Predicted Melt Curve and Liquid Shear Viscosity of RDX up to 30 GPa

Abstract

Recent grain scale simulations of HMX and TATB have shown that predictions for hot spot formation in high explosives are particularly sensitive to accurate determinations of the pressure-dependent melt curve and the shear viscosity of the liquid phase. These physics terms are poorly constrained beyond ambient pressure for the explosive RDX. We adopt an all-atom modeling approach using molecular dynamics (MD) simulations to predict the melt curve of RDX near to detonation conditions (30 GPa) and determine the shear viscosity of the liquid as a function of temperature and pressure above the melt curve. Phase-coexistence simulations were used to determine the melt curve, which is predicted to vary by almost 1100 K as the pressure increases from 0 GPa to 30 GPa. Equilibrium MD simulations and the Green-Kubo formalism were used to obtain the pressure-temperature dependent shear viscosity. The shear viscosity of RDX is predicted to be of similar magnitude to the viscosity of TATB at low GPa-range pressures, and to be roughly an order of magnitude lower than the viscosity of HMX. The temperature dependence of the shear viscosity is Arrhenius at a given pressure, and the exponential pre-factor and activation term exhibit a strong, yet complicated, pressure dependence. An empirical pressure-temperature dependent function for RDX shear viscosity is developed that simultaneously captures a wide range of MD predictions while taking an analytic form that extrapolates smoothly beyond the fitted regime. The relative strength of the pressure and temperature dependencies of these two physics terms is found to be of similar magnitude for RDX, HMX, and TATB, which motivates incorporating these results in future RDX grain scale modeling.

Predicted Viscosity of Liquid HMX up to 40 GPa

“…Classical, non-reactive MD simulations were performed using the LAMMPS code [43] and a variant [44] of the Smith-Bharadwaj force field (FF) for HMX and RDX [45], which has been widely validated and used to model both materials in the solid and liquid states [4,6,7,9,27,28,33,34,36,[46][47][48][49][50][51]. Simulations of liquid HMX were performed using a cubic, 3D periodic simulation cell (see Figure 1) that contained 250 molecules.…”

“…Additional details concerning implementation of the HMX FF in LAMMPS can be found in Ref. [36]. Isochoric-isothermal (NVT) and isothermal-isobaric (NPT) trajectories were integrated at various points in this study.…”

“…During a pore collapse event, solid material can transition to a liquid state before reacting and viscous flow of that liquid serves as a dissipative work contribution that can increase peak hot spot temperatures. It is further understood that the shear viscosity of organic high explosives is a strong function of temperature and pressure [33][34][35][36], while the states accessed during hot spot formation span many thousands of Kelvin and 10s of GPa. Despite its importance to the physics of hot spot formation, the shear viscosity of most explosives is poorly constrained in explosive material models, especially at the extreme conditions of shock initiation.…”

“…While statistical mechanical treatments of shear viscosity, such as the Green-Kubo formalism [40], are not parameter-free, the information they require as input can be readily obtained from MD simulations. Viscosity data used in material models for common explosives such as RDX (1,3,5-trinitro-1,3,5-triazinane) [36], HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) [34,33], and TATB (1,3,5-triamino-2,4,6-trinitrobenzene) [35] all stem from MD simulations and only the viscosity of RDX has been determined past low GPa-range pressures. The lack of high-pressure viscosity data leaves considerable uncertainty in the material property parameters used in simulations of common explosives such as HMX at typical operational conditions.…”

“…Those authors noted that the activation term obtained by Bedrov et al at 0 GPa was distinctly larger than would be expected based on an extrapolation of the trend identified over 1-5 GPa. Moreover, the empirical viscosity model form developed by Kroonblawd and Austin for HMX was unable to capture the pressure-dependent trends in the viscosity of RDX up to 30 GPa [36]. While the viscosity model by Kroonblawd and Austin was designed to smoothly extrapolate to higher pressures, it remains untested beyond its fitted regime.…”