Nitinol Fractography

Fracture in nitinol, particularly that resulting from cyclic loading (fatigue), has been well studied in the literature in part due to its many applications in medical devices. Relatively fewer examples of overload fractures (and transitions from fatigue to overload) exist in the literature for both laboratory-tested samples as well as commercially available devices retrieved from patients. As a result, the characteristic features of differing fracture modes have been well documented for nitinol and a sizeable library of fractographic images can be consulted as part of a failure analysis investigation.

In this presentation, a few examples of nitinol fractures are presented, including those exhibiting features of a nitinol-specific mechanism known as compression cracking that occurs as a result of nitinol’s tension-compression asymmetry. Fractures that occur as a result of this mechanism initiate on the intrados of a bend and typically exhibit a smooth surface before transitioning to microvoid coalescence (MVC) over most of the fracture surface.