Implementation of Multi-Axial Fatigue Theory in FE Packages

Many mechanical or structural components are subjected to multi-axial, irregular cyclic loading during service. The direction and amplitude of principal stress and strain vary over a period of time results in non-proportional cyclic loading on the component. At geometrical discontinuities, even a monotonic load will result in multi-axial state of stress. In general, the life of the components subjected to multi-axial stress loadings, are evaluated using classical yield theories. The Tresca and von Mises criterions along with Basquin-Coffin and Manson life curve are widely used in commercially available Finite Element Analysis (FEA) tools. These classical methods are conservative and may not yield good experimental correlation at all the loading conditions and this augments the need for robust life estimation methodology.There are many commercially available FEA tools to estimate the multi-axial fatigue life viz. nCODE ® which uses Wang-Brown method [1]. However, it has been found that for shear dominated fatigue material Fatemi-Socie criteria is more suitable. So an attempt is made to develop a an algorithm to implement Fatemi-Socie criteria in a commercially available generic FEA software in a cost effective way. This paper discusses how to estimate the life of a sample specimen subjected to multi-axial and non-proportional loading conditions. The classical yield criteria based on von-Mises stress with Basquin-Coffin and Manson equation and critical plane method viz, Fatemi-Socie criteria are implemented in to commercial FEA tool, ANSYS. This paper also attempt to see how these theories compare with experimental data. Results of this study would help in leveraging the established process of implementing custom based life estimation method in ANSYS for the estimation of the life of the mechanical components.