Microstructural model of mechanical initiation of energetic materials

Mechanical initiation of chemical reactions in energetic materials depends on microstructural details of the materials. Several models are described in the literature that are appropriate for a continuum, such as energy dissipation from plastic flow, or shear bands. A technique is presented here for developing initiation models using relations between macroscopic variables and conditions at inter-grain contact areas in a granular material. The chemical processes that lead to initiation are included by using a 2-D numerical heat transfer model of energy flux on a surface spot of a half-space medium with multiple species chemical reactions. A number of calculations are done and the time to ignition at a given fluence is obtained as a function of spot size. Then simple kinematic relations between macroscopic stresses and the inter-grain contact forces are developed and the shearing velocity at the contact region is related to the macroscopic shear strain rate. Combining these relations leads to an ignition criterion in terms of macroscopic pressure, shear-strain rate and time. Even though very simple approximations for most relations are used, the overall result is similar to commonly used detonation initiation criterion. Experiments to define the constants in the model are under development and will be described