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

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.