A phenomenological model for functional fatigue of superelastic-plastic Nitinol

Mechanistic models for the simulation of the mechanical behavior of Nitinol have been around for 30 years now. In this time researchers have continued to uncover more intricate characteristics of that behavior, including functional fatigue. A new model has been created which tries to address some of these developments, focused on the superelastic-plastic regime common in medical devices. Experiments show that the behavior in compression is different from tension, with non-trivial correlation between the two. The model allows for two different stress-strain curves in tension and compression. The end of the upper plateau does not correspond to 100% martensite, rather a ‘saturated’ value. Then transformation continues along a relatively straight line, until plasticity initiates. This is responsible for different loading and unloading slopes after the end of the plateau. Accumulated martensite transformation, either through single large load, or repeated lower loads, is responsible for two other phenomena. One is the development of residual martensite, meaning martensite that does not transform back into austenite upon unloading. The other is the lowering of the plateaus (or equivalently, increases in transformation temperatures). Due to the complexity of these two phenomena, the model allows the user to control their modeling based on his/her experimental data and loading conditions. It is possible that the lower plateau becomes ‘negative’, either due to cycling, or lowering the temperature. The model allows for such behavior in which the material would become purely martensitic.