Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H2: Materials in Extremes IVFocus
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Sponsoring Units: DCOMP DMP SHOCK Chair: Brian Barnes, Army Research Laboratory Room: 261 |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H2.00001: Laser-shocked energetic materials for laboratory-scale characterization and model validation Invited Speaker: Jennifer Gottfried The development of laboratory-scale methods for characterizing the properties of energetic materials, i.e., using only milligram quantities of material, is essential for the development of new types of explosives and propellants for use in military applications. Laser-based excitation methods for initiating or exciting the energetic material offer several advantages for investigating the response of energetic materials to various stimuli: 1) very small quantities of material can be studied prior to scale-up synthesis, 2) no detonation of bulk energetic material is required, eliminating the need for expensive safety precautions, and 3) extensive diagnostics can be incorporated into the experimental setup to provide as much information as possible per shot. In this presentation, progress in our laboratory developing three laser-based methods for characterizing energetic materials will be discussed. Direct excitation of a sample residue using a focused nanosecond laser pulse enables estimation of the performance of the energetic material based on the measured shock wave velocity with a technique called laser-induced air shock from energetic materials (LASEM); recent LASEM results on novel energetic materials will be presented. Impact ignition of energetic materials has also been investigated using laser-driven flyer plates. High-speed schlieren imaging of the flyer plate launch has demonstrated that late-time emission from the impacted energetic material is caused by the reaction of particles ejected off the sample surface with the flyer plate launch products. Finally, the role of a rapid temperature jump (10$^{\mathrm{14}}$ K/s) in the initiation of the explosive cyclotrimethylenetrinitramine (RDX) has been investigated by indirect ultrafast laser heating. Although the temperature jump was insufficient to decompose the RDX, it did induce a temporary electronic excitation of the heated explosive molecules. These results are being used to validate multiscale models in order to understand initiation mechanisms for explosives. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H2.00002: Reduced Order Models for Reactions of Energetic Materials Edward Kober The formulation of reduced order models for the reaction chemistry of energetic materials under high pressures is needed for the development of mesoscale models in the areas of initiation, deflagration and detonation. Phenomenologically, 4-8 step models have been formulated from the analysis of cook-off data by analyzing the temperature rise of heated samples. Reactive molecular dynamics simulations have been used to simulate many of these processes, but reducing the results of those simulations to simple models has not been achieved. Typically, these efforts have focused on identifying molecular species and detailing specific chemical reactions. An alternative approach is presented here that is based on identifying the coordination geometries of each atom in the simulation and tracking classes of reactions by correlated changes in these geometries. Here, every atom and type of reaction is documented for every time step; no information is lost from unsuccessful molecular identification. Principal Component Analysis methods can then be used to map out the effective chemical reaction steps. For HMX and TATB decompositions simulated with ReaxFF, 90{\%} of the data can be explained by 4-6 steps, generating models similar to those from the cook-off analysis. By performing these simulations at a variety of temperatures and pressures, both the activation and reaction energies and volumes can then be extracted. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H2.00003: Abstract Withdrawn
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Tuesday, March 14, 2017 3:30PM - 3:42PM |
H2.00004: Shock Waves and Defects in Energetic Materials, a Match Made in MD Heaven. Mitchell Wood, David Kittell, Cole Yarrington, Aidan Thompson Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. In this talk the shock response of Hexanitrostilbene (HNS) is studied through large scale reactive molecular dynamics (RMD) simulations. These RMD simulations provide a unique opportunity to elucidate mechanisms of viscoplastic pore collapse which are often neglected in larger scale hydrodynamic models. A discussion of the macroscopic effects of this viscoplastic material response, such as its role in hot spot formation, will be provided. Through this work we have been able to map a transition from purely viscoplastic to fluid-like pore collapse that is a function of shock strength, pore size and material strength. In addition, these findings are important reference data for the validation of future multi-scale modeling efforts of the shock response of heterogeneous materials. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H2.00005: Towards validated chemistry at extreme conditions: reactive MD simulations of shocked Polyvinyl Nitrate and Nitromethane Md Mahbubul Islam, Alejandro Strachan A detailed atomistic-level understanding of the ultrafast chemistry of detonation processes of high energy materials is crucial to understand their performance and safety. Recent advances in laser shocks and ultra-fast spectroscopy is yielding the first direct experimental evidence of chemistry at extreme conditions. At the same time, reactive molecular dynamics (MD) in current high-performance computing platforms enable an atomic description of shock-induced chemistry with length and timescales approaching those of experiments. We use MD simulations with the reactive force field ReaxFF to investigate the shock-induced chemical decomposition mechanisms of polyvinyl nitrate (PVN) and nitromethane (NM). The effect of shock pressure on chemical reaction mechanisms and kinetics of both the materials are investigated. For direct comparison of our simulation results with experimentally derived IR absorption data, we performed spectral analysis using atomistic velocity at various shock conditions. The combination of reactive MD simulations and ultrafast spectroscopy enables both the validation of ReaxFF at extreme conditions and contributes to the interpretation of the experimental data relating changes in spectral features to atomic processes. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H2.00006: Initiation of Insensitive High Explosives Using Multiple Wave Interactions Elizabeth Francois Insensitive High Explosives (IHEs) increase safety in many types of weapons. However, the safety comes at the cost of performance. Initiation of IHE requires large boosters and powerful detonators as well. Multipoint initiation is being utilized to exploit explosive wave interactions to create overdriven states, greatly facilitating the initiation of IHEs. This presentation will focus on recent explosive experiments where the minimum spot size for single-point initiation in PBX 9502 was determined. Below this threshold, PBX 9502 could not be initiated. This was then expanded to three initiation points, which were smaller this threshold. Measurements of the velocity and pressure of the wave interactions were measured using Photon Doppler Velocimetry (PDV). Initiation was observed, and the resulting pressures at the double and triple points were found to be above the CJ state for PBX 9502. Further testing will be performed using cutback experiments to isolate the overdriven state, and quantify the duration of the phenomenon. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H2.00007: Microstructural Effects on Initiation Behavior in HMX Christopher Molek, Eric Welle, Barrett Hardin, Jim Vitarelli, Ryan Wixom, Philip Samuels Understanding the role microstructure plays on ignition and growth behavior has been the subject of a significant body of research within the detonation physics community. The pursuit of this understanding is important because safety and performance characteristics have been shown to strongly correlate to particle morphology. Historical studies have often correlated bulk powder characteristics to the performance or safety characteristics of pressed materials. We believe that a clearer and more relevant correlation is made between the pressed microstructure and the observed detonation behavior. This type of assessment is possible, as techniques now exist for the quantification of the pressed microstructures. Our talk will report on experimental efforts that correlate directly measured microstructural characteristics to initiation threshold behavior of HMX based materials. The internal microstructures were revealed using an argon ion cross-sectioning technique. This technique enabled the quantification of density and interface area of the pores within the pressed bed using methods of stereology. These bed characteristics are compared to the initiation threshold behavior of three HMX based materials using an electric gun based test method. Finally, a comparison of experimental threshold data to supporting theoretical efforts will be made. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H2.00008: Impact-induced initiation and energy release behavior of pre-stressed aluminum reactive materials Kevin Hill, Dylan Smith, Michelle Pantoya One approach to improving aluminum (Al) particle reactivity is to anneal and quench particles in order to increase dilatational (volumetric) strain which has also been linked to increased combustion performance. This study compares the reactivity of pre-stressed to as-received Al particles, when each is combined with a solid oxidizer, copper oxide (CuO). Reactivity was examined under dynamic testing techniques for ignition and energy release characteristics. Experiments utilized a drop weight impact apparatus that provided up to 50 J impact energy and included a pressure cell that contained the Al $+$ CuO composite. The cell was instrumented with PCB 101A06 dynamic pressure sensors and measured the pressurization rate and peak pressure from ignited samples. Using fundamental relationships between maximum pressure and the energy deposited into the material, a reaction efficiency is derived. Results show a significant increase in ignition sensitivity and reaction efficiency for the annealed and quenched aluminum particles. Changes in physiomechanical properties of Al particles upon pre-stressing affect particle hardness. These changes lead to significant enhancements in ignition sensitivity, with differences of more than 10 J between pre-stressed and untreated samples. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H2.00009: Comparison of Laser-Induced Plasmas and Electrostatic Discharges for Ignition of Energetic Materials Eric Collins, Jennifer Gottfried Ignition and deflagration experiments with small quantities (5-10 mg) of energetic materials were conducted using either a laser-induced plasma or an electrostatic discharge for ignition. High-resolution emission spectra, time-resolved temperatures, and combustion emission from the deflagration of energetic materials were measured using advanced diagnostics. Ignition of the energetic materials from the extreme environments created by the laser-induced plasma and electrostatic discharge showed similar behavior including particle ejection, heating of particles from the plasma/spark, and a shockwave formation. The shock waves generated by the laser-induced plasma and the electrostatic discharge were analyzed at various energy levels using schlieren imaging. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H2.00010: Particle size and surface area effects on the thin-pulse shock initiation of Diaminoazoxy Furazan (DAAF) Rosemary Burritt, Elizabeth Francois Diaminoazoxy furazan (DAAF) has many of the safety characteristics of an IHE: it is extremely insensitive to impact and friction and is comparable to triaminotrinitrobezene (TATB) in this way. Conversely, it demonstrates many performance characteristics of a CHE. DAAF has a small failure diameter of about 1.25 mm and can be sensitive to shock under the right conditions. Large particle size DAAF of 40 $\mu $m has been ball milled and crash precipitated into a variety of smaller particle sizes. DAAF pellets were tested in an exploding foil initiator configuration, by varying flyer thickness and diameter, the relation to pulse duration and flyer diameter was examined. Larger particle sized DAAF requires more energy to initiate and a flyer diameter. We will present the initiation characteristics of DAAF, and the parameter space in which it can be initiated in a slapper configuration.. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H2.00011: Nanoscale Heat Conduction in Crystalline Solids Joel Christenson, Ronald Phillips Heat conduction in crystalline solids occurs through the motion of molecular-scale vibrations, or phonons. In continuum scale problems, there are sufficient phonon-phonon interactions for local equilibrium to be established, and heat conduction is accurately described by Fourier's law. However, at length scales comparable to the phonon mean free path, Fourier's law becomes inaccurate, and more fundamental descriptions of heat transfer are required. We are investigating the viability of the phonon Boltzmann Transport Equation (BTE) to describe heat conduction in nanoscale simulations of the high-explosive material $\beta $-HMX. By using a combination of numerical and analytic solutions of the BTE, we demonstrate the existence of physical behavior that is not qualitatively captured by the classical Fourier's law in the nanoscale regime. The results are interpreted in terms of continuum-scale simulations of shock-induced collapse of air-filled pores in $\beta $-HMX, which is believed to be a precursory step towards complete detonation of the material. [Preview Abstract] |
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