Wilfried Eichlseder

Fatigue analysis by local stress concept based on finite element results

Abstract The assessment of finite element results for dynamically loaded components requires local S/N-curves, which depend, under further effects, on the type of loading and the stress flow within the component. These influences are described with help of the stress gradient, which can easily be determined with finite element calculations. A model for the calculation of S/N-curves is presented, which takes into account the stress gradient to define the local stress limit, the number of cycles at the fatigue limit and the slope.

Effect of high-strength filler metals on the fatigue behaviour of butt joints

Welded structures made of high-strength steel offer benefits in fatigue strength for finite life applications. The high-cycle fatigue limit, however, depends mostly on the geometry, and the metallurgy of the notch is of little account. Therefore, an optimised weld process is required to achieve an improvement in the fatigue strength. This paper contributes to the field of fatigue behaviour of thin-walled, high-strength steel butt joints, with regard to an optimisation of the gas metal arc weld process. An existing methodology was extended to manufacture welded specimens with minimised production scatter. The majority of the butt joint samples were dynamically tested, with the root surface ground flush to plate, to study the effect of the weld process on fatigue. The investigated specimen were carefully analysed by metallographic studies along with hardness, distortion, and weld toe topography measurements. This facilitated in finding a relation between experimental fatigue life and the weld parameters. The nominal stress approach, including a benign, nonconservative thinness correction, and the recommended notch stress concept, were applied to assess the fatigue behaviour of the thin-walled, high-strength steel butt joints. The experimental results showed that in case of high-quality welds with negligable geometric notch factor, a small, but distinct influence of the filler metal on fatigue is observable. The highest fatigue strength for the investigated butt joint design was obtained with a high-strength metal-cored wire filler in combination with a three-component shielding gas.

Failure Mechanism of Pure Nickel (Ni 200/201) under Thermo-Mechanical Loading

Cyclic loading of metallic engineering components at constant elevated or fluctuating temperature causes a complex evolution of damage which be can hardly be described in a unique and straightforward manner. Often the thermal behaviour of the base metals is to weak, so thermal barrier coatings were needed. Nickel is generally used for such thermal barrier coatings. Therefore it is necessary to study the thermo-mechanical fatigue (TMF) of this material. The lifetime of these coatings is very strong affected by the temperature loading in general, both described by nodal temperatures and their local gradient. The thermal cyclic loading takes place as thermo-mechanical and low cycle fatigue (LCF) damage regime. To classify the thermo-mechanical failure mechanism of pure nickel, OP (out of phase) and IP-TMF (in phase) test series were examined. The use of damage parameters like the unified energy approach make sense, a more detailed life time calculation for pure Nickel can be done by using the Neu-Sehitoglu model. Summary, thermomechanical loadings activate multiple damage mechanism. Surface embrittlement by oxidation is the major distinctive mechanism in addition to pure fatigue damage. Different lifetime approaches were tested and analysed to fulfil the requirements for the fatigue analysis of nickel made components.

Neuer Ansatz zur Bewertung von Stützwirkung und statistischem Größeneinfluss im Auslegungsprozess

Kurzfassung Im Bauteilauslegungsprozess spielt bei der Übertragung von Schwingfestigkeitsergebnissen aus Probenversuchen auf zulässige Bauteilbemessungskennwerte die Beschreibung des spannungsmechanischen und statistischen Größeneinflusses eine wesentliche Rolle. Dazu wird im vorliegenden Beitrag ein neuer Ansatz zur getrennten Bewertung dieser beiden Größeneinflüsse vorgestellt sowie die Vorteile gegenüber klassischen Modellen an Schwingfestigkeitsversuchsergebnissen demonstriert.

Lifetime Calculation of Thermo-Mechanically Loaded Materials (Al, Cu, Ni, and Fe Alloys) Based on Empirical Methods

Cyclic loading of metallic engineering components at constant elevated or fluctuating temperatures causes a complex evolution of damage, which cannot be described easily. In many engineering components, thermo-mechanical loading occurs, e.g., cooling components in metallurgy and metal forming, turbine blades, cylinder heads, exhaust systems, etc. At the same time, the thermal expansion is restricted in some regions due to the complex geometry of the components. Therefore, mechanical stresses take place, and the cyclic plastic deformation leads to thermo-mechanical fatigue of the material. A careful analysis and comparison of the experimental results, based on a systematic variation of the relevant influence factors, allow to develop empirical models for computing the fatigue life of thermo-mechanically loaded components made of AlSi cast alloys, Cu alloy, Ni alloy, and cast iron. Based on stress-strain loops from low cycle fatigue tests at different temperatures, a nonlinear combined material model was adopted to describe the cyclic deformation behavior. The simulated loading parameters of stress and strain were the basis for the subsequent lifetime simulation. Different lifetime approaches were tested and analyzed to fulfill the requirements for the fatigue analysis of components made of these alloys. In particular, strain based criteria, damage parameters as well as hysteresis energy criteria were investigated. Also, a new energy based parameter was developed to optimize the scatter band and standard deviation. In order to verify the simulation model for components, numerical results have to be compared, and—if necessary—it also has to be adapted to experimental results from component tests. In addition, the model parameters can be optimized by using these results.

Evaluation of fillet weld properties and fatigue behaviour in dependence of welding parameters

Numerous different design codes can be used to describe the durability of welded structures. One wide spread approach is the local notch stress approach, which calculates the fatigue lifetime in dependency of the notch stress factor using different effective radii. To calculate the fatigue behaviour by using the local notch stress approach, the RIMS-concept is commonly used [1] . The evaluation of the influence due to the welding process parameters, especially for high-strength steels, the effect of both the geometrical and metallurgical notch is studied in a parametric way for selected weld joints. Experimental fatigue tests have been performed to investigate the link between fatigue life and manufacturing process dependent weld toe notch design. To be able to capture the influence of welding parameters, as energy input per unit length, welding velocity, angle of blowpipe, size and shape of the heat input zone in a numerical way, a local coupled thermo-mechanical simulation is build-up. The complexity of this modelling increases very strong by the temperature dependency of the multitudinous phase material properties. The material and manufacturing properties were adjusted by comparison of the temperature profiles. This experimental based procedure defines the simulation base for more complex welding seams.

Liftime Optimization of Hot Forged Aerospace Components by Linking Microstructural Evolution and Fatigue Behaviour

Heavy-duty aerospace components are frequently hot forged to satisfy the high requirements concerning their mechanical behaviour. Only the usage of high-performance materials together with a near-optimum manufacturing process enables the production of parts that are at the same time lightweight and mechanically extremely durable. Not only the static properties, but also the fatigue behaviour of Inconel718 is strongly influenced by the material’s microstructure resulting from the forging and heat treatment processes. Therefore, the static and fatigue properties may be controlled via the microstructural properties by suitably adjusting the parameters of the manufacturing processes. The present work links the complete forging and heat treatment process to the local distribution of the material’s fatigue strength within a component; the effect of the operating temperature is also considered. To this purpose, an empirical model is derived from fatigue tests on specimens with different microstructures at different temperatures. The resulting fatigue strength model is implemented, along with a microstructural evolution model from earlier work [1], into a finite element code in order to predict the local fatigue strength distribution in a component after being subjected to an arbitrary forging process. In a further step, the finite element code is linked to an optimization tool for determining the optimum set of manufacturing process parameters such that the component lifetime is maximized while taking process constraints into consideration.

The Fatigue Behaviour of a High Strength CrNi-Steel Regarding Fretting Fatigue

In many assemblies of moving components, contact problems under various lubrication conditions are lifetime-limiting. There, relative motion of contacting bodies, combined with high loads transmitted via the contact surface lead to fretting fatigue failure. For a reliable prediction of in service performance load type, different damage and failure mechanisms that may be activated during operation have to be known. In this contribution selected results of a currently conducted research project are presented. The aim of this study was to examine the material behaviour of a surface stressed steel. The influence of the fretting regime on fatigue properties has been investigated.Copyright

Schädigungstolerante Auslegung von Aluminium-Druckgusskomponenten

Kurzfassung Für eine verbesserte betriebsfeste Auslegung von Aluminium-Druck-Kurzfassung gussbauteilen muss die Porenverteilung im Bauteil abgeschätzt werden können. Ein existierendes Porositätsmodell wurde für die Berechnung der Porenverteilung in einem Referenzbauteil verwendet. Ein Vergleich mit der Porenverteilung im realen Bauteil zeigt eine sehr gute Übereinstimmung. Der Einfluss der Poren auf die Schwingfestigkeit wird anhand des Kitagawa-Diagramms beschrieben. Daraus kann, zusammen mit der Porenverteilung im Bauteil, die lokal ertragbare Spannungsamplitude abgeleitet werden.

Importance of Residual Stresses and Surface Roughness regarding Fatigue of Titanium Forgings

This paper presents the results of a long-term research program aimed at developing qualitative and quantitative design guidelines for the use of mechanical surface treatments designed to improve the fatigue life of structural components. High cycle fatigue tests were performed on planar four-point bending specimens derived from Ti-6Al-4V pancake forgings with a mill-annealed microstructure. The high cycle fatigue behavior of specimens with different surface conditions (as-forged and machined) in both an unpeened and a shot peened state was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layers were carried out. The as-forged surface state exhibits a stress distribution with significant compressive stresses near the surface, resulting in equilibrium tensile stresses in the depth. When the tensile stresses were exposed by machining a bordering surface, a distinct decrease in the fatigue strength was observed. In such cases, a shot peening treatment was shown to improve the fatigue strength. Square edges lead to a decrease in the fatigue strength, which could be aggravated by shot peening.