Issue 38

T. Morishita et alii, Frattura ed Integrità Strutturale, 38 (2016) 289-295; DOI: 10.3221/IGF-ESIS.38.39 289 Focussed on Multiaxial Fatigue and Fracture Fatigue strength of SS400 steel under non-proportional loading T. Morishita, T. Takaoka Graduate School of Science and Engineering, Ritsumeikan University, Japan gr0202xp@ed.ritsumei.ac.jp, rm0033px@ed.ritsumei.ac.jp T. Itoh College of Science and Engineering, Ritsumeikan University, Japan itohtaka@fc.ritsumei.ac.jp A BSTRACT . 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 Δεeq and the non-proportional strain range ΔεNP. A modified strain parameter is presented which can evaluate the failure life in high and low strain levels under non-proportional loading. K EYWORDS . Fatigue; Multiaxial stress; Non-proportional loading; Life evaluation; Carbon steel. Citation: Morishita, T., Takaoka, T., Itoh, T., Fatigue strength of SS400 steel under non- proportional loading; Frattura ed Integrità Strutturale; 38 (2016) 289-295. Received: 30.07.2016 Accepted: 31.08.2016 Published: 01.10.2016 Copyright: © 2016 This is an open access article under the terms of the CC-BY 4.0; which permits unrestricted use; distribution; and reproduction in any medium; provided the original author and source are credited. I NTRODUCTION on-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 [1-12]. 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 [10-12]. They presented a strain parameter for life evaluation under non-proportional loading; a non- proportional strain range Δε NP ; which includes a non-proportional factor taking into account the effect of loading path N