Pressure peaks

Shock Synthesis. When graphite is strongly compressed and heated by the shock produced by an explosive charge, some (up to 10%) diamond may form (26,27). These crystaUite diamonds are small (on the order of 1 llm) and appear as a black powder. The peak pressures and temperatures, which are maintained for a few microseconds, are estimated to be about 30 GPa (300 kbar) and 1000 K. It is beheved that the diamonds found in certain meteorites were produced by similar shock compression processes that occurred upon impact (5). 

Figure 1.1 shows a typical stress-volume relation for a solid which remains in a single structural phase, along with a depiction of idealized wave profiles for the solid loaded with different peak pressures. The first-order picture is one in which the characteristic response of solids depends qualitatively upon the material properties relative to the level of loading. Inertial properties determine the sample response unlike static high pressure, the experimenter does not have independent control of stresses within the sample. 

The peak pressures attainable with explosive facilities can be greatly enhanced, and the initial peak pressure can be better sustained by using a plane-wave generator to accelerate a flyer plate, which then impacts a flat specimen as shown in Fig. 3.2. This technique will generate peak pressures up to a few hundred GPa. (McQueen and Marsh, 1960 McQueen et al., 1970). 

In this chapter, we will review the effects of shock-wave deform.ation on material response after the completion of the shock cycle. The techniques and design parameters necessary to implement successful shock-recovery experiments in metallic and brittle solids will be discussed. The influence of shock parameters, including peak pressure and pulse duration, loading-rate effects, and the Bauschinger effect (in some shock-loaded materials) on postshock structure/property material behavior will be detailed. 

To illustrate the effect of radial release interactions on the structure/ property relationships in shock-loaded materials, experiments were conducted on copper shock loaded using several shock-recovery designs that yielded differences in es but all having been subjected to a 10 GPa, 1 fis pulse duration, shock process [13]. Compression specimens were sectioned from these soft recovery samples to measure the reload yield behavior, and examined in the transmission electron microscope (TEM) to study the substructure evolution. The substructure and yield strength of the bulk shock-loaded copper samples were found to depend on the amount of e, in the shock-recovered sample at a constant peak pressure and pulse duration. In Fig. 6.8 the quasi-static reload yield strength of the 10 GPa shock-loaded copper is observed to increase with increasing residual sample strain. 

Figure 6.12. Stress-strain behavior of shoek-loaded NijAl as a funetion of peak pressure.

Figure 6.13, Brightfield electron micrographs of dislocations and stacking faults in NijAl as a function of peak pressure, (a) 14 GPa and (b) 23.5 GPa.

G.T. Gray III and P.S. Follansbee, Influence of Peak Pressure on Substructure Evolution and Mechanical Response of Shock-Loaded 6061-T6 Aluminum, in Shock Waves in Condensed Matter 1987 (edited by S.C. Schmidt and N.C. Holmes), Elsevier Science, New York, 1988, 339 pp. 

G.T. Gray III and C.E. Morris, Influence of Peak Pressure on the Substructure Evolution and Shock Wave Profiles of Ti-6A1-4V, in Sixth World Conference on Titanium (edited by P. Lacombe, R. Tricot, and G. Beranger), Les Editions de Physique, France, 1989, 269 pp. 

L.E. Murr, Effects of Peak Pressure, Pulse Duration, and Repeated Loading on the Residual Structure and Properties of Shock Deformed Metals and Alloys, in Shock Waves and High-Strain-Rate Phenomena in Metals (edited by M.A. Meyers and L.E. Murr), Plenum, New York, 1981, 753 pp. 

Figure 7.8. Material hardness of Hadfield steel subject to shock compression of various peak pressures and pulse durations.

Figure 7.10. Reload stress-strain curves for recovered OFE copper subject to shock compression of 10 GPa peak pressure and pulse durations of 0.1 /rs, 1.0 /rs, and 2.0 /rs.

Column 6 provides the operating pressure and peak pressures that the process may go to. 

Data are available only for simple building geometries. In Allard," a tool for the calculation of wind pressure coefficients for simple geometries is made available, and another tool is described in Knoll et al. Existing wind pressure data have to be examined carefully, because many data represent peak pressure values needed for static building analysis. Real cases with obstructions and buildings in the close surroundings are difficult to handle. Wind-tunnel tests on scale models or CFD analysis will be required. 

In extreme cases, very high pressure waves are encountered in which the time to achieve peak pressure may be less than one nanosecond. Study of solids under the influence of these high pressure shock waves can be the source of information on high pressure equations of states of solids within the framework of specific assumptions, and of mechanical, physical, and chemical properties under unusually high pressure. 

The strong shock regime is the classic archetype and is characterized by a single narrow shock front that carries the material from its initial condition into a new high pressure, elevated temperature, high kinetic energy state. Following a quiescent period at peak pressure, whose duration depends upon... 

The effect of such a transformation on a pressure-volume relation and on wave profiles is shown in Fig. 2.12. Above the transformation, its characteristics dominate the wave profile. At sufficiently high pressure, the peak pressure wave will move at higher speeds and a strong shock regime can be encountered. 

The ramp of pressure to about 3 GPa observed in shock-loaded fused quartz has been used very effectively in acceleration-pulse loading studies of viscoelastic responses of polymers by Schuler and co-workers. The loading rates obtained at various thicknesses of fused quartz have been accurately characterized and data are summarized in Fig. 3.6. At higher peak pressures there are no precise standard materials to produce ramp loadings, but materials such as the ceramic pyroceram have been effectively employed. (See the description of the piezoelectric polymer in Chap. 5.)... 

As the current pulse is largely dominated by the stress differences, a short duration current pulse is observed upon loading with a quiescent period during the time at constant stress. With release of pressure upon arrival of the unloading wave from the stress-free surface behind the impactor, a current pulse of opposite polarity is observed. The amplitude of the release wave current pulse provides a sensitive measure of the elastic nonlinearity of the target material at the peak pressure in question. 

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