Recent developments in local concepts of fatigue assessment of welded joints

Probabilistic framework for fatigue life assessment of notched components under size effects

Very high cycle fatigue measuring techniques

The notch stress approach for fatigue assessment of welded joints is based on the highest elastic stress at the weld toe or root. In order to avoid arbitrary or infinite stress results, a rounded shape with a reference radius, instead of the actual sharp toe or root, is usually assumed. Different proposals for reference radii exist, e.g. Radaj proposed a fictitious radius of 1 mm to consider micro-structural support effects for steel. The present guideline reviews different proposals for reference radii together with associated S-N curves. Detailed recommendations are given for the numerical analysis of the notch stress by the finite or boundary element method. Several aspects are discussed, such as the structural weakening by keyhole-shaped notches and the consideration of multiaxial stress states. Regarding the fatigue strength, appropriate S-N curves are presented for different materials. Finally, four examples illustrate the application of the approach, as well as the variety of structures that can be analysed and the scatter of results obtained from different models.

IIW recommendations for the fatigue assessment of welded structures by notch stress analysis: IIW-2006-09

In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii r ref = 1.00 and 0.05 mm

In dem vorliegenden Beitrag werden Methoden, auf Basis des bezogenen Spannungsgradienten χ* und des hochbeanspruchten Werkstoffvolumens HBV90%, zur betriebsfesten Vorbemessung von Bauteilen aus hochfesten Spharogusswerkstoffen vorgestellt. Aufbauend auf umfangreichen Schwingfestigkeitsuntersuchungen unter konstanten Amplituden mit aus dem Bauteil entnommenen Materialproben wurden fur die Werkstoffe EN-GJS-500-7, SiboDur 700-10 und MADI die zugehorigen Werkstoffkennwerte experimentell bestimmt und Naherungslosungen fur die Ubertragung auf beliebige Bauteilgeometrien abgeleitet. Die Validierung der Treffsicherheit bei der Abschatzung der Schwingfestigkeiten von innendruckbelasteten Bauteilen erfolgte an einem praxisnahen Pulsdummy-Modellkorper, der als versagenskritisches Detail eine Bohrungsverschneidung im Winkel von 45° aufweist.



In the present paper methods based on the related stress gradient χ* and the highly stressed material volume HBV90% for the assessment of the structural durability of high-strength ductile iron components are presented. Approximate solutions were formulated on the basis of extensive fatigue tests under constant amplitude loading using specimens from the alloys EN-GJS-500-7, SiboDur 700-10 and MADI. The estimated fatigue strengths were validated by tests with the so-called “Pulsing Dummy Model Bodies”, samples with a bore intersection with an angle of 45° as critical area.

Methoden zur betriebsfesten Bauteilvorbemessung von innendruckbeaufschlagten Bauteilen aus hochfesten Eisengusswerkstoffen

Virtual product development aims at the use of information modeling techniques and computer-aided (CAx-) tools during the product development process, to represent the real product digitally as an integrated product model (Anderl and Trippner 2000). Thereby, data related to the product as well as product properties are generated and stored as result of the product development process (e.g., product planning, conceptual design) (Pahl et al. 2007; VDI 2221 1993). Within virtual product development CAx process chains have been established. They comprise the concatenating of the applied tools and technologies within the steps of the virtual product development process enabling the consistent use of product data (Anderl and Trippner 2000). The computer-aided design (CAD) technology aims at the integration of computer systems to support engineers during the design process such as design conceptualization, design, and documentation. It provides the geometry of the design and its properties (e.g., mass properties, tolerances) which is abstracted to be used in computer-aided engineering (CAE) systems (e.g., finite element method (FEM)) for design analysis, evaluation, and optimization. The computer-aided process planning (CAPP) technology provides tools to support process planning, Numerical Control (NC) programming, and quality control (Hehenberger 2011; Lee 1998; Vajna 2009). The advantages are continuous processing and refinement of the product model, minimizing the modeling efforts regarding time as well as costs and avoiding error sources. In addition, all relevant data and information related to the product can be provided for subsequent processing (Anderl and Trippner 2000). CAx technologies have been widely established within the product development processes in industry. They have been further developed in the last years; however efforts to integrate and to automate them are still a topic of research. Especially, with the introduction of innovative manufacturing technologies such as linear flow and bend splitting require new methods and tools for the virtual product development process. These technologies enable the production of a new range of sheet metal products with characteristic properties (e.g., Y-profile geometry, material properties) that are not addressed in state-of-the-art methods and tools.

Computer-Integrated Engineering and Design

Wesentlich fur die betriebsfeste Auslegung von Bauteilen sind geeignete Methoden und Ubertragbarkeitskriterien sowie die Kenntnisse uber das zyklische Werkstoffverhalten. Aus diesem Grund werden die zyklischen Werkstoffkennwerte in Abhangigkeit der Produkteigenschaften (Umformgrad, Gefuge, Oberflachenrauhigkeit, Eigenspannungen) und unterschiedlichen Belastungsparametern ermittelt. Da jedoch die Ermittlung der zyklischen Kennwerte mit hohen Versuchsaufwand und Kosten verbunden ist, wird nach Methoden gesucht, diese kostengunstig und schnell zu bestimmen. Ein viel versprechender Ansatz hierfur ist der Einsatz von kunstlichen neuronalen Netzen (KNN), da sie in der Lage sind, die Einflusse aus Fertigungs- und Umweltparametern auf die Schwingfestigkeit aus sinnvoll gewahlten Eingangsparametern zu generieren. Sie bieten die Moglichkeit, auf erlerntes Wissen zuruckzugreifen und somit, auf Basis weniger Versuche ein mehrdimensionales Kennfeld zu erstellen. Durch eine Kombination aus wenigen experimentellen Versuchen und dem kunstlich neuronalen Netz kann die Treffsicherheit der Abschatzung erhoht und der erforderliche experimentelle Aufwand reduziert werden.



Suitable methods and transferability criteria and knowledge of the cyclic material behaviour is essential for a durable design of a component. For this reason, the cyclic material parameters are determined as a function of the product's properties (level of deformation, microstructure, surface finish, residual stresses) and different loading parameters. However, since the determination of the cyclic parameters is associated with considerable experimental effort and costs, a cost-effective and easy method is sought to determine these parameters. A very promising approach for this is the application of artificial neural networks (ANN) since they have the ability to generate the influences on the fatigue strength from the manufacturing and environmental parameters using sensibly selected input parameters. They offer the possibility to access acquired knowledge and to thus construct a multidimensional map based on a few tests. By combining a few experimental tests with the ANN, the result of the estimation can be improved and the experimental effort can be reduced.

Einfluss der Versuchsunterstützung auf die Abschätzungsgenauigkeit mittels künstlich neuronaler Netze

Die weit verbreiteten Methoden zur Beschreibung des zyklischen Werkstoffverhaltens nach Ramberg-Osgood und Manson-Coffin-Basquin sind schon mehrere Jahrzehnte alt. Fur verschiedene Werkstoffe wird jedoch festgestellt, dass die sich laut Kompatibilitatstheorie [1, 2, 3] ergebenden Spannungs-Dehnungs-Kurven nicht mit den Versuchsergebnissen ubereinstimmen. In fruheren Arbeiten wurde eine laut Theorie notwendige Methode zur kompatiblen Auswertung vorgestellt [4] und κ als Mas fur das inkompatible Werkstoffverhalten eingefuhrt [5]. In dieser Arbeit wird auf die Bedeutung von κ naher eingegangen und dem Ingenieur die Auswirkungen bei der Vernachlassigung dieser neuen Kenngrose verdeutlicht.

κ – Ein Maß für nicht kompatibles Werkstoffverhalten

A bainitic forging steel, which enables the TRIP-effect, has been developed and can be used for lightweightconstruction for forged components under cyclic loading. The high potential under cyclic loading conditions of the new generation of TRIP forging steels is shown and also the improved results under service loads, especially under overloads and misuse. A comparison of linear damage accumulations by Palmgren-Miner will depict the differences in characteristic damage sums between quenched and tempered and the new bainitic steel. For TRIP steels properties the process parameters are very important. Hence, the influence of the cooling conditions of TRIP will be discussed.

3rd International Conference on Material and Component Performance under Variable Amplitude Loading, VAL2015 New Bainitic Steel for cyclic loaded safety part s with improved cyclic material behaviour

Heinz Kaufmann

Papers

Methoden zur betriebsfesten Bauteilvorbemessung von innendruckbeaufschlagten Bauteilen aus hochfesten Eisengusswerkstoffen

Computer-Integrated Engineering and Design

Einfluss der Versuchsunterstützung auf die Abschätzungsgenauigkeit mittels künstlich neuronaler Netze

κ – Ein Maß für nicht kompatibles Werkstoffverhalten

3rd International Conference on Material and Component Performance under Variable Amplitude Loading, VAL2015 New Bainitic Steel for cyclic loaded safety part s with improved cyclic material behaviour