Next Generation Experiments and Models for Shock Initiation and Detonation of Solid Explosives

Current phenomenological hydrodynamic reactive flow models, such as Ignition and Growth and JohnsonTang-Forest, when normalized to embedded gauge and laser velocimetry data, have been very successful in predicting shock initiation and detonation properties of solid explosives in most scenarios. However, since these models use reaction rates based on the compression and pressure of the reacting mixture, they can not easily model situations in which the local temperature, which controls the local reaction rate, changes differently from the local pressure. With the advent of larger, faster, parallel computers, microscopic modeling of the hot spot formation processes and Arrhenius chemical kinetic reaction rates that dominate shock initiation and detonation can now be attempted. Such a modeling effort can not be successful without nanosecond or better time resolved experimental data on these processes. The experimental and modeling approaches required to build the next generation of physically realistic reactive flow models are discussed. INTRODUCTION Phenomenological hydrodynamic reactive flow models, such as Ignition and Growth (1) and Johnson-Tang-Forest (2), have been very successful in predicting shock initiation and detonation in solid explosives. These models use compression and pressure of the reacting mixture in their reaction rate equations. The main experimental tools available to study shock initiation and detonation have been embedded manganin pressure gauges (3), embedded particle velocity gauges (4), and various applications of laser velocimetry, such as Fabry-Perot (5) and VISAR (6). Thus, when normalized to the measured pressure and/or velocity versus time data, the pressure and compression dependent reaction rates have been able to predict shock initiation and detonation wave propagation in one, two and three dimensions for most initial conditions in most applications. Of course, the modeling, especially in 3D, was limited by the size and speed of the available computers. With the advent of teraflop, parallel computers, these size and speed limitations have largely disappeared. The Ignition and Growth and Johnson-Tang-Forest reactive flow models are now being used on the large parallel machines. However, it has long been known that shock initiation of solid explosives is controlled by local reaction sites called "hot spots" (7) and that detonation waves have complex, 3D structures containing many Mach stem interactions (8). To model more exactly the physical and chemical processes that control reactions in solid explosives, the next generation of reactive flow model is required. The new generation of computers certainly allows such microscopic models to be built and tested in a timely manner. To really benefit liom these models, the next generation of experimental tools with improved spatial and time resolutions must also be developed. This paper discusses some of the properties that are desired in the next generation of microscopic reactive flow models and the associated experimental techniques. NEW REACTIVE FLOW MODELS Chemical reaction rates are always governed by the local temperature of the reacting material. Therefore, the local reaction rates in the heated regions which either ignite and form growing "hot spots" or fail to ignite due to conductive heat losses are intimately coupled to the physical mechanisms that create such heated regions. The next generation reactive flow models must accurately describe the physical processes (void collapse, friction, shear, viscosity, etc.) that form hot spots and the states (temperature, dimensions, geometry, pressure, etc.) that these hot spots attain. Various hot spot formation models have been proposed The usefulness of such modeling has been limited by both experimental and computational factors. The main experimental limitation is the lack of local time resolved temperature measurements. This and other experimental requirements are discussed in the next section. The main computational obstacle was the lack of coupling of thermal-mechanical codes to hydrodynamic codes. This obstacle has recently been overcome in the ALE3D, LS-DYNA2D/3D, and other hydrodynamic codes. This coupling has allowed modelers to study the hydrodynamic formation of hot spots using various dissipation mechanisms and heat transfer to the surrounding cooler material in 3D mesoscale meshes containing over one billion computational explosives using elements (12). Critical conditions for the subsequent growth or failure of the heated regions for HMX-and TATB

Molecular dynamics simulation of shocks in porous TATB crystals

We report molecular dynamics results on the shock structure of 2-D crystals of triaminotrinitrobenzene (TATB). We find that the shock front broadens to approx. 30 nm in materials with a 20% random void distribution. As expected from bulk experiments, the shock velocity decreases with increasing porosity and the temperature behind the shock front increases with increasing porosity. Shock equilibration times increase from 1 ps to greater than 10 ps

Floret Test, Numerical Simulations of the Dent, Comparison with Experiments

The Floret test has been developed as a screening test to study the performance of a small amount of HE. Numerical simulations have been performed recently using CTH. The objective of this study is to perform numerical simulations in order to better understand the shock waves interactions, involved in the dent formation. Different 3D wedge configurations have been tested using the Ignition and Growth reactive flow model for the HE receptor with Ls-Dyna

Simulating Thermal Explosion of Octahydrotetranitrotetrazine-based explosives: Model Comparison with Experiment

The authors compare two-dimensional model results with measurements for the thermal, chemical and mechanical behavior in a thermal explosion experiment. Confined high explosives are heated at a rate of 1 C per hour until an explosion is observed. The heating, ignition, and deflagration phases are modeled using an Arbitrarily Lagrangian-Eulerian code (ALE3D) that can handle a wide range of time scales that vary from a structural to a dynamic hydro time scale. During the pre-ignition phase, quasi-static mechanics and diffusive thermal transfer from a heat source to the HE are coupled with the finite chemical reactions that include both endothermic and exothermic processes. Once the HE ignites, a hydro dynamic calculation is performed as a burn front propagates through the HE. Two octahydrotetranitrotetrazine (HMX)-based explosives, LX-04 and LX-10, are considered, whose chemical-thermal-mechanical models are constructed based on measurements of thermal and mechanical properties along with small scale thermal explosion measurements. The present HMX modeling work shows very first violence calculations with thermal predictions associated with a confined thermal explosion test. The simulated dynamic response of HE confinement during the explosive phase is compared to measurements in larger scale thermal explosion tests. The explosion temperatures for both HE's are predicted to within 1 C. Calculated and measured wall strains provide an indication of vessel pressurization during the heating phase and violence during the explosive phase

Are textbook lungs really normal? A cadaveric study on the anatomical and clinical importance of variations in the major lung fissures, and the incomplete right horizontal fissure.

INTRODUCTION: The lungs have three main fissures: the right oblique fissure (ROF), right horizontal fissure (RHF), and left oblique fissure (LOF). These can be complete, incomplete or absent; quantifying the degree of completeness of these fissures is novel. Standard textbooks often refer to the fissures as complete, but awareness of variation is essential in thoracic surgery. MATERIALS AND METHODS: Fissures in 81 pairs of cadaveric lungs were classified. Oblique fissures were measured from lung hila posteriorly to the lung hila anteriorly; and the RHF measured from the ROF to the anteromedial lung edge. The degree of completeness of fissures was expressed as a percentage of the total projected length were they to be complete. The frequency and location of accessory fissures was noted. RESULTS: LOF were complete in 66/81 (81.5%), incomplete in 13/81 (16.0%) and absent in 2/81 (2.47%); ROF were complete in 52/81 (64.2%), incomplete in 29/81 (35.8%) and never absent; RHF were more variable, complete in 18/81 (22.2%), incomplete in 54/81 (66.7%) and absent in 9/81 (11.1%). LOF and ROF were on average 97.1% and 91.6% complete, respectively, being deficient posteriorly at the lung hila. The RHF on average 69.4% complete, being deficient anteromedially. There were accessory fissures in 10 left and 19 right lungs. CONCLUSIONS: This study provides a projection of the anatomy thoracic surgeons may encounter at operation, in particular the variable RHF. This knowledge is essential for optimal outcomes in both benign and oncological procedures influenced by the fissures