Nitinol belongs to a class of materials called Shaped Memory Alloys (SMA). SMA's have interesting mechanical properties.
Nitinol for example, contracts when heated, which is the opposite of what standard metals do when heated which is expand.
Not only does this alloy contract, but does so with a 100 times greater thermal movement (expansion-contraction) than standard metals.
This nitinol metal changes phase around 50oC.
Bend it, then drop it into hot water and watch it return to its original shape!
Can be set into different shapes by heating with a candle flame.
The wire may be contracted up to 8%-10% of its length.
However, for longer lifetime (greater than 1,000,000 cycles), you should restrict the contraction to only 5-6% of its length.
While the length of the wire may change, the absolute volume of nitinol material of the wire remains constant:
Contraction and relaxation depend solely on the temperature of the nitinol alloy wire.
Any method of heating and cooling the wire may be used.
Nitinol wire has a high electrical resistance, approximately 1.25 ohms resistance per inch for the 6-mil wire.
The resistance of the wire to the electric current quickly generates sufficient heat (ohmic heating) to bring the wire through its transition temperature.
So many times an electric current is passed through nitinol wire to heat the wire electrically.
When the material is allowed to cool, the wire can easily be stretched back it its original length.
Nitinol wire usually has a counter-force applied to it in the opposite direction of its contraction.
The counter force resets, or stretches the wire back to its original length when in the low temperature phase.
This is called the bias force.
If the nitinol wire is brought to its transition temperature without a bias force it will contract, however, when it cools it will not return to its original length.