Thermomechanical behavior of pseudoelastic NiTi shape memory alloys

Abstract

The thermomechanical behavior of polycrystalline Ni-rich pseudoelastic NiTi shape memory alloys is analyzed. Special focus is on regions within the stress strain diagram which are regarded as linear elastic in common phenomenological material models, i.e. the region between zero stress and the beginning of the pseudoelastic plateau as well as the region within the hysteresis. In both cases, severe temperature changes can be observed. A possible explanation for this effect is twofold: On the one hand, it might be explained by the presence of an R-phase transformation. On the other hand, unstructured martensite of the B19' phase may form. However, the assumption of a purely thermo-elastic material behavior in those regions does not seem to hold true in general. Experimental analysisThe setup for the following experiments is described in [2]. Two inductive transducers measure the axial displacement between two aluminum discs, which are clamped on the gauge length of the tubular sample. The sample has an hourglass shape with a radius-to-wall-thickness ratio of 1.5. Concerning the heat treatment of the 50.2at% NiTi material, the sample was solution annealed for one hour at 850 • C and aged for 30 minutes at 350 • C. Between these two steps the sample was quenched by water at 21 • C. During the experiments the temperature is measured by eight thermocouples distributed on the gauge length.

A non-local implicit gradient-enhanced model for thermomechanical behavior of shape memory alloys

“…This finding implies that beside the applied strain rate, the surrounding ambient affects the temperature variations too. A similar conclusion can be drawn based on the works of Morin et al (2011) and Luig et al (2007). Morin et al (2011) observed that the rate dependency disappears for low to moderate strain rates when the temperature of the test sample is kept constant.…”