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Shape Memory Alloys



Shape Memory Alloys
How much "expansion" can
I expect from SMAs?
SMAs do not operate based on volumetric expansion as in paraffin or wax type actuators. Rather Shape Memory Alloys are able to recover from "deformation" by heating from one state to another. The mode of deformation and subsequent recovery can be in all directions (i.e. tensions, compression, bending, twisting, or any combination thereof). Recoverable strains for binary Nitinol is in the order of 4% where single crystal derivatives can exceed 10%.
Is the recovery of SMAs linear with temperature? No, unlike bi-metallic actuators or devices which operate based on material CTE (Coefficient of Thermal Expansion) SMAs recovery from one state to another occurs over a narrow temperature range (approximately 10C). This is analogous to water boiling when it gets to 100C (i.e. there is a "state change" which occurs relatively abruptly when the material hits a critical temperature).
How high can you make the actuation temperature of SMA elements? Titanium Nickel based alloys can generally go up to 100C. Copper based alloys (such as our SCSMA) can go up to 200C.
Does the applied load or stress affect the transition temperature? Yes, the transition temperature can be raised with load or stress. The value is typically in the order of 1C/ksi. As an example, an element that has a free recovery transition temperature of 70C will exhibit an In-Situ actuation temperature of 100C if under a load of 30ksi.
What is meant by
Super Elasticity refers to the ability of Shape Memory Alloys to spring back after exposure to significant deformation. For SCSMAs this can be 9% strain (or more) which is approximately 20 times greater than normal metals.

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