Fatigue-crack propagation in Nitinol, a shape-memory and superelastic endovascular stent material

Improving the design and performance of medical stents for implantation in the human body is of current interest. This paper describes a study of fatigue-crack propagation behavior in the superelastic alloy Nitinol. Specifically, the objective of this work was to study the effect of environment on cyclic crack-growth resistance in an approximately 50Ni-50Ti (atom %) alloy and to provide the necessary data for the safe life prediction of Nitinol endovascular stents. The material selected for this study was heat treated such that it was superelastic at human body temperature; this was confirmed with monotonic uniaxial tensile tests. Characterization of fatigue-crack growth rates was performed at 37 degrees C on disk-shaped compact-tension samples in environments of air, aerated deionized water, and aerated Hank's solution (a simulated body fluid). The effect of cyclic loading on the uniaxial constitutive behavior was investigated at a strain range of 6.4%, and results indicate that the magnitude of available superelastic strain ( approximately 5.0%) is maintained even after cyclic softening. However, despite the persistence of nucleating the stress-induced martensitic phase after cycling with a maximum strain slightly below the plastic yield point, Nitinol was found to have the lowest fatigue-crack growth resistance of the principal metallic alloys currently used for implant applications.