Dynamic behavior of electro-rheological fluid sandwich beams

Abstract

Electrorheological (ER) fluids are fluids with controllable rheological properties. The study of change in rheology of suspensions due to application of electric field is of interest both from academic and industrial points of view. When an electric field is applied to these fluids, they respond by forming chain-like structures which results in enhancement of apparent viscosity of the fluid by as high a factor as five orders of magnitude. Just as warm butter progressively changes from a mobile liquid to a plastic solid as the temperature falls, so an ER fluid progressively solidifies as the electric field increases. ER fluids exhibit yield behavior analogous to that of a Bingham plastic. It is found that the yield stress has quadratic dependence on the applied voltage. Typical values of the yield stress are of the order of 10 kPa under static conditions and 5 kPa under dynamic conditions. The interplay between electric fields, chain formation and viscosity gives a wide range of applicability to these fluids in various industries. Economy and relative ease of preparation are other features of ER fluids which make them an attractive candidate for various applications. Control over a fluid’s rheological properties offers the promise of new possibilities in engineering for actuation and control of mechanical motion. Any device that relies on hydraulics can benefit from ER fluid’s quick response times and reduction in device complexity. Devices designed to utilize ER fluids include shock absorbers, active dampers, and clutches. One of the engineering applications of ER fluids is vibration control. The dependence of stiffness and damping properties of ER fluids on applied electric field makes them an attractive candidate for active vibration control schemes. In automotive and aeronautics industries they are mainly used as active dampers for vibration control. Efforts are being made to embed ER fluids in various structures to mitigate vibration problems. Sandwich beams, because of their high strength-to-weight ratio, prove to be an appropriate choice for applications in aeronautics industry. However, the successful application of ER fluids in vibration damping thrives on the understanding of the underlying ER mechanisms that influence properties such as stiffness and damping. The current level of understanding in this area is not conclusive. It has been observed that ER dampers exhibit nonlinear characteristics such as Coulomb damping, hysteresis, and other types of material nonlinearities. The ER effect is influenced by factors such as temperature, humidity, pressure, particle concentration, and electric field. It is very essential to investigate these phenomena for successful implementation of ER fluid-based technologies in vibration control. Towards this end, experiments are conducted on sandwich beams with embedded ER fluids to understand and characterize their dynamic behavior. The focus of this thesis is to investigate the dynamic behavior of sandwich beams embedded with ER fluids, specifically:

To embed ER fluids in sandwich beams and study their vibration characteristics by experiments for various voltage inputs and concentrations of fluids. To study nonlinear effects and identify the system parameters such as stiffness and damping by force state mapping technique. To model the dynamic behavior of sandwich beam for vibration control problems.

Force state mapping technique is found to capture the dynamic characteristics of the ER fluid sandwich beam accurately. The results of the experiments conducted on the ER fluid sandwich beam for different voltages, different concentrations, and different excitation levels show the strong presence of ER effect and its ability to control the vibrations of the sandwich beams by altering the energy dissipation mechanisms. At low amplitudes, Coulomb damping seems to have some effect on the dynamics. At higher amplitudes, its effect diminishes relative to the viscous damping force. With the increase in electric field across the ER fluid, an increase of 25% to 50% in equivalent viscous damping is observed. Similarly, it is observed that with the increasing concentration of starch, the ER effect grows stronger. With increase in the amplitude of excitation, there is a decrease in the ER effect from the consideration of energy dissipation due to the fact that the stress experienced by the ER chains increases resulting in the weakening of the chain structure. Chapter 1 is a brief introduction to ER fluids. A review of the literature both on ER fluids and their application in vibration control problems is undertaken. The objectives and the outline of the thesis are given. Chapter 2 deals with the Force State Mapping (FSM) identification technique to identify nonlinear single degree of freedom (SDOF). Simulations on known systems are presented to show that FSM technique can identify the nonlinear characteristics of vibrating systems. Chapter 3 deals with the experiments on ER fluid sandwich beams. Fabrication of specimen and preparation of fluid are explained followed by a description of experimental setup and tests undertaken. Results of the experiments at various voltages and fluid concentrations are presented followed by discussion. Chapter 4 deals with the modeling of ER fluid sandwich beams as well as comparison of numerical simulations using FSM estimates of modal parameters with experimentally recorded displacements and velocities. Chapter 5 is a summary of the work reported in this thesis, and also lists suggestions for future research.