Despite our best efforts, all things wear out eventually. The same is true for the shape memory effect. Each cycle is not quite 100% recovered, and eventually the effect disappears altogether. So, what causes shape memory fatigue?
The way shape memory fatigue works is similar but different from how mechanical fatigue works. In mechanical fatigue, first the atoms slip, causing the chemical bonds to be broken and re-formed. After sufficient slip, micro fractures begin to form. Micro fractures are typically less than 1mm long and 0.001mm wide. After there are enough micro fractures, they begin to connect to each other, forming large scale fractures, eventually leading to the catastrophic failure (breaking) of the part.
Shape memory fatigue occurs well before microfractures start to become a problem. Simply, enough slip occurs to dislocate the shape memory effect.
Fatigue Life:When designing with shape memory materials, one of the first questions that need to be asked is, “How many cycles does this need to last?” Some applications may only need to cycle once in their entire service life, while many applications have a reasonable fatigue life requirement of 10,000 cycles. In energy applications, 10 million or more cycles are expected.
As a general rule of thumb, NiTi can recover 12% deformation once—great if you have an application that needs to work once! 5% deformation will give you a cycle life of approximately 10,000 cycles and 3% deformation will give you a fatigue life of ~10 million cycles. Adding aluminum, copper, or magnesium to the matrix will improve the fatigue life, but will generally limit recoverable deformation to 3%.
There is some bad news: drift. After each cycle, the SMA loses some of its effect. In NiTi, 50% of the shape memory effect is lost after just 10% of fatigue life. Adding copper to the matrix stabilizes drift, losing just 5% of the shape memory effect at 50% of fatigue life, with the trade-off of much smaller deformations.
If you are only interested in very high cycle life and can live with 2.5-3% recovery, adding 5% copper is an excellent option. Conservative researchers are reporting 10 million cycles to failure while some researchers are reporting fatigue life in excess of 1 billion cycles!
All of these deformation numbers may seem small, but it is important to compare them to other materials. For example, 316 medical grade stainless steel fractures at 0.5% deformation. So, the fact that nitinol can recover 10x that 10,000 times is very impressive! Other than nitinol, only plastics can withstand such deformation.
Now for some good news. Nitinol can be rehabilitated. Did you exceed the fatigue life on your nitinol during your development process? Simply set the memory shape again and the bonds will re-form, eliminating the effects of fatigue.