Dean L. Preston

Model of plastic deformation for extreme loading conditions

We present a model of metallic plastic flow suitable for numerical simulations of explosive loading and high velocity impacts. The dependence of the plastic strain rate on applied stress at low strain rates is of the Arrhenius form but with an activation energy that is singular at zero stress so that the deformation rate vanishes in that limit. Work hardening is modeled as a generalized Voce law. At strain rates exceeding 109 s−1, work hardening is neglected, and the rate dependence of the flow stress is calculated using Wallace’s theory of overdriven shocks in metals [D.C. Wallace, Phys. Rev. B 24, 5597 (1981); 24, 5607 (1981)]. The thermal-activation regime is continuously merged into the strong shock limit, yielding a model applicable over the 15 decades in strain rate from 10−3 to 1012 s−1. The model represents all aspects of constitutive behavior seen in Hopkinson bar and low-rate data, including a rapid increase in the constant-strain rate sensitivity, with 10% accuracy. High-pressure behavior is co...

Charged particle motion in a highly ionized plasma

Abstract A recently introduced method utilizing dimensional continuation is employed to compute the energy loss rate for a non-relativistic particle moving through a highly ionized plasma. No restriction is made on the charge, mass, or speed of this particle. It is, however, assumed that the plasma is not strongly coupled in the sense that the dimensionless plasma coupling parameter g = e 2 κ D / 4 π T is small, where κ D is the Debye wave number of the plasma. To leading and next-to-leading order in this coupling, d E / d x is of the generic form g 2 ln [ Cg 2 ] . The precise numerical coefficient out in front of the logarithm is well known. We compute the constant C under the logarithm exactly for arbitrary particle speeds. Our exact results differ from approximations given in the literature. The differences are in the range of 20% for cases relevant to inertial confinement fusion experiments. The same method is also employed to compute the rate of momentum loss for a projectile moving in a plasma, and the rate at which two plasmas at different temperatures come into thermal equilibrium. Again these calculations are done precisely to the order given above. The loss rates of energy and momentum uniquely define a Fokker–Planck equation that describes particle motion in the plasma. The coefficients determined in this way are thus well-defined, contain no arbitrary parameters or cutoffs, and are accurate to the order described. This Fokker–Planck equation describes the straggling—the spreading in the longitudinal position of a group of particles with a common initial velocity and position—and the transverse diffusion of a beam of particles. It should be emphasized that our work does not involve a model, but rather it is a precisely defined evaluation of the leading terms in a well-defined perturbation theory.

Melting as a dislocation-mediated phase transition

We present a theory of the melting of elements as a dislocation-mediated phase transition. We model dislocations near the melt as noninteracting closed loops on a lattice. In this framework we derive simple expressions for the melting temperature and latent heat of fusion that depend on the dislocation density at melt. We use experimental data for more than half the elements in the periodic table to determine the dislocation density from both relations. Melting temperatures yield a dislocation density of (0.61{+-}0.20)b{sup -2}, in good agreement with the density obtained from latent heats, (0.66{+-}0.11)b{sup -2}, where b is the length of the smallest perfect-dislocation Burgers vector. Melting corresponds to the situation where, on average, half of the atoms are within a dislocation core. (c) 2000 The American Physical Society.

Solving φ1,24 field theory with Monte Carlo

We study lattice g0φ4 field theory for all g0 and fixed renormalized mass M in one and two dimensions using Monte Carlo techniques. We calculate the dimensionless renormalized coupling constant gR = gRM4−d, where d is the dimension of space—time, at fixed small values of the lattice spacing a for various g0 and lattice sizes. Our results are in quantitative agreement with the analyses of high temperature and strong coupling series which rely on extrapolation from large to small lattice spacing.

Analysis of dislocation mechanism for melting of elements: Pressure dependence

In the framework of melting as a dislocation-mediated phase transition we derive an equation for the pressure dependence of the melting temperatures of the elements valid up to pressures of order their ambient bulk moduli. Melting curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar, Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated melting curves are in good agreement with existing data. We also discuss the apparent equivalence of our melting relation and the Lindemann criterion, and the lack of the rigorous proof of their equivalence. We show that the would-be mathematical equivalence of both formulas must manifest itself in a new relation between the Gruneisen constant, bulk and shear moduli, and the pressure derivative of the shear modulus.

An analytic model of the shear modulus at all densities and temperatures

An analytic model of the shear modulus applicable at temperatures up to melt and at all densities is presented. It is based in part on a relation between the melting temperature and the shear modulus at melt. Experimentaldata on argon are shown to agree with this relation to within 1%. The model of the shear modulus involves seven parameters, all of which can be determined from zero-pressure experimental data. We obtain the values of these parameters for 11 elemental solids. Both the experimental data on the room-temperature shear modulus of argon to compressions of ∼2.5, and theoretical calculations of the zero-temperature shear modulus of aluminum to compressions of ∼3.5 are in good agreement with the model. Electronic-structure calculations of the shear moduli of copper and gold to compressions of 2, performed by us, agree with the model to within uncertainties.

A model of the shear modulus

The shear modulus is modeled at all temperatures and densities. Parameters are provided for twenty-two metals.

Analysis of dislocation mechanism for melting of elements

The melting of elemental solids is modeled as a dislocation-mediated transition on a lattice. Statistical mechanics of linear defects is used to obtain a new relation between melting temperature, crystal structure, atomic volume, and shear modulus that is accurate to 17% for at least half of the Periodic Table.

The study of high-speed surface dynamics using a pulsed proton beam

We present experimental results supporting physics based ejecta model development, where we assume ejecta form as a special limiting case of a Richtmyer-Meshkov (RM) instability with Atwood number A = -1. We present and use data to test established RM spike and bubble growth rate theory through application of modern laser Doppler velocimetry techniques applied in a novel manner to coincidentally measure bubble and spike velocities from shocked metals. We also explore the link of ejecta formation from a solid material to its plastic flow stress at high-strain rates (

Explosively driven two-shockwave tools with applications

107/s) and high strains (700%).