Impact Induced Microstructural and Crystal Anisotropy Effects on the Performance of HMX Based Energetic Materials
Abstract
Accurate predictions of the behavior of energetic materials necessitates incorporation of all possible variables affecting the mechanical behavior of such materials into numerical models. This work presents findings in the combined experimental and computational study of the effects of anisotropy and microstructure on the behavior of HMX-based energetic materials. Large single crystal samples of β-HMX were meticulously created by solvent evaporation for experimental purposes, and respective orientations were identified via x-ray diffraction. Indentation modulus and hardness values were obtained for different orientations of β-HMX via nanoindentation experiments. Small-scale dynamic impact experiments were performed, and a viscoplastic power law model fit, to describe the anisotropic viscoplastic properties of the crystal. The anisotropic fracture toughness and surface energy of β-HMX were calculated by studying indentationnucleated crack system formations and fitting the corresponding data to two different models, developed by Lawn and Laugier. It was found that the {011} and {110} planes had the highest and lowest fracture toughnesses, respectively. Drop hammer impact tests were performed to investigate effects of morphology on the impact-induced thermal response of HMX. Finally, the anisotropic properties obtained in this work were applied in a cohesive finite element simulation involving the impact of a sample of PBX containing HMX crystals with varying orientations. This showed temperature spikes to occur where the crystals are in closer proximity. Cohesive finite element models were generated of separate microstructure containing anisotropic or isotropic properties of HMX particle. In comparison, the isotropic model had more even distributions of stress through the material as expected, however, the anisotropic model withstood larger amounts or stress for longer than the isotropic model.
Degree
Ph.D.
Advisors
Tomar, Purdue University.
Subject Area
Mechanics|Polymer chemistry
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