Design Principles

Basic Theory

The shape memory effect is defined as the property of material that can change to temporary shapes and then recover their memorized shapes under external stimuli. Actuators are devices that perform a task, like moving an object, either on-demand or in response to certain changes in their environment (temperature, pressure, etc).

There are only two commercially available groups of shape memory alloys for actuator applications today, the Cu-Zn-Al alloys and the Ni-Ti alloys. For most of the applications, Ni-Ti is preferred because of a number of advantages like high strength, high electrical resistivity, large recovery strains, easy workability, and excellent corrosion resistance. Therefore, we will only be focusing on Ni-Ti alloys.

Design Consideration

The design of shape memory actuators (SMA) is generally based on the different stress-strain curves of the material in its austenitic and martensitic condition. They display two distinct crystal structures or phases. Martensite exists at lower temperatures, and austenite exists at higher temperatures. When an SMA is in martensite form at lower temperatures, the metal can easily be deformed into any shape. When the alloy is heated, it goes through the transformation from martensite to austenite. In the austenite phase, the memory metal “remembers” the shape it had before it was deformed.

Figure 1 shows the stress vs temperature graph. We can observe that the martensite state exists at low stress and low temperature and austenite state exists at higher stress and temperature. Austenite is not stable at room temperature, and because systems always seek lower energy states, the austenite will change back to the martensite phase. For example, eyeglass frames are in a martensite phase, bending the arms in half (at room temperature) introduces a phase change at the bend to austenite.

References

1. Zhang, Jun., et al., “Robotic Artificial Muscles: Current Progress and Future Perspectives,” IEEE Transaction on Robotics, pp. 1-21, 2019.

2. Swardz, Savanah. "How Shape Memory Alloys Work." Ceramic Engineering, University of Washington, depts.washington.edu/matseed/mse_resources/Webpage/Memory%20metals/how_shape_memory_alloys_work.htm.

3. Christopher, Brianne. "The Elephants of Materials Science: SMAs Never Forget Their Shape." COMSOL Multiphysics, 11 May 2018, www.comsol.com/blogs/the-elephants-of-materials-science-smas-never-forget-their-shape/.