Evaluation and visualization of multiaxial fatigue behavior under random non-proportional loading condition

In cyclic multiaxial stress/strain condition under non-proportional loading in which principal direction of stress/strain are changed in a cycle, it becomes difficult to analyze stress/strain ranges because of complexity of multiaxial stress/strain states depending on time in cycles. In order to evaluate stress/strain simply and suitably under non-proportional loading, Itoh and Sakane have proposed a method called as IS-method and a strain parameter for life evaluation under non-proportional loading _NP. In the method, 6-components of stress/strain are converted to an equivalent stress/strain indicating the amplitude and the direction of principal stress/strain as a function of time as well as an intensity of loading non-proportionality f_NP. Based on IS-method, the authors also have developed a tool which enables to analyze multiaxial stress/strain condition with the non-proportionality of loading history and evaluate failure life under non-proportional multiaxial loading. The tool indicates the analyzed results on monitor and users can understand visually not only variation of the stress/strain conditions but also non-proportionality during the cycle, which helps the design of material strength.