Shape memory alloy properties
Figure: Shape memory alloy hysteresis loop
This curve presents a wide hysteresis loop. Its width varies with the alloy system. For NiTi alloys, the temperature hysteresis is usually between 30 and 50 degreeC.
One-Way Shape Memory Effect
After deformation, the one-way shape memory effect allows a material to return to its original shape by simply increasing the temperature.
Two-Way Shape Memory Effect
As in the one-way shape memory, the two-way shape memory effect allows the material to return to its original shape by simply increasing the temperature. In addition, it also permits the return of a second shape by cooling.
Sometimes called superelasticity, is an elastic (reversible) response to an applied stress. Unlike the shape memory effect, pseudoelasticity occurs without a change in temperature.
Shape setting in Shape Memory alloys
The Shape Memory Effect is programmed into the shape memory alloy with an appropriate thermal procedure. First the alloy is formed into desired austenite form which is needed to be remembered and heated above a specific temperature. The temperature and the duration of the heating depend on the alloy and the required properties. For a NiTi alloy temperature above 400 degC and heating duration of one to two minutes can be sufficient, but in practice temperature above 500 degC and over 5 minutes time interval is used. Higher heat treatment times and temperatures will increase the actuation temperature of the element and often give a sharper thermal response, but may reduce the maximum output force.
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