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Conference Article: Development and validation of an improved creep-fatigue interaction rule for lifetime determination of high temperature components

Lehmann, H.J.; Becker, M.; Luedeke, M. (Rheinisch-Westfaelischer Technischer Ueberwachungs-Verein e.V., Essen (Germany, F.R.))

Abstract

For design purposes and for lifetime assessments of components operating at elevated temperatures the creep-fatigue interaction rule is widely used. Although this rule - because of its shortcomings - does not provide sufficient results, the world-wide use is due to the simplicity of its application in connection with inelastic lifetime analyses. In view of this, the creep-fatigue interaction rule was improved. A significant shortcoming is the missing strain rate dependence on creep damage during a relaxation phase. The influence of the strain rate on the creep damage in a material is caused by different deformation modes. At high strain rates the motion and formation of dislocation dominates, whereas grain boundary sliding increases with decreasing strain rate. These basic deformation modes cause different creep damage. Since the strain rate during a relaxation phase correlates with holdtime, an interaction term c(t) was introduced, describing the fractional change of creep damage in dependence of timedependent damage due to fatigue, with time as variable. The interaction term c(t) represents a weight function for the differential creep damage dt/t and varies between zero and unity, nonlinear with respect to holdtime. The effect of the interaction term c(t) consist in a lower accumulated creep damage in comparison with the unmodified rule in the region of fast strain rates. This fact is an agreement with the experience, that dislocation controlled deformation is less damaging than grain boundary sliding. With increasing holdtime corresponding with decreasing interaction between creep and fatigue the term c(t) tends to unity. In case of pure creep the improved rule turns over to the modified one. The improved rule has been validated by evaluation of uniaxial low-cycle fatigue holdtime experiments. The following materials have been investigated: Inconel 617 (NiCr22Co12Mo). AISI 304ss (X6CrNi1811). 12%Cr-Mo-V-steel (X20CrMoV121). A comparison with the results of the unmodified rule is given.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
High temperature metallic materials for gas-cooled reactors. Proceedings of a specialists meeting held in Cracow, 20-23 June 1988
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--18, pp:190-194