Impact of Hydrogen on the Fatigue Behavior of Nitinol Wire and Diamond Shape Samples

For the production of permanent medical devices made from Nitinol, such as stents, filters or heart valve frames, surface-finishing processes like electropolishing or chemical passivation are commonly performed in hydrogen-containing media. Under certain conditions Nitinol is prone to hydrogen uptake, which can lead to embrittlement as a result of hydride formation. According to ASTM F2063-12 the hydrogen content in wrought Nitinol is limited to 50 wppm. Previous studies have shown that hydrogen reduces the low-cycle fatigue resistance of Nitinol in zero-mean strain wire fatigue tests.

Nitinol cardiovascular implants like stents typically operate at a certain mean strain level while implanted in the human body. Thus, it is of particular interest to study effects of mean strain on fatigue behavior of hydrogen-loaded samples.

In this study, the zero-mean strain fatigue behavior of Nitinol wires, hydrogen-loaded by cathodic charging, is compared to the fatigue behavior of hydrogen-loaded Nitinol diamond shape samples processed from Nitinol tubing, which are subjected to fatigue testing at non-zero mean strain conditions. Strain amplitudes are covering both low and high cycle fatigue behavior. For the diamond shape testing mean strains range from 1 % to 5 %. All samples are tested in electropolished surface condition.