Nitinol Fatigue: What Do We Really Understand?

The field of Nitinol fatigue and durability is ripe with contributions from academia, government laboratories, and multiple industries. These contributions include results from benchtop testing, mechanistic modeling, and computational analyses. To date, however, there are no international standards that guide testing or analysis of a two-phase, transforming material, such as shape memory and superelastic Nitinol. Whereas the damage tolerant concept has gained traction with other engineering materials, the continuum mechanics used for linear elastic materials may not be applicable to understand durability of a Nitinol device. Consequently, a majority of tests in the Nitinol literature employ more conventional total-life approaches, although the chosen mean stress/strain-stress/strain amplitude may only be meaningful to demonstrate fatigue performance at a certain lifetime for a given material or product. Neither of these fatigue testing approaches address that the actual stress/strain state in a given region may vary dramatically due to crystallographic orientation, martensite variant selection, presence/absence of dislocations and proximity to oxide/carbide inclusions. Furthermore, specimen geometry selection may lead to results that conflict dramatically with other studies. As such, a coherent multi-scale model that incorporates microstructural effects to predict Nitinol fatigue is still lacking. Therefore, the researcher and practitioner are left to sort through the literature to determine what might be applicable for their particular need. This presentation will review the literature for best practices and will highlight the areas that require improvement in order to gain further insights into Nitinol durability.