Theoretical Investigation of Mechanically Coupled Chemical Kinetics and Phase Transitions in Energetic Materials
Abstract
Understanding and predicting the underlying mechanisms that lead to premature initiation of explosion in energetic materials can significantly affect safety in handling and transporting them. In particular, these materials can be sensitive to mechanical loading that, in presence of defects or heterogeneity, can lead to localization of mechanical energy which can trigger chemical reactions at `hot spots and to explosion. The proposed work will employ a combination of advanced theoretical tools of continuum mechanics and high precision experimentation (in inert materials), to understand how the mechanical response of energetic materials affects the chemical kinetics, and to determine in what scenarios this effect is significant. This will be done in accord with recent accumulation of experimental evidence of the coupling between the extreme mechanical response on the resulting chemical kinetics (by Dana Dlott and co-workers). These observations of the explosive process have been captured with high time and space resolution in both single explosive crystals and at the aggregate level and will thus serve as basis for verification of the models derived in the proposed activity. Then, the models will be further investigated to explain the observed phenomena and to examine the constitutive sensitivities of the explosion process. The proposed work will revolve around three thrusts. Thrust 1 will consider the explosive process of a single explosive particle embedded in a polymer binder material. This process, also referred to as a `micro-explosion , has been shown to be highly influenced by the mechanical response of the binder material. A theoretical model that captures dynamic cavity expansion response, while accounting for the embedded explosive as a reactive mixture of two species, will be developed. As observed in recent experiments, the dynamic cavity expansion process, when coupled to the chemical kinetics, can exhibit complex multi-stage explosions and fracture. Based on preliminary investigation, multi-stage explosions can be captured by the proposed model, however the transition to fracture in such extreme settings is not well understood. Nonetheless, it can significantly influence the chemical kinetics by suddenly removing the confinement. Thrust 2 will thus study the transition to fracturing via high precision experimentation in inert materials subjected to cavity expansion at different rates. By combining the experimental data and the theoretical models, a scaling relationship will be developed based on dimensionless groups, to determine the nature of the transition and its threshold. Thrust 3 of the proposed work will combine results from both Thrust 1 and Thrust 2 to investigate the response of an aggregate explosive material. The chemo-thermo-mechanically coupled hot spot formation and solid phase transformation due to collapse of an isolated pore will be considered. This will be achieved by applying continuum theories of phase-transitions in solids to investigate explosion nucleation and dynamics as well as the coupling between the extreme dynamic process in the solid and the chemical kinetics. Finally, the models derived in this work will lead to rigorous and thermodynamically consistent investigation of the sensitivities of the explosion process and its nucleation criteria. By accounting for different material and gas properties, it will be possible to compare and validate the model against the increasing amount of available experimental research and thus to evaluate available kinetic relations that describe the chemical response as well as present explanation of available observations. Furthermore, the sensitivity analysis can uncover untapped regimes of the response to guide future experimentation and can possibly lead to the design of safer and more predictable polymer bounded explosives.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 13, 2019
- Source ID
- W911NF1910275
Entities
People
- Tal Cohen
Organizations
- Army Contracting Command
- Massachusetts Institute of Technology
- United States Army