Experimental Investigation of Hotspot Formation in HMX-Based Plastic-Bonded Explosives Under Shock Compression

This study focuses on real-time measurements of ignition growth in HMX (cyclotetramethylene-tetranitrate)-based microstructured composites (abbreviated PBX plastic-bonded explosives) as they undergo shock compression. A small wafer of PBX is encased in a transparent polymer so that all the HMX crystals in the PBX can be observed under a microscope. Hot spots are produced by hypervelocity (2-6 km/s) impacts with laser-launched flyer plates. The hot spots are observed via their intense thermal emission either by a 32-channel visible (450-825 nm) optical pyrometer or a nanosecond video camera. Combining these two detectors allows hot spots to be observed with nanosecond time and micrometer space resolution. Because the pyrometer measures the absolute spectral radiance, the graybody approximation can be used to determine real-time temperatures. Producing hot spots with previously developed 500m diameter flyer plates lead to a complex ignition field that includes ignited HMX and other kinds of hot spots such as those produced by gas compression in voids. The complex hot spot field makes it difficult to determine the mechanism of ignition growth. Recently we developed a method to generate tiny flyer plates whose diameter is comparable to the size of single HMX particles in the PBX, and preliminary results are presented here showing we can generate a single spot of deflagrating HMX. In other words, we can watch ignition growth in a reactive microstructured medium originating from a point source that is instantaneously ignited by mechanical means.