CHARACTERIZING THE EFFECTS OF ANNEALING TEMPERATURE ON KNITTED SHAPE MEMORY ACTUATORS

Advancements in the design of knitted Shape Memory Alloy (SMA) actuators have produced highly functional and easy-towear advanced textiles with variable fit/stiffness properties. Annealing SMA at high temperatures can alter their thermomechanical properties; however, heat treatments of SMA knitted actuators have yet to be systematically studied. This study investigates the relationship between annealing temperatures and the resultant SMA knit thermomechanical behavior (maximum force and actuator transformation temperature). Twelve SMA knitted samples were manufactured using Dynalloy FlexinolR 70°C (0.012 h). Nine samples were annealed at three different temperatures (350°C, 450°C, 550°C), with the remaining samples acting as controls. The blocked force mechanical performance was characterized by varying the thermal load across various applied structural strains. The samples were thermally cycled between 20°C and 90°C at each strain level. The results indicate that samples annealed while stretched along the knit-course direction enabled large-force actuation at smaller displacements than the control samples. Annealed samples showed higher peak force than control samples as annealing temperature increased until the 450° condition; however, the 550° samples showed a dip in force. The actuator transformation temperatures showed reduced Austenite start (As) temperature compared to the control sample, enabling actuation at a lower temperature while also reducing the Martensite Finish (Mf) temperatures. This work allows for greater tuning of knitted SMA actuators to meet the needs of researchers and product developers.