Multiaxial fatigue assessment of crankshafts by local stress and critical plane approach

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 Citation: Leitner, M., Grün, F., Tuncali, Z., Steiner, R., Chen, W., Multiaxial fatigue assessment of crankshafts by local stress and critical plane approach, Frattura ed Integrità Strutturale, 38 (2016) M. Leitner et alii, Frattura ed Integrità Strutturale, 38 (2016) [47][48][49][50][51][52][53] 48 experiments showing that the Huber-Mises-Hencky criterion fits well to the test results in case of proportional rotating bending and torsion loading.

Fatigue Life Properties of Stainless Steels in Wide Ranged Biaxial Stress State

“…Those pipes receive multiaxial loading by movement of internal fluid [1]. However, most of the fatigue data which are used for those strength designs are obtained by uniaxial loading tests, and there is limited data under multiaxial loading [2][3]. The equivalent stress based on von Mises is generally used in the design for structures under multiaxial loading.…”

“…The equivalent stress based on von Mises is generally used in the design for structures under multiaxial loading. In recent literatures of multiaxial fatigue, it is reported that fatigue life evaluated by the Mises' equivalent stress is overestimated [2][3][4][5][6][7][8][9] and the degree depends on loading path [10][11]. In the multiaxial fatigue under non-proportional loading in which directions of principal stress and strain are changed in a cycle, it has been reported that fatigue lives are reduced accompanying additional hardening which depends on both loading path and material.…”