Bending of Superelastic Shape Memory Alloy Tubes

Many shape memory alloy (SMA) applications exploit superelasticity in a bending mode, yet the large displacements and rotations associated with bending of slender structures make controlled experiments difficult. A custom pure bending fixture was built to perform experiments on superelastic NiTi tubes. To understand the bending results, the tubes were also characterized in uniaxial tension and compression, where a custom fixture was utilized to avoid buckling. In addition to measuring the global mechanical response, stereo digital image correlation (DIC) was used in all the experiments to capture the local surface displacement and strain fields. Consistent with the tension/compression data, our bending experiments showed a significant shift of the neutral axis towards the compression side. Also, the tube had strain localization on the tension side, but no such localization on the compression side. 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 and can be used to predict the global moment–curvature response. However, this assumption should be used with caution since it can under/over predict local strains by as much as 2× due to the localized deformation morphology.