Alexander Bosch

Numerical Investigations of Seam Welds Under Low Cycle Fatigue: Proposal for Lifetime Estimation and Recommendations for Design With Commonly Used Guidelines

High stress induced by thermal loadings is a relatively rare but still present and decisive load case for seam welds in technical plants. Especially in nuclear power plants, regular maintenance and replacing of components must be done, before failure can occur. 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 addi- tional 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 (K_e -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 life estimation.

Fatigue Behavior of Butt Weld Seams: Experimental Investigation and Numerical Simulation

Austenitic stainless steel of type X6CrNiNb18-10 (1.4550) is a widely used material in piping and components of nuclear power plants. The fatigue behavior of these components is often operationally determined by thermomechanical strains and corresponding stresses. Welded structures lead to complex stresses in the component and potential fatigue lifetime reductions. Various geometrical and microstructural inhomogeneities in welded structures represent the main factors of influence. Nevertheless, clear identification and quantification of various factors of influence are issues still to be resolved.Within the framework of an ongoing research project, the experimental investigation comprises uniaxial and biaxial fatigue experiments on welded joints which cover temperatures from 25°C to 350°C. Furthermore, a key issue deals with the thermomechanical fatigue behavior of machined and unmachined butt weld seams. A special focus is set on typical low cycle fatigue (LCF) tests in order to explain the behavior of the base material and the weld material to identify the influence of microstructural inhomogeneities. In addition, specimens manufactured directly from the pipe components are tested to examine the influence of the butt weld seam geometry. For a better understanding of the local strain effects, optical strain field measurements (OSFM) are conducted and used to validate numerical simulation. The finite element method (FEM) is utilized to expand the parameter space and identify the main parameters.Experimental and numerical results show that fatigue failure occurs either in the base metal in the vicinity of the welded zone or in the top layer of the weld, depending on the loading conditions. This knowledge is used to develop an approach to fatigue lifetime estimation.Copyright

Schwingfestigkeit von thermo-mechanisch beanspruchten Stumpfschweißverbindungen austenitischer Werkstoffe

Kurzfassung Stumpfschweißverbindungen austenitischer Werkstoffe sind häufig Teil von Komponenten der Kraftwerkstechnik oder des chemischen Apparatebaus. In diesem Bereich unterliegen die Bauteile zyklischen mechanischen und thermischen Lasten. Vornehmlich bedingt durch hohe Temperaturschwingweiten, resultieren hohe elastisch-plastische Dehnungsschwingweiten. Die Bereiche Grundmaterial und Schweißnaht zeigen ein unterschiedliches plastisches Deformationsverhalten, die daraus resultierende metallurgische Kerbe kann versagensmaßgebend sein. Im Rahmen der begleitenden numerischen Untersuchungen wurde ein Modell entwickelt, um die metallurgische Kerbe abzubilden.