Effect of initial damage variability on hot-spot nucleation in energetic materials

Abstract

Mechanical insult may be able to produce chemical transformations in solids when the energy is released in highly localized regions. This phenomenon is responsible for the nucleation of hot-spots that are responsible for ignition of energetic materials. The concentration of energy at microstructural defects leads to the probabilistic nature of ignition. The effect of the microstructure of the energetic particles, specifically the influence of the initial crack distribution on the sensitivity to ignition, is studied for a particle embedded in a polymeric matrix at impact velocities 100 m/s and 400 m/s with finite element simulations that couple fracture dynamics and heat transport. A phase field damage model that includes heat sources due to frictional heating at the crack surfaces and heat dissipation during crack propagation is developed and verified. These heat sources are compared and, in the range of impact velocities studied, heat generation due to friction is more important than dissipation due to crack propagation. Hot-spots nucleated at 100 m/s do not reach the critical temperature while conditions consistent with the Lee-Tarver criterion for ignition are observed at 400 m/s impact velocity. The variability observed due to the stochasticity of the initial crack distribution is studied and it increases with a higher impact velocity. In particular, regions of high temperature develop close to cracks intersecting the particle polymer interface. Therefore, controlling the surface quality of the energetic particles may lead to a reduction on the sensitivity uncertainty in polymer-bonded explosives.