Electrically-induced twist in geometrically incompatible dielectric elastomer tubes

Dielectric elastomers (DEs) are materials that experience large deformations in response to electric excitation. In this work, we introduce and investigate a DE-based bi-layer tubular design that can be used as an electrically activated torsional actuator. Specifically, the bi-layer tube is constructed as follows: an isotropic Gent tube is twisted, inserted into another tube, and the two layers are adhered along the interface, resulting in a bi-layer with residual stress. The two layers are referred to as geometrically incompatible. Next, the bi-layer tube is subjected to an electric field and, due to deformation-induced anisotropy, experiences twist. This work begins with a summary of the governing electro-mechanical equations. The overall electrically-induced twist depends on the ratios between the radii, the stiffnesses, the lock-up stretch parameters, and the permittivities of the two layers. Thus, we employ the Gent model and follow by investigating the role of each one of these four key ratios. To illustrate the feasibility of the proposed design, three bi-layer tubes from commonly used DE materials are studied. We show that twists ranging from 25 deg /mm to 130 deg/mm can be achieved in response to voltages of-6-15 kV. The behaviors that can be obtained with commonly used DEs highlight the merit of the proposed configuration. The findings and insights from this work can be used in a wide range of applications that require a twist motion, including soft rotational robots, artificial muscles, and soft torsional actuators.