Material Characterization of Phase Change Materials for Munitions Safety Applications

Abstract

Ammunition packaging is a critical safety component throughout a munitions lifecycle. Packaged munitions are subjected to a series of harmonized Insensitive Munitions (IM) and Final Hazard Classification (FHC) tests that dictate limits on storage and transportation operations. System level IM tests include bullet and fragment impact, fast and slow heating and sympathetic detonations among others. The reaction severity of packaged ammunition to each external stimulus creates the basis for the final hazard classification. Detonations and explosions result in restrictive shipping and storage quantities. Benign reactions result in less restrictive final hazard classifications that allow for improved logistical efficiencies. Significant studies are being conducted to improve insensitivity and hazard classifications of legacy munitions without redesigning the ammunition or energetic material. This work investigates the integration of phase change materials (PCM) into munitions packaging to improve IM reactions during fast and slow heating. Both fast and slow heating are possible occurrences in the military ammunition lifecycle due to vehicle accidents, fuel spills or enemy actions. The materials in question are a solid, wax-like substance that begin to melt at a specific temperature. Once the PCM reaches it latent heat of fusion it acts as a heat sink that can absorb large amounts of energy. This property may help improve cook-off reactions of packaged ammunition that is exposed to an uncontrolled external heat source such as a fuel fire. Limiting and delaying heat transfer to extremely sensitive primary explosives and igniters may allow less sensitive components to burn out and prevent a detonation or explosion. Material testing was conducted to quantify the thermal characteristics of several PCM configurations. A legacy mortar package was selected as the test bed with a focus on the propulsion charge and its ignition train. A numerical model was utilized to identify potential designs for evaluation. Limited free volume created a challenge to fit enough PCM into the required areas needed to achieve the desired result. Full scale heating tests were conducted with an inert munition to collect system thermal data, including interactions of multiple layers of packaging materials. The PCM influenced the thermal response of the legacy packaging system as compared against baseline data. When used in specific locations and quantity for the packaging system in question, the PCM absorbs enough heat energy to show a measurable decrease in munition skin temperature at critical points of interest. The findings show that phase change materials may reduce reaction severity of legacy munitions by influencing heat transfer in designated areas. A robust and economical containment method for PCM is still required for munition applications.