Bulletin of the American Physical Society
16th APS Topical Conference on Shock Compression of Condensed Matter
Volume 54, Number 8
Sunday–Friday, June 28–July 3 2009; Nashville, Tennessee
Session Y1: DC-6: Detonation Initiation |
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Chair: Craig Tarver, Lawrence Livermore National Laboratory Room: Magnolia Ballroom |
Friday, July 3, 2009 8:00AM - 8:30AM |
Y1.00001: Modeling thermal ignition and the initial conditions for internal burning in PBX 9501 Invited Speaker: Work has been ongoing in our group for several years to produce a thermal ignition model for HMX based plastic bonded explosives valid over the entire temperature range of energetic response. We have made considerable progress recently, resulting in both the first broadly accurate model of this type and the possible identification of a crucial component of the chemical mechanism responsible for thermal ignition and decomposition. I will present a new model of thermal ignition for HMX formulations that is based on this recent progress.~ The model is similar in kind, but very different in detail from previous models produced by us and others.~ As has been the case for our previous models it is based entirely on known processes in the decomposition of HMX and is highly constrained by independent measurements. We have applied the model in simple calculations of ignition time over the full temperature range of energetic response for HMX, including directly observed ignition induced by fast shear and compression. I will also present new calculations relevant to the initial conditions for internal burning subsequent to ignition in low boundary temperature thermal explosion experiments. Simplified gas phase chemistry relevant to both dark and bright zone burning in HMX has been included and leads to a second, high temperature and pressure ignition zone in this environment. I will discuss experimental support for these calculations and the ramifications for internal pressures at ignition responsible for driving initial subsonic burning subsequent to ignition. [Preview Abstract] |
Friday, July 3, 2009 8:30AM - 8:45AM |
Y1.00002: Influence of hot spot features on the shock initiation of heterogenous nitromethane Dana Dattelbaum, Stephen Sheffield, David Stahl, Andrew Dattelbaum The shock initiation sensitivity of heterogeneous explosives is known to be strongly related to the confluence of ``hot spots'' or localized regions of high pressure and temperature. Physical origins of hot spots within a material include dynamic pore collapse, friction from motion along closed cracks, and wave reflections from other \textit{in situ} interfaces. A complex interplay among numerous physical and chemical factors, spanning several length scales, determines whether or not a hot spot will quench or lead to initiation. To further elucidate key features of hot spots on energetic materials sensitivity and initiation mechanisms, we have intentionally introduced well-defined particles into the homogeneous liquid explosive nitromethane which has been gelled so the particles are somewhat stationary. Gas-gun driven shock initiation experiments using embedded electromagnetic gauging methods have been performed on these materials, revealing new insights into the role of heterogeneities on the sensitivity of the explosives through shock input-to-run distance relationships (Pop-plots), and reactive chemistry growth in and behind the incident shock front. By logically mapping out these relationships, the data provide a scientific foundation for the development of predictive capabilities for modeling new formulations, and designing next-generation energetic materials. [Preview Abstract] |
Friday, July 3, 2009 8:45AM - 9:00AM |
Y1.00003: Initiation of Secondary Explosives Measured Using Embedded Electromagnetic Gauges Chris Stennett, Gary Cooper, Paul Hazell, Gareth Appleby-Thomas There is considerable evidence that secondary explosive materials having a relatively large (10-12\%) proportion of HTPB binder do not exhibit DDT under cook-off. However, the understanding of the mechanisms controlling the growth of reaction in such experiments is incomplete. Most importantly, it is not known whether a mechanistic reason exists to preclude DDT; it is possible that existing techniques to explore cook-off simply do not offer the correct conditions to allow DDT to occur. We present experiments in which impacts were made against an RDX/HTPB PBX using a single-stage light gas gun. Electromagnetic particle velocity gauge packages were embedded within the targets, placing sensing elements at different distances from the impact face, to record the onset of reaction, and in some cases detonation. These experiments were also performed against RDX/TNT 60:40 targets. The time-resolved particle velocity histories have allowed comparison of some of the factors governing growth of reaction, and have provided run distance to detonation data for different impact stresses. [Preview Abstract] |
Friday, July 3, 2009 9:00AM - 9:15AM |
Y1.00004: Shock Initiation Experiments Plus Ignition and Growth Modeling of Damaged LX-04 Charges Steven Chidester, Frank Garcia, Kevin Vandersall, Craig Tarver Shock initiation experiments were performed on mechanically or thermally damaged LX-04 (85{\%} LX-04, 15{\%} Viton by weight) to obtain in-situ manganin pressure gauge data and run distances to detonation at various shock pressures. The LX-04 charges were damaged mechanically by applying a compressive load of 600 psi for 20,000 cycles, thus creating many small narrow cracks, or by cutting wedge shaped parts that were then loosely reassembled, thus creating a few large cracks. The thermally damaged LX-04 charges were heated to 190$^{\circ}$C for long enough for the beta to delta phase transition to occur, and then cooled to ambient temperature. The densities of the damaged LX-04 charges were measured before shock initiation. Mechanically damaged LX-04 exhibited only slightly increased shock sensitivity, while thermally damaged LX-04 was much more shock sensitive. The Ignition and Growth model calculated this increased sensitivity by igniting more damaged LX-04 near the shock front. This work was performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Friday, July 3, 2009 9:15AM - 9:30AM |
Y1.00005: Modeling Short Shock Pulse Duration Initiation of LX-16 and LX-10 Charges Chadd May, Craig Tarver Short pulse duration shock initiation experiments were performed on LX-16 (96{\%} PETN, 4{\%} FPC 461 binder by weight) and LX-10 (95{\%} HMX, 5{\%} Viton binder by weight) using electrically driven kapton and Parylene-C flyer plates. Critical impact velocities for initiation at several flyer plate thicknesses and diameters were determined. The Ignition and Growth reactive flow model parameters previously developed for LX-16 and LX-10 were used to calculate these experiments. Good agreement was obtained between the experimental and calculated critical impact velocities that caused shock initiation of the explosive targets. This work was performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Friday, July 3, 2009 9:30AM - 9:45AM |
Y1.00006: Changes in Run Distance Observed in two explosives at the threshold for sustained ignition using the Modified Gap Test Richard Lee, Jerry Forbes, Douglas Tasker, Rebecca Orme The Modified Gap Test was used to quantify different levels of partial reaction for various input stresses. This test configuration has been historically useful in highlighting thresholds for first reaction, sustained ignition, and detonation. Two different HMX based compositions were studied; a cast-cured composition with 88{\%} HMX and a pressed composition with 92{\%} HMX. The final ingredients of each comprised different unreactive polymeric binder systems. Short samples (50.8 mm in diameter and 12.7 mm thick) were shock loaded using the standard large scale gap test donor system. Product-cloud blow-off velocities at the opposite end of the sample were measured using a high-speed digital-camera. Velocity versus input pressure plots provided changes in reactivity that had developed by the 12.7 mm run distance. Results were fairly consistent for the lower input pressures. In contrast, the results varied widely in a range of input stresses around the transition threshold for sustained ignition in both explosives. These results indicate that both explosives are subject to variation in run to detonation distance in a range of input stresses just prior to prompt detonation. [Preview Abstract] |
Friday, July 3, 2009 9:45AM - 10:00AM |
Y1.00007: Characterisation of an Exploding Foil Initiator (EFI) system H.R. Davies, D.J. Chapman, T.A. Vine, W.G. Proud Exploding Foil Initiators (EFIs) provide a safe and reliable means of detonation of explosives. They are highly insensitive to mechanical shock and electrical interference, requiring a specific high current pulse for initiation. The use of only insensitive secondary explosives and not more sensitive primary explosives further improves safety. When a high current is passed through the metal bridge, a plasma is formed as the metal can not expand beyond the polymer film layer above. This causes the film to expand forming a bubble or shearing off to form a flyer. These flyers can then be used to initiate secondary explosives. Due to the very high speed at which these systems operate, high speed streak photography was used to characterise the behaviour of the polymer film flyers produced. This paper will report the preliminary findings on the mechanical, electrical and velocity changes seen in some proprietary systems. [Preview Abstract] |
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