Implications of Shock Wave Experiments with Precompressed Materials for Giant Planet Interiors

This work uses density functional molecular dynamics simulations of fluid helium at high pressure to examine how shock wave experiments with precompressed samples can help characterizing the interior of giant planets. In particular, we analyze how large of a precompression is needed to probe a certain depth in a planet's gas envelope. We find that precompressions of up to 0.1, 1.0, 10, or 100 GPa are needed to characterized 2.5, 5.9, 18, to 63% of Jupiter's envelope by mass.Comment: Submitted As Proceedings Article For The American Physical Society Meeting On Shock Compression Of Condensed Matter, Hawaii, June, 200

Faunal studies of the type Chesteran, Upper Mississippian of southwestern Illinois

48 p., 7 pl., 4 fig.http://paleo.ku.edu/contributions.htm

Measuring Hugoniot, reshock and release properties of natural snow and simulants

We describe methods for measuring dynamical properties for underdense materials (e.g. snow) over a stress range of roughly 0. 1 - 4 GPa. Particular material properties measured by the present methods include Hugoniot states, reshock states and release paths. The underdense materials may pose three primary experimental difficulties. Snow in particular is perishable; it can melt or sublime during storage, preparation and testing. Many of these materials are brittle and crushable; they cannot withstand such treatment as traditional machining or launch in a gun system. Finally, with increasing porosity the calculated Hugoniot density becomes rapidly more sensitive to errors in wave time-of-arrival measurements. A family of 36 impact tests was conducted on snow and six proposed snow simulants at Sandia, yielding reliable Hugoniot states, somewhat less reliable reshock 3 states, and limited release property information. Natural snow of density {approximately}0.5 gm/cm{sup 3}, a lightweight concrete of density {approximately}0.7 gm/cm{sup 3} and a {open_quotes}snow-matching grout{close_quotes} of density {approximately}0.28 gm/cm 3 were the subjects of the majority of the tests. Hydrocode calculations using CTH were performed to elucidate sensitivities to edge effects as well as to assess the applicability of SESAME 2-state models to these materials. Simulations modeling snow as porous water provided good agreement for Hugoniot stresses to 1 GPa; a porous ice model was preferred for higher Hugoniot stresses. On the other hand, simulations of tests on snow, lightweight concrete and the snow-matching grout based on (respectively) porous ice, tuff and polyethylene showed a too-stiff response. Other methods for characterizing these materials are discussed. Based on the Hugoniot properties, the snow-matching grout appears to be a better snow simulant than does the lightweight concrete

Simulation of Particle Size Effect on Dynamic Properties and Fracture of PTFE-W-Al Composites

Recent investigations of the dynamic compressive strength of cold isostatically pressed composites of polytetrafluoroethylene (PTFE), tungsten (W) and aluminum (Al) powders show significant differences depending on the size of metallic particles. The addition of W increases the density and overall strength of the sample. To investigate relatively large deformations multi-material Eulerian and arbitrary Lagrangian-Eulerian methods, which have the ability to efficiently handle the formation of free surfaces, were used. The calculations indicate that the increased strength of the sample with fine metallic particles is due to the formation of force chains under dynamic loading. This phenomenon occurs even at larger porosity of the PTFE matrix in comparison with samples with larger particle size of W and higher density of the PTFE matrix.Comment: 5 pages, 6 figure

Atomistic simulations of dislocation mobility in Al, Ni and Al/Mg alloys

Dislocation velocities and mobilities are studied by Molecular Dynamics simulations for edge and screw dislocations in pure aluminum and nickel, and edge dislocations in Al-2.5%Mg and Al-5.0%Mg random substitutional alloys using EAM potentials. In the pure materials, the velocities of all dislocations are close to linear with the ratio of (applied stress)/(temperature) at low velocities, consistent with phonon drag models and quantitative agreement with experiment is obtained for the mobility in Al. At higher velocities, different behavior is observed. The edge dislocation velocity remains dependent solely on (applied stress)/(temperature) up to approximately 1.0 MPa/K, and approaches a plateau velocity that is lower than the smallest "forbidden" speed predicted by continuum models. In contrast, above a velocity around half of the smallest continuum wave speed, the screw dislocation damping has a contribution dependent solely on stress with a functional form close to that predicted by a radiation damping model of Eshelby. At the highest applied stresses, there are several regimes of nearly constant (transonic or supersonic) velocity separated by velocity gaps in the vicinity of forbidden velocities; various modes of dislocation disintegration and destabilization were also encountered in this regime. In the alloy systems, there is a temperature- and concentration-dependent pinning regime where the velocity drops sharply below the pure metal velocity. Above the pinning regime but at moderate stresses, the velocity is again linear in (applied stress)/(temperature) but with a lower mobility than in the pure metal.Comment: PDF, 30 pages including figures, submitted to Modelling Simul. Mater. Sci. En