Tandem Molecular Dynamics and Continuum Studies of Shock‐Induced Pore Collapse in TATB

Abstract

All‐atom molecular dynamics (MD) and Eulerian continuum simulations, performed using the LAMMPS and SCIMITAR3D codes, respectively, were used to study thermo‐mechanical aspects of the shock‐induced collapse of an initially cylindrical 50 nm diameter pore in single crystals of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Three impact speeds, 0.5 km s−1, 1.0 km s−1 and 2.0 km s−1, were used to generate the shocks. These impact conditions are expected to yield collapse mechanisms ranging from predominantly visco‐plastic to hydrodynamic. For the MD studies, three crystal orientations (relative to shock‐propagation direction) were examined that span the limiting cases with respect to the crystalline anisotropy in TATB. An isotropic constitutive model was used for the continuum simulations, thus crystal anisotropy is absent. The evolution of spatiotemporally resolved quantities during collapse is reported including local pressure, temperature, pore size and shape, and material flow. Multiple models for the melting curve and specific heat were studied. Within the isotropic elastic/perfectly plastic continuum framework and for the range of impact conditions studied, the specific heat and melting curve sub‐models are shown to have modest effects on the continuum hotspot predictions in the present inert calculation. Treating the MD predictions as ‘ground truth’, albeit with a classical rather than quantum‐like heat capacity, it is clear that – at a minimum – an extension of the constitutive model to account for crystal plasticity and anisotropic strength will be required for a high‐fidelity continuum description.

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

“…The relative strength of the pressuretemperature functional dependencies of the RDX melt curve and shear viscosity are similar in magnitude to recent determinations for HMX [24] and TATB [21]. Thus, it can be expected that grain scale simulation predictions of hot spot formation and growth in RDX will exhibit similar sensitivity as was found for those explosives [28,29,24]. Both the melt curve and shear viscosity were shown to independently alter peak hot spot temperatures by hundreds of Kelvin in HMX and TATB, which provides strong motivation for including the present results in future grain scale models of RDX.…”

“…In the case of HMX (octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine), estimates of the melt curve based on the Lindemann law [32] were shown to be substantially lower than predictions obtained from MD simulations at GPa-range pressures [24]. Application of MD-derived melt curves for HMX [24] and TATB (1,3,5-triamino-2,4,6-trinitrobenzene) [21] in grain scale simulations of supported shocks were shown to effectively suppress melting during the collapse of micron-and sub-micron scale pores [24,28,29].…”

“…[a] M. P. Kroonblawd length and time scales has recently been used to calibrate and validate grain scale model terms through direct comparison [25][26][27][28][29][30][31]. Despite the promise of a one-and-done approach to calibration through scalebridging simulations, it is often more advantageous to gradually increase grain scale model complexity and independently determine physics terms due to the highly coupled and nonlinear nature of dynamic material response [24,28,29]. This is especially true for material parameters that vary significantly within the applicable temperature and pressure domain such as the melt curve and liquid shear viscosity [15,21,24].…”

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

“…A study of shock-induced pore collapse in TATB using both continuum and MD simulations was reported by Zhao et al 15 A quasi-2D simulation cell containing a cylindrical pore in the MD simulations (for three different crystal orientations) and a 2D cell containing a circular pore in the continuum treatment were studied at three impact speeds chosen to span collapse mechanisms ranging from predominantly visco-plastic to hydrodynamic. The MD simulations used an all-atom non-reactive force field (the same one used here) and the continuum calculations were performed using a baseline isotropic, elastic-perfectly plastic strength model.…”

Hugoniostat and Direct Shock Simulations in cis‐1,4‐Polybutadiene Melts

“…Alternatively, note that other works have relied on very short-range repulsion corrections to circumvent the high pressure limitation of the original EXP6 potential. [22,42] It will be specified explicitly which flavor of the potential was used in each section of this paper. Note that the OPLS-AA potential is non-reactive and therefore can describe only chemically inert processes: however, Mattsson et al [18] estimated numerically that reactions begin to take place above 15 GPa for polybutadiene which is around the highest pressures obtained in our simulations.…”