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

Local and global strain field maps of Nickel Titanium (NiTi) were examined during stress-induced phase transformation using digital image correlation (DIC). 2-D strain field maps of the sample surface during tensile deformation were correlated to the recoverable superelastic phase transformation. Localized stress concentrations were examined by machining a 500μm diameter hole in the center of the gage section. Axial and transverse strain measurements from DIC were used to calculate a localized strain ratio as a function of global strain. This strain ratio includes both the elastic and inelastic transformation strain contributions. It can be readily compared to typical modeling assumptions, which input Poisson's ratio as constant values for transforming NiTi and elastically deformingNiTi.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.