Computational Study of the Structure and Mechanical Properties of the Molecular Crystal RDX

Report No. ARL-TR-6241
Authors: Lynn Munday
Date/Pages: December 2012; 180 pages
Abstract: Molecular crystals are commonly used as active pharmaceutical ingredients and high explosive materials. Like simpler crystalline materials, they possess a repeating lattice structure. However, the complexity of the structure—due to having several entire molecules instead of atoms at each lattice site—significantly complicates the relationship between the crystal structure and mechanical properties. This work used molecular dynamics to study the structural response due to large deformations in the energetic molecular crystal RDX, C3H6N6O6, a commonly used military high explosive. Molecular dynamics was used to determine the crystal response to deformation by verifying elastic constants, polymorph transitions, cleavage properties, and energy barriers to slip. The energy barriers to slip are determined through the generalized stacking fault (GSF) procedure. To account for the steric contributions and elastic shearing due to flexible molecules, a modified calculation procedure for the GSF energy is proposed that distinguishes elastic shear energy from the energy associated with the interfacial displacement discontinuity at the slip plane. The unstable stacking fault energy from the GSF simulations is compared to the free surface energy to differentiate cleavage and slip planes. The results are found to be largely in agreement with available experimental data.
Distribution: Approved for public release
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Last Update / Reviewed: December 1, 2012