A Rigorous Thermodynamic Description of Dielectric Elastomer Actuators Meets Capacitive Extensometry for Transient Strain Analysis
Sprache des Vortragstitels:
2009 MRS Spring Meeting, San Francisco, USA
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Dielectric elastomer actuators (DEAs) are promising for bionic and robotic applications, energy harvesters and adaptive optical elements. Reliability requirements in such applications necessitate better knowledge of the safe operation regimes and possible failure modes of DEAs. Elastomers consist of a network of entangled and cross-linked polymer chains, able to sustain spatially varying stress and strain fields that store elastic energy. We present a rigorous thermodynamic description of DEAs, based on statistical mechanical models including cross-links and entanglements. This provides valuable insight for material development and optimization. We investigate the dependence of the DEA’s energy on stretch and voltage and combine our models with experimental observations applying real-time in situ electrical capacitive measurements. DEAs are based on deformable capacitors with highly compliant electrodes. When a voltage is applied between these electrodes a significant Maxwell stress is acting on the DEA. Thereby the elastomer film is squeezed in the thickness direction and expanded in the film plane, due to the incompressibility of the material. The deformation of the elastomer and thus the actuation state of the DEA is determined by the applied voltage. Minimization of the free energy of an elastomer actuator yields its deformation state as a function of the applied voltage. The predicted values are compared to measured transient strain data obtained by capacitive extensometry, showing a good agreement between theory and experiment. In addition, we investigate multistabilities in the energy landscapes of elastomer actuators. Unwanted switching between multiple states is a key issue in applications. One example, which frequently occurs in electromechanical systems, is the so called pull-in instability, which is observed experimentally for circular actuators. It can cause a dielectric breakdown of the elastomer leading to a failure of the system.