(434a) Multiscale Modeling of the Shock Compression of Energetic Materials Using Particle Based Constant Energy Dissipative Particle Dynamics With Local Density Dependent Potentials

Moore, J. D., U.S. Army Research Laboratory
Izvekov, S., U.S. Army Research Laboratory
Lisal, M., Academy of Sciences of the Czech Republic
Brennan, J. K., U.S. Army Research Laboratory

Energetic materials often contain nano- and microscale defects, such as voids and grain boundaries.  Mechanical stimulation of these materials often incites responses over a wide range of spatial and temporal scales, with a strong dependence upon the defects.  Modeling these materials atomistically remains a challenge due to the length and time scales required, as billions of molecules would be necessary to model micron-size defects. To overcome these challenges, we have implemented multiscale techniques to bridge the atomistic and mesoscale descriptions by coarse-graining RDX through a force-matching technique [1,2], resulting in a density-dependent potential which spans ambient to high pressures (>10 GPa) [3].

Our resulting model reasonably reproduces several atomistic properties [3,4], but only those properties which depend on intermolecular interactions.  Properties that depend on the coarse-grained intramolecular degrees-of-freedom (e.g., the heat capacity) are underestimated [3].  Implementing traditional molecular dynamics to simulate the mechanical response of such models inevitably results in inaccurate energy and momentum exchange due to these unaccounted degrees of freedom.  To correct this, we utilize the constant-energy Dissipative Particle Dynamics method (DPD-E) [5-7], which provides a mechanism to account for all coarsened degrees of freedom through the inclusion of a coarse-grain particle internal energy.  This work presents results for the shock compression of RDX using DPD-E with results assessed by direct comparison to fully atomistic simulations.  An additional focus will also be made concerning the derivation and implementation of the density-dependent potential, which will be shown to conserve energy.

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