Deformation and life assessment of high temperature materials under creep fatigue loading

Abstract

The knowledge of mechanical long term behaviour under static and cyclic loading for high temperature components requires methodologies for life assessment in order to employ the full potential of materials. A phenomenological life time prediction concept which was developed for multi-stage creep fatigue loading demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a creep fatigue interaction concept which was also developed covers a wide range of creep dominant loading as well as fatigue dominant loading.Service-type experiments conducted at different strain rates and hold times for verification purposes demonstrate the acceptability of life prediction method for variation of conventional 1 %Cr-steels as well as modern high chromium 9-10 %Cr-steels.Generally, the service life of components is influenced by multiaxial behaviour. Multi-axial experiments with e.g. notched specimens and with cruciform specimens accompanied by advanced methods for calculation of stress strain path and life time prediction stress conditions are of future interest.

The influence of temperature transients on the lifetime of modern high-chromium rotor steel under service-type loading

“…In the case of 2%Cr steel of type 23CrMoNiWV8-8, the life ratio N i TMF /N i iso is observed to be close to 1 [20]. Earlier investigations of a low-chromium steel 1%Cr of type 28CrMoNiV4-9 exhibit values of about 1.2 [10]. Therefore, in terms of 10%Cr-steel (type: X12CrMoWVNbN10-1-1) and 12%Cr-steel (type: X22CrMoWV12-1) [20], the data on isothermal tests for a component may constitute an unsafe approach.…”

“…With the requirements of frequent start-up und shut-down processes of power plants due to the flexibility of electrical power demand, in addition to creep and creep fatigue endurance, thermal mechanical fatigue cracking behavior has increasingly become an area of great interest (e.g. [6][7][8][9][10][11][12][13][14]). These investigations usually describe TMF life behavior independently of microstructural damage mechanisms.…”

Crack initiation behavior of notched specimens on heat resistant steel under service type loading at high temperature

“…The fatigue damage D f is calculated by a reference number of cycles to crack initiation Nio, which is determined under consideration of creep fatigue interaction and mean stress effect. The creep fatigue interaction was developed in a previous paper according to metallographic damage [8]. The material-specific mean creep fatigue damage of 0.68 for the material X12CrMoWVNbN10-1-1 determined by uniaxial service-type tests [8] was used as a critical damage value Dcrit in this paper.…”

“…The creep fatigue interaction was developed in a previous paper according to metallographic damage [8]. The material-specific mean creep fatigue damage of 0.68 for the material X12CrMoWVNbN10-1-1 determined by uniaxial service-type tests [8] was used as a critical damage value Dcrit in this paper. Phenomenological life time calculation requires scalar stress and strain values.…”

“…10 observed, a significant drifting of the mean axial deformation is observed. Therefore, the Smith-Waston-Topper parameter Pswt [8] is modified as follows:…”