Microstructure and Dynamic Failure Properties of Freeze-Cast Materials for Thermobaric Warhead Cases

We consider the microstructure and dynamic failure of ice-templated freeze cast alumina materials that are currently being studied for novel warhead cases. The freeze-cast matrix is a porous, cellular structure of overlapping lamellae similar to many biomaterials such as nacre. This lightweight matrix provides a high-toughness shell that can be filled with polymers or combustible reactive materials. Three porosities of alumina freeze-cast structures were studied, and a systematic variation in microstructural properties such as lamellar width and thickness was observed with changing porosity. Dynamic impact tests were performed in a single stage light-gas gun to examine the failure properties of these materials under high strain-rate loading. Nearly complete delamination was observed under impact, along with characteristic cracking across the lamellar width. Average fragment size decreases with increasing porosity, and a theoretical model was developed to explain this behavior. Based on an energy balance between kinetic, strain, and surface energies within a single lamella, we are able to predict the characteristic fragment size using only standard material properties of bulk alumina.