Bulletin of the American Physical Society
22nd Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 67, Number 8
Monday–Friday, July 11–15, 2022; Anaheim, California
Session Z03: Molecular Dynamics Simulations of High ExplosivesRecordings Available
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Chair: Brenden Hamilton, Purdue University Room: Anaheim Marriott Platinum 1 |
Friday, July 15, 2022 11:00AM - 11:15AM |
Z03.00001: Reduced Reaction Chemistry Models for TATB and CL-20 Edward M Kober Reactive Molecular Dynamics (RMD) simulations with the ReaxFF-lg force field are used to simulate the cook-off chemistry of CL-20 and TATB at a variety of fixed density and fixed temperature conditions. The chemical transformations are monitored using a Coordination Geometry Analysis (CGA) approach which tracks the number and types of bonds associated with each atom. Correlations between these chemical transformations are identified using a Non-negative Matrix Factorization (NMF) approach. These identify reduced chemistry models for these systems containing 6-7 steps. The time histories of these transformations show exponential growth/decay properties that could be fit with Arrhenius rates. From those fits, both activation energies (Ea) and activation volumes (Va) can be extracted to explain the temperature and pressure dependence of the reactions. Equation of States for the intermediates and products in gamma-law forms can also be derived from the simulations. Both materials show separate low and high-density pathways that can be associated with initial reaction pathways that are dissociative and associative, respectively. This behavior is similar to what has been previously been observed for HMX and RDX, and the reaction rates reflect the observed sensitivities of the materials. |
Friday, July 15, 2022 11:15AM - 11:30AM |
Z03.00002: Molecular dynamics analysis of shock passage through (010)-oriented β-HMX containing fields of pre-existing dislocations Tommy Sewell, Dilki Perera, Zhaocheng Zhang, Catalin Picu We report all-atom simulations of shock propagation in quasi-2D samples of (010)-oriented β-HMX (P21/n) containing fields of pre-existing dislocations. The goal is to understand the mechanical evolution as functions of dislocation density ρ and impact speed up. Dislocation fields were generated by inserting N = 44, 22, or 12 dislocations with mixed edge {(010)[001], (001)[010]} and screw {(010)[100]} character randomly into the sample, which was 5 nm × 150 nm × 150 nm in size. The corresponding ρ values are 2 × 1015, 1 × 1015, and 0.5 × 1015 m-2. Above a threshold shock intensity, the dislocations split into <111> partials. Impacts with up = 1.00 km s-1 were studied for all three ρ, along with 0.75 and 0.50 km s-1 for the largest ρ. Effective plastic strain rates (PSRs) were obtained from stress profiles, whence average dislocation velocities <v> were obtained using the Orowan equation. For ρ = 2 × 1015 m-2, the PSR and <v> increase with increasing up. For up = 1.00 km s-1, increasing ρ leads to an increase in PSR but a decrease in <v>. Resolved shear stress (RSS) profiles for the partials show that dislocation motion ceases when the RSS falls below a threshold that is consistent with published critical RSS values for the same slip systems under non-shock conditions. |
Friday, July 15, 2022 11:30AM - 11:45AM |
Z03.00003: Melt Curves of RDX and HMX Computed by Molecular Simulation Garrett M Tow, James P Larentzos, Michael S Sellers, Martin Lίsal, John K Brennan In this talk, we show how the solid–liquid coexistence curves of classical fully flexible atomistic models of α-RDX and β-HMX can be calculated using thermodynamically rigorous methodologies that identify where the free energy difference between the phases is zero. The free energy difference between each phase at a given state point was computed using the pseudosupercritical path (PSCP) method, and Gibbs–Helmholtz integration was used to evaluate the solid–liquid free energy difference as a function of temperature. This procedure was repeated for several pressures to determine points along the coexistence curve. While effective, this method is computationally expensive. To trace out the coexistence curve in a more computationally economical manner, Gibbs–Duhem integration was used starting from a coexistence point determined by the PSCP method. For α-RDX, the predicted melting temperature increases significantly more for a given increase in pressure when compared to available experimental data. |
Friday, July 15, 2022 11:45AM - 12:00PM |
Z03.00004: Predicted melt curve and liquid shear viscosity of RDX and HMX up to detonation pressures Matthew P Kroonblawd, Dilki Perera, Tommy Sewell, Ryan Austin, H. Keo Springer Recent grain-scale simulations have shown that hot spot formation mechanisms in high explosives are sensitive to the pressure-dependent melt curve and the shear viscosity of the liquid phase. Despite this importance, these physics terms are poorly constrained at GPa-range pressures due to a lack of experiments. Through molecular dynamics (MD) modeling, we provide the first direct predictions of the melting curve for RDX and HMX up to detonation-like pressures. Equilibrium MD simulations and the Green-Kubo formalism are applied to obtain the liquid shear viscosity as a function of temperature and pressure above the melt curve. Pressure sensitivity of the melt curve is found to be substantially greater than widely used estimates based on the Lindemann law. At the same time, the viscosity exhibits Arrhenius temperature dependence with a complicated pressure dependence. At a given pressure, the melting point of HMX is hundreds of Kelvin higher than RDX and the shear viscosity of HMX is similarly larger by an order of magnitude. Grain scale simulations showing the implications of these findings on hot spot formation processes are discussed. |
Friday, July 15, 2022 12:00PM - 12:15PM |
Z03.00005: Thermal conductivity tensor of β-HMX as a function of pressure and temperature from equilibrium molecular dynamics simulations Andrey Pereverzev, Tommy Sewell We apply the Green–Kubo (G-K) approach to obtain the thermal conductivity tensor of β-1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (β-HMX) as a function of pressure and temperature from equilibrium molecular dynamics (MD) simulations. Direct application of the G-K formula exhibits slow convergence of the integrated thermal conductivity values even for long (120 ns) simulation times. To partially mitigate this slow convergence, we developed a numerical procedure that involves physically justified filtering of the MD-calculated heat current to remove contributions which do not contribute to the conductivity tensor. Also, by combining keyword options in LAMMPS, which was used as the MD engine, we obtained a “hybrid” heat current that is physically more realistic for β-HMX than those predicted by either keyword alone. The heat-current correlation functions from the filtered hybrid heat current exhibit significantly reduced oscillations and lead to a much smoother behavior of the G-K time integrals. A physically motivated double-exponential function was fitted to the integrated time-dependent thermal conductivity tensor components to obtain the asymptotic values for the tensor. This procedure was used to determine the thermal conductivity tensor of β-HMX as a function of pressure and temperature for 0 ≤ ?? ≤ 30 GPa and 300 K ≤ ?? ≤ 900 K. The thermal conductivity increases with increasing ??, by approximately an order of magnitude over the interval considered, and decreases with increasing temperature. The predictions are compared to experimental and other theoretically determined values for the thermal conductivity.
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