Modelling the effects of temperature-dependent material properties in shear melt layers

2016-12-08T11:03:18Z (GMT) by Robert Timms
<p>Technical paper presented at the 2016 Defence and Security Doctoral Symposium. This paper won second place in the Technical Paper category.<br></p><p>The mechanisms responsible for ignition of explosive materials in response to low energy stimuli, known as “insults" in the literature, are still not well understood. It is in general believed that explosive ignition is of thermal origin, with mechanical energy being converted into heat energy in localised regions, forming so-called \hot spots". When an explosive sample is subject to a mechanical insult pre-existing, or new, microcracks will be in compression and shear. It is possible for such microcracks to grow in size if the local stress is great enough and, due to friction between solid surfaces, heat is released during the growth process. Subsequent to sufficient heat release, the crack surface temperature will be raised to the solid melting point and a thin sheared melt layer will be formed, separating the solid surfaces. This thin melt layer will continue to be heated through viscous dissipation and subsequent chemical reaction, and is thought to be a prime location for so-called hot spot generation. </p> <p>Mechanical insults, resulting from low-speed impacts which shear an explosive, have been identified as a possible ignition source. However, modelling such an ignition mechanism numerically with hydrocodes proves to offer some considerable challenges. To supplement the numerical approach, we develop an analytical model of the shearing, melting and subsequent ignition of an explosive material. We consider the melting of a thin viscous layer of explosive material due to an applied shear in an idealised planar geometry. The model accounts for self-heating due to mechanical dissipation, and a single-step Arrhenius reaction is used to describe the heating of the explosive due to subsequent chemical reaction. A solution is sought by considering perturbations from a melt layer of uniform width. In particular, we consider the effects of modelling the temperature dependence of the liquid viscosity and specific heat are studied. In contrast to previous work which does not account for the temperature dependence of material properties, it is shown that allowing the viscosity to vary with temperature can lead to non-uniform mechanical heating in the layer to leading order. Such localised heating may be associated with generation of localised hot spots which give rise to ignition.</p>