Tension, Compression, and Bending of Superelastic NiTi Tubes

Many shape memory alloy applications use bending to leverage superelasticity, yet large displacements and rotations associated with bending of slender structures make controlled experiments difficult. A custom 4-point bending fixture was built to perform experiments on superelastic NiTi tubes (3.176mm outer diameter by 14.77mm gage length) within a standard mechanical test frame. The fixture maintained a constant moment in the gage section during large rotations, while avoiding axial loading of the tube specimen. The strain fields on the surface of the tube were measured using stereo digital image correlation (DIC), along with the moment-end rotation response. As is known under uniaxial loading, <111>-textured NiTi has lower transformation strains in compression than in tension, which leads to "tension-compression asymmetry" in which transformation stresses are significantly higher in compression than in tension. In addition, strain localization and propagating transformation fronts have been observed in NiTi tubes in tension, but not in compression. Consistent with these behaviors, the tube specimen in our bending experiments had larger strains on the tension side than the compression side during stress-induced transformation, and exhibited strain localization on the tension side but no such localization on the compression side. A significant shift of the neutral strain axis towards the compression side was observed and quantified. Detailed analysis of the strain distribution across the tube diameter revealed that the usual assumption of beam theory that plane sections remain plane, did not hold along the tension side. Averaged over a few diameters of gage length, plane sections remain plane is a reasonable assumption, but should be used with caution since it can under/over predict local strains by as much as 2x due to the localized deformation morphology. This result may have implications for bending fatigue.