In recent years, one of the most promising new actuator technologies is based on the use of Shape Memory Alloys (SMA). The main challenge for the application of devices that use these materials is the hysteresis in the phase transition they suffer during actuation.

Finite element method (FEM) analysis is an important aid in the simulation of mechanical properties and thermal fields in actuators. Dynamic simulations give in many cases enough information without the necessity of building a prototype. We have used ANSYS and its FEM material module to simulate a Nitinol wire used in micropositioning actuator. The accurate model parameters, not given by the supplier but required by the FEM analysis program, have been obtained by thermal and mechanical characterization of the material used.

Phase transition temperatures for wire specimens have been examined by differential scanning calorimetry (DSC) over a temperature range from -30ºC to 180ºC, with a heating rate of 5ºC per minute. These results have been corroborated by electrical resistivity measurements in a climatic chamber (Angelantoni Industrie Challenge 250). Resistivity has been characterized in the range of 25ºC to 130ºC. Cyclic deformation of the Nitinol wire above the austenite finish temperature has been measured in a SHIMADZU AGS-5 mechanical testing machine.

When the SMA is resistively heated by an electric current applied to the wire the phase transition is induced and results in a contraction of the wire length. When the material cools off, a load applied to the wire stretches it back to the original left. The hysteresis of the system has been measured under different applied loads. These measured hysteresis loops can be compared to the loops obtained from the FEM simulations.

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2. Micropositioning using shape memory alloy actuators
3. Neutron study of the martensitic transformation in Ni-Fe-Ga alloys