Investigation on local and global behaviors of pseudoelastic shape memory alloy wires in simple tensile test considering stress concentration of grippers

Abstract

To investigate the local phenomena of nucleation and propagation in the simple tensile test of shape memory alloys, the existing models consider geometrical imperfections in the specimen to indirectly take the effects of grippers’ stress concentration into account. In this article, the grippers are directly modeled in studying shape memory alloy wires in simple tensile test. A finite element model is created in ABAQUS to determine the distributions of cross-sectional area and stress in a shape memory alloy wire when tightening the grippers. Then, using this stress distribution as the pre-stress, tension of the specimen is studied by employing a coupled thermomechanical model to obtain distributions of the stress, strain, martensite volume fraction, and temperature at any strain rate. The results indicate that the nucleation and propagation of transformation fronts are observed using this method of directly considering the pre-stress and initial area distribution due to fastening the grippers. It is also shown that the local and the nominal strains are not the same during nucleation and propagation of phase fronts. A simple tensile test is conducted on an NiTi wire, and a good agreement is seen between the experimental and the numerical stress–strain responses and local–nominal strain diagrams.