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Properties of heterogeneous energetic materials under high strain, high strain rate deformation


Heterogeneous energetic materials have many applications. Their dynamic behavior and microstructural evolution upon plastic deformation have remained not fully understood. The following heterogeneous materials were investigated in the this study: the pure PTFE (usually a mixture of crystalline and amorphous phases), PTFE-Sn, PTFE-Al, PTFE- Al-W, and carbon fibers filled Al alloy. Sample manufacturing processes involving ball milling and Cold Isostatic Pressing were employed. Quasi-static and Hopkinson bar tests were carried out to obtain the compressive strengths of composites. The Conventional Thick-walled Cylinder (TWC) method and newly developed small-scale Hopkinson bar based TWC experiments were conducted to investigate single shear bands and their assembly. Conventional and "soft" drop-weight tests were performed to examine the mechanical properties and the initiation of chemical reactions. Scanning Electron Microscopy was used to detect the details of the microstructures and failure mechanisms of heterogeneous materials. New features in the dynamic behavior of heterogeneous materials were observed. They include the following: Strain softening, instead of thermal softening, is the main mechanism in the initiation of shear bands in explosively driven TWC tests of solid PTFE. Cold isostatically pressed PTFE-Sn samples were more stable with respect to shear localization than solid PTFE. The dynamic collapse of solid PTFE-Al samples with different particle sizes was accomplished with the shear localization bands and cracks. Force chains in the fine W and Al particles were attributed to the high strength of the porous PTFE-Al-W composite containing fine W particles in comparison with composites with coarse W particles. .Debonding of metal particles from the PTFE matrix and the fracture of the matrix were identified to be two major mechanisms for the failure of the PTFE-Al-W composites. The formation of PTFE nano-fibers during high strain flow was detected. The orientation of carbon fibers did not influence the strength and reaction of carbon fibers filled Al alloys, but the strength of carbon fibers did

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