Thermomechanical behavior of 55Ni45Ti nitinol

Extensive and full range mechanical testing of 55Ni45Ti nitinol alloy wire up to failure over a wide range of testing temperatures is reported. Thermomechanical properties are obtained including: initial modulus, secondary modulus, critical martensitic start and finish stresses, plastic flow stress, recovery strain limit, failure stress and plastic modulus. The data is correlated with the Brinson constitutive model modified to account for full range loading. The ratio of austenite to martensite moduli was found to be 5.8 and the secondary modulus (stress-induced martensite) was found to be smaller by a factor of two when compared to the modulus of (stress-free) thermally induced martensite. The influence of the initial temperature treatment of the nitinol was shown to be pronounced within the transformation range. Reasonably accurate correlation is obtained with the modified Brinson constitutive model although there is some discrepancy in critical stress predictions which is partially attributed to experimental scatter.