A rheological investigation of electrorheological materials subjected to small strains

The present investigation focused on the study of the small strain, dynamic behavior of electrorheological (ER) materials at room temperature. Emphasis was placed on the influence of ER rheology on the long-term frequency response of ER material based -adaptive structures. Both Alternate Current (AC) and Direct Current (DC) ER materials were tested for linearity (strain dependence) at small strain amplitudes. The results of the rheological experiments indicated that both AC and DC ER materials behaved in a nonlinear viscoelastic fashion; this rendered the linear viscoelastic theory invalid for the description and characterization of ER materials under dynamic shear. Factors which influenced the nonlinear viscoelastic properties were electric field strength, strain amplitude, and strain frequency. Investigations were conducted on adaptive structures which utilized ER materials as the damping layer. The reliability of these structures in terms of variations in vibration modal frequency and loss factor over time were found. The rheology of the two ER materials was found to have caused fundamental differences in the frequency response of the respective ER adaptive structures. Factors which affected the time response of these structures to on-off states in electric field were: electric field strength, electric field application time, and amplitude of excitation. The response of this class of adaptive structures to the-applicatiOn ofeIedricfIeld ~a'y be approximated as instantaneous, while the response to the removal of electric field could be extraordinarily long-especially at high excitation frequencies. CHAPTER 1: INTRODUCTION OF ELECTRORHEOLOGICAL MATERIALS AND THEIR APPLICATIONS 1.1 ER Materials-What are They and What are They Used For? Since Winslow's discovery of electrorheological (ER) fluids in 1949, much has been conducted in this relatively new field. Although first called "electroviscous" fluids by Winslow, "electrorheological" fluids soon replaced the previous name. This is because while the apparent viscosity of these fluids does increase dramatically with the application of an electric field, "electrorheological" better describes the changes in viscoelastic properties of these fluids under deformation. The studies first conducted by Winslow (1949) were performed on silica powder in kerosene under the influence of an electric field. Winslow reported that the formation of the fibrils in the direction of electric field was responsible for the increase in shear resistance of the bulk fluid. The shear resistance of the fibrils, or chains, was an active agent in affecting the fluid's apparent viscosity under varying degree of electric field intensity. The reversibility of this phenomenon upon electric field removal was also reported in this pioneering work. _ •• _ ~_ c . • ¢". '~ By definition, ER fluids are primarily a colloidal suspension of polarizable particles , in a dielectric medium. In the absence of an electric field, the fluid flows freely, and the particulate distribution is random. But upon application of an external electric field, the particles align themselves in the direction of the field and form fibrils or chain-like structures between the electrodes. The formation of these fibrils or chains increases the viscosity of the fluid and changes the rheological behavior of the ER material, i.e. the ER material's ability to resist shear and deformation increases. When the electric field is removed,-thebulkfluid re1lims to zero-field behavior. The response time of this reversible phenomenon is generally claimed throughout available literature to be on the order of a few milliseconds. The phenomenon described above is illustrated in Figure 1.1.