Atomic Simulation Of Meting And Spalling In Single Crystal Copper Under Cylindrical Impact

The research on the structure and mechanical response of cylindrical shells under implosions or explosions is significant to weapon design,material synthesis,safety protection,etc.Affected by the extreme condition and ultrafast response process in shock experiments,it is difficult to get insights into the atomic processes and mechanism of shock melting.In our work,Non-equilibrium molecular dynamics combined with embedded atom method(EAM)potential is u sed to study the response behavior of cylindrical single crystal copper under cylindrical impact by introducing a cylindrical potential wall.The results are as follow:By applying implosion loading to cylindrical sample,we discuss the propagation of shock waves and the process of shock melting in detail.In the case of low impact strength,the plastic waves in the <110> directions fall behind the elastic wave,forming an obvious two-wave structure.Simultaneously,the plastic waves in <100> directions catche up with the elastic wave,which leads to a single wave structure.Because the shock-wave propagation speed is different in all directions,the wave profile evolves from an initial cylindrical shape to a non-cylindrical shape.When the impact strength increases,the anisotropy of the shock wave disappears,and a single wave structure appears in all directions,where the wave profile maintains the initial cylindrical shape.In the case of low impact strength,premelting-recrystallization phenomenon occurs in wave head along the <110> directions,plastic deformation mainly occurs in the <100>directions.As the impact strength increases,impact melting occurs in all directions.We discuss the melting process of cylindrical sample under cylindrical diverging shock compared with the results of implosion loading.Under cylindrical diverging shock,the front of the plastic wave releases stress through plastic deformation and impact melting.Compared with the implosion loading,the shock wave propagates from the inside to the outside under cylindrical diverging shock.During shock wave propagation,the energy gradually dissipates,resulting that the impact velocity required for melting under diverging shock is higher than that of planar loading and even much higher than implosion loading.The spalling behaviors under implosive and diverging shock are different.Under the implosion loading,the cylindrical potential wall is on the cylindrical outer surface.When unloading,the particles on the outer surface spontane ously expand outward,forming unloading wave.The reflected wave and unloading wave lead to the formation of tensile regions.Premelting occurs in the <110> directions of the tensile region and serves as the nucleation point for subsequent spallation.Unde r diverging shock,the cylindrical potential wall is on the cylindrical inner surface.When unloading,the inner surface spontaneously expands toward the central axis.Differently,the outer surface expands outward under the action of the compression wave.The shock wave propagates faster in the <110> directions,resulting in a greater speed of particles on the outer surface in <110> directions.Under the action of tangential stress,a relatively strong tensile state is formed in the <100> directions.The s pallation eventually nucleates and grows at the twin boundary in the <100> directions.