Superelastic Applications of Nitinol Alloys

While most of the attention that nitinol has received from both researchers and press alike is in regards to the shape memory and superelastic phenemona. However, possibly the greatest commercial applications of nitinol lie outside these phenomena.

Vibration Reduction:Vibration kills products and all products with moving parts experience vibration.  Generally speaking, vibration is mitigated by more accurately producing the parts.  However, vibration damping materials are sometimes used.  Nearly all vibration damping materials are also soft, meaning that while they are excellent at eliminating vibrations, they cannot be used to provide structural support.

Not only does nitinol provide good structural support, the damping characteristics are excellent.  When nitinol vibrates, the crystal lattice detwins—meaning that the twinned bonds mentioned in Chapter 2 rotate back and forth.  This motion creates an internal friction, which converts the motion to heat, which can then be removed.  Since nitinol generally has a natural frequency in the region of 5-15Hz, it can be used to eliminate audible vibrations (20-20,000Hz) as well as low frequency (<1Hz) vibrations quite effectively.  Since nitinol is highly tunable, the damping characteristics can be tailored to the specific application.

One retrofit option that is frequently implemented at Kellogg’s Research Labs is to replace steel washers with nitinol washers.  This simple retrofit reduces wear and tear by up to 90% with little to no engineering work required.  Unfortunately, the vibration mitigation phenomenon of nitinol is still relatively poorly understood, so a more sophisticated vibration reduction project may require significant work.

Shock Mitigation: Sudden impact or mechanical shock can cause a product to fail catastrophically.  For buildings, this may come from an earthquake. For cars, this may come from hitting a pothole.  Whatever the source is, shock is catastrophic to products.  Of course, there are certain shock mitigation technologies already available.  For example, cars have shock absorbers.

By properly implementing nitinol shock mitigation elements, products can survive dramatically higher impacts.  For example, Kellogg’s Research Labs, along with various other researchers, have embedded nitinol into structures in a laboratory environment.  Without the nitinol, the structure failed (broke) at 7.6 magnitude on the Richter scale.  However, after nitinol had been implemented, it survived 9.2 magnitude on the Richter scale!

While the shock mitigation properties of nitinol are far from well understood, some things are known.  While both superelastic and shape memory nitinol are excellent at shock mitigation, shape memory is better as long as you have a way to heat the nitinol to restore it to its original shape.

Wear Reduction: Wear reduction by itself doesn’t really justify the expense of nitinol.  However, applications where wear reduction where the vibration reduction and shock mitigation properties are also beneficial is the real gold mine.  For example: snow plow blades.  The road quickly wears out the cutting edge of snow plow blades, so that they must be replaced frequently.  Many highway departments use carbide edges because they wear much more slowly. However, if anything impacts the carbide, they quickly chip or crack, rendering the blade useless.  A nitinol edge would have a wear resistance similar to the carbide edge but with the ability to take impacts.

 

How will these properties affect your product design process?

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