For modelling overdriven detonation of explosives in AUTODYN 11.0, using classical JWL EOS. Is the 'burn on compression' process ok just with specifying a burn on compression fraction in the material data input?




In most applications of explosives to flyer plates, the detonation of explosives is usually regarded as a steadily progressing wave phenomenon in which the pressure of the detonation products immediately behind the wave front is characterized by the so-called Chapman-Jouguet (C-J) pressure value. This type of detonation is therefore routinely referred to as the C-J detonation behavior and the detonation products start to expand from this C-J state to accelerate the flyer plates to a high velocity status.

Overdriven detonation, however, is a detonation process that can provide a higher or much higher pressure than does the C-J detonation. Taking use of the detonation products from the overdriven detonation to push the plate may lead the plate to reach a hypervelocity status not achievable by means of the usual explosive acceleration techniques. An overdriven detonation can be achieved in many ways, for example when an explosive is shocked by a flyer plate and after an induction time the thermal explosion occurs in the explosive material that is pre-compressed by the initial shock.

The JWL explosive EOS should be used with care for pressures significantly above the CJ pressure, since the exponential terms can give an unrealistic behavior at higher pressures. Some workers have modified the form of the JWL equation to extend the JWL EOS with a straight line adiabat above the CJ point, determining the constants for the extrapolation from the continuity condition at the CJ point.
When you use the standard detonation on time logic you start of from an uncompressed (reference density) explosive and the theoretical maximum achievable detonation pressure would be the CJ pressure.
The only way to take into account pre-compression of the explosive before detonation is to use a sympathetic detonation approach, where the explosive material needs to be compressed first to some extend before detonation takes place. AUTODYN offers two ways of doing this:
1. Usethe JWL material with the burn on compression logic, where the pre-burn Bulk Modulus is required to pre-compress the inert explosive before a detonation compression threshold is reached.
2. Use the Lee-Tarver model
The first approach is a simple numerical methodology and not really physical and this optionmay give unrealistic results for unconfined regions of explosive since the material is free to expand at the time of initial shock arrival and may not achieve sufficient compression to initiate energy release in a realistic time scale. Typically, a burn logic based upon compression is more successful in a Lagrangian frame rather than in an Eulerian frame.

The Lee-Tarver model takes into account compression, ignition and growth and needs quite some material data, which not always is available.





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