This basic question is discussed in a paper with S. Osovski and A. Venkert, to appear in Mechanics of Materials.
http://dx.doi.org/10.1016/j.mechmat.2012.09.008
We definitely enjoyed working on this aspect of the dynamic shear localization failure process (aka adiabatic shear) and I hope you will enjoy reading it! Comments are welcome :=)
Dynamic shear localization
D. Rittel
merittel [at] technion.ac.il
www.technion.ac.il/~merittel
D.
D. Rittel
merittel [at] technion.ac.il
www.technion.ac.il/~merittel