Fatigue of Notched Superelastic Nitinol

A hybrid photomechanics technique employing moire interferometry and digital image correlation is used to study fatigue crack initiation in superelastic Nitinol. Surface images from the interferometer are digitally correlated and used to enhance the quantification of strain gradients at sharp bimaterial interfaces. Phase shifted moire interferometry is used to calculate strains along these boundaries at the minimum and maximum loading conditions of the fatigue cycle. Samples were machined from seamless drawn superelastic Nitinol tubing into notched four-point bend coupon samples and processed according to two thermochemical process histories.  Prior to testing they were subjected to a range of loading histories following a multi-level amplitude test protocol tailored for medical implant applications.  The novel empirical approach is used to explore the evolution of local driving forces preceding fatigue crack initiation for two distinct material process histories.