Elastic precursor amplitude

Asay and Gupta [25] measure elastic precursor amplitudes as functions of propagation distance for two different divalent impurity concentrations in <100)-loaded LiF. It is shown that not only does the presence of divalent ions affect the precursor amplitude, but also that the state of the dispersion plays an important part. It is concluded that, for a given concentration of defects, the rate of precursor attenuation is reduced if the defects are clustered. 

At loading stresses between the HEL and the strong shock threshold, a two-wave structure is observed with an elastic precursor followed by a viscoplastic wave. The region between the two waves is in transition between the elastic and the viscoplastic states. The risetime of the trailing wave is strongly dependent on the loading stress amplitude [5]. 

Figure 4.16. Free-surface velocity profiles measured on 1400° C molybdenum. The free-surface velocity profile is characterized by an 0.05 km/s amplitude elastic precursor, a plastic wave front, and a spall signal (characteristic dip) upon unloading. The dashed lines represent the expected free surface velocity based on impedance-match calculation [Duffy and Ahrens, unpublished].

Fig. 2.7. Elastic precursor decay in which elastic waves are observed to decrease in amplitude with propagation distance is a typical behavior. The data of this figure describe the behavior of crystalline LiF samples of different yield strengths (after Asay et al. [72A02]).

Typical stress-time profiles for the various materials (28.5-at. % Ni, fee and bcc) and various stress regions are shown in Fig. 5.12. The leading part of the profile results from the transition from elastic to plastic deformation. The extraordinarily sharp rise in stress for the second wave in Fig. 5.12(a) and the faster arrival time compared with that in Fig. 5.12(b) is that expected if the input stress is above the transition, whereas the slower rise in Fig. 5.12(b) is that expected if the stress input to the sample is below the transition. The profile in Fig. 5.12(c) for the bcc alloy was obtained for an input particle velocity approximately equal to that in Fig. 5.12(a) for the fee alloy. The bcc alloy shows a poorly defined precursor region, but, in any event, much faster arrival times are observed for all stress amplitudes, as is indicative of lower compressibility. 

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