Detwinning of preloaded martensite in shape memory alloys and its effect on the cyclic behavior of NiTi cylindrical actuators

A multi-mechanism material model is used to investigate the effect of the degree of martensitic detwinning/reorientation occurring at the end of mechanical preloading, on the performance of shape memory tubular and solid cylindrical actuators. A high-temperature ternary Ni _50.3 Ti _29.7 Hf ₂₀ alloy, showing almost complete transformation behavior, and an ordinary binary Ni _49.9 Ti _50.1 material, depicting incomplete martensitic transformation, are used. The results indicated a clear correlation between the shape memory cyclic actuation behavior and the martensitic deformation character of the selected alloy. More specifically, while the actuation strokes produced by the Ni _50.3 Ti _29.7 Hf ₂₀ systems consistently followed a direct pattern for the different geometries and torque magnitudes, the results for the Ni _49.9 Ti _50.1 cases indicated a behavior that is counterintuitive to what may be expected under pure mechanical loading conditions. In particular, when subjected to the same, high preload torques, Ni _49.9 Ti _50.1 solid actuator under lesser stresses generated higher twist strokes than a tubular counterpart experiencing higher stresses.