Research On The Electromechanical Behavior And Dynamic Performance Of Dielectric Elastomers

In the mechanical engineering design and manufacture, hard materials like metals and ceramics are widely used. However, the main structures of animals and plants in the nature are constituted by soft materials. As one of the typical soft materials, dielectric elastomers have advantages such as large deformation, fast response, low cost, lightweight, high energy density, etc. Thus, it has been primarily used to make diverse practical applications such as artificial muscles, energy harvesters, soft robotics, adaptive optics, etc.Many problems in the research of dielectric elastomers are still required to solve. Most of the studies on the dielectric elastomers only analyze d its performance in static model, without taking the time-dependent dissipative processes such as viscoelastic relaxation into consideration. In addition, the form of applied voltage is obscure, and the performance of dielectric elastomers under the conditions of different voltages is rarely studied. Furthermore, as an important factor that affects the performance of dielectric elastomers, temperature is often neglected. In the static model, pre-stretch has been approved to be a help to the realization of large deformati on of dielectric elastomers. However, these results are necessary to be further studied in the dynamic model with viscoelastic relaxation considered.To address these problems, the electromechanical behavior and the dynamic performance of dielectric elastomers are studied in this thesis. The typical failure modes include loss of the tension, rupture by stress, electromechanical instability and electrical breakdown are studied in detail. The allowable area that enclosed by these failure modes is obtained. The effects of temperature on the failure modes and allowable areas are also analyzed. Considering the dissipative processes such as dielectric relaxation, the effect of temperature and pre-stretch on the dynamic performance of dissipative dielectric elastomers are studied, respectively. The results should give a guidance to the design and manufacture of high performance dielectric elastomer actuators, as well as the realization of large deformation.Firstly, the basic theories of the dielectric elastomers are introduced. The work principles for dielectric elastomers acted as generators and actuators are explained, respectively. Different free energy expressions and various relative dielectric constant expressions are categorized. The methods to deduce the equations of state under the equilibrium and nonequilibrium thermodynamic frameworks are also researched respectively. The dissipative processes such as viscoelastic relaxation are explained, with the rheological model outlined.Secondly, the effect of temperature on the electromechanical behavior is studied. The specific equations for different failure modes: loss of the tension, ruptured by stress, electromechanical instability(EMI) and electrical breakdown(EB), are obtained to analyze the effect of temperature on them. The influence of temperature on the allowable areas that enclosed by the failure modes is investigated accordingly. In addition, the phenomenon of electrical breakdown(EB) resulted from electromechanical instability(EMI) and the effect of temperature on this phenomenon are studied.Thirdly, the impact of temperature on the dynamic performance of a dissipative dielectric elastomer is analyzed. The viscoelastic relaxation is specifically researched with the influence of temperature on it. Under two voltage actuation conditions: constant voltage and ramping voltage, the impact of temperature on a dissipative dielectric elastomer is separately studied.Lastly, the dynamic influence of pre-stretch on the dynamic performance of a dissipative dielectric elastomer is studied. The join effects of voltage and pre-stretch on the deformation and maximum actuation strain are analyzed for the constant and ramping voltage actuation methods, respectively. Consequently, the best join conditions of voltage and pre-stretch are separately recommended for the two actuation methods, in order to realize the large deformation of dissipative dielectric elastomers.