D. W. Norwich, Memry Corporation, Bethel, CT; A. Fasching, Memry Corporation, Menlo Park, CA
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 37oC 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.
Summary: Fatigue testing of various diameter wire drawn from the same starting material show that fatigue life increases as diameter decreases.
