Preliminary experimental study of the effect of shape setting on knitted SMA structures

Smart materials can be integrated into textile structures to produce active textiles with tailored mechanical properties and large, complex actuation motions. Active textiles have the potential to enable a wide range of applications including wearable technologies, soft robots, medical devices, and aerospace structures. One type of active textile is the shape memory alloy (SMA) knitted structure. SMA knitted structures produce a range of kinematic actuation motions as a result of the bending, torsion, extension, and buckling of the SMA wire during the loop-based knitting manufacturing process. The kinematic motions of several different patterns of SMA knitted actuators have been cataloged, and the mechanical performance of basic knitted patterns have been characterized. However, the effect of shape-setting of knitted SMA structures has not been explored. This paper investigates the effect of postmanufacturing shape-setting on the kinematic and kinetic performance of basic SMA knitted structures. A design of experiment methodology was employed to isolate the impact of knitted pattern, SMA wire diameter, and shape-set curvature on mechanical performance. The introduction of a large curvature shape-set in the SMA wire resulted in a very stiff textile structure with a minimal change in length between the austenite and martensite states, thus, minimal capacity for large actuation deformations. Meanwhile, the introduction of a small curvature in the SMA wire resulted in a nearly constant force plateau and a larger change in length between the austenite and martensite state for the same applied load, and the potential for enhanced structural actuation deformations. Shape-setting is an additional design parameter that can be employed to enhance and tune the mechanical performance of knitted SMA structures.