The Effect of Maximum Incremental Forming Strain on the Fatigue Properties of Nitinol Wire

Many devices manufactured from Nitinol wire, tube, and sheet are formed and shape set with very high overall strains.  These cumulative forming strains can reach 40% or more for a finished device.  An incremental forming strain as high as 40% will likely cause cracking and failure of the device.  In practice the devices are formed and heat treated in stages of lesser strains.  This goal of this research is to determine if the maximum incremental forming strain between heat treat cycles has an effect on the fatigue life of Nitinol wire.  A commercial Nitinol wire was formed at room temperature to a maximum strain of 20% and then back to straight.  Increments of 5%, 10%, and 20% strain were evaluated.  Each group of samples was given the same number of heat treat cycles. An additional group of wires was cooled to below its martensitic transformation temperature before forming to 20% strain to determine if cooling before forming has an effect on fatigue life.   All samples were fatigue tested using a zero mean strain machined block rotating beam method to generate Ɛ-N curves.  SEM analysis and metallography was also used to evaluate the samples with different maximum incremental forming strains.