Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G27: Matter at Extreme Conditions: Energetic Materials IIFocus Recordings Available
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Sponsoring Units: GSCCM Chair: Santanu Chaudhuri, Argonne National Laboratory Room: McCormick Place W-187C |
Tuesday, March 15, 2022 11:30AM - 12:06PM |
G27.00001: Accelerating reactive Monte Carlo simulation by machine learning Invited Speaker: Ryan B Jadrich Upon detonation, high explosives exhibit a near discontinuous change from reactant material, at ambient conditions, to very hot (thousands of Kelvin) small molecule detonation products at highly over-compressed (gigapascal pressure) conditions. Ab Initio density functional theory simulations are the current state of the art for modeling detonation product mixtures; however, they are 1) inherently expensive and 2) must be performed over a vast range of thermodynamic state points to create a usable products equation of state. To circumvent this limitation, we have developed a nested Monte Carlo, machine learning accelerated simulation protocol that exactly retains the fidelity of the quantum mechanical simulations while avoiding having to actually perform quantum mechanical calculations at every step in the simulation, yielding significant computational savings. Furthermore, leveraging smart data science strategies, we are able to maximize the equation of state information extracted from the simulations, including chemical composition. We demonstrate our methodology for the high explosive PETN and discuss the quantitative limitations of density functional theory-based simulations. First principles corrections to DFT are proposed, yielding quantitative detonation performance predictions. |
Tuesday, March 15, 2022 12:06PM - 12:18PM |
G27.00002: Many Bodied Mechanochemistry Simulations: Exploring the initial events inside a hotspot Brenden W Hamilton, Alejandro H Strachan When molecular materials experience high velocity impacts, the resulting shockwave can induce a variety of complex intra-molecular deformations, leading to mechanochemistry. Mechanochemistry can enable novel reaction pathways, lower the thermal energy cost of reactions, and is relevant to thermochemical phenomena such as detonation, with mounting indication that these effects are highly relevant to the initiation of "hotspots" in high explosives. However, these effects are difficult to assess under non-equilibrium and shock loading simulations, and the state-of-the-art simulation techniques for mechanochemistry typically apply linear forces to individual molecules. Therefore, we develop a novel technique of an external biasing potential to apply 'many-bodied steered molecular dynamics' in which we mimic intra-molecular deformations of a shock induced hotspot in TATB. Independent simulations of two different deformation types show different levels of acceleration of reaction kinetics for the applied work and results in different alterations to first-step reaction pathways. We believe these results help to solve the puzzling difference between thermal and shock-loaded kinetics in HEs and provide a more general methodology for assessing mechanochemical affects in bulk, covalent solids. |
Tuesday, March 15, 2022 12:18PM - 12:30PM |
G27.00003: Visualizing Phase Change and Temperature Rise during Shock Loading of an Energetic Material Sample using Laser Array Raman Spectroscopy Abhijeet Dhiman, Tyler A Dillard, Vikas Tomar Experimental quantification of thermo-mechanical field and chemical analysis during shock compression of a heterogeneous domain of material requires diagnostics with ultra-fast acquisition within nanoseconds. Time-gated Raman spectroscopy has been used in the past for single laser spot chemical analysis at a nanosecond time scale. Such analysis is only limited to a small domain or single measurement point on the microstructure. This work presents a novel experimental capability to perform time-gated Raman spectroscopy over multi-locations on microstructure using a laser array method. The excited Raman signal from each spot on the array was collected simultaneously on the spectrometer using a custom design of optical path. The quantification of phase change of material during the shock compression is important to model the temperature rise and to understand reaction mechanisms under shock loading of energetic materials. This technique was used to quantify the effect of friction and shock confinement at the interface between two energetic crystals. Shock-induced temperature distribution and phase-field were measured as a function of proximity between the energetic particles. The results show a strong experimentally measured correlation between temperature rise and melting of at energetic material interface as a function of microstructure variation. |
Tuesday, March 15, 2022 12:30PM - 12:42PM |
G27.00004: Characterization of temporal species and temperature evolution during nitromethane fireballs Robert Greene, Nishan Khanal, Marc Etienne, Subith Vasu Nitromethane (NM) as an energetic material has been investigated over the past several decades for its high oxygen concentration. This enables it to be used in several applications ranging from: high-performance fuel additive used to reduce soot formation and prevent engine knocking, to monopropellant due to the ability to burn in anoxic environments. However, despite the sustained interest in the fuel over this time, there is still a lack of detailed understanding of the combustion kinetics of the fuel. This is especially relevant at extreme conditions such as those described in this work. Temperature and H2O time-histories will be collected during nitromethane rich detonations (φ=1.75, not accounting for local spatial variations) and results are to be presented. A two-color scheme is to be used to determine the temperature of water using lasers centered about 3920.1 cm-1 and 4028.2 cm-1 (2551.0 and 2482.5 nm respectively). Simulated conditions range pressures of .2 – 16 atm, and temperatures of 273 – 2500 K. These data is to be collected at the Air Force Research Laboratory (AFRL) facility located at Eglin Air Force Base (AFB), and are to be used in the development and refinement of blast models of NM. |
Tuesday, March 15, 2022 12:42PM - 12:54PM |
G27.00005: Simulation of Post-Detonation Reaction and Afterburning Process of High-Explosive Charges with Detailed Chemical Kinetics Ryan Houim, Anthony Egeln The posted detonation reaction processes of high-explosive charges involves reaction inside of the high-pressure detonation products, dissociation of air by intense shock waves, and afterburning when fuel-rich detonation products mix with the surrounding air. The details of these reaction processes are not well understood. We present two-dimensional axisymmetric simulations of the reaction and afterburning that occurs in the expanding flow produced by the detonation of a PETN charge. The simulations used the Becker-Kistiakowky-Wilson real-gas equation of state with detailed chemical kinetics with 59 species and 368 reactions. A novel programmed burn model was used to approximate the detonation while allowing the gas-phase detonation products to react. The computed results show significant air dissociation by the shock wave as well as turbulent mixing and afterburning. Initially when the shock breaks out of the charge, the degress of air dissocision is large producing a N, O, and NO. Most of the N and O recombine as the shock expands and cools down. However, a large amount of NO remains for a significant period of time. Significant quantities of OH are produced in regions where the shock-heated air and fuel-rich detonation products mix and burn. The computed results also show that the chemical composition in the detonation products are essentially frozen once the temperature falls below 2000 K, which provides some evidence for the freeze-out assumption used in chemical equilibrium codes. |
Tuesday, March 15, 2022 12:54PM - 1:06PM |
G27.00006: Simulations of Deflagration Waves in HMX Edward M Kober Large-scale 1D simulations (≥1M atoms) of deflagration waves in the energetic material HMX were performed using ReaxFF-lg. These were ignited either by thermal hot spots or shock-induced collapse of voids (i.e. gaps). For ignition temperatures >1200K, the deflagration waves reached steady-state fairly rapidly, with deflagration velocities ≥ 50 m/s and quite thin reaction fronts of ~10 nm. The propagation velocities were highly dependent on the state of the material in front of the wave. In the void collapse simulations, very rapid (~2,000 m/s) deflagration waves were observed propagating backwards (compared to the shock direction) into the doubly-shocked material that filled the void space, and which was at ~1500K. The reaction chemistry could be mapped cleanly onto a six-component reduced chemistry model that had been formulated from smaller scale NVT simulations using the Non-negative Matrix Factorization technique. This model includes pressure dependent Arrhenius rates and gamma-law EOS forms for the intermediates. This demonstrates a clean connection between thermal and shock-induced chemistry and enables the formulation of a deflagration model for this material. |
Tuesday, March 15, 2022 1:06PM - 1:18PM Withdrawn |
G27.00007: Cellular structures found in the detonation front of liquid nitromethane with tabletop shock compression Erin J Nissen, Mithun Bhowmick, Dana D Dlott After accumulating data on hundreds of nitromethane (NM) experiments, it was verified the tabletop shock compression microscope could produce detonations with properties that are consistent with bomb sized charges. This was not initially obvious, due to the multiple orders of magnitude difference in material amount (0.09 – 0.7 µL) and shorter input shock duration (4 ns). Due to the short shock duration, the time it takes the shock to transition into a detonation, the shock-to-detonation, was reduced by orders of magnitude, nearing an ultimate minimum. Using photon Doppler velocimetry (PDV), optical pyrometry and a nanosecond imaging this work shows that the NM reaction falls into three specific regimes dependent on input shock strength. Images of these regimes revealed patterns that form on a shock font when oblique reactions waves race forward and collide with the front, referred to as cellular structures. Although these structures have been speculated to exist in pure liquid NM, they have not been directly imaged until now because high spatial (2 µm) and temporal (5 ns) resolution was needed to capture these dynamic events. This work shows that spherical explosions behind the planar shock font in print unique cellular patterns on the front. These patterns can determine exactly when, where, and how many explosions occurred. |
Tuesday, March 15, 2022 1:18PM - 1:30PM |
G27.00008: Drop weight impact thermomechanics with GnarlyX multi-physics hydrocode simulations Roseanne M Cheng, Milovan Zecevic, Marc J Cawkwell, Frank Marrs, Virginia W Manner Drop weight impact experiments are critical in assessing handling safety when developing new explosives and formulations. However, they convolute large deformations, heat generation and chemistry that lead to a "Go" or "No-Go" signal for the onset of reactions. Although the current interpretation is useful in screening for high explosive (HE) safety and sensitivity, we push towards a fundamental understanding of these tests in investigating the thermomechanical conditions that lead to a chemical reaction. This talk focuses on large deformation and heat localization behavior through multi-physics hydrocode simulations of a related system, the Viscoplastic flow Ignition and Propagation Imaging of Reactions (VIPIR) experiment. We use GnarlyX, a multi-dimensional, multi-material and strength hydrodynamics code for high performance computing platforms. It is based on a Helmholtz free energy formulation which gives thermodynamically consistent temperature. We simulate the crushing of an HE sample between two anvils with simple to complex geometries and material interaction. This approach is necessary in estimating the computational expense of a suite of multi-physics simulations important in revealing the mechanisms that lead to temperatures sufficient for ignition. |
Tuesday, March 15, 2022 1:30PM - 1:42PM |
G27.00009: 3-Phonon Scattering Pathways for Vibrational Energy Up-pumping in Crystalline RDX Gaurav Kumar, Francis G VanGessel, Lynn B Munday, Peter W Chung Up-pumping of shock energy to intramolecular vibrational modes may lead to breaking of critical bonds and phenomenon resulting in initiation in energetics. In this work, Fermi’s Golden Rule based 3-phonon scattering model is used over 216 kpoints throughout the Brillouin zone to investigate ~12 billion pathways for vibrational energy up-pumping in crystalline RDX [1]. On average, modes with frequencies up to 102 cm-1 have the highest scattering rates and redistribute over 99% of the vibrational energy to other low frequency modes up to 102 cm-1 within 0.16 ps. These low frequency modes transfer less than 0.5% of their vibrational energy directly to the NN stretching modes. The mid-frequency modes from 102 to 1331 cm-1 further up-pump the energy to the NN stretching modes within 5.6 ps. The mid-frequency modes between 457 and 462 cm-1 and between 831 and 1331 cm-1 are the most critical for vibrational heating of the NN stretching modes and phenomena leading to initiation in energetics. |
Tuesday, March 15, 2022 1:42PM - 1:54PM |
G27.00010: Coarse-Grained Reactive Model of RDX: Conjoining the Continuum and Atomistic Resolution Brian H Lee, Michael N Sakano, James P Larentzos, John K Brennan, Alejandro H Strachan Accurate models for predicting the thermal, chemical, and mechanical properties of high explosive (HE) materials are of great interest for both military and civilian usage of these materials in a safe and efficient manner. Computational models with atomistic resolutions have demonstrated accurate portrayal of HE, but their computational cost constrain their application to systems with O(100 nm, ns). To overcome such limitation, reduced-order chemistry Arrhenius-like models that can predict the sensitivity of μm-scale hotspots have been developed. However, such models are incapable of capturing phenomena where the material behavior at the molecular level significantly affects the system. Here, we bridge the gap between atomistic and continuum models by developing a particle-based, coarse-grained model of an HE material that utilizes generalized dissipative particle dynamics with reactions (GenDPD-RX). By incorporating the chemical kinetics from the Arrhenius-like models while parametrizing the GenDPD-RX model based on atomistic simulations, our model accurately captures the material response to shocks with multiple orders of improvement in computational efficiency. We expect that such models will enable future investigation of HE at length and time scales previously inaccessible. Approved for public release; distribution is unlimited. |
Tuesday, March 15, 2022 1:54PM - 2:30PM |
G27.00011: Explosive mechanochemistry: Foundations for strength-aware chemical kinetics Invited Speaker: Matthew P Kroonblawd Molecular dynamics shows strong synergy between high-rate strength behavior and chemistry in explosives, with plastic deformation localizing heat into hot spots and reducing reaction barriers through mechanochemistry. Nanoscale shear bands form dynamically in many shocked explosives, and we show that these regions are 200 times more reactive than bulk crystal in TATB. Enhanced reactivity is traced to molecular deformations that are identified based on intramolecular strain energy. Axial compression simulations show that substantial molecular deformations are generated for all crystal orientations at pressures greater than 10 GPa, which qualitatively explains unusual changes in TATB reactivity with increasing shock pressure. Despite fine-scale complexity, the global stress-strain response is straightforward with a flow stress that scales with the shear modulus. This suggests a plausible route to simplify treatments of strength, which when coupled to our measured intramolecular strain energy distributions, provides grounds for parameterizing a “strength-aware” chemical kinetics model for TATB-based explosives. |
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