Browsing by Author "Schopf, Tim"

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Open Access
Experimentelle und numerische Untersuchungen zum Ermüdungsverhalten austenitischer Werkstoffe im HCF- und VHCF-Bereich
(Stuttgart : Materialprüfungsanstalt (MPA), Universität Stuttgart, 2024) Schopf, Tim; Weihe, Stefan (Prof. Dr.-Ing.)
Ziel dieser Arbeit ist die Formulierung einer Ermüdungsbewertungsmethodik zur Berücksichtigung von Beanspruchungen im Bereich hoher Lastwechselzahlen und deren Überlagerung mit Beanspruchungen im Zeitfestigkeitsbereich für austenitische Werkstoffe und deren Schweißverbindungen. Anhand umfangreicher experimenteller Untersuchungen konnten das komplexe zyklische Werkstoffverhalten sowie signifikante plastische Dehnungsanteile bei sehr geringen Beanspruchungen identifiziert werden. Diese bilden die Grundlage zur Berechnung der Ermüdungsbeanspruchung und der Verbesserung der Bewertungsmethodik. Zusätzlich leisten Erkenntnisse aus Ermüdungsversuchen mit betriebsrelevanten Belastungskollektiven einen wesentlichen Beitrag zu einer konsolidierten Abdeckung von nichtlinearen Schadensakkumulationseffekten und der transienten Dauerfestigkeit. Ein weiterer Baustein stellt die elastisch-plastische Bewertung der experimentellen Ergebnisse aus der Literatur dar. Hiermit können validierte Ermüdungskurven mit einem Gültigkeitsbereich bis in den VHCF-Bereich, gestützt auf einer experimentellen Datenbasis, bereitgestellt werden. In numerischen Untersuchungen wurde gezeigt, dass mit Hilfe der abgeleiteten Materialmodelle und eines nichtlinear isotrop-kinematisch kombinierten Verfestigungsansatzes das transiente Verhalten der Spannungs-Dehnungs-Tensoren aus den Experimenten abgebildet werden kann. Somit lassen sich die notwendigen zyklischen Beanspruchungsgrößen zur Berechnung der Ermüdungsschädigung aus numerischen Analysen ableiten und zur Verfügung stellen. Des Weiteren kann durch die Modifikation von Schädigungsparametern anhand mehrparametrischer Formulierungen der zyklischen Spannungs-Dehnungs-Kurve und einer Anpassung zur verbesserten Beschreibung der Ermüdungskurven im VHCF-Bereich eine höhere Genauigkeit in der Lebensdauerberechnung erreicht werden. Die Ergebnisse der entwickelten Ermüdungsbewertungsmethodik zeigen, dass die nichtlinearen Schädigungseffekte sowie das transiente Dauerfestigkeitsverhalten von kombinierten Beanspruchungen aus LCF-/HCF- und VHCF-Bereich, respektive Beanspruchungen unterhalb der Dauerfestigkeit, konsistent abgedeckt werden können. Die Lebensdauer der Versuche wird innerhalb eines engen Streubereichs vorhergesagt.
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
Microstructure-based lifetime assessment of austenitic steel AISI 347 in view of fatigue, environmental conditions and NDT
(2021) Acosta, Ruth; Heckmann, Klaus; Sievers, Jürgen; Schopf, Tim; Bill, Tobias; Starke, Peter; Donnerbauer, Kai; Lücker, Lukas; Walther, Frank; Boller, Christian
The assessment of metallic materials used in power plants’ piping represents a big challenge due to the thermal transients and the environmental conditions to which they are exposed. At present, a lack of information related to degradation mechanisms in structures and materials is covered by safety factors in its design, and in some cases, the replacement of components is prescribed after a determined period of time without knowledge of the true degree of degradation. In the collaborative project “Microstructure-based assessment of maximum service life of nuclear materials and components exposed to corrosion and fatigue (MibaLeb)”, a methodology for the assessment of materials’ degradation is being developed, which combines the use of NDT techniques for materials characterization, an optimized fatigue lifetime analysis using short time evaluation procedures (STEPs) and numerical simulations. In this investigation, the AISI 347 (X6CrNiNb18-10) is being analyzed at different conditions in order to validate the methodology. Besides microstructural analysis, tensile and fatigue tests, all to characterize the material, a pressurized hot water pipe exposed to a series of flow conditions will be evaluated in terms of full-scale testing as well as prognostic evaluation, where the latter will be based on the materials’ data generated, which should prognose changes in the material’s condition, specifically in a pre-cracked stage. This paper provides an overview of the program, while the more material’s related aspects are presented in the subsequent paper.
Open Access
A short-time approach for fatigue life evaluation of AISI 347 steel for nuclear power energy applications
(2021) Bill, Tobias; Acosta, Ruth; Boller, Christian; Donnerbauer, Kai; Lücker, Lukas; Walther, Frank; Heckmann, Klaus; Sievers, Jürgen; Schopf, Tim; Weihe, Stefan; Starke, Peter
AISI 347 austenitic steel is, as an example, used in nuclear energy piping systems. Piping filled with superheated steam or cooled water is particularly exposed to high stresses, whereupon local material properties in the pipes can change significantly, especially in the case of additional corrosive influences, leading to aging of the material. In the absence of appropriate information, such local material property variations are currently covered rather blanketly by safety factors set during the design of those components. An increase in qualified information could improve the assessment of the condition of such aged components. As part of the collaborative project “Microstructure-based assessment of the maximum service life of core materials and components subjected to corrosion and fatigue (MiBaLeB)”, the short-time procedure, StrainLife, was developed and validated by several fatigue tests. With this procedure, a complete S-N curve of a material can be determined on the basis of three fatigue tests only, which reduces the effort compared to a conventional approach significantly and is thus ideal for assessing the condition of aged material, where the material is often rare, and a cost-effective answer is often very needed. The procedure described is not just limited to traditional parameters, such as stress and strain, considered in destructive testing but rather extends into parameters derived from non-destructive testing, which may allow further insight into what may be happening within a material’s microstructure. To evaluate the non-destructive quantities measured within the StrainLife procedure and to correlate them with the aging process in a material, several fatigue tests were performed on unnotched and notched specimens under cyclic loading at room and elevated temperatures, as well as under various media conditions, such as distilled water and reactor pressure vessel boiling water (BWR) conditions.