Thermomechanical properties and equation of state of γ-RDX

In order to simulate the detonation pathways of an energetic material in continuum models, it is important to know the constitutive relations and the sensitivity of the model to pressure and temperature. The γ-polymorph plays an important role in the detonation mechanism of RDX. In our present work, we predict the thermomechanical properties of γ-RDX and their variation with pressure (4-11GPa) and temperature (200-550K) using a non-reactive fully flexible Smith and Bharadwaj (SB) molecular potential using molecular dynamics simulations (LAMMPS) [1]. The simulation cell is modeled after experimentally obtained crystal structure definition of γ-RDX [2] with non-orthogonal periodic boundary conditions. After equilibrating the system at 5.2GPa and 300K using isothermal-isostress ensemble, the pVT data is collected for calculating equation of state by equilibrating the system from 4-11GPa in increments of 1GPa and 200-550K in increments of 100K. At each equilibration point, a strain is applied on the system up to 10% in increments of 1% without straining the molecules internally and equilibrated using isothermal-isochoric ensemble at each increment. Then, the stress-strain data is used to calculate the elastic modulus tensor. The lattice parameters and unit cell volume of γ-RDX predicted by the simulation are consistent with experimental [2] and other simulation results [3]. The elastic modulus tensor calculated at 5.2GPa and 300K compares well with previous simulation data [3]. It indicates linear hardening and mild softening with pressure and temperature, respectively. This is consistent with similar observations of the α-polymorph of RDX. A third order Birch Murnaghan equation of state has been proposed for γ-RDX which agrees well with previous simulation results. However, it over predicts the volume when compared with experimental data [2]. The coefficients of thermal expansion of γ-RDX are small in comparison to α-RDX and show negligible variation with pressure and temperature. In particular, the b-lattice parameter is approximately constant with temperature, especially at high pressures. The present study provides invaluable information for modeling constitutive relations in continuum simulations for RDX, especially during shock loads. The absence of experimental material data on γ-RDX makes it even more significant. The stable and consistent results obtained using the SB potential, indicate the appropriateness of the potential in studying other molecular phenomena of RDX.

References:
1. Josyula et al., RSC Adv. 4, 41491 (2014).
2. Davidson et al., CrystEngComm 10, 162 (2008).
3. Munday et al., J. Phys. Chem. B 115, 4378 (2011).