A novel shape memory alloy curved strip actuator: Enhancing the bending actuation by SME-activated cross-sectional curvature transition

This paper presents a method for significantly enhancing the bending actuation capacity of tape actuators through a shape memory effect (SME)-activated cross-sectional curvature transition from a flat configuration to an arc-shaped one. NiTi shape memory alloy (SMA) sheets, with a thickness of 0.3 mm and a width of 24 mm, were trained to acquire SME along the transverse direction by annealing at 550 °C for 30 min while constrained within stainless steel molds. These SMA variable curvature strips (SMA-VCS) were subsequently subjected to folding and unfolding tests, monitored by a high-speed camera and an infrared camera. The folding process exhibited behavior similar to that of a traditional tape spring, while the unfolding process under applied loads proved significantly more complex. Upon heating, the SMA-VCS initially demonstrated stable bending recovery with its cross-section remaining flat. Once a critical angle was reached, a snap-through transition occurs, causing the SMA-VCS to “jump” back to its nearly original unbent position as the cross-section abruptly shifted from flat to arc-shaped. A significant improvement in bending actuation capacity was observed: under a constant load of 200 g applied to the ends of a 0.3 mm thick beam, traditional SMA actuators with a flat cross-section achieved a recovery angle of only 56.77° upon efficient heating, whereas the SMA-VCS returned to its nearly original unbent position (90°), representing an improvement of 32.32°. Finally, an SMA transformation-point bending actuation model is proposed to describe the critical points governing the bending performance. The predicted actuation angles and corresponding temperatures align closely with experimental results.