Dielectric elastomer actuators

Stress and strain are introduced as tensor properties. Several theoretical models of stress-strain behaviour are presented, of which the Ogden model is capable of fitting the observed behaviour in the entire range of measurement. Dielectricity is defined, and Maxwell stress is derived. The theories of elasticity and Maxwell stress are combined, in the special case of a dielectric elastomer actuator (DEA) constrained in width. The resulting model has no adjustable parameters. Percolation properties are introduced, and an experiment is presented, in which the fraction of conducting graphite in insulating silicone rubber is varied. Mixtures of conducting carbon black in insulating polymer were developed and used as compliant electrodes. In order to produce stable electrodes, a system of one-component silicone glue and Ketjenblack suspended in heptane is developed. This mixture is sprayed on pre-strained silicone elastomer films, providing stable compliant electrodes. Procedures for manufacturing of DEA with two kinds of elastomer film are presented. Silicone actuators are made from spun elastomer films. Handling of thin silicone elastomer films is possible if temporary frames are used. VHBTM4910 actuators are made from a commercial glue, which has a higher dielectric constant and stretches farther than silicone. The manufacture and handling of VHB4910 actuators is easier, since the film itself provides gluing, such that frames and electrodes are very easy to apply. These DEA structures are simple, making it easier to describe them theoretically. Force-strain measurements are obtained using an actuator test bench. For a silicone actuator, the force-strain curve included artefacts of the additional stress from the compliant electrodes. Introducing a cut-off function, similar to the Fermi-Dirac distribution function, it is possible to fit the force-strain curve in the whole range of strain. The blocking force of a VHBTM4910 actuator is presented. The dependence of the blocking force on applied voltage was parabolic, as expected from the Maxwell stress equation. There are discrepancies, which are explained as a channelling of the developed actuation stress in the transversal direction. Measurements of electric breakdown strength of pre-strained VHBTM4910 are presented, for isotropic pre-strains. The electric breakdown strength is shown to be near inversely proportional to the thickness of the stretched elas-

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