Shock Initiation Criteria

James, H.R. Critical energy criterion for the shock initiation of explosives by projectile impact. Propellants, Explosives, Pyrotechnics 13, 35-41 (1988). 

ABSTRACT RARDE work on the ignition or initiation of cased explosives and rocket propellants is reviewed. Attention is focussed on criteria to predict the threshold impact velocity for detonation. Critical energy and power concepts are considered. Experiments show that, for thin cases, a critical energy criterion is adequate but clear evidence is provided for a non-shock initiated detonation regime with thicker cases. Existing criteria cannot describe this phenomenon. The need for a quantitative description of hot spot formation and growth is highlighted. 

The work reported here indicates that a critical energy criterion as postulated by Walker and Wasley and modified by James can provide a useful model of prompt shock initiation in projectile impact experiments. However, it is important that the shocked volume is accurately estimated. It is at least likely that it is the failure to take into account rarefactions in the barrier material which account for the different critical energy values required to give the best fits to our data with metal and PMMA barriers. Figs 6-8. 

Firstly, a non-shock initiated detonation regime has been identified which cannot be described by the essentially shock-based Walker-Wasley criterion. Beyond this we must also recognise that impacts at lower velocities than those required to produce detonation can for many explosives nevertheless result in violent deflagrations. The description of such a complex phenomenon is unlikely to be possible in terms of single parameter. Finally, even when the Walker-Wasley criterion can be used, its value must be derived from experimental studies. We have no a priori methods for the estimation of the critical energy. 

A corollary, often appended to the second RPT criterion, is that RPTs are more difficult to achieve as the temperature of the hot liquid increases beyond the threshold temperature. For very high hot-liquid temperatures, some external pressure pulse or shock becomes necessary to initiate an RPT. 

Here the three ascending curves represent the shock waves in He, Air and CO2, which are regarded as ideal gases with constant specific heats, the initial pressure pQ being 1 atm in each case. The two descending curves provide p -Wj relations for waves reflected into the deton products. The criterion for shock reflection is that w at the intersection is smaller than the C-J value w. The C-J values... 

Much effort is being invested on the development of computer programs for the prediction of initiation thresholds of expls and proplnts but none to date are satisfactory because of the lack of a clear threshold criterion. Shock induced mechanical failure (spall) has been correlated with stress and time by varying the thickness of the flyer plates. Likewise Walker and Wasiey (Ref 15) have attempted to use a P2t criterion (where P is the scalar shock pressure... 

M, Cowperthwaite, Single-Shock Curve Buildup and a Hydrodynamic Criterion for Initiation of Detonation, in Gasdynamics of Detonations and Explosions, vol. 75 of Progress in Astronautics and Aeronautics, J. R. Bowen, N. Manson, A. K. Oppenheim, and R. I. Soloukhin, eds.. New York American Institute of Aeronautics and Astronautics, 1981, 269-281. 

An experimental study of the projectile impact initiation of detonation in cased explosives has revealed both shock and non-shock regimes. The shock regime can be described by an empirical critical energy criterion for detonation. However, a full description of the processes leading to detonation or to violent reaction cannot be based on such a criterion and requires a full description of hot spot formation and growth mechanisms. 

Once a therapy has been delivered the ICD makes a determination of whether the tachycardia episode has been successfully terminated. Criterion for redetection of an unsuccessfully terminated tachycardia may be less rigorous than that for initial detection. Following a shock redetection typically begins after an extended refractory period (on the order of 500-1000 ms). This period serves to prevent the shock energy itself from being detected as a ventricular event. 

Unbeknownst to the surgeon, the ICD was programmed committed to shock once detection criterion was satisfied. Detection was quickly satisfied with even this short burst of cautery, which initiated capacitor charging, and then the subsequent committed shock. 

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