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We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 41 GPa.

The physics of hypervelocity impact into foams are of interest because of application to comet dust capture during flyby encounters. Particles much larger than the foam cells behave as if the foam were a continuum, so that standard equations of fluid mechanics describe the effects of drag and ablation. Calculations based on these arguments accurately reproduce experimental results.

Traditionally, most dynamic shock compression experiments are conducted using a plane one-dimensional wave of uniaxial strain. In this case, the evaluation of the equation of state is simplified due to the geometry, but the amplitude of the induced shock wave is limited by the magnitude of the input load. In an effort to dramatically increase the range of pressures that can be accessed by traditional loading methods, a composite target assembly is examined. The target consists of two concentric cylinders aligned with the axial direction parallel to the loading. The target is designed such that on initial loading, the outer cylinder will have a higher shock velocity than the inner material of interest. Conically converging shocks will be generated at the interface between the two materials due to the impedance mismatch. Upon convergence, an irregular reflection occurs and the conical analog of a Mach reflection develops. The Mach reflection will grow until it reaches a steady state, at which point the wave configuration becomes self similar. The resulting high pressure Hugoniot state can then be measured using velocity interferometry and impedance matching. The technique is demonstrated using a planar mechanical impact generated by a powder gun to study the shock response of copper. Two systems are examined which utilize either a low impedance (6061-T6 aluminum) or a high impedance (molybdenum) outer cylinder. A multipoint VISAR experiment will be presented to validate the technique, and will be compared to numerical simulations. The feasibility of measuring an entire Hugoniot curve using full field velocity interferometry (ORVIS) will also be discussed.

QC 20190212

We report density functional theory (DFT) results for the shear stresses of uniaxially compressed diamond under conditions corresponding to strong shock wave compression. A nonmonotonic dependence of shear stresses on uniaxial strain was discovered in all three low-index crystallographic directions: <100>, <110>, and <111>. For <100> compression the shear stress even becomes negative in the region near the minimum of the shear stress-strain curve. The DFT results suggest that anomalous elastic regime observed in recent molecular dynamics shock simulations is a real phenomenon caused by a significant delay or even freezing of the plastic response.

We report the first experimental observation of highly nonlinear signals propagating in a two dimensional system composed of granular chains. In this system one of the chains contacts two others to allow splitting and redirecting the solitary-like signal formed in the first chain. The system consists of a double Y-shaped guide in which high- and low-modulus chains of spheres are arranged in various geometries. We observed fast splitting of the initial pulse, rapid chaotization of the signal and sharp bending of the propagating acoustic information. Pulse and energy trapping was also observed in composite systems assembled from hard- and soft-particles in the branches.

This paper presents results of first-principles density functional calculations of the equation of state (EOS) of PETN-I and beta-HMX. The isotropic EOS for hydrostatic compression has been extended to include uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions up to compression ratio V/V0 = 0.70. Equilibrium properties, including lattice parameters and elastic constants, as well as hydrostatic EOS are in good agreement with available experimental data. The shear stresses of uniaxially compressed PETN-I and beta-HMX have been evaluated and their behavior as a function of compression ratio has been used to make predictions of shock sensitivity of these EMs. A comparison of predicted sensitivities with available experimental data has also been performed.

We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear-deformation and overdriven rehybridization shock wave regimes.

I rapporten beskrivs nuvarande kunskapsläge när det gäller metallers uppträdande imark. Riskbedömningar för metaller i förorenad mark diskuteras.De flesta metaller binds i viss utsträckning i marken, oftast genom olika ytreaktioner med markens organiska material eller med järn- och aluminiumoxider, ochibland även genom utfällningsreaktioner. I vilken omfattning detta sker beror avfaktorer som t.ex. pH, redoxförhållanden, löst organiskt material (DOC) i markvattnet, samt förekomst av konkurrerande joner. Det är också viktigt att beaktavilka former metallerna förekommer som i markvattnet. Komplexbildning medt.ex. DOC minskar i regel toxiciteten. För att bättre ta hänsyn till markkemiskaförhållanden i riskbedömningen bör geokemiska modeller komma till ökad användning.Metaller lösta i vattnet följer med när vattnet strömmar och kan transporterastill grund- och ytvatten eller tas upp av växter. Hur snabbt detta går beror, förutomkemiska och biologiska processer, även på en rad olika jordegenskaper. Till exempel innehåller vissa jordar s.k. makroporer vilka ger upphov till preferentiellt flöde.Detta innebär att en del av metallerna snabbare kan transporteras genom marken,förbi jordpartiklar där metallerna annars skulle bindas.De olika modeller som används för att beskriva påverkan av markförorening pågrundvatten kan delas in i två grupper: stationära modeller antar att källtermen ärkonstant och tar enbart hänsyn till den utspädning som sker i grundvattnet. Exempel på sådana modeller är den svenska riktvärdesmodellen, JAGG och RBCA. I detvå senare modellerna finns möjlighet att ta hänsyn till nedbrytning av organiskaämnen. Tidsberoende modeller antar en avklingande källterm och en tidsberoendetransport i grundvattnet, exempelvis RISC och TAC-modellen. För beräkning avtransport i grundvattnet tas hänsyn till flödes- och fastläggningsmekanismer somadvektion, dispersion och linjär sorption.Med hjälp av laktester kan man uppskatta den andel av metallföroreningen somär löslig i, QC 20120112

For the first time, molecular dynamics simulations (MD) are applied to study the evolution of shock-driven Richtmyer-Meshkov instability of the perturbations at a solid surface and at the interface separating two solids of different densities. We study the dependence of the instability evolution on the strength and orientation of the shock, and analyze the structure of the material in a vicinity of the interface perturbations (i.e. the spikes and bubbles).

We develop strain-driven compression-expansion technique using molecular dynamics (MD) with reactive force fields (ReaxFF) to study the impact sensitivity of energetic materials. It has been applied to simulation of 1,3,5-trinitrohexahydro-s-triazine (RDX) crystal subjected to high-rate compression typical at the detonation front. The obtained results show that at lower compression ratio x = 1-V/V0<40% only a few of RDX molecules are decomposed, while for higher compressions (x>40%) all molecules decompose very quickly. We have observed both primary and secondary reactions during the decomposition process as well as production of various intermediates (NO2, NO, HONO, OH) and final products (H2O, N2, CO, CO2). The results of strain-driven compression-expansion modeling are in a good agreement with previous ReaxFF-MD shock simulations in RDX. Proposed approach might be useful for a quick test of sensitivity of energetic materials under conditions of high strain rate loading.

We report the study of thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine (RDX) bonded with polyurethane (Estane) and of the bulk hydrazine by molecular dynamics (MD) simulations equipped with the reactive force field (ReaxFF). For the polymer binder explosive, the simulation results show that the thermal decomposition of RDX is affected by the presence of the polymer binder Estane. Generally, with addition of Estane the decomposition of RDX slows down. Final products including N2, H2O, CO, CO2 and intermediates NO2, NO and HONO have been identified from the thermal decomposition processes. For the bulk hydrazine, it is found that with the increase of temperature, its decomposition increases and more N2 and H2 are generated, but NH3 molecules are consumed much faster at higher temperatures. This simulation work provides us an approach to quickly test the response of various energetic materials to thermal conditions.

Johnson-Holmquist constitutive model for brittle materials, coupled with a crack softening model, is used to describe the deviatoric and tensile crack propagation beneath impact crater in granite. Model constants are determined either directly from static uniaxial strain loading experiments, or indirectly from numerical adjustment. Constants are put into AUTODYN-2D from Century Dynamics to simulate the shock-induced damage in granite targets impacted by projectiles at different velocities. The agreement between experimental data and simulated results is encouraging. Instead of traditional grid-based methods, a Smooth Particle Hydrodynamics solver is used to define damaged regions in brittle media.

We apply molecular dynamics (MD) simulations to study the evolution of the shock-driven Richtmyer-Meshkov instability (RMI) in the cylindrical and planar geometries. Compared to traditional hydrodynamic simulations, MD has a number of fundamental advantages: it accounts for strong gradients of the pressure and temperature, and captures accurately the heat and mass transfers at the early stage (shock passage) as well as the late stage (perturbation growth) of the instability evolution. MD has no hydrodynamic limitations for spatial resolution and thermodynamic quasi-equilibrium at atomic scale. We study the instability evolution for different perturbation modes and analyze the role of the vorticity production for RMI dynamics.

Shear stresses are the driving forces for the creation of both point and extended defects in crystals subjected to high pressures and temperatures. Recently, we observed anomalous elastic materials response in shock-compressed silicon and diamond in the course of our MD simulations and were able to relate this phenomenon to non-monotonic dependence of shear stress on uniaxial compression of the material. Here we report results of combined density functional theory (DFT) and classical interatomic potentials studies of shear stresses in shock compressed covalent solids such as diamond and silicon for three low-index crystallographic directions, <100>, <110>, <111>. We observed a non-monotonic dependence of DFT shear stresses for all three crystallographic directions which indicates that anomalous elastic response of shock compressed material is a real phenomenon and not an artifact of interatomic potentials used in MD simulations.

Strongly nonlinear phononic crystals were assembled from stainless steel spheres. Single solitary waves and splitting of an initial pulse into a train of solitary waves were investigated in different viscous media using motor oil and non-aqueous glycerol to introduce a controlled viscous dissipation. Experimental results indicate that the presence of a viscous fluid dramatically altered the splitting of the initial pulse into a train of solitary waves. Numerical simulations qualitatively describe the observed phenomena only when a dissipative term based on the relative velocity between particles is introduced.

We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated energetics and isothermal equation of state (EOS) upon hydrostatic compression obtained by DFT and ReaxFF are both in good agreement with available experimental data. Our calculations of the hydrostatic EOS at zero temperature are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression were also calculated by both ReaxFF and DFT.

We report results of molecular dynamics simulation of shock wave propagation in silicon in [100], [110], and [111] directions obtained using a classical environment-dependent interatomic potential (EDIP). Several regimes of materials response are classified as a function of shock wave intensity using the calculated shock Hugoniot. Shock wave structure in [100] and [111] directions exhibit usual evolution as a function of piston velocity. At piston velocities 1.25< vp < 2.75 km/s the shock wave consists of a fast elastic precursor followed by a slower plastic front. At larger piston velocities the single overdriven plastic wave propagates through the crystal causing amorphization of Si. However, the [110] shock wave exhibits an anomalous materials response at intermediate piston velocities around vp ~= 1.75 km/s which is characterized by the absence of plastic deformations.

Understanding the response of energetic materials (EM) to thermal or shock loading at the atomistic level demands a highly accurate description of the reaction dynamics of multimillion-atom systems to capture the complex chemical and mechanical behavior involved: nonequilibrium energy/mass transfer, molecule excitation and decomposition under high strain/heat rates, formation of defects, plastic flow, and phase transitions. To enable such simulations, we developed the ReaxFF reactive force fields based on quantum mechanics (QM) calculations of reactants, products, high-energy intermediates and transition states, but using functional forms suitable for large-scale molecular dynamics simulations of chemical reactions under extreme conditions. The elements of ReaxFF are: - charge distributions change instantaneously as atomic coordinates change, - all valence interactions use bond orders derived uniquely from the bond distances which in turn describe uniquely the energies and forces, - three body (angle) and four body (torsion and inversion) terms are allowed but not required, - a general "van der Waals" term describes short range Pauli repulsion and long range dispersion interactions, which with Coulomb terms are included between all pairs of atoms (no bond or angle exclusions), - no environmental distinctions are made of atoms involving the same element; thus every carbon has the same parameters whether in diamond, graphite, benzene, porphyrin, allyl radical, HMX or TATP. ReaxFF uses the same functional form and parameters for reactive simulations in hydrocarbons, polymers, metal oxides, and metal alloys, allowing mixtures of all these systems into one simulation. We will present an overview of recent progress in ReaxFF developments, including the extension of ReaxFF to nitramine-based (nitromethane, HMX) and peroxide-based (TATP) explosives. To demonstrate the versatility and transferability of ReaxFF, we also present applications to silicone polymer poly-dimethylsi

One dimensional "sonic vacuum"-type phononic crystals were assembled from chains of polytetrafluoroethylene (PTFE) beads and Parylene coated spheres with different diameters. It was demonstrated for the first time that these polymer-based granular system, with exceptionally low elastic modulus of particles, support the propagation of strongly nonlinear solitary waves with a very low speed. They can be described using classical nonlinear Hertz law despite the viscoelastic nature of the polymers and the high strain rate deformation of the contact area. Trains of strongly nonlinear solitary waves excited by an impact were investigated experimentally and were found to be in reasonable agreement with numerical calculations. Tunability of the signal shape and velocity was achieved through a non-contact magnetically induced precompression of the chains. This applied prestress allowed an increase of up to two times the solitary waves speed and significant delayed the signal splitting. Anomalous reflection at the interface of two "sonic vacua"-type systems was reported.