Hypervelocity acceleration and impact experiments with the LLNL electric guns

In the electric gun, the explosion of an electrically heated metal foil and the accompanying magnetic forces are used to accelerate a thin flyer plate to velocities as high as 18 km/s. Here we report preliminary results of a study to extend this capability to the acceleration of projectiles or particles ranging in size from micrometeorites to chunky projectiles with a mass as high as 0.5 g. We also have started code calculations of projectile impacts on thick aluminum witness plates for comparison with observations of experimentally produced cratering.     

Sensitivity of hypervelocity impact simulations to equation of State

We investigate the sensitivity of hypervelocity impact simulations to differences in equations of state (EOS's), comparing our hydrocode calculations to recent experiments on lead at G. M. Delco (Pomykal, 1986). The results suggest that the hydrocode does not correctly predict all the details of debris cloud structure, even when these are significant EOS differences. We used three different equations of state, one of which is new. It was generated just for sensitivity studies using the most recent experimental EOS data.     

Mechanochemical reaction at the interface between a metal plate and oxide powders

A new geometry is proposed and explored for the investigation of mechanochemical reactions. Typical displacement reactions between an oxide (PbO, CuO, and WO₃) and Al as the reducing metal are studied, but instead of starting with a mixture of two powder reactants, Al is introduced in the form of a plate attached to the inside wall of the milling vial and only the oxide is milled in powder form. Consequently, the reaction takes place at the surface of the aluminium plate, where the microstructure can be investigated easily using SEM. The phase composition is followed by XRD. Of the three oxide components, the reaction is the fastest with PbO due to the intense ductile mixing at the interface. The current configuration can be utilized for the investigation of any mechanochemical or mechanical alloying process where at least one of the components can be prepared in the form of a ductile plate that is capable of withstanding the multiple impacts from the milling balls.     

The universal equation of state applied to analysis of EOS data for solid molybdenum and tungsten

The universal equation of state of solids (UEOS) proposed by Vinet is applied to analysis of EOS data for Mo and W in literature obtained experimentally and by ab initio theoretical calculations reported by different groups. It is demonstrated that the experimental measurements and theoretical calculations are consistent with each other and UEOS is a good analytical expression of EOS for solid Mo and W. The bulk modulus and thermo-expansion coefficients calculated from EOS analysis for Mo and W are in agreement with values obtained by direct measurements with other techniques with high accuracy.     

金属簧片阻尼隔振器性能分析

建立了金属簧片隔振器振动系统的动力学模型,通过有限元计算得出摩擦力曲线,进一步采用动力学方程研究了系统在简谐激励和冲击激励下的响应.结果表明:电子机柜采用该隔振器,能使最大加速度降低72%,说明该隔振器具有良好的减振隔振和抗冲击性能,为干摩擦隔振器优化设计提供了理论依据.     

Excitation of plate waves in thickness measurements of layers deposited on thin plates

Layer thickness measurements with an acoustic micrometer using pseudo-Sezawa waves in which ultrasonic waves are obliquely applied to a layered surface of a specimen have been proposed. A case in which the plate thickness of the specimen is so thin that it cannot be regarded as a half space is studied. A number of modes of plate waves are then excited in addition to pseudo-Sezawa waves. The plate waves, giving rise to the appearance of extra dips in the power spectrum of reflected waves, cause difficulties in the measurements. To prevent the excitation of plate waves, it is proposed that a mask of a sound-insulating material with a slit aperture should be placed on the layered surface of the specimen. Experiments and theoretical calculations, using lead frames of LSI chips as typical test specimens with thin substrates, were performed to demonstrate the effectiveness of the present method in preventing the excitation of plate waves.     

The effects of strength models in numerical study of metal plate destruction by contact explosive charge

Based on a complete mathematical model, the authors set up a problem of metal plate destruction by contacting explosive charge in highly nonlinear dynamic software AUTODYN and solved in two cases using the Johnson–Cook and von Mises strength models. The numerical simulation results were compared with the experimental results and showed a good fit of numerical calculations versus experiments by using the von Mises strength model. The study also shows that the Johnson–Cook strength model, if applied unreasonably, will lead to large errors, which would help to avoid mistakes in the future high speed impact study.     

The role of electromagnetic radiation in hypervelocity impact mechanics

This paper is based upon an acceptance presentation I made for the 1998 Distinguished Scientist award presented by the Hypervelocity Impact Society. Nearly thirty years ago, I recorded a cine sequence of a gaseous debris plume expanding behind a thin cadmium plate impacted by a cadmium sphere. The plume stagnated on a second plate where it glowed so brightly that it must have been heated to over 50,000 ^oK. The fact went unnoticed for nearly twenty years until it was predicted theoretically and then finally observed. The theoretical analysis showed that virtually all gaseous impact debris that is intercepted gives up its energy to electromagnetic radiation. Potential exists to use the resulting radiation for a variety of research opportunities. More recently, tungsten plasma accelerated electrically with a 10 MV pulse generator at Sandia National Laboratories has been imploded onto the axis of an evacuated cylindrical cavity where it stagnates against itself to produce enormous pulses of incoherent electromagnetic radiation. The intensity of the radiation is so great that it vaporizes and launches surface material off samples exposed to it. Extremely intense shockwaves are produced that are being used for extending material equation-of-state determinations well above pressure levels reachable by laboratory impacts.     

Metal nanolayer formed by tunnelling current through thin oxide in the electrolyte–oxide–silicon system

We propose a method of forming metal nanoparticles or layers on the oxide by tunnelling current of the EOS (electrolyte–oxide–silicon) system. Electrical characteristics of the metal layer and particles obtained experimentally by the proposed method are compared with the electrolyte–metal–oxide–silicon and the metal–oxide–silicon systems. Also, it is shown that the instability of the EOS system is caused by the H⁺ penetration into the oxide and is largely cured by applying alternative voltage to extract the H⁺ ions from the oxide. We show that the proposed technique can selectively deposit extremely thin metal layers on the active sites of the silicon surface in a self-alignment manner.     

Acoustic impact localization in plates: properties and stability to temperature variation

Localizing an impact generated by a simple finger knock on plate-shaped solid objects is made possible in an acoustic time reversal experiment. It is shown that the technique works with a single accelerometer. To better understand the phenomenon and to know exactly the nature of the created waves, a two-dimensional (2-D) elastic simulation is used, showing that in a very good approximation the A0 Lamb mode is the only propagating one. However, it is shown that, within one wavelength distance from the edges, evanescent waves must be taken into account. As a first consequence, the ability to distinguish two neighboring impacts improves when the plate thickness decreases and the frequency increases. As a second consequence, it is expected theoretically that temperature variations lead to a stretching or a contraction of acoustic signatures. The experimental demonstration used a heterodyne interferometer to measure the impulse responses created by a knock on a plate during the cooling. A simple algorithm is shown to perfectly compensate for temperature impacts, which demonstrates the feasibility of the technique for outdoor time reversal interactive experiments.     

Dispersion characteristics of the surface electromagnetic-spin waves in ferrite-ferroelectric-dielectric-metal structures

Dispersion characteristics of the surface hybrid electromagnetic-spin waves propagating in a layered structure comprising a ferrite film on a ferroelectric plate and a metal screen at a certain distance from the ferroelectric plate surface were experimentally studied. The experimental data are compared to the results of calculations performed within the framework of a previously developed theory. The theoretical and experimental dispersion characteristics show good coincidence.     

Shock evolution after shaped charge jet impact and its relevance to explosive initiation

When a shaped charge jet impacts a target containing explosive, the explosive may be initiated by one of three shocks: the impact shock, a bow shock that forms in the inert plate covering the exlosive, or a bow shock that forms in the explosive. In this paper, numerical calculations are used to determine how thick the cover plate must be to prevent initiation by the impact shock and how much time (or distance) is required to form a bow shock in the explosive. The results show that the cover plate must be from 4 to 12 jet diameters (depending on jet velocity) thick to sufficiently attenuate the impact shock so that it will not cause initiation in a common secondary explosive. For a 7-km/s copper jet, a distance of about 8 jet diameters was required to form a bow shock in the explosive. This corresponds well to experimental data reported elsewhere.     

A quasi-optical phase shifter based on an active grating for the 8-mm wavelength range

A new quasi-optical phase shifter for the 8-mm wavelength range, which allows the phase of electromagnetic radiation in a wave beam to be controlled, is described. The phase shifter operation is based on the phenomenon of wave phase variation upon reflection from an active grating that is formed by the layers of plasma in channels of a dielectric plate arranged above a flat metal mirror. It is shown that the phase of the reflected wave can be effectively controlled by changing the grating-mirror distance and the gas pressure in the channels. The experimental data are compared to the results of numerical calculations.     

Nondestructive laser-induced plasma formation at a target surface

The parameters of ultrashort laser pulses and the thermal characteristics of a metal target were determined, which provide that the laser-induced plasma formation at the target surface does not lead to damage of the metal surface. The results of calculations are compared to experimental data.     

Simulation of explosive welding using the Williamsburg equation of state to model low detonation velocity explosives

Explosive welding is a process for producing bi-metallic plates and tubes. Whilst well established it has been essentially an empirical process. In most welding operations, low-speed explosives such as ammonium nitrate fuel oil (ANFO) mixtures, are used. Such explosives have a low velocity of detonation with an appreciable detonation zone. The usual Jones–Wilkins–Lee equation of state (EOS) is not valid. A more representative EOS is the Williamsburg EOS.Recent work to numerically model the process is described. A notable advance is the use of the finite difference engineering package AUTODYN with a Williamsburg-type EOS to model low detonation velocity ANFO explosive and perlite mixtures. In this work, the Williamsburg EOS was coded as a subroutine in the AUTODYN software package which was then used to simulate most aspects of the explosive welding process. The computed results were validated by explosive welding trials.The phenomenon of jetting and the interfacial waves usually observed in explosive welding were successfully reproduced.     

Applicability of equations of state for modeling helium systems

Proper design of helium systems with large number of components and involved configurations such as helium liquefiers/refrigerators requires the use of tools like process simulators. The accuracy of the simulation results, to a great extent, depends on the accuracy of property data. For computation of thermodynamic properties of helium, the 32-parameter MBWR equation of state proposed by McCarty and Arp [1] is widely used. However, it is computationally involved, makes the simulation process more time-consuming and sometimes leads to computational difficulties such as numerical oscillations, divergence in solution especially, when the process operates over a wide thermodynamic region and is constituted of many components. Substituting MBWR EOS by simpler equations of state (EOS(s)) at selected thermodynamic planes, where the simpler EOS(s) have the similar accuracy as that of MBWR EOS may enhance ease of computation. In the present paper, the methodology to implement this concept has been elucidated with examples of steady state and dynamic simulation of helium liquefier/refrigerator based on Collins cycle. The above concept can be applied to thermodynamic analysis of other process cycles where computation of fluid property is involved.     

Melting of metallic and intermetallic solids: An energetic view from DFT calculated potential wells

Potential wells of metallic and intermetallic phases are constructed by ab initio calculation using the density functional theory. The potential wells are presented as a function of a characteristic length scale equivalent to the cubic root of the average atomic volume, to reflect the importance of density on binding. The results show that the characteristics of a potential well dictates the melting temperature of a crystalline solid metal or an intermetallic phase. The energetic requirement for melting is equivalent to an activation state where the slope of the potential well gets close to the positive maximum, or where the well experiences a step change in energetic state. The work offers a way to predict melting points of metallic/intermetallic phases, which is particularly useful to metastable phases, for which it is usually difficult to measure the melting points directly. Such predicted data on melting points are useful for establishing thermodynamic databases for phase diagram calculations.     

Analytic Equation of State for Lennard-Jones Fluids Based on the Ross Variational Perturbation Theory

By using a previously developed analytic expression for the radial distribution function of hard spheres, a simple analytic equation of state (EOS) for fluids with a continuous Lennard-Jones potential is established based on Ross's variational perturbation theory. The main thermodynamic quantities have been analytically derived, the resulting expressions are surprisingly simple, the variational procedure is greatly simplified, and the calculations are absolutely convergent. The numerical results are compared with the Monte-Carlo data and the original Ross variational theory. It is shown that the precision of the analytic EOS is as good as the original non-analytic one, and their applicable range is believed identical. A comparison with the recently proposed mean-sphere-approximation theory shows that the analytic equation of state developed here has wider applicability and precision.     

An alternative numerical approach for computing eddy currents: Case of the double-layered plate

A numerical approach is presented for cases where induced eddy current densities are to be computed in axisymmetric situations. Conductors are represented as stacks of inductively coupled, concentric, metal rings in which current is excited by an external coil. Matrix techniques are used to solve the resulting set of circuit equations. Sinusoidally induced currents in a sandwich plate consisting of two layers of different metals are discussed as an application. Comparison is made with a finite element calculation and with results obtained via an analytical approach. Nonsinusoidal excitation is also briefly discussed.     

Electro-mechanical vibration of smart piezoelectric FG plates with porosities according to a refined four-variable theory

Here, free vibration analysis of functionally graded piezoelectric (FGP) plates with porosities is carried out based on refined four-unknown plate theory. The present plate theory captures shear deformation impacts needless of shear correction factor. A modified power-law model is adopted to describe the graded material properties of a functionally graded piezoelectric plate. Implementing an analytical approach, which satisfies different boundary conditions, governing equations derived from Hamilton's principle are solved. The obtained results are compared with those provided in the literature. The impacts of applied voltage, porosity distribution, material graduation, plate geometrical parameters, and boundary conditions on vibration of porous FGP plate are discussed.