Localization of Deformation During Pseudo Elastic Cycling of Ultra Fine Grained NiTi Shape Memory Wires (bibtex)

by M. Wagner, J. Frenzel, G. Eggeler

@InProceedings{Wagnera,
  Title                    = {{Localization of Deformation During Pseudo Elastic Cycling of Ultra Fine Grained {NiTi} Shape Memory Wires}},
  Author                   = {Wagner, M. and Frenzel, J. and Eggeler, G.},
  Booktitle                = {Proc. of FATIGUE 2006},
  Year                     = {2006},

  Address                  = {Atlanta, USA},
  Pages                    = {FT196 (auf CD)},

  Abstract                 = {The present study considers cyclic loading of pseudo elastic NiTi shape memory wires, where L\"{u}ders-like deformation may readily be observed under uniaxial tension: The stress-induced martensitic transformation is concentrated in shear bands, and the deformation of the material in strain-controlled testing proceeds via the growth of one or several shear bands. We discuss the formation and propagation of these shear bands during pseudo elastic cyclic loading, and we characterize the fatigue behavior by mechanical testing and by optical microscopy. Our results demonstrate how a localization of the martensitic transformation can simultaneously lead to a localization of functional fatigue. We present a microstructural scenario on three length scales which can account for several observations during mechanical testing of NiTi wires, and, most importantly, rationalize the evolution of pseudo elastic stress-strain curves during complex cyclic loading. Functional fatigue during tensile pseudo elastic cycling of ultra fine grained NiTi shape memory wires was studied experimentally. It was demonstrated that localization of transformation and deformation leads to a localization of functional degradation. The functional fatigue behavior was discussed on three relevant length scales (macroscale: stress strain data; mesoscale: localization of deformation inside the gage length; microstructure: generation of dislocations and stabilization of martensite) in order to rationalize the effect of cyclic loading with constant or variable strain amplitude. It was shown in particular that different parts of the material within the gage length may be subjected to a locally differing number of transformation cycles N\textsubscript{L}. The mechanical properties of these different sections may vary considerably. The formation of multiple plateaus in the stress strain curves after complex cycling with increasing strain amplitude is closely related to the growth of the transformation bands through regions with different N\textsubscript{L}.}
}