Creep-Fatigue Behaviour under Service-Type Strain Cycling

Abstract

High temperature components are normally operating under variable loading conditions [1,2,3]. Load changes lead to temperature transients and thus at the heated surface of the components strain cycling can be critical. Additionally to fatigue, creep occurs due to relaxation in hold phases. To simulate this type of loading a service-type strain cycle has been developed earlier [1,4] (Fig. 1). It contains a compressive strain hold phase 1 simulating start-up condition, a zero strain hold phase 2 characterizing constant load and a tensile strain hold phase 3 referring to shut-down. Anisothermal strain cycling is of special interest because it is nearer to practice than isothermal testing. With regard to the design life of power plants up to 200 000 h long-term testing is important. As demonstrated earlier [2,3,5] creep-fatigue life under such conditions can be analysed with the aid of the generalized damage accumulation rule if internal stress and a damage interaction concept are considered. An extension of this concept to longer test durations and from single-stage to three-stage strain cycling is of interest.