Effects of Microstructure and Binder on the Shock-to-Detonation Behavior of Hexanitrostilbene (HNS)*

We developed a mesoscale shock-to-detonation model for HNS that predicts the effects of microstructure and binder on the ignition and growth processes. In this model, shock-wave energy is localized by voids and defects in the HNS, creating hotspots that release energy to support shock-wave growth to steady detonation. An Arrhenius reaction rate, based on the local temperature in the HNS, was calibrated using experimental threshold flyer-velocity data from exploding foil initiator (EFI) tests. We simulated flyers of various thicknesses impacting the HNS and obtained reasonably good agreement between the predicted and experimental threshold velocities. Moreover, we compared simulations of both fine-grained and coarse-grained HNS where the only difference between the simulations was the initial microstructure. For very thin flyers, fine-grained HNS was more sensitive than coarse-grained HNS. For thicker flyers, a cross-over in sensitivity was predicted, making coarse-grained HNS more sensitive. We also performed three-phase simulations of the same coarse-grained HNS with a small amount of binder added to the HNS and pore interfaces. These simulations enabled detailed predictions of the relative change in initiation sensitivity and threshold flyer-velocity due to the binder content.