The influence of hysteresis loop in a NiTi shape memory oscillator behavior

This work consists of an experimental and a numerical investigation on how heat treated Ti-50.7% at Ni springs can affect the dynamical behavior of a single degree of freedom (1DOF) oscillator. Initially, the springs are solution annealed at 300°C, 400°C and 500°C over 30, 60, 90, and 120 min. After, the springs are characterized, through quasi-static loadings following a cyclic thermomechanical loading path until the inelastic strains saturate and the behavior of the springs stabilizes. The results demonstrate that springs aged at 400°C and 500°C present a larger hysteresis loop area when compared with those aged at 300°C. Then, the springs aged at 500°C are used in an experimental apparatus that represents a 1DOF system—the SMA spring represents the element of stiffness—and are analyzed in an up and down sweep frequency at two different temperatures. It is observed that the maximum amplitudes strongly depends on the temperatures and on the frequency test. It is also noticeable that the springs with higher hysteresis loop area results in lower maximum amplitudes. Finally, the 1DOF system is numerically modeled, in which a constitutive model is used to describe the SMA spring behavior. When the numerical and experimental results are compared, it is observable a difference, suggesting that the hysteresis loop may present a very different and counter intuitive behavior when subjected to dynamic loads. (paper)