HIGH STRAIN-RATE RESPONSE OF FREEZE-CAST COMPOSITES FOR USE IN THERMOBARIC WARHEADS

CPT Seonwoo Nam, Korean Army

We consider the dynamic fragmentation of freeze-cast alumina materials currently under consideration as reactive materials in thermobaric warheads. We have performed experiments to determine whether fragment recovery in a split Hopkinson pressure bar can adequately reproduce more complex and expensive ballistic experiments from a single-stage gas gun. We impacted cylindrical samples of freeze-cast material and determined that the pattern of dynamic failure closely matches previous experiments. The fragment distribution itself resembles previous data at lower porosities, but differs for highly porous samples; this is likely due to the lamellar orientation in Hopkinson bar testing, rather than an intrinsic difference in fragmentation behavior. Second, we consider samples in which the porosity of the freeze cast samples is filled with PMMA; previous reports on the quasistatic properties of this composite suggested considerably improved strength and toughness. For these samples, we found a bimodal fragmentation pattern consisting of a small-fragment region that does not differ dramatically from unfilled samples (due to residual unfilled zones), and second distribution at larger sizes arising from filled areas. Fragment distributions can be modeled by an energy-based fragmentation theory. Finally, we present preliminary data on tantalum samples in advance of tests on reactive tantalum composites currently under study.