Modeling The Effects of Shock Pressure and Pore Morphology on Hot Spot Mechanisms in HMX

We investigate the effects of shock pressure and pore morphology on the formation and growth of hot spots in HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine). Both non-reactive and reactive ALE3D simulations are used in these studies. Our non-reactive simulations show a viscous-dominated pore collapse mode at lower shock pressures (2-10GPa) with shear band formation and a hydrodynamic-dominated mode at higher shock pressures (20-40GPa) due to bulk melting. When normalized by bulk shock heating, viscous-dominated pore collapse modes are more efficient at generating hot spots. Pore morphology influences the post-collapse temperature distributions and reaction rate for a fixed pore area and shock pressure. We find that multiple surface pores at the binder-grain interface tend to react the fastest. Due to their upstream location in the HMX grain, the surface pores collapse sooner than interior pores; thus, the extent of reaction will generally favor these morphologies because they have more time to grow. In general, multiple smaller hot spots tend to react faster than a single larger hot spot because they accelerate one another's burning. The rank order of morphology effects, however, is not the same for non-reactive and reactive simulations. For example, while multiple surface pores produce the highest reaction rates they do not produce the highest (non-reactive) hot spot temperatures. Our numerical studies provide insights on hot spot mechanisms in lieu of direct measurements and can be used to develop advanced shock initiation models.