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 E6: TMS: First-principles and Molecular Dynamics I |
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Chair: Aidan Thompson, SNL Room: Broadway III/IV |
Monday, June 17, 2019 3:30PM - 3:45PM |
E6.00001: Non-Schmid effect of pressure on plastic deformation in molecular crystal HMX. Anirban Pal, Catalin Picu The energetic molecular crystal HMX is a key constituent in common plastic bonded explosives. Its plastic deformation under shock conditions is important in reaction initiation and detonation. Here we study the effect of high pressure on dislocation slip using isothermal-isobaric atomistic simulations. We consider two slip planes, (011) and (101), that are reported to be most active under ambient conditions. For all slip systems considered, the effect of pressure is to increase the critical resolved shear stress for dislocation slip. Pressure may fully inhibit dislocation-based plasticity if the resolved shear stress is not increased in proportion. On the other hand, at sufficiently high shear stresses the crystal loses shear stability. Therefore, in a broad range of shock conditions, plastic deformation takes place by a combination of dislocation glide in some slip systems and localization in some other systems, with dislocation activity being gradually inhibited as the shock pressure increases. This provides new data on the physical basis of plastic deformation in HMX, indicating that mesoscale representations of plasticity must include shear localization which is more important under these conditions than dislocation plasticity. [Preview Abstract] |
Monday, June 17, 2019 3:45PM - 4:00PM |
E6.00002: Anisotropic Thermomechanical Response to Shock Wave Loading in TATB Puhan Zhao, Matthew Kroonblawd, Nithin Mathew, Tommy Sewell Molecular dynamics simulations were used to study shock loading in oriented crystalline 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 crystal orientations, with limiting cases that correspond to propagation exactly perpendicular and exactly parallel to the molecular layers. The simulations were performed for initially pristine crystals using a reverse-ballistic configuration with an impact speed of 1 km/s. Orientation-dependent properties are reported including pressures, temperatures, compression ratios, shock speeds, and local strain rates. Analysis of temperature, stress, material flow, and molecular orientations reveal complicated processes that arise for specific shock directions. The shock response is highly sensitive to crystal orientation, with significant qualitative differences for the evolution of stress and temperature, elastic/inelastic compression response, defect formation and growth, and strain rates. Several inelastic deformation mechanisms are identified, ranging from twinning to dislocation-mediated plasticity to intense shear strain localization. [Preview Abstract] |
Monday, June 17, 2019 4:00PM - 4:15PM |
E6.00003: Pressure, temperature, and orientation dependence of the thermal conductivity of $\alpha $- and $\gamma $-RDX Romain Perriot, Marc Cawkwell, John Lazarz, Shawn McGrane, Kyle Ramos Thermal conductivity is one the important component to devise a mesoscale model of high explosives (HE) response, notably to investigate accident scenario and ignition under weak stimuli. We present results from molecular dynamics (MD) simulations in RDX (1,3,5-trinitro-1,3,5-triazinane), using the Muller-Plathe, or \textit{reverse} non-equilibrium molecular dynamics (rNEMD), method to determine the orientation, pressure, and temperature dependence of the thermal conductivity. We find that $\alpha $-RDX exhibits anisotropy between the (100) and (001) directions, and a monotonic decreasing behavior between 225 and 400 K at zero pressure. Increasing pressure also leads to a monotonic response, this case resulting in an increase of the thermal conductivity. Simulations performed in the $\gamma $-phase (above 3 GPa), show a similar response between the (100) and (001) directions, suggesting an isotropic response of $\gamma $-RDX. Results are compared to experiments performed at LANL. [Preview Abstract] |
Monday, June 17, 2019 4:15PM - 4:30PM |
E6.00004: Theoretical study of interfacial thermal conductance for $\beta $-HMX/PVDF interface Hang Fan, Fude Nie, Pengwan Chen The heat conduction across the heterogeneous interface between $\beta $-HMX and PVDF is investigated. We use the diffuse mismatch model incorporating exact phonon dispersion and polarization to predict the interfacial thermal conductance. Based on lattice dynamics, the phonon dispersion relation is calculated from 2nd order interatomic force constants under harmonic approximation. Phonon density of states contributation is analysed from the view of assigned phonon mode. The calculated interface phonon transmission indicates that low frequency phonon modes dominant the interface thermal transport, while energy conducted by frquency mismatch phonon mode is refected by the interface. The interfacial thermal conductance as a function of temperature and accumulated interfacial thermal conductance are also calculated. Combined with the calculated thermal conductivity of $\beta $-HMX and PVDF by equlibrium molecular dynamics, the thermal conductivity of mixture $\beta $-HMX/PVDF system is analysed with simple series model. Particle size effect is also analysed. [Preview Abstract] |
Monday, June 17, 2019 4:30PM - 5:00PM |
E6.00005: A mesoscopic model with non-linear elasticity and phase transformation framework for the twinning-buckling behavior of TATB under dynamic loading: A Molecular Dynamics inferred constitutive law Invited Speaker: Paul Lafourcade A mesoscopic modeling of 1,3,5-TriAmino-2,4,6-TrinitroBenzene, a very anisotropic energetic molecular crystal, is proposed and validated on MD simulations. The two dominant deformation modes observed at nanometric scale and limited stress (less than one GPa) are a buckling instability and a non-symmorphic twinning (irreversible) transformation. \\ \\ A thermodynamically consistent continuum model is detailed, with a non-linear elasticity in pressure constructed to reproduce a cold equation of state. The twinning-buckling phase transition observed in MD is modeled by using a Phase-Field by Reaction-Pathway (PFRP) formalism. To validate the present constitutive law, we study the response of the single crystal under constant strain-rate uniaxial compressions for various directions in the basal plane and present one to one comparisons between both techniques. These comparisons are done by prescribing the same non-proportional strain tensor, insuring a strictly constant strain-rate. We then compare the Green-Lagrange strain tensor between PFRP and MD simulations, the latter obtained through a local least-square estimate. \\ \\ Finally, an upscale is performed and simulations using the mesoscopic constitutive law are presented for time and space scales non reachable in MD. A large TATB polycrystal is generated and two types of loadings are considered: a hydrostatic compression and a shock compression at different impact velocities. We show that in each case, the twinning-buckling phase transition is activated and this phenomenom is explained in detail. \\ \\ \vfill \noindent \textbf{References} \\ \noindent P. Lafourcade ; C. Denoual ; J.-B. Maillet, J. Phys. Chem. C. 121 (2017), no. 13, 7442-7449. \\ \noindent P. Lafourcade ; C. Denoual ; J.-B. Maillet, AIP Conference Proceedings. 1979 (2018), 180005. \\ \noindent P. Lafourcade ; C. Denoual ; J.-B. Maillet, J. Phys. Chem. C. 122 (2018), no. 26, 14954-14964. \\ [Preview Abstract] |
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