The stress relaxation phenomenon (time-dependent drop of the stress), which is commonly known as viscoelastic material behavior of polymers, occurs in metallic materials as well. In NiTi shape memory alloys (SMAs), the time-dependent stress relaxation under uniaxial tension is observed even at room temperature. Previous studies have shown that the relaxation phenomenon in martensitic NiTi SMAs may be caused by the viscous motion of the different martensite variant interfaces and of the thermally activated dislocations during loading. However, in contrast to polymers, the time-dependent relaxation behavior of NiTi SMAs has not yet been sufficiently understood. In this study, we characterize the stress relaxation behavior of martensitic NiTi shape memory actuators as a function of different total strains (from 0.2 up to 10%) and strain rates (10-5, 10-4 and 10-2 s-1) under uniaxial tensile loading. During the displacement-controlled relaxation tests on NiTi actuator wires, the samples are initially loaded to a specified total strain (I) and the strain is held constant during a period of 1800 s. When the strain remains constant, a pronounced stress drop (relaxation) occurs (II); this is measured as a function of time. After a certain hold/relaxation time, the material is completely unloaded (III). By way of example, the above-mentioned sequences (tensile loading I, relaxation II and unloading III) of such a relaxation experiment (at total strain of 6% and strain rate of 10-4 s-1) are shown in Fig. 1. Our results indicate that the stress relaxation during the hold time strongly depends on the total strain as well as on the strain rate during prior loading in martensitic NiTi SMAs. Based on the experimental results of numerous stress relaxation tests and on microstructural considerations, we argue that the stress relaxation in martensitic NiTi SMAs is produced by two complementary mechanisms: stress-induced motion of martensitic twin boundaries vs. thermally-activated dislocation motion. The insights gained from these observations provide a deeper understanding of the relaxation phenomenon that is also of practical importance in NiTi SMAs.
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