Molecular Dynamic Simulation On Shock Response Of Nanoporous Cu

Shock response of porous materials can be of crucial significance for shock physics and bears many practical applications in materials synthesis and engineering. Molecular dynamics simulations are carried out to investigate shock response of nanoporous metal materials and metal nano-honeycomb materials, including elastic-plastic deformation, Hugoniot states, shock-induced melting, partial or complete void collapse, hotspot formation, nanojetting, and vaporization. The influences of microstructure effects, anisotropy and porosity are discussed.A model nanoporous Cu with cylindrical voids and a high porosity under shocking is established to investigate such physical properties as velocity, temperature, density, stress and von Mises stress at different stages of compression and release. The elastic-plastic and overtaking shocks are observed at different shock strengths. A modified power-law P-a model is proposed to describe the Hugoniot states. The Gruneisen equation of state is validated. Shock-induced melting shows no clear signs of bulk premelting or superheating. Void collapse via plastic flow nucleated from voids, and the exact processes are shock strength dependent. With increasing shock strengths, void collapse transits from the "geometrical" mode (collapse of a void is dominated by crystallography and void geometry and can be different from that of one another) to "hydrodynamic" mode (collapse of a void is similar to one another). The collapse may be achieved predominantly by flow along the{111} slip planes, by way of alternating compression and tension zones, by means of transverse flows, via forward and transverse flows, or through forward nano-jetting. The internal jetting induces pronounced shock front roughening, leading to internal hotspot formation and sizable high speed jets on atomically flat free surfaces.Microstructure effects on shock response of nanoporous Cu are investigated, including pore shape, size and arrangement, as well as grain boundaries. The void collapse jetting and vaporization are dependent on the microstructure, although to some different extents. The void arrangement and aspect ratio play an important role. The effects of grain boundaries and void size are less pronounced. The high pressure Hugoniot states are not sensitive to microstructure. Jetting during void collapse is due to tensorial velocity gradients (direction and amplitude), and a combined result of forward, divergent and convergent flows with varying contributions. Free surface jetting involves necking and cavitation. Elliptical voids with large aspect ratios, and with their centers aligned linearly with the shock direction, are particularly efficient in inducing high speed jetting and vaporization.A serial of models of hexagonal nano-honeycombs Cu with porosities varying from0.1to0.9are established to simulate their shock response with molecular dynamic method. The shock velocities of elastic and plastic waves, stress, density and temperature at different shock velocities are compared. Similarities and differences for three shock directions are also considered. The shock results of nano-honeycombs with various relative densities are also compared. It shows that the relative density and impact velocity play primary roles in shock response of Cu nano-honeycombs.