Low pressure shock initiation of porous HMX for two grain size distributions and two densities

Abstract

Shock initiation measurements have been made on granular HMX (octotetramethylene tetranitramine) for two particle size distributions and two densities. Samples were pressed to either 65% or 73% of crystal density from fine (≈10 μm grain size) and coarse (broad distribution of grain sizes peaking at ≈150 μm) powders. Planar shocks of 0.2−1 GPa were generated by impacting gas gun driven projectiles on plastic targets containing the HMX. Wave profiles were measured at the input and output of the ≈3.9 mm thick HMX layer using electromagnetic particle velocity gauges. The initiation behavior for the two particle size distributions was very different. The coarse HMX began initiating at input pressures as low as 0.5 GPa. Transmitted wave profiles showed relatively slow reaction with most of the buildup occurring at the shock front. In contrast, the fine particle HMX did not begin to initiate at pressures below 0.9 GPa. When the fine powder did react, however, it did so much faster than the coarse HMX. These observations are consistent with commonly held ideas about burn rates being correlated to surface area, and initiation thresholds being correlated with the size and temperature of the hot spots created by shock passage. For each grain size, the higher density pressings were less sensitive than the lower density pressings.