Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A23: Materials in Extremes: Energetic Materials - IFocus Live
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Sponsoring Units: GSCCM Chair: Kien Nguyen Cong, University of South Florida |
Monday, March 15, 2021 8:00AM - 8:12AM Live |
A23.00001: Hotspot formation in amorphous energetic materials due to shock-induced void collapse CHUNYU LI, Michael Sakano, Alejandro Strachan High energy explosives are typically used in their common crystalline form. Most studies have been focused on crystalline phases. Molecular crystals tend to exhibit polymorphism and can often be engineered to form amorphous solids, lacking long-range order. Amorphous high-energy density materials open interesting opportunities in terms of processing and properties not accessible to crystals. While amorphous formulations lack defects typical of crystals such as dislocations and grain boundaries, and their processing can minimize cracking and sharp surface angles, porosity is a common defect. The collapse of porosity is known to play a central role in the initiation of detonation and this contribution uses molecular dynamics simulations to characterize this process. We find that, just as in crystals, the collapse of cylindrical voids exhibits a transition from viscoelastic to a hydrodynamic with increasing shock strength. Interestingly, in the viscoelastic regime the collapse of pores in the amorphous HMX results in higher temperatures as compared the crystalline case. In contrast, the hotspot temperature is lower in the amorphous phase than in the crystal for strong shocks, in the hydrodynamic regime. The simulations reveal the molecular level mechanisms behind these observations. |
Monday, March 15, 2021 8:12AM - 8:24AM Live |
A23.00002: The dependence of deflagration reaction properties on initiation method Olivia Morley, David Williamson Deflagration reactions are very complicated; efforts to model temperature, pressure and chemistry are underway with the aim of simulating the reaction under a range of conditions. In the current study, deflagration of the explosive HMX was initiated using a BAM impact tester, as well as a Split Hopkinson pressure bar (SHPB), to investigate the dependence of reaction conditions on the initiation method. The BAM impact tester initiated reaction at 0.6 GPa, whereas the SHPB required a pressure of 1 GPa to initiate. Under higher pressure, SHPB deflagration had a lower reaction temperature of 2900 ± 200 K compared to 3900 ± 400 K in BAM impact, with optical spectroscopy also indicating the presence of more solid particles compared to the BAM impact. Use of a mass spectrometer showed the two reactions occupy different areas in products-temperature space; they have different ‘typical’ chemical pathways, or at least the same multiplicity of reactions in differing proportions. As such, deflagration cannot be considered to have the same chemistry across our experiments; the overall ‘ideal’ reaction, and therefore other properties such as temperature, are dependent on initiation method and the prevailing local environment. |
Monday, March 15, 2021 8:24AM - 8:36AM Live |
A23.00003: PDV measurements of the chemical reaction zone of pressed TNT Arnaud Sollier, Philippe Hébert, Eric Bouton In order to have a better insight into the chemical reaction zone of pressed 2,4,6-trinitrotoluene (TNT), laser interferometry has been used to record nanosecond time resolved particle velocity profiles of the free surface of detonating trinitrotoluene (TNT) charges and of the interfaces between TNT detonation products and both lithium fluoride (LiF) and PMMA windows. The experiments consisted in initiating a detonation wave in a cylinder (75 mm length, 15 mm diameter) of pressed TNT (ρ0=1.55 g/cm3) using an explosive wire detonator associated with an explosive booster, and in measuring either the explosive free surface velocity or the explosive/window interface velocity by means of a 1550 nm frequency shifted photonic Doppler velocimetry (PDV) system. The direct comparison of free surface and interface velocity wave profiles allows us to precisely determine the von Neumann spike parameters, and also to derive values for the Chapman-Jouguet pressure and for the reaction time. Our experimental records for pressed TNT were found to be consistent with earlier reports. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A23.00004: A study of energy localization mechanisms in PBXs under shock loading through interface-resolved reactive simulations Shobhan Roy, Pratik Das, Xuan Zhou, Belinda Pacheco, Hoya Ihara, Dana D Dlott, H. S. Udaykumar The shock-induced initiation of plastic bonded explosives (PBXs) is a complex phenomenon wherein various mechanisms are at play at the grain-scale. For example, the interaction of binder-crystal and crystal-crystal interfaces, the shock focusing due to tortuosity of the embedded microstructures, and the collapse of pores may lead to energy localization followed by ignition of HMX crystals. However, the relative importance of these mechanisms is not well understood. In this work, we take a computational approach to investigate the mechanistic details of shock response of HMX crystals embedded in polyurethane binder, for a set of actual images of PBX samples. We have employed interface-resolved reactive simulations using a sharp-interface Eulerian framework. The interfaces are represented using levelsets, with appropriate boundary conditions being enforced using the ghost fluid method. High-resolution calculations are performed on crystals of different shapes, sizes, and crystals with/without pores. The results show that energy localization due to collapse of pores in HMX crystals is the dominant mechanism of hotspot formation at the short timescales relevant to shock-to-detonation transition in PBXs. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A23.00005: Observing and Modelling Hot Spot Formation in Individual HMX Grains Belinda Pacheco, Xuan Zhou, Hoya Ihara, Shobhan Roy, Pratik Das, H.S. Kumar, Dana D Dlott Microstructure influences the ignition and growth of hot spots in energetic materials (EM) which in turn effects their sensitivity and performance. We aim to better understand the underlying physics behind EM sensitivity by: 1) conducting high-throughput experiments on individual grains of HMX embedded in polymer revealing hot spot locations and temperatures, and 2) comparing experimental results with microstructurally informed, reactive simulations. The experiments employed laser-driven impactors to shock individual HMX grains embedded in polymer and fabricated into arrays. Simultaneous gated, multi-frame imaging and optical pyrometry were used as the primary diagnostics. By combining fast-frame imaging with pyrometry, we can identify regions of preferred hot spot formation with 1 µm resolution, while tracking hot spot temperatures every 2 ns. Single and defective HMX crystals were tested across several polymer and at varying impactor velocities to identify critical pressure thresholds for hot spots. Simulations were conducted using interface-resolved reactive simulations using a sharp-interface Eulerian framework. This new methodology provides the means to evaluate the influence of microstructural energy localization and predict mesoscale behavior of plastic-bonded explosives. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A23.00006: Formulation of Reduced Order Chemistry Models for Energetic Materials Edward Kober A reduced order reactive chemistry model for the energetic material HMX has been formulated based on reactive molecular dynamics simulations using ReaxFF-lg. Simulations on 240 molecules were performed over a large range of initial densities and temperatures, and the results were analyzed by classifying each atom in the simulations according to its coordination environment. The time evolution of these geometries during a molecular dynamics simulation creates an array of data that can be transformed into a reduced chemistry model using the Non-negative Matrix Factorization technique. This captures the general reaction characteristics (e.g. reduction of nitrogen, oxidation of carbon) in a series of correlated chemical waves. From all of the simulations, a 7-component, pressure-dependent Arrhenius reaction rate model was formulated. In particular, separate high- and low-pressure reaction branches were identified. Large-scale 1D simulations (≥1M atoms) of both thermal ignition and void collapse were then performed and analyzed with this model. The flame structure found in both cases mapped quite well onto the derived chemistry model and reflected the expected pressure dependence. At higher pressures, the flame was quite thin (~10 nm) and propagated at ~100 m/s. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A23.00007: Implications of Solvent Inclusions on Shock Initiation of HMX David Hardin, Matthew Stuthers, Jim Vitarelli, Christopher Molek We have detected and propose a plausible understanding of the impacts of solvent entrapment in HMX. Solvent inclusions can have significant impacts on aging and performance, as observed with RDX. In HMX, it was determined that solvent inclusions generate tortuous void structures. The structures are on the scale which may impact the initiation behavior and performance. A detailed look at the solvent inclusions revealed buoyant bubbles which orient themselves depending on the crystal position. Impacts of these findings have been systematically studied using CTH modeling and simulations. Overall results are described with potential impacts on initiation. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A23.00008: Void collapse in shocked β-HMX single crystals across scales Camilo Duarte, Chunyu Li, Brenden Hamilton, Marisol Koslowski, Alejandro Strachan Heat generation in the vicinity of a void during shock compression plays a critical role in the initiation of high explosives (HE). Atomistic simulations of β-HMX under shock compression have shown that the void collapse regime transitions from viscoplastic to hydrodynamic jetting as the shock strength increases. However, they are limited to very small length and time scales and are computationally costly. Then, a mesoscale model informed with atomistic simulations results is needed to study the anisotropic response of shocked single β-HMX crystals at larger scales, and to understand similarities\differences of the deformation response across scales. In this work, the shock response of a β-HMX single crystal containing a void is studied with finite element simulations that include plasticity and heat transport. The effects of crystal orientation and impact velocity on the deformation response of the single crystal are discussed. The model is calibrated with non-reactive molecular dynamics simulations. Results are compared with both atomistic simulations and gas gun experimental results of β-HMX containing a single void. |
Monday, March 15, 2021 9:36AM - 10:12AM Live |
A23.00009: Time resolving the loss of crystallinity during detonation in a secondary solid explosive Invited Speaker: Pamela Bowlan There are still significant uncertainties in our ability to predict and control detonation in secondary solid explosives which has serious implications for the safety and performance of explosives. One reason is for this uncertainty is that while chemical kinetics are well understood in gases and liquids, much less is known about how chemistry proceeds within a crystalline lattice. Secondly, events like detonation, where a bulk material can go from ambient conditions to pressures of Gigapascals (GPa) and temperatures of about 4000 kelvin (K) within nanoseconds (ns), are extremely difficult to directly observe. To better understand the role of the loss of crystallinity and how this affects temperature and chemical kinetics during a detonation, we developed a technique using visible laser scattering to probe morphology changes on a nanosecond time scale before and during a detonation. We will present our results applying this to several common secondary solid explosives, PETN, HMX and TATB, and considering steady detonation, initiation of detonation, and failure scenarios. These measurements reveal when during a detonation wave, and how fast that the initial crystals change into the product fluid giving new insight into the microscopic mechanism of a detonation in solid explosives. |
Monday, March 15, 2021 10:12AM - 10:24AM Live |
A23.00010: Machine-Learning Improved Density Functional Tight Binding Models for Energetic Materials Rebecca Lindsey, Sorin Bastea, Nir Goldman, Laurence E. Fried Atomistic energetic materials (EM) modeling efforts are often encumbered by a lack of efficient interatomic interaction potentials (IAPs) suitable for describing molecular materials under extreme conditions. The machine-learned ChIMES IAP was recently developed to overcome this challenge and has been shown highly effective for many materials under high T and p, but EM parameter set generation is non-trivial. ChIMES development hinges upon Kohn-Sham density functional theory (DFT) molecular dynamics reference data. Ideally, reference simulation lengths would be set by the target system’s characteristic relaxation times, but chemical evolution in EM often occurs on timescales beyond the reach of DFT. Density functional tight binding (DFTB) is a practical alternative to DFT which is ≈103 more efficient and capable of similar predictive power. However, DFTB parameter sets are not designed for EM or extreme conditions. Here, we show how ChIMES can be used to generate corrections to standard DFTB models with relatively little DFT training data, enabling quantum-accurate EM simulations on timescales inaccessible to DFT. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-816398 |
Monday, March 15, 2021 10:24AM - 10:36AM Live |
A23.00011: Experimental Observations of Exploding Bridgewire Detonator Function Laura Smilowitz We have studied the behavior of exploding bridgewire detonators using a variety of observables for measuring temperature and density during detonator function. Continuous density movies have been collected using either proton or x-ray radiography and continuous light emission videos simultaneously measured using ultra-high speed video cameras. Spectrally resolved and single broadband pyrometric measures of temperature are also measured. We attempt to provide a description of the mechanism of function for EBW detonators based on the full suite of observations made and comparisons to studies in the literature. In this talk, we will show the results of measurements on PETN based EBW detonators run in both hardfire and near threshold conditions. In a companion talk, we will show the results of PETN response as a function of detonator voltage and temperature. |
Monday, March 15, 2021 10:36AM - 10:48AM Live |
A23.00012: The Role of Soot Formation in TATB Detonation Joel Christenson, Laurence E. Fried The detonation of (CHNO)-based insensitive high explosives (IHEs) yields carbon-rich soots, the formation of which may increase the reaction zone compared to conventional high explosives. In particular, triaminotrinitrobenzene (TATB)-based IHEs produce soots with a relatively large nitrogen content, which may further affect explosive performance by attenuating energy release during detonation. The composition and thermodynamic properties of TATB-based soots are still under investigation. Therefore, to assess the validity of thermochemical codes, which rely on experimental and theoretical measurements of these properties, it is of interest to perform a sensitivity analysis on the effects of soot properties on detonation response. In this work, we use recent measurements of soot composition, in conjunction with the thermochemical code Cheetah, to examine the sensitivity of soot parameters on measures of detonation performance, including the detonation velocity and Chapman-Jouguet pressure. These results will inform future experimental and theoretical efforts towards enhancing the predictive capabilities of thermochemical calculations. |
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