Strength of Metals and Alloys at High Strains and Strain Rates

Abstract

The effects of strain rate and strain-rate history on the strength of metals at high strains have been investigated in two series of tests at room temperature using the split Hopkinson-bar method to obtain shear strain rates up to 3000 s-1. In the first series, various materials were subjected to nearly constant low and high strain rates in torsion, and apparent rate sensitivities were determined by comparison of the flow stresses at a given strain. The flow stresses were found to increase with rate even at large shear strains (~0.5), the effect of adiabatic heating being relatively small. In the second series, strain-rate changes of about six orders of magnitude were rapidly imposed at shear strains up to 0.6. The results indicate that the flow stress depends on the strain-rate history, and may be greater or less than that obtained at the same strain and strain rate in a constant-rate test. The differences in the observed behavior of copper, titanium, and mild steel are discussed, and it is shown that the data for copper are consistent with a proposed form of constitutive relation involving a functional of the strain rate.