Using Electrical Resistance For Control Of Partial Transformations Of Smas Based On Percolation Theory

The shape memory effect possessed by shape memory alloys makes this group of materials suitable for actuation purposes. A remaining challenge for the application of these smart materials for actuators is the limited functional life. It has been proposed by others that partial transformation cycles can mitigate damage and thus extend the functional life. However, when controlling partial transformation cycles, issues arise due to shifting transformation temperatures and required energy.

Utilising the difference in electrical resistivity between the martensitic and austenitic structures allows for accurate control of the extent of transformation. However, because a connected path of austenitic grains severely lowers the resistance, a correction factor has to be imposed. Since the resistance is severely lowered by connected paths of austenite, the electrical resistance of a shape memory alloy reaches its lowest value before full transformation is achieved. The electrical resistance corresponding to a degree of transformation should therefore be predicted in agreement with the percolation theory. With accurate measurement of both current and voltage, the resistivity can then be monitored and used as a control parameter.

It is shown that resistivity can be used for consistently controlling the extent of transformation. Because the percolation theory is highly dependent on the grain structure, some scatter in the results can be expected among different samples. However, for a single sample the method shows good repetitiveness when subjected to cyclic loading.