An investigation of the shape of the titanium stress strain curves after a strain rate change

Abstract

Strain rate changes during a tensile test of commercial purity polycrystalline α-titanium do not have the same effect on the shape of the stress-strain diagram at all temperatures. Only within rather limited temperature ranges does the flow stress change smoothly and continuously to a value characteristic of the new deformation rate. At intermediate temperatures, a rate increase normally produces a small transient flow stress maximum similar in appearance to a yield point, while equivalent minima may be observed on a decrease in rate. These effects have sometimes been ascribed to testing machine transients. However, simultaneous autographic measurements of load against time, and specimen elongation against time, made during rate changes at slow strain rates, indicate the observed maxima and minima are probably characteristic of the material. The transients are analyzed using a modified Johnston-Gilman approach assuming that the stress acting on the dislocations is the effective stress and not the total stress.