Accelerating Molecular Dynamics Simulations to Model Defect and Damage Evolution in Lightweight Metallic Materials Under Shock Loading Conditions

Abstract: The capability to predict the impact tolerance of next-generation lightweight metallic materials for protective armor requires a fundamental understanding of the deformation and failure behavior of these materials under dynamic loading (impact/shock) conditions.Iimpact/shock loading conditions result in complex stress states that range from uniaxial compression to uniaxial tension at very high strain rates ranging from 10⁵ s^-1 to 10¹⁰ s^-1. The deformation response of these materials is determined by the capability of the metallic microstructure to nucleate dislocations during shock compression, and the failure response is determined by the creation of weak sites for nucleation of voids during triaxial expansion. A critical challenge in the experimental characterization of the mechanisms of plastic deformation and onset of dynamic failure (spallation) is attributed to the short time scales associated with the phenomena.

Molecular dynamics (MD) simulations can provide atomic-level insights into the micromechanisms for the evolution, interaction and accumulation of defects (dislocations, twins, interfaces) and damage (voids) in the microstructure. However, the capabilities of MD simulations are limited to small system sizes and small simulation times.

This talk will demonstrate the capability of the novel mesoscale modeling method, quasi-coarse-grained dynamics (QCGD), to scale up the predictive capability of classical MD simulations to the mesoscale and, at the same time, retain the atomic-scale characteristics of the deformation processes involved. The QCGD method is based on coarse-graining the atomic-scale microstructure by using a reduced number of representative atoms (R-atoms) and scaling relationships for interatomic potentials to retain the atomic-scale energetics of R-atoms at the mesoscale. The capability of the QCGD method to probe the effect of loading conditions (shock pulse, shock pressure) and the microstructure (grain size) on the evolution of dislocation density, elastic wave amplitude, and spall strength in polycrystalline aluminum microstructure at the mesoscale will be presented. In addition, the capabilities and limitations of the current interatomic potentials to model the deformation behavior of metallic microstructure at the atomic scale will be discussed.