Constitutive and Structural Modeling of SMA-Driven Knitted Soft Actuator Unit Cell Under Large Deformation

초록

Shape memory alloy (SMA)-integrated knitted actuators can generate large, compliant deformation through temperature-induced phase transformation; however, their response is strongly nonlinear owing to loop kinematics and large rotations. This study develops a constitutivestructural modeling methodology for an SMA-driven knitted soft actuator unit cell under large deformation, with emphasis on the plain-pattern P-loop as a baseline architecture. In experimentally reported knitted SMA actuators, a NiTi (55wt% Ni-45wt% Ti) core wire wrapped with polyester fibers is heated by electrical current, thus resulting in martensite-to-austenite transformation. In the present numerical study, electrical heating is not modeled explicitly; instead, thermally driven actuation is represented by a prescribed thermal load. A 3 × 3 P-loop unit cell is constructed using a weft-knit geometry pipeline and imported into ANSYS for nonlinear analysis. The SMA response is captured using the built-in SMA material capability in ANSYS under a temperature increase from 25°C to 90°C, and the structural problem is solved with large-deformation kinematics to capture out-of-plane loop deformation. Contact and self-contact between yarn segments are disregarded in the current model to isolate the primary thermally driven deformation coupling. The simulations reproduce the expected loop-level tightening and geometry-driven amplification of deformation in the out-of-plane direction, thus demonstrating that the unit-cell model can quantify actuation-induced shape changes in knitted architectures. The proposed methodology provides a practical approach for evaluating and comparing knitted patterns at the unit-cell level. Additionally, it serves as a basis for future extensions, including contact, electro-thermal coupling, and experimental validation.

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