(228c) Multiscale Modeling of the Shock Compression of Energetic Materials Using Constant Energy Dissipative Particle Dynamics

Energetic materials often contain nano- and microscale features, such as voids and grain boundaries.  Consequently, mechanical stimulation of these materials (i.e., shock) often incites responses over a wide range of spatial and temporal scales.  Many energy transfer processes within energetic materials are atomistically governed, yet the explosive response is manifested at the micro- and mesoscale.  The existing state-of-the-art computational methods include continuum level approaches that rely on idealized field-based formulations that are empirically based.  Our goal is to connect the atomistic and macroscales by developing microscale methods that can remove this empiricism, by supplying the dynamic responses of energetic materials measured on the microscale directly to macroscale continuum models.  However, significant technical challenges exist, including that the multiscale methods linking the atomistic and microscales for molecular crystals are immature or nonexistent.  To overcome these challenges, we have implemented a bottom-up approach to derive microscale coarse-grained models directly from quantum mechanics-derived atomistic models. 

The multiscale coarse-graining method (MS-CG) [1,2], based on force-matching of an atomistic model, has been utilized to derive particle-based coarse-grained potentials for hexahydro-1,3,5-trinitro-s-triazine (RDX), where one RDX molecule is mapped onto a single interaction site.  To improve transferability of the MS-CG model (required to accurately capture thermodynamic responses during mechanical shock), explicit mapping from the atomistic to the microscale is made from ambient to high pressures (>60 GPa) [2,3].  The resulting density dependent model for RDX reproduces several properties of the atomistic model within reasonable agreement, including the molecular crystal lattice structure, melting point, elastic and vibrational properties, and thermal conductivity [3,4].  However, properties which depend on the coarse-grained degrees of freedom (e.g., the heat capacity) are underestimated up to an order of magnitude due to coarse-graining of the intramolecular degrees of freedom.  Thus, the model inevitably cannot account for accurate energy and momentum exchange during mechanical shock using traditional dynamics methods.  To correct this deficiency, we account for momentum and energy transfer under mechanical shock by utilizing the constant-energy Dissipative Particle Dynamics (DPD-E) method [5-7].  The DPD-E method conserves both energy and momentum through the inclusion of a meso-particle equation of state, which for the MS-CG RDX model, helps to recapture the thermal and dynamic effects of the coarse-grained degrees of freedom.  In this talk, we will present results for mechanical shock of the MS-CG model of RDX using DPD-E.  The effect of various model parameters as well as their parameterization will be discussed, including the mesoparticle equation of state and DPD dissipative terms.  Results will be compared directly to atomistic simulation.

This effort is supported by the Institute for Multi-Scale Reactive Modeling of Insensitive Munitions (MSRM), which is a multi-team effort led by the U.S. Army Research Laboratory and the U.S. Army Armament Research, Development, and Engineering Center, involving various other national laboratories and academic groups totaling over 20 scientists.  J.D.M. gratefully acknowledges support, in part, by an appointment to the Internship/Research Participation Program for the U.S. Army Research Laboratory administered by the Oak Ridge Institute of Science and Education through an agreement between the U.S. Department of Energy and U.S. ARL (Project ID: 201003211).

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[2] S. Izvekov, P.W. Chung, and B.M. Rice, J. Chem. Phys., 133, 064109 (2010).

[3] S. Izvekov, P.W. Chung, and B.M. Rice, J. Chem. Phys., 135, 044112 (2011).

[4] S. Izvekov, P.W. Chung, and B.M. Rice, Int. J. Heat and Mass Transfer, 54, 5623 (2011).

[5] J. Bonet Avalos and A.D. Mackie, Europhys. Lett., 40, 141 (1997).

[6] P. Español, Europhys. Lett., 40, 631 (1997).

[7] M. Lísal, J.K. Brennan, and J. Bonet Avalos, J. Chem. Phys., 135, 204105 (2011).