The Effects of Varying Active Af Temperatures on the Fatigue Properties of Nitinol Wire

The influence of deformation temperature on mechanical properties of Shape Memory and Superelastic NickelTitanium (Nitinol) alloys has been studied and is well documented. In determining the effectiveness of a device when it is deployed and maintained under strain at ambient or body temperature, both the material properties and the environments encountered by the final device must be taken into account. In device design, it is important to understand the thermomechanical history of the wire supplied. However, data is lacking in terms of deriving a relationship between transition temperatures to a breadth of mechanical properties. Engineers of medical devices tailor the final material properties of wires used in a medical device application typically through a shape-setting process. Heat treating Nitinol wires in this type or a similar process aids in obtaining a desired shape as well as in reaching a target Active Af prior to final surface preparation. These processing steps can affect fatigue life. By applying alternating tension and compression states through rotary beam fatigue testing, one may generate data to predict the life expectancy of Nitinol wires. Specimens with varying Active Austenitic Finish temperatures have been subjected to multiple strain levels. The relevancy of this type of study, which involves the generation of fatigue data, supplements thermal and mechanical data to provide design engineers additional information for the development of Nitinol wire implants.