Impedance shock

Acoustic impedance The shock impedance in the limit of an infinitesimal disturbance. Independent of pressure. 

The shock pressures attainable with direct explosive contact depend on the shock impedance (shock velocity times material density) of the specimen material, and on the explosive energy of the contacting explosive. High-energy explosives placed directly on high-shock impedance materials can produce shock pressures of several tens of GPa. 

To achieve pressures intermediate to those achieved by direct contact with a given metal plate, use is often made of alternate layers of various shock impedance materials. Table 3.2 gives a summary of experimental arrangements that have been used in materials studies to achieve pressures from 3 to 80 GPa. 

Alternately (and showing the versatility of the impact technique), impac-tors can be designed to achieve structured loading. The pillow technique of Barker used a graded shock impedance to achieve a small amplitude shock followed by a slowly increasing pressure [88C04]. Materials synthesis studies... 

Fig. 5.6. Typical current-time responses from impact-loaded PVDF are shown for samples on the standard materials indicated. In each record the upper traces are the full record showing short duration negative and positive current pulses due to loading and release in the standard. Time increases from left to right. The detail of each pulse depends upon the shock impedance of the materials. In each record, enlarged views of loading and release pulses are shown.

Fig. 6.9. Two-dimensional numerical simulations are depicted for the Sandia Momma Bear fixture. Pressure contours within one-half the powder compact are shown at various times. The principal feature shown is the development of a radial-mode loading due to the low shock impedance of the powder (after Graham [87G03]).

Intermetallics also represent an ideal system for study of shock-induced solid state chemical synthesis processes. The materials are technologically important such that a large body of literature on their properties is available. Aluminides are a well known class of intermetallics, and nickel aluminides are of particular interest. Reactants of nickel and aluminum give a mixture with powders of significantly different shock impedances, which should lead to large differential particle velocities at constant pressure. Such localized motion should act to mix the reactants. The mixture also involves a low shock viscosity, deformable material, aluminum, with a harder, high shock viscosity material, nickel, which will not flow as well as the aluminum. 

The response of titanium-aluminum powder mixtures in a 3 1 molar ratio was investigated under the same shock-loading conditions as in the nickel aluminides. Such mixtures are especially interesting in that the shock impedances of the materials are approximately equal and both are relatively hard and difficult to deform. In addition to any chemical differences, such materials should prove to be difficult to mix with the shock conditions. 

On detonation of charge the foil moves behind the shock front with particle velocity, and the potential on its end is recorded on an oscilloscope. It is claimed that this method has a better precision than the shock-impedance method ... 

An interpretation of the fact that, for some explosives at least, the detonation velocity does not continue to rise with rise in density, but goes thru a maximum and detonation finally fails when the density exceeds a critical value is reptd by Dunkle (Ref 5) and Price Refs 9 10). Roth (Ref 4), on the basis of results reported in Refs 1, 2 St 3, suggests the existence of a property he calls Widerstand ("resistance or "impedance ) of value equal to the product of loading density and detonation velocity, analogous to acoustic impedance and shock impedance (See abstract of Roth s paper at the end of this item)... 

Detonation, Shock Impedance and Acoustic Impedance in. Acoustic impedance is the ratio between sound pressure and particle velocity. A sound wave, on reaching a boundary between two media, has part of its energy reflected at the boundary and part transmitted into the 2nd medium. The relationships depend on the values of the acoustic impedance in the two media. Swenson (Ref 2) showed that ... 

If shock impedances in both media are the same (impedance match), no wave is reflected (Ref 3, pp 80-1)... 

Shock impedance of a material influences its action as a casing material for explosive charges. While pressures of detonation are sufficient to burst or deform any container, the duration of the detonation process is of the same order of magnitude as the expansion times of the usual containers. The rate at which the container expands is inversely related to the mass of container material which is moved. For a thin-walled container the mass is essentially that of the wall. For one having thick walls, the "effective mass is proportional to p U, because only the material which has been reached by the shock front is affected... 

In conclusion, Dunkle remarked that the shock impedance is a good measure of the effectiveness of a material as a confining medium for detonation (Ref 3, p 81) Refs 1) H. Eyring et al, "The Stability of Detonation , ChemRevs 45, 69-178(1949)... 

Shock impedance is the product and graphically it is represented by the slopes of the broken lines in Figs 3 4. Note that in Fig 3 the reflected wave is a rarefaction rather than a shock. Figure 5 (also from Ref 21) is a useful summary of shock effects in metals, rocks, plastics etc in contact with some common explosives... 

Impedance, Acoustic and Shock is the product of density and sound velocity, namely pc. Analogously shock impedance is pQU where U is the shock- velocity in a medium whose density (ahead of the shock)is p0. Both acoustic shock impedances are used to estimate the interface stress, a, and interface particle velocity, u, for planar shocks moving from one medium into another medium. A simple method of doing this, based on the so-called acoustic approximation, is illustrated below... 

Detonation, shock impedance and acoustic impedance in 4 D518... 

This contains an interesting factor, the product poU. This product is called the shock impedance and is designated by the letter Z. 

The impact of a finite-thickness flyer on a thick target we will see how the flyer thickness and the relative shock impedances of the flyer and target affect duration and shape of the target shock that is produced. 

Here, as in the problem involving a shock at an interface of two materials, we will get different behavior when the relative values of the shock impedances of target and flyer are reversed. So we must break this example into three cases ... 

By following this process in Figure 19.21, you can see that the final state of the flyer-target interface will converge at P = 0, = 0, and that the flyer will remain in contact with the target. You will also surmise that both the number and size of the steps produced on the back of the shock wave in the target will depend upon both the flyer thickness and the relative values of the shock impedance, Z, of the flyer and target. 

The major interaction of interest with explosives is the case where an explosive is in contact with another material, and the detonation wave interacts at that interface. As with the analogous nonreactive shock, it is important to know the relative shock impedance of the material and the explosive reaction products. The shock impedance of the products is Z et = PoD-... 

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