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.
Multifield theories are powerful frameworks able to manage complexities arising in the modelling of
physical phenomena. In the mechanics of active materials a multifield theory, consisting in balance and constitutive equations,
may be constructed by assigning to each point of the solid an additional parameter (the morphological descriptor) which accounts
for the activation mechanisms at the level of microstructure. The main elements of multifield theory applied to continuum mechanics
are given. The relevant equations of the problem of a cantilever beam of
conductive polymer subject to flux of ions are reported to show the coupling between chemical (ion concentration) and mechanical
behaviour (end deflection).