Nonlinear behavior of magnetorheological fluids (MR) and MR dampers for vibration control of structural systems

This dissertation focuses on basic understanding of the behavior of Magnetorheological (MR) fluids and their applications in MR fluid dampers for vibration control of structural systems. Rheological properties, such as apparent viscosity, of a MR fluid change when it is subjected to an applied magnetic field. These changes are fast (on the order of milliseconds), reversible and repeatable.; A flow mode type rheometer is designed and built to evaluate the tunable rheological properties of MR materials in the presence of a magnetic field. The rheological properties of a commercial MR fluid and a newly developed magneto-rheological polymeric gel (MRPG) are investigated. Experimental shear stress results are presented for a shear strain rate range of 20s−1 to 20,000s−1. The results for apparent viscosity, dynamic and static shear yield stress under the influence of different applied magnetic fields are reported. All samples show strong shear thinning effect.; In order to incorporate the shear-thinning behavior of MR fluids, the Herschel-Bulkley model is proposed to analyze the flow behavior of MR fluids through pipes and parallel plates. A quasi-static model based on Herschel-Bulkley flow analysis is then applied to an MR fluid damper, which was designed and developed at the University of Nevada, Reno. The Herschel-Bulkley quasi-steady analysis is extended to consider the effect of fluid compressibility. A fluid mechanics-based model is also developed to account for an MR damper dynamic behavior. This model is based only on the physical parameters of the device and material properties of MR fluids. It is demonstrated that the proposed model can accurately predict the dynamic response of an MR damper over a wide range of operating conditions.; To evaluate the performance of MR dampers in structural vibration control, a 1/12 scaled, two-span bridge equipped with prototype MR dampers is also investigated. A theoretical study, incorporating the dynamic model of MR dampers is presented. A feedback on-off control law is employed based on Lyapunov approach. Both the theoretical and experimental studies show that the Lyapunov based control systems can effectively reduce the relative displacement between the deck and the abutment of the scaled bridge when subjected to various input motions.