The fatigue behavior of the austenitic stainless steel X6CrNiNb18-10 (1.4550, AISI 347) is investigated. Experimental data are generated at room temperature, at elevated temperatures and further under thermo-mechanical conditions. Research is focused on generating data for parameter identification, especially for the improvement of material models. The influence of temperature on the stress-strain behavior and the fatigue life is presented.Finite element simulations are used to describe fatigue crack growth under thermo-cyclic loading conditions. Using the example of a thick-walled tube, the essential parameter for the crack growth rate, the effective cyclic J-integral, is determined for eight temperature transients (defined by fluid temperature, heat transfer coefficient and inner pressure). An approximation of the effective cyclic J-integral allows modeling without consideration of the crack geometry in the model. Keywords: Thermo-mechanical fatigue / biaxial behavior / effective cyclic J-integral / AISI 347 / fatigue crack growth Schlüsselwörter: Thermomechanische Ermüdung / biaxiales Verhalten / effektives zyklisches J-Integral / X6CrNiNb18-10 / Ermüdungsrisswachstum
Elastic-plastic strains at points relevant for structural failures are usually approximated using formulas based on the stresses determined by elasticity theory. For this purpose, the Neuber approximation is a common method to estimate the local elastic-plastic strains in the notch root, although this is currently only approved for homogeneous components. For surfacehardened notched components, these approximation formulas need to be modified to cover two potential failure points: The notch root as well as the interface between the stronger surface layer and the weaker core material. In the following, a multi-step algorithm is shown that allows the estimation of elastic-plastic local strains at these two points, based on a single elasticitytheoretical solution. A comparison of the approximated values with those from finite element analyses (FEA) reveals that this results in only minor inaccuracies, while the usability is remarkable.
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