Dislocation Mechanics of Shock-Induced Plasticity

The constitutive deformation behavior of copper, Armco iron, and tantalum materials is described over a range of strain rates from conventional compressive/tensile testing, through split Hopkinson pressure bar (SHPB) test results, to shock-determined Hugoniot elastic limit (HEL) stresses and the follow-on shock-induced plasticity. A mismatch between the so-called Zerilli–Armstrong (Z-A) constitutive equation description of pioneering SHPB measurements for copper provided initial evidence of a transition from the plastic strain rate being controlled by movement of the resident dislocation population to the strain rate being controlled by dislocation generation at the shock front, not by a retarding effect of dislocation drag. The transition is experimentally confirmed by connection with Swegle–Grady-type shock vs plastic strain rate measurements reported for all three materials but with an important role for twinning in the case of Armco iron and tantalum. A model description of the shock-induced plasticity results leads to a pronounced linear dependence of effective stress on the logarithm of the plastic strain rate. Taking into account the Hall–Petch grain size dependence is important in specifying the slip vs twinning transition for Armco iron at increasing strain rates.