Volume 47, Issue 5

The present overview covers the fundamentals of multiaxial loading and the state of the art of the theoretical assessment of fatigue strength.

This paper is focused on a research project supported by the German automotive industry and international software producers. Six multiaxially loaded components were tested under random multiaxial service loading conditions and evaluated by different algorithms. The results reveal a large scatter of calculated fatigue lives which are, in most cases, on the unsafe side. The contradictory unsatisfactory results are determined by the complex interaction of the multiaxial stress state with cumulative damage. Therefore, in the case of safety critical components, experimental structural durability tests must be carried out.

Within the more general field of structural durability research, multiaxial fatigue has been one of the most active research topics over the past 20 yeaars. The current paper presents a limited review of both the current state-of-the-art for multiaxial fatigue and attempts to present several areas where continued advances are needed. While any number of topics could be selected, this review considers the technical fields of multiaxial stress-strain, mixedmode crack growth, multiaxial damage parameters, multiaxial fatigue of welds, and computer implementation.

An extension of several algorithms is presented for estimating elastic-plastic notch stresses and strains for metallic materials under cyclic multiaxial nonproportional loading. The extension allows for using advanced models of incremental plasticity. The accuracy of the algorithms, i.e. the structural yield surface approach according to Köttgen et al. and the incremental Neuber approach according to Glinka et al., is shown by comparisons with finite element calculations and notch strain measurements. The accuracy of both approaches is comparable. The differences in predicted stress-strain paths are in the same order as those to measured and FE-calculated paths. These differences lead to a factor of up to two in calculated fatigue lives. Glinka's approach can be applied much more easily; Köttgen's approach offers the opportunity for further improvements in accuracy.

This paper aims to propose a new interpretation of several energy-based critical plane high cycle fatigue criteria. More exactly, the meaning of several global energy-based criteria is detailed from the calculation of normal and shear work on a critical plane. This study is limited to multiaxial fatigue under constant amplitude and synchronous loadings (proportional or non-proportional), with or without mean load. After a short review of some energy-based criteria using the critical plane concept or a global approach, the reasons why energy-based critical plane criteria are more and more developed in literature are summarized. It is shown that a direct link exists between critical plane energy parameters and global ones. A critical examination of the famous Findley et al. experiments is proposed and the previous link is illustrated with two mesoscopic criteria developed by Papadopoulos and the global energy-based criterion of Banvillet et al. This leads us to propose an interpretation of fatigue crack initiation from the mesoscopic scale up to the macroscopic scale either with stress and strain quantities or with strain energy parameters.

Kurzfassung Der im Fahrzeugbau zunehmende Einsatz von Bauteilen aus Leichtmetallen wie Aluminium- und Magnesiumlegierungen kann beim Verspannen mit den zurzeit üblichen Schrauben aus niedrig legierten hochfesten Stählen zu erheblichen Problemen führen. Eine Reihe dieser Probleme kann durch Verwendung von Schrauben aus Aluminiumlegierungen verringert oder gar vermieden werden.

The adhesion strength as well as the crack resistance of hard CVD coatings on cutting tools are widely influenced by the appertaining residual stress states. They are generated during cooling down from coating temperature, or during a subsequent heat treatment. To enhance the knowledge of the residual stress development in such CVD coatings on steels, X-ray residual stress analyses were performed during the cooling process. Therefore, a high temperature chamber for a Ψ-diffractometer was developed and manufactured. It allows X-ray residual stress analyses at temperatures of up to at least 1000°C using a measuring range of 2θ ≥ 94° at |Ψ| ≤ 60°.