Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session Y4: TM Continuum Modeling IV |
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Chair: Tim Germann, Los Alamos National Laboratory Room: Vashon |
Friday, July 12, 2013 9:15AM - 9:30AM |
Y4.00001: Combined Hydrodynamic and Diffraction Simulations of Femtosecond X-Ray Scattering from Laser-Shocked Crystals Justin Wark, Andrew Higginbotham, Despina Milathianaki, Arianna Gleason We describe a simple hydrocode based on a two-step integration scheme that models the evolution of elastic and plastic strains in crystals subject to rapid laser-shock loading. By monitoring the elastic strains during plastic flow we track the rotation and spacing of lattice planes within the polycrystalline sample, and can thus predict the signal that would be produced by X-ray diffraction in a variety of experimental geometries. By employing a simple Taylor-Orowan dislocation model we simulate diffraction patterns from in a Debye-Scherrer geometry to track the orthogonal strain states within a laser-shocked sample. The yielding rate is approximately matched to those observed in multi-million atom MD simulations, allowing movies to be made of the diffraction images that would be seen in a real experimental geometry, and illustrating the pertinent experimental requirements, including target texture. Judicious choice of geometry allows clear demarcation of the initial elastic response of the target to be made from the subsequent plastic relaxation. We discuss the simulations in the context of the novel experimental capabilities that have recently become available with the advent of 4th generation light sources, which allow single-shot diffraction with sub-100-fs resolution. [Preview Abstract] |
Friday, July 12, 2013 9:30AM - 9:45AM |
Y4.00002: Modelling of the Pele Fragmentation Dynamics Jimmy Verreault The Penetrator with Enhanced Lateral Effect (PELE) is a type of explosive-free projectile that undergoes radial fragmentation upon an impact with a target plate. This type of projectile is composed of a brittle cylindrical shell (the jacket) filled in its core with a material characterized with a large Poisson's ratio. Upon an impact with a target, the axial compression causes the filling to expand in the radial direction. However, due to the brittleness of the jacket material, very little radial deformation can occur which creates a radial stress between the two materials and a hoop stress in the jacket. Fragmentation of the jacket occurs if the hoop stress exceeds the material's ultimate stress. The PELE fragmentation dynamics is explored via Finite-Element Method (FEM) simulations using the AUTODYN explicit dynamics hydrocode. The numerical results are compared with an analytical model based on wave interactions, as well as with the experimental investigation of Paulus and Schirm (1996). The comparison is based on the mechanical stress in the filling, the resulting radial velocity of the fragments, the number of fragments generated and their mass distribution. [Preview Abstract] |
Friday, July 12, 2013 9:45AM - 10:00AM |
Y4.00003: On the scaling of the magnetically accelerated flyer plate technique to currents greater than 20 MA R.W. Lemke, M.D. Knudson, K. Cochrane, M.P. Desjarlais, J.R. Asay In this talk we discuss scaling the magnetically accelerated flyer plate technique to currents greater than are available on the Z accelerator. Peak flyer plate speeds in the range 7-46 km/s are achieved in pulsed power driven, hypervelocity impact experiments on Z for peak currents in the range 8-19 MA. The highest (lowest) speeds are produced using aluminum (aluminum-copper) flyer plates. In either case, the $\approx $1 mm thick flyer plate is shocklessly accelerated by magnetic pressure to ballistic speed in $\approx $400 ns; it arrives at the target with a fraction of material at standard density. During acceleration a melt front, due to resistive heating, moves from the drive-side toward the target-side of the flyer plate. The speed of the melt front increases with increasing current. Peak flyer speeds on Z scale quadratically (linearly) with current at the low (high) end of the range. Magnetohydrodynamic simulation shows that the change in scaling is due to geometric deformation, and that linear scaling continues as current increases. However, the combined effects of shockless acceleration and resistive heating lead to an upper bound on the magnetic field feasible for pulsed power driven flyer plate experiments, which limits the maximum possible speed of a useful flyer plate to \textless 100 km/s. [Preview Abstract] |
Friday, July 12, 2013 10:00AM - 10:15AM |
Y4.00004: Generation of isentropic compression by use of multi-layer composite flyer and its influence on system thermodynamics: A simulation study Aditi Ray Recently the possibility of achieving quasi-isentropic compression using functionally graded materials, in both gas gun and explosive driven systems was explored by hydrodynamic simulations. In the current paper, we show that multi-layered composite flyer with progressively increasing shock impedances, referred to as graded density impactor (GDI), has the potential to enable increased flexibility in suitably tailoring applied-pressure profiles, further relaxing constraints on the thermodynamic path of compressed material. Present simulation pertaining to constant velocity impact of GDI reveals that linear ramp pulses of different pressure rise times with comparable peak pressures can be realized only by changing the layer thicknesses of a particular GDI. We report generation of three different slope ramp pulses by a five layer GDI made of PMMA, Al, Ti, Cu and Ta with different set of optimum thicknesses obtained with genetic algorithm based optimization technique. Generation of long duration ($\sim \mu$s) isentropic pressures using discrete GDI is a significant step, since it is devoid of fabrication difficulties of ultra-thin lamellae of FGM. Signatures of isentropic compression of a thin Cu target under different slope ramp loadings are identified from basic thermodynamic aspects in terms of temperature rise and entropy production. It is shown that the extent of entropy increase is closely related to the slope of ramping pulse. Further, a physical model has been constructed to determine the approximate time profile of pressure pulse generated by equal layer-width GDI. [Preview Abstract] |
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