Mechanical Properties Measurement of Electroactive Polymers

Electroactive polymers (EAPs), such as piezoelectric polymer, polyelectrolyte gels, dielectric elastomer and conducting polymer etc., are emerging as a new type of actuation materials for a broad range of actuator and transducer applications, because of their higher strain, higher response and higher efficiency. Acrylic elastomer films have demonstrated higher specific energy density (0.4J/g) and more than 100% actuated strains, and have been recommended for the artificial muscle actuators. Much research has been carried out to investigate the actuation properties of EAP films, however, little information is available for the mechanical properties of EAP films, which are crucial for designing EAP actuators. This work focuses on developing a means of characterizing the mechanical properties of EAP thin film materials, describing the mechanical behavior with the suitable constitutive models and determining the material parameters for the development of actuators. To measure the mechanical properties of EAP films, a uniaxial testing system is developed, which consists of a small-scale force transducer, a CCD camera, a National Instruments card and a laser displacement transducer. The loading and unloading cycles on film specimens are controlled by an Instron Machine. The applied force and the total are stored in the computer by the National Instrument card. A sequence of 2D images are recorded by the CCD camera to capture the deformation process of the film sample. Then, the displacements of the marks on the film surface vertical to the thickness plane are calculated from the sequential images by image analysis techniques. There are several well-known models available to describe the mechanical behaviors of the EAP films, such as Neo-Hookean, Mooney-Rivlin and Ogden models etc. To determine the most suitable constitutive models and corresponding material constants, a generalized method based on finite element analysis is proposed and implemented by interfacing with ABAQUS finite element package. The kernel of the method is to minimize the difference between the measured displacement field and the computed displacement field. A global optimisation algorithm, simulated annealing (SA), is used to minimize the objective. The experimental investigation on the mechanical properties of the dielectric elastomer film (VHB4910) is presented as an example to demonstrate the functions of the testing system and the developed method. The developed testing system and method can also be used for characterizing the mechanical properties of other EAP film materials.