Effects of microstructure and chemistry on the ignition sensitivity of PBX under shock loading
Miller, Christopher M.
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The ignition sensitivity of heterogeneous energetic materials subject to shock loading is analyzed using both a Lagrangian and Eulerian computational framework. The specific focus here is on the various microstructure heterogeneities (including cracks, granular anisotropy, voids, and aluminum additives) and their relative contributions to the development of critical hotspots and macroscale detonation behavior characteristics, such as the run distance to detonation. A probabilistic approach is developed by generating statistically equivalent microstructure sample sets (SEMSS) and measuring the ignition behavior of each one under similar impact conditions. By varying the material and microstructural characteristics in a controlled fashion, the contribution to ignition of each specific type of microstructural defects is rank-ordered. The Lagrangian-based cohesive finite element method (CFEM) is used to track material response prior to the onset of chemical reaction. A probability threshold is proposed based on a modified form of the Hugh James and Walker-Wasley energy-based ignition criterions. The computations focus on both 100% packed energetic grains (HMX) as well as aluminized polymer-bonded explosives (APBXs). The exact physical mechanisms governing the development of hotspots are quantified, and the friction is found to be the dominant dissipation mechanism. The Sandia National Laboratories Eulerian hydrocode, CTH, is then used to simulate the entire shock to detonation transition (SDT) of pressed HMX. The run-to-detonation distance is predicted as a function of shock pressure. The initial probability analysis is expanded upon to generate a predictive map of the SDT threshold for both 2D and 3D samples. The probability thresholds proposed in this study serve as a useful design metric and may directly influence future shock experimentation as well as the development of new insensitive high explosives design metric and may directly influence future shock experimentation as well as the development of new insensitive high explosives.