Actuation mechanisms in twisted and coiled polymer actuators using finite element model

Twisted and coiled polymer actuators (TCPAs) offer the advantages of large stroke and large specific work as compared to other actuators. Despite extensive experimental investigations towards understanding their actuation response, a computational model with a full material description has not been utilized to probe into the underlying mechanisms responsible for their large actuation. In this work, we develop a three-dimensional finite element model that includes the physics of the fabrication process to simulate the actuation of TCPA under various loading and boundary conditions. We implemented a novel approach to estimate the temperature-dependent anisotropic elasticity tensor based on experimental observations. The model is validated against the experimental data and used to explore the factors responsible for actuation under free and isobaric conditions. The model captures the physics of the angle of twist in the fiber, and the distinct response of the homochiral and heterochiral nature of TCPAs. The results show that there exists an optimal angle of twist at which the actuation peaks, and it can be determined through these simulations for any novel TCPA concept. The simulations reveal that the anisotropy in the thermal expansion coefficient (CTE) matrix plays a major role in large actuation irrespective of the anisotropy or isotropy in the elasticity tensor. For the first time, the studies on the extent of anisotropy in thermal expansion show that the key for TCPA actuation is the absolute value of mismatch in thermal expansion irrespective of the sign of CTE in axial and transverse directions of the fiber. Furthermore, to demonstrate the application of this model on a non-homogeneous cross-section, we propose a new shell-core composite-based TCPA concept by combining the epoxy and hollow Nylon tubes to suppress the creep in TCPA. The results show that the volume fraction of epoxy-core can be tuned to attain a desired actuation while offering a stiffer and creep-resistant response. Therefore, this work sets forth a framework for probing into various kinds of TCPAs and opens opportunities to enhance their actuation performance under complex loading conditions as an element of a device or robotic systems.