Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session V6: Explosives Thermal/Mechanical Response |
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Chair: Laura Smilowitz, Sandia National Laboratories Room: Fairmont Orchid Hotel Promenade I/II |
Friday, June 29, 2007 10:30AM - 11:00AM |
V6.00001: Violent Reactions from Non-Shock Stimuli Invited Speaker: Most reactions are thermally initiated, whether from direct heating or dissipation of energy from mechanical, shock, or electrical stimuli. For other than prompt shock initiation, the reaction must be able to spread through porosity or over large surface area to become more violent than just rupturing any confinement. While burning rates are important, high-strain mechanical properties are nearly so, either by reducing existing porosity or generating additional surface area through fracture. The first example is deflagration-to-detonation transition (DDT) in porous beds. During the early stages, weak compressive waves ahead of the convective ignition front will reduce porosity, thereby restricting the spread of combustion and the pressure buildup. If, however, pressure increases faster than can be relieved by loss of confinement, coalescing compressive waves can initiate reaction at hot spots from rapid pore collapse. This compressive reaction can drive a shockwave that transits to detonation, the most violent reaction in any scenario. It has been shown that reaction violence is reduced in DDT experiments if the binder is softened, either by raising the initial temperature or adding a solvent. An example of the role of mechanical properties in enhancing reaction violence through fracturing occurs when cavities in projectile fills collapse during acceleration in the gun barrel, which is referred to as setback. Explosives with soft rubber binders will deform and undergo mild reaction from shear heating within the explosive and adiabatic compression of any gas in the cavity. Stiff explosives are similarly ignited, but also fracture and generate additional surface area for a violent event. The last example to be considered is slow cook-off, where thermal damage can increase burning rate as well as provide porosity to enhance the pressure buildup. As reaction spreads from the zone of thermal run-away, an explosive binder that resists breakup will limit the violence. [Preview Abstract] |
Friday, June 29, 2007 11:00AM - 11:15AM |
V6.00002: ABSTRACT WITHDRAWN |
Friday, June 29, 2007 11:15AM - 11:30AM |
V6.00003: Thermal properties of a UK PBX and binder system Stewart Palmer, David Williamson, William Proud The thermal conductivity, diffusivity and heat capacity of a UK PBX and binder system have been measured over a temperature range of ambient to approximately 120 \r{ }C. Independent measurements of any two of the above, and knowledge of the density, allows the third to be calculated. Comparisons between the directly measured and calculated values give an indication of the reliability of such data. Thermal conductivity measurements were made using the Lee's disc method, thermal diffusivity via {\AA}ngstr\"{o}m's method and heat capacity via Differential Scanning Calorimetry (DSC). Such data are required for the development and validation of PBX thermal models. This paper outlines the current state of research and details the important observations to date. [Preview Abstract] |
Friday, June 29, 2007 11:30AM - 11:45AM |
V6.00004: Critical Temperature Formula for a Body of Arbitrary Size and Shape Larry Hill The Frank-Kamenetskii thermal explosion model provides a framework for calculating the critical temperature for an energetic material body of any size and shape. The calculation involves finding a dimensionless shape parameter, which, except for the case of an infinite cylinder, must be determined numerically. This exercise is easy enough for simple symmetric geometries such as the infinite slab, infinite cylinder, and sphere, and these results are well known. But for arbitrary bodies the manipulations are cumbersome, to the extent that they are almost never undertaken in practice. It is therefore desirable to deduce a formula that can, to a good approximation, predict the critical temperature of an arbitrary body without the necessity of a heat transfer calculation. Over the past $\sim $60 years, several attempts to find a universal formula have been made---none of which have been completely successful. It is not too difficult to develop a methodology that can reproduce the shape factor of the three canonical objects---infinite slab, infinite cylinder, and sphere. The challenge is to develop a methodology, which, while reproducing the three canonical objects, can \textit{also} correctly distinguish the shape factors of, say, the sphere, cube, and a unity aspect ratio cylinder. I will present a method that can do so. [Preview Abstract] |
Friday, June 29, 2007 11:45AM - 12:00PM |
V6.00005: Burn Propagation in a PBX 9501 Thermal Explosion Bryan Henson, Laura Smilowitz, Jerry Romero, Blaine Asay, Mary Sandstrom We have measured burn velocities in a series of radially heated PBX 9501 thermal explosion experiments. Burn fronts have been imaged in these experiments using proton radiography. The velocities observed imply a convective burn front moving at approximately 200m/s. A compendium of burn velocities verses pressure show two distinct burn mechanisms: convection and conduction. The 200m/s velocity places the PBX 9501 radial thermal explosion experiment in the convective regime with an implied pressure on the order of 1 Gpa. The density evolution observed shows that HE continues to be consumed behind the convective front. The HE consumption follows the approximate radial symmetry of the heating profile. A hypothesis for the material consumption is made based on the implied pressures and material state. Implications for incorporating PBX 9501 thermal ignition and burn into larger scale models are discussed. [Preview Abstract] |
Friday, June 29, 2007 12:00PM - 12:15PM |
V6.00006: Non-random Crack Opening in Partially-Confined, Thermally-Damaged PBX 9501 and Observations on its Effects on Combustion Gary Parker, Blaine Asay, Peter Dickson, Philip Rae, Axinte Ionita During thermal insult to PBX 9501, damage in the form of cracking has been shown to modify the combustion mode and violence after ignition, since flames can intrude at lower pressures through cracks than the more tortuous matrix porosity. However, for fluid transport processes, including volumetric combustion and gas permeation, to occur the porosity must be open and accessible to the surfaces of the charge. Because many charges are encased (confined), there are mechanical limitations on the extent to which thermally-induced cracks can open, therefore diminishing their effect on transport to some concomitant extent. In this work, we present evidence for how strong radial confinement can result in aligned crack opening, despite the existence of plentiful randomly-oriented, though apparently closed, cracks. We damage cylinders in tight-fitting glass or metal sleeves open on both ends and observe the occurrence of preferential crack-opening normal to the cylindrical axis. This geometry and confinement is common in experimental arrangements such as strand burners and DDT tubes. Further, we observe, with high speed photography, how this non-random crack opening affects combustion and propose a mechanism, garnered from linear elastic finite element modeling, for why it occurs. [Preview Abstract] |
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