Although the tensile behavior of shape memory alloys (SMAs) is relatively well understood, the effects of other modes of loading on SMA behavior are still unknown. To examine the influence of loading and specimen geometry on structural behavior and phase transformation morphology, this work presents an experimental characterization of commercially-available superelastic NiTi rods and tubes under tensile, compressive, pure bending and column buckling loading. At room temperature and under isothermal loading conditions, the mechanical response for each loading configuration and structural geometry is measured and stereo digital image correlation is used to capture local displacement and strain phenomena (such as strain localization and propagating phase boundaries).
It is found that the length and cross-sectional geometry of NiTi structures affects the nature of stress-induced phase transformation; in particular, transformation induced strain localization. During uniaxial tension, the rod specimens exhibit strain localization and propagating phase transformation fronts along well-defined load plateaus, and phase boundaries manifest as diffuse propagating necks. The tube specimens also exhibit localization/propagation phenomena, but fronts consist of distinct angled finger-like features due to the relatively thin-walled cross-section. During uniaxial compression, both forms exhibit (now well-known) tension-compression asymmetry in their mechanical responses, no propagating fronts are observed, and distinct load plateaus are absent. During pure bending, the moment-curvature responses of both forms exhibit plateaus during localizing forward and reverse transformations. In the case of the rod, high curvature localizes and propagates along the specimen; whereas, finger-like high strain regions develop along the tensile side of the tube. During column buckling, the structures are loaded into the post-buckling regime yet recover their straight forms upon unloading. Localization is observed, but only for high length/diameter aspect ratio tubes and has little affect on the mechanical response. Therefore, due to the varying phase transformation behaviors observed, understanding geometric and material effects on phase transformation, along with the resulting effects on the material response, is necessary to predict SMA beam behavior.