The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability. Those components are subjected to low cycle (LCF), high cycle (HCF), very high cycle fatigue (VHCF) and their combination in terms of complex loading collectives and nonlinear fatigue damage accumulation. Reactor internals, for example, are subjected to thermo-mechanical fatigue induced by operational temperature transients such as stratification loads and to flow induced vibrations. Obviously, methodological gaps within the current fatigue assessment approach, i.e. the assessment of the VHCF behavior, the treatment of the fatigue damage accumulation in combination with the transient endurance limit are to be bridged. Additionally, environmentally assisted fatigue is to be considered properly.

Design fatigue curves in international design codes were extended from originally 106 up to 1011 load cycles. Nevertheless, the existing data base for load cycles above ≈ 107 is still insufficient. The cyclic deformation behavior of the material under discussion (AISI 347 within the recent project and AISI 304 within a follow-up project) is different depending on the fatigue regime respectively to the applied load or deformation amplitude. While the LCF behavior is already well investigated and the basic behavior in the HCF regime is fairly well known the VHCF cyclic deformation behavior has not been characterized so far. As a consequence, the real damage accumulation of variable amplitude combinations consisting of LCF- and HCF/VHCF loads is still widely unknown.

A cooperative R&D project of MPA Stuttgart, WKK University of Kaiserslautern and Framatome GmbH addresses the existing gaps of knowledge discussed above and has recently been finished.

The following topics will be discussed in detail in the paper:

• Basic characterization of the HCF and VHCF behavior at relevant operational temperatures in air at 106 – 2·109 load cycles (base material and welded material)

• Fatigue behavior at variable amplitude loading (combination of LCF / HCF and LCF / VHCF)

• Numerical simulations and fatigue assessment of VHCF-regime loadings

• Development of a fatigue assessment methodology under consideration of the transient endurance limit and damage accumulation effects including assessment and adaptation of appropriate fatigue damage parameters

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