Nonlinear vibration analysis and optimal damping design of sandwich cylindrical shells with viscoelastic and ER-fluid treatments

Viscoelastic and smart fluid materials such as electro-rheological (ER) and magneto-rheological fluids have been used in many applications in industry to suppress vibration in sandwich shell structures. The main objective of this dissertation is to investigate and develop analysis models and design optimization strategies to optimally suppress the vibration of cylindrical shell/panel type structures using both passive and semi-active treatments. This dissertation constitutes two major related parts. In the first part, passive treatment using viscoelastic layer is studied for sandwich cylindrical shell using semi-analytical finite element modeling. In order to provide more accuracy a higher order Taylor’s expansion of transverse and in-plane displacement fields is developed for the core layer of sandwich cylindrical shell structures including the least number of degrees of freedom. The developed model is then employed to formulate cut and partial treatment modeling which are applied to increase damping and reduce the weight of the structure. The formulations are also modified in order to consider the slippage between layers at the interfaces. A systematic parametric study is presented to investigate the effect of main parameters such as temperature, vibration amplitude, pre-stress components, slippage and etc on vibration damping characteristics of sandwich shell structure. The temperature distribution at each layer is obtained by solving the transient heat transfer equation for axisymmetric cylindrical structure based on the finite difference method using irregular grid. Finally, by combining the semi-analytical finite element method and the optimization algorithms a design optimization methodology has been formulated to maximize the damping characteristics in sandwich cylindrical shell using the optimal number and location of cuts and partial treatments with the optimal thicknesses of the treating layers. In the second part of the dissertation, semi-active treatment using smart ER fluid layer is studied. The shear stress response and the dynamic mechanical properties of the ER fluid created by dispersing cornstarch into corn oil are experimentally explored for small/large shear strain amplitude, moderate range of frequencies and different field intensities. A new constitutive model has been also proposed to predict accurately the measured experimental data in both frequency and time domains. Then, the nonlinear vibration analysis of sandwich shell/panel structure with constrained electrorheological (ER) fluid is investigated for different boundary conditions using the finite element method. In order to reduce the computational costs, a new notation referred to as H-notation is also developed over the two well known notations referred to as B and N notations in order to represent the nonlinear equations of motion. Finally, a design optimization methodology has been presented to maximize damping in sandwich cylindrical panel using both unconstrained viscoelastic and constrained ER fluid damping layers. The unconstrained viscoelastic layer is employed in order to practically seal the constrained ER fluid patches and boundaries of the ER based sandwich structure. Then, an optimization problem has been formulated to find simultaneously the optimum number and distribution of unconstrained viscoelastic and constrained ER fluid patches, electric field intensity and thickness ratios of the treating layers.