At Kellogg’s Research Labs, we often get people asking for the Youngs modulus of nitinol. Often the question is asked why we don’t publish the Youngs modulus of our nitinol. There are two answers and I’ll take some time to answer each one individually.
- Nitinol is functionally non-linear. Most materials follow Hooke’s Law, meaning that force increases proportionate to the displacement. This is represented by the equation . For most materials, k is a constant related to the Youngs modulus. Nitinol, however, does not follow this linear relationship. Yes, there is an area of the stress/strain curve that is linear elastic, but almost no applications fall in this region. This means that, in order to adequately model nitinol, k is not a constant but an equation.
- The shape of the stress/strain curve is unique to every preparation method. This means that for each heat treatment profile, each fixture design, each surface treatment method, etc. the stress/strain curve is unique. For this reason, we only measure stress/strain curves after the component is fully manufactured.
So, how do I go about setting up my simulation software in order to design my nitinol component? First, you need to build a material definition function (MDF). In order to have a reasonably accurate model, you should include at least four independent variables in the MDF. Some suggestions include
- Strain rate
- Number of cycles
- Grain orientation
- Thermal history
This is not a complete list and some of the variables will not be important to your application.