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Abstract
A discussion is presented regarding interferometer experiments conducted on free surfaces which are impulsively loaded with high amplitude shock waves. It is shown that material ejection from shocked surfaces can significantly degrade interferometer experiments. In particular, loss of both light intensity and contrast of interferometer signals can result from various scattering and absorption processes occurring in a cloud of ejected material. An experimental technique is presented which allows determination of the mass and velocity of material ejected from free surfaces during shock loading. The technique has been applied to a study of mass ejection occurring naturally from shocked surfaces of two aluminium alloys and from lead. These results show that the total ejected mass ranges from a few ..mu..g/cm/sup 2/ in the aluminum alloys studied to a few mg/cm/sup 2/ in lead, for shock pressures ranging from about 10 to 50 GPa (100 to 500 kbar). Surface defects, such as pits and scratches, are thought to strongly influence mass ejection in aluminum; whereas in lead, localized shock-induced melting and vaporization are thought to be the dominant mechanisms at the higher shock pressures. Experimental results are also presented for aluminum surfaces which contain artificial defects in the form of wedge-shaped cavities. These results show that the maximum ejecta velocities of approximately two to four times the free surface velocity which are observed in these experiments can be correlated with predictions of steady jetting theory.
... 11,12 We refer to it as an empirically fitted model rather than a theory because it is a linear regression of empirical data observed by Asay and Bertholf 8 in their Fig. 9. The first theory is known variously as "steady jetting theory," 13 "steady-state theory," 14 or "steady-jet theory," 15 and it is shaped charge liner jet theory, 16,17 originally formulated to predict jets from thin metal groove liners surrounded by high explosives upon detonation, but has been applied to relatively thick metal plates with grooves [13][14][15]18,19 (the geometry shown in Fig. 1). The second theory is a limiting case of RMI 4 theory applied to the geometry shown in Fig. 1 by assuming At % À1 for shocks that propagate in the "heavy to light" direction. ...
... Both theories have been developed for at least six decades and have many contributors. The contributors to the first theory include Birkhoff, 16 MacDougall, 16 Pugh, 16,20 Taylor, 16 Eichelberger, 20,21 Asay, 13 de Rességuier, 14 Lescoute, 14 Sollier, 14 Prudhomme, 14 Mercier, 14 Soulard,18 and probably others. The contributors to the second theory include Meyer, 22 Blewett, 22 Dimonte, 23 Ramaprabu, 6,23 Mikaelian, 24 Zhang, 25 Velikovich, 26 Buttler, 5 Oró, 5 Preston, 5 Cherne, 5,6 Hixon, 5 Mariam, 5 Morris, 5 Stone, 5 Terrones, 5 Tupa, 5 Karkanis, 6 Hammerberg, 6 Andrews, 6 Taylor, 27 Richtmyer, 2 and probably others. ...
... The liner converges at the angle β relative to the pre-shock geometry [shown in Fig. 2(a)], where β has a constant value under the steady flow assumption (assumption 6). However, β is not analytically tractable and can be determined: (a) by direct empirical fitting 13,20,21 (denoted by subscript δ, 1); (b) by semi-empirical methods that use additional kinetic equations, an equation of state for the metal liner, and additional conservation equations 14,28 (denoted by subscript δ, 2); or (c) by relaxing the isentropic assumption and using a modified form of the Bernoulli equation appropriately coupled with the equation of state. 18 A final assumption (not explicitly discussed by Birkhoff et al. 16 ) is that the shock velocity tangent to the free surface inside the groove must be greater than the speed of sound behind the shock, referred to as the "regular" case. ...
Many studies have investigated the mass outflows generated when a planar shock transits an imperfect (“defected”) metal surface, where the defects are symmetric triangular or sinusoidal grooves. Yet a fundamental question remains unanswered: how does the quantity of outflow mass and its maximum velocity vary as a function of the groove cross-sectional aspect ratio? We identify two sets of missing experiments that must be addressed to answer the question. The aspect ratio (groove depth over width) is equivalently represented by θ, the cross-sectional half angle, or by η 0 k, the amplitude multiplied by an effective wavenumber. Low θ (high η 0 k) grooves comprise the first set of missing experiments, which are necessary to determine the validity of theoretical predictions of the nonlinear regime ( η 0 k ≥ 1, θ < 57.5 °). The second set of missing experiments are those in which the volume of the groove (or equivalently, the axial cross-sectional area) has been held constant as θ or η 0 k are varied. Such experiments are necessary to independently measure the effects of variations in groove volume and groove aspect ratio on the resulting jets.
... Another less known phenomenon is the ejection of matter from the free surface, producing what is known as ejecta. The formation of ejecta was explored by Asay and associates in the 1970s [1][2][3] and is a special case of the Richtmeyer-Meshkov Instability (RMI) [4][5][6] . RMI occurs when the shock front interacts with a roughened surface, causing the peaks and valleys of the surface to invert when impacted, forming fingerlike jets that grow and may eventually break-up into smaller clusters of atoms. ...
... Theoretical and experimental studies on crystalline metals have explored the role of surface roughness, particle velocity, and crystalline phases but have historically neglected material microstructure. This can mostly be attributed to the fact that total ejected mass significantly increases once the material melts while a negligible mass is usually produced in the solid state [1] . Few studies mention the importance of heterogeneities such as voids or inclusions to ejecta production, but even those that do mainly focus on the role of surface rough-ness and particle velocity [1,8,10,11] . ...
... This can mostly be attributed to the fact that total ejected mass significantly increases once the material melts while a negligible mass is usually produced in the solid state [1] . Few studies mention the importance of heterogeneities such as voids or inclusions to ejecta production, but even those that do mainly focus on the role of surface rough-ness and particle velocity [1,8,10,11] . As materials are developed for use in extreme environments, where heterogeneities are formed through sustained damage, understanding the role of microstructure in dynamic behavior and strength is crucial. ...
The effect of He concentration and morphology on ejecta production is investigated via molecular dynamics simulations. Identical He concentrations are inserted into Cu single crystals as interstitial atoms or bubbles near a flat free surface. The resulting ejecta is quantified through total mass, cluster size, and velocity of ejected particles. The presence of He increases total ejected mass as compared to pure Cu; He bubbles produce 56% more mass than atomic He. This increase is attributed to non-planarities in the shock front and reflected pulse due to He bubbles, akin to ejecta resulting from traditional Richtmeyer–Meshkov instabilities.
... To this day, extensive experimental and theoretical researches have been conducted, for understanding the ejection mechanism comprehensively and developing a predictive ejecta model that can be applied in hydrodynamic calculations. Previous works demonstrate that the mass and velocity of ejecta are relevant to various factors, such as surface defects [2][3][4][5][6], shock pressure and profile [7,8], formation of hot spots and release melting [9,10]. ...
... It is known that the machined metal surfaces are normally characterized by periodic wedge grooves of micronscale [6,9]. There, the shock-induced microjetting from those grooves becomes a main ejection mode [2][3][4]. ...
... The above jetting velocity and mass are compared to the experimental dates [3], where the lead specimens with a surface finish of 1 micrometer were adopted. Our simulations show that the ratio of jetting velocity peak to surface velocity remains relatively constant (1.5-1.55). ...
This work investigates the shock-induced microjetting from a grooved surface (10 nm, 120 degree) of low-melting metal Pb with molecular dynamics simulations. The microjetting processes under surface/release melting conditions are presented in detail, and some properties on the microjet mass and velocity are revealed for different shock pressure and profile cases. It is found that the increase of microjet mass with shock pressure experiences three stages: rapid increase (solid phase), slowdown increase (release melting) and almost no increase (shock melting). For all cases, the ratio of the maximal jetting velocity to the surface velocity approximately keeps a constant (1.5–1.55), but this value undergoes a degree of exponential decay with time for the solid release cases. In addition, the temperature of the microjet is found to be always above the melting point (zero pressure) and keep a continuous increase towards the microjet tip. When introducing slow decaying profiles, the microjet mass begins to increase with the decay rate, which is dominated by the deformation of bubble during pull-back. When the decay rate becomes fast enough, the microspall occurs as expected, meanwhile the microjet appears to reduce because of the shock energy reduction. But that cannot cut off the microjet completely. The velocity distribution along the loading direction shows two linear regions corresponding to the microspall and microjet, and the latter seems to have a greater velocity gradient.
... It has long been known that a shock-driven material can emit a fine spray of particles (ejecta) from its free surface, [1][2][3][4][5][6][7][8] which can corrupt optical and electrical measurements at metal surfaces. The process can also afflict applications like inertial confinement fusion (ICF) by mixing cold ablator material into the hot thermonuclear fuel. ...
... The shock strength is also important because r max ej is found to increase dramatically when the material melts upon release and loses its material strength. 2,8 Similarly, a supported shock produces more ejecta above melt than a Taylor wave. 8 In addition, we believe that r max ej should depend on the postshock density rather than the initial density since the rarefaction wave that reflects from the free-surface can decompress the material significantly. ...
... Moreover, the DMA model does not address the ejecta velocity, which has been observed to approach a maximum of 3Â the free-surface velocity for small h. 2,4,5 Such velocity enhancements are calculated by an incompressible steady-state jetting theory for explosively driven lined cavities at macroscopic scales. [10][11][12] However, the result is not in closed form and may not strictly apply to shock-driven micro-jets since they are compressible, transient, and nonuniform. ...
Previous work employed Richtmyer-Meshkov theory to describe the development of spikes and bubbles from shocked sinusoidal surfaces. Here, we discuss the effects of machining different two-dimensional shaped grooves in copper and examine the resulting flow of the material after being shocked into liquid on release. For these simulations, a high performance molecular dynamics code, SPaSM, was used with machined grooves of
kh
0
= 1 and
kh
0 = 1/8, where 2h
0 is the peak-to-valley height of the perturbation with wavelength λ, and k = 2π/λ. The surface morphologies studied include a Chevron, a Fly-Cut, a Square-Wave, and a Gaussian. We describe extensions to an existing ejecta source model that better captures the mass ejected from these surfaces. We also investigate the same profiles at length scales of order 1 cm for an idealized fluid equation of state using the FLASH continuum hydrodynamics code. Our findings indicate that the resulting mass can be scaled by the missing area of a sinusoidal curve with an effective wavelength, λeff
, that has the same missing area. Our extended ejecta mass formula works well for all the shapes considered and captures the corresponding time evolution and total mass.
... Wang et al. 45 investigated the impact of varying defect angles under the same shock pressure on the head velocity and mass of microjets using SPH method. Zhang et al. 46 proposed a splashing velocity model and compared it with experimental data, 47 showing a good agreement. According to Zhang's model, the theoretical relationship between particle velocity and head velocity can be expressed as follows: ...
... The ejecta mainly originates from surface or near-surface micrometer and submicrometer-scale defects. 11,47 To determine the source locations of different parts of the microjet, the material sources in the z 1 region (red area) were compared with those in the z 2 region (yellow area). Furthermore, the microjet mass was characterized using the jetting factor. ...
The structure of surface defects is one of the primary focuses in exploring the mechanism behind microjetting phenomena. However, the influence of defect size on microjets remains understudied. This work investigated the correlation between shock-induced microjets and surface defect scales under continuous approximation with the smoothed particle hydrodynamics method. The physical properties of microjets from generation to fragmentation were analyzed in detail. A relationship between the mass of different parts of the microjet and the defect size was established. The results indicate that the length of microjets and the jetting head velocity increase with the increase in the defect sizes. The jetting head velocity increases significantly when the defect depth is less than 4 μm, and the increase slows down when the defect depth is greater than 4 μm. This is attributed to the pressure and energy variations in the defect surface layer. A transition in the mass distribution of the microjet occurs when the defect depth reaches 7 μm. The jetting factor exhibits a trend of decreasing first and then increasing with the enlargement of defect size. The time of microjet fragmentation shows a proportional relationship with the defect size. By statistically analyzing the distribution of microjet fragmentation aggregates, it is found that the dispersal degree of microjet fragmentation aggregate sizes increases with defect size. This research reveals the correlation between the microjet and defect size.
... Mass ejection experiments indicated that the micro-jet from a defect on the specimen surface was the main source of ejecta when surface melting didn't occur (Asay [3]). The ejection experiment conducted on a groove specimen showed that the groove angle affected the maximum ejecting velocity and total ejecting mass (Asay [4]). Mass ejection experiments conducted on Sn indicated that the surface roughness had great effect on the mass ejection (Zellner et al. [5]). ...
... The study was focused on the maximum ejecting velocity and the mass distribution of the micro-jet, when the groove angle and the groove wavelength changed. The specimens used in the experiment were conducted on aluminum with artificial defects of a parallel V-groove (Asay [4]). The groove depth was 55um, the groove wavelength was 130um and the half groove angle varied from 15 to 45. ...
... It was not until 2004 that the problem was first studied with numerical simulations by using molecular dynamics (MD). 11 Up to now, the problem has been studied by experiments 10,[12][13][14][15][16][17] and numerical simulations based on the hydrodynamic approach [18][19][20][21][22][23] or the MD method, [24][25][26][27][28][29][30] and in the meantime, some models on the jet velocity and ejecta mass have also been presented. 18,20,31,32 Although the phenomenon of ejections from shock free surfaces is complex and involves multiple physical processes as in recent works, 18,[20][21][22] it can be considered a limiting case of the RMI when the problem focuses on the ejection velocity, mass, and shape. ...
... In order to further verify the application, we compared the empirical models [Eqs. (24), (20), and (19)], the numerical results (FLASH 20 and current results) and the experimental results (LANL, 12 SNL, 16 CEA 10,17 ) in Fig. 18. It can be found that the numerical results from FLASH 20 and the current method are all consistent with Eq. (24), and these results are also consistent with the experiment from LANL, but some slightly lower estimates can also be found than the experiment from SNL and CEA with a large scaled amplitudes. ...
A numerical study of the metal jet induced by a shock wave ABSTRACT In this work, a metal jet induced by a shock wave is studied numerically. Different from the previous works on metal jets, we apply a cut-cell based sharp interface numerical method for the study. The evolution of jets is simulated by the in house code CCGF [X. Bai and X. Deng, Adv. Appl. Math. Mech. 9(5), 1052-1075 (2017)], and the interfacial growth rate is computed and compared with some theoretical models. Various initial conditions, including disturbance amplitude and shock wave strength, are considered here. Based on the model of Karkhanis et al. [J. Appl. Phys. 123, 025902 (2018)], a modified model of the spike velocity is presented to achieve better consistency between the numerical simulation and the model formula under more wide initial conditions (here, the scaled perturbed amplitudes involved are 0.125 and 4, and the incident shock wave Mach number is from 2.5 to 8) in this paper. In order to extend the applicability of the empirical models, an approximate formula for the initial velocity V 0 is also obtained; a direct prediction of the spike velocity will become possible when the initial perturbed amplitude and incident shock intensity are known. Relevant figures show that the modified model can estimate a more consistent result with the numerical simulation than the VK or GD model. Published under license by AIP Publishing. https://aip.scitation.org/doi/10.1063/5.0019811
... In the early research, experimental methods were mainly adopted, and over the years the experimental technology of the microjet has been made great progress. Asay et al [2,[15][16][17] developed the thin foil and thick plate techniques, and studied the mass and velocity distribution of the ejecta from surface. The results indicated that the substantial part of the ejection occurred at the pits on the surface. ...
... The jetting factor R is introduced to describe the differences in ejected mass. The jetting factor R of microjet is defined as the ratio of ejected mass to micro defect mass [53,54], which is proposed by Asay [16]. In the simulation results, we define the mass from the spike to the bubble as M J , and the mass from the spike to the theoretical free surface as M F . ...
The shock-induced microjet phenomenon has attracted much attention due to its importance in shock compression science and technology. The existing researches have shown that shockwave profile has a significant effect on the microjet formation. This work investigates the influence of shock pulse duration on the microjet systematically based on smoothed particle hydrodynamics simulations and theoretical analysis. In fact, when the pulse duration of shock wave is more than 7.31 times the time of shock front passing through groove, the microjet mass and its time-space evolution will be consistent with the case under supported shock. It is shown that there is a critical value for the shock pulse duration (about 4.21 times the time of shock front passing through groove), below which the effect of shock pulse duration is distinct. With decreasing the shock pulse duration, the mass from spike to bubble experiences a rapid increase due to the increase of the velocity gradient behind the free surface, while the mass from spike to the theoretical free surface experiences a gradual reduction because the shock energy reduces. As a result, the spike becomes very thinner and the bubble amplitude is lengthened. The damage will be localized around the groove region. Besides, with the interaction between the release stage of incident waves and the release waves from the groove, the cavities and different low-density fragments are observed.
... When a shock wave breaks out at a free surface, ejected matter (ejecta) can be emitted from the surface through different physical processes [1,2]. The impacts of such high velocity debris can cause severe damage to nearby diagnostics and structures, which is a major safety issue for various applications, including 5 pyrotechnics [3,4,5] and inertial confinement fusion experiments [6,7,8,9]. ...
... Microjetting is one of the processes governing such debris generation. It is due to the interaction of a shock wave with a free surface presenting geometrical defects such as pits, cavities, scratches, or grooves, leading to material ejection from these defects, in the form of thin jets expanding ahead of the main surface 10 and breaking up into small particles (typically µm-scale) with high velocities (a few km.s −1 ) [2]. Over the last decades, microjetting has been extensively studied, using mainly explosive or plate impact loading drivers (see reference [10] for a brief history of ejecta physics). ...
A high resolution picosecond laser imaging diagnostic has been developed for making high-resolution spatial measurements of ejecta particles moving at high velocities (a few km.s⁻¹). Preliminary results obtained with both visible (532 nm) and UV (355nm) lighting are presented for laser shock-driven tin ejecta experiments performed with different kind of surface defects. These results are compared with those obtained at LANL under high explosive loading using ultraviolet in-line Fraunhofer holography, and also with molecular dynamics (MD) simulations performed at lower space and time scales.
... There has been significant recent interest in the ejecta problem, encompassing theory, 7-13,15,16 simulations, 7,13,16-22 and experiments. [13][14][15][23][24][25][26][27] For recent detailed reviews, we refer the reader to Refs. 28 and 29. ...
... The effective perturbation wavelength λ eff for use in Eqs. (11) and (12) is then computed as λ eff A sh /h 0 using the missing area given in Eq. (15) and h −− bu . Finally, we note that the above analysis is only valid for bubbles that satisfy R > 2h −− bu . ...
We apply a hydrodynamic approach to analyze ejecta emanating from doubly shocked liquid
metals. In particular, we are interested in characterizing ejecta velocities in such
situations by treating the problem as a limiting case of the Richtmyer–Meshkov
instability. We find existing models for ejecta velocities do not adequately capture all
the relevant physics, including compressibility, nonlinearities, and nonstandard shapes.
We propose an empirical model that is capable of describing ejecta behavior across the
entire parameter range of interest. We then suggest a protocol to apply this model when
the donor material is shocked twice in rapid succession. Finally, the model and the
suggested approach are validated using detailed continuum hydrodynamic simulations. The
results provide a baseline understanding of the hydrodynamic aspects of ejecta, which can
then be used to interpret experimental data from target experiments.
... Therefore, since being discovered by Walsh et al [1], this phenomenon has been attached much attention. Previous experiments have shown that the factors affecting the ejection are various, such as surface defects [2][3][4], shock strength and profile [5,6], formation of hot spots and release melting [7]. ...
... Some experimental and theoretical studies on the jetting mechanism have already been reported. Asay [2,3] investigated the effect of the groove angle on the total jetting mass and mass-velocity distribution. Han [8] presented a semi-rational formula for calculating the above jetting mass, based on the classical constant jetting theory. ...
Nowadays, molecular dynamics simulation has been a significant method in detecting the atomistic-scale kinetics of materials. The objective of this work is to detect the dynamic properties of micro-jet from a nano-grooved aluminium surface by molecular dynamics simulations. For a large range of shock pressure, we investigate the variation of micro-jet mass and morphology. The stacking fault, amorphous state, and release melting during the jetting are all observed, and the effect of release melting on the micro-jet is analyzed. It is found that, the micro-jet mass keeps a linear increase with the piston velocity prior to release melting, the occurrence of release melting can intensify the jetting evidently, and when the velocity of release melting is above a threshold, the jetting mass shows a linear increase with the piston velocity again.
... 24,25 It is known that the phase of the bulk shocked material affects the resulting jet characteristics, such as particulate phase, mass-velocity distributions, particle size distributions, and total mass. 14,[26][27][28][29] But unknown is how and whether the phase of the particulates within the jets affects the collective particle interaction behavior. Further studies on jet interactions over such transitional pressures could provide key insights into how tin phase affects the bulk collisional properties. ...
Ejecta microjets offer an experimental methodology to study high-speed particle laden-flow interactions, as microjets consist of millions of particulates traveling at velocities of several kilometers per second and are easily generated by most common shock drives. Previous experiments on the OMEGA Extended Performance laser found that collisions between two counter-propagating laser-driven tin ejecta microjets varied as a function of drive pressure; jets generated near shock pressures of 10 GPa passed through each other without interacting, whereas jets generated at shock pressures of over 100 GPa interacted strongly, forming a cloud around the center interaction point. In this paper, we present a more systematic scan of tin ejecta microjet collisions over intermediate pressure regimes to identify how and at what shock pressure interaction behavior onsets. Radiographs of interacting microjets at five different laser drive energies qualitatively demonstrate that interaction behavior onsets slowly as a function of laser drive energy. Quantitative mass and density metrics from each radiograph provide trends on jet characteristics and collisional mass dispersion. It is observed that jetting mass, jet densities, and mass dispersion increase with increasing drive pressures and that the increased jet density at the higher drive energies may account for the increased mass dispersion. This work provides an important step in the understanding of tin ejecta microjet collisions and paves the way for future studies on the physics dominating high-speed particle-laden flow interactions.
... An interesting effect in shock-wave phenomena is the behaviour of surfaces shocked to high pressures such as hundreds of kilobars. When a shock wave propagating in a solid sample reflects from a free surface, rapid ejection of fine fragments usually happens [1]. It is an extremely complicated and challenging subject in shock physics. ...
... [6][7][8][9] All studies consistently agree that for a given surface finish, the maximum amount of ejecta is produced when the material is in the liquid rather than the solid state. The work by Asay 10 showed that Pb, which melts at a lower stress, produced a significantly higher amount of ejected mass as compared to Al. ...
The interaction of shock waves with non-planar free surfaces can cause atoms to eject from the surface, leading to the formation of ejecta. These non-planarities in the free surface can occur due to machining of the free surface or can be induced in the shock wave itself due to the presence of heterogeneities in the material. Both cases lead to the formation of ejecta. While the effect of machining on ejecta has been well studied, the latter has not been a topic of significant investigations. In this work, molecular dynamics simulations are used to systematically investigate the effect of size and concentration of He bubbles in Cu with planar free surfaces on ejecta production. It is shown that the presence of defects leads to the formation of non-planarity in the shock wave itself producing ejecta as the front reaches the flat free surface. The cluster size and velocity of ejected particles greatly exceeds that of pure Cu; the radius, density, and nature of the helium-filled voids alter the mass, velocity, and size distribution of the ejected matter.
... For example, 1970s research on new experimental ejecta mass diagnostics developed at Sandia National Laboratories -notably what is now known as the Asay-foil [23,24] -were published. Further, Asay, et al. used their new foil diagnostic to study ejecta formation from Al, Au, Cu, Pb and W on flyer-plate (gun) experiments [25][26][27]. Results from that research were used to build a prescriptive ejecta model that Asay and Bertholf describe in [27]. ...
This manuscript investigates reactive- versus hydrodynamic-breakup processes of ejecta. For this study, the reactive metal is cerium (Ce) and the nonreactive metal is tin (Sn), the nonreactive gas is helium (He) and the reactive gas is deuterium (D2) or hydrogen (H2). Experiments were performed in vacuum and the reactive- and nonreactive-gases at various pressures, where we endeavored to match the post-shock gas densities to differentiate between reactive- versus hydrodynamic-breakup processes. Hydrodynamic breakup sensitively links to the Weber number (gas density, liquid fragment diameter, surface tension, and the square of the relative velocity between the fragment and the gas), whereas reactive breakup links to the reactive dynamics which includes two processes. In one case the reactive metal breaks up into smaller fragments as rapidly as the reaction rate, and in the other a crust grows on the liquid fragments as the reactions occur, a diffusion limited process. In the latter case, the particle diameters increase with time as the crust grows. In this process, which is indicated by the data, particles breakup as the CeD2 loses strength with increasing temperature, leaving an exponentially increasing diameter.
... 大量的实验研究表明 [10−24] , 微喷射是一种 极其复杂的界面动力学行为, 主要影响因素包括表面缺 陷结构、冲击加载压力与波形、材料强度以及卸载熔 化等. 在不断总结实验规律的基础上, 研究人员逐渐发 现冲击波与表面微结构缺陷相互作用形成的微射流 [2,4] 以及衰减冲击波加载下近表面熔化物质动态拉伸破坏 引起的微层裂 [17] 是造成物质喷射的两种主导机制. 随着对微喷射认识的不断深入以及工程研究的迫 切需要, 充气条件下的微喷混合问题亦备受关注. ...
When a strong shock wave releases from material surface, material ejection usually takes place where a great amount of high-speed particles can be ejected from the material surface. This phenomenon is called micro-ejection and the ejected particles is named as ejecta. In gas environment, the interaction between ejecta and gas can lead to the formation of a layer of gas-ejecta mixture and this process is called mixing. Micro-ejection and mixing are of great importance in many fields such as inertial confinement fusion and implosive dynamics, and have attracted intense research activities both experimentally and theoretically in the past several decades. The author and collaborators have been studying the microejection and mixing phenomena for years, and some valuable achievements have been obtained. In the present review, we introduce some of our theoretical works on the two main mechanisms of micro-ejection (i.e., micro-jetting and microspallation) as well as the gas-ejecta mixing process, including mechanisms of micro-jetting and micro-spallation and their dominant factors, modeling of micro-jetting based on the Richtmyer-Meshkov theory in elastic-plastic solids, and some dynamic characteristics of mixing.
... Of interest is what happens to metals when a shock wave encounters different perturbed metal/vacuum interfaces. The phenomenon, which rapid ejection of micron-scale fragments usually happens from material surface, has attracted much attention since being discovered by Asay [1,2]. These ejected high-speed particles often have negative effects on optical and electrical measurements at metal surfaces, and are relevant to inertial confinement fusion. ...
This work investigates the mass ejected from surface perturbations as the shockwave reaches the AL-vacuum interface, which originates from unstable Richtmyer-Meshkov (RMI) impulse phenomena. The main purpose is to explore the relationships between the shockwave impulse and the geometric properties of surface perturbations, and how those relationships drive the total ejected mass, directionality and velocity distribution. We discuss in detail different types of surface geometry (sinusoidal, square-wave, chevron and semicircle), as well as the wavelengths and amplitudes of surface perturbation. The time evolutions of micro-jet ejection are simulated using a hydrodynamic Lagrangian-Remapping Eulerian method. The calculated results show that primary jetting ejection can be formed from the different shapes, and with increasing wavelength, the ejection mass keeps an increase while the jet head-velocity decreases. However, not all periodic perturbations behave similarly, and masses ejected from irregular surface cannot be normalized to its cross-sectional areas. The square-wave surface may yield pronounced, velocity-enhanced secondary jetting, which is a result of collision of primary jets.
... En 1974, Asay et Barker [7] s'intéressent à la répartition spatiale gaussienne en vitesse des particules éjectées à l'arrière d'un échantillon d'Aluminium poreux impacté par un projectile, en les mesurant par la technique interférométrique du VISAR. En 1976, Asay [8] et al. [9] remplacent la fenêtre transparente de protection du VISAR par une feuille mince de Cuivre. La mesure de la mise en vitesse de cette feuille par les particules éjectées fournit également la répartition de masse éjectée : cette technique de « pesée cible mince » est depuis appelée « feuille d'Asay ». ...
Lorsqu’un matériau solide est soumis à un chargement dynamique (par l’impact d’un projectile, la détonation d’un explosif ou l’irradiation par un laser intense), il se forme une onde de choc, qui se propage dans le matériau depuis la surface chargée. Si cette onde débouche sur une surface libre comportant des défauts géométriques tels que des rugosités, des rayures ou des cavités, son interaction avec ces défauts conduit à l’éjection, sous forme de jets de matière, de débris dont la taille caractéristique est de l’ordre du micromètre et dont la vitesse est typiquement de quelques km/s. La maîtrise de ce processus, appelé microjetting ou micro-éjection, est essentielle pour de nombreuses applications (conception de blindage, découpe pyrotechnique, usinage à très haute vitesse, expériences de Fusion par Confinement Inertiel…). Dans le cadre de cette thèse, menée en collaboration avec le centre CEA de Bruyères-le-Châtel, ce phénomène est étudié dans quatre métaux (Aluminium, Etain, Cuivre et Plomb) à partir de rainures calibrées de deux types : triangulaires isolées de demi-angles d’ouverture contrôlés (20°, 30° et 45°) ou sinusoïdales périodiques. Les influences du matériau, de la forme et de l’ouverture des défauts, de la pression de choc et de l’état du milieu (solide ou fondu sous choc ou en détente) sur les propriétés balistiques des éjectas (vitesses de jet, distribution en taille et densité surfacique des débris constituant les jets) sont évaluées via trois approches complémentaires : expérimentale, théorique et numérique.L’étude expérimentale comporte plusieurs campagnes de chocs laser, effectuées sur l’installation LULI2000 du Laboratoire pour l’Utilisation des Lasers Intenses (Ecole Polytechnique, Palaiseau), avec plusieurs techniques de diagnostic : Ombroscopie Transverse, Vélocimétrie Hétérodyne, radiographie X rapide in-situ, récupération d’éjectas dans des gels (analysés ensuite en microtomographie). Les résultats sont confrontés à des prédictions théoriques (hydrodynamique des chocs obliques et des charges creuses pour les rainures triangulaires, instabilités de Richtmyer-Meshkov pour les rainures sinusoïdales). Enfin, les simulations numériques réalisées avec le code Radioss utilisent deux approches complémentaires : les Eléments Finis Lagrangiens et la formulation SPH (Smoothed Particles Hydrodynamics), encore très peu appliquée au microjetting, plus empirique que la première mais mieux adaptée aux grandes déformations dans les jets et permettant d’accéder à des distributions de tailles de fragments
... A variety of measurement techniques have been developed to determine the properties of ejecta. Radiography, 1-2 PZT and LiN pins, [3][4][5] and Asay foils [6][7][8] have been used to determine areal mass densities. Visible imaging and shadowgraphy have been used to measure the spatial extent of ejecta clouds (this measurement being extremely sensitive to the smallest amounts of ejecta particles). ...
An ultraviolet (UV) in-line Fraunhofer holography diagnostic has been developed for making high-resolution spatial measurements of ejecta particles traveling at many mm/μs. The diagnostic will be described and results from high-explosive shock-driven tin ejecta experiments will be presented.
... Although the laser facility has additional capability, higher pressures cannot be achieved. Similar experimental results have been reported in shock compression studies, [32][33][34] but no such phenomena have been reported in ramp compression experiments. In shock compression experiments, the cause of the missing VISAR fringes has been found to be mass ejection, which has been widely studied in experiment and theory. ...
Laser-direct-driven ramp compression experiments were performed on long temporally shaped laser pulses based on an analytical isentropic compression model. Upper pressure limits, the ablation pressure scaling law, and stress-density curves were studied. The validity of the analytical model used, the ablation pressure scaling law, and the phenomena of missing line-imaging velocity interferometer (VISAR) fringes in the experimental results are discussed.
... Ejection is a kind of complex dynamic phenomenon that occurs when a shock wave interacts at a material/vacuum or material/gas interface accompanied by the emitting of a great amount of matter (ejecta). Extensive studies have been conducted on the phenomenon since the pioneering studies of Asay and co-workers in the 1970s, [1][2][3] and significant achievements have been gained. Previous research on the ejection of roughened surface has shown that the mass, size, and velocity of the ejecta vary depending closely on the surface status, the initial shock conditions, and the material properties of the sample. ...
Large-scale non-equilibrium molecular dynamics simulations are performed to explore the jet breakup and ejecta production of single crystal Cu with a triangular grooved surface defect under shock loading. The morphology of the jet breakup and ejecta formation is obtained where the ejecta clusters remain spherical after a long simulation time. The effects of shock strength as well as groove size on the steady size distribution of ejecta clusters are investigated. It is shown that the size distribution of ejecta exhibits a scaling power law independent of the simulated shock strengths and groove sizes. This distribution, which has been observed in many fragmentation processes, can be well described by percolation theory.
... The phenomena of shock-induced ejecta is an active area of shock-physics research involving different shock generators and diagnostics, which leads to great progress in the construction of dynamic material models [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The shockwave breakout and reflection at the metalvacuum interface (free-surface) may cause ejection of liquid or solid metal fragments into the vacuum beyond the metal interface. ...
This effort investigates the dynamic properties of ejecta from explosively shocked, melted Pb targets. The study shows that the ejecta cloud that expands beyond the shocked surface is characterized by a high density and low velocity fragment layer between the free-surface and the high velocity micro-jetting particle cloud. This slow, dense ejecta layer is liquid micro-spall. The properties of micro-spall layer, such as the mass, density and velocity, were diagnosed in a novel application of an Asay window, while micro-jetting particles by lithium niobate piezoelectric pins and high speed photography. The total mass-velocity distribution of ejecta, including micro-spall fragments and micro-jetting particles, is presented. Furthermore, the sensitivity of ejecta production to slight variations in the shockwave drive using the Asay foil is studied.
... For these experiments, particle sizes are measured using holography. Ejecta experiments similar to these have been performed at other facilities [1,2,3,4,5] however, only a few measurements of particle sizes have been carried out. The experiments were performed at the Pegasus Pulsed Power Facility (PPPF) [6,7] at Los Alamos National Laboratory. ...
When a shock wave
interacts at the surface of a metal sample “ejected matter” (ejecta) can be emitted from the surface. The mass, size, shape, and velocity of the ejecta varies depending on the initial shock conditions and the material properties of the target. To understand this phenomena, experiments have been conducted at the Pegasus Pulsed Power Facility (PPPF) located at Los Alamos National Laboratory (LANL). The facility is used to implode cylinders to velocities of many mm/μsec. The driving cylinder impacts a smaller target cylinder where shock waves of a few hundreds of kilobars can be reached and ejecta formation proceeds. The ejecta particle sizes are measured for shock loaded Sn and Al metal samples using an in‐line Fraunhofer holography technique. The distributions will be compared to calculations from 3 and 2 dimensional percolation theory.
... In the region of P SB ≤ 195 kbar, Region 1, the Sn shocks up to a solid material state and releases to a solid material state [10][11][12]. It was previously shown that metals prepared with large, "classical" grooves (defects of order 10 to 100 µm), that the total ejected mass could be approximated by the missing mass associated with the defect volume, i.e., R(Θ) ~ 1 [15][16]. Our measurements present quantities of ejecta slightly lower than those published results. ...
This effort investigates the relation between shock-pulse shape and the amount of micron-scale fragments ejected (ejecta) upon shock release at the metal/vacuum interface of shocked Sn targets. Two shock-pulse shapes are considered: a supported shock created by impacting a Sn target with a sabot that was accelerated using a powder gun; and an unsupported or triangular-shaped Taylor shockwave, created by detonation of high explosive that was press-fit to the front-side of the Sn target. Ejecta production at the back-side or free-side of the Sn coupons were characterized through use of piezoelectric pins, Asay foil, optical shadowgraphy, and X-ray attenuation.
... Recently, some studies on the jetting mechanism have been reported. Asay [1,2] investigated the effect of the groove angle on the total jetting mass and mass-velocity distribution. Based on the classical constant jetting theory, Han [3] presented a semi-rational formula for calculating the above jetting mass. ...
This work investigates the relation between shock wave risetime and the amount of micro-scale fragments ejected from a grooved aluminium surface under shock loading condition. Using smoothed particle hydrodynamics, we calculate the formation of micro-jet from the groove of metal surface, and analyze the dependence on the width of loading wave front. The simulation results compare well with the experiment ones, and reveal the dependence of the micro-jet on the wave front: both the mass and the maximal velocity of the ejection will decrease with the increasing of the width of wave front. It is also found that the micro-jet originates from the folium near the groove, which can acquire axial velocity and impact at axis when the shock wave releases at the metal/vacuum interface. The folium becomes smaller as the wave front widens. This is because some matter will satisfy the lock condition of jet strength and can eject no more.
... Ejecta production has been linked to the material phase upon shock release 19 as well as properties such as surface defects, and surface inhomogeneities such as surface roughness, inclusions, and voids. [20][21][22][23] Localized melting and development of hot spots have also been shown to contribute to ejecta formation. 24 This experimental effort focuses on surface-preparation methods to reduce or homogenize surface topography, defects, and stress state ͑i.e., homogenize and/or control surface residual stresses͒, and therefore sup-press ejecta production. ...
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.
By using molecular dynamics, we have investigated the effect of nanoscale helium (He) bubbles on the formation of micro-jets and the various physical mechanisms under supported and unsupported shock wave loading. Our simulations suggest that the micro-jet is primarily influenced by the local dynamics of the nano-He bubbles, as the velocity of the shock wave in copper–helium (Cu–He) system is slightly slower than in pure Cu. The expansion of He bubbles can accelerate the velocity of the jet head, but this effect disappears during the released tensile stage. We categorize the behavior of nano-He bubbles into three types: Type A bubbles are in the micro-jet forming region, and their expansion increases the velocity and rupture of the jet. Type B bubbles are located between the micro-jets, and their compression and rapid bursting accelerate the free surface. Type C bubbles are situated far from the free surface and mainly affect the propagation of the shock wave and the released damage process. The global effects of the He bubble are similar under both supported and unsupported shock wave loading. However, the evolution of Type C He bubbles is significantly different under unsupported shock wave loading, with pressure-atom volume and density attenuated to zero and temperature reduced to the initial temperature due to the strong tensile effect. Overall, our study has revealed the differences in the evolution process of He bubbles and their dynamic effects during the formation of micro-jets under different compressed and released paths.
The micro jetting from a grooved aluminum surface under impact loading is investigated by using Eulerian peridynamics (PD). The simulation results are compared with the published experimental data and the spike velocity model, exhibiting qualitative agreement. The governing mechanism accounting for the formation of micro jetting is elucidated from the perspective of the shock wave interaction with the surface groove. The PD simulation results indicate that the incident shock wave induces progressive groove collapse along the direction of shock wave propagation. The rarefaction waves reflected from the groove edges cause the variation of the velocity vector of PD material points, leading to the material points above and below the symmetric axis of the groove converging toward the symmetric axis and colliding with each other. Then, those collided material points are driven by the incident shock wave propagating along the horizontal symmetric axis and eventually ejected from the groove. The effects of the groove dimensions and the impact velocity on the spike velocity and the ejected mass are discussed. The results show that spike velocity decreases with an increasing groove angle but increases with increasing impact velocity. Furthermore, the ejected mass increases with increasing impact velocity. However, when the depth of the surface groove is fixed and the groove angle increases, the ejected mass first increases and then decreases with the turning point at ∼120°. As the depth of the surface groove increases, the ejected mass increases. The simulation results provide a mechanistic understanding of the micro jetting phenomena and instructive guidance for developing better ejecta models.
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Using molecular dynamics simulation, we studied the dynamic behavior and microscopic mechanisms of near-surface helium bubble in single crystal Cu subjected to shock loading. The whole process of bubble evolution includes three stages: shock compression, equilibration, and expansion-rupture-ejection. When shock wave reaches the bubble, it is compressed and an internal jetting is formed, leading to the increases of velocity and internal pressure of the bubble. The internal pressure then reduces slightly due to the relaxation of helium atoms. After the reflected rarefaction wave reaching the bubble, the internal pressure greatly decreases to several GPa and the volume expands a lot. Because of the pressure gradient between the bubble and the free surface, the velocity of the bubble increases again, squeezing the upper thin metal layer continuously and finally making it plastically fail or melt. Then the bubble ruptures, ejecting helium atoms and some Cu clusters. Impact strength has significant effects on this dynamic process. Higher impact velocity leads to stronger jet and more significant expansion and rupture of the bubble. The latter is attributed to the microstructure evolution of the surrounding metal atoms and the upper metal layer, for which plastic failure or melt is observed at higher impact velocity. The influences of helium bubble size and number ratio of He/vacancy are also investigated. We find that the internal jet is stronger and temperature rise and local melting of metal is more obvious for larger bubble, resulting in easier failure of the metal layer and thus more remarkable expansion and rupture of the bubble. Higher number ratio results in weaker internal jet but stronger expansion and rupture of the bubble. This is because more helium atoms in the bubble for higher ratio can impact on the upper metal layer, leading to easier failure of the metal layer.
Metal plates containing penetrating gap may eject high velocity material outward from the gap opening under strong impact loading. Experiments have observed that some facts significantly affect the gap ejection behavior, such as metal properties, gap size, and loading method. Since the shape of the gap is the strip with a much greater depth than width, the mature instability theory is hard to apply, and it is difficult to accurately predict the mass and the velocity of the ejecta under this situation. In this paper, we develop a model for this case based on the shaped charge jet theory. The modeling is divided into two parts: the approximately steady jet formed by the long distance closure of gap in the depth direction, and the overturning of the interface after the gap closure reaches the surface. The theoretical model can precisely predict the total mass and maximum velocity of the ejecta. Thereafter, the verification of the theoretical model is carried out with the experiments and the simulation of the detonation-driven lead and copper metal plates containing penetration gaps. The total mass and maximum velocity of the ejecta obtained from the theoretical model agree well with the experimental and simulation results. The experimental phenomenon of the needle-like and mushroom-like ejecta formation is interpreted by the jet incoherence theory, and we proposed a method to determine the ejection coefficient of the theoretical model from this. Finally, a theoretical estimate model for metal gap ejection under sliding detonation loading is presented. The model in this paper can also be applied to the metal gap ejection phenomena formed by non-penetrating elongated gaps closure that satisfying the conditions of steady jet formation.
Understanding dynamic fragmentation in shock-loaded metals and predicting properties of the resulting ejecta are of considerable importance for both basic and applied science. The nature of material ejection has been shown to change drastically when the free surface melts on compression or release. In this work, we present hydrodynamic simulations of laser-driven microjetting from micron-scale grooves on a tin surface. We study microjet formation across a range of shock strengths from drives that leave the target solid after release to drives that induce shock melting in the target. The shock-state particle velocity ( U p) varies from 0.3 to 3 km/s and the shock breakout pressure is 3–120 GPa. The microjet tip velocity is 1–8 km/s and the free-surface velocity varies from 0.1 to 5 km/s. Two tin equations of state are examined: a “soft” model (LEOS 501) where the target melts for U p > 1 km/s and a more detailed multiphase model (SESAME 2161) that melts for U p > 1.4 km/s. We use these two models to examine the influence of phase change and the choice of the material model on microjet formation and evolution. We observe in our computational results that jet formation can be classified into three regimes: a low-energy regime where material strength affects jet formation, a moderate-energy regime dominated by the changing phase of tin material, and a high-energy regime where results are insensitive to the material model and jet formation is described by an idealized steady-jet theory. Using an ensemble of 2D simulations, we show that these trends hold across a wide range of drive energies and groove angles.
The effect of helium (He) concentration on ejecta production in OFHC-Copper was investigated using Richtmyer–Meshkov Instability (RMI) experiments. The experiments involved complex samples with periodic surface perturbations machined onto the surface. Each of the four target was implanted with a unique helium concentration that varied from 0 to 4000 appm. The perturbation’s wavelengths were λ ≈ 65 μ m, and their amplitudes h 0 were varied to determine the wavenumber ( 2 π / λ ) amplitude product k h 0 at which ejecta production beganfor Cu with and without He. The velocity and mass of the ejecta produced was quantified using Photon Doppler Velocimetry (PDV) and Lithium-Niobate (LN) pins, respectively. Our results show that there was an increase of 30% in the velocity at which the ejecta cloud was traveling in Copper with 4000 appm as compared to its unimplanted counterpart. Our work also shows that there was a finer cloud of ejecta particles that was not detected by the PDV probes but was detected by the early arrival of a “signal” at the LN pins. While the LN pins were not able to successfully quantify the mass produced due to it being in the solid state, they did provide information on timing. Our results show that ejecta was produced for a longer time in the 4000 appm copper.
The surface jets induced by an explosion below an immersed gas bubble in water are investigated experimentally. Typical phenomena including the bubble evolution and the jet formation are observed through high-speed photography. It is found that the inner jet resulting from the shock bubble interaction is the main cause of the surface jet. The velocity of the surface jet decreases with the initial depth of the bubble, and there exists a maximum bubble depth above which no surface jet occurs.
Most previous development of the peridynamic theory has assumed a Lagrangian formulation, in which the material model refers to an undeformed reference configuration. In the present work, an Eulerian form of material modeling is developed, in which bond forces depend only on the positions of material points in the deformed configuration. The formulation is consistent with the thermodynamic form of the peridynamic model and is derivable from a suitable expression for the free energy of a material. It is shown that the resulting formulation of peridynamic material models can be used to simulate strong shock waves and fluid response in which very large deformations make the Lagrangian form unsuitable. The Eulerian capability is demonstrated in numerical simulations of ejecta from a wavy free surface on a metal subjected to strong shock wave loading. The Eulerian and Lagrangian contributions to bond force can be combined in a single material model, allowing strength and fracture under tensile or shear loading to be modeled consistently with high compressive stresses. This capability is demonstrated in numerical simulation of bird strike against an aircraft, in which both tensile fracture and high pressure response are important.
Sn ejecta particle-size distributions from the break-up of a microsheet in a vacuum will be presented. The micro-sheet was created from a high explosive driven shock wave passing through a precision groove machined into a Sn sample. The particle-size measurements were carried out using an ultraviolet in-line Fraunhofer holography diagnostic. The diagnostic will be presented along with particle-size distributions over most of the ejecta velocities throughout the microsheet. © 2017, Society for Experimental Mechanics, Inc (outside the US).
The evidence and the understanding of the predominant physical mechanisms which control the surface phenomena induced by shock wave reflection on a metallic sample free surface, require a comprehensive experimental approach. Optical shadowgraphy with a high speed framing camera associated with an electronic flash, 600 kV X-ray transmission analysis and the Laser Doppler Interferometry (LDI) thin foil method have been developed in our laboratory. Shock loading of samples has been carried out with a 60 mm in diameter powder gun providing a pressure range from 5 to 40 GPa or with explosive generators providing a pressure range between 40 GPa and 60 GPa. These experimental studies are mainly concerned by the matter ejection from the free surface and the bulk microspalling of the sample near the free surface. These two surface phenomena essentially depend on sample melting in release and also on other parameters such as the incident shock level, the nature and the thickness of the sample and the rugosity of the free surface.
Based on the cluster multiple labeling technique, a novel cluster detection algorithm is presented as an analysis subroutine in two-and three-dimensional molecular dynamic simulations of ejecta that take place as a planar shock wave encounters a free metal surface. The algorithm is described, tested, and used to detect cluster distribution of ejecta from copper and aluminum under a shock loading. The information obtained about the size, distribution, evolution of the cluster is helpful in the understanding of ejection.
According to smoothed particle hydrodynamics, we investigate numerically the micro-jets from grooved surfaces of different metals, where the velocities of jet head and material sources are discussed in detail. Our simulation results suggest that the jetting factor reaches its maximum at the half angle 45 degree, and the jetting factor reduces with the increase or decrease of the groove angle; also, the maximum velocity of jet shows a linear reduction with the increase of groove angel. Those results are consistent with the corresponding experimental results. The jetting material source and its dynamical process are analyzed. It is shown that with the increase of groove angel, the jet material sources transfer to the bottom of groove from two-side layer, while at the groove angel near 45 degree, a homogenous source layer throughout the groove comes into being. Finally, we further explain the jetting dynamics from different grooves by particle trajectory and its mechanical quantity history.
Ejecta mixing takes place at the interface between metal and gas under shock loading, i.e., the transport process of ejecta from metal surface appears in the gas. In this paper, adopting disperse particles instead of the initial ejecta, we simulate the ejection mixing process according to two-phase flow of gas and particle. We give the numerical results of the evolution process of the mixing, and analyze the effects of initial gas pressure and particle size on the mixing zone. The pneumatic break is observed from the numerical simulations, which can lead to evident reduction of the particle and then become an important factor affecting the evolution of mixture; also, our simulations are consistent with the corresponding measurements, showing that the gas and particle two-phase flow model is an effective method to simulate the ejection mixing.
Via molecular dynamics simulations employing an embedded-atom-method potential, we investigate the microscopic process and dynamical properties of shock-induced micro-jet from a grooved aluminum surface. For a large range of shock pressure, we obtain the micro-jet morphology variation, its mass spatial distribution and mass-velocity distribution. The amorphous state and release melting during the jetting are both analyzed using the central symmetry parameter, where the effect law of release melting on the micro-jet is obtained. It is found that the micro-jet mass keeps a linear increase with the piston velocity prior to release melting; the micro-jet mass is enhanced evidently after release melting; while the velocity of release melting is above a threshold, the jetting mass shows a linear increase with the piston velocity again, where the strength of material can be neglected.
Employing an embedded-atom-method potential and molecular dynamics simulations, we have simulated the microscopic process and dynamical properties of the dynamic failure of metal Al specimens under triangular wave loading. The microstructure evolution of the sample is analyzed using the central symmetry parameter, while the difference of morphology between non molten and molten states is also explained. The pressure profiles were calculated based on the virial theorem, and the results show that the tensile strength of the material is decreased considerably in its molten state. Using the simulation results for different impact velocities, we discuss the variation of morphology and density distribution, from which the change of damage depth in the process from non molten to molten states is obtained. Our simulations also suggest that: the tensile strength of material derived from acoustic approximation is distinctively higher than the peak of internal stress from virial theorem for the melted state.
A metal plate subjected to a shock (tin, 10 GPa) undergoes a variety of damages such as spalling or the ejection of a cloud of particles. Two main mechanisms govern the formation of this cloud: the micro-jetting and the melting under shock. Photonic Doppler Velocimetry (PDV, a.k.a. LDV or het-V) is a multi-velocity time-resolved diagnostic. Developed from 2000s, the all-fibered conception makes its integration easy into shock experiments. The purpose of the thesis is to describe the contributions of PDV systems for high-velocity (several km/s) particle-cloud characterization, including micro-jetting cloud.This document presents a state of the art of shock generators, diagnostics and (numerical and experimental) studies involved in metallic micro-machined jetting. An extensive study of a PDV system is proposed. It leads to the definition of time-velocity spectrogram, evaluated in units of collected power, and a detectivity limit. Thanks to photon diffusion models, a threshold in the diameter of the measured particle is estimated. A PDV spectrogram simulation program is shown within the framework of particle clouds. Finally, several experimental campaigns are exposed. They emphasize the remarkable capacities of the system; results are compared to simulations. Diameter distributions are inferred using slowing down in air or in other gazes. Some radiometric analyses are also performed.
Molecular dynamics simulations are employed to examine the relation between ejecta production and shock-breakout pressure for single crystal Pb subjected to a decaying shockwave loading. To better understand the physical mechanism of ejecta formation, a surface with multiple triangular grooves representing the imperfections left from machining finish is taken into consideration. It is found that the ejecta volume density distribution displays a smooth nature and the amount of ejecta increases significantly after melting on release or shock. Additionally, the ejecta particle mass distribution is captured by a power law scaling, revealing the self-similarity. These results are in reasonable agreement with the characteristics of experimentally diagnosed findings. (C) 2014 AIP Publishing LLC.
Using molecular dynamics methods, we simulate and compare the microjetting from a grooved Al surface induced by supported and unsupported shocks at different breakout pressures. Via the analysis on the microjetting morphologies and mass distributions, we find that the threshold of shock breakout pressure for the microjetting formation is almost same, but the variation of microjet mass with shock pressure shows a great difference for the two loading patterns. Under supported shock loading, the microjet mass keeps a continuous increase with increasing shock pressure, and release melting can enhance it markedly. By contrast, the microjet mass under unsupported shocks is smaller and seems no remarkable increase with shock pressure in our simulations (at extremely short pulses), implying the shock decaying can weaken the microjetting. Of course, a large area of fragments near the surface may form in this case. The microjet source distributions corresponding to supported and unsupported shocks are presented. It is found that the former becomes apparently broader than the latter with increasing shock pressure. Besides, the microjet tip velocity under supported shocks may appear a reduction because of the material strength effect below release melting. While under unsupported shocks, all the microjets in solid and melted states will experience the reduction of tip velocity. These decrements of tip velocity can be fitted by an exponential function.
We have investigated the failure modes of single crystal aluminium under decaying shock loading by using molecular dynamics simulations. The microstructure evolution during the failure is presented in terms of the central symmetry parameter, and the corresponding pressure and temperature profiles are calculated and discussed. These results explain the failure morphology and mechanical properties under dynamic tension and especially the difference between solid and melted states. In addition, the fracture strength of aluminium is analyzed from surface velocity within acoustic approximation and virial theorem.
Molecular dynamics simulations have been used to study the microjet from a grooved aluminium surface under shock loading. Plastic deformation and release melting during microjetting are both presented by the centrosymmetry parameter, where the effect of release melting is discussed in detail. Consequently, we obtain the change law of microjet morphology and mass with the shock strength. The microjet mass is found to keep a linear increase with the post-shock particle velocity prior to release melting, and the release melting can evidently enhance the microjet. However, while the release melting speed is fast, the microjet mass shows a linear increase again, because the material strength can already be neglected. Also, our simulations suggest that the head speed of microjet always keeps a linear increase with the post-shock particle velocity, nearly independent of melting. Finally, the mechanical evolution of microjet matter with time is also discussed.
We describe a simple algebraic model for the particulate spray that is ejected from a shocked metal surface based on the nonlinear evolution of the Richtmyer-Meshkov instability (RMI). The RMI is a shock-driven hydrodynamic instability at a material interface in which the dense and tenuous fluids penetrate each other as spikes and bubbles, respectively. In our model, the ejecta areal density is determined by the product of the post-shock metal density and the saturated bubble amplitude, which depends on both the amplitude and wavelength of the initial surface imperfections of the metal. The maximum ejecta velocity is determined by the ever-growing spikes, which are accelerated relative to the RMI growth rate by the spatial harmonics that sharpen them. The model is formulated to fit new hydrodynamics and molecular dynamics simulations of the RMI and validated by existing ejecta experiments over a wide range of material properties, shock strengths, and surface perturbations. The results are also contrasted with existing ejecta source models
We investigate the quantitative reliability and precision of three different piezoelectric technologies for measuring ejected areal mass from shocked surfaces. Specifically we performed ejecta measurements on Sn shocked at two pressures, P ≈ 215 and 235 kbar. The shock in the Sn was created by launching a impactor with a powder gun. We self-compare and cross-compare these measurements to assess the ability of these probes to precisely determine the areal mass ejected from a shocked surface. We demonstrate the precision of each technology to be good, with variabilities on the order of ±10%. We also discuss their relative accuracy.
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