Multiaxial low cycle fatigue tests with degradation were performed on duplex stainless steels(CF8M). The fatigue properties of CF8M to predict multiaxial fatigue life are obtained from the pure axial and torsional low cycle fatigue. CF8M is used in pipes and valves in a nuclear reactor coolant system. The specimens to be used at the experiment are virgin and degraded of CF8M. The degraded material was aged at 430℃ for 3600 hrs. In the present study, von Mises equivalent strain and a critical plane method proposed by Fatemi-Socie(FS) and Smith-Watson-Topper(SWT) are used to calculate fatigue damage. The predicted result by von Mises equivalent strain and FS parameter show good agreement with experimental results. In conclusion, fatigue life by material degradation is decreased and multiaxial fatigue life prediction which was used FS parameter is not conservative but the best result. Materials and Test ProcedureSpecimen. Table 1 shows the chemical composition of the CF8M used in this study. The study was performed on the virgin material and the degraded material produced by the thermal aging for 3600 hours at 430℃ to evaluate the effect of degradation to the axial-torsional fatigue life [8,9,10].The profile of the used specimen was a thin walled tubular specimen as shown in Fig. 1. The inside of the specimen was finished with electrolytic polishing to prevent the crack from initiation
A combined axial-torsional low cycle fatigue test was carried out to predict the fatigue life under in-phase and out-out-phase loading conditions for CF8M cast stainless steels. The Fatemi-Socie (FS) parameter which is based on the critical plane approach is not only one of the many methods but also the best method that can predict the fatigue life under a biaxial loading condition. But the result showed that, a biaxial fatigue life prediction by using the FS parameter with several different parameters for the CF8M cast stainless steels is not conservative enough but at the same time it was the best result so far. So in this present research, we proposed a new fatigue life prediction parameter (Park-Kwon parameter) by considering effective the shear stress instead of the FS parameter which considers the maximum normal stress acting on the maximum shear strain and its effectiveness was verified.
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