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Ding J.L.,Washington State University | Asay J.R.,Ktech Corporation | Ao T.,Sandia National Laboratories
Journal of Applied Physics | Year: 2010

In a previous study by Asay [J. Appl. Phys. 106, 073515 (2009)], the inelastic response of annealed and cold-rolled pure polycrystalline tantalum at intermediate strain rates was characterized with ramp wave loading to peak longitudinal stresses of 17 GPa. It was found that the annealed Ta at strain rates of about 106/s exhibited pronounced elastic overshoot, followed by rapid stress relaxation and the amplitude of the elastic precursor depicted essentially no dependence on sample thickness for samples with controlled initial properties, in contrast to the precursor attenuation typically observed in shock wave experiments. The precursor for the cold-rolled sample was more dispersive and did not exhibit the characteristics depicted by the annealed samples. A principal objective of the present study was to gain some insights into this behavior and its implication on the deformation mechanisms for tantalum. Another objective was to gain a fundamental understanding of the dynamic inelasticity of polycrystalline tantalum, its evolution with the processing history, and the resultant thermomechanical behavior. The approach used to achieve these objectives was to first develop a material model that captured the observed material characteristics and then to use numerical simulations of dynamic experiments to gain additional insights into the observed material behavior. The constitutive model developed is based on the concept of dislocation generation and motion. Despite its simplicity, the model works quite well for both sets of data and serves a valuable tool to achieve the research objectives. The tantalum studied here essentially exhibits a strong rate sensitivity and this behavior is modeled through the low dislocation density and the strong stress dependence of the dislocation velocity. For the annealed material, the mobile dislocation density is assumed to be essentially zero in the model. This low dislocation density combined with strong stress dependence of dislocation velocity results in a metastable elastic response and a precursor that shows little attenuation. The increase of mobile dislocations through the cold-rolling process leads to a less rate-sensitive behavior for the cold-rolled tantalum and also the disappearance of the precursor behavior observed for the annealed samples. Both the low dislocation density and the strong rate dependence of the dislocation velocity may be related to the low mobility of the screw dislocations in bcc metals. This low mobility results from its extended, three-dimensional core structure. © 2010 American Institute of Physics.


Ding J.L.,Washington State University | Asay J.R.,Ktech Corporation
Journal of Applied Physics | Year: 2011

In a previous study, the behavior of single crystal tantalum under ramp wave loading along the [100] and [110] orientations was characterized. The principal objective of the present study is to gain some insights on the observed single crystal behavior particularly on its precursor response and strong orientation dependence, and the implication of the macroscopic behavior on the possible underlying deformation mechanisms. The approach used to achieve this objective is through the material model development and numerical simulation. A continuum model developed in a previous work for polycrystalline tantalum was first modified to describe the experimental data and extract the material information associated with the data. A rigorous finite deformation single crystal model based on dislocation slip was then developed to gain physical insights into the possible deformation mechanisms. The slip systems considered were the {110}〈111〉 and {112}〈111〉 systems. Dislocation density and its evolution by nucleation or multiplication were incorporated as a key mechanism for describing the precursor behavior in both models. The orientation dependence was modeled through the assumption of anisotropic dislocation nucleation. In the continuum model, different nucleation rates were assumed for the [100] and [110] orientation. In the single crystal model, this anisotropy is assumed to be associated with the twinning/ antitwinning asymmetry of the BCC crystals. The precursor for the [100] orientation is attributed mainly to the slips along the antitwinning direction and that for the [110] is to the slips along the twinning direction. The anisotropic dislocation nucleation leads to the orientation dependence of the rate sensitivity of single crystal Ta and the subsequent deformation behavior. Both models were demonstrated to be able to generate reasonably consistent results and to capture the observed material features. Through the developed models, a reasonable understanding was achieved for the evolution of stress, strain, strain rates, strength, temperature, and stress strain relations for single crystal tantalum under ramp wave loading and the possible correlation between the macroscopic behavior and microscopic deformation mechanisms. © 2011 American Institute of Physics.


Wolfer W.G.,Ktech Corporation | Wolfer W.G.,Sandia National Laboratories
Acta Materialia | Year: 2011

Lattice parameter changes in nanoparticles can be used to determine the surface stress of solids. In the past a Laplace-Young relationship has been employed to interpret the lattice parameter changes as a function of the particle size. In the meantime, however, atomistic calculations revealed a purely mechanical origin of the surface stress that is consistent with elasticity theory for solid surfaces as developed by Gurtin and Murdoch. In this theory the equilibrium distance for surface atoms may differ from that in the bulk solid, and the elastic properties of the surface layer may also deviate from bulk values. We apply this Gurtin-Murdoch theory to spherical nanoparticles and reanalyze past data as well as results from recent theoretical calculations on lattice parameter changes, thereby enabling us to determine surface properties commensurate with the mechanical interpretation of surface stress. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


Cochrane K.R.,Ktech Corporation | Desjarlais M.P.,Sandia National Laboratories | Mattsson T.R.,Sandia National Laboratories
AIP Conference Proceedings | Year: 2012

An accurate equation of state (EOS) for polyethylene is required in order to model high energy density experiments for CH2 densities above 1 g/cc, temperatures above 1 eV, and pressures above 1 Mbar. Density Functional Theory (DFT) based molecular dynamics has been established as a method capable of yielding high fidelity results for many materials at a wide range of pressures and temperatures and has recently been applied to complex polymers such as polyethylene [1]. Using high density polyethylene as the reference state, we compute the principal Hugoniot to 350 GPa, compression isentrope, and several release isentropes from states on the principal Hugoniot. We also calculate the specific heat and the dissociation along the Hugoniot. Our simulation results are validated by comparing to experimental data [2, 3] and then used to construct a wide range EOS. © 2012 American Institute of Physics.


Bulatov V.V.,Lawrence Livermore National Laboratory | Wolfer W.G.,Ktech Corporation | Kumar M.,Lawrence Livermore National Laboratory
Scripta Materialia | Year: 2010

Recently it was proposed that voids in crystals could grow by emission of shear dislocation loops [V.A. Lubarda, M.S. Scheider, D.H. Kalantar, B.A. Remington, M.A. Meyers, Acta Materialia 52 (2004) 1397-1408]. Even more recently, this proposal was ostensibly supported by molecular simulations of voids in strained single crystals [S. Traiviratana, E.M. Bringa, D.J. Benson, M.A. Meyers, Acta Materialia 56 (2008) 3874-3886]. The purpose of this comment is to dispute this recent assertion as unfounded. © 2010 Acta Materialia Inc.


Mattsson T.R.,Sandia National Laboratories | Lane J.M.D.,Sandia National Laboratories | Cochrane K.R.,Ktech Corporation | Desjarlais M.P.,Sandia National Laboratories | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Density functional theory (DFT) molecular dynamics (MD) and classical MD simulations of the principal shock Hugoniot are presented for two hydrocarbon polymers, polyethylene (PE) and poly(4-methyl-1-pentene) (PMP). DFT results are in excellent agreement with experimental data, which is currently available up to 80 GPa. Further, we predict the PE and PMP Hugoniots up to 350 and 200 GPa, respectively. For comparison, we studied two reactive and two nonreactive interaction potentials. For the latter, the exp-6 interaction of Borodin showed much better agreement with experiment than OPLS. For the reactive force fields, ReaxFF displayed decidedly better agreement than AIREBO. For shocks above 50 GPa, only the DFT results are of high fidelity, establishing DFT as a reliable method for shocked macromolecular systems. © 2010 The American Physical Society.


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 99.99K | Year: 2010

Our recent developmental efforts have successfully produced the design for a chip slapper with a reduction in the average size (miniaturization), weight and cost of these particular chip slappers. The processes that we have established in the production of this chip slapper have reduced the number of assembly stages for this particular assembly configuration and/or application. Production of this style chip slapper has also been modified with the reduction of manufacturing stages and more stringent quality controls during the manufacturing, providing a more robust cost effective and smaller component. We propose to reduce the standard size chip slapper by approx. 84% with acceptable industry standard practices. The following is a review of standard industry manufacturing techniques for the production of the chip slappers. We have included our proposal for a smaller chip relative to the process design we have come up with. We believe that we have also increased yield and not compromised the quality of these components but have in fact improved the performance. There are several unique practices which we feel have improved our production, miniaturization, and cost reduction all of the following technical details are proprietary. BENEFIT: Improved guidance and navigation has given us the ability to deliver precision effects, and has allowed us to reduce our ordnance payloads and, thereby, reduce collateral effects. This trend toward ever smaller munitions is being driven by our need to conduct military operations in urban terrain and to severely control collateral effects. The goal in this effort is to further miniaturize ordnance components (damage mechanisms, energetic, fusing) for delivery by micro air vehicles. Damage mechanisms other than blast/fragmentation may be proposed.


Grant
Agency: Department of Defense | Branch: Army | Program: SBIR | Phase: Phase II | Award Amount: 382.38K | Year: 2011

The US Army has programs that require very compact explosive drive power supplies. One such power supply is the Flux Compression Generator (FCG). Flux compression generators convert the chemical energy of explosives into electrical energy by compressing an initial magnetic field. A major advantage of FCGs is that they can be relatively small and can fit into platforms of interest, unlike conventional power supplies such as battery powered Marx generators. Unfortunately, as the size of FCGs decrease, they have higher losses due, in part, to size and tolerance scaling. However, it may be possible to take advantage of this characteristic for higher losses to couple part of the energy that would have been lost out of the flux-trapped region into Radio Frequency (RF) energy. For Army platforms of interest, the geometries of interest are 1.5 inches (40 mm) in diameter and 1 inch (25 mm) in length. We propose to explore innovative ways to convert energy typically lost due to mechanisms such as stator clocking and flux pocketing.


Grant
Agency: Department of Defense | Branch: Missile Defense Agency | Program: SBIR | Phase: Phase II | Award Amount: 999.79K | Year: 2010

Design and build compact, pulsed power devices for High Power Microwave (HPM) Payloads for small rockets and/or the Multiple Kill Vehicles (MKVs).The overall requirements for this solicitation are very demanding. Overall, an explosive pulsed power system is required to be able to drive an RF source. This eventually will include prime power, the generator, and a pulse forming circuit. While the design and test of a explosive generator that does not produce debris was the main focus of the Phase I effort, we proposed and worked toward bringing along the energy storage capacitor as well. In Phase II, we will continue this parallel development. For the explosive generator, the requirements are extremely challenging. These may be summarized as four separate requirements to be satisfied within one design. 1. The system must be able to power an HPM source, implying the generation of 10’s to 100’s of joules of energy. 2. It must be compact enough to fit within a 3” diameter and 24” long enclosure and be capable of G hardening for the anticipated missions. 3. The system must be low mass to satisfy the needs of the small smart communicating kill vehicles, such as the Multiple Kill Vehicle (MKV). 4. The system, once initiated, must produce little or no debris. These are each very challenging requirements. However, to achieve all of them simultaneously is a challenge indeed! As noted in the solicitation, the “current FCG designs are not satisfactory and new innovative designs are required.”


Grant
Agency: Department of Defense | Branch: Army | Program: SBIR | Phase: Phase I | Award Amount: 69.98K | Year: 2010

The US Army has programs that require very compact explosive drive power supplies. One such power supply is the Flux Compression Generator (FCG). Flux compression generators convert the chemical energy of explosives into electrical energy by compressing an initial magnetic field. A major advantage of FCGs is that they can be relatively small and can fit into platforms of interest, unlike conventional power supplies such as battery powered Marx generators. Unfortunately, as the size of FCGs decrease , they have higher losses due, in part, to size and tolerance scaling. However, it may be possible to take advantage of this characteristic for higher losses to couple part of the energy that would have been lost out of the flux trapped region into Radio Frequency (RF) energy. For Army platforms of interest, the geometries of interest are ¡Ý1.5 inches (40 mm) in diameter and ¡Ý1 inch (25 mm) in length. We propose to explore innovative ways to convert energy typically lost due to mechanisms such as stator clocking and flux pocketing.

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