Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session E2: Energetic and Reactive Materials II: Shocked Materials |
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Chair: Jon Maienschein, Lawrence Livermore National Laboratory, Shawn McGrane, Los Alamos National Laboratory Room: Grand F |
Monday, June 15, 2015 3:30PM - 3:45PM |
E2.00001: Study of shock initiation in pressed energetic materials using mesoscale simulations H.S. Udaykumar, Nirmal Rai, E.J. Welle, C.D. Molek Pressed energetic materials have complicated microstructure and contain various forms of heterogeneities such as voids, micro-cracks, binders, energetic crystals etc. Shock interaction with the heterogeneities leads to the formation of local heated regions known as hot spots. There are different mechanisms which can lead to the formation of hot spots. However, for pressed energetic materials viscoplastic deformation of voids leading to collapse has been considered to be the most important mechanism. The reaction and specifically its growth in the pressed energetic materials depends on the temperature and location of the hot spots. Hence, an accurate representation of the microstructure is desired for mesoscale study of shock initiation. In the present work, shock initiation on pressed HMX has been studied and ignition threshold for two types of HMX materials have been established. The microstructure geometry is accurately represented using image processing algorithms employed on SEM images of both explosives. The image processing framework is incorporated in a massively parallel Eulerian code SCIMITAR3D for the mesoscale simulations. The chemical decomposition of HMX has been modeled using Henson-Smilowitz multi-step mechanism. The ignition threshold obtained for pressed HMX is compared with experimental results. [Preview Abstract] |
Monday, June 15, 2015 3:45PM - 4:00PM |
E2.00002: Ignition of pressed granular explosives due to short-duration pulse loading Christopher Miller, Seokpum Kim, Min Zhou We report the results of micromechanical simulations of a series of experiments on the ignition of pressed granular HMX under loading due to impact by thin flyers. The conditions analyzed concern loading pulses on the order of 50 nanoseconds to 1 microsecond and impact velocities on the order of 200-1600 m/s. The materials studied have average grain sizes of 50-200 microns. The model provides phenomenological account of defects in the forms of microcracks, voids, interfacial debonding, and constituent property variations and material attributes including constituent shock and non-shock responses, fracture, internal contact, frictional heating, and heat conduction. The analysis focuses on the development of hotspots under different material settings and loading conditions. In particular, a hotspot-based ignition criterion developed recently [Barua et al., Ignition criterion for heterogeneous energetic materials based on hotspot size-temperature threshold, J. Applied Physics; 113, 064906 (2013)] is employed to determine the probability of ignition of each material design under combinations of impact velocity and load duration. The results of parametric studies are compared with experimental observations reported in the literature. [Preview Abstract] |
Monday, June 15, 2015 4:00PM - 4:15PM |
E2.00003: Shock Initiation Experiments with Ignition and Growth Modeling on Low Density Composition B Kevin S. Vandersall, Frank Garcia, Craig M. Tarver Shock initiation experiments on low density ($\sim$1.2 and $\sim$1.5 g/cm$^{3})$ Composition B were performed to obtain in-situ pressure gauge data, characterize the run-distance-to-detonation behavior, and provide a basis for Ignition and Growth reactive flow modeling. A 101 mm diameter gas gun was utilized to initiate the explosive charges with manganin piezoresistive pressure gauge packages placed between packed layers ($\sim$1.2 g/cm$^{3})$ confined in Teflon rings or sample disks pressed to low density ($\sim$1.5 g/cm$^{3})$. The shock sensitivity was found to increase with decreasing density as expected. Ignition and Growth model parameters were derived that yielded reasonable agreement with the experimental data at both initial densities. The shock sensitivity at the tested densities will be compared to prior work published as near full density material. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work was funded in part by the Joint DoD-DOE Munitions Program. [Preview Abstract] |
Monday, June 15, 2015 4:15PM - 4:30PM |
E2.00004: Phase-field modeling of shock-induced $\alpha $-$\gamma $ phase transformation of RDX - Rahul, Suvranu De A thermodynamically consistent continuum phase field model has been developed to investigate the role of shock-induced $\alpha $-$\gamma $ phase transition in the sensitivity of RDX. Dislocations and phase transformations are distinguished and modeled within a crystal plasticity framework. The Landau potential is derived for the finite elastic deformation analysis. The response of the shock loaded RDX crystal is obtained by solving the continuum momentum equation along with phase evolution equation using a Helmholtz free energy functional, which consists of elastic potential energy and local interfacial energy that follows from the Cahn--Hilliard formalism. We observe that the orientations for which there is a resolved shear stress along the slip direction, the material absorbs large shear strain through plastic deformation, allowing it to be less sensitive as less mechanical work is available for temperature rise. Therefore, plastic slip should be associated with greater shear relaxation and, hence, decreased sensitivity. For elastic orientations, large shear stress arises from steric hindrance that may provides much more mechanical work to increase the temperature and hence more sensitive to detonation. Our simulations suggest that the $\alpha $-$\gamma $ phase transformation in RDX may be associated with the increased temperature rise and hence the shock sensitivity. [Preview Abstract] |
Monday, June 15, 2015 4:30PM - 4:45PM |
E2.00005: Theoretical study of $\beta $-HMX decomposition mechanism of the solid phase under shock loadings Guangfu Ji, Nina Ge, Xiangrong Chen Study material properties under extreme conditions is a fundamental problem in the field of condensed matter physics. The decomposition mechanisms of energetic materials under the shock wave become a hot topic in recent years. In this paper, molecular dynamics simulations combined with multi-scale shock technology (MSST) are used to study the decomposition mechanism, shock sensitivity and electronic structure of$\beta $-HMX. First, the decomposition mechanism of $\beta $-HMX perfect crystal were studied at different shock speeds. We found that when the shock wave at a speed 8 km / s is loaded, the decomposition reaction start at N-NO2 bond breakage; when the shock wave at a speed of 10 km / s and 11 km / s is loaded, the the first decomposition reaction is CH bond breaking, and accompanied by the formation of five-membered ring and transfer of hydrogen ions. The simulation results also show that when the shock wave velocity is increased, the higher the pressure generated in the high-pressure N-NO2 bond cleavage was inhibited significantly. Secondly, the impact of its initial chemical reaction process along different crystal axis directions were studied, the results showed that along the a-axis and c-axis shock sensitivity is higher, and along the b-axis sensitivity is lower. We believe that the system of all sensitivity of direction is due to the rotation of the friction between the slip plane of crystals and molecules. Finally, we discussed the solid phase $\beta $-HMX electronic properties change under the shock wave loadings. We found that in the 11 km / s under the impact load, when the pressure reaches 130 GPa , zero bandgap is reached. [Preview Abstract] |
Monday, June 15, 2015 4:45PM - 5:00PM |
E2.00006: Shock response of single crystal and nanocrystalline pentaerythritoltetranitrate: Implications to hotspot formation in energetic materials Yang Cai, Sheng-Nian Luo We investigate shock response of single crystal and nanocrystalline pentaerythritol tetranitrate (PETN) with a coarse-grained model and molecular dynamics simulations, as regards mechanical hotspot formation in the absence or presence of grain boundaries (GBs). In single crystals, shock-induced plasticity is consistent with resolved shear stress calculations and the steric hindrance model, and this deformation leads to local heating. For regular-shaped hexagonal columnar nanocrystalline PETN, different misorientation angles lead to activation of different/same slip systems, different deformation in individual grains and as a whole, different GB friction, different temperature distributions, and then, different hotspot characteristics. GB friction alone can induce hotspots, but the hotspot temperature can be enhanced if it is coupled with GB-initiated crystal plasticity, and the slip of GB atoms has components out of the GB plane. The magnitude of shearing can correlate well with temperature, but the slip direction of GB atoms relative to GBs may play a critical role. GB-related friction and plasticity induce local heating or mechanical hotspots, which could be precursors to chemical hotspot formation related to initiation in energetic materials, in the absence of other, likely more effective, means for hotspot formation such as void collapse. [Preview Abstract] |
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