Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 319 Focussed on multiaxial fatigue Microstructural study of multiaxial low cycle fatigue Masao Sakane, Takamoto Itoh Ritsumeikan University, Department of Mechanical Engineering, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan sakanem@se.ritsumei.ac.jp, itohtaka@fc.ritsumei.ac.jp A BSTRACT . This paper discusses the relationship between the stress response and the microstructure under tension-torsion multiaxial proportional and nonproportional loadings. Firstly, this paper discusses the material dependency of additional hardening of FCC materials in relation with the stacking fault energy of the materials. The FCC materials studied were Type 304 stainless steel, pure copper, pure nickel, pure aluminum and 6061 aluminum alloy. The material with lower stacking fault energy showed stronger additional hardening, which was discussed in relation with slip morphology and dislocation structures. This paper, next, discusses dislocation structures of Type 304 stainless steel under proportional and nonproportional loadings at high temperature. The relationship between the microstructure and the hardening behavior whether isotropic or anisotropic was discussed. The re-arrangeability of dislocation structure was discussed in loading mode change tests. Microstructures of the steel was discussed in more extensively programmed multiaxial low cycle fatigue tests at room temperature, where three microstructures, dislocation bundle, stacking fault and cells, which were discussed in relation with the stress response. Finally, temperature dependence of the microstructure was discussed under proportional and nonproportional loadings, by comparing the microstructures observed at room and high temperatures. K EYWORDS . Multiaxial fatigue; Microstructure; Stainless steel; Dislocation; Stacking fault energy; Twin I NTRODUCTION ultiaxial low cycle fatigue (LCF) is one of the key issues for appropriate designs and safe operations of machines since some of machines undergo multiaxial LCF damage. Stress parameters have been widely used to describe high cycle fatigue damage and strain parameters to describe LCF damage. In high cycle fatigue, crack nucleation and extension at localized areas are a major damage mechanism, whereas the deformation of a whole part remains in elastic regime. Therefore, the microstructural change in high cycle fatigue is limited in the vicinity of a crack. In LCF, however, whole parts elasto-plastically deform nucleating many cracks in the early stage in fatigue process and some of them propagate to bring failure of machines and structures. Manson-Coffin law, where the damage parameter is written with strain, has been widely used but stress has an important role in LCF failure, especially nonproportional low cycle fatigue. Additional hardening occurred under nonproportional loading, where principal axes change their directions with time. The additional hardening was reported to have a close connection with failure lives in nonproportional LCF, and M