Session J1: EM-6: Thermal Decomposition

Deflagration rates of secondary explosives under static MPa - GPa pressure

We discuss our measurements of the chemical reaction propagation rate (RPR) as a function of pressure using diamond anvil cell (DAC) and strand burner technologies. Materials investigated include HMX and RDX crystalline powders, LX-04 (85{\%} HMX and 15{\%} Viton A), and Comp B (63{\%} RDX, 36{\%} TNT, 1{\%} wax). The anomalous correspondence between crystal structure, including in some instances isostructural phase transitions, on pressure dependant RPRs of TATB, HMX, Nitromethane, and Viton are elucidated using micro -IR and -Raman spectroscopies. The contrast between DAC GPa and strand burner MPa regime measurements yields insight into explosive material burn phenomena. Here we highlight pressure dependent physicochemical mechanisms that appear to affect the deflagration rate of precompressed energetic materials.   

Time-sequenced X-ray observation and modeling of a thermal explosion

The evolution of a thermally-initiated explosion is studied using a multiple-image x-ray system. PBX-9501 is used in this work, enabling direct comparison to recently-published data obtained with proton radiography. For each observed explosion, four x-ray images of the explosive are obtained, each image spaced by tens of microseconds. The multi-physics code, ALE3D, is used to model the pre-ignition thermal profile and post-ignition deflagration of the solid explosive. The model incorporates chemical decomposition, thermal transport, and implicit hydrodynamics to enable accurate prediction of ignition time and temperature. A convective burn model is also implemented in ALE3D to simulate the post-ignition deflagration of thermally-damaged solid energetic materials.   

Radial Combustion Dynamics in Fe2O3/Al Thermite Mixtures: Variability of the Flame Propagation Profiles

Radial combustion in thin circular samples of stoichiometric and over aluminized Fe$_2$O$_3$/Al mixtures is studied. Two confinement materials are tested: stainless steel and PVC. The combustion front profiles are registered by digital video-crono-photography. The radial geometry allows an easy detection of sample heterogeneities, since they cause distortions on the combustion front profiles circularity. The influence of the mixtures Al content and type of confinement on the combustion propagation dynamics is analyzed. In addition, an asymmetry analysis of the combustion front profiles is performed, defining an asymmetry parameter and using ANOVA. Although the type of confinement contributes more than the mixture composition to the variability of the asymmetry parameter, they both have a weak influence. The main source of variability is the intrinsic variations of the samples, which are due to their heterogeneous character.   

On the Burn Topology of Hot-Spot Initiated Reactions

The bulk rate of heterogeneous reaction of an energetic material depends on both the decomposition chemistry and the physical microstructure. Simple thermal decomposition models and most detonation reactive burn models express the reaction rate as the product of two functions. One expresses the sensitivity of the rate to the thermodynamic state; the other expresses the effect of reactant depletion. For a homogeneous reaction, the depletion function structure depends on the reaction pathways (overall reaction order, autocatalysis, etc.). For a heterogeneous reaction, the depletion function also depends on the reaction topology (e.g., how reaction spreads from nucleation sites to consume the material). We numerically generate depletion functions for simultaneously initiated, randomly oriented hot spots, and compare the result to the analytic solution for regularly spaced hot spots. The effect of randomization is substantial. We also compare the depletion function for ideal randomly located hot spots to those employed by various reactive burn models that are calibrated to detonation experiments.   

A Comparison of Thermal Explosions in HMX Based Formulations

Radial thermal explosion experiments have been run using different HMX based formulations. The reaction violence of the different HMX based formulations under identical boundary conditions varies dramatically. In this talk, PBX 9501 and PBX N9 deflagrations will be compared. Diagnostics include proton radiography, case strain, burn front velocities, wall velocities, and post shot case fragments. The difference in reaction violence will be explored in the framework of our current understanding of burn mechanism.   

Thermal Damage Characterization of Energetic Materials

Incidents caused by fire or other thermal events would expose energetic materials to unexpected heat that may damage explosive charges. The thermal damage may affect material handling safety, material properties, and degrade its performance. We recently conducted some thermal damage experiments on several high explosives including HMX-based formulations and TATB-based formulations, with temperatures from 150 C to 190 C. We also evaluated the handling safety, some physical properties (density, porosity, permeability), and detonation velocity of the damaged energetic materials. In this paper, we will describe our approach, instruments and equipment used for the study and share our experimental results.