A Method for the design of Shape Memory alloy spring actuators

Today, Shape Memory Alloys based on Nickel Titanium are mostly known for applications in the medical field, using the superelastic behaviour, good biocompatibility and excellent mechanical and functional properties of NiTi. For actuator applications, two geometries are predominant, tensile wires and compression spring actuators, which are both used in automotive applications.

While the geometric diversity of NiTi actuators expands, spring actuators become a key component in technical SMA systems. NiTi spring actuators and its ternary and quaternary derivatives allow high displacements, but in return reduce the possible actuator force. Regarding the current state of the art in literature and publications, there are suggestions for determining the SMA spring properties and geometries which, however, clearly contradict experimental results.

Publications in this area often suggest characterizing an austenitic NiTi spring like a conventional steel spring, but the reality shows a much more complex behaviour of these components. Characteristics of SMA in general, like functional fatigue, Clausius-clapeyron dependency of the phase transformation temperatures, and different mechanical poperties of Martensite and Austenite are completely neglected. Although this observation shows interesting results for specific spring actuators, the exact composition of the alloy does seem to affect the force-displacement characteristics under the same conditions as well.

One way to understand the actual properties is to contrast the key differences and use them to develop a method that allows the design of components closer to real-world behaviour. In this work, we present our findings for this new design method for this growing field of applications of SMA.