Observations Relating to the Post-Transformation Straining of Nitinol Wires

The design of next generation stent-grafts is directed towards a very low-profile. Such devices are capable of treating narrow access vessels and tortuous anatomies while inducing less trauma to patients. Therefore, Nitinol stents are required to undergo greater deformations during manufacture without compromising the performance and durability of the device. The present work seeks to produce useful recommendations associated with the design of modern cardiovascular devices. The uniaxial high-strain deformation of superelastic Nitinol wires is examined to investigate the effect of compaction and sterilization on their properties. Two specimen types manufactured by Fort Wayne Metals are used, namely NiTi#1-SE and USN®. The former is a commonly used medical grade wire whereas the latter is recommended for applications that require greater stiffness in the modulus region upon loading. Specimens are subjected to three load-unload cycles up to 12% strain with increasing 0.5% strain increments. Loading to failure is conducted at temperatures up to 55°C. Results show that compaction beyond 10% strain is expected to have a significant impact on the stiffness and unload plateau stress of the material, especially if more than one loading cycles are applied. USN® specimens exhibit greater stiffness and unload plateau stresses during high-strain cycling. However, this material also experiences greater residual strains during unloading. Temperature sensitivity is not seen within the post-transformation region of Nitinol wires.