Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session T2: TMS: Mesoscale Explosive Initiation II |
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Chair: Angela Diggs, AFRL Room: Grand Ballroom II |
Thursday, June 20, 2019 2:00PM - 2:15PM |
T2.00001: Investigating Shock Response of a PETN Based Explosive with Grain-Scale Simulations Graham Kosiba, Keo Springer, William Shaw, Richard Gee The shock initiation of heterogeneous solid explosives, such as binderized PETN-based explosives, is governed by mesoscopic processes. However, there is a scarcity of direct measurements probing these processes so we employ modeling to improve our understanding of it. We perform grain-scale simulations in the multi-physics code, ALE3D, to investigate the shock response of explicitly resolved energetic grains and pores following planar impact. Non-reactive simulations are used to determine an unreacted equation of state (EOS) for the explosive. The calculated unreacted EOS is compared to historic data as well as an engineering mixture model based on constituent properties and weight fractions. Reactive simulations are performed to investigate the effects of pore size distribution on the temperature field and bulk reaction rate in the shocked explosive. Finally, we probe the combined effects of pulse duration and microstructure on the shock initiation response. These studies are important because they provide insight on mesoscopic processes and form the basis for new morphology aware explosive models. [Preview Abstract] |
Thursday, June 20, 2019 2:15PM - 2:30PM |
T2.00002: Pore collapse in single-crystal TATB under shock compression Matt Nelms, Matthew Kroonblawd, Ryan Austin High explosive crystals often contain many defects, such as pores, cracks, and interfaces. When shock-compressed to sub-detonative pressures, the hot spots formed in the vicinity of these defects are responsible for triggering chemical reactions. In this work, we study thermal localization resulting from the collapse of a single pore in TATB crystal. A continuum-based crystal model is employed, which accounts for anisotropic elastic/plastic responses and melting in regions of sufficient dissipation. The mechanical description is informed, in part, by atomistic calculations as experimental data are lacking for many of the properties needed for simulation. A parametric study is performed to assess the sensitivity of the predicted thermal localization to model parameters and assumptions. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 (LLNL-ABS-768217). [Preview Abstract] |
Thursday, June 20, 2019 2:30PM - 2:45PM |
T2.00003: Hot spot ignition through shear banding in TATB single crystals shocked to detonation pressures Matthew Kroonblawd, Laurence Fried Detonating high explosives exhibit different reaction responses than those undergoing shock initiation. Shear banding in the bulk crystal, as opposed to void collapse, is a plausible mechanism for ignition that could become significant at higher shock pressures. Using all-atom molecular dynamics simulations, we demonstrate the dynamic formation of shear bands in TATB single crystals shocked to 30 GPa, which corresponds to the steady detonation pressure. Structural analysis reveals that shear bands in TATB develop and grow as regions of amorphous material that are substantially hotter than the crystalline bulk surroundings. Through scale-bridging with semi-empirical quantum-based molecular dynamics, we show that the amorphous shear bands are more chemically reactive than the bulk. An Arrhenius kinetics analysis of multiple simulations reveals that increases in shear band reactivity manifest as a simultaneous reduction in the activation energy and increase in the pre-exponential factor. Connections to continuum-level models for shock initiation are described. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It is approved for unlimited release with document number LLNL-ABS-768121. [Preview Abstract] |
Thursday, June 20, 2019 2:45PM - 3:00PM |
T2.00004: Molecular dynamics and continuum studies of shock-induced pore collapse in TATB Tommy Sewell, Puhan Zhao, S. Lee, H.S. Udaykumar Molecular dynamics (MD) and continuum simulations, performed using LAMMPS and SCIMITAR3D, respectively, were used to study thermo-mechanical aspects of shock-induced pore collapse in oriented single crystals of TATB. The simulations were performed using (almost) the same system dimensions, and exactly the same pore size/location and impact speeds, to enable close comparisons between the atomistic and continuum predictions. Two impact speeds, 1 km/s and 2 km/s, were used to generate the shocks; these yield predominantly visco-plastic and hydrodynamic-like collapse, respectively. The initial pore, with a diameter of 50 nm, was located at the center of a sample that was 150 nm \texttimes 150 nm along two edges. The MD simulations were ``quasi-2D'' with a length of $\approx $4 nm in the third direction ($\approx $300,000 fully flexible molecules) whereas the continnum simulations were 2D. For the MD studies, three crystal orientations (i.e., shock-propagation directions) were studied that span the limiting cases with respect to the crystal anisotropy. For the continuum simulations an isotropic constitutive model was used, thereby requiring only two simulations per impact strength (two different specific heat models were used). The evolution of spatio-temporally resolved properties during collapse will be reported including local stress tensors, temperatures, pore size and shape, and velocity fields. [Preview Abstract] |
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