Characterizing Dynamic Changes in Superelastic Properties in-situ High Cycle Linear Fatigue Testing of Nitinol Wire Z-Specimen

The effects of inclusions within a Nitinol material on the fatigue performance has been largely accepted by the Nitinol community. With great improvements in material quality, the statistical probability of an inclusion being at the region of high stress and facilitating fatigue failure is much reduced. The role of accumulative damage within the microstructure of the Nitinol material in a high cycle fatigue situation should therefore be looked at more closely.

In this study the accumulative microstructural damage is quantified for the entire typical lifecycle of a Nitinol component which is represented by a wire Z-specimen. A novel-design linear fatigue tester enables in-situ measuring of dynamic changes in the thermomechanical properties of the material. Thermomechanical properties are characterized during a 1 Million cycle test with an initial characterization prior to cycling, several intermediates and a final one after 1 Million cycles.

Through this work, a better understanding of the mechanisms behind fatigue failure of a superelastic Nitinol device tested under strain-controlled condition is obtained.