Explorations of knitted shape memory alloy actuation behavior under high loads via finite element analyses and experimental studies

The properties of shape memory alloy (SMA) wires have long been leveraged across a variety of industries. While the response of such SMA forms implemented as straight axial actuators is well understood, curved and complex configurations such as knits have received far less attention. Considering 2D configurations, it is well known that knits exhibit more in-plane compliance than weaves and meshes, the curved wires comprising the former being much more flexible than the straight wire segments in the latter. In addition, knitted structures are uniquely highly tailorable. Knitting techniques and patterns developed in the textile industry allow for variable materials and geometries in the same structure, allowing for a large range of tailored macro-structure responses. Existing efforts to model the behavior of knitted SMA structures are lacking; though finite element analysis (FEA) models have been presented for knit SMAs, these models either only consider superelastic SMA behavior, or, in those that account for actuation behavior, the applied load conditions studied are insufficient to fully leverage the thermally induced strain recoverability of SMAs. This work seeks to develop and validate a finite element model for the actuation of SMA knitted structures where individual SMA wire components are axially stressed to more than 100 MPa. A representative volume element is developed for a common knit pattern, and macro-structure responses are explored and compared with experiments. This research provides a foundation for better understanding fundamental capabilities and responses of knitted SMA structures, allowing for better design, functionality, and customizability of the applications into which they are incorporated, enabling development of unique soft actuators. A shape-set sample examined herein generated 13% extension (analogous to strain) and recovered more that 6% under a load associated with 100 MPa stress in a straight wire, and a sample knit off-the-spool generated over 20% extension and recovered 9% for the same load.