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 or e-N curves for engineering metals. Independence of wire diameter on fatigue life may not hold true for Nitinol. If so, care must be used when applying generic Nitinol S-N or e-N curves to product design. To provide a more comprehensive understanding of this issue, a comparison study was conducted on the fatigue properties of various diameters of NiTi (Ti-55.8/55.9wt.%Ni) wire drawn from the same starting material. e-N curves were generated for various diameters. The wire diameter reduction from 0.030¢¢ to 0.010¢¢ increased the 0.8% strain 80k cycle survival rate from 35 to 96%. For the scope of this study, all factors that may have an effect on fatigue life except for wire diameter were held constant.
Development of a superelastic material with higher stiffness and plateau stresses than binary nitinol is of interest to the medical device industry because it may allow for lower profile, less intrusive devices without compromising the materialÕs characteristics. This project studied the effect of cobalt (Co) alloying additions on the stiffness and plateau stresses of a superelastic nickel-titanium alloy. In addition, the general physical, mechanical, corrosion, and biocompatibility properties of the alloy were compared to binary nitinol. The results of this study showed Co to be an interesting alloying addition that should be considered for future medical devices in applications, where stiffness is of concern.
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