Characterizing Fatigue and Fracture Response of Medical Grade Nickel-Titanium Alloys by Rotary Beam Testing

With respect to a dynamic test protocol, fatigue information has been reliable in predicting wire wear longevity as well as critical structural integrity. It has been noted that the efficacy of modern medical devices are confined by the life cycle of the material of which they are composed. The aim of this study is to expound on the means of how rotary beam fatigue test methods merit attention as a viable characterization technique. By simulating alternating tension and compression states through rotary beam fatigue testing, it is possible to predict the life expectancy of Nitinol wires. Fatigue testing has been employed to characterize the influence of subtle changes in ingot inclusion content, chemistry variations of raw material, ingot transformation temperatures of Nitinol, and surface finish conditions. Arrays of specimens have been subjected to multiple strain levels, yielding calculations of the approximate stress/strain values of ultimate failure at the outer surface of monofilament wires. Test protocols were administered up to 100 million alternating cycles where desired. If preferred, fatigue testing may be conducted in temperature-controlled ambient air or liquid environments. Characterization of fracture surfaces has proven useful in evaluating the factors influencing failures. The utilization of fatigue data and fracture mechanics surpasses tensile testing in providing information to the design engineer. Results from studying flexural endurance, statistical Weibull life prediction analysis, fracture analysis, and a determination of stress/strain levels at the site of failure may prove useful in determining desired material properties for next generation medical devices.