Fatigue Performance of Nitinol Round Wire with Varying Cold Work Reductions

The role of cold work reduction and its effect on fatigue properties are examined by means of rotary beam fatigue testing. In this study, processing parameters from the initial ingot to the final size before finish drawing were held constant. Fatigue specimens were prepared with between 20 and 55% cold work in five percent increments. These samples were subsequently heat treated, cut into test sections at room temperature, and subjected to strain-controlled fatigue testing in a temperature-controlled RO water bath at a constant 10 C above the measured active austenitic finish temperature (Af). An empirical analysis of the relationship between final reduction prior to heat treatment and fatigue lifetime is presented. Under alternating strain amplitudes greater than 1%, the 20 to 30% cold work level was found to deliver the greatest mean cycles to failure. Supplemental to the fatigue data, mechanocyclic and thermomechanical data are also presented as they evolve with prior retained cold work.