A study on the effects of final diameter on the fatigue properties of NiTi wire

It is a generally held belief that controlled strain fatigue life is independent of wire diameter.  This has led to the widespread use of general S-N curves for Engineering metals.  This independence of wire diameter may not hold true for Nitinol.  If this is true, then much care must be used when applying generic Nitinol S-N curves to product design.  In hope to provide a more comprehensive understanding of this issue, a study on the fatigue properties of NiTi wires, Ti-55.8/55.9 wt %Ni in composition, comparing various diameter wires drawn from the same starting material was conducted using a controlled strain rotating beam test method in a 37^oC water bath.  Where enough data was gathered S-N curves were generated for the various diameters.  Surface, microstructure as well as fracture analyses were conducted to support the cyclic fatigue data.