The belt type specimens of TiNi shape memory alloy are subjected to the strain-controlled tension test at room temperature with the strain rate of 5x10-2 s-1.Thermomechanical aspects of martensitic and reverse transformations in TiNi SMA during tension test with stress relaxation sequences are investigated using a thermovision camera. Stress relaxation stages in loading and unloading branches are induced, respectively. The mechanical and the temperature data enable the study of nucleation and development of the phase transformations as well as analysis of the impact of the stress relaxation brakes on both the mechanical specimen behavior and the phase transformations progress. The uniform temperature distribution indicates homogeneity of the stress and the strain state along the specimen. The inhomogeneous temperature distribution on the specimen surface reflects immediately the origin and development of the new phase, due to the significant temperature variations between the parent phase and the new one. A monotonic stress drop is observed during the stress relaxation in both the course of loading and unloading, while, for the temperature-controlled test, in the unloading branch the stress increases under constant strain due to the recovery. Furthermore, on thermograms obtained from the loading branch at the end of relaxation brake, fine bands of deeper temperature occur starting from the grip area. They may manifest nucleation of the reverse transformation at the end of the relaxation brake, caused by temperature decrease due to the relaxation. After relaxation is completed, in both loading and unloading branches of loading, the phase transition processes develop, initiating however at substantially lower stresses and temperatures of the SMA sample.
This research has been carried out with the support of Polish Grant No. 4 T08A 060 24, and of the JSPS Grants: No. 13650104 (C), Post-doc PO4774 and Joint Research supported by JSPS and PAS: No. 6612.
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