Session: MF-12-01 Materials and Fabrication: General Topics I

Paper Number: 62005

Start Time: Wednesday, July 14, 2021, 05:00 PM

62005 - Fatigue Behavior and Lifetime Assessment of the Austenitic Stainless Steel AISI 347 and its Associated Filler Metal ER 347 Under Low-, High- and Very High Cycle Fatigue Loadings 

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability as well as Long Term Operation (LTO) and Load Follow operational modes. Reactor internals are subjected to thermo-mechanical fatigue induced by operational temperature transients (high amplitudes, low frequencies) and to superposed flow induced vibrations (low amplitudes, high and very high frequencies). The resulting complex loading collectives induce  low cycle (LCF), high cycle (HCF) and even very high cycle (VHCF) fatigue and their interaction. The existing methodological gaps within the current fatigue assessment approach with regard to these complex fatigue loading situations are to be closed.

Design code fatigue analyses are based on defined loads and frequencies of occurrence (specified or measured). High cycle and very high cycle fatigue loadings are not explicitly considered except for endurance limit studies of reactor internals. Furthermore, design fatigue curves in the applicable 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 generally highly insufficient. The cyclic deformation behavior of the material under discussion (austenitic stainless steel 1.4550) is different depending on the fatigue regime respectively 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, TU Kaiserslautern and Framatome GmbH addresses the existing gaps of knowledge discussed above and has recently been finished. Results of the following topics will be discussed in detail in the paper:

· Basic characterization of the HCF and VHCF fatigue behavior at relevant operational temperatures in air at 10⁶ – 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 non-linear damage accumulation effects including sequence effecs.

Keywords: Reactor internals, austenitic stainless steel 1.4550, welds, combined low cycle (LCF) – high cycle (HCF) and very high cycle (VHCF) loading and related non-linear damage accumulation, material characterization, cyclic deformation behavior, fatigue assessment methodology for reactor internals, transient endurance limit

Presenting Author: Tim Schopf Materials Testing Institute University of Stuttgart

Authors:

Tim Schopf Materials Testing Institute University of Stuttgart
Stefan Weihe Materials Testing Institute University of Stuttgart
Tobias Daniel Institute of Materials Science and Engineering Technical University of Kaiserslautern
Marek Smaga Institute of Materials Science and Engineering Technical University of Kaiserslautern
Tilmann Beck Institute of Materials Science and Engineering Technical University of Kaiserslautern
Jürgen Rudolph Framatome GmbH