Evolution of localization in pseudoelastic NiTi tubes under biaxial stress states

• Biaxial experiments of radial stress paths with internal pressure and axial load.
• Inhomogeneous deformation except for small region around equibiaxial tension.
• Localization bands vary with stress state.
• Mild stress anisotropy, strong strain anisotropy.
• Transformation surface deviates from von Mises.

Nearly equiatomic NiTi can be strained in tension to several percent and fully recover upon unloading (pseudoelastic behavior). This property is derived from solid-state transformations between the austenitic (A) and martensitic (M) phases, which can be induced by either changes in temperature or stress. It is well established that stress-induced phase transformation leads to localized deformation associated with the nucleation and propagation of the M-phase during loading and the A-phase during unloading. It has been demonstrated that this partially unstable material behavior strongly influences the stability of NiTi structures, motivating the need for the development of constitutive models that can capture this instability. To support the calibration of such constitutive models, the behavior of 6.35 mm diameter pseudoelastic NiTi tubes loaded under combined axial load and internal pressure has been examined. Tubes were loaded under radial stress paths in the axial-hoop stress space using stereo digital image correlation for full-field monitoring of the evolution of transformation-induced deformation. Results from 26 experiments spanning axial-to-hoop stress ratios from −1.0 to uniaxial tension revealed that, but for a narrow region near equibiaxial tension, transformation leads to localized helical deformation bands with inclinations dependent on the stress ratio. In the process, the stresses remain nearly constant until transformation is completed. In the vicinity of equibiaxial tension, the material exhibits hardening and homogeneous deformation. Loci of the transformation stresses, while exhibiting very modest anisotropy, traced an elongated trajectory in the axial-hoop stress space not captured by a von Mises representation. By contrast, the transformation strains exhibited significant anisotropy between axial and hoop dominant stress paths. Moreover, strains around the equibiaxial stress state, where material hardening and homogeneous deformation was observed, were significantly smaller than in the rest of the stress space. The strain anisotropy had a corresponding reflection on the energy dissipated during transformation with axial dominant stress paths dissipating significantly more energy than hoop dominant ones, with less dissipation observed in the neighborhood of equibiaxial stress.