On the Torsional Creep Characters of Torsionally Prestrained Steel

Behaviors of Over-strained Metals

Abstract

In order to investigate the effect of prestrain, especially large amount of prestrain on the creep phenomena the authors performed the torsional creep test, in the direction of working, of the small round solid bar of mild steel previously twisted at room temperature.
1. The transition phenomenon in the creep rate (measured after one hour) vs. prestrain relation was observed, and the creep rate of material above the transition prestrain range is about 2 times larger than below the range (see Figs. 4-6).
2. The ratio of two creep rates above and below the transition range is approximately constant independent of stress and temperature. And on these creep rates the general equation
r= KPⁿ,
P=τ⋅e-Q/RT
where K and n are the constants, γ is the creep rate, τ is the shear stress, T is the absolute temperature, Q is the activation energy and R is the gas constant, is satisfied in this experiment.
3. It was found that both the decreasing point of the strain hardening rate in a torsional test and the ultimate point in a tensile test correspond to the begining point of the transition.