Direct Experimental Evaluation of High-cycle Fatigue Indicator Parameters in Nickel-Titanium Shape Memory Alloys

A key step in the design of implantable Nitinol medical devices is to assess the fatigue safety limit of the underlying superelastic material. This is typically achieved using a combination of simulation and fatigue testing. Using simulation, mean strain and strain amplitude in surrogate specimens such as diamonds is estimated for a set of displacement boundary conditions. Fatigue testing is subsequently performed under these displacement boundary conditions to generate a strain-limit diagram, which is a plot of survival under fatigue loading vs. a fatigue indicator parameter such as the imposed strain amplitude. We have recently demonstrated that the phase transformation volume amplitude (PTVA) — an alternate fatigue indicator parameter for NiTi — provides a robust indication of fatigue life. In this work, we demonstrate a digital image correlation (DIC) based experimental method to directly quantify PTVA. We also show that there is substantial specimen-to-specimen variation in the measured PTVA values on diamond-type specimen. We build fatigue failure probability maps for NiTi using this method. Finally we demonstrate that PTVA allows connecting the fatigue fracture probability for a given expected cycle life to the statistics of impurity inclusion content in the NiTi material. This approach opens a new paradigm for directly measuring fatigue indicator parameters in NiTi device surrogate samples and estimating fatigue fracture probability based on such measurements.