Tensile Stress Wave Research Articles

Experimental and numerical study of failure behavior and mechanism of coal under dynamic compressive loads

A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space. In this study, the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated. The experiments were conducted using a split Hopkinson pressure bar (SHPB) system. The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates. The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture. In addition, the typical failure modes of coal under dynamic load were numerically reproduced. The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar. Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.

Dynamic response and failure of rock in initial gradient stress field under stress wave loading

Once an opening is created in deep underground, the stresses surrounding the opening will be redistributed, inducing a gradient stress field. To understand how the ground rock in such a gradient stress field responses to dynamic stress loading, the gradient stress distribution at a circular opening was first analyzed and the propagation of 1D stress wave in rock mass under gradient stress field was theoretically derived. By using an implicit to explicit solution method in LS-DNA code, the dynamic mechanical behaviors of rock in gradient stress field were numerically investigated. The results indicate that the damage is mainly produced at or near the free face, partly due to the straight action of compressive stress wave and the tensile stress wave generated at the free face. The range of the induced damage zone is narrowed under the conditions of higher gradient stress rate and lower dynamic stress amplitude. However, under lower gradient stress field and higher dynamic stress, the damage becomes severer and wider with discontinuous failure regions.

Molecular dynamics simulations on shock response and spalling behaviors of semi-coherent {111} Cu-Al multilayers

Shock response and spalling behaviors of semi-coherent {111} Cu-Al multilayer are investigated by molecular dynamics simulations. Firstly, the influences of interfaces on shock wave propagation are studied. The simulations show that reflection-unloading happens when the shock wave propagates from Cu to Al and reflection-loading occurs when the shock wave propagates from Al to Cu. Strong discontinuities of stress components at the interfaces are revealed as well, the discontinuities are correlated to mismatches of Hugoniot curves and elastic-plastic properties of the two materials adjacent to the interfaces. Secondly, shock induced dislocation activities are studied. It is found that the stress barrier offered by the interface Cu-Al is smaller than that offered by Al-Cu. In fact, the resistance for dislocations to transmit through interfaces is determined by the coherency stresses, Koehler stresses and misfit dislocations at the interface. In addition, spalling fracture of Cu-Al multilayers after stress wave reflection at the free surface is studied. It is observed that ductile damage only nucleates in Al layer, rather than in Cu layers. This observation agrees well with the experimental results of Han (Acta Mater., 2014 (63) 150–161). Due to complex reflection and transmission of the stress waves at the interfaces, the profile of the free surface velocity histories of the multilayer target is quite different from that of the single-crystalline counterpart. In order to study thermal dissipation process during cavitation, analysis on thermal dissipation in the spalling damage zones is also provided in this paper. Moreover, contour plots and profiles of stress are illustrated to understand the spallation process of the multilayer target deeply. Tensile stress emerges in Al1 layer first, causing voids to nucleate; then the tensile stress wave attenuates rapidly and propagates to Cu1 and Cu2 layers. With the dissipation effect of damage and plastic deformation in the target, the tensile stress wave dissipates ultimately.

Measuring method of dynamic fracture toughness of mode I crack under blasting using a rectangle specimen with a crack and edge notches

In order to study fracture toughness of mode I crack under blast load, a new specimen of a rectangle plate with a crack and edge notches (RPCEN) was proposed in this study. The PMMA was selected to prepare the RPCEN specimens. The testing system consists of a dynamic strain amplifier, an oscilloscope, a constant source, strain gauges and crack propagation gauges. The strain gauges and crack propagation gauges (CPGs) were used to measure the blast load and crack propagation speed, respectively. The scanning electron microscope was applied in studying the property of fracture surface. The explicit dynamic software AUTODYN was applied in simulating crack dynamic propagation behavior. The JWL equation of state (EOS) was applied to the explosive detonation products, and a linear EOS was used to describe the relationship between pressure and density of PMMA. A modified principle stress failure criterion was employed to assess the material state. The experimental-numerical method was used to determine the dynamic initiation and propagation toughness of the PMMA. The dynamic stress intensity factors (DSIFs) were calculated by the finite element code ABAQUS. The experimental and numerical results show that: (1) The RPCEN specimen proposed in this paper has the function to minimize the reflected tensile stress waves; (2) During crack propagation the fracture surface roughness and the nucleation rate increase with crack propagation speed; (3) Crack propagation speed is not a constant, and propagation toughness is related to the crack propagation speed.

Fracture Evaluation and Dynamic Stress Concentration of Granite Specimens Containing Elliptic Cavity under Dynamic Loading

The Split-Hopkinson pressure bar (SHPB) was used to determine the fracture characteristics of a long bar rock specimen with an elliptical cavity under different axial ratios and dip angles. A high speed camera was applied to record the fracturing process of the granite specimen around the cavity. The experimental results showed that the fracture characteristics around the elliptical cavity were closely related to the axial ratio and dip angle. A three-dimensional numerical model was established using LS-DYNA to quantitatively analyze the dynamic stress state around the cavity. The numerical results indicate that the dip angle and axial ratio of the elliptical cavity significantly affected the dynamic stress concentration factor (DSCF), then affected the rock failure. The location of higher DSCF led to a higher possibility of spalling failure. The maximum DSCF remarkably decreased with a decreasing dip angle and increased the axial ratio. In the dynamic loading propagate process, the stress concentration distribution around the cavity formed by a compression stress wave had a certain damaging effect on the destruction of rock around the cavity, and the stress concentration generated by the tensile stress wave was the main factor of the rock fracture, which was most notable in the peak area of the stress concentration.

Experimental investigation of the dynamic response of prestressed rockbolt by using an SHPB-based rockbolt test system

A new rockbolt test system was developed to investigate the dynamic response of prestressed rockbolts. The test system involved the modification of a split Hopkinson pressure bar, such that the static prestress and dynamic tensile stress wave could be simultaneously applied on the rockbolt specimen. Strain gauges were mounted to the threaded bolt to trace the stress wave propagation in the rockbolt. Prestressed rockbolt specimens with various prestress levels were subjected to repeated impact loading. The effects of the prestress and number of impacts on the dynamic response of the rockbolt were investigated. The results indicated that with increase in the prestress, the resistance of the prestressed rockbolt to the dynamic impact is enhanced, and the slippage of the bolt decreases. Under repeated impacts, the slippage of the rockbolt increases exponentially with increase in the number of times that impact occurs. Finally, the failure mechanism of the prestressed rockbolt subjected to the dynamic impact was clarified.

Analyzing dynamic disturbance fragmentation mechanism of surrounding rock in roadway roof

In this paper, a numerical simulation is conducted to analyze an engineering practice pertaining to the in panel 2347, the discussion is focused on the following aspects: the influence of fault mining activation on fragmentation of surrounding rock in roof, surrounding rock failure of unsupported roadway under static load alone, surrounding rock failure of unsupported roadway under dynamic load and static load combination, damage of surrounding rock in supporting roadway under dynamic load and static load combination. The results show that, for the roadway surrounding rock with obvious dynamic disturbance, the compressive stress value can eliminate the influence of the tensile stress wave generated by the reflection of the compressive stress disturbance wave on the roadway wall. In the roadway support, it should provide a certain compressive stress to the surrounding rock of the surrounding roadway wall. An anchor support surrounding rock can significantly inhibit the tensile crack, shear the crack expansion and dislocation slip of coal gang, and it can also alleviate the tensile, compressive and shear failure of the roof carbonaceous mudstone. Because the dynamic load has a significant damage to the carbonaceous mudstone between roof anchors, it is necessary to reduce the anchor spacing and row spacing or enhance the stiffness and active stress of the protective surface member.

Solid-to-super-critical phase change and resulting stress wave during internal laser ablation

The phase change, stress wave generation and propagation, and stress-induced structure damage in a model material (argon) during internal picosecond laser ablation are studied by molecular dynamic simulations. The propagation of the stress wave and fluctuations in temperature is periodic and tightly correlated. The phase-change process from solid to liquid and supercritical fluid and then back to solid occurs as a coupled consequence of heat transfer and the tensile stress wave induced by the laser pulse and the limited internal space. The orientation–radial distribution function is applied to provide substantial information about the atomic density and angle distribution of the crystal structure. It reveals the strong material twisting and permanent damaged region induced by the very high stress fields.

Dynamic crack propagation of granite subjected to biaxial confining pressure and blast loading

Abstract To great understanding the dynamic crack propagation of rock under in-situ stress and blasting, the static-dynamic loading experimental platform was built. With different confining pressure (0MPa, 2MPa, 4MPa, 6MPa, 8MPa) and different ratios (K) of horizontal-to-vertical pressure (0, 0.25, 0.5, 0.75, 1), nine texts have been carried out in quadrate granite samples. The dynamic strain gauges and high-speed (HS) camera were used to measure strain field and observe crack growth. According to experimental results, the centrosymmetric damage zone is generated around borehole when K=1, the circumference compressive stress is formed by confining pressure, and it reduces the circumference tensile failure by blast loading. The number and size of the broken radius significantly reduce with confining pressure increases. As K decrease from 1 to 0, the tensile stress concentration is obviously around borehole in the vertical direction, and radial cracks grow from the stress concentration zone and develop into boundary under tensile stress wave from blast loading. From the numerical analysis based on Johnson-Holmquist in AUTODYN, crack branching appear near free boundary obviously, meanwhile the radial cracks grow and run through sample by reflected wave at last. Compared to the experimental results, the evolution and final shape of cracks are agree with the experimental results, the central damage area is less than latter because JWL equation neglect the gas loading in numerical calculation.

Experimental Study on Dynamic Fracture Toughness of Compact Tension Specimens

In this paper, the dynamic fracture toughness test of 2A12T4 aluminum alloy compact tension (CT) specimens is carried out by tensile stress waves, based on a Hopkinson experimental device. The crack propagation of the CT specimen is recorded by a high-speed camera. The dynamic fracture toughness of CT specimens with two different crack forms was studied by using the digital image correlation method and the strain gauge method. The study analyzed the CT specimen dynamic fracture toughness of different crack types, different thicknesses and prefabricated lengths were discussed. Research results indicate that, under the same loading conditions, the dynamic fracture toughness of prefabricated fatigue crack specimens is less than of the wire cutting gap specimens. With an increase in the length of fabricated fatigue crack, the dynamic fracture toughness decreases. The dynamic fracture toughness of 15 mm thick CT specimens is greater than that of 20 mm thick specimens.

Rockburst characteristics and numerical simulation based on a strain energy density index: A case study of a roadway in Linglong gold mine, China

With an increase in mining depth, rockburst is becoming an important problem in roadways of Linglong gold mine, where the overburden is up to 1000m. Field observations indicate that rock failures are often triggered by blasting at the adjacent workface. In order to understand the rockburst mechanism, numerical investigation associated with an energy index, strain energy density (SED), is conducted to simulate the energy accumulation and dissipation characteristics of the failure process. The results show that rock failure follows different mechanisms under dynamic loading as a result of unloading and loading zones generated around the cavity. Numerical modelling demonstrates that strain burst occurs at the corner and the floor with the release of massive energy, while dynamic spalling occurs at the lower sidewall due to the reflected tensile stress wave at the free surface. The failure types and damage extend from simulated results agree well with field observations. The findings presented in this study will contribute to effective support design strategies in deep underground openings.

Study on the anisotropic response of condensed-phase RDX under repeated stress wave loading via ReaxFF molecular dynamics simulation.

Anisotropic mechanical response and chemical reaction process of cyclotrimethylene trinitramine (RDX) along crystal orientations were studied with molecular dynamics simulations using ReaxFF potential under repeated stress wave loading. In the simulations, shocks were propagated along the [010], [001], [210], [100], [111], and [102] orientations of crystal RDX at initial particle velocity Up in the range of 1∼4km/s. For shocks at Up ≤ 2km/s, local stacking fault and molecular conformational change can only cause marginal temperature and pressure increase without molecular decomposition. As shocks increase to Up ≥ 2.5km/s, rupture of N-NO2 bond accompanied by partial HONO elimination dominates the main chemical reactions at the initial stage. The ordering of the follow-up consumption of NO2 and ring-breaking rate is directly consistent with that of increasing rate in temperature and pressure. The (210) and (100) planes are more sensitive to shocks in temperature and pressure profiles than the (111) plane, which agrees well with experimental observations and theoretical results in the literature. Therefore, the repeated dynamic loading model in conjunction with MD simulation using ReaxFF potential for crystal RDX indicates that these methods can be applied to study the mechanical response and chemical reaction process of polymer bonded explosives that are commonly subjected to compressive and tensile stress waves observed in practice.

The dynamical response characteristics of elastic–plastic coal under dynamic load

At present, it has been confirmed that tensile stress can be generated in coal bodies through instantaneous unloading, which leads to severe damage in certain areas of the coal. However, the process for the generation of this tensile stress is still unclear. In this study, by taking the elastic–plastic coal in the front of the tunneling face and the rock cross-cut coal uncovering region as examples, and by applying the rules of reflection and transmission to the stress waves, the concrete propagation process of stress waves in coal under dynamic loading was examined. At the same time, the formation and acting mechanisms of the tensile stress waves under the interaction of the loading and unloading stress waves were studied. The results indicated that compression-shear, tension, and tension-shear failures in the elastic–plastic coal were evident, under the role of the instantaneous loading and unloading. Due to the fact that the wave impedance of the coal tended to distribute unevenly in the front of the tunneling face, the loading and unloading stress waves were thereby transmitted and reflected in the propagation process of the coal body, and formed unloading stress waves. The tensile stress generated in the coal under the unloading waves, which were prone to severe damage to coal body, led to dramatic increases of the gas desorption rate, as well as a significant rising of the gas pressure. These results led to the occurrences of the coal and gas outburst accidents. Therefore, the development process of the coal and gas outbursts was determined to have begun on the inside of the coal, then to gradually develop towards the tunneling face. Multi-beam unloading waves were formed by the instantaneous unloading of the blast-hole wall, as well as the pressure released around the coal. Radial tensile stress waves in the coal were prominent, under the role of the unloading waves, which led to tension failure in the coal, which in turn generated circumferential cracks. This caused the spallation phenomena of the coal around the outburst-hole wall during the process of the coal and gas outbursts. The conclusions of this study have important theoretical guiding significance for the prevention of coal and rock dynamic disasters.

Quantitative evaluation of adhesion quality of surface coating by using pulse laser-induced ultrasonic waves

Abstract This study aims to evaluate delamination characteristics of surface coating by using strong ultrasonic wave induced by pulsed laser irradiation. Strong tensile stress wave is induced by pulse laser irradiation, and delamination of the coating layer/substrate interface (or spallation) is produced by confined silicone grease breakdown (i.e. ablation). Upon various levels of energy of irradiation laser, the coating delamination and displacement are measured in-situ. Parallel computation of elastic wave propagation using FDTD (finite difference time domain) yields the wave propagation in the specimen, and estimates the interfacial strength with stress wave distribution. The delamination area is visualized by laser ultrasonic wave scanning technique after the laser spallation test. The interaction between displacement waveform and delamination is explored. The present technique may shed some light on various coating depositions for assessing the coating adhesion quality.

Longitudinal impact of piezoelectric media

We consider the elastodynamic impact problem involving a one-dimensional finite-thickness piezoelectric flyer traveling at initial velocity $$V_0$$ that collides with (and adheres to) a stationary piezoelectric target of finite thickness backed by a semi-infinite non-piezoelectric elastic half-space. We derive expressions for the stress, velocity, and electric displacement in the target at all times after impact. A combined d’Alembert and Laplace transform method is used to derive new numerically based solutions for this class of transient wave propagation problems. A modified Dubner–Abate–Crump (DAC) algorithm is used to invert the analytical Laplace transform domain solutions to the time domain. Unlike many authors who neglect electromechanical coupling in the initially unstressed region ahead of the shock, we consider this effect, which gives rise to the development of a tensile stress wave within the piezoelectric target ahead of the shock. To solve the problem, we derive a new piezoelectric impact boundary condition and apply it to the problem of a finite quartz (Si $$\text {O}_2$$ ) flyer impacting a lead zirconate titanate (PZT-4) target and find that the solutions obtained using the modified DAC algorithm compare well with those obtained using both a finite-difference time-domain method, and the commercial finite element code, COMSOL multiphysics.

Numerical simulation of stress wave propagating through filled joints by particle model

This paper presents a numerical simulation of stress wave propagation through filled joints using the particle flow code (PFC2D 3.10). A thin layer of granular material without tensile strength is used to model the natural filled joint. The Kelvin viscous–elastic contact model is developed with C++ code and embedded as the dynamic link library in PFC2D 3.10 to simulate the particle deformation behavior of the filled joint. It has been proved that the PFC2D is competent in simulating the stress wave propagating through a filled joint according to the comparison between the forward fitting data and the experiment results. The influence of amplitude and frequency on the transmission coefficient is analyzed, and the results show that the transmission coefficient is amplitude and frequency dependent. Additionally, the effect of the tensile stress wave, loading history and filled thickness on the transmission waves is also evaluated. It has been found that the transmission coefficient decreases with the increase of the filled thickness. The tensile stress wave cannot propagate through the filled joint but can tear apart the filled layer, which weakens the multiple reflections in the filled layer. When the incident wave is composed of multiple pulses, the loading history has an important effect on the transmitted waves. For multiple parallel filled joints, the variation trend of the transmission coefficient versus the dimensionless joint spacing is similar to the analytical result obtained by Zhu et al. (2011) except that the loading mode and amplitudes have an important effect on the magnitude of the transmission coefficient. Finally, the deformation process of the filled layer under different loading modes was examined microscopically.

Mesoscale Modeling of Spallation Failure in Fiber-Reinforced Concrete Slab due to Impact Loading

AbstractIn this paper, a model for damage and fracture, considering the heterogeneity of material properties, is validated and used to investigate the mechanism of spallation in fiber-reinforced concrete (FRC) slabs with various fiber and aggregate contents under impact loading. Numerical simulations show that there is a marked difference in the failure patterns among slabs with various fiber and aggregate contents, and the fibers can remarkably improve the tensile strength of the concrete slab, effectively prevent the initiation and propagation of cracks, and inhibit the occurrence of spallation. Moreover, numerical simulations capture the whole process of the propagation of incident compressive stress waves in the FRC and the reflection of stress waves upon concrete surfaces and the spallation failure of FRC induced by the reflected tensile stress wave, which is obviously different from the failure pattern of FRC under static loads. The results of this study can also provide a valuable reference for stu...

Plastic deformation and microstructural evolution during the shock consolidation of ultrafine copper powders

Shock consolidation of ultrafine copper powders at room temperature for bulk nano/ultrafine structured materials is achieved in a gas gun system. The stress states in the powders during the shock consolidation process are analyzed using the finite element method associated with the dynamic densification model (P-α model). The mechanical properties of the shock-consolidated copper are evaluated in terms of hardness, static tensile and compressive strengths, and dynamic compressive strength. The microstructures are characterized using an optical microscope, a scanning electron microscope, and X-ray diffraction (XRD). The XRD patterns are quantitatively analyzed in order to estimate the crystallite sizes and dislocation densities using the Convolution Multiple Whole Profile method. The shock-consolidated specimens were highly densified over 98% of relative density with uniform spatial distributions of high hardness. However, insufficient consolidation due to the tensile stress wave induced by the interactions between shock waves in the powders and due to the ultrafine particles requiring high pressure for good bonding has resulted in several defects in the consolidated specimens. These defects cause tension–compression asymmetry in the shock-consolidated materials. Compared with the tensile results, where fractures occurred at low stresses without plastic deformation due to weak interparticle bonding, the high compressive yield stresses of 600 and 900MPa with large plastic strains are achieved in the static and dynamic compression results, respectively. These high compressive flow stresses are attributed to the extremely high dislocation density and the refinement of the crystallite size via the shock deformations. A microstructure model is proposed for the extremely high dislocation density, where dislocations are generated not only by shock waves but also by plastic flow during the void collapses. The strengths of the shock-consolidated specimen are slightly decreased during the post-shock deformations due to decreases in the excess dislocations despite further refinement of the crystallite size.

Experimental Investigation on the Basic Law of the Fracture Spatial Morphology for Water Pressure Blasting in a Drillhole Under True Triaxial Stress

The present literature on the morphology of water pressure blasting fractures in drillholes is not sufficient and does not take triaxial confining stress into account. Because the spatial morphology of water pressure blasting fractures in drillholes is not clear, the operations lack an exact basis. Using a large true triaxial water pressure blasting experimental system and an acoustic emission 3-D positioning system, water pressure blasting experiments on cement mortar test blocks (300 mm × 300 mm × 300 mm) were conducted to study the associated basic law of the fracture spatial morphology. The experimental results show that water pressure blasting does not always generate bubble pulsation. After water pressure blasting under true triaxial stress, a crushed compressive zone and a blasting fracture zone are formed from the inside, with the blasting section of the naked drillhole as the center, to the outside. The shape of the outer edges of the two zones is ellipsoidal. The range of the blasting fracture is large in the radial direction of the drillhole, where the surrounding pressure is large, i.e., the range of the blasting fracture in the drillhole radial cross-section is approximately ellipsoidal. The rock near the drillhole wall is affected by a tensile stress wave caused by the test block boundary reflection, resulting in more flake fractures appearing in the fracturing crack surface in the drillhole axial direction and parallel to the boundary surface. The flake fracture is thin, presenting a small-range flake fracture. The spatial morphology of the water pressure blasting fracture in the drillhole along the axial direction is similar to a wide-mouth Chinese bottle: the crack extent is large near the drillhole orifice, gradually narrows inward along the drillhole axial direction, and then increases into an approximate ellipsoid in the internal naked blasting section. Based on the causes of the crack generation, the blasting cracks are divided into three zones: the blasting shock zone, the axial extension zone, and the orifice influence zone. The explosion shock zone is the range that is directly impacted by the explosive shock waves. The axial extension zone is the axial crack area with uniform width, which is formed when the blasting fracture in the edge of the explosion shock zone extends along the drillhole wall. The extension of the orifice influence zone is very large because the explosion stress waves reflect at the free face and generate tensile stress waves. In the water pressure blasting of the drillhole, the sealing section should be lengthened to allow the drillhole blasting cracks to extend sufficiently under the long-time effect of the blasting stress field of quasi-hydrostatic pressure.

補強量の違いが耐爆補強効果に及ぼす影響

When designing blast-resistant reinforced concrete (RC) structures, it is important to reduce spall damage due to reflected tensile stress waves. In this study, to devise a blast-resistant strengthening method for existing RC slabs, contact detonation tests using 200 g of explosive charges were conducted on RC slabs with rear surfaces strengthened with continuous fiber sheets/meshes. High-strength carbon fiber (HSCF) and high-modulus carbon fiber (HMCF) sheets were employed to strengthen 75-mm-thick RC slabs, and the number of layers used for strengthening them was varied between 2, 4, and 6 layers. Two types of carbon fiber reinforced plastic (CFRP) meshes, in which the weight per unit area of carbon fibers was varied between 215 and 430 g/m2, were also used to strengthen 100-mm-thick RC slabs. The results showed that the spall failure of the 75-mm-thick RC slabs could be prevented by bonding more than four HSCF sheets onto the rear side of the slab; however, the HMCF sheets had no effect on the blast resistance of the slabs because of fiber rupture. Further, the spall failure of the 100-mm-thick RC slabs could be prevented by bonding the CFRP meshes with fibers 430 g/m2 in volume. On the basis of the test results, the criteria for fiber rupture in CF sheets due to detonation loading was discussed in terms of the fracture energy of the sheets and the modified-scaled concrete thickness of the slab. Furthermore, high-speed photography was employed to investigate the spall failure process of each strengthened slab.