Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session J6: EM.1 Ignition II |
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Chair: Harold Sandusky, Naval Surface Warfare Center - Indian Head Room: Cascade II |
Tuesday, July 9, 2013 11:00AM - 11:15AM |
J6.00001: Initiation Mechanisms in IHE and CHE Materials Andrew Jardine, David Williamson, Stephen Walley, Stewart Palmer, Claire Leppard Impact sensitivity and subsequent impact initiation is one of the key characteristics of explosive materials. Various standardised tests exist, such as the Rotter or BAM impact tests, which allow the relative sensitivity of different materials to be characterised. However, these provide little insight into the underlying behaviour of the material. The use of a periscopic glass-anvil drop-weight apparatus has proven to provide valuable information about the hotspot initiation of many materials [1,2]. In this paper we describe experiments which apply the technique, in conjunction with high speed video and additional diagnostic instrumentation, to study the mechanism of initiation of modern explosive materials including TATB, LLM-105, Fox-7, HMX, RDX and PETN. \\[4pt] [1] J.E. Field, N. K. Bourne, S. J. P. Palmer, S. M. Walley, Hot-Spot Ignition Mechanisms for Explosives and Propellants, Phil. Trans. R. Soc. Lond. A 339, 269-283 (1992).\\[0pt] [2] J. E. Field, Hot Spot Ignition Mechanisms for Explosives, Acc. Chem. Res. 25, 489-496 (1992). [Preview Abstract] |
Tuesday, July 9, 2013 11:15AM - 11:30AM |
J6.00002: A framework for analyzing the ignition response of energetic materials under dynamic loading Seokpum Kim, Ananda Barua, Min Zhou A multiphysics finite element framework is developed to analyze the ignition response of energetic materials under dynamic loading. The framework uses a cohesive finite element method (CFEM) to capture large deformation, microcracks, and frictional heating. Chemical reactions are incorporated into this framework by accounting for the decomposition of energetic granules according to chemical kinetic models. As an application, the dynamic response of HMX-Estane polymer-bonded explosive (PBX) is analyzed. The focus is on the effect of loading intensity and microstructural attributes on hot spot evolution, coalescence, and ignition. Results suggest that the time taken to form critical hotspots (order of microseconds) from thermo-mechanical dissipation processes is several orders of magnitude smaller than the time taken for ignition to occur (order of milliseconds). Microstructure-performance relations obtained from this analysis can be used to design explosives with tailored attributes and safety envelopes. [Preview Abstract] |
Tuesday, July 9, 2013 11:30AM - 11:45AM |
J6.00003: Modeling pore collapse and chemical reactions in shock-loaded HMX crystals Ryan Austin, Nathan Barton, William Howard, Laurence Fried The collapse of micron-sized pores in crystalline high explosives is the primary route to initiating thermal decomposition reactions under shock wave loading. Given the difficulty of resolving such processes in experiments, it is useful to study pore collapse using numerical simulation. A significant challenge that is encountered in such calculations is accounting for anisotropic mechanical responses and the effects of highly exothermic chemical reactions. In this work, we focus on simulating the shock-wave-induced collapse of a single pore in crystalline HMX using a multiphysics finite element code (ALE3D). The constitutive model set includes a crystal-mechanics-based model of thermoelasto-viscoplasticity and a single-step decomposition reaction with empirically determined kinetics. The model is exercised for shock stresses up to $\sim$10 GPa to study the localization of energy about the collapsing pore and the early stages of reaction initiation. [Preview Abstract] |
Tuesday, July 9, 2013 11:45AM - 12:15PM |
J6.00004: Large-Scale Reactive Atomistic Simulation of Shock-induced Initiation Processes in Energetic Materials Invited Speaker: Aidan Thompson Initiation in energetic materials is fundamentally dependent on the interaction between a host of complex chemical and mechanical processes, occurring on scales ranging from intramolecular vibrations through molecular crystal plasticity up to hydrodynamic phenomena at the mesoscale. A variety of methods (e.g. quantum electronic structure methods (QM), non-reactive classical molecular dynamics (MD), mesoscopic continuum mechanics) exist to study processes occurring on each of these scales in isolation, but cannot describe how these processes interact with each other. In contrast, the ReaxFF reactive force field, implemented in the LAMMPS parallel MD code, allows us to routinely perform multimillion-atom reactive MD simulations of shock-induced initiation in a variety of energetic materials. This is done either by explicitly driving a shock-wave through the structure (NEMD) or by imposing thermodynamic constraints on the collective dynamics of the simulation cell e.g. using the Multiscale Shock Technique (MSST). These MD simulations allow us to directly observe how energy is transferred from the shockwave into other processes, including intramolecular vibrational modes, plastic deformation of the crystal, and hydrodynamic jetting at interfaces. These processes in turn cause thermal excitation of chemical bonds leading to initial chemical reactions, and ultimately to exothermic formation of product species. Results will be presented on the application of this approach to several important energetic materials, including pentaerythritol tetranitrate (PETN) and ammonium nitrate/fuel oil (ANFO). In both cases, we validate the ReaxFF parameterizations against QM and experimental data. For PETN, we observe initiation occurring via different chemical pathways, depending on the shock direction. For PETN containing spherical voids, we observe enhanced sensitivity due to jetting, void collapse, and hotspot formation, with sensitivity increasing with void size. For ANFO, we examine the effect of reaction rates on shock direction, fuel oil fraction, and crystal/fuel oil/void microstructural arrangement. [Preview Abstract] |
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