In the field of soft dielectric elastomers, the notion 'electrostriction' indicates the dependency of the permittivity on strain. The present paper is aimed at investigating the effects of electrostriction onto the stability behaviour of homogeneous electrically activated dielectric elastomer actuators. In particular, three objectives are pursued and achieved: i) the description of the phenomenon within the general nonlinear theory of electroelasticity; ii) the application of the recently proposed theory of bifurcation for electroelastic bodies in order to determine its role on the onset of electromechanical and diffuse-mode instabilities in prestressed or prestretched dielectric layers; iii) the analysis of band-localization instability in homogeneous dielectric elastomers. Results for a typical soft acrylic elastomer show that electrostriction is responsible for an enhancement towards diffuse-mode instability, while it represents a crucial property -necessarily to be taken into account -in order to provide a solution to the problem of electromechanical band-localization, that can be interpreted as a possible reason of electric breakdown. A comparison between the buckling stresses of a mechanical compressed slab and the electrically activated counterpart concludes the paper.
Electromechanical harvesters based on dielectric electroactive polymers are promising devices for the production of electrical energy by the conversion of abundant sources of mechanical work available in Nature. However, severe limitations to the performance of these devices arise from various sources of dissipation and failure of the polymeric material. By making use of an energetic approach, we establish a direct and quantitative connection between the Mullins effect taking place in the polymeric material and the harvesting efficiency, showing the prominent role of rate-independent effects in the hysteretic behavior of electromechanical harvesters.
Predictive models for Dielectric Elastomer Actuators require the nonlinear solid mechanics theory of soft dielectrics. This is certainly true for homogeneous systems, but also for devices made of composite materials, where the insertion of stiff conductive particles in the soft matrix may help to improve the overall actuation performance. In this note, we present a theoretical framework to investigate a wide range of instabilities in both homogeneous and composite-manufactured actuators: pull-in/electromechanical instability, buckling-like modes and band-localization failure, that can be analyzed taking into account all the geometric and electromechanical properties of the device such as i) nonlinearities associated with large strains and the employed material model; ii) initial prestretch applied to the system; iii) dependency of the permittivity on the deformation (electrostriction). In particular, we focus on the general expression which gives the condition for pull-in instability, also valid for anisotropic composite soft dielectrics. In the second part, we show that in a layered composite an electromechanical/snap through instability can be designed and possibly exploited to conceive release-actuated systems.
Electroactive polymer energy harvesters are promising devices for the conversion of mechanical work to electrical energy. The performances of these devices are strongly dependent on the mechanical response of the polymeric material and on the type of electromechanical cycle, and these are limited by the occurrence of dielectric breakdown, compression induced wrinkling and electromechanical instability (pull-in). To identify the optimal electromechanical cycle that complies with all of these limitations, we set-up and solve a constraint optimization problem and we critically discuss the influence of material behavior of the polymer in the optimal performances of the energy harvesting device. Finally, we show that if the rate-independent dissipative behavior of the polymer (Mullins effect) is neglected, the optimization procedure may lead to quite unsatisfactory predictions: by making reference to explicit experimental data from literature we show that an optimal harvesting cycle deduced by neglecting the Mullins effect is far from being optimal when this is taken in consideration.
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