B. Maass, J. Frenzel, T. Depka, Ruhr University Bochum, Bochum, Germany; G. Eggeler, Ruhr-University Bochum, Bochum, Germany
In actuator applications, shape memory wires are often used as helical coil springs. In the present work, we investigate the thermomechanical behavior of NiTiCu spring actuators in order to understand their functional fatigue characteristics. NiTiCu springs were produced by induction melting, swaging, wire drawing and shape setting treatments. The spring actuators were tested in an automated test rig in which the springs are forced to contract by electric current heating and a reverse stroke is provided by a counter weight arrangement on cooling. We show that the actuators receive an irreversible extension mainly caused by dislocation plasticity and changes in transformation temperatures. In the present work, we investigate how these effects depend on testing parameters. We concentrate on the influence of (1) actuator load, (2) mixed load scenarios (increasing / decreasing loads after certain cycle numbers) and (3) displacement limits. It was found that a higher load is associated with a faster degradation of the functional properties while displacement limits can partially improve actuation stability. We provide phenomenological equations which account for the effect of different testing conditions on functional fatigue.
Summary: In the present work, we investigate the thermomechanical behavior of NiTiCu spring actuators in order to understand their functional fatigue characteristics. The spring actuators were tested in an automated test rig in which the springs are forced to contract by electric current heating and a reverse stroke is provided by a counter weight arrangement on cooling. We show that the actuators receive an irreversible extension mainly caused by dislocation plasticity and changes in transformation temperatures.
