Shock processes

The Rankine-Hugoniot curve is sometimes referred to as the shock adiabat (especially in the Soviet literature). This terminology reflects the fact that the shock process is so fast that there is insufficient time for heat... 

Stdffler, D. (1972), Deformation and Transformation of Rock-Forming Minerals by Natural and Experimental Shock Processes, I, Fortschr. Miner. 49, 50-113. 

This phenomena has been attributed to the very high strain rates associated with shock loading and the subsonic restriction on dislocation velocity requiring the generation and storage of a larger dislocation density during the shock process than for quasi-static processes [1], [2], [12],... 

The structure/property relationships in materials subjected to shock-wave deformation is physically very difficult to conduct and complex to interpret due to the dynamic nature of the shock process and the very short time of the test. Due to these imposed constraints, most real-time shock-process measurements are limited to studying the interactions of the transmitted waves arrival at the free surface. To augment these in situ wave-profile measurements, shock-recovery techniques were developed in the late 1950s to assess experimentally the residual effects of shock-wave compression on materials. The object of soft-recovery experiments is to examine the terminal structure/property relationships of a material that has been subjected to a known uniaxial shock history, then returned to an ambient pressure... 

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. 

We first consider strain localization as discussed in Section 6.1. The material deformation action is assumed to be confined to planes that are thin in comparison to their spacing d. Let the thickness of the deformation region be given by h then the amount of local plastic shear strain in the deformation is approximately Ji djh)y, where y is the macroscale plastic shear strain in the shock process. In a planar shock wave in materials of low strength y e, where e = 1 — Po/P is the volumetric strain. On the micromechanical scale y, is accommodated by the motion of dislocations, or y, bN v(z). The average separation of mobile dislocations is simply L = Every time a disloca-... 

The scientific enterprise is concerned with the identification, interpretation, and quantification of observed responses in terms of mechanical, physical, and chemical materials properties. The technological enterprise is concerned with the utilization of materials responses or distinctive shock processes. 

A range of complex, elastic-plastic behaviors are observed experimentally they are perhaps the most widely encountered and most typical of shock behaviors, but they are perhaps the least understood of the materials responses. Unfortunately, nonspecialists seldom consider realistic elastic-plastic descriptions of shock processes. This section summarizes the very large body of information available in this area. The metallurgical mud is most viscous in this area. 

Thermal treatment of shock-modified theta-phase alumina, which initially contained about 30% alpha phase, showed a dramatic change in the rate of transformation to the alpha phase [90B01]. As shown in Fig. 7.13, the shocked sample showed no evidence for an incubation period and displayed a rapid conversion to the alpha phase, in sharp contrast to the unshocked sample. Such behavior clearly indicates that the shock process resulted in formation of larger concentrations of alpha-phase nuclei. 

The reaction is significantly exothermic with a heat of reaction of about 40 kcalmol . This energy will produce a sufficiently high temperature to melt the product and will allow the influence of thermochemical factors to be investigated. The temperature required to initiate the Ni-Al reaction at atmospheric pressure is about 660 °C. This reaction temperature threshold will be encountered in the shock processing, but it should be recognized that the conventional synthesis process is preceded by melting of the aluminum. At the pressure of the shock compression, the melt temperature of the aluminum will be approximately doubled to a value above the mean-bulk tempera-... 

Perhaps the most definitive result to come from the early nickel-aluminia synthesis work was the thermal analysis investigation of Hammetter [88HO 88W01], which showed explicit data on substantial changes in the shockec-but-unreacted mixtures. Differential thermal analysis was carried out on th -starting powder compacts of both the mechanically mixed and composite powders. Shocked and unreacted powders were compared to provide direc evidence for substantial changes introduced by the shock process. 

Fig. 8A. The exothermic energy release of shocked but unreacted Ni-Al mixtures shows a profound change. A preinitiation" phenomenon in which reaction temper ture is reduced by over 200 °C is caused by the shock process. A compact composer of composite particles that inhibit mixing shows no such effect [88H01].

Shock-compressed solids and shock-compression processes have been described in this book from a perspective of solid state physics and solid state chemistry. This viewpoint has been developed independently from the traditional emphasis on mechanical deformation as determined from measurements of shock and particle velocities, or from time-resolved wave profiles. The physical and chemical studies show that the mechanical descriptions provide an overly restrictive basis for identifying and quantifying shock processes in solids. These equations of state or strength investigations are certainly necessary to the description of shock-compressed matter, and are of great value, but they are not sufficient to develop a fundamental understanding of the processes. 

Major efforts have developed descriptions of shock processes with molecu-... 

Shock-compression science, which has developed and matured since its inception in 1955. has never before been documented in book form. Over this period, shock-compression research has provided numerous major contributions to scientific and industrial technology. As a result, our knowledge of geophysics, planetary physics, and astrophysics has substantially improved, and shock processes have become standard industrial methods in materials synthesis and processing. Characterizations of shock-compressed matter have been broadened and enriched with involvements of the fields of physics, electrical engineering, solid mechanics, metallurgy, geophysics, and materials science... 

Adamczyk (1976) is one of the few who tried to incorporate energy losses from the irreversible shock process into the calculation. He proposes to use the work done by gas volume in a process illustrated in Figure 6.12 and described below. 

Calculate the mole fraction of nitric oxide that ultimately will form, assuming that the elevated temperature and pressure created by the shock are sustained indefinitely. Calculate the time in milliseconds after the passage of the shock for the attainment of 50% of the ultimate amount this time may be termed the chemical relaxation time for the shock process. Calculate the corresponding relaxation distance, that is, the distance from the shock wave where 50% of the ultimate chemical change has occurred. 

Accdg to Dunkle (Ref 11, p 156 and private communication), high pressures and temperatures may not be enough to cause a detonation and it seems to require some "shock processes . Dunkle also stated that accdg to G.B. Kistiakowsky, as stated in Ref 8,... 

The velocity of advance of the front is super sonic in a detonation and subsonic in a deflagration. In view of the importance of a shock process in initiating detonation, it has seemed difficult to explain how the transition to it could occur from the smooth combustion wave in laminar burning. Actually the one-dimensional steady-state combustion or deflagration wave, while convenient for discussion, is not easily achieved in practice. The familiar model in which the flame-front advances at uniform subsonic velocity (v) into the unburnt mixture, has Po> Po> an[Pg.249]

Its definition is given, together with definition of detonation head, at the end of the item entitled "Detonation (and Explosion) Initiation of Explosives and Shock Processes"... 

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