Bewertung der Schwingfestigkeit scharf gekerbter Bauteile unter hoher Mittelspannung mit dem Örtlichen Konzept

In this paper a concept is introduced, which can be used for the assessment of sharply notched components under high mean stress. The concept is based on the local strain approach. The extension of the concept is basically the consideration of the stress relaxation in the notch root. The concept is explained on a screw nut connection which is a very good example for a cyclically loaded component. The transition to the nominal stress approach, which is generally used for screw‐nut connections, is done by using the example of a combined bending and axial loaded screw‐nut connection.     

Anwendung des Örtlichen Konzepts in der Turbinenschaufelauslegung

Die Anforderungen an große Kraftwerksturbinen (Gas‐ und Dampfturbinen) sind im Zusammenhang mit der Energiewende in den letzten Jahren signifikant gestiegen. Durch die wachsende Einspeisung von Strom aus erneuerbaren Energien wie Wind und Sonne gewinnt die Erhöhung der Flexibilität im Kraftwerksbetrieb immer mehr an Bedeutung. Dafür müssen zuverlässige und robuste Turbinen entwickelt werden. Turbinenschaufeln sind eine der Schlüsselkomponenten einer Turbine und ein entscheidender Erfolgsfaktor, um die aktuellen und zukünftigen Markt‐ und Kundenanforderungen zu erfüllen und die damit verbundenen Herausforderungen zu meistern. Die Auslegung von Turbinenschaufeln ist komplex. Turbinenschaufeln sind im Betrieb häufig extremen, hohen sowie komplexen mechanischen und thermischen Belastungen ausgesetzt. Diese mechanischen und thermischen Betriebsbelastungen führen zu hohen Bauteilbeanspruchungen und häufig zu einem Ermüdungs‐ und/oder Kriechlebensdauerverbrauch. Hersteller großer Kraftwerksturbinen können ihre Turbinen und deren Komponenten in den allermeisten Fällen nicht im Vorfeld auf Prüfständen testen. Außerdem können Fehler in der Auslegung der Komponenten zu deren Versagen im Betrieb und damit zumindest zu erheblichen wirtschaftlichen Schäden führen. Aus diesen Gründen und vor dem Hintergrund der zunehmend flexibleren Fahrweise von Kraftwerken und der damit verbundenen höheren Anzahl an Lastwechseln sind für die Entwicklung und Auslegung von Turbinenschaufeln sowie deren Lebensdauervorhersage geeignete, zuverlässige und abgesicherte Berechnungs‐ und Bewertungskonzepte sowie Regeln unerlässlich, um den sicheren Betrieb der Turbinenschaufeln zu ermöglichen. Dies gilt insbesondere für die genaue Beschreibung des Ermüdungsverhaltens unter Betriebsbelastung. Der vorliegende Artikel beschäftigt sich mit den hier verwendeten Konzepten zur Lebensdauervorhersage und zum Betriebsfestigkeitsnachweis von Gas‐ und Dampfturbinenschaufeln.     

Berechnung von Anrisslebensdauern auf Basis des Örtlichen Konzepts

Die Berechnung von Anrisslebensdauern auf Basis des Örtlichen Konzepts ist aktueller Stand der Technik. Im Rahmen des AiF‐Projektes Nr. 17.612 N/1 ?Richtlinie nichtlinear“ wurden Lebensdauerberechnungen für einstufige und Betriebslastfolgen für die Werkstoffgruppen Aluminium und Stahl durchgeführt und analysiert. Die dabei verwendeten Algorithmen und ausgewählte Ergebnisse sollen innerhalb dieses Aufsatzes präsentiert und diskutiert werden.     

Lebensdauervorhersage für Getriebegehäuse aus Aluminiumdruckguss mit dem Örtlichen Konzept

Pressure cast transmission housings are structural components of complex shape, which have to fulfil, amongst others, the requirements of structural durability, saving as much weight as possible. The numerical proof of structural durability is carried out using the local strain approach. One challenge is to consider the inhomogenious material properties, as a consequence of the casting process. It shows, that local porosity is the determining factor for the components fatigue life. Coming from the experiences made using the basic version of the local strain approach the methods used to successfully adjust the procedure for calculating pressure cast components under constant and variable amplitudes are described.     

Vergleichende Untersuchungen zur Anrisslebensdauervorhersage gekerbter Proben mit dem Örtlichen Konzept und dem Nennspannungskonzept am Beispiel des Stahls Cm15

Comparative Investigations on Service Life Assessment of Notched Specimens Based on the Local Strain and the Nominal Stress Approach to Fatigue for a Steel SAE 1017 It is still unclear whether the strain based approach to fatigue or the stress based approach to fatigue should be preferred for service life assessment of notched components. In order to clarify the similarities and differences between these concepts stress and strain controlled fatigue experiments have been performed with notched specimens. It has been found, that stress and strain controlled fatigue testing results in the same number of cycles until failure. Essential for this correlation is that the cyclic stable strain amplitude at the notch root is taken for the entry into the strain‐life diagram in both cases. Starting from an elastic‐plastic analysis of the material behaviour at the notch root it is shown, how the strain‐life curve can be converted into a stress‐life curve. Based on that result service‐life is calculated from both approaches mentioned above. The calculation gives nearly the same service‐lives for both cases, but overestimates the measured data. It becomes obvious, that a S‐N curve determined under one‐level loading doesn’t provide a proper basis for service life assessment. While strain or stress‐life curves always contain crack initiation phase as well as crack propagation phase, the fatigue process under irregular loads is mainly governed by crack propagation. As a consequence, the damage per cycle is underestimated for loads near the fatigue limit, if Miner’s rule is used.     

Influence of porosity and environmental impact on fatigue life of magnesium alloys

Based on experimental investigations using specimens with different grades of porosity, a significant influence of the material condition of die‐cast magnesium alloys on the accuracy of fatigue‐life estimation according to the local‐strain approach is verified if the data‐base is not related to the microstructure of the component. In order to address this transferability issue, a porosity‐depending formulation is proposed. Based on a sensitivity analysis, a simplified, empirical engineering approach has been derived that describes the effect of the inherent porosity on fatigue strength via the fatigue‐strength exponent b. Furthermore, it is shown that the degradation of the fatigue‐behaviour in chloride‐containing media is not simply determined by the external corrosive attack, but also influenced by inner defects competing with the environmental impact.     

Nachhaltigkeitsaspekte unter der Lupe – Ökobilanzen im Gebäudevergleich

Checking out sustainability aspects – life cycle assessments in building comparisons The topic of life cycle assessments at building level plays an increasing role in environmental and socio‐economic terms. This article analyzes the aspect of climate protection in the context of sustainability assessments of buildings. A summary of five life cycle assessment studies accepted by experts (LCEE, RUB, KÖNIG, FIW, STEINBEIS) is given with reference to building‐related life cycle assessments. In particular, the assumptions and framework conditions made in these studies and their effects on the results are assessed. Similarities as well as essential differences of the individual studies are shown in form of a comparative analysis.     

Die Anrisslebensdauerabschätzung unter Berücksichtigung des statistischen Größeneinflusses am Beispiel der AlMgSi0,7‐Legierung

The fatigue lifetime estimation in consideration of the statistical size effect as example for the AlMgSi0,7 alloy The prediction of the crack initiation lifetime in consideration of the statistical and technological size effects will be introduced in course of an analytical model. To verify this method results from fatigue tests at constant and variable amplitude loads for unnotched and notched specimens of the aluminum alloy AA6009 will be considered.     

Modellierung der Stängelkristallitbildung beim Hartlöten von Kohlenstoffstählen mit Kupfer

Simulation of columnar crystallite formation in brazed seams of copper‐brazed carbon steels When brazing steels of different carbon content with copper filler metal, columnar crystallites form on the carbon‐rich iron surface if the width of the brazing gap is smaller than 100 μm. Braze seams with such microstructures were described as early as the 1950ies and it was found out, that the strength of such a joint is significant enhanced, if this crystallites penetrate the entire seam. Extensive experimental investigations in recent years confirm, that the final average length of the crystallite increases superproportionally with decreasing brazing gap width and is almost inversely proportional to the difference in carbon content of the joined steels. Although many attempts to explain this phenomenon are known from literature, the mechanism of columnar structure formation has not been clarified properly until now. The aim of the present work was to develop an appropriate physical model, that describes the growth of crystallites as a function of carbon content in the base materials, the initial brazing gap width and the applied process parameters (temperature, time). The model is an appropriate tool for a general choice and development of filler metal‐base material combinations forming columnar crystallites in the braze seam.     

Die Werkstoffbeanspruchung im Wälzkontakt bei hoher Flächenpressung,ermittelt nach Hertz und nach der EHD‐Strömungshypothese

Material stressing at rolling contact with high specific load determined by HERTZian‐ and elastohydrodynamic‐ (EHD) flow hypothesis For the calculation of the material stressing at rolling contact HERTZian distribution of the normal stresses and the tangential stresses within the contact area, which are proportional to the normal stresses, are assumed. This numerical explication is based upon the calculation of the contact fatigue endurance. Taking in consideration the real stress distribution of forces at EHD contact, it can be explained by the EHD flow hypothesis, that at special load constellations the materials stressing leads to higher values.     

Struktur der Randschicht nichtrostender Stähle nach Tieftemperatur‐Nitrierung – eine Mößbauerstudie

Case structure of stainless steel after low temperature nitriding – a Mössbauer study Due to the nitriding of stainless steel at temperatures between 300 °C and 400 °C cases of high hardness and nitrogen contents ranges between 8wt.% and 12 wt.% could be prepared. In the present work the nitriding was performed by gas and plasmanitriding. The phase generation was investigated by use of the Conversion‐Mössbauer‐specrtoscopy (CEMS and XCMS) and the X‐ray diffraction. The chemical composition was determined by GDOS and the hardness test using the Martens‐hardness under load. In spite of different principles of action no significant differences between the structure and properties of the gas and plasmanitrited samples could be observed within the bounds of the used test methods. Clear differences were found in the nitriding behaviour of different steels.     

Literature study on the rate and mechanism of carbonation of lime in mortars / Literaturstudie über Mechanismus und Grad der Karbonatisierung von Kalkhydrat im Mörtel

The hardening kinetics of a lime based mortar is based on the uptake of carbon dioxide from the ambient air. The presence of watervapour is required in order to enable the reaction between the CO₂ and the lime (calcium hydroxide). Via this reaction the hardening of air lime is net uptaker of CO₂. An extensive literature study was made on the fundamentals of the carbonation process in mortars with different compositions. The results of the study indicate that carbonation ranges from 80 % up to 90 %. It is clear that the mechanism and the kinetics of the carbonation depend strongly on the mineralogy, texture of mortars, type of additive used, the lime use for the mortar, the width of the walls, thickness of the mortar (less carbonation when mortar depth increases) as well as the timeframe allowing for the carbonation process to take place. Under natural conditions, actual building practice and depending on the thickness of the mortar/plaster, carbonation takes between a few weeks and several years. The results of this study were used for the environmental footprint study in order to calculate the capture of CO₂ that occurs progressively during the hardening of a building materials containing lime.