Improvements to the Fatigue Life of Shape Memory Alloys using Partial Transformation Cycles Based on Energy Output per Cycle

Shape memory alloys (SMAs) are useful materials for actuators due to their high energy density. However, functional degradation is an important drawback of SMAs. Damage is created during the transformation process that inhibits the transformation in subsequent cycles. Because of this process the functional life of SMAs is finite and should be considered during the design of an SMA actuated device. Improving the functional life of SMAs will allow broader applicability.

Since the material transforms over a temperature range, the energy differential increases during the transformation. At the end of the transformation the higher energy differential will cause more damage than at the beginning of the transformation. Using partial transformation cycles instead of full transformation cycles eliminates the most damaging part of the transformation and allows the functional life to be extended significantly.

SMA wires are loaded by a dead weight and cycled at different degrees of transformation. By varying the weight, the work done per cycle is kept the same to allow a fair comparison among different specimen. It is shown that despite the same amount of work done, functional degradation can be significantly mitigated by employing partial transformation cycles.