Seam welds in technical plants are often subjected to thermal loading conditions, e.g. thermal transients in the low cycle fatigue range. Corresponding to this fact, the failure of seam welds under Low Cycle Fatigue (LCF) must be predictable, to avoid cases of damage. A commonly used material for pipelines of power plants is the austenitic stainless steel of type X6CrNiNb18-10 (1.4550). Based on experimental investigations on machined and as-welded seam welds by Lang and co-authors, a parametric model was developed to consider geometrical and metallurgical notches in finite element analyses. Through additional experimental investigations (including several specimen shapes and temperature levels on 20°C, 200°C and 350°C) by Langschwager, Bosch and co-authors the assumptions and model parameters have been validated. The consideration of the microstructural support effect at the sharp notch on as-welded seam welds was realized by the reduction of local strain amplitudes by averaging stresses over a structural support length. The additional surface influence was determined on specimens with machined seam welds and is describable by a factor on local strain amplitudes. Using a local approach by using measured geometry-parameters, the selected distribution of material parameters and in consideration of the additional influences, acceptable accordance between experimentally and numerically calculated fatigue lives was obtained. Moreover, the results of the experimental investigations were used to compare them to allowable loading cycles, calculated by the simplified elastic plastic fatigue analysis (Ke-procedure) of the German KTA 3201.2 standard (similar to the procedure in ASME Section III and French RCC-M Code). The results of this comparison were not satisfactory, because the allowable number of loading cycles was mostly higher than the experimental number of loading cycles. The simplified elastic plastic fatigue analysis was modified in order to correct the results of the numerical lifetime estimation.

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