This manuscript investigates reactive- versus hydrodynamic-breakup processes of ejecta. For this study, the reactive metal is cerium (Ce) and the nonreactive metal is tin (Sn), the nonreactive gas is helium (He) and the reactive gas is deuterium (D2) or hydrogen (H2). Experiments were performed in vacuum and the reactive- and nonreactive-gases at various pressures, where we endeavored to match the post-shock gas densities to differentiate between reactive- versus hydrodynamic-breakup processes. Hydrodynamic breakup sensitively links to the Weber number (gas density, liquid fragment diameter, surface tension, and the square of the relative velocity between the fragment and the gas), whereas reactive breakup links to the reactive dynamics which includes two processes. In one case the reactive metal breaks up into smaller fragments as rapidly as the reaction rate, and in the other a crust grows on the liquid fragments as the reactions occur, a diffusion limited process. In the latter case, the particle diameters increase with time as the crust grows. In this process, which is indicated by the data, particles breakup as the CeD2 loses strength with increasing temperature, leaving an exponentially increasing diameter.

Strain rate, temperature, and microstructure play a significant role in the mechanical response of materials. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate. Molecular dynamics calculations combined with reported experimental measurements show power-law kinetic relationships that vary as a function of dominant defect mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the cohesive energy, or the energy required to separate atoms from one another. At strain rates below the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress concentrations at grain boundaries that arise due to compatibility requirements; and the growing fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the present cases, voids nucleate at defect structures present in the microstructure. The exact makeup and distribution of defects is controlled by the initial microstructure and the plastic deformation during both compression and expansion, where grain boundaries and grain orientation play critical roles.

Ductile tensile failure of tantalum is examined through large scale non-equilibrium molecular dynamics simulations. Several loading schemes including flyer plate impact, decaying shock loading via a frozen piston, and quasi-isentropic ( constant strain-rate) expansion are employed to span tensile strain-rates of 108 to 1014 per second. Single crystals of < 001 > orientation are specifically evaluated to eliminate grain boundary effects. Heterogeneous void nucleation occurs principally at the intersection of deformation twins in single crystals. At high strain rates, multiple spall events occur throughout the material and voids continue to nucleate until relaxation waves arrive from adjacent events. At ultra-high strain rates, those approaching or exceeding the atomic vibrational frequency, spall strength saturates near the maximum theoretical spall strength.

We investigate the relation between shock-pulse shape and the amount of micron-scale fragments ejected upon shock release at the metal/vacuum interface of shocked Sn targets. These micron-scale particles are commonly referred to as ejecta. Two shock-pulse shapes are considered: a supported shock created by impacting a Sn target with a sabot that was accelerated using a powder gun; and an unsupported or Taylor shockwave, created by detonation of high explosive that was press-fit to the front-side of the Sn target. Ejecta production at the back-side or free-surface of the Sn coupons were characterized through use of piezoelectric pins, Asay foils, optical shadowgraphy, and x-ray attenuation.

We apply the Lindstedt method to the one dimensional Fermi-Pasta-Ulam β lattice to find fully general solutions to the complete set of equations of motion. The perturbative scheme employed uses ϵ as the expansion parameter, where ϵ is the coefficient of the quartic coupling between nearest neighbors. We compare our nonsecular perturbative solutions to numerical solutions and find striking agreement.

We present the results of large-scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag interfaces sliding in the velocity regime 0 ≤ v ≤ 1 km/sec. System sizes of 2.8 × 10 6 atoms are considered using Embedded Atom Method (EAM) potentials. Single crystals with (010) interfaces sliding along the (100} direction are considered. We discuss the observed velocity weakening in the tangential force at high velocities, and its connection with the observed dislocation structure and nanostructure that are nucleated during dry sliding.

Large-scale atomistic simulations by non-equilibrium molecular dynamics have revealed that shock-wave loading and high-speed friction between dry metal interfaces have surprising similarities, in that plastic deformation occurs by the violent birth of dislocations. Shock-wave deformation is initiated at the shock front, while in sliding friction, the interface produces dislocations that move first within the plane and then out of it, so as to generate a microstructure that accommodates the slippage. For both shocks and friction in perfect, or nearly perfect, crystals, there is a threshold driving force that needs to be overcome in order to induce plastic flow. Below that threshold, pre-existing extended defects are able to trigger plastic microstructure that resembles the kind seen above the threshold.