Fatigue Analysis of Nitinol Stent

Strain-based total life approach has been proven as the most effective fatigue assessment for Nitinol stent and similar small implant devices.  However, fundamental material knowledge on Nitinol fatigue and cyclic behaviors remains a research challenge lack of material forms suitable for traditional fatigue tests.  Alternative methods, such as displacement controlled fatigue tests on stent like specimens are therefore used to established fatigue safety boundaries.  Finite Element Analysis is used to convert these displacement controlled test data into strain-life curves to extend the test results as strain based fatigue safety boundaries for fatigue analysis.

However, nonlinearity and hysteresis of Nitinol indicate that the fatigue strains of a Nitinol stent are path-dependent.  Despite this phenomenon was well-known, there are lack of systematic researches that quantify the material behavior as loading history.  The current material model simulates a few cycles of the material behavior very well, but it fails to cover the change of material properties as loading history lack of testing inputs.

The simple solution to the complicated material response that is essential to fatigue computation, though it was not clearly stated in majority of the publication, was to use the loading path of the stress-strain curve in fatigue strain computations and subsequent fatigue analysis [1].  This treatment provides unique “computed fatigue strains”, and has been proven to be very successful in predicting the stent and stent like device fatigue life [2].  Yet these “computed fatigue strains” are not the strains that one usually measures during the fatigue tests as those real fatigue strains vary with loading history and cycles.