Anisotropic Shock Sensitivity of Cyclotrimethylene Trinitramine (RDX) from Compress-and-Shear Reactive Dynamics

• Published in: Journal of physical chemistry. C Vol. 116; no. 18; pp. 10198 - 10206
• Main Authors: An, Qi, Liu, Yi, Zybin, Sergey V, Kim, Hyungjun, Goddard, William A
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 10-05-2012
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Mechanical and acoustical properties of condensed matter; Mechanical properties of nanoscale materials; Physics; Strain energy; Digital simulation; Slip; Mechanical properties; Energy density; Steric hindrance; Chemical reactions; Crystallographic plane; Anisotropy; Detonations; Slip system; Plane strain; Shear stress; Stress effects; Crystal faces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp300711m

We applied the compress-and-shear reactive dynamics (CS-RD) simulation model to study the anisotropic shock sensitivity of cyclotrimethylene trinitramine (RDX) crystals. We predict that, for mechanical shocks between 3 and 7 GPa, RDX is most sensitive to shocks perpendicular to the (100) and (210) planes, whereas it is insensitive for shocks perpendicular to the (120), (111), and (110) planes. These results are all consistent with available experimental information, further validating the CS-RD model for distinguishing between sensitive and insensitive shock directions. We find that, for sensitive directions, the shock impact triggers a slip system that leads to large shear stresses arising from steric hindrance, causing increased energy inputs that increase the temperature, leading to dramatically increased chemical reactions. Thus, our simulations demonstrate that the molecular origin of anisotropic shock sensitivity results from steric hindrance toward shearing of adjacent slip planes during shear deformation. Thus, strain energy density, temperature rise, and molecule decomposition are effective measures to distinguish anisotropic sensitivities. We should emphasize that CS-RD has been developed as a tool to distinguish rapidly (within a few picoseconds) between sensitive and insensitive shock directions of energetic materials. If the high stresses and rates used here continued much longer and for larger systems, it would ultimately result in detonation for all directions, but we have not demonstrated this.