Multiaxial fatigue modeling for Nitinol shape memory alloys under in-phase loading.

The realistic loading condition for many components is multiaxial arising from multidirectional loading or geometry complexities. In this study, some multiaxial stress-based classical and critical plane fatigue models are briefly reviewed and their application for martensitic Nitinol under torsion and in-phase axial-torsion loading is evaluated. These models include von Mises equivalent stress, Tresca, Findley, McDiarmid, and a proposed stress-based Fatemi-Socie-type model. As the fatigue cracks appear to be on the maximum shear plane for the martensitic Nitinol, all the models examined here consider the shear stress as the primary damage parameter. Among all the models considered in this study, the proposed Fatemi-Socie-type model provides a better prediction for fatigue lives when compared to torsion and in-phase multiaxial fatigue experimental data from literature. Analyses indicate that critical plane approaches are more appropriate for multiaxial fatigue prediction of Nitinol alloys, at least in martensitic phase. Finally, recommendations are made to calibrate more reliable multiaxial fatigue models for Nitinol.