Macroscopic Thermomechanical Model Suitable for Simulations of Anisotropic NiTi Shape Memory Alloys with R-Phase

Due to unique mechanical properties (e.g. superelasticity) and excellent biocompatibility, NiTi shape memory alloys are utilized in medical applications as surgical tools or reinforcements for arteries and veins (stents). The components are usually made from polycrystals, which are frequently strongly textured, thus anisotropic, and in which R-phase tranformation often occurs. Such phenomena influence the response of the material through modification of elastic and tranformation properties even in superelastic loading mode, and should be taken into account in realistic simulations of these components.
This motivated development of a rate-independent three-dimensional macroscopic model for NiTi shape memory alloys, which is capable to capture complex material behavior when multiple loading modes are involved, R-phase transition is present and highly anisotropic behavior is exhibited [1]. The model was formulated within the thermomechanical framework of generalized standard materials by explicit formulation of energy and dissipation functions. Phase transformation and evolution of martensite structure is effectively captured by introduction of two dissipative internal variables. An additional, non-dissipative internal variable represents volume fraction of R-phase. Anisotropic material behavior and tension-compression asymmetry can be easily adjusted by a particular choice of input material parameters. The model was successfully tested through simulations of a NiTi textured wire subjected to complex thermal and mechanical loadings. Simulations presented in this talk will also show high influence of the aforementioned phenomena on the performance of NiTi components in general loading modes.

[1] Sedlak, P., Frost, M., Benesova, B., Sittner, P., Ben Zineb, T. (2012). Int. J. Plast. in press.