Melvin R. Baer

A thermomechanical analysis of hot spot formation in condensed-phase, energetic materials

This article discusses the mechanics of shock-induced hot spot formation in porous, energetic materials. A hollow sphere configuration is used to simulate the dynamic and thermodynamic response of a single void centered in a field of condensed-phase energetic material. In addition to treating pore dynamics, the hot spot model includes energy balances for the pore gas and surrounding material. Important thermal processes such as viscoplastic heating, finite rate chemical effects, and heat exchange between the pore gas and surrounding material are also evaluated. The governing conservation equations together with the initial and interface conditions are solved numerically for a series of test cases for a nitramine material. The results show that viscoplastic heating is an effective mechanism for shock initiation of porous, energetic materials. In addition, it is demonstrated that the initial porosity of the material, the initial pore size and the material viscosity have strong influences on hot spot formation.

A compact strip-line pulsed power generator for isentropic compression experiments.

Veloce is a medium-voltage, high-current, compact pulsed power generator developed for isentropic and shock compression experiments. Because of its increased availability and ease of operation, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed with larger pulsed power machines such as the Z accelerator. Since the compact pulsed power technology used for dynamic material experiments has not been previously used, it is necessary to examine several key issues to ensure that accurate results are obtained. In the present experiments, issues such as panel and sample preparation, uniformity of loading, and edge effects were extensively examined. In addition, magnetohydrodynamic simulations using the ALEGRA code were performed to interpret the experimental results and to design improved sample/panel configurations. Examples of recent ICE studies on aluminum are presented.

Investigation of the mesoscopic scale response of low-density pressings of granular sugar under impact

The mesoscopic scale response of low-density pressings of granular sugar (sucrose) to shock loading has been examined in gas-gun impact experiments using both VISAR and a line-imaging, optically recording velocity interferometer system in combination with large-volume-element, high-resolution, three-dimensional numerical simulations of these tests. Time-resolved and spatially resolved measurements of material motion in waves transmitted by these pressings have been made as a function of impact velocity, sample thickness, and sample particle size distribution. Observed wave profiles exhibit a precursor regime arising from elastic stress wave propagation and a dispersive compaction wave with superimposed localized particle velocity fluctuations of varying amplitude. Material motion associated with dynamic stress bridging leads compaction wave arrival by ∼2μs at the lowest impact velocity (0.25kms−1) employed in this study and <200ns at the higher values (0.7–0.8kms−1). Over the same range, the compaction wa...

Isentropic loading experiments of a plastic bonded explosive and constituents

The plastic bonded explosive PBX 9501 and its constituents [cyclotetramethylene tetranitramine (HMX) crystals, nitroplasticized Estane ®5703 and a fine-crystallite HMX laden binder mixture] were subjected to a ramped quasi-isentropic compression load using the Z machine at Sandia National Laboratories to determine equation of state and constitutive property data. Various sample thicknesses of these materials were subjected to an identical ramp loading history up to 4.5 GPa over 350 ns and particle velocities were measured using a velocity interferometry technique to assess material response. Upon defining appropriate constitutive relationships for the individual constituents, a topologically disordered model of the composite material was numerically simulated and details of the mesoscale simulation indicate that much of the plastic deformation first occurs locally at the large HMX crystal contacts points and subsequently by the deformation of the interstitial fine-crystallite/binder material.

JCZS: An Intermolecular Potential Database for Performing Accurate Detonation and Expansion Calculations

Exponential-13,6 (EXP-13,6) potential pammeters for 750 gases composed of 48 elements were determined and assembled in a database, referred to as the JCZS database, for use with the Jacobs Cowperthwaite Zwisler equation of state (JCZ3-EOS)~l) The EXP- 13,6 force constants were obtained by using literature values of Lennard-Jones (LJ) potential functions, by using corresponding states (CS) theory, by matching pure liquid shock Hugoniot data, and by using molecular volume to determine the approach radii with the well depth estimated from high-pressure isen- tropes. The JCZS database was used to accurately predict detonation velocity, pressure, and temperature for 50 dif- 3 Accurate predictions were also ferent explosives with initial densities ranging from 0.25 glcm3 to 1.97 g/cm . obtained for pure liquid shock Hugoniots, static properties of nitrogen, and gas detonations at high initial pressures.

An experimental and theoretical study of deflagration-to-detonation transition (DDT) in the granular explosive, CP☆

Abstract In this paper, we present results of an experimental and theoretical study of the combustion processes associated with deflagration-to-detonation transition in the granular explosive, CP. Image-enhanced, high-speed streak photography recorded the various stages of flame spread within heavily confined charges as viewed through a Lexan window in a thick-wall, stainless steel tube. To characterize the phenomena, we applied a multiphase reactive flow model based on the theory of mixtures. This nonequilibrium model treated each phase as fully compressible and incorporated a compaction model for the granular reactant. Formulation of the constitutive models included a pressure-dependent burn rate and experimentally determined porous bed permeability. Predictions of this model were in good agreement with experimental observations.

Investigation of Dispersive Waves in Low‐Density Sugar and HMX Using Line‐Imaging Velocity Interferometry

A line‐imaging optically recording velocity interferometer system (ORVIS) has been used in gas‐gun impact experiments to compare the mesoscopic scale response of low‐density (65% theoretical maximum density) pressings of the explosive HMX to that of an inert simulant (granulated sugar). Dispersive waves transmitted through 2.27‐ to 6.16‐mm‐thick beds of the porous sugar typically include mesoscale fluctuations that occur on length scales consistent with those seen in 3‐D numerical simulations. Conditions that approximate steady wave behavior occur at a sample thickness ⩾4 mm. Transmitted wave profiles in HMX include complex effects of chemical reaction. For coarse‐grain HMX samples, reaction expands vigorously over a narrow range (0.4–0.47 km‐s−1) of impact velocity. Localized regions of reaction growth are evident in the spatially resolved velocity‐time data.

Interaction of a planar shock with a dense field of particles.

Understanding the particle-particle and shock-particle interactions that occur in dense gas-solid flows is limited by a lack of knowledge of the underlying phenomena. Gas-solid flows are characterized by the particle volume fraction φp of the flow [1]. For particle volume fractions less than about 0.1%, flow is considered dilute and the effects of particle collisions are negligible [2]. For packed particles, where the φp is greater than about 50%, the flow regime is said to be granular. The dilute and granular regimes have been well studied, but conversely, a substantial knowledge gap exists for dense gas-solid flows, which have intermediate particle volume fractions of about 0.1 to 50%. This regime exists at microsecond time scales during blast-induced dispersal of material when the shocked particles are closely spaced. Very little experimental data exist for the interaction of a shock wave with a dense gas-solid field of particles, with rare exceptions such as Rogue et al. [3]. Though they provided useful observations of particle trajectories and pressures following the impingement of a shock on a granular bed of particles, much remains unknown regarding the interactions that are involved in the dense gas-solid flow regime. Without data specifically acquired for such flows, simulations of energetic material detonation during the early-time expansion will continue to suffer from limited physical fidelity. To fill the gap in data for shock-particle interactions with initial volume fractions residing between the dilute and granular limits, a multiphase shock tube was recently constructed [4]. The unique facility uses a gravity-fed seeding method to generate a dense, spatially isotropic field of 100-micron diameter particles into which a planar shock is driven. High-speed schlieren imaging and pressure data are used to provide insight into the flow and particle behavior in the dense gas-solid regime.

Interaction of a Planar Shock with a Dense Field of Particles in a Multiphase Shock Tube

A novel multiphase shock tube has been constructed to test the interaction of a planar shock wave with a dense gas-solid field of particles. The particle field is generated by a gravity-fed method that results in a spanwise curtain of 100-micron particles producing a volume fraction of about 15%. Interactions with incident shock Mach numbers of 1.67 and 1.95 are reported. High-speed schlieren imaging is used to reveal the complex wave structure associated with the interaction. After the impingement of the incident shock, transmitted and reflected shocks are observed, which lead to differences in flow properties across the streamwise dimension of the curtain. Tens of microseconds after the onset of the interaction, the particle field begins to propagate downstream, and disperse. The spread of the particle field, as a function of its position, is seen to be nearly identical for both Mach numbers. Immediately downstream of the curtain, the peak pressures associated with the Mach 1.67 and 1.95 interactions are about 35% and 45% greater than tests without particles, respectively. For both Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced by about 30-40% by the presence of the particle field.

Development of a Multiphase Shock Tube for Energetic Materials Characterization

A novel multiphase shock tube to study particle dynamics in gas-solid flows has been constructed and tested. Currently, there is a gap in data for flows having particle volume fractions between the dusty and granular regimes. The primary purpose of this new facility is to fill that gap by providing high quality data of shock-particle interactions in flows having dense gas particle volume fractions. Towards this end, the facility aims to drive a shock into a spatially isotropic field, or curtain, of particles. Through bench-top experimentation, a method emerged for achieving this challenging task that involves the use of a gravity-fed contoured particle seeder. The seeding method is capable of producing fields of spatially isotropic particles having volume fractions of about 1 to 35%. The use of the seeder in combination with the shock tube allows for the testing of the impingement of a planar shock on a dense field of particles. The first experiments in the multiphase shock tube have been conducted and the facility is now operational.