Assessing ultrahigh cycle fatigue strength of nitinol: from a strain-based to a stress-based approach

Fatigue strength of Nitinol at very high cycle ranges (6.0 x 10⁸) is used for assessing fatigue safety margins for structural heart implants, and is assessed using strain-based metrics to construct a constant life curve. For transcatheter implants that undergo small pre-strains as they are crimped into small-diameter catheters for delivery into patients, the strain-based methodology has proven successful. As next generation devices are demanding smaller profiles for reduced invasiveness, high crimp-strains leading to substantial permanent deformation must be considered, along with the path dependence of Nitinol’s behavior. Development of residual stresses and change in plateau stress will alter the cyclic stresses significantly, and result in the dependence of fatigue strength on pre-strain and/or specimen geometry. Strain-based approaches are unable to capture these effects.

We demonstrate the use of an alternative fatigue indicator parameter based on stress that reflects the fatigue driving force resulting from prior deformation history and accounts for large crimp strain and residual stress impact. This framework will enable a master S-N curve that can be independent of crimp strain, and may reduce the amount of testing to the minimum necessary. We will illustrate the effectiveness of this methodology in both low cycle and very high cycle fatigue.