Cyclic Compressive Responses of NiTi Shape Memory Alloy—Effects of Loading Frequency

Abstract

This paper investigates the interaction between cyclic phase transitions and plasticity, focusing on the synchronized stress strain evolution with temperature profiles of polycrystalline superelastic NiTi SMA. NiTi column is subjected to displacement-controlled cyclic compressive loading in the room temperature ambient air over the frequency range of 0.0007–40 Hz. At low frequencies (0.0007–0.02 Hz) and intermediate frequencies (0.04–7 Hz), the deformation is dominated by phase transition which couples with the temperature variation. The previously established model in cyclic loading can well predict these results. However, for frequency above 8 Hz, both the temperature and stress oscillations experienced a non-monotonic transient stage in which the plastic deformation of the austenite phase takes place and accumulates due to the more rapid accumulation of hysteresis heat and the resulting increase of temperature at the higher frequency region. Such plastic deformation, as a positive feedback to the heat source under the displacement controlled cyclic loading, in turn helps reduce both the transformation strain and hysteresis heat and eventually reduces the temperature and thus brings the stress below the plastic yielding stress of the material. After this transient stage a new steady-state temperature and stress oscillations are established with pure cyclic phase transition of reduced transformation strain. We conclude that, for given material properties, specimen geometries and ambient, a broader scenario of frequency dependent cyclic behavior of SMA can be established, and in particular, the occurrence of austenite plastic yielding at high frequency region can be used as a positive feedback to protect the material from over-stressing and permanent damage.