Ejecta production from the free surface of metals under shock loading is investigated using large-scale molecular dynamics (MD) simulations performed with a new (hybrid) method. A copper crystal, in contact with vacuum and with a sinusoidal surface finish representative of the roughness produced by a machine polishing, is divided in two zones, bulk and surface, calculated with, respectively, Hugoniostat and NVE ensembles. The bulk part is simulated using the Hugoniostat technique, which allows a very large number of particles to reach a Hugoniot equilibrium state in a short physical time by the mean of a quasi-equilibrium MD simulation. The surface part is simulated with the NVE ensemble (microcanonical ensemble in which the total number N of particles, the total volume V, and the total energy E of the system are constant) in order to account for the non-equilibrium character of the ejection process. With this method, the morphology and the size distribution of the ejecta cloud generated by a system with 125 × 106 atoms are studied over 1 ns. The simulations show that the ejection phenomenon tends toward a steady state on long times (typically above 200 ps). The ejected particles remain spherical with time and their size distribution exhibits a power law scaling followed by a large-size residual in the large size limit. This behavior is in good agreement with most of distributions measured in fragmentation processes. In particular, the power law scaling reflects a self-similar behavior which seems to be successfully reproduced within the framework of a 2D percolation model although a direct analogy is still difficult to establish.

Large scale molecular dynamics (MD) simulations are performed to study and to model the ejecta production from the dynamic fragmentation of shock-loaded metals under melt conditions. A generic 3D crystal in contact with vacuum containing about 10 8 atoms and

Abstract:The mass (volume and areal densities) versus velocity as well as the size versus velocity distributions of a shock-induced cloud of particles are investigated using large scale molecular dynamics (MD) simulations. A generic three-dimensional tin crystal with a sinusoidal free surface roughness (single wavelength) is set in contact with vacuum and shock-loaded so that it melts directly on shock. At the reflection of the shock wave onto the perturbations of the free surface, two-dimensional sheets/jets of liquid metal are ejected. The simulations show that the distributions may be described by an analytical model based on the propagation of a fragmentation zone, from the tip of the sheets to the free surface, within which the kinetic energy of the atoms decreases as this zone comes closer to the free surface on late times. As this kinetic energy drives (i) the (self-similar) expansion of the zone once it has broken away from the sheet and (ii) the average size of the particles which result from fragmentation in the zone, the ejected mass and the average size of the particles progressively increase in the cloud as fragmentation occurs closer to the free surface.Though relative to nanometric scales, our model may help in the analysis of experimental profiles.2

We compare, at similar scales, the processes of microjetting and ejecta production from shocked roughened metal surfaces by using atomistic and continuous approaches. The atomistic approach is based on very large scale molecular dynamics (MD) simulations with systems containing up to 700 × 106 atoms. The continuous approach is based on Eulerian hydrodynamics simulations with adaptive mesh refinement; the simulations take into account the effects of viscosity and surface tension, and the equation of state is calculated from the MD simulations. The microjetting is generated by shock-loading above its fusion point a three-dimensional tin crystal with an initial sinusoidal free surface perturbation, the crystal being set in contact with a vacuum. Several samples with homothetic wavelengths and amplitudes of defect are simulated in order to investigate the influence of viscosity and surface tension of the metal. The simulations show that the hydrodynamic code reproduces with very good agreement the profiles, calculated from the MD simulations, of the ejected mass and velocity along the jet. Both codes also exhibit a similar fragmentation phenomenology of the metallic liquid sheets ejected, although the fragmentation seed is different. We show in particular, that it depends on the mesh size in the continuous approach.