Investigations on Multi-Stage Tests and Transient Endurance Limit Behavior Under Low-, High- and Very High Cycle Fatigue Loads

The fatigue analyses included in nuclear rules of KTA [1]–[2] and ASME [3] are based on defined loads (specified or measured loads and frequencies). It is assumed that highly cyclic loadings or resonance vibrations are avoided by appropriate design. Often these loads are recorded by measurements during commissioning or during operation. In particular, in pressure vessel and reactor internals such vibrational excitations cannot be excluded, so that fatigue loadings in the HCF regime and even in the VHCF regime can occur. Since these kinds of loading situations also appear in combination with fatigue loadings in the LCF regime, load collectives are to be considered, as they are not explicitly taken into account in the current analysis of the nuclear regulations. Furthermore, no generally validated method, especially a consolidated damage accumulation model is available. Furthermore, design fatigue curves for austenitic steels in the applicable international design codes were extended by extrapolation from originally 10⁶ up to 10¹¹ load cycles [1]–[3]. However, the existing database for load cycles equal to or above 10⁷ is still insufficient. Therefore, international efforts are currently ongoing in order to expand the database through international co-operations and compile a safe high cycle fatigue (HCF) database [4]. This is particularly important in combination with the influence of the cooling medium and its consideration according to established international standards as the database of the Argonne National Laboratory ANL [5] for fatigue behavior under medium conditions. For the range from HCF to VHCF and for their combination with LCF loads (collective effect) and the currently discussed limit values above which the cooling medium has an effective influence on the fatigue strength are not sufficiently consolidated. These aspects gain in importance particularly in the long-term operation context. A recently finished cooperative research project aims at contributing to closing these mentioned gaps by generation of a data and assessment basis for the fatigue behavior of welded austenitic stainless steels at high numbers of load cycles [6].

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

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

• 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