Dynamic yield behavior of shock-loaded iron from 76 to 573°k

Abstract

The dynamic yield behavior of Ferrovac E iron was studied over a temperature range of 76–573°K. Large diameter-to-thickness ratio discs were shock loaded, producing a state of uniaxial strain and strain rates of ~10 5 sec −1. The dynamic yield stress for 12.6-mm thick discs was found to be 9.61 ± 0.13 kbar regardless of temperature. This was in contrast to the highly temperature sensitive quasi-static yield stress which decreased by a factor of 2.5 between 76 and 298°K. The shock-loading data were analyzed in terms of dislocation models which previously have been applied in describing the decay of the dynamic yield point at constant temperature with increasing propagation distance. None of the models provided an adequate fit to both the new data involving the effect of temperature on the dynamic yield stress at a constant propagation distance and previous data of Taylor and Rice on the effect of propagation distance on the dynamic yield stress at a constant temperature. From the failure of these various models to correctly describe the dynamic yielding of iron, it would seem that some of the assumptions utilized in applying the dislocation theories to the description of dynamic yielding under shock loading need further study. Of particular importance are the assumptions that dislocation acceleration and relativistic effects are negligible. Twinning was observed to occur at shock-loading stress levels slightly above the dynamic yield stress but did not occur at lower stresses. It was concluded that the motion of twinning dislocations is the mechanism of dynamic yielding of iron.