Junjie Sheng

Dynamic electromechanical performance of viscoelastic dielectric elastomers

Because of their viscoelasticity, dielectric elastomers (DEs) are able to produce a large time-dependent electromechanical deformation. In the current study, we use the Euler-Lagrange equation to characterize the influence of temperature, excitation frequency, and viscoelasticity on the dynamic electromechanical deformation and stability of viscoelastic dielectrics. We investigate the time-dependent dynamic performance, hysteresis, phase diagram, and Poincare map associated with the viscoelastic dissipative process. The results show that the dynamic response has strong temperature and frequency dependencies. It is observed that the natural frequency of the DE decreases with increasing temperature and the maximal amplitude increases at higher temperatures. At relative low frequencies, the amplitude is very small and the viscoelasticity has a significant effect on the oscillation of the system. Furthermore, the results show that the viscoelasticity has a relatively major influence on the dynamic performance for DEs that have very low relaxation times.

Nonlinear dynamic characteristics of a dielectric elastomer membrane undergoing in-plane deformation

This paper proposes a free energy model to study the dynamic characteristics of a dielectric elastomer membrane undergoing in-plane deformation, subject to the combined loads of a mechanical press and an electric field. The natural frequency of the small-amplitude perturbation around the state of equilibrium is calculated with focus on the damping effects and the resonance phenomenon. The numerical results, such as the oscillation, phase diagrams and Poincare maps, are presented to show the influence of the damping on the nonlinear dynamic characteristics of the dielectric elastomer. The numerical results indicate that pre-stresses, damping effects and applied voltages could tune the natural frequency and modify the dynamic behavior of the dielectric elastomer. There is a stability transition when taking the damping effect into account. The damping effect could cause the dynamic responses to constant vibration and decrease the amplitude. These conclusions may guide the exploration of high-performance dielectric elastomers under dynamic mechanical and electrical loads.

Experimental study on the dynamic response of in-plane deformation of dielectric elastomer under alternating electric load

Recently, dielectric elastomer actuators (DEAs) have garnered remarkable attention mainly due to their ability of large deformation. Previously, the dynamic responses of out-of-plane deformations of inflated and clamped dielectric elastomer (DE) membranes were experimentally investigated, and a quasi-static model of large deformation concerned with the configuration was derived. However, the research work on the time-varying response of in-plane deformation of DE is insufficient. In this paper, we studied the dynamic response of the in-plane deformation of a dielectric elastomer membrane under a pure-shear state. We experimentally analysed how this response was affected by the peak voltage, frequency, pre-stretching, and signal waveform. The deformation equilibrium position of the membrane drifted severely during vibration, which may be attributable to the high viscoelasticity of the membrane and may lead to issues when designing precise instruments. We also studied how the peak voltage, frequency, pre-stretching, and waveform affected this viscoelastic drifting.

Effect of temperature on the stability of dielectric elastomers

Dielectric elastomer (DE) is a kind of electroactive polymer material, capable of large deformation up to 380%. However, under conservative operating conditions, DE is susceptible to instability with a small deformation due to various modes of failure, including electrical breakdown, electromechanical instability (EMI), loss of tension and rupture by stretch. This paper proposes a free energy model in the thermodynamic system of DE involving thermoelastic strain energy, electric energy and purely thermal contribution energy to obtain the stability conditions of all failure modes. The numerical results indicate that the increase in temperature can markedly contribute to improving the entropy production, the actuation stress and the critical nominal electric field of the DE. The increase in temperature could modify the failure modes of loss of tension and the EMI, which consequently enhances the stability of DE. Simultaneously, estimations on the maximal energy to be converted can be made from the theoretical formulation of the stability states. These conclusions may guide the exploration for high-performance DE materials under thermo, mechanical and electrical loads.

Thermal, Mechanical, and Dielectric Properties of a Dielectric Elastomer for Actuator Applications

Dielectric elastomers (DE) are a new type of electro-active material, which is able to produce a large degree of deformation under electrical stimulation. The thermal, mechanical, and dielectric properties of the most widely used dielectric acrylic elastomer (VHB 4910), commercially available from the company 3M, were studied by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and broadband dielectric spectroscopy (BDS) analyzer, respectively. DSC experiments on the VHB 4910 showed a glass transition at about −40°C. VHB 4910 started to lose weight at about 250°C from the TGA study. The results of DMA indicated the storage modulus of VHB 4910 increased with frequency and had a strong temperature dependence of elasticity. The dielectric constant of VHB 4910 increased as a function of temperature up to 0°C, followed by a drop till 100°C. The mechanical and electrical efficiency of dielectric elastomer actuators (DEA) of VHB 4910 were analyzed. It was demonstrated that the actuation performance is dominated by the mechanical properties of the elastomer and is less influenced by the frequency and the temperature dependence of the dielectric properties; this may be used to guide the design of actuator configurations, as well as the choice of actuator materials.

Constitutive relation of viscoelastic dielectric elastomer

In this paper, we present a modified model describing the constitutive relation of viscoelastic dielectric elastomer (DE). The uniform uniaxial tension-recovery experiment was carried out at different stretching rates. Based on Yeoh hyper-elastic model, model-fitting approach is put forward to obtain the relationship between parameters of Yeoh model and stretching rate, thus the modified model was obtained. From the approximate relationship between harmonic motion and uniform reciprocating motion, the stress—strain curve in the recovery process was also identified through the hysteresis between stress and strain. The modified model, with concise form and evident physical concept, can describe the strong nonlinear behavior between deformation and mechanical stress of the material in a common stretching rate range (from 0.01s−1 to 0.8s−1 at least). The accuracy and reliability of the modified model was examined.

Effect of temperature on the electromechanical actuation of viscoelastic dielectric elastomers

The electromechanical deformation of viscoelastic dielectric elastomers (DEs) is primarily governed by three material parameters: permittivity, Youngs modulus, and relaxation time. All three parameters are functions of temperature, so a complete description of the electromechanical behaviour of a DE must take thermal effects into account. In this paper, we have established a physical model for viscoelastic DEs that takes temperature effects into consideration. The actuation of a DE was measured under different temperatures to verify the model. A peak actuation stretch was obtained at around 363 K both experimentally and theoretically. Moreover, we also demonstrate the contribution of strain-stiffening induced by greater pre-stretching to the improvement of thermostability.

Effect of viscoelastic relaxation on the electromechanical coupling of dielectric elastomer

Dielectric elastomer is able to produce a large electromechanical deformation which is time-dependent and unstable due to the visco-hyper-elasticity. In the current study, we use a thermodynamic model to characterize the viscoelastic relaxation in the electromechanical deformation and instability of a viscoelastic dielectric. The parameters in the model were verified experimentally. We investigate the time-dependent mechanical deformation, electrical breakdown strength, polarization, and the electromechanical stability which are coupled by viscoelastic relaxation. The results show the electromechanical stability has strong time-dependence, due to the stress relaxation when the pre-stretch is applied.

Viscoelastic performance of dielectric elastomer subject to different voltage stimulation

Dielectric elastomer (DE) is capable of giant deformation subject to an electric field, and demonstrates significant advantages in the potentially application of soft machines with muscle-like characteristics. Due to an inherent property of all macromolecular materials, DE exhibits strong viscoelastic properties. Viscoelasticity could cause a time-dependent deformation and lower the response speed and energy conversion efficiency of DE based actuators, thus strongly affect its electromechanical performance and applications. Combining with the rheological model of viscoelastic relaxation, the viscoelastic performance of a VHB membrane in a circular actuator configuration undergoing separately constant, ramp and sinusoidal voltages are analyzed both theoretically and experimentally. The theoretical results indicated that DE could attain a big deformation under a small constant voltage with a longer time or under a big voltage with a shorter time. The model also showed that a higher critical stretch could be achieved by applying ramping voltage with a lower rate and the stretch magnitude under sinusoidal voltage is much larger at a relatively low frequency. Finally, experiments were designed to validate the simulation and show well consistent with the simulation results.

Effect of temperature on the electric breakdown strength of dielectric elastomer

DE (dielectric elastomer) is one of the most promising artificial muscle materials for its large strain over 100% under driving voltage. However, to date, dielectric elastomer actuators (DEAs) are prone to failure due to the temperature-dependent electric breakdown. Previously studies had shown that the electrical breakdown strength was mainly related to the temperature-dependent elasticity modulus and the permittivity of dielectric substances. This paper investigated the influence of ambient temperature on the electric breakdown strength of DE membranes (VHB4910 3M). The electric breakdown experiment of the DE membrane was conducted at different ambient temperatures and pre-stretch levels. The real breakdown strength was obtained by measuring the deformation and the breakdown voltage simultaneously. Then, we found that with the increase of the environment temperature, the electric breakdown strength decreased obviously. Contrarily, the high pre-stretch level led to the large electric breakdown strength. What is more, we found that the deformations of DEs were strongly dependent on the ambient temperature.