Research On Electrostrictive Strain Characteristics Of Electroactive Membraniform Dielectric Elastomer Material

Dielectric elastomer (DE) as a new kind of intelligent polymer material belongs to electric-filed-type electroactive polymers. When the material is put into electric field, it will be compressed in the direction of electric field and meanwhile be stretched in the plane perpendicular to the field, according to Maxwell effect.Dielectric Elastomer shows most promising in the manufacturing of actuators. With excellent performance parameters such as large and reversible elastic deformation, short response time, high energy desity, good flexibility and matching of load, high efficiency, high reliability and fatigue life, strong environmental tolerance and high actuation presure, etc., it is widely used in the design of all types of intelligent actuators.The potential applications of the electroactive material includes small size detecting robots, micro-pumps, micro-values, disk driving system, face robots, bend rollers, loudhailer, micro-aerocrafts, unidirection linear actuators, artificial limb equipments, loud speaker, tube robots and rulable medicine pipes, etc.This work was supported by National Natural Science Fundation of China (grant No.50777028). Relevant theoretical and experimental research were carried out to figure out characteristics of electrostrictive strain in Electro-active membraniform dielectric elastomer material. Main research work of this thesis can be summed up as following:1. Basic Concernning Factors of Electrostrictive Strain on Dielectric Elastomer Actuating UnitDielectric elastomer actuating unit is a basic unit of dielectric elastomer actuators (DEAs) which is composed of DE film and two compliant electrodes adhering to its upper and bottom sides. Once a high voltage difference is placed on the both electrodes, the unit will change its shape according to Maxwell stress. On the basis of introduing basic configuration and working principle of dielectric elastomer unit, qualitative analysis of main factors that might influence characteristics of its electrostrictive strain were analysised, such as dielectric permitivity, elastic modulus, thickness of the film, preload, etc..2. Research on Mechanical Characteristics of Dielectric Elastomer MaterialStarting from mechanical characteristics of DE material such as nonlinear, volume incompressible and large and reversible elastic deformation etc., several hyper-elastic material models such as Mooney-Rivlin, Neo-Hookean, Yeoh and Ogden models were selected to describe mechanical property of it. Relationship between nominal stress and practical strain of this volume compressible hyper-elastic material were derived from the bounduary condition of uniaxial pre-stretch, on the basis of strain energy function of Yeoh and Ogden models. Also, method to determine material coefficients of strain energy function in these two hyper-elastic materials were given. Material coefficients of these two strain energy function about a kind of typical DE material, Acrylic elastomer VHB4910, were obtained by fitting experiment data from uniaxial tension. This work laid the foundation for modeling and numerical simulation of characteristics of electrostrictive strain in electro-active membraniform DE materials.3. Research on Static Stress of Dielectric Elastomer Actuating UnitSimulation program COMSOL Multiphysics 3.5a was used in simulating distribution of the static stress on dielectric elastomer actuating unit. Simulation result indicates that static stress caused by electric filed is homogenously distributing on the the internal suface area and lateral surface area of the unit. Base on the hypothesis of the distribution of static stress on dielectric elastomer actuating unit only existing along the direction of electric field, the vertical static equivalent stress on the unit was obtained from derivation, and also the horizontal one, according to the Energy Conversion and Conservation Law.A method to obtain the relative permittivity of the film under different stretch rate by fitting the data from experiment was adopted considering the fact that the relative permittivity of the material is changing during deformation. According to the method, the law of relative permittivity changing with stretch rate of acrylic elastomers VHB4910 was obtained through experiments and it was found out that the relative permittivity of acrylic elastomers VHB4910 keep linearly decreasing with the increasing of stretch rate. On that basis, the method to calculate the vertical static equivalent stress on the unit more accurately has been proposed with revised relative permittivity. The work is useful for furhter calculating and simulating of electrostrictive strain of membraniform dielectric elastomer material.4. Research on Characteristics of Electrostrictive Strain in Electro-active Membraniform Dielectric Elastomer MaterialsCharacteristics of electrostrictive strain in electro-active membraniform DE materials were studied under the boundary condtions of free boundary, uniaxial and equi-biaxial pre-stretch, respectively. Mathematical models about characteristics of electrostrictive strain in DE material were proposed, based on Yeoh and Ogden strain energy function, respectively.The model took into account the influence of non-linearity mechanical property of membraniform material, decreasing of relative permittivity and changing of electrostatic equivalent stress during deformation process on characteristics of electrostrictive strain of electroactive DE material, synthetically. Thus it can efficiently describe the relationship among pre-stretch rate, actuation voltage and electrostrictive strain of the material under actuating process.The thesis took acrylic elastomer VHB4910, a typical kind of DE material, as an example, demonstrated a research method for electrostrictive strain characteristics based on Ogden strain energy function, under the boundary condtions of free boundary, uniaxial and equi-biaxial pre-stretch, respectively. The results show that the best atrain on one axis strain and area electrostrictive strain of the material can be obtained under the boundary condiction of equi-biaxial pre-stretch. By properly selecting pre-stretch rate, the electroactive film can achieve higher electrostrictive strain as well as lower the possibility of pull-in failure.The reason of pull-in failure was analysised by explaining the deformation process of electroactive dielectric elastomer material as a positive feedback electromechanical couple process.By numerical simulation of electrostrictive area strain with the mathematic model, stable and instable zone of the unit can be devided and also the limit stable voltage and top area strain can be achieved.Three main failure modes of electroactive film were analysised, that is mechanical strength, dielectric strength and pull-in instability. Also, their corresponding failure mechanism were set forth. Limit electrostrictive area strain of each failure mode can be obtained by experience formula, mathematic model and experiment research, synthetically. According to numerical simulation, the unit built with acrylic elastomer VHB4910 can obtain its highest area strain under boundary condition of equi-biaxial pre-stretch rateλopt=3.17. The result can be used in the design of DEAs with substrate material of VHB4910.5. Experiment Research on Characteristics of Electrostrictive Strain in Electro-active Membraniform Dielectric Elastomer MaterialsA new type of electroactive membraniform functional material (E-ACE) was bulit with acylic VHB4910 and silicon-gel mixture. A method for pre-stretching dielectric elastomer was proposed and also all kinds of circular actuator with different pre-stretch rate was built with the funtional material.Different boundary conditions, actuation voltages and compliant electrodes were chosen in the experiments and relative experiments were carried out to find out the influence of them on electrostriction strain of DEAs.Experiment results indicated that electrostrictive area strain of the actuators under equi-biaxial pre-stretch condition was much larger than non-prestretch and uniaxial pre-stretch condition, and the largest area strain of the DEAs occured between 250%-320% under equibiaxial pre-stress condition. Electrostrictive area strain of the DEAs can be improved by increasing bias voltage for any boundary condition. The influence of compliant electrodes material on the characteristics of electrostrictive area strain of DEAs also can not be ignored.Experiment results coincide with numerical simulation results, which further validates the applicability of the mathematics model. The result of this paper can provide theoretical guidance and application reference for the design of DEAs.