L-062 Probing Critical Ignition, Reaction Growth And Deflagration Mechanisms

March 2000
Dr Peter R. Lee (Project Manager), Dr Michael W. Sharp (Energetic Materials)

This paper is intended to set the scene and to highlight issues of which an understanding is essential in order to aid development of techniques to predict the response of energetic materials in munitions subjected to thermomechanical stimuli. There are two main problems:

  • Prediction of whether the stimulus has sufficient power to initiate the energetic material in the specific environment of the munition; governed by the power per unit mass (Watts/kg) required to damage or initiate the material
  • Prediction of the fate of the deflagrative process which has been initiated; the options are that it fades or grows.

Once deflagration has started, the fate of the process depends upon a huge number of factors, some of which are:

  • The thermochemical characteristics and decomposition kinetics of the energetic material as functions of temperature and pressure Whether the energetic material has been damaged and to what extent; governed by high rate of strain mechanical and fracture mechanics properties of the energetic material
  • Whether the casing has been damaged; governed by the high rate of strain mechanical and fracture mechanical properties of the casing
  • the level of dynamic confinement of the energetic material; governed by the combination of the dynamic strength of the casing and the charge itself and, possibly mitigation materials and/or devices

It is not intended to discuss shock initiation of detonation in detail during this workshop, although the response of systems to low intensity shocks, which are insufficient to lead to prompt detonation, may be considered. We wish to avoid discussion of areas of shock physics for which adequately developed theory already exists and for which suitable codes are available. Shock to deflagration can be handled very well by hydrocodes and will not be considered here. However, under shock pressures and duration close to the threshold level of shock sensitivity, there is some doubt as to exactly where shock to detonation transition (SDT) occurs. As a rough guideline, it is suggested that any situation where shock levels are sufficiently great to enable hydrocode prediction to be carried out reliably should not be considered here. In contrast, any initiation process which is sufficiently slow to include compressive or tensile fracture, or other movement of the energetic material, either internally, involving density discontinuities, or relative to its container, will be considered. One area of shock-based initiation which will be considered is the so-called XDT, where reflected weak shocks may combine to cause initiation, usually associated with some sort of damage process, such as occurs in the rocket motor "Bore Effect, because movement of fractured, ignited propellant may be involved.

Contact us for more information

Christopher Hollands
Energetic Materials TSO
United Kingdom
+32 2 707 56 30