Vibration control is very fundamental and significant issue in various engineering fields such as flexible structures, rotor systems, cable and bridge, vehicle suspension and so on. So far, there are three different recipes to suppress or control unwanted vibrations are used: passive, semi-active and active. As well known, the passive method has several limitations such as the lack of real time avoidance of the time-varying resonances. On the hand, active vibration control method is very effective, but it is not attractive in terms of cost due to the use of several actuators and sensors. Therefore, recently semi-active vibration control method is popularly used in many practical environment. Prior to deeply investigating the semi-active method, some vibration control works are reviewed. Yanik et al 1 proposed a simple method to determine required damping and control parameters for the passive control of the structures based on optimal control gain of closed loop classical control and found that the damping and stiffness parameters could be modified. Routaray et al 2 developed eddy current damper to control the vibrations of cantilever beam by supplying electric current. Beltran et al 3 undertook an active vibration absorption scheme for the linear-spring damper mechanical system subjected to exogenous multi-frequency harmonic excitations and found that the proposed scheme was very useful to suppress the vibration and hence to achieve desired position. Roy et al 4 investigated the dynamic behaviour of a stepped 2-material composite beam by observing the first natural frequency and frequency response amplitude. Huang et al 5 investigated the transient vibrations of viscoelastic beam subjected to both actions of an alternating uniform transverse magnetic field and axial periodic force. They have found that the effects of viscosity parameter, axial load and magnetic field on the transient amplitudes and vibration frequencies of the first mode are significant.Aravindhan and Gupta 6 addressed on vibration control of rotors using MR Fluid dampers during transient operating conditions and a flexible symmetric rotor with one control disk was considered. Computer simulation results were carried out using finite element method for the different configurations and it has been identified that the vibration controllability of the rotor system is possible using MR damper. Gupta et al 7 also investigated MRF based finite squeeze film damper integrated in the rotor-bearing system and developed an useful a mathematical model in which film pressure, stiffness and damping coefficients can be obtained. Xu et al 8 investigated the principle and dynamic properties of semi active vibration absorber (SAVA) using an optimum variable step size control strategy of SAVA was investigated. Zhang et al 9 studied for vibration reduction of the flexible structures using Fourier response functions equivalent damping ratios by adopting a single-degree-of-freedom and two-degree-of-freedom systems.
Recently, the authors of this paper published on vibration control of flexible sandwich structures containing controllable magneto-rheological (MR) fluid core 10, 11. It has been verified that the natural frequency can be appropriated tuned by supplying proper input current (or magnetic field) to MR fluid domain.In this review article, theresearches on the semi-active vibration control of flexible sandwich structures in which damping treatment and stiffness change are controllable are deeply investigated. Specifically, these structures can be effectively used in aerospace, automotive, construction industry, marine and even sports equipment. Sandwich construction offers designers high strength to weight ratio which is of extremely important in aerospace application, buckling resistance, formability to complex shapes and easy reparability. Sandwich construction can also offer energy and vibration damping when the viscoelastic core layer is used. Due to many advantages from the sandwich construction, the analysis of sandwich beams or plates has been investigated by a large number of scholars for more than four decades. In general, the viscoelastic materials such as many polymeric and glassy materials are frequently used as layers to augment damping property of sandwich structures.