Considerations in Shock Compression of Solids

The subject of this chapter is the relationship between macroscale observations and the underlying microscale processes in shock compression. Since the greater part of our current experimental knowledge of the shock compression process involves macroscale observations, we try to infer microscale phenomena from these data. A much more satisfactory approach is the direct real-time observation of microscale processes themselves. This is difficult to do in most cases, so we must still rely on a combination of macroscale measurement, microscale theory, and whatever direct observations of microscale processes that can be made. 

An example of research in the micromechanics of shock compression of solids is the study of rate-dependent plasticity and its relationship to crystal structure, crystal orientation, and the fundamental unit of plasticity, the dislocation. The majority of data on high-rate plastic flow in shock-compressed solids is in the form of  

Unfortunately, in many cases, one or more of the above three components are unavailable, and we resort to substantial subjective guesswork in establishing underlying micromechanical causes of a particular macroscale observation. 

We will attempt to address a number of these phenomena in terms of their micromechanical origins, and to give the essential quantitative ideas that connect the macroscale (continuum description) with the microscale. We also will discuss the importance of direct observations, wherever possible, in establishing uniqueness of scientific interpretation. 

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Micromechanical Considerations in Shock Compression of Solids 253 From p = Gb/R... 

In addition to these micromechanical considerations, low pressure shock compression of porous powder compacts has distinctive features not encountered in low pressure solid density samples. Basically, the sample is dominated by the pores, and the wavespeed at pressures less than those required to crush the sample to solid density is unusually low and is little dependent on the properties of the solid. 

Shock-induced solid state chemistry represents the most complex fundamental problem ever encountered in shock-compression science. All the mechanical and physical complications of other work are present, yet the additional chemical complications are added. Indeed, all mechanical, physical, and chemical aspects of the problem are intimately intertwined. Chemical investigations promise to provide a description of shock compression that differs considerably from that to which we have become accustomed. Nevertheless, a full description of the process requires contributions from a number... 

Among the newer probes now being developed, spectroscopic observations of crystals in the elastic-plastic regime hold promise for limited development of atomic level physical descriptions of local defects [91S02]. It is yet to be determined how generally this probe can be applied to solids. The electrochemical probe appears to have considerable potential to describe shock-compressed matter from a radically different perspective. 

Table 6.2 summarizes the low pressure intercept of observed shock-velocity versus particle-velocity relations for a number of powder samples as a function of initial relative density. The characteristic response of an unusually low wavespeed is universally observed, and is in agreement with considerations of Herrmann s P-a model [69H02] for compression of porous solids. Fits to data of porous iron are shown in Fig. 6.4. The first order features of wave-speed are controlled by density, not material. This material-independent, density-dependent behavior is an extremely important feature of highly porous materials. 

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