Finite element analysis of straight twisted polymer actuators using precursor properties

Twisted polymer actuators (TPAs) are a promising new low-cost actuation technology that can generate torsional or linear actuation. While analytical models for TPAs have been explored to predict their thermal actuation responses, these models are either based on elastic properties of the twisted monofilament, which does not allow for design of initial twist, or ignore the internal stresses between layers. How the elastic properties of the precursor (untwisted) monofilament affects internal stresses is not well understood. Our goal is to better understand how the actuation response is affected by internal stresses. To study the internal stresses in TPAs, we simulate a straight twisted polymer actuator (STPA) using a finite element (FE) model. The linear elastic mechanical properties of the untwisted monofilament as a function of temperature are needed as inputs to the FE model, and are obtained experimentally. FE results are compared with experimental actuation responses and closed-form predictions using models which neglect internal stresses, namely the Shafer et al and the averaged Shafer et al models. The simulations and experiments use a STPA to generate torsional actuation under free torsion conditions. Under these conditions, we show that interlayer stresses are not the main driver for the torsional actuation of STPAs and FE model predicts experimental data about as well as closed-form models. Additionally, we use the internal stresses to better understand the actuation mechanism of TPAs and evaluate their effects on STPAs torsional actuation under free torsion conditions.