Tensile Wave Research Articles

Experimental study on stress wave attenuation and energy dissipation of sandstone under full deformation condition

Excavation behaviours not only destroy the balance of original stress in rock mass but lead to the redistribution of initial stress, resulting in rock mass in a new stress environment. The static stress in rock mass has the characteristics of dynamic change. For the purpose of investigating the effect of increasing static stress on attenuation mechanism and energy dissipation of stress wave in sandstone under the overall deformation condition, small disturbance experiments on sandstone bar are carried out with a modified split-Hopkinson pressure bar test system. The stress waveform, p wave velocity, temporal and spatial attenuation of amplitude, and energy dissipation are studied under various static stresses. Results show that stress waveforms at different locations are approximately the same, those at the same locations vary greatly, and the tensile waves that appeared at the tails of stress waves are larger with increasing static stress. With increasing static stress, p wave velocity experiences a dramatic increase, then develops gently, and finally a sharp decrease; the stress demarcation points are σs/σc = 30% and σs/σc = 55%, respectively. P wave velocity can be predicted by a quadratic equation when σs/σc > 63.69%. The relation between longitudinal wave velocity and static stress can be described by a quadratic function. Stress wave amplitudes decrease exponentially with increasing distance and time. Values of temporal spatial response intensity (RI) and spatial RI are approximately equal and present the same development tendency of nonlinear and linear stages due to the same dimensions. Values of temporal attenuation characteristic (AC) and spatial AC are differed by 2 or 3 orders of magnitude although they present the same development tendencies. Ratios of RI and AC can indicate the sensitivity of temporal and spatial attenuation to different static stresses. Full-wave energy declines in an exponential function with increasing static stress and decays as a logarithmic function with increasing distance. Widespread attenuation characteristics like these should provide a theoretical basis for rock mass stability analysis.

The influence mechanism of the strain rate on the tensile behavior of copper nanowire

The influence mechanism of the strain rate on the tensile behavior of the copper nanowires is investigated with molecular dynamics (MD) simulations. Three failure modes are observed at the loading strain rates from 5×106 to 1×109 s−1. The three modes are named slipping mode, mixed mode and necking mode, respectively. The evolution of atomic configurations show that the competition of the lattice recovery and the dislocation multiplication determines the fracture mode, and the importance of lattice recovery decreases with the increase of strain rate. The input and the reflection of the tensile wave, which is induced by tensile loading, also plays an important role in the failure mechanism. The location of necking tends to approach the two ends of nanowire at higher strain rates.

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.

Cladding of a crack-free W plate on Cu plates using explosive welding at higher collision velocity with lower collision angle

This study aimed to obtain a W/Cu clad with high bending strength using explosive welding (EW), since its thermo-physical property combination is attractive for industrial applications. Cladding both metal plates using EW results cracks in W plates from the surface toward interface. Eliminating the cracks in W plate of clads was a major challenge in this study. The formation of cracks is fundamentally due to generation of high-intensity reflected tensile waves as a consequence of the undissipated imparted kinetic energy (KE) at the collision interface. A crack-free W plate as well as higher bending strength have been obtained in the W/Cu clads produced at the highest collision velocity with lower collision angles among three samples produced at different horizontal collision velocities. The EW clad interface exhibited an intermediate layer (IL), which was associated with higher consumption of KE at the interface, leading to reduction of the intensity of reflected tensile waves. The IL consisted of nanometer- and submicrometer-sized W fragments distributed in a Cu matrix, leading to higher hardness than a Cu plate. This could have contributed to the increase in the bending strength of the clads. (187 words).

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.

Development of a 3D Hybrid Finite-Discrete Element Simulator Based on GPGPU-Parallelized Computation for Modelling Rock Fracturing Under Quasi-Static and Dynamic Loading Conditions

As a state-of-the-art computational method for simulating rock fracturing and fragmentation, the combined finite-discrete element method (FDEM) has become widely accepted since Munjiza (2004) published his comprehensive book of FDEM. This study developed a general-purpose graphic-processing-unit (GPGPU)-parallelized FDEM using the compute unified device architecture C/C ++ based on the authors’ former sequential two-dimensional (2D) and three-dimensional (3D) Y-HFDEM IDE (integrated development environment) code. The theory and algorithm of the GPGPU-parallelized 3D Y-HFDEM IDE code are first introduced by focusing on the implementation of the contact detection algorithm, which is different from that in the sequential code, contact damping and contact friction. 3D modelling of the failure process of limestone under quasi-static loading conditions in uniaxial compressive strength (UCS) tests and Brazilian tensile strength (BTS) tests are then conducted using the GPGPU-parallelized 3D Y-HFDEM IDE code. The 3D FDEM modelling results show that mixed-mode I–II failures are the dominant failure mechanisms along the shear and splitting failure planes in the UCS and BTS models, respectively, with unstructured meshes. Pure mode I splitting failure planes and pure mode II shear failure planes are only possible in the UCS and BTS models, respectively, with structured meshes. Subsequently, 3D modelling of the dynamic fracturing of marble in dynamic Brazilian tests with a split Hopkinson pressure bar (SHPB) apparatus is conducted using the GPGPU-parallelized 3D HFDEM IDE code considering the entire SHPB testing system. The modelled failure process, final fracture pattern and time histories of the dynamic compressive wave, reflective tensile wave and transmitted compressive wave are compared quantitatively and qualitatively with those from experiments, and good agreements are achieved between them. The computing performance analysis shows the GPGPU-parallelized 3D HFDEM IDE code is 284 times faster than its sequential version and can achieve the computational complexity of O(N). The results demonstrate that the GPGPU-parallelized 3D Y-HFDEM IDE code is a valuable and powerful numerical tool for investigating rock fracturing under quasi-static and dynamic loading conditions in rock engineering applications although very fine elements with maximum element size no bigger than the length of the fracture process zone must be used in the area where fracturing process is modelled.

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.

Numerical simulation of underwater explosion wave propagation in water–solid–air/water system using ghost fluid/solid method

The analysis of deformations of structures immersed in water due to explosion waves is important for off-shore oil and marine applications. A challenging issue in these problems is the satisfaction of continuity conditions at the fluid/structure interface. We numerically analyze these transient problems by using the ghost fluid/solid method, and coupling the isobaric fixing technique with the real ghost fluid method. We verify accuracy of the developed algorithm by comparing predictions from it with the analytical solution for a one-dimensional problem in which a plane wave traveling in water impinges upon a water/solid interface. We show that the coupled method has first order accuracy. Numerical results for the propagation of a spherical wave interacting with a fluid/solid interface computed with the developed method are found to compare well with those from the commercial software, ANSYS, that uses the arbitrary Eulerian–Lagrangian method. It is found that the intensity of the reflected tensile wave at a solid/water interface is less than that at a solid/air interface. Factors motivating this work include (i) using only the finite difference method for numerically solving equations governing transient deformations of fluids and solids that employs the ghost fluid/solid method to accurately satisfy continuity conditions at the fluid–solid interface, (ii) avoiding the need to refine the grid near the fluid–solid interface, and (iii) having an in-house capability of modeling detonation of a charge and study the interaction of the shock wave produced with the deformable structure. A novelty of the work is that the pressure due to a shock wave is not modeled as the product of a function of time and a function of space coordinates but its spatial and temporal dependence is found by solving the fluid–structure interaction problem.

Defect characterization, mechanical and thermal property evaluation in CVD-W after low-dose neutron irradiation

Ultra-high purity (> 99.9999 wt%) chemical vapour deposited tungsten (CVDW) samples were neutron irradiated in the BR2 reactor (Belgium) at Tirr < 210 °C to ~ 0.15 dpa, followed by isochronal annealing at 500, 800 and 1100 °C. Defect characterization showed that dislocation loops dominated the as-irradiated damage microstructure and were mostly ≤5 nm. Void formation was observed after post-irradiation annealing at 1100 °C. The mechanical and thermal properties of CVD-W were evaluated based on tensile tests, Vickers hardness and temperature wave analysis. Fractography study suggested that a transition from intergranular fracture to cleavage fracture took place in the material after neutron irradiation. Hardening was found ~ 23% after irradiation. Subsequent annealing below 800 °C saw further increase in hardness, featuring a maximum value of about Hv = 487. Softening occurred at 1100 °C. Thermal diffusivity dropped by ~ 65% after irradiation and ~ 40% of this degradation recovered at 1100 °C.

Toughening Mechanism and Application Analysis of ZrO2 Ceramics under High Speed Strain Condition

In order to study the impact properties of ZrO2 ceramics and analyze the failure characteristics to improve the brittleness of ZrO2 ceramics, we used a high-power glass pulsed laser to test the impact of ZrO2 ceramics. Observing the fracture morphology by scanning electron microscopy, we found that the failure of ceramics is mainly caused by spallation. The cracked surface is dominated by typical brittle fractures, and there are more crystals appearing and cracking. The surface does not expand along a single plane, but forms a large number of steps. The direction of the crack and the density distribution indicate that the failure is caused by the reflected tensile wave. At the same time, the surface layer of the material generates a dense compression layer of about several tens of micrometers due to compressive stress, and no cracking occurs in the compression layer. This mechanism indicates that ZrO2 ceramic materials with surface strengthening and matrix toughening can be formed under suitable laser energy.

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.

Visualization of impact damage in annealed and chemically strengthened aluminosilicate glass

Abstract The damage propagation and fracture process of monolithic annealed and chemically strengthened aluminosilicate (ALS) glass plates are characterized using Edge-on impact (EOI) testing technique. The EOI experiments on two types of ALS glass plates at a medium impact velocity of ∼180 m/s with spherical and cylinders steel projectiles were completed using pneumatic gas-gun. The high-speed images in shadowgraph mode during the impact process were recorded at the framing rate of 1,000,000 frames per second (fps) to visualize the fracture and damage front in annealed and strengthened glass plates. The real-time captured shadowgraphs in case of annealed glass (AG) showed that the damage front initially starts to develop from the impact end and then after traveling some distance arrested in few microseconds and measured average front velocity against cylindrical and spherical projectile was 2658.91 m/s and 1710.0 m/s, respectively. The tensile wave reflection in AG impact case against both types of projectiles was observed. In a chemically strengthened glass (CSG), the damage front shape and propagation against both types of projectiles were quite different than the AG, front traveled in a self-sustained mode due to the release of elastic stored energy with the velocity of 1746.92 m/s and 1690.12 m/s, respectively. The damage evolution curves plotted from captured high-speed images show that damage traveled slower in case of the spherical projectile which suggests that the shape of the projectile has a great influence on the fracture process of annealed and strengthened glass. The damage front shape in CSG against cylindrical projectile impact was more dominant along the glass boundaries and the spallation effect was not observed in CSG against spherical projectile impact. Additionally, the damage-free zones and fracture surfaces were studied using a scan electron microscope (SEM) from collected post-test debris to discuss the fracture process.

Pressure wave propagation effects in the eye after photoablation

A transient pressure wave created in photoablation-based laser corneal refractive surgery propagates along the optical axis of the eye from the cornea toward the fundus. Along its path, it is attenuated, reflected several times at tissue interfaces, focused due to the spherical shape of the cornea, and partially phase-converted to a tensile wave after passing the acoustic focus. Employing the numerical acoustic eye model, the dynamics of the acoustic focusing is described in detail. The simulated pressure waveforms are first validated against the existing measurements. Then, the calculated full pressure fields are used to obtain the extreme values of the pressure amplitude at every point within the eye over the whole timespan of interest. Over these pressure maps, various threshold contours obtained from the existing literature are superimposed to help ophthalmologists investigate the consequences of acoustic focusing within the human eye during laser keratorefractive surgery. The comparison indicates that the simulated negative peak pressure values used in the contemporary broad beam and flying spot laser-assisted in situ keratomileusis/photorefractive keratectomy treatments exceed some of the reported threshold values. The critical volume where the thresholds are surpassed is located near the lens-vitreous boundary and extends to the retina for wide beams. The photoablation-induced pressure wave focusing is, therefore, a mechanism that needs to be researched carefully and included in the list of potential risks accompanying refractive laser surgery.

Shock-Induced Hot Spot Formation and Spalling in 1,3,5-trinitroperhydro-1,3,5-triazine Containing a Cube Void.

The initial reaction mechanism of energetic materials under impact loading and the role of crystal properties in impact initiation and sensitivity are still unclear. In this paper, we report reactive molecular dynamics simulations of shock initiation of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) crystals containing a cube void. Shock-induced void collapse, hot spots formation and growth, as well as spalling are revealed to be dependent on the shock velocity. The void collapse times are 1.5 and 0.7 ps, for the shock velocity of 2 and 4 km·s–1, respectively. Results indicate that the initial hot spot formation consists of two steps: one is the temperature rise caused by local plastic deformation and the other is the temperature increase resulting from the collision of upstream and downstream particles during the void collapse. Whether hot spots will continue to grow or quench depends on sensitive balance between energy release caused by local physical and chemical reactions and various heat dissipation mechanisms. In our simulations, hot spot would grow for Up = 4 km·s–1; hot spot is weak to some extent for Up = 2 km·s–1. The tensile wave reflected by the shock wave after reaching the free surface causes the spalling, which depends on the initial shock velocity. Typical spalling occurs for the shock velocity 2 km·s–1, while the tensile wave induces the microsplit region in RDX crystals in the case of Up = 4 km·s–1. Chemical reactions are studied for Rankine–Hugoniot shock pressures Ps = 14.4, 57.8 GPa. For the weak shock, there is almost no decomposition reaction of the RDX molecules near the spalling region. On the contrary, there are large number of small molecule products, such as H2O, CO2, NO2, and so forth, around the microsplit regions for the strong shock. The ruptures of N–NO2 bond are the main initial reaction mechanisms for the shocked RDX crystal and are not affected by shock strength, while the microsplit slows down the decomposition rate of RDX. The work in this paper can shed light on a thorough understanding of thermal ignition, hot spot growth, and other physical and chemical phenomena of energetic materials containing voids under impact loading.

ПЕРСПЕКТИВЫ РАЗВИТИЯ ТЕХНОЛОГИИ И ТЕХНИКИ ГОРИЗОНТАЛЬНО-НАПРАВЛЕННОГО БУРЕНИЯ ПИЛОТНЫХ СКВАЖИН ДЛЯ БЕСТРАНШЕЙНОЙ ПРОКЛАДКИ ТРУБОПРОВОДОВ

Актуальность. В процессе реализации технологии горизонтально-направленного бурения скважин нередко возникают проблемы, связанные с необходимостью преодоления включений или горных пород высокой твердости, что требует использования рационального способа разрушения их ударом. Однако существующие ударные механизмы современных буровых установок формируют силовые импульсы нерациональной формы, не учитывающей свойства горной породы к сопротивлению для её разрушения. Такая форма силовых импульсов и кратковременность их воздействия на горную породу снижает глубину внедрения в нее коронки, формируя волну растяжения, разрушающую буровой инструмент, прежде всего соединения бурильных труб. В связи с этим возникла необходимость в научном обосновании и разработке принципиально нового формирователя силовых импульсов, соответствующих процессу сопротивления горной породы внедрению в нее породоразрушающего инструмента, а также резьбовых соединений бурильных труб, учитывающих волновой характер распространения силовых импульсов по бурильной колонне. Цель: научное обоснование и разработка принципиально нового формирователя силовых импульсов в бурильной колонне, учитывающего характер изменения сопротивления внедрению породоразрушающего инструмента в грунт, и новой конструкции резьбовых соединений бурильных труб, учитывающей волновой характер распространения через них энергии силовых импульсов на забой применительно к бурению пилотной скважины при бестраншейной прокладке трубопроводов. Объекты: технология и техника процесса горизонтально-направленного бурения пилотных скважин с использованием энергии силовых импульсов, распространяющихся по бурильной колонне. Методы: обобщение и анализ научно-технической информации; методы теоретической и прикладной механики; тензометрирование процессов передачи упругих волн деформаций на стенде, имитирующем искусственные скважины; математическое моделирование работы системы формирования силовых импульсов и процесса распространения их через резьбовые соединения бурильной колонны. Результаты. Приведен обзор способов бестраншейной прокладки трубопроводов. Определены перспективы совершенствования технологии и техники горизонтально-направленного бурения пилотных скважин как начального этапа технологии бестраншейной прокладки трубопроводов. Предложены технические решения по совершенствованию технологии и техники горизонтально-направленного бурения пилотных скважин, обеспечивающих существенное увеличение механической скорости проходки скважины.

Numerical Simulation Study of Target Response under Underwater Explosion

Studying the structural dynamic response characteristics under the action of underwater explosion load had great significance for improving the anti-explosion and anti-impact performance of ships. The autodyne numerical simulation software was used to calculate the dynamic response of target under the underwater explosion. A series of movable observation points are set on the target and the y-axis coordinate-time image of each observation point is extracted after the target plate is completely broken at the boundary. We find that there is a tensile wave propagating in the y-axis direction inside the target plate before the target plate is in a severely bent state. The intensity variation of the tensile wave along the thickness direction of the target plate is studied. The tensile wave has less influence on the back-explosion surface and have an obvious effect on the destruction of the detonation surface. The effect of the tensile wave on the boundary was studied. The tensile wave contributes to the fracture of the target at a fixed boundary.

A unified phase field damage model for modeling the brittle-ductile dynamic failure mode transition in metals

Dynamic brittle fracture and shear banding are typical failure modes in metals under high strain rate loading. In this paper, a unified phase field damage model is developed to capture the above two failure modes’ transition naturally by allowing the critical energy release rate to vary with the stress triaxiality to distinguish the material failure properties of tensile fracture and shear banding. Besides this, compared with the existing elastic-plastic phase field fracture models, the plastic dissipation energy before the damage evolution is excluded in the failure energy to model ductile failure more physically, and the degradation function of the yield stress is introduced which not only provides a damage softening mechanism for the ductile failure but also ensures a proper simulation of the brittle fracture. The unified phase field damage model is applied to study the brittle-ductile failure mode transition in Kalthoff test. The results show that stress wave propagation plays an important role in the brittle-ductile failure mode transition. The shear banding requires more energy for initiation and will be activated straightly as the plastic wave passes the notch tip for high impact velocity. While for low impact velocity, the energy will be released by selecting the brittle fracture mode which requires less energy when the reflected tensile wave superimposes at the notch tip. For our simulation, the damage softening contributes considerably more to the shear banding than the thermal softening. This model can be used to simulate the complex dynamic failure of metal under impact loading.

An Innovative Approach of Using Continuous Impedance-Graded Metallic Composite System for Attenuation of Stress Waves

This paper presents an innovative approach of stress attenuation through a continuous impedance-graded material system for high strain-rate events. High energetic dynamic events such as blasts and impact could cause stress waves—in the form of elastic, plastic, and shock—to propagate in a solid material. An impedance-graded composite is created by arranging different metallic alloys in the reducing order of their impedance through the system. Impedance, which is the product of volumetric mass density and wave velocity, is chosen as the function as it plays a governing role in elastic, plastic, and shock waves. An analytical framework to quantify the stress wave propagation through an impedance-graded multimaterial system is developed based on the principles of shock and elastic wave theories. The numerical simulations carried out using nonlinear finite element code, LS-DYNA, were able to capture and quantify the elastic, plastic, and shock waves and their reflections at different interfaces. It was identified that the final transmitted stress wave, which could comprise elastic, plastic, and shock waves, as well as the reflected tensile elastic wave at each material interface, needs to be controlled in order to develop a robust multimaterial system.

The microstructure and ballistic performance of B4C/AA2024 functionally graded composites with wide range B4C volume fraction

In this research, the researchers applied the squeeze casting technology to integrally produced the B4C/AA2024 functionally graded materials (FGMs). The volume fraction of B4C were 70 vol%, 47 vol% and 25 vol% in the top, middle and bottom layer respectively. The ballistic tests of different samples with areal density of 80 kg/m2 and 38 kg/m2 were carried out using 7.62 mm armor-piercing incendiary (API). For comparison, homogeneous 70 vol% and 47 vol% B4C/AA2024 targets were tested under the same conditions. The results demonstrate that the three layers 70-47-25 vol% B4C/AA2024 FGMs has a better ability to maintain integrity than homogeneous 70 vol% B4C/AA2024 target and the areal density of penetration is reduced by 14% compared to homogeneous 47% B4C/AA2024 composites. The microstructure of targets and projectiles were investigated. Large amounts of fragmentation of B4C particles and plastic deformation of composites are observed near the bullet hole at the top and bottom layer respectively. Crack deflection appears in the interface of the top and middle layer. By stress wave analysis, it is found that the existence of gradient structure reduces the magnitude of reflected tensile wave by 12%, which improves the ability of the target to resist fracture. The results indicate the FGMs with high ceramic content at the top layer were of good application prospects in lightweight armor.

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.