Long-term behaviour of binary Ni-Ti SMA wires

One of the main obstacles for broader utilization of SMA based actuators in industrial applications is the missing knowledge about the long-term behaviour of the SMA wires. In this work the aim is to study systematically the long-term behaviour and fatigue endurance of the binary Ni-Ti SMA wires. Study concentrates on the effects of three major design parameters on the long-term behaviour of binary Ni-Ti-based actuators: the effect of the stress level used, the effect of the temperature interval used on thermal cycling and the effect of the heat treatment state of the wire used. The long-term behaviour of the wires was studied in a custom-built fatigue test frame in which the wires were thermally cycled. The fatigue life and the evolution of transformation and plastic strains were carefully examined for the tested wires. Also the micro-scale changes and damage patterns of fractured materials were investigated. One of the major conclusions of the study is that designing and controlling long-term behaviour of binary Ni-Ti actuators is very challenging because the properties are highly sensitive to the heat-treatment state of the wires. A major effect on fatigue endurance has the temperature interval used for thermal cycling: decreasing the temperature interval used increased the fatigue life markedly. The stress level and heat-treatment state used had a marked effect on the actuator properties of the wires, but the effects on fatigue endurance were minor. The fatigue test results reveal that the amount of plastic strain can be stated as one of the failure criteria for binary Ni-Ti wires, but that every heat-treatment state and loading level should be treated and presented separately. Fractographic analysis performed with SEM reveal similar damage patterns for fractured wires. Surface cracking and subsequent crack growth proved to be responsible for the accumulation of fatigue damage in the studied wires. The accumulated plastic strain seems to be the major factor for nucleating the surface cracking.