Experimental observations of aluminum particle combustion during hypervelocity water impact and penetration

Experiments conducted at the University of Illinois Urbana-Champaign have investigated the reaction between Al shaped charge jets and underwater environments. Although many qualitative and semi-quantitative characteristics are known, including pressure field augmentation, light emission from combusting material, and oxide-containing powder residues, the actual combustion mechanism has not been isolated. The difficulty of studying combustion in this physical situation is due to the extremely transient nature of the combustion and the difficulty of implementing common diagnostics. This research presents a novel approach to isolating the combustion which occurs between particulated jet material traveling at high velocities (1500 - 3000 m/s) in underwater environments as would be found in shaped charge jet penetration. A method of accelerating small packets (10 mg - 20 mg) of Al particles using a light gas gun was developed to simulate conditions experienced by particulated shaped charge jet material during water penetration. In particle sizes tested (75 mum - 5 mum), only large particles at high velocities (75 mum - 45 mum at velocities greater than 2500 m/s) exhibited evidence of combustion. XRD and SEM analyses were used to verify residue composition and to characterize individual particle morphology. XRD analysis yielded a global residue composition while SEM analysis gave a single particle composition. Both analyses verified the presence or absence of oxidized material. Surprisingly, no oxidation was indicated in particles smaller than 45 mum even at velocities greater than 3000 m/s. Images of shaped charge residue and light gas gun experiment residue qualitatively verified similar particle oxidation and surface morphologies characterized by the presence of numerous hollow nodules and porous, oxidized surfaces. In addition, controlled experiments involving Al and Cu shaped charges fired into H2O and oil verified the particle sizes created during penetration and residue composition. It was found that 7% of an Al shaped charge liner reacted during penetration in open water targets. Oxidized and unoxidized particles in sizes ranging from 425 mum - 10 mum were created during penetration and combustion. No reaction was observed with Al jets fired into oil.