Torsional strength and ductility of mild steel in quasi-static, dynamic and strain-rate-jump loading at different temperatures

Abstract

Tubular mild-steel specimens have been subjected to twist at constant strain rates of 10 −3 and 10 3 s −1, over the range of temperatures from −150 to 400°C, using a modified torsional split Hopkinson-bar. Additional specimens were prestrained quasi-statically to large strains before continuing the deformation to fracture at the dynamic strain rate. Under constant dynamic loading, thermal softening leads to an instability which affects the shape of the hardening curve: adiabatic stress-strain relations were derived in differential form to describe this behavior. It is also shown that a mechanical equation of state is not applicable to mild steel since the strain-rate history influences both the strength and the ductility at all temperatures. Upon changing the strain rate from quasi-static to dynamic, an elastic jump in stress takes the flow stress well above the corresponding level at the same strain, had the test been conducted exclusively at the dynamic rate. This is followed immediately by softening at sub-ambient temperatures and moderate hardening at room temperature and higher temperatures. Tentative, but inconclusive, physical explanations of this behaviour are presented. At low temperatures, quasi-static ductility is found to be greater than dynamic ductility, whilst at 200 and 400°C the opposite is observed because of quasi-static strain ageing. At any given temperature, various amounts of quasi-static prestraining were found to reduce the retained ductility at the dynamic strain rate. Nevertheless, the toughness appeared to be relatively unaffected.