Dielectric Elastomer: Pre-straining
Pre-straining is done as it improves the performance of the film . With pre-straining, the thickness of the elastomer decreases. So comparatively less voltage is required to receive a specified strain. Pre-straining reduces the buckling of the electrode and also increases the dielectric-breakdown strength of the film . Another effect of pre-straining is reduction in dielectric constant of the DE film .
In our work, the sample has been pre-strained bi-axially using the equipment shown in the following . The equipment is developed in house and a patent is filed for it. The equipment consists of couple of clamps which are connected to a circular shaft. By rotating the circular shaft the clamps will move towards or away from the centre. The film must be placed on the clamps as shown in Figure.
Before placing the DE film the dimensions of the rectangular area formed between the clamps must be taken. These dimensions serve as initial reading of the DE film .i.e at no-strain condition. By slowly rotating the circular shaft the pre straining of the DE film is done. Using vernier from time to time, strain measurement is done in individual directions. Figure shows the DE film while pre-straining.
Fig 1. Biaxial pre-straining apparatus
After pre-straining the DE film to the required value, to maintain the strain on the film, 2 circular shaped cardboard's is fastened on either side of the film. After fastening the cardboard, the film is removed from the pre-straining device and the slack part which is outside the cardboard region is removed. Figure 3.5 shows the pre-strained DE film after fastening cardboards. Now the pre-strained DE film is ready for electrode coating.
Fig 2. DE sample after fixing cardboard
 Raj Kumar Sahu, Munshi Imran Hossain, Karali Patra and Shovan Bhaumik, "Dielectric elastomers: materials for future actuators", 10th National Conference on Industrial Problems on Machines and Mechanisms (IPRoMM 2010), MNIT, Jaipur (Rajsthan) December 17-18, 2010.
 Alexander Barnes, Qiang Liu, George Young, Tien Fu Lu, "Evaluation of selected dielectric elastomer for use in an Artificial Muscle Actuator", Proceedings of the Australasian Conference on Robotics and Automation, pp. 1-9, 2007.
 Michael Wissler, Edoardo Mazza, "Electromechanical Coupling in dielectric elastomer actuators", Sensors and Actuators A, pp. 384-393, 2007.