Giuseppe Zurlo

Pull-in and wrinkling instabilities of electroactive dielectric actuators

We propose a model to analyse the insurgence of pull-in and wrinkling failures in electroactive thin films. We take into consideration both cases of voltage and charge control, the role of pre-stretch and the size of activated regions, which are all crucial factors in technological applications of electroactive polymers (EAPs). Based on simple geometrical and material assumptions we deduce an explicit analytical description of these phenomena, allowing a clear physical interpretation of different failure mechanisms such as the occurrence of pull-in and wrinkling. Despite our simple assumptions, the comparison with experiments shows a good qualitative and, interestingly, quantitative agreement. In particular our model shows, in accordance with experiments, the existence of different optimal pre-stretch values, depending on the choice of the actuating parameter of the EAP.

Electromechanical instability and oscillating deformations in electroactive polymer films

Based on an energetic approach, we analytically determine inhomogeneous equilibrium configurations of thin electroactive polymeric films of under assigned voltage. We show that our results are useful in the analysis of well known failure phenomena taking place in this type of devices. Moreover, we demonstrate that neglecting inhomogeneity effects may lead to a drastic overestimate of the activation performances.

The stretching elasticity of biomembranes determines their line tension and bending rigidity

In this work, some implications of a recent model for the mechanical behavior of biological membranes (Deseri et al. in Continuum Mech Thermodyn 20(5):255–273, 2008) are exploited by means of a prototypical one-dimensional problem. We show that the knowledge of the membrane stretching elasticity permits to establish a precise connection among surface tension, bending rigidities and line tension during phase transition phenomena. For a specific choice of the stretching energy density, we evaluate these quantities in a membrane with coexistent fluid phases, showing a satisfactory comparison with the available experimental measurements. Finally, we determine the thickness profile inside the boundary layer where the order–disorder transition is observed.

Compression-induced failure of electroactive polymeric thin films

The onset of compression induces wrinkling in actuation devices based on electroactive polymer thin films, which leads to a sudden decrease in performances and, eventually, to failure. Inspired by the classical tension field theory for thin membranes, we provide a general framework for the analysis of the insurgence of in-plane compressions. Our main result is the analytical deduction of a voltage-dependent domain of tensile configurations in the principal stretches plane.

Catastrophic thinning of dielectric elastomers

We provide an energetic insight into the catastrophic nature of thinning instability in soft electroactive elastomers. This phenomenon is a major obstacle to the development of giant actuators, yet it is neither completely understood nor modeled accurately. In excellent agreement with experiments, we give a simple formula to predict the critical voltages for instability patterns; we model their shape and show that reversible (elastic) equilibrium is impossible beyond their onset. Our derivation is fully analytical, does not require finite element simulations, and can be extended to include prestretch and various material models.

Failure modes in electroactive polymer thin films with elastic electrodes

Based on an energy minimization approach, we analyse the elastic deformations of a thin electroactive polymer (EAP) film sandwiched by two elastic electrodes with non-negligible stiffness. We analytically show the existence of a critical value of the electrode voltage for which non-homogeneous solutions bifurcate from the homogeneous equilibrium state, leading to the pull-in phenomenon. This threshold strongly decreases the limit value proposed in the literature considering only homogeneous deformations. We explicitly discuss the influence of geometric and material parameters together with boundary conditions in the attainment of the different failure modes observed in EAP devices. In particular, we obtain the optimum values of these parameters leading to the maximum activation performances of the device.

Damage induced dissipation in electroactive polymer harvesters

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.

Fine tuning the electro-mechanical response of dielectric elastomers

We propose a protocol to model accurately the electromechanical behavior of dielectric elastomer membranes using experimental data of stress-stretch and voltage-stretch tests. We show how the relationship between electric displacement and electric field can be established in a rational manner from this data. Our approach demonstrates that the ideal dielectric model, prescribing linearity in the purely electric constitutive equation, is quite accurate at low-to-moderate values of the electric field and that, in this range, the dielectric permittivity constant of the material can be deduced from stress-stretch and voltage-stretch data. Beyond the linearity range, more refined couplings are required, possibly including a non-additive decomposition of the electro-elastic energy. We also highlight that the presence of vertical asymptotes in voltage-stretch data, often observed in the experiments just prior to failure, should not be associated with strain stiffening effects, but instead with the rapid development of electrical breakdown.