Abstract

The MR fluids can change their rheological behavior rapidly and reversibly under an applied magnetic field. Due to their unique characteristics, these promising controllable fluids can be effectively utilized in devices and structures in a reliable and fail-safe manner to suppress vibration with minimal power requirement. While modeling of MR dampers and their integration in systems and structures to control vibration have been widely studied, there are relatively few studies on MR based sandwich structures. MR sandwich structures can provide better vibration control capability as their damping and stiffness characteristics can be simultaneously varied. Considering this, the main objectives of this dissertation are to develop accurate models to predict the vibration characteristics of MR based sandwich plates under varying magnetic field and also develop design optimization strategies to identify optimal MR fluids treatments.
MR fluids typically experience low shear strain in sandwich structures and thus they operate in pre-yield region and behave like visco-elastic materials. In this study, first MR fluids have been accurately characterized in pre-yield region. Particularly new frequency-magnetic flux dependent constitute models for both loss and storage moduli have been proposed. To accomplish this, an experiment is conducted on a sandwich beam structure with aluminum face layer and MR fluid as the core layer, under different magnetic field densities. The frequency response characteristics of the sandwich cantilevered beam are subsequently measured under harmonic base excitations. Dynamic responses of the structure are also obtained using the developed finite element (FE) model. The frequency and field dependent complex shear moduli of the MR fluids (MRF 132DG and MRF 122EG) are then identified by minimizing the error between natural frequency and damping parameters obtained by experiment and FE model. The validity of the proposed constitute models is demonstrated by comparing the FE model results with the experimental data for a copper sandwich structure comprising the two MR fluids.
Next, the characterized MR fluids have been used as the core layer in the fully and partially treated sandwich plates. The goal is to predict the dynamic responses of the MR sandwich plates under different levels of magnetic field. To accomplish this, finite element and Ritz models based on the classical plate theory are formulated to obtain governing equations of motion of the multi-layer rectangular and circular sandwich plates fully and partially treated with MR fluids as the core layer under different boundary conditions. Extensive experimental studies have been conducted to validate the developed models. Then, the validated models have been effectively utilized to conduct comprehensive investigation on the effect of MR fluid properties, geometry of the face layers, thickness of the core layers and magnetic flux density on vibration suppression capability of the sandwich plate structures. Finally, an optimization problem is formulated based on genetic algorithm (GA) to identify optimal locations for the MR fluid treatments, resulting in maximum variations in the stiffness and damping of the structure, corresponding to the lower three modes of flexural vibration in response to the applied magnetic field. The effects of shear deformation on the vibration properties of fully and partially treated sandwich structures are discussed, comprehensively.