This paper describes a multiaxial low cycle fatigue parameter for correlating Hues under nonproportional loadings. Constant amplitude low cycle fatigue tests were carried out under 14 proportional and complex nonproportional cyclic strain paths using type 304 stainless steel hollow cylinder specimens at room temperature. In nonproportional loading tests, fatigue lives are decreased by as much as a factor of 10 in comparison with those in proportional loading tests with the same strain range. Reduction in fatigue life due to nonproportional loading is closely related to additional nonproportional cyclic hardening. The product of the maximum principal stress and strain ranges correlated the nonproportional fatigue data. A nonproportional cyclic hardening parameter computed from the strain path is also proposed that allows life estimates to be obtained directly from the strain history without the need for a cyclic plasticity model.

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This study discusses evaluation of material dependence of multiaxial low cycle fatigue (LCF) to develop a suitable strain parameter for life estimation under non-proportional loading. It has been reported that fatigue lives are reduced accompanying an additional hardening under strain controlled non-proportional loading in which principal directions of stress and strain are changed in a cycle. Strain controlled multiaxial LCF tests using proportional and non-proportional strain paths were carried out using hollow cylinder specimens of several materials. The reduction in low cycle fatigue life due to non-proportional loading is discussed relating to the additional cyclic hardening behaviors and its material dependence. Material constant, α, used in strain parameter for life estimation under non-proportional multiaxial LCF is also discussed.

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