Influence of Hold-Time and Strain Rate on the LCF Behaviour of Alloy 617 at 950 °C

Synopsis

In the present study the influence of different hold times (tension, compression and symetrical hold times), reduced strain rates, and a combination of both was examined for alloy 617 at 950 °C in a helium atmosphere. The highest fatigue life reduction was observed for tension hold times, followed by compression hold times, and to a much smaller extent by smmetrical hold times and reduced strain rates. The saturation stress during fatigue loading was found to depend on the strain rate by a logarithmic law, indicating that dynamic recovery determines the hardening behaviour at 950 °C. An evaluation of the stress relaxation behaviour and of the failure modes at the different types of loading enabled a separation of the damage into a plastic and creep part depending on the actual deformation rate. Based hereupon a consistent fatigue life curve for different loading rates and a modified evaluation of the linear creep-fatigue interaction is presented.