Viscoelastic effects on electromechanical instabilities in dielectric elastomers

We present a computational study of the effects of viscoelasticity on the electromechanical behavior of dielectric elastomers. A dynamic, finite deformation finite element formulation for dielectric elastomers is developed that incorporates the effects of viscoelasticity using the nonlinear viscoelasticity theory previously proposed by Reese and Govindjee. The finite element model features a three-field Q1P0 formulation to alleviate volumetric locking effects caused by material incompressibility. We apply the formulation to first perform a fundamental examination of the effects of the viscoelastic deviatoric and volumetric response on dielectric elastomers undergoing homogeneous deformation. Specifically, we evaluate the effects of the shear and bulk relaxation times on the electromechanical instability, and demonstrate that while the bulk relaxation time has a negligible impact, the shear relaxation time substantially increases the critical electric field needed to induce electromechanical instability. We also demonstrate a significant increase in the critical voltage needed to induce electromechanical instability in the presence of a distribution of relaxation times, compared to a single relaxation time, where the former is more representative of viscoelastic behavior of polymers. We then study the effects of viscoelasticity on crack-like electromechanical instabilities that have recently been observed in constrained dielectric films with a small hole containing a conductive liquid. Viscoelasticity is shown again to not only significantly increase the critical electric field to induce the electromechanical instability, but also to substantially reduce the crack propagation speeds in the elastomer.