Nucleation, bifurcation and propagation of local deformation arising in NiTi shape memory alloy

The main purpose of this paper is to investigate the local deformation arising in NiTi plate subjected to stress concentration during tensile loading. First, a test system is constructed on the basis of digital image correlation (DIC) in order to measure the local strain distribution over the surface of the specimen. Second, we measure the local deformation (local strain distributions) arising in a 50.5Ni49.5Ti plate with a double notch under tensile loading. The results are as follows. (1) NiTi shows local deformation, such as nucleation and propagation of local strain bands, and its bands have a fixed angle (about 54°–55°) relative to the loading direction. On the other hand, the distribution of local shear strain is almost uniform when the local longitudinal strain bands propagate during tensile loading. (2) NiTi with stress concentration shows that the local longitudinal and lateral strain bands initiate at the stress concentration part, and propagate with a symmetrical shape. Later, the local strain band shows bifurcation and propagates toward both ends of the specimen with a fixed angle relative to the loading direction. The distribution of local shear strain is almost uniform when the local longitudinal strain bands propagate, but it is not uniform when the phase transformation initiates.

NiTi with stress concentration shows local deformation, such as nucleation, bifurcation and propagation of phase transformation, and its bands have a fixed angle relative to the loading direction. The transition of the local deformation mode has an effect on the macroscopic stress–strain curve. When predicting the macroscopic deformation behavior of NiTi structures, these local deformation modes arising in NiTi should be considered.