Thermomechanical characterization of plastic deformation and transformation fatigue in shape memory alloys

Presented in this research is the thermomechanical characterization of plastic deformation and thermal transformation fatigue in NiTi based Shape Memory Alloys (SMAs). Experiments are performed on specimens of 50at%Ni-50at%Ti and 50at%Ni-40at%Ti-10at%Cu. First, the effect of a non-proportional loading on the thermomechanical response of a NiTi specimen without the effects of plastic deformation is presented. This is followed by the effect of plastic deformation on the transformation properties of NiTiCu and NiTi specimens through four different loading paths. Pure austenitic plastic deformation, pure martensitic plastic deformation, stress-induced martensitic plastic deformation, and plastic deformation through thermally induced phase transformations are performed. The third topic presented in this research is thermomechanical tests performed on fully annealed NiTi SMA wire specimens of identical chemical compositions, which were cold rolled and then annealed. The fourth topic discusses the selection of material constants in a thermomechanical constitutive model for polycrystalline SMAs and a parametric study to determine the influence of each material parameter on the material response is presented. Finally, results are presented on the thermal transformation fatigue of NiTiCu SMAs. The difficulties associated with performing such a test are discussed as well as the novel test frame developed to perform these tests. A testing protocol for the identification of the SMA thermal transformation fatigue is identified, and thermal transformation fatigue results are presented showing the effect of the applied stress on the thermal transformation fatigue life. Additionally, the effect of the material processing, specifically the heat treatment temperature, on the thermal transformation fatigue life is identified.