Heat Generation Mechanisms in Energetic Composite Materials Under Ultrasonic Excitation
Thermal dissipation of mechanical energy from periodic loading in energetic materials (EMs) leads to the creation of areas of intense, localized heating, called hot spots. The impact and shock conditions for the hot spot initiation of solid explosives have been extensively explored, but little work has focused on high-frequency contact loading. In order to design formulations to address unintentional initiation by mitigating heating in polymer-bonded explosives (PBXs) and other heterogeneous EMs, the mechanisms of heat generation which lead to the thermal initiation of energetic composites under ultrasonic excitation were explored. Heat generation mechanisms which may lead to unintentional initiation were identified through the diagnostic techniques of second harmonic generation (SHG) of δ-HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine) crystals; X-ray phase contrast imaging (PCI) performed at the Argonne National Laboratory Advanced Photon Source; infrared (IR) thermography; and optical microscopy. This work concludes with high-speed mesoscale observations of dense layers of PETN (pentaerythritol tetraniterate), CL-20 (hexanitrohexaazaisowurtzitane), RDX (1,3,5-trinitro-1,3,5-triazine), and HMX which were damaged or driven to decomposition under acoustic insult using the non-intrusive imaging technique of shadowgraphy to detect hot spots within the transparent binder. Recommendations are formed which address binder adhesion, energetic material properties, and particle morphology on the vibration sensitivity of a PBX formulation.