Impact-induced friction ignition of an explosive: Infrared observations and modeling

Abstract

A contaminant (grit) trapped between an explosive and an impacted surface can significantly reduce the impact energy required to initiate a secondary high explosive. Several severe accidents have occurred when an explosive charge was dropped from a height insufficient to cause ignition by heating due only to plastic deformation; the frictional heating from embedded grit has been implicated. Here, we describe an idealization of this situation where a small sample of a polymer-bonded cyclotetramethylenetetranitramine explosive (HMX-PBX 9501), with a 400 μm diameter sphere of silica embedded in the surface, was impacted between instrumented transparent anvils and infrared images were recorded. The instrumentation provided temperature and the work done by the friction between the grit and the anvil surface for the impact process, up to ignition. All experiments were conducted under impact conditions insufficient to cause ignition without grit. Ignition occurred at approximately 500 μs, a grit temperature of 1000 K, and an impact load of 12 kN. A high-fidelity numerical heat transport model, using four-step reversible decomposition kinetics for HMX, clarified the physical mechanism of ignition in the experiment. The model suggested that only a very small part of the silica sphere was heated by the friction process and residual heat in the impacted surface behind the moving grit caused ignition. The model agreed well with the experiment in terms of time and temperature, and we have good confidence in the mechanistic picture provided by the model.