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 Y1: DSIC: Initiation and Growth |
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Chair: David Damm, SNL Room: Grand Ballroom I |
Friday, June 21, 2019 9:15AM - 9:45AM |
Y1.00001: What we can learn from quantum molecular dynamics simulations of detonation chemistry: extracting reaction rates, and the search for intermediates Invited Speaker: Romain Perriot The chemistry of energetic materials (EM) is characterized by rapid exothermic reactions that lead to dramatic increase of the pressure and temperature on the pico- to nanosecond timescales. Under these conditions, experiments have struggled to provide detailed insights into early and intermediate processes, and simulations have thus become a valuable tool to help interpret experiments and parameterize mesoscale models. We have performed molecular dynamics (MD) cook-off simulations of EM with DFTB, a parameterized form of DFT that allows to simulate systems with hundreds of atoms, over hundreds of picoseconds, with explicit treatment of the electronic interactions and an accuracy close to that of DFT. We find drastically different times-to-explosion, even for the same initial T/P conditions, due to multiple complex and competitive chemical pathways. However, a simple effective reaction rate can be extracted, as long as multiple simulations are performed at each T/P to account for the stochastic component of detonation chemistry in EM. We apply this methodology to nitromethane and RDX, and determine pressure-dependent activation energies and volumes that can be compared to experimental results and used in higher scale models. Additionally, the atomistic MD resolution allows us to simulate time-resolved IR spectra, that are compared to experiments, and can be used to guide the search for metastable intermediates during the reaction. [Preview Abstract] |
Friday, June 21, 2019 9:45AM - 10:00AM |
Y1.00002: Probabilistic Assessment of the Effects of Microstructure and Voids on the Pop Plot of Heterogeneous Energetic Materials Christopher Miller, Daniel Olsen, Yaochi Wei, David Kittell, Cole Yarrington, Min Zhou We report the results of mesoscale simulations of the shock-to-detonation transition (SDT) of pressed HMX using CTH. Variations in the run distance due to material heterogeneities are systematically quantified using a probabilistic approach, allowing confidence levels to be established as a function of material and shock pressure. Grain size, grain morphology, voids, and grain-to-grain variations in the properties of the constituent are considered. The grains have a mean size of 220 $\mu$m and the voids have a diameter of 50 $\mu$m. The samples considered are 3 by 15 millimeters. The SGL viscoplasticity model, the Mie-Gr\"{u}neisen EOS, and the History Variable Reactive Burn (HVRB) chemistry model are used. Shock loading is effected via the imposition of piston velocities of 600-1200 m/s, leading to shock pressures of 4-11 GPa. Four cases are considered: homogeneous material, material with microstructure but no voids, homogeneous material with voids but no microstructure, and material with both voids and microstructure. The results are in good agreement with experimental data and show that both microstructure and voids strongly influence SDT behavior and decrease the run distance. The importance of both factors is delineated and becomes less pronounced at higher shock pressures. [Preview Abstract] |
Friday, June 21, 2019 10:00AM - 10:15AM |
Y1.00003: The Mechanism of Response of Detonators to AC Mains Voltage Elizabeth Lee High voltage EBW and EFI detonators are designed to fire from low inductance, low resistance energy sources with small capacitors delivering a few thousand volts in tens of nanoseconds. However, other electrical sources will always be present during a device's lifecycle. One such source is mains voltage, the AC mains test applies, in this case, 240V to the bridge of the device. AC mains can deliver electrical energies to detonators which can be greater than the detonation threshold energies but on a longer timescale. Although the delivered energies are greater than the energies required for detonation, typically the reactions are violent, sub-detonative reactions. Therefore, understanding the response of detonators to AC Mains provides valuable insight to the mechanisms determining the safety and performance characteristics. A number of different detonator types and designs have been subjected to different signals, the degree of response measured and possible design related trends identified. Where there was no external disruption X-ray CT scanning has been used to assess the internal damage to the detonator. The electrical, chemical and physical processes determining the response of detonators to mains electricity are identified. \copyright British Crown Owned Copyright / 2019. [Preview Abstract] |
Friday, June 21, 2019 10:15AM - 10:30AM |
Y1.00004: Understanding Spark formation and Growth in Inert Porous Media and the Implications for the Response of Low Density PETN to ESD. Rodney Drake, Daniel Chester, Anthony Glauser, John Richardson, Neil Watkins, Lee Webb Electrostatic discharges can readily initiate energetic materials. The detonation thresholds are dependent on complex interacting mechanisms. As a first step to unravelling the mechanisms, an integrated experimental and modelling study has been undertaken to characterise spark formation and growth characteristics in air and inert porous beds as a function of the circuit parameters (capacitance, resistance, inductance, voltage). An electrical model of the circuit was developed which enabled the various time dependent properties of the spark (for example, resistance and energy) to be calculated. As part of this study alternative spark resistance, including the Braginski-Martin and the Rompe-Weizel models were evaluated. The experimental and modelling results are discussed together with the implications for the spark initiation response of low density PETN. British Crown Owned Copyright 2019. [Preview Abstract] |
Friday, June 21, 2019 10:30AM - 10:45AM |
Y1.00005: Probing Shock-Initiation of Plastic-Bonded Explosives with a Tabletop Microscope Lawrence Salvati, Will Bassett, Belinda Johnson, Zhiwei Men, Dana Dlott Highly heterogeneous energetic materials like plastic-bonded explosives (PBX) are an important example of shock-initiated materials where both microscale and molecular factors couple to its shock-to-detonation transition. The complicated nature of this process necessitates high quality data on the nanosecond timescale, where the shock-to-detonation transition can be directly observed. We explore this time and space scale though a tabletop laser-driven flyer plate initiation process to generate 5-nanosecond shock waves over 40 to 165-micron distances of PBX. Using coupled spectroscopic probes, including nanosecond-scale particle velocity, emission spectroscopy and high-speed gated photography measurements, we probe shock-to-detonation transition of a PETN-based PBX under detonation success and failure conditions. Detonation success cases were characterized by a dramatic rise in bulk cooling rates and emissivity, likely attributed to creation and expansion of product gases. Failure cases were observed to have longer emission lifetimes whose initial hot spot growth can be observed by camera. [Preview Abstract] |
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