Modeling of Properties of Energetic Materials: Influence of Surrounding Temperature and Container Properties on the Time to Self-Ignition

During computational simulation of the thermal ignition of energetic materials two important issues have to be considered: (i) the application of advanced kinetics which properly describes the complicated, multistage course of the decomposition process and (ii) the effect of heat balance in the system, as the sample mass is increased by a few orders of magnitude compared to the thermoanalytical experiments used for the determination of kinetic parameters. The application of both, advanced kinetic description and determination of the non-uniform temperature distribution within the solid energetic materials by solving the heat conduction problem has been already presented by us elsewhere [1-2]. The current study presents the extension of these methods for the computational prediction of the time to self-ignition of ammunition systems due to the accumulation of the heat at several constant surrounding temperatures occurring e.g. during the cook-off in a hot loading chamber. The geometry and dimension of the ammunition container and, additionally, the amount, properties and thickness of the layers of different materials used for the construction of the ammunition container have been taken into account during calculations. Application of Finite Element Analysis (FEA) and the appropriate decomposition kinetics enabled the determination of the effect of scale and geometry of the container as well as the influence of thermal conductivity, heat transfer and surrounding temperature on the heat accumulation in the sample. The results of modeling enabled the rigorous analysis of the design of the container parameters such as radius and the type and thickness of the insulation. Proposed computational methods can be used for any profile of the surrounding temperature such as isothermal, stepwise, modulated or temperature profiles reflecting the real daily minimal-maximal fluctuations for different localizations. Additionally, the temperature profiles due to the thermal shocks can be taken into account what enables the determination of the time to ignition in case of an accident during e.g. transportation. The calculations have been verified by the comparison with the experimentally determined values of the time to ignition in a hot loading chamber under isothermal conditions at several temperatures for a 5.56 mm small caliber system and a new 155 mm artillery charge for the Swiss army. The simulations have been done for the sample in the form of cylinders containing three layers of materials possessing significantly different thermal properties: single-base propellant, combustible cartridge case and steel container. The very good computational prediction of the experimental time to thermal ignition indicates the high accuracy of the applied method.