Influence of Strain-Rate History and Temperature on the Shear Strength of Copper, Titanium and Mild Steel.

摘要

Experiments have been carried out to determine the strain-rate sensitivity of the shear flow stress of OFHC copper, commercially-pure titanium and mild steel over the temperature range -150 to 400 C. All the tests were performed on thin-walled tubular specimens of short gauge length, mounted in a torsional split Hopkinson-bar apparatus adapted to permit quasi-static straining as well as dynamic straining. For each material, the constant-rate behaviour was first measured at nominal strain rates of ten to the minus 3rd power and ten to the 3rd power s to the minus 1st power, for six different temperatures. Tests were then carried out in which the strain rate was suddenly increased from ten to the minus 3rd power to ten to the 3rd power s to the minus 1st power at various values of plastic strain. Comparison of the results obtained in the two series of tests shows that the response of all three materials depends on the strain-rate history, so that a 'mechanical equation of state', relating stress to strain, strain rate and temperature is not valid; however, the influence of strain-rate history is less marked for titanium than for copper or mild steel. The results are discussed in terms of plastic flow mechanisms involved, thermal activation, changes in microstructure, strain ageing and dynamic recovery. Possible forms of macroscopic constitutive relation are also discussed.