Effect of Prestrain on the Fatigue Life of Superelastic Nitinol

Three types of fatigue testing are performed to elucidate the effects of prestraining superelastic Nitinol on its subsequent fatigue lifetime: rotary bending and tension–tension testing of wire, and beam bending using diamond-shaped specimens fabricated from tubing. Results show that local plastic deformation during prestraining induces residual stresses that have a pronounced effect on fatigue performance, enhancing performance when the fatigue duty cycle is of the same sense as the prestraining (tensile prestraining followed by a tensile duty cycle, for example), and decreasing fatigue lifetime when the sense of the duty cycle is opposite to that of prestraining. This provides an avenue to increasing fatigue lifetime, but more importantly it highlights the need to fully understand the nature of the duty cycle: for example, prestraining a stent by crimping it into a delivery catheter induces favorable residual stresses with respect to subsequent pulsatile fatigue, but might accelerate fracture in other modes, such as axial or crush fatigue. Caution is also advised when trying to apply data from “constant life diagrams” derived from the literature (Ref 1 , 2 for example) that may not properly reflect the strain history of the device being analyzed.