Chemical kinetic thermal decomposition models of pressed solid high explosives containing octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (HMX) and triaminotrinitrobenzene (TATB), which accurately calculate the times to explosion at various initial temperatures measured in the one-dimensional time to explosion (ODTX) test, are extended to higher temperatures to predict the critical temperatures, times to explosion, and dimensions of the impact-and shock-induced hot spots that are known to control the ignition of exothermic reaction in solid explosives. The effects of hot spot geometry and surrounding temperature on the critical hot spot conditions are investigated. Since hot spot temperatures and dimensions cannot be measured experimentally, these estimated temperatures, sizes, and times required for exothermic chemical reaction provide a means to evaluate proposed physical mechanisms of hot spot formation in accident scenarios involving impact (friction and shear) and shock compression of solid explosives.

An ignition and growth concept is used, within the framework of a one-dimensional Lagrangian hydrodynamic code, to model the shock initiation of heterogeneous solid explosives. The leading shock wave of an initiating pulse is assumed to ignite a small fraction of the explosive at localized heated regions. These ignited regions then grow as material is consumed at their boundaries. The growth rate for a particular material is assumed to have the characteristic pressure dependence of high-pressure laminar burning experiments. Results of the model calculations are in good quantitative agreement with recent manganin pressure gage and particle velocity gage measurements of the buildup of the initiating shock front to detonation for both sustained and short duration pulses in four solid explosives: PBX−9404, TATB, cast TNT, and PETN. The predicted run distances to detonation as functions of shock pressure at various initial densities and the predicted reaction zone lengths of the fully developed detonation waves also correlate well with experimental data for these four solid explosives.

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A first-order group additivity approach was used to estimate the densities of 188 explosives and related compounds of very diverse compositions. Of the 173 compounds for which direct comparisons could be made, 40.5 % of the estimated densities were within 1 % of the measured densities, 33.0% were within 1 to 2%, 16.8% were within 2 to 3%, and 9.8% deviated more than 3% from the measured densities. The average absolute error in density was 0.0191 g/cm3, and the absolute error In density exceeded 0.05 g/cm3 for only 14 of the 173 compounds (8.1%). The largest errors occurred for compounds with several bulky highly polar groups in close proximity and for compounds containing groups whose calculated molar volumes were based on density data for a small number of compounds. Inclusion of second-order effects, such as nearest neighbor interactions, phase transitions, and crystalline structure in a second-order group additivity model, appears necessary for accurate density estimations in certain types of compounds.

Recent experimental and theoretical advances in the understanding of high-pressure, high-temperature chemical kinetics are used to extend the nonequilibrium Zeldovich−von Neumann−Doring (NEZND) theory of self-sustaining detonation in liquid and solid explosives. The attainment of vibrational equilibrium behind the leading shock front by multiphonon up-pumping and internal vibrational energy redistribution establishes a high-temperature, high-density transition state or series of transition states through which the chemical decomposition proceeds. The reaction rate constants for the initial unimolecular decomposition steps are accurately calculated using high-temperature, high-density transition-state theory. These early reactions are endothermic or weakly exothermic, but they channel most of the available energy into excited vibrational states of intermediate product species. The intermediate products transfer some of their vibrational energy back into the transition states, accelerating the overall reaction rates. As the decomposition progresses, the highly vibrationally excited diatomic and triatomic molecules formed in very exothermic chain reactions are rapidly vibrationally equilibrated by “supercollisions”, which transfer large amounts of vibrational energy between these molecules. Along with vibrational−rotational and vibrational−translational energy transfer, these excited vibrational modes relax to thermal equilibrium by amplifying pressure wavelets of certain frequencies. These wavelets then propagate to the leading shock front and reinforce it. This is the physical mechanism by which the leading shock front is sustained by the chemical energy release.