  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 with a sinusoidal free surface roughness is
shock loaded so as to undergo a solid-liquid phase change on shock. The
reflection of the shock wave at the interface metal/vacuum gives rise to the
ejection of 2D jets/sheets of atoms (Richtmyer-Meshkov instabilities in the
continuum limit) which develop and break up, forming ejecta (fragments) of
different volumes (or mass). The fragmentation process is investigated by
analyzing the evolution of the resulting volume distribution of the ejecta as a
function of time. Two metals are studied (Cu and Sn) and the amplitude of the
roughness is varied. The simulations show that the associated distributions
exhibit a generic behavior with the sum of two distinct terms of varying
weight, following the expansion rate of the jets: in the small size limit, the
distribution obeys a power law dependence with an exponent equal to 1.15 plus
or minus 0.08 and in the large size limit, it obeys an exponential form. These
two components are interpreted, with the help of additional simple simulations,
as the signature of two different basic mechanisms of fragmentation. The power
law dependence results from the fragmentation of a 2D network of ligaments
arranged following a fractal (scale free) geometry and generated when the
sheets of liquid metal expand and tear. The exponential distribution results
from a 1D Poisson fragmentation process of the largest ligaments previously
generated. Unlike the power law distribution, it is governed by a
characteristic length scale which may be provided by energy balance principle.
