Strain Rate Sensitivity of Flow Stress at Very High Rates of Strain

Summary

In order to clarify the mechanism of the steep rise in the flow stress, widely seen in metallic materials at strain rates above about 5×10³/sec, two strain rate change tests were conducted. The strain rate reduction test was made for polycrystalline aluminium, copper, iron and niobium at strain rates up to about 2×10⁴/sec. The measurement of the attenuation of the ultrasonic pulse superimposed upon the dynamic plastic deformation, a kind of strain rate change test using a high frequency perturbation in the strain rate, was made for polycrystalline aluminium and copper at strain rates up to 1×10⁴/sec. The results of the above two experiments show that the instantaneous strain rate plays a more important role than the strain rate history in the steep rise in the flow stress. By comparing the results of the above experiments with the results of theoretical analyses made for fcc and bcc metals on the basis of the dislocation kinetics, it is shown that, in fcc metals, the steep rise in the flow stress is caused by the phonon drag upon dislocations while, in bcc metals, the rise is a little gentler and governed by the stress dependency of the activation energy for the kink pair generation on screw dislocations.