Hysteresis testing of Nitinol wires

Shape memory alloy (SMA0 wires are one class of materials being proposed as actuator elements in smart structures. They undergo a reversible phase transformation from a martensitic structure to an austenitic structure as the temperature is increased, and the material properties change as a function of the martensitic fraction. If the wires are properly conditioned through a training process, large reversible strains can also be realized through this transformation. Nitinol wires composed of 55% Nickel and 45% Titanium were trained and tested to determine the transformation temperatures. A two way shape memory (TWSM) was trained into the wires with varying initial strain and number of training cycles. The wires were placed in a cryogenically cooled environment and heated by electrical resistance while the deformation was measured. An indication of martensitic fraction is the recovery strain, and graphs of this strain vs. temperature reveal four distinct transformation temperatures that characterize the material. The effect of initial strain, number of training cycles, stability for extended cycling, and low levels of stress are presented.