Abstract
ABSTRACT. For multiaxially-loaded parts several stress-based fatigue assessment concepts are applicable mostly taking uniaxial test results as basis. These approaches work well in case of proportional loading states, but on contrary, for non-proportional stress conditions, implying a change of the principal stress direction, deviations in the fatigue life estimation may occur. The aim of this study is to evaluate the cyclic multiaxial material behavior experimentally and to proof the applicability of stress-based methods to assess the fatigue strength. The investigated base materials incorporate the commonly applied crankshaft steels 50CrMo4 and 34CrNiMo6 without surface-layer post-treatments. Extensive fatigue tests with small-scale specimens are performed to evaluate the material behavior under cyclic loading. The experiments include basic uniaxial characterizations, such as notch stress sensitivity and effect of loading type, including tests under tension, rotating bending, and torsion loading. Additionally, combined loading tests with proportional and non-proportional situations are presented to reveal the fatigue resistance for multiaxial stress states. Significant loading conditions, such as proportional stress under rotating bending and torsion, and further on, non-proportional effects like phase shifts and varying frequency ratios are presented. The local fatigue strength assessment is performed on the basis of the critical plane approach, whereat the normal and shear stresses are transformed in numerous cutting planes. Equivalent stress hypotheses are applied and compared with the experiments showing that the Huber-Mises-Hencky criterion fits well to the test results in case of proportional rotating bending and torsion loading. KEYWORDS. Multiaxial fatigue assessment; Local stress concept; Critical plane approach; Crankshaft.
Highlights
The local fatigue strength assessment is performed on the basis of the critical plane approach, whereat the normal and shear stresses are transformed in numerous cutting planes
This paper presents a multiaxial fatigue strength assessment of crankshafts based on different local stress approaches
The results reveal that a phase shift of 45° (π/4) leads to an increase of 3 % and a phase shift of 90° (π/2) of 6 % compared to the fatigue strength of proportional normal and shear stress loading
Summary
This paper presents a multiaxial fatigue strength assessment of crankshafts based on different local stress approaches. The application of the model in order to assess the local notch fatigue strength is depicted in Fig. 2 showing a good accordance to the fatigue test results. Thereby, the determination is performed on the basis of the evaluated support effect and an exponential interpolation among a minimum and maximum value kmin and kmax, and NDmin and NDmax These values are statistically evaluated by numerous fatigue tests, whereby kmin and NDmin represent a technically highest-notched case, and kmax and NDmax the unnotched condition.
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