Superelastic Nitinol: The Frontiers of Lifetime Prediction

At the same time, predicting the fatigue behavior of Nitinol is far more complicated than for most metals, requiring new ways to model, test and report lifetime. Perhaps the greatest of these difficulties is in establishing the duty cycles to which devices are exposed; fatigue models are generally stress-based, or sometimes strain-based, but most Nitinol devices are compliance controlled, having to blend into their anatomical environment. 

Once a duty cycle is established regulatory agencies expect to see a Goodman-style safety factor. But then non-linear response of Nitinol means that areas of high mean strain are generally not those with high cyclic strain. Moreover, prestrains have been shown to have an enormous impact on fatigue life, and again, the areas with maximum prestrain are not those with the highest cyclic or mean strain. The combination of these factors make it extremely difficult to identify the most fatigue-critical location, and in turn, a safety factor. 

The third major issue to be considered is the role of inclusions. While everybody strives toward the “fewer and smaller” goal, inclusions will remain part of life for quite some time. Fatigue then becomes a Monte Carlo game of sorts, where each microscopic volume of a device has a determinable compliance, mean strain, cyclic strain, and pre strain, and thus critical flaw size. Predicting lifetime is then a statistical calculation to determine the probability of finding a critical flaw size in a given micro-volume of material. Finally, while the above factors affect device performance, they also impact testing methodology. While rotary beam testing is easy and fast, given the above, it is irrelevant to many devices, So-called “diamond testing”—testing geometries that emulate the device—are useful, but these tests do not produce material performance data that can be extended to other geometries and thermomechanical processing conditions. 

This presentation cannot claim to provide a solution to any of these problems; its intent is only to provide a way of thinking that may help people approach the problem in a more organized way, as well as helping people understand the limitations of the tools we have today.