24-11-2012, 03:32 PM
SHAPE MEMORY ALLOYS
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Definition of a Shape Memory Alloy
Shape Memory Alloys (SMAs) are a class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature.
They can retain their properties at high temp.
Basic working principle
SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite.
the martensite can be in one of two forms: twinned and detwinned, as shown in Figure 1.
A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs.
Selection Criteria of SMA’S
Range
Sensitivity
Repeatability
Linearity and Accuracy
Impedance
Nonlinearities
Static and Coulomb Friction
Frequency Response
Ni-Ti Alloy
NITINOL means Nickel Titanium Naval Ordnance Laboratory and it is discovered in 1965.
Its composition is about 56% Nickel and 44% Titanium.
Nitinol “remembers” its original shape and springs back up to temperatures up to 500 degrees C. Can be strained 8 to 10 times more than spring steel without permanent deformation.