Browsing by Author "Smaga, Marek"

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Open Access
Fatigue behavior and lifetime assessment of an austenitic stainless steel in the VHCF regime at ambient and elevated temperatures
(2023) Schopf, Tim; Weihe, Stefan; Daniel, Tobias; Smaga, Marek; Beck, Tilmann
While the LCF behavior of austenitic steels used in nuclear power plants is already well investigated, the VHCF regime has not been characterized in detail so far. For this, fatigue tests on the metastable austenitic steel AISI 347/1.4550 were performed with a servo‐hydraulic testing system at test frequencies up to 980 Hz and with an ultrasonic fatigue testing system at a test frequency of 20,000 Hz. To compare these test results to the ASME standard fatigue curve (total strain amplitude vs. load cycles to failure), a fictitious‐elastic and an elastically plastic assessment method was used. The elaborated elastic-plastic assessment method generates good results, while a purely elastic assessment in the VHCF regime, commonly used in literature, leads to significantly nonconservative results. Moreover, phase transformation from metastable austenite into stable α′‐martensite can take place, and no specimen failure occurs in the VHCF regime. Consequently, for this material, a real endurance limit exists.
Open Access
Investigations of metallurgical differences in AISI 347 and their influence on deformation and transformation behaviour and resulting fatigue life
(2024) Veile, Georg; Regitz, Elen; Smaga, Marek; Weihe, Stefan; Beck, Tillmann
Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α’-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α’-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component’s metastability is required. In less metastable batches and for lower stress levels, α’-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α’-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.