Novel method to control explosive shock sensitivity: A mesoscale study to understand the effect of thermally expandable microsphere (TEM) inclusions in high explosives (HE) microstructure

Abstract

When the void content and/or void structure of a high explosive (HE) is altered by some means (i.e., bulk heating or mechanical damage), the shock initiation behavior of the material changes. The ability to precisely predict the change in shock sensitivity after an HE has undergone microstructural changes is a crucial capability in multi-scale reactive flow models. Here, we utilize thermally expandable microspheres (TEMs) as a dopant in a polymer bonded explosive (PBX) matrix to alter the shock initiation properties in a controlled fashion. Using a mesoscale modeling approach, we evaluated how a single TEM (before and after thermal expansion) behaves under shock compression, as well as how the matrix PBX in the direct vicinity of the TEM is affected. We first examined the effect of an unexpanded TEM in the explosive matrix and found that its presence does not significantly perturb the bulk flow and by extension will not affect bulk sensitivity. Next, we examined the effect of an expanded TEM and found that its presence significantly perturbs the flow via hydrodynamic jetting, which causes a secondary shock wave with a strength that exceeds that of the incident wave. Finally, we showed that this secondary shock interacts with the downstream porosity to ignite a larger fraction of the overall pore volume, commensurate with the secondary shock strength and the affected volume, increasing the global (bulk) shock sensitivity.

Modifying explosive behavior with additives

“…TEMs are comprised of a hydrocarbon encapsulated by thermoplastic which expand up to 120 times its volume upon heating between 85 to 230 °C [21]. Thus, after the formulation has been created, void volume, therefore shock sensitivity, can be modified [18]. Although TEMs have been used in the mining industry [22], their effects on condensed explosive formulations remains unexplored.…”

“…Chemically inert, glass microballoons have been reported to increase the detonation velocity in an ammonium nitrate emulsion explosive [15]. Few studies of low-impedance polymer microspheres have been reported and even fewer polymer balloons [18]. It has been reported that low-melting particles are not as effective in sensitizing a formulation as high melting ones [16,17].…”