Theory of Shock Transfer

Available evidence indicates that secondary explosives in detonators are initiated by shock waves. This evidence consists of streak traces showing an accelerating shock front passing from the azide to the secondary explosive, and flash X-ray studies show the secondary explosive detonating at the interface between the lead azide and the secondary. Since the theory of shock waves at interfaces has been adequately treated in other texts [27,32,39], only the major aspects will be presented. 

Hydrodynamic theory defines the behavior of shock waves at interfaces between elements of exqilosive trains in terms of curves that relate the shock velocity to the particle velocity, called Hugoniot curves. Assuming that mass and momentum are conserved across the shock front, one can write 

A plot of P vs. Up has the slope poDs and passes through the origin. If the Hugoniot curve for the detonation products of lead azide at the C-J point were known (the curve for unreacted RDX is known [40]), it would be possible to determine the strength of the shock wave generated in RDX by the detonation wave from lead azide. It is to be recalled that the Chapman-Jouget model as modified calls for a reaction zone at the end of which the reactants have been completely converted to products in equilibrium and travel at the local sonic velocity. The end of the reaction zone is often called the C-J plane, and the associated pressure and temperature called the C-J pressure and temperature. 

The detonation velocity refers to the velocity of the shock front relative to the unreacted material. Peak shock pressure needed to initiate RDX and other secondary explosives as a function of density and particle size have been measured by the small scale gap test [48]. For RDX, which is commonly used in detonators, peak pressure for initiation is found to range from 9 to 16 kbar. Since even under nonideal conditions lead azide produces over 100 kbar, as shown in Table VI, if enough azide is present, it readily initiates RDX, and other secondary explosives, as shown in Table VII. 

F lire 20. Distance-time plots for shock initiation of homogeneous (a) and heterogeneous (b) explosives. 

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