A microscopic model for predicting hot-spot ignition of granular energetic crystals in response to drop-weight impacts

A micro-mechanics model describing hot-spot formation in the energetic crystal powders cyclotetramethylene tetranitramine (HMX) and pentaerythritol tetranitrate (PETN) subjected to drop-weight impact is developed. Considering contact deformation, friction and chemical reactions at the particle level during the impact loading process, three hot-spot sources are included, namely, the micro-particle contact deformation between two equal-sized particles with no relative sliding, the contact sites between the impacting surface and the particles, and the contacting zone between particles with a sliding velocity along the maximum shear-stress direction. Drop-weight impacts on samples composed of equal-sized particle layers are considered. The temperature rise due to plastic and frictional dissipation is estimated, and melting is included. Hot spot ignition is predicted via thermal explosion, using an Arrhenius thermochemical model. The effects of drop height and particle size on the ignition processes are analyzed. The hot-spot sources at the interface between the particles and the impacting surfaces play the most important role during the early ignition stage. The time-to-ignition of the three sources increases with decreasing drop height. Calculated results show that the HMX and PETN crystals demonstrate monotonously increasing time-to-ignition with a reduction in particle size. The effect of particlesize on the hot spot ignition threshold can be predicted. Samples with a smaller particle size undergo larger localized deformations and lower average pressures.