experiments were conducted through Split Hopkinson Pressure Bar of gas-bearing coal (SHPB-GAS) experimental system. The results showed that the filtered stress waveform presented to be sine-like. With the impact load increasing, the amplitude of incident wave, reflected wave and transmission wave also increased. After calculation of stress wave data, the stress-strain curves were obtained, which included linear elastic stage, step stage, yield and rupture stage. The dynamic mechanical strength increased with the increase of impact load, and the specimen developed from stratiform rupture with macro cracks to crush rupture in fragments. To verify the experimental results, COMSOL Multiphysics software was adopted to conduct numerical simulation. The simulation results indicated that the high stress area was formed at the end faces of specimen firstly and expanded to interior of specimen gradually under different impact load. High stress induced the plastic deformation in coal specimen, which also developed from end faces to specimen inside. These results were consistent with the phenomenon of experiments, which explained the causes of failure process of gas-bearing coal. Based on the experimental and simulation achievements, the formed mechanism of dynamic disaster induced by impact load were discussed.

Study on the breaking process of jointed rock mass by disc cutters based on digital image correlation and infrared thermography

Jointed rock masses are frequently encountered during the TBM excavation. To investigate the rock breakage mechanism of jointed rock mass by disc cutter and precursory information before rock failure, a series of indentation tests were performed on shear rheological test system. During the test, breaking process of specimens were recorded by digital image correlation system and thermal infrared imager simultaneously. Therefore, the strain fields and thermal infrared images of jointed rock mass by disc cutters were obtain. The experimental results show that the failure process of jointed rock mass can be divided into four stages: compaction stage, linear elastic stage, residual failure transition stage and post-failure stage. Besides, the characteristic of strain fields shows a good accordance with that of infrared radiation temperature fields at each stage. The failure of rock samples is dominated by shear failure, and the localized temperature rise is an important infrared precursor of rock fracture and instability. The experimental results are of great significance to the deep understanding of the breaking mechanism of jointed rock mass and warnings of engineering disasters.

Failure mechanics and infrared radiation characteristics of soft coal at various moisture contents

Soft coal is characterized by low strength and weak bonds, which play a key role in the occurrence and development of dynamic disasters. A more thorough understanding of the failure mechanics and infrared radiation characteristics of soft coal at various moisture contents is needed. In this study, infrared radiation experiments were conducted for soft coal at various moisture contents. The results indicate that moisture content affects compressive strength and elastic modulus of soft coal with compressive strength and elastic modulus being highest at moderate. Moisture has a substantial influence on the infrared radiation of compressed coal samples. Soft coals with high moisture contents show a smaller fluctuation of the average infrared radiation temperature curve and a stronger direct relationship of temperature and stress in the linear elastic stage. The coal samples with various moisture contents show a warming trend during loading. The increased amplitude of infrared radiation temperature per unit stress shows a linear relationship with moisture content. A large number of high temperature red spots appear along the diagonal line of coal samples with 0% moisture content before fracture, while changes in the infrared thermal image of the coal sample with 4% moisture content were negligible during loading.

Experimental Study on the Relationship between Compression Stability and Deformation Localization of Viscous/Brittle Rock‐Like Materials

Rock‐like materials often exhibit irregular failure deformation under long‐term service conditions, and the deformation and failure of asphalt and concrete materials is a serious problem that leads to subgrade failure. In this study, two different viscous/brittle rock‐like materials were prepared by the in situ loading and optical speckle synchronous monitoring test method, and the evolution characteristics of the deformation field were studied during compression. The formation process of the compression deformation localization of rock‐like materials and their relationship with stability were analyzed. A quantitative description of the compression deformation stage and localization characteristics of the viscous/brittle rock‐like materials is presented. The results can be summarized as follows. At the initial stage of compression, the deformation localization zone of viscous/brittle rock‐like materials begins to expand from the middle area to the surrounding area. Preliminary results of the deformation localization of the linear elastic deformation stage were obtained. The failure cloud image is completely formed at the peak, which is consistent with the failure physical map. The deformation process of compression can be quantitatively described using the deformation localization characteristics of rock‐like materials.

Fracture evolution and failure mechanism of rock-like materials containing cross-flaws under the shearing effect

Shear failure of rock masses along discontinuities is one of the dominant failure modes of underground tunnels and rock slopes. In this paper, the shear fracture evolution and failure mechanism of rock-like materials containing cross-flaws are first reported based on a developed CZM-FEM method. The laboratory uniaxial compression and corresponding numerical tests were initially performed to acquire the mechanical parameters of rock-like materials. Subsequently, the direct shear test of rock-like materials containing two sets of cross-flaws was conducted, the effect of the main flaw and secondary flaw angles were considered. At last, the mechanical properties and fracture behaviors, and the coalescence mechanism were investigated and concluded. The results indicate that four typical stages are observed during the shearing process of rock-like materials with cross-flaws, which are the linear elastic stage, crack strengthening stage, plastic softening stage, and residual strength stage, respectively. Note that the mechanical properties (i.e., peak, residual shear strength, and the crack initiation stress) and cracking behaviors are strongly dependent on the angles of the main and secondary flaws as well as the loading conditions (i.e., shear rates and constant applied normal stresses). In addition, the coalescence mechanism of rock bridge between the two cross-flaws can be classified into three types, which are the mixed shear tensile-tensile damage, shear-tensile damage, and tensile damage, respectively; Similarly, the coalescence paths can also be identified as three types, respectively, the connection between the main flaw and the secondary flaw, linkage by the two main flaws, and the penetration dominated by the coalescence of the main flaw and the secondary flaw.

Dynamics behaviour of gas-bearing coal subjected to SHPB tests

During impact dynamics experiments, the stress waves such as incident wave, reflected wave, and transmission wave were recorded by using a strain data-collecting subsystem. Experimental results showed that the amplitude of reflected wave and transmission wave was lower than that of incident wave, which was closely related to loading conditions such as axial static load, confining pressure, gas pressure, and impact load. After processing of data on aforesaid waves, stress–strain curves of gas-bearing coal in the experiments were obtained, which presented the following stages: linear elastic stage, “step” characteristic stage, and postpeak decrease stage. Furthermore, relationships between dynamic strength, failure strain, and strain rate presented an exponential trend under different axial static loads. However, the dynamic strength and failure strain increased linearly with strain rate under different confining pressures, gas pressures, and impact loads. It can be concluded that the strain rate effects enabled dynamic characteristics to be comparable at different loading conditions. The dynamic constitutive equation of gas-bearing coal under impact load was then built and verified by comparison between theoretical and experimental results. This effectively illustrated the effects of axial static load, confining pressure, gas pressure, and impact load on dynamic mechanical characteristics of coal samples.