The influence of microstructure and conformational polymorph on the drop-weight impact sensitivity of δ-phase HMX

The influence on HMX impact sensitivity resulting from damage due to phase transition versus that of the polymorphic conformer and crystal lattice change in δ-HMX are quantified in this work. Microstructural imaging techniques are used to characterize 30 crystals from three groups of distinctly large and small HMX crystals which have undergone a thermally induced phase transition, as well as some which have undergone spontaneous reversion from δ→β-phase. It is found that large HMX crystals incur a significant increase in cracks during the transition from β→δ, while much smaller crystals appear to have no distinguishable increase in voids after undergoing a β→δ-phase change. Drop-weight impact experiments were performed on 46 to 68 samples from each of six sample types of HMX, three types of which are composed of very small HMX crystals and three of which are composed of large HMX crystals. It is found that small HMX crystals, which do not appear to incur a significant increase in voidspace after undergoing the phase transition, do not have statistically distinguishable changes in drop-weight impact sensitivity between β, δ, or β-reverted phase HMX; however, large crystal HMX impact sensitivity increases significantly when transitioned to δ-phase and furthermore when allowed to revert to β-phase. This indicates that the morphological change is the dominant effect of β→δ transformation on impact sensitivity and that the δ-conformer and crystal packing structure have little or no effect on the impact sensitivity of HMX.