Tension-Tension Fatigue of Physical Vapor Deposited Superelastic Nitinol

Over recent years, superelastic Nitinol tubing and wire manufactures have strived to reduce the size and distribution of non-metallic inclusions in their material to increase material fatigue limits for biomedical applications. Most techniques to reduce the size and distribution of inclusions have focused primarily on ingot starting material purities and ingot process optimization. Advances in physical vapor deposition (PVD) of superelastic Nitinol have allowed for an alternative manufacturing method for creating high purity raw tubing material. PVD superelastic Nitinol tubing offers nanometer sizescale inclusions compared to the micron size “stringer” and particle void assemblies (PVAs) often found in wrought drawn standard or high purity grade Nitinol. Therefore, the aim of this study was to quantify the fatigue performance of PVD Nitinol material compared wrought and drawn commercial and high purity varieties

Tensile dogbone specimens were laser cut from as deposited and heat-treated PVD Nitinol tubing measuring approximately 5 mm in diameter and 110 µm in wall-thickness. Specimens were manufactured to have a gauge length of 10 mm and gauge width of 250 µm. Specimens were tension-tension fatigue tested on a Bose ElectroForce 3330 load frame in 37°C PBS at 60 Hz. Specimens were first loaded to a tensile mean strain of 2.5% and then cycled at five to six strain amplitudes in displacement-controlled fatigue to a run-out of 10-million cycles. A minimum of four specimens were tested per amplitude to generate S/N curves for the materials.