Multiaxial fatigue of cast aluminium EN AC-42000 T6 (G-AlSi7Mg0.3 T6) for automotive safety components under constant and variable amplitude loading

Regarding the fatigue behaviour of EN AC-42000 T6 (A 356 T6), which is the most frequently used cast aluminium alloy for automotive safety components, especially under non-proportional constant and variable normal and shear stress amplitudes with changing principal stress directions, a poor level of knowledge was available. The reported investigations show that, under non-proportional normal and shear stresses, fatigue life is increased in contrast to ductile steels where life is reduced due to changing principal stress directions. This behaviour caused by the low ductility of this alloy (e < 10%) compared to quenched and tempered steels suggests the application of the Normal (Principal) Stress Hypothesis (NSH). For all of the investigated stress states under multiaxial constant and variable (Gaussian spectrum) amplitudes without and with mean stresses, the NSH was able to depict the life increase by the non-proportionality and delivered, for most cases, conservative but non-exaggerated results

Multiaxial fatigue assessment of welded joints: A review of Eurocode 3 and International Institute of Welding criteria with different stress analysis approaches

This paper reviews quantitively the multiaxial fatigue assessment of steel and aluminum welded joints, focusing on Eurocodes (ECs) and the recommendations of the International Institute of Welding (IIW). Extensive fatigue data under constant and variable amplitude loading are reanalyzed using stress analysis approaches such as nominal stress, hot spot stress, and effective notch stress. Evaluation of the ECs and IIW criteria reveals an effective assessment of multiaxial fatigue, with a satisfactory level of conservatism. Further research is needed especially for variable amplitude (VA) loading to enhance the precision and reliability of assessments, contributing to improved design practices and structural integrity in welded joint applications

Fatigue design for powder metallurgy

This paper presents design relevant material data like S-N curves, effect of notches and mean stresses and fracture mechanical properties. The influence of different manufacturing parameters on the fatigue properties of sintered steels is discussed. The necessity to determine material properties, under different loading modes, is outlined against the background of different service conditions and component geometries. The fatigue design starts out by determining the local stresses and their distributions in the most critical areas of the parts due to the loading. For the fatigue life assessment the local stresses have then to be compared with adequate material properties in order to evaluate design, material and technology selection and to decide whether production can be started or further optimization is necessary. Within the design procedure, the importance of the scatter of material properties, manufacturing and service loading which determine the safety margin is also discussed fr om the standpoint of component reliability and economy. The design procedure for PM components is illustrated by an example

Effect of residual stresses on the fatigue behaviour of welded joints depending on loading conditions and weld geometry

The structural durability of welded structures is determined by the interaction of different influencing parameters such as loading mode, spectrum shape, residual stresses and weld geometry among others. Examples from plant, offshore, transportation and automotive engineering show how these parameters influence the fatigue life and to what extent they are considered in design codes. Especially, under spectrum loading, the stress decreasing effect of tensile residual stresses is not as high as under constant amplitude loading; this knowledge benefits light weight design. The overloads harmed only the low strength joints under pulsating bending. In all other cases investigated, with low, medium and high-strength steels, a significant decrease of fatigue life was not observed; on the contrary, significant improvement of fatigue life could even be observed. However, a systematic interaction with material strength, loading mode and residual stresses was not apparent

Einfluss der Werkstoffzähigkeit auf das Festigkeitsverhalten unter mehrachsiger Beanspruchung am Beispiel von Schweißverbindungen aus Stahl und Aluminium

The multiaxial fatigue behaviour of materials with different ductility under constant and changing principal stress directions is also applicable to welded joints of different materials. For this, welded flange tube connections of the fine grained steel StE 460 and the artificially aged aluminium alloy AlSilMgMn T6 were investigated under constant amplitude combined bending and torsion. Out-of-phase loading, i.e. changing principal stress directions, of the steel joints led to a decrease of fatigue life, which is observed at ductile material states. However, for the aluminium joints out-of-phase loading resulted same behaviour as in-phase loading, which indicates a semi-ductile material behaviour. The results for the welded steel joints were evaluated on basis of local stresses by the integral hypothesis of the Effective Equivalent Stress EES (WVS). This hypothesis for ductile material states takes into account the life decreasing influence of out-of-phase loading by considering the interaction of the shear stresses on different planes. The fatigue behaviour of the aluminium welds is described by the critical plane based combination of shear and normal stresses (KoNoS), which is valid for semi-ductile material states

A consideration of allowable equivalent stresses for fatigue design of welded joints according to the notch stress concept with the reference radii rref = 1.00 and 0.05 mm

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 r(ref) = 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 r(ref) = 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