The Fragmentation of Al-W Granular Composites Under Explosive Loading
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The Fragmentation of Al-W Granular Composites Under Explosive Loading Karl L. Olney1, Po-Hsun Chiu2, Vitali F. Nesterenko1,2, David J. Benson3, Chris Braithwaite4, Adam Collins4, David Williamson4, and Francesca McKenzie5 1 Department of Mechanical and Aerospace Engineering, University of California, San Diego, San Diego, California, USA 2 Material Science and Engineering program, University of California, San Diego, San Diego, California, USA 3 Department of Structural Engineering, University of California, San Diego, San Diego, California, USA 4 Fracture and Shock Physics, SMF Group, Department of Physics, Cavendish Laboratory, Cambridge, CB3 0HE, United Kingdom 5 Institute of Shock Physics, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom ABSTRACT Small scale explosively driven fragmentation experiments have been performed on Aluminum (Al)-Tungsten (W) granular composite rings processed using cold isostatic compression of Al and W powders with a particle size of 4-30 microns. Fragments collected from the experiments had a maximum size of the order of a few hundred micrometers. This is a dramatic reduction in the fragment size when compared to the 1-10 mm typical for a homogeneous material such as solid aluminum under similar loading conditions. Numerical simulations of the experiment were performed to elucidate the mechanisms of fragmentation that were responsible for this shift in fragmentation size scales. Simulations were performed with a significantly stronger explosive driver to examine how the mechanisms of fragmentation change when the detonation pressure increases. INTRODUCTION Reactive metal based materials combine the ability to survive a launch environment while at the same time undergoing a rapid release of chemical energy at a desired time. Aluminum has a large combustion energy per gram (7422 cal/g), approximately 5 times that of traditional high explosives. In order for this energy to be utilized effectively in a relevant application, the release of chemical energy needs to be on the order of 1ms to have an effect on the impulse wave. Previous work examining the oxidization rate shows a relationship between the size of the Al particle and the oxidization rate [1]. For example, to attain an oxidization rate of ~1 ms, the Al particles need to be on the order of ~20 microns in size. Generation of Al fragments of this size in situ with fresh, non-oxidized surfaces is a nontrivial matter. In typical applications where strain rates in the materials are ~103 s-1 [2], the fragment size scale estimated by the Grady-Kipp equations [3] is ~1-10 mm. To address this discrepancy, a highly heterogeneous granular composite material system composed of Aluminum and Tungsten particles was proposed to activate a mesoscale based mechanism of fragmentation [2]. Leveraging the very different properties of the Al (low strength, low density) and W (high strength, high density) and their significantly different responses to shock loading,
fragments may be generated with a size scale approx
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