Direct observation of frictional ignition in dropped HMX-based polymer-bonded explosives

Abstract

This work concerns the response of high explosive (HE) charges to oblique impact events, as would occur during a drop accident. We describe a rigid-arm pendulum that was used to examine mechanisms for frictional heating, ignition and reaction propagation during drop-skid insults to HE from low drop heights ( ≤ 3.66 m). Drops onto clean surfaces never produced ignition. On dirty surfaces, ignition was frequently observed, sometimes developing into damaging explosions. The mechanisms for hotspot formation and ignition involved hard, high-melting-point grit particle surface contamination. Two mechanisms are described. (1) The grit-substrate mechanism operates when the impacted surface is a high-melting-point, hard material and frictional work is localized forming hotspots when grit rubs against the substrate. Material properties of the HE, grit, and impact surface, such as indentation hardness, thermal conductivity and melting point, were also explored showing how propensity for ignition is affected. (2) The grit–grit mechanism operates when an impacted surface has similar hardness to the explosive; under these conditions grit particle collisions are the hotspot-forming frictional interaction. Through direct observation, this work confirms some of the early work on frictional ignition mechanisms, and offers insight for how these types of dangerous explosive handling accidents can be mitigated in HE-operations environments.