INNOVATIVE MATERIALS 1 2020 Figure 6 shows how this happens as a function of temperature. At low temperature 100% of the volume is in the Martensite phase. If the sample is heated up, you can observe that at a temperature ‘As’ the austenite phase formation starts. (‘As’ stands for the Austenite start). At the temperature Af, Austenite final, the materials are fully transformed to the austenite form. If it now cools down, you can see the start of formation of Martensite at Ms. This in complete at M final. So there are no sharp transitions of one phase to the other, but there is a big hysteresis effect. For SMA actuators there are two working principles. The first is the single way actuation where the deformation takes place below the Martensite final temperature. (Figure 7, at the top.) If it’s heated above the Austenite final temperature it will return to its original shape and stays there also upon cooling. For the so-called two way actuation, the change in shape takes place when the part is heated above the Austenite final temperature. This has been programmed in the material by special thermomechanical treatments - the so called training of the SMA. With both principles it is possible to make actuators.
Figure 6
If an actuator is made of a trained spring, forces up to about 20 N can be obtained. Figure 7
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Figure 8
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