Dislocation Mechanics Based analysis of Material Dynamics Behavior : Enhanced Ductility, Deformation Twinning, Shock Deformation, Shear Instability, Dynamic Recovery

Abstract
Further developments are described for the dislocation mechanics based constitutive equation analysis previously used to describe the separate dynamic stress-strain behavior of fcc and bcc metal polycrystals. An enhanced hardening and ductility in copper and certain tantalum materials at higher strain rates in split Hopkinson pressure bar tests and in shock loading are attributed to enhanced dislocation generation rather than to dislocation drag. Added material strengthening is accounted for also by deformation twiming in ARMCO iron and titanium and in shocked copper and tantalum. The separate equations are applied to calculate the critical strain for shear banding in copper, iron, and the titanium alloy, Ti-6Al-4V. In the two latter cases, the results are very sensitive to the details of the strain-hardening behavior and the need is demonstrated for a dynamic recovery factor to account for the onset of shear banding. Consideration is given also to the possibility that shear band behavior requires explanation on a more fundamental Hall-Petch dislocation pile-up basis.