Shock-Induced Dynamic Yielding and Phase Transitions

Shock-Induced Dynamic Yielding and Phase Transitions 

The condition that gives rise to multiple shock fronts (i.e., allows a shock wave to bifurcate as indicated in Fig. 4.10(b)) will occur when the second wave propagation velocity (with respect to the laboratory) is given by (4.39). How- 

Bancroft et al. (1965), and in the case of CaO and the B1 to B2 transition discovered by Jeanloz and Ahrens (1979), complete reversion of the low-pressure phase occurs upon unloading. These latter transitions involve rearrangement of the lattice which can occur via its deformation rather than complete reconstruction. The volume change in the Si02 transition is much larger than in the case of CaO, as seen in Fig. 4.15. In contrast to the pressure-volume plane when plotted in the Ph-u, plane, the occurrence of these transitions is less striking in this representation (Fig. 4.14). 

In the case of most nonporous minerals at sufficiently low-shock stresses, two shock fronts form. The first wave is the elastic shock, a finite-amplitude essentially elastic wave as indicated in Fig. 4.11. The amplitude of this shock is often called the Hugoniot elastic limit Phel- This would correspond to state 1 of Fig. 4.10(a). The Hugoniot elastic limit is defined as the maximum stress sustainable by a solid in one-dimensional shock compression without irreversible deformation taking place at the shock front. The particle velocity associated with a Hugoniot elastic limit shock is often measured by observing the free-surface velocity profile as, for example, in Fig. 4.16. In the case of a polycrystalline and/or isotropic material at shock stresses at or below HEL the lateral compressive stress in a plane perpendicular to the shock front 

---