Direct observations of ultrasonically generated hot spots in polymer composite energetic materials

Thermally dissipated energy from periodic mechanical action can lead to the creation of areas of intense, localized heating in heterogeneous composite materials, called hot spots. Measurements of hot spots have been performed in shock, impact, and friction studies of energetic materials, amongst others. In this work, high-frequency contact loading allows for the controlled development of hot spots near explosive crystals embedded in polymer with temperature gradients suitable for shadowgraph visualization. Heat dissipation initiated the delamination of crystal-binder interfaces, which led to increased heating from interfacial motion and further delamination propagation. A variety of explosives were considered. Low melting point explosives were driven to reaction despite a hypothesized reduction in interfacial friction upon melting, and crystals with controlled surface morphology still experienced damage. Shadowgraphy allows nonintrusive access to internally developing hot spots which, in turn, enables the identification of heat generation mechanisms to inform our understanding of explosive initiation.