Gradient-based topology optimization of soft dielectrics as tunable phononic crystals

Dielectric elastomers are active materials that undergo large deformations and change their instantaneous moduli when they are actuated by electric fields. By virtue of these features, composites made of soft dielectrics can filter waves across frequency bands that are electrostatically tunable. To date, to improve the performance of these adaptive phononic crystals, such as the width of these bands at the actuated state, metaheuristics-based topology optimization was used. However, the design freedom offered by this approach is limited because the number of function evaluations increases exponentially with the number of design variables. Here, we go beyond the limitations of this approach, by developing an efficient gradient-based topology optimization method. The numerical results of the method developed here demonstrate prohibited frequency bands that are indeed wider than those obtained from the previous metaheuristics-based method, while the computational cost to identify them is reduced by orders of magnitude.

Instabilities of soft dielectrics

The basic modern theory of nonlinear electroelasticity and its use in the formulation of constitutive laws governing the behaviour of dielectric elastomer materials was summarized in a recent review article by Dorfmann & Ogden (Dorfmann & Ogden 2017 Proc. R. Soc. A 473 , 20170311 ( doi:10.1098/rspa.2017.0311 )). The theory is used, in particular, to analyse the behaviour of transducer devices such as actuators and sensors. Important considerations for the design and effective functioning of such devices are the issues of material and geometric instabilities. Following on from the above-cited work, the present paper provides a detailed account of the types of instabilities that arise for some of the geometries used in transducer devices and the theory that is adopted for the analysis of such instabilities. The theory is then used in two illustrative examples: (i) determination of instabilities of a thin electroelastic plate with flexible electrodes attached to its major surfaces, in particular comparison of the results for the so-called Hessian approach and a general incremental bifurcation analysis in respect of an equibiaxially stretched plate, with numerical results presented for a Gent electroelastic model; (ii) a general analysis of axi-symmetric bifurcation from a circular cylindrical configuration of a thin-walled tube of an electroelastic material with flexible electrodes on its curved surfaces, illustrated by numerical results for neo-Hookean and Gent electroelastic models. This article is part of the theme issue ‘Rivlin's legacy in continuum mechanics and applied mathematics’.

Analysis of multilayer electro-active tubes under different constraints

Dielectric elastomers are an emerging class of highly deformable electro-active materials employed for electromechanical transduction technology. For practical applications, the design of such transducers requires a model accounting for insulation of the active membrane, non-perfectly compliant behavior of the electrodes, or interaction of the transducer with a soft actuated body. To this end, a three-layer model, in which the active membrane is embedded between two soft passive layers, can be formulated. In this article, the theory of non-linear electro-elasticity for heterogeneous soft dielectrics is used to investigate the electromechanical response of multilayer electro-active tubes—formed either by the active membrane only ( single-layer tube) or by the coated active membrane ( multilayer tube). Numerical results showing the influence of the mechanical and the geometrical properties of the soft coating layers on the electromechanical response of the active membrane are presented for different constraint conditions.

Revisiting the Instability and Bifurcation Behavior of Soft Dielectrics

Development of soft electromechanical materials is critical for several tantalizing applications such as human-like robots, stretchable electronics, actuators, energy harvesting, among others. Soft dielectrics can be easily deformed by an electric field through the so-called electrostatic Maxwell stress. The highly nonlinear coupling between the mechanical and electrical effects in soft dielectrics gives rise to a rich variety of instability and bifurcation behavior. Depending upon the context, instabilities can either be detrimental, or more intriguingly, exploited for enhanced multifunctional behavior. In this work, we revisit the instability and bifurcation behavior of a finite block made of a soft dielectric material that is simultaneously subjected to both mechanical and electrical stimuli. An excellent literature already exists that has addressed the same topic. However, barring a few exceptions, most works have focused on the consideration of homogeneous deformation and accordingly, relatively fewer insights are at hand regarding the compressive stress state. In our work, we allow for fairly general and inhomogeneous deformation modes and, in the case of a neo-Hookean material, present closed-form solutions to the instability and bifurcation behavior of soft dielectrics. Our results, in the asymptotic limit of large aspect ratio, agree well with Euler's prediction for the buckling of a slender block and, furthermore, in the limit of zero aspect ratio are the same as Biot's critical strain of surface instability of a compressed homogeneous half-space of a neo-Hookean material. A key physical insight that emerges from our analysis is that soft dielectrics can be used as actuators within an expanded range of electric field than hitherto believed.