Strongly Anisotropic Thermomechanical Response to Shock Wave Loading in Oriented Samples of the Triclinic Molecular Crystal 1,3,5-Triamino-2,4,6-trinitrobenzene

Abstract

All-atom molecular dynamics (MD) simulations were used to study shock wave loading in oriented single crystals of the highly anisotropic triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The crystal structure consists of planar hydrogen-bonded sheets of individually planar TATB molecules that stack into graphitic-like layers. Shocks were studied for seven systematically prepared crystal orientations with limiting cases that correspond to shock propagation exactly perpendicular and exactly parallel to the graphitic-like layers. The simulations were performed for initially defect-free crystals using a reverse-ballistic configuration that generates explicit, supported shocks. Final longitudinal stress components are between ≈8.5 and ≈10.5 GPa for the 1.0 km s–1 impact speed studied. Orientation-dependent properties are reported including shock speeds, stresses, temperatures, compression ratios, and local material strain rates. Spatiotemporal maps of the temperature, stress tensor, material flow, and molecular orientations reveal complicated processes that arise for specific shock directions. The results indicate that TATB shock response is highly sensitive to crystal orientation, with significant qualitative differences for the time evolution of the stress tensor and temperature, elastic/inelastic compression response, defect formation and growth, critical von Mises stress, and strain rates during shock rise that span nearly an order of magnitude. A variety of inelastic deformation mechanisms are identified, ranging from crumpling of graphitic-like layers to dislocation-mediated plasticity to intense shear strain localization. To our knowledge, these are the first systematic MD simulations and analysis of explicit shock wave propagation along nontrivial crystal directions in a triclinic molecular crystal.

The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

“…Recent work from Lafourcade et al showed a strong orientation dependence for deformation mechanisms in TATB under controlled strain conditions 55 that leads to analogous deformations under shock conditions. 56 For instance, compressive stresses along [100] result in an inelastic chevron-like buckling of the basal planes, whereas resolved shear stresses along (011)-type planes result in a nonbasal gliding of the planes. Under weak stresses, the TATB crystal layers will glide in-plane 55,57,58 while detonation-level shocks lead to the formation of nanoscale shear bands.…”

“…All-atom simulations were used to study the perfect crystal shock response in a variety of crystallographic orientations. 56 This showed significant effects on the wave structure (single vs two-wave response), elastic wave speeds, and deformation mechanisms, which ranged from a variety of crystal-level defect formations to plasticity and intense shear localization. Coupled MD and continuum simulations explored the mechanics of pore collapse for various orientations and shock speeds for cylindrical pores.…”

Systematic Builder for All‐Atom Simulations of Plastically Bonded Explosives

“…The wavefront does not reach a clean, nearly planar shape until it reaches the closely packed central region of our sample (40–60 ps). At this point, a two wave (elastic‐plastic) shock structure can be seen, with plastic particle velocities mostly independent of the grain despite high levels of anisotropy in TATB [91]. In the final two frames, when the shock front re‐enters a porous region, molecular jetting into voids accelerates the wave locally, which leads to the bands of higher velocity (red) regions seen in the 80 ps frame.…”

Head‐To‐Head Comparison of Molecular and Continuum Simulations of Shock‐Induced Collapse of an Elongated Pore in an Energetic Molecular Crystal

“…Several mechanisms are responsible for the sensitivity which, in addition to dependence on the underlying molecular structures of the ingredients, stems from energy localization at defects and the formation of hotspots [2,3]. Collapse of pre-existing pores in the crystal [4][5][6][7][8], formation of shear bands [9][10][11][12], friction between sliding crystals and other interfaces [13][14][15], and plastic dissipation [16][17][18] are thought to be the most important mechanisms for hotspot formation. Among these, pore collapse and shear bands predominate at high shock intensities.…”