Heterogeneities in explosives affect detonation process

The propagation of a one-dimensional shock moving through a uniform gas is well-described, but a shock moving through real materials that may be heterogeneous is not so well-understood. A study published in the Journal of Applied Physics reports the results of new calculations that incorporate heterogeneities in a computational model of a widely used energetic material.

Investigators at Sandia National Laboratories developed a grain-scale model of hexanitrostilbene (HNS), which is widely used in explosives applications. Several different grades of HNS are available, but scanning electron microscopy (SEM) images of these different grades reveal that some have a needlelike structure while others are more plateletlike. The authors used loose HNS powder pressed to within its theoretical maximum density to obtain SEM images of two-dimensional HNS cross sections. These images were then used to create a computational model of the substance involving circular pores distributed in a manner consistent with these experimental observations.

A solid mechanics code developed at Sandia and known as CTH calculated the propagation of shocks through this heterogeneous model energetic material. A density functional theory-molecular dynamics (DFT-MD) approach was used to determine the properties according to the Hugoniot equation on either side of the shock wave. This then permitted parameterizing an equation of state suitable for use in CTH.

Initial reactive simulations resulted in threshold flyer velocities that match experimental data. For the thin flyers used, at approximately 11 micrometers, the shock to detonation transition process was heterogeneous. The complete model, which involved the inclusion of a global kinetics model, accurately reproduced the expected response of the heterogeneous material to shock loading, and the trends in sensitivity for different pore size distributions. These insights may prove particularly insightful for applications such as oil and gas extraction.

Source: “Shock interactions with heterogeneous energetic materials,” by Cole D. Yarrington, Ryan R. Wixom, and David L. Damm, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5022042 .