Non-equilibrium Simulations of Material Heterogeneities in Extreme Regimes
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Non-equilibrium Simulations of Material Heterogeneities in Extreme Regimes

  • Author(s): Flanagan, Rachel Marie
  • Advisor(s): Meyers, Marc A.
  • et al.
Abstract

The role of material heterogeneities in the shock behavior of materials has presented a great challenge to the shock community since World War II. Experimental, analytical, and computational methods have matured such that systematic studies of materials under shock are feasible and well-informed by the application of analytical and computational models. The greatest challenge of current experimental diagnostics for studying shock is that it is impossible to get both spatial and temporal resolution that allows for analysis on a scale smaller than a nanometer or a nanosecond simultaneously. To this end, computational simulations consisting of billions of atoms over thousands of time steps are able to provide the spatial resolution of individual atoms at times faster than their vibrational period. Although copper has been well-studied by both experiments and simulations, its dynamic behavior containing heterogeneities such as helium continues to be of great interest to the materials science community. Using atomistic models of copper, the role of pre-existing material heterogeneities in shock behavior is explored. The mechanisms responsible for the collapse of helium-filled bubbles and empty voids during the passage of shock waves in monocrystalline copper are revealed. The internal pressure (caused by pre-existing helium atoms), defect concentration, and bubble size are each varied in molecular dynamics simulations to understand the atomistic scale deformation as they are subjected to a range of peak shock stresses. It is shown that both empty and helium filled bubbles serve as dislocation sources, generating an intense, localized plastic region. A generalized model for dislocation emission from helium bubbles is proposed, where the inclusion of the shear stress generated by the helium bubble is shown to increase the critical stress to generate dislocations at the defect surface, demonstrating the change in plastic deformation. The production of ejecta, which is formed when a planar shock wave reaches a free surface, is of particular interest since these results describe the dependence of ejected mass on the shock strength, as well as the size and velocity distributions of the ejected mass. The goal is to systematically understand how material heterogeneities such as voids and helium interstitials alter the shock behavior.

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