A finite element study on localized deformation and functional fatigue in pseudoelastic NiTi strips and plates

Finite element simulations provide realistic models of transformation and deformation in complex pseudoelastic NiTi specimens, parts and devices. Most current approaches, however, do not allow simulation of localization phenomena which are frequently observed in NiTi: When thin NiTi specimens are subjected to uniaxial tension, they deform by the formation and propagation of distinct transformation bands. Within these bands, the stress-induced martensitic transformation is complete, whereas in the adjacent regions, the material is still almost fully austenitic. A similarly striking phenomenon is functional fatigue, where repeated cycling through the pseudoelastic hysteresis results in considerably decreasing plateau stresses, and in increasing residual strains. When localization of transformation and functional fatigue occur simultaneously, only the transforming regions of a specimen are prone to fatigue degradation, whereas the untransformed material remains in an unaltered microstructural state. And while it has been clearly recognised that fatigue generally limits the service life of NiTi devices, there are at present no finite element models that reflect both localization and functional fatigue. In this contribution, we present finite element simulations on localized transformation and functional fatigue in pseudoelastic strips and plate specimens subjected to cyclic loading. The constitutive modeling is based on a total transformation strain model developed by Azadi and co-workers, which we extend to also reflect decreasing plateau stresses associated with functional fatigue. We demonstrate how localization of transformation/functional fatigue results in characteristic changes in the macroscopic stress-strain behavior of uniaxially loaded specimens, such as multiple plateaus during cycling with increasing strain amplitudes, and we relate these changes to the movement of individual transformation bands. Moreover, we study how multiaxial stress states near holes or notches affect fatigue degradation in the presence of localization phenomena. Finally, we discuss the consequences of localized fatigue evolution for service lives of geometrically more complex NiTi parts and devices.