Local and global strains and strain ratios in shape memory alloys using digital imagecorrelation

The recoverable superelastic phase transformation was found to initiate at the gripped ends and propagate towards the middle of the tensile sample. With the added stress concentration, transformation was initiated at the hole, followed by secondary nucleation at the gripped ends and elsewhere. Localized strain ratios, which include contributions from inelastic strain, were found to increase with local transformation, plateauing at 0.45 at 4.7% global strain. Deformation at the granular length scale was compared to finite element modeling of a circular hole in a NiTi plate. Discrepancies between the observed DIC strain field and the modeled strain field are due to intergranular stresses disturbing the strain field, which cannot be captured with continuum modeling techniques.