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Experimental diagnostic of melting fragments under explosive loading
Abstract
We have conducted experiments to study the melting fragments from explosively shocked melting Pb targets. Based on the traditional Asay-Window technique, Asay-F-window was designed, which is suitable to investigate high-density melting fragments of metal sample. The areal mass and volume density of melting fragments from the Pb target were presented, which is also compared with that of micro-jetting and solid spallation. The results may contribute to the understanding of physical mechanism and construction of dynamic model for melting fragments. Additionally, the Asay-F-Window technique is an effective supply for proton photography technique to study the dynamic fragmentation of melting metal.
... Kury [7] proposed cylinder tests that served as the general references for studies on the shells driven by explosive detonation. Recent studies mainly focused on the fracture mode and crushing behavior of materials [8][9][10][11][12][13][14][15]. Based on the development of experimental methods, they gained in-depth insights from the analysis of macroscopic tests into the micro-mechanisms, analyzed the detonation driving processes and metal cracks by high-speed photography and micro-mechanism analysis by scanning electron microscopy (SEM), and investigated many fracture modes of metals driven by detonation. ...
... . 这 里需要指出的是, 金属微层裂现象与受冲击后金属 表面向外形成的物质喷射、微喷射现象 [8,15,16] 是 不同的, 金属微层裂可理解为熔化金属受拉伸后由 外表面向内的层裂现象. ...
After high pressure shock, the shock wave in the metal is unloaded at the metal-gas interface, and micro spallation occurs when the metal melts. When the micro spallation develops to a certain extent, the high pressure gas penetrates the zero pressure vacuum gap between the metal melt droplets. In this paper, the phenomenon of gas penetrating metal micro spallation zone is analyzed theoretically. Based on the regular hexahedron periodic arrangement of metal droplets, the calculation formulas of the maximum penetration depth, the sealing time of the penetration channel and the maximum mass of the gas penetrating the metal micro spallation zone are given through theoretical analysis under the quasi-static and semi-dynamic conditions. The quasi-static process is considered to be the gas penetration process that can be approximated as the escape process of gas into the vacuum, and the gap in the metal micro spallation zone will be filled with gas. The semi-dynamic analysis is based on two basic assumptions: one is the equal droplet size and spacing in the micro spallation zone and the other is the critical sealing condition of gas penetration. In the process of semi-dynamic analysis it is demonstrated that the initial critical sealing distance is independent of the shape factor of the droplet single control volume. The semi-dynamic analysis can give various critical sealing information when the gas stops penetrating the metal micro spallation zone. The results of quasi-static analysis can be used as the upper limit of gas penetration, and the semi-dynamic analysis results can be used as the lower limit of gas penetration. From the sensitivity analysis, it can be seen that the change law of physical phenomena given by theoretical analysis accords with the basic physical understanding of the problem. Through this study, the upper and lower limit of the mixed state of gas penetrating the metal micro spallation zone can be estimated, which can provide more accurate initial metal-gas mixed state for subsequent research of the evolution of mixed state. The theoretical analyses given in this paper are based on a lot of uncertain assumptions, and the in-depth study of this phenomenon is still needed based on the law summary and mutual confirmation of experiment and simulation.
A strong shock-wave, produced by plate impact, explosive detonation or laser irradiation, can induce metal materials to melt. Reflection of the triangular pressure wave from the free surface generates a strong tensile stress in the liquid state, resulting in the creation of an expanding cloud of liquid debris. This phenomenon is called micro-spalling. The understanding of spall damage evolution and dynamic fragmentation of melted metal under shockwave loading and subsequent releasing is an issue of considerable importance for both basic and applied science, to predict the evolution of engineering structures subjected to explosive detonation in implosive dynamics or inertial confinement fusion (ICF), the latter involving high energy laser irradiation of thin metallic shells. For dynamic failure processes, spall fracture in solid material has been extensively studied for many years, while scarce data can be found about how such a phenomenon can evolve after being melted partially or fully when being compressed or released. In this paper, by studying the physical laws of void evolution in melted metals, we expect to reveal the mode and criterion of void coalescence, inertial and temperature effects on void distribution and evolution, and the relationship between fragment distribution and characteristics of breakup of damaged material. According to these physical laws, we can develop theoretical model to describe the damage evolution and fragment distribution of metal that melts when shock releases. This model is implemented as a failure criterion in a one-dimensional hydrocode. The experimental results and computational results are in fairly good agreement with each other. Some discrepancies are explained by using both experimental uncertainties and model limitations which are carefully pointed out and discussed. We believe that these results can deepen our physical understanding of the damage evolutions of metals and improve the credibility of numerical simulation on the damage and fragmentation of materials under implosive loading.
In this paper, a stability analysis is given to study the unstable mechanism of the Richtmyer-Meshkov flow of explosion-driven copper interface. The Richtmyer-Meshkov flow refers as an interfacial instability growth under shockwave incident loading. Numerical investigations are performed to check the applicability of the two-dimensional hydrocode, which is named AFE2D, and the physical models of detonation waves propagating in the high explosives, equations of state and the constitutive behaviors of solids in the analysis of Richtmyer-Meshkov flow problems. Here we theoretically analyze the two key issues of the unstable mechanism in Richtmyer-Meshkov flow in solids. The unstable mechanism includes temperature related melting mechanism and the plastic evolution related tensile fracture mechanism. In the analysis of the temperature related unstable mechanisms, the calculated temperature increase during the shockwave compression from the shock Hugoniot data in the shockwave physics is not enough to melt the material near the perturbed interface. On the other hand, the temperature increase from the translation of plastic work during perturbation growth which relats to the distribution of the cumulative effective plastic strain is also not enough to supply the thermal energy which is needed to melt the crystal lattice of solid, either. Therefore, the temperature related melting mechanism is not the main factor of the unstable growth of copper interface under explosion driven. In the analysis of the plastic tensile fracture related unstable mechanism, a scaling law between the maximum cumulative effective plastic strain and the scaled maximum amplitude of spikes is proposed to describe the relationship between the plastic deformation of material and the perturbation growth of interface. Combined with a critical plastic strain fracture criterion, the unstable condition of the scaled maximum amplitude of spikes is given. If the spikes grow sufficiently to meet the unstable condition, the interfacial growth will be unstable. Numerical simulations with varying initial configurations of perturbation and yield strength of materials show good agreement with the theoretical stability analysis. Finally, a criterion to judging whether the growth is stable is discussed in the form of competition between the temperature related unstable mechanism and the tensile fracture unstable mechanism.
Ejecta production from the metal surface under shock-loading is currently a focused issue both at home and abroad. However, the traditional experimental techniques, such as piezoelectric pin, only diagnose the ejected data for low-density ejecta but not for high-density ones, giving a poor understanding of this process. Particularly, when ejecta production increases significantly as the loaded metal melts on release or shock, the measurement carried out by the traditional piezoelectric pin becomes worse, and brings further missing knowledge in the ejecta evolution. In this paper, an Asay-F window designed earlier by the authors based on the traditional Asay-window, is employed to investigate the formation process of the ejecta from the melted Sn metal. As indicated by previous experimental findings on shocked Pb sample, the Asay-F window is a reliable and effective tool for measuring the high-density ejecta by comparing the result with those of the piezoelectric pin. The interface velocity within the Asay-F window measured by Doppler pin system, is obtained. On the basis of momentum conservation condition, the physical quantities of ejecta, such as accumulative areal mass, volume density and velocity, are derived from the interface velocity. By analyzing the experimental data diagnosed by the Asay-F window, which is placed at different offsets from the free surface of Sn sample, the expansion evolution of the ejecta is obtained. Through transforming the dynamic volume density to the static one, the picture of the ejecta density distribution changes with the spatial distance at a specific moment, which is explicitly displayed. It is found that the ejecta density distributions gained from the different offsets at the uniform moment are consistent. As a consequence, the self-similar expansion evolution of the ejecta is experimentally confirmed, which successfully avoids the unclear understanding of this process if only examined by the piezoelectric pin. This experiment may lay the foundation of the formation of the ejecta production for the metal sample subjected to high pressure loading.
This effort investigates the relation between ejecta production and shock-breakout pressure (PSB) for Sn shocked with a Taylor shockwave (unsupported) to pressures near the solid-on-release/partial melt-on-release phase transition region. The shockwaves were created by detonation of high explosive (HE) PBX-9501 on the front side of Sn coupons. Ejecta production at the backside or free side of the Sn coupons was characterized through use of piezoelectric pins, optical shadowgraphy, x-ray attenuation radiography, and optical-heterodyne velocimetry. Ejecta velocities, dynamic volume densities, and areal densities were then correlated with the shock-breakout pressure of Sn surfaces characterized by roughness average of Ra = 16 μin or Ra = 32 μin.
This effort investigates surface-preparation methods to enhance dynamic surface-property measurements of shocked metal surfaces. To assess the ability of making reliable and consistent dynamic surface-property measurements, the amount of material ejected from the free surface upon shock release to vacuum (ejecta) was monitored for shocked Al-1100 and Sn targets. Four surface-preparation methods were considered: Fly-cut machine finish, diamond-turned machine finish, polished finish, and ball rolled. The samples were shock loaded by in-contact detonation of HE PBX-9501 on the front side of the metal coupons. Ejecta production at the back side or free side of the metal coupons was monitored using piezoelectric pins, optical shadowgraphy, and x-ray attenuation radiography.
This effort investigates the underlying physics of ejecta production for high explosive (HE) shocked Sn surfaces prepared with finishes typical to those roughened by tool marks left from machining processes. To investigate the physical mechanisms of ejecta production, we compiled and re-examined ejecta data from two experimental campaigns [W. S. Vogan etal, J. Appl. Phys. 98, 113508 (1998); M. B. Zellner etal, ibid. 102, 013522 (2007)] to form a self-consistent data set spanning a large parameter space. In the first campaign, ejecta created upon shock release at the back side of HE shocked Sn samples were characterized for samples with varying surface finishes but at similar shock-breakout pressures P SB . In the second campaign, ejecta were characterized for HE shocked Sn samples with a constant surface finish but at varying P SB .
This effort investigates ejecta cloud expansion from a shocked Sn target propagating into vacuum. To assess the expansion, dynamic ejecta cloud density distributions were measured via piezoelectric pin diagnostics offset at three heights from the target free surface. The dynamic distributions were first converted into static distributions, similar to a radiograph, and then self compared. The cloud evolved self-similarly at the distances and times measured, inferring that the amount of mass imparted to the instability, detected as ejecta, either ceased or approached an asymptotic limit.
With smoothed particle hydrodynamics, we calculate the formation of micro-jet from the groove of metal surface, and reveal the dependence on the width of loading wave front. The calculated results show that, with the decreasing loading speed,both the mass and the front velocity of ejection decrease, and the mass fraction with low velocity increases. We draw the conclusion that the formation of micro-jet is a result of the particles near the groove acquiring axial velocity and impacting at the axis, and the efflux derives from a layer near the groove. The thin layer becomes smaller when widening the wave front, which is because some particles can no longer satisfy the lock condition of jet strength with the decreasing impact velocity and the increasing impact angle, and then can jet no more.
High-speed photography and pulsed in-line holography were applied in the diagnosis of microjet from shocked metal surface. Microjet loading driver, high-speed photography system, pulsed in-line holography system, and high-accuracy synchronization timing system were established. Shown as the experiment, the evolutionary images of microjet can be gained by high-speed photography, and the clear image of microjet can be gained by pulsed in-line holography. The two diagnosis methods are effective means for microjet.
With smoothed particle hydrodynamics, we calculate the formation of micro-jet from the groove of metal surface, and reveal the dependence on the width of loading wave front. The calculated results show that, with the decreasing loading speed, both the mass and the front velocity of ejection decrease, and the mass fraction with low velocity increases. We draw the conclusion that the formation of micro-jet is a result of the particles near the groove acquiring axial velocity and impacting at the axis, and the efflux derives from a layer near the groove. The thin layer becomes smaller when widening the wave front, which is because some particles can no longer satisfy the lock condition of jet strength with the decreasing impact velocity and the increasing impact angle, and then can jet no more.
Smoothed Particle Hydrodynamics (SPH) is a meshless Lagrangian hydrodynamic computational technique, and the form of SPH equations is simple. An obvious advantage of SPH is that it is meshless, thus large deformation calculations can be easily done. The dynamic process of microjet from shocked aluminum with grooved surface is simulated with two-dimension SPH code. Jet mass, tip velocity and jet mass-velocity curve were obtained. Numerical results of microjet from grooves with the same depth and different angle are also presented and compared with those of experimental. It is shown that SPH technique is an effective means for simulating microjet from metal free surface under shock loading.
When a metal is shocked above its melting pressure or melted on release, the tensile stresses generated upon reflection of the compressive pulse from a free surface are induced into a liquid state. Instead of the well-known spallation process observed in solid targets, cavitation is expected in the melted material, and liquid fragments are ejected from the free surface. Their size, velocity, and temperature distributions are issues of increasing interest, as well as their impact on other nearby materials, but data are limited on the subject. Here, we present an experimental study performed on tin samples subjected to high pressure laser shocks (ranging from about 50 to 200 GPa) of short duration ({approx}5 ns). The results include post-test observations of the ejecta recovered after impact on a polycarbonate shield and time-resolved measurements of the free surface velocity through the shield. For shock pressures below some 80 GPa, the velocity profiles are compared to the predictions of one-dimensional simulations involving a multiphase equation of state. For higher loading pressures, the emergence of the shock at the free surface produces a rapid loss of reflectivity so the particle velocity cannot be determined. In all cases, solidified fragments of tin are recovered on the shield. Their sizes, their shapes, and the induced damage depend significantly on shock pressure, and are indicative of a very wide range of ejection velocities. The data provide a basis for a phenomenological description of the process.
Dynamic fragmentation in liquid metals is an issue of increasing interest in shock physics, both for basic and applied scientific motivations. In a recent paper, we have reported an exploratory study of liquid spall in tin samples melted upon laser-driven shocks. The need for further experiments and analyses to answer some questions raised by that work had been pointed out. Thus, we present here some complementary results, including specific recovery tests as well as high speed transverse shadowgraphy to visualize both the expanding cloud of fast droplets and the late motion of large fragments that had been inferred from postshock observations.
An experimental technique is discussed for measuring the mass‐velocity distribution of material ejected from strongly shocked surfaces. The two major advantages of this technique are, first, that it is useful for measuring large amounts of surface ejecta and, second, that it allows a direct determination of the mass‐velocity distribution of ejecta, without the need for differentiating experimental data. The method has been applied to a measurement of surface ejecta from shock‐loaded porous tungsten.
Following the introduction of the concept of continuous spall damage as a replacement for the customary discrete description, existing spall criteria are generalized to continuous measures of damage and are classified according to their dependence on the history of the continuum field variables. A compound‐damage‐accumulation theory is proposed in which the rate of damage accumulation depends on the existing damage, in addition to the applied stress. Several examples of the application of the new theory to the correlation of existing data are given.
Velocity interferometry and double‐pulse holography have been used to study material ejected from surfaces which are impulsively loaded with plane shock waves. Experiments performed on aluminum shocked to 25 GPa (250 kbar) provide the average mass and velocity distribution as well as the spatial distribution of material ejected from the surface. A total mass of about 3 μg/cm2 was ejected when the shock arrived at the surface in the present experiments, and a substantial part of this material resulted from jetting at small pits on the surface.
This paper reviews recent attempts to construct a microstatistical fracture mechanics; that is, a methodology that relates the kinetics of material failure on the microstructural level to continuum mechanics. The approach is to introduce microstructural descriptions of damage into the continuum constitutive relations as internal state variables. The microstructural damage descriptions are based on dynamic and quasi-static experiments with carefully controlled load amplitudes and durations. The resulting constitutive relations describe the nucleation, growth, and coalescence of the microscopic voids and cracks, and therefore in principle describe both quasi-static and dynamic fracture on the continuum scale. The paper describes several such kinetics models in detail, shows examples of several engineering applications, and discusses the link between microstatistical fracture mechanics and continuum fracture mechanics.
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