Consideration of the Significance of Cold Work for Fatigue of Nitinol Medical Implants

The purpose of this study is to investigate the significance of the cold work on the fatigue behavior of Nitinol under loading conditions that are equivalent for self-expanding medical devices. For the present review, Nitinol wire was processed to a final dimension (0.52 mm) with 30, 40 and 50% cold work conditions. The wires were all shape set at 500˚C in two steps for a total of six minutes to achieve an "S" shape in accord with earlier investigations (Pike, et al. 2011, Launey, et al. 2014) and then electropolished to 0.5 mm. The transformation temperature of the wires was evaluated with DSC and the A_f temperatures from all three conditions was 15˚C. However, the martensitic transformation temperatures decreased with increasing CW. Furthermore, uniaxial tensile testing demonstrated that the there was a systematic increase in the initial modulus, upper plateau stress as well as the ultimate tensile stress and decrease in total elongation with increasing CW. This article will discuss the fatigue response for different cold work conditions due to combination of 6% pre-strain, 3% mean strain and various strain amplitudes to 10M fatigue cycles.