Large-scale molecular dynamics study of jet breakup and ejecta production from shock-loaded copper with a hybrid method

Abstract

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.

Communication: Slab thickness dependence of the surface tension: Toward a criterion of liquid sheets stability

“…As an example, in case where the defects are parallel grooves, Durand et al have recently used Molecular Dynamic (MD) simulations of large systems to show that the ejection process is as follows: (i) the ejected material forms sheets of liquid metal which go thinner and thinner; (ii) when sheets are sufficiently thin, holes appear and sheets break to form filaments; and (iii) those filaments stretch and finally break also to form spherical clusters. 1,4 It appears that the breaking of the sheets starts when the thickness is about several nanometers. So, to study the last parts of this phenomenon, microscopic simulations seem to be the good tool, and those MD simulations made possible to get the entire size distribution of the final spherical aggregates.…”

Mass-velocity and size-velocity distributions of ejecta cloud from shock-loaded tin surface using atomistic simulations

“…In particular the classical hydrodynamics approaches suffer a lack of modeling and characterization of the rheological behavior of metals after shock loading; and data on surface tension and viscosity are needed. To our knowledge, only recent MD simulations performed on atomistic systems allowed to provide directly from computations ejecta size distributions [25][26][27]. Particulate…”

“…[25]. The analysis tools implemented for the characterization of the particles/aggregates/ejecta (these terms refer to the same thing) in size, velocity, position and mass are also described in Ref.…”

Emergence of energy dependence in the fragmentation of heterogeneous materials

“…During the past decades research on fragmentation mainly focused on the statistics of fragment masses (sizes) obtained by the breakup of heterogeneous materials [1,9,10]. A large number of experimental [1,[5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical studies [13,[18][19][20][21][22][23][24] have confirmed that the mass distribution of fragments is described by a power law functional form. The exponent of the distribution was found to show a high degree of robustness, i.e., investigations revealed that the value of the exponent does not depend on the type of materials, amount of input energy, and on the way the energy is imparted to the system until materials of a high degree of heterogeneity are fragmented [1,9,10].…”