Abstract
This study discusses fatigue properties of low carbon steel, type SS400 steel, under non-proportional loading. Multiaxial fatigue tests under proportional and non-proportional loading conditions with various stress amplitudes were carried out using a hollow cylinder specimen at room temperature. In the test, three types of stress paths were employed. They are a push-pull, a reversed torsion and a circle loading. The circle loading is a cyclic loading combined the push-pull and the reversed torsion loading in which axial and shear stress waveforms have 90 degrees phase differences. From the obtained test results, poor evaluations of failure life under non-proportional loading are indicated when the life is correlated by the equivalent strain range based on von Mises ??eq and the non-proportional strain range ??NP. A modified strain parameter is presented which can evaluate the failure life in high and low strain levels under non-proportional loading.
Highlights
Non-proportional loading of which directions of principal stress and principal strain are changed in a cycle occurs in various structures such as machinery for construction and transportation
Failure life can be correlated by a unique line independent of loading path in the stress region over the fatigue strength σw
The strain range in the circle loading test becomes smaller in comparison with that in the push-pull loading test at the same stress range because of additional hardening caused by non-proportional loading
Summary
Non-proportional loading of which directions of principal stress and principal strain are changed in a cycle occurs in various structures such as machinery for construction and transportation. In studies of multiaxial fatigue under non-proportional loading conditions; it has been reported that failure life is reduced accompanying with an additional hardening depending on both strain path and material [1,2,3,4,5,6,7,8,9,10,11,12]. Itoh et al have carried out series of multiaxial low cycle fatigue (LCF) tests under non-proportional loading conditions to examine properties of cyclic deformation and failure life [10,11,12].
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