## 2 Active Systems
### Field-Active Fluids in Energy Dissipating Systems
#### 2.2.2 Active Sandwich Structures  
A sandwich structure consists of two skin layers and a core. Typically, the skin layers are thin but stiff, while the mechanical characteristics of the core material will depend on the application itself. The concept of sandwich structures containing ERFs and MRFs was developed by [PERSON] et al. [95] and [PERSON] et al. [96], respectively. The active fluid is operatively enclosed between containment layers, as depicted in Figure 2 for the use of an ERF. In this case, the containment layers contribution is two-fold, providing for structural integrity and serving as conductive electrodes. An electric potential is then developed between them for providing the electric field across the electrorheological fluid. The non-conductive encasement prevent electrical arcing and failure of the system. Thus, the sandwich structure has complex shear and tensile modulus properties controlled by varying the electric (or magnetic in the case of using MRFs) field applied thereto. These active sandwich structures can be applied into a variety of extended mechanical systems such as plates, panels, beams and bars or structures and there are countless engineering applications for these materials with controllable structural behavior, which include the control of vibrations and sound propagation [95]. Other type of active sandwich structure consists of a viscoelastic damping solid layer constrained by a piezoelectric film [97; 98], whose damping effect can be controlled by varying the voltage applied to the film. This latter approach may be somewhat effective, but it shows various shortcomings related with the performance of the piezoelectric ceramic materials, which maybe either inapplicable because of their weight, their brittleness or the fact that large, thin sections are difficult to fabricate. Alternative piezoelectric polymers could be used instead, due to they are light-weight, flexible and available in large, thin sheets; but they are impractical because they are unable of producing the required forces. Therefore, active sandwich structures using ERFs provide tunable, reversible and flexible mechanical structures which substantially minimize the above mentioned and other problems typically associated with conventional construction and operation of active sandwicles controlled by a piezoelectric film [95]. Unfortunately, the complex shear and compression/tension moduli exhibited by the ERFs are several orders of magnitude smaller than that exhibited by conventional viscoelastic solids or polymers. The use of MRFs also provides the controllable characteristics exhibited by ERFs, but with complex shear and compression/tension moduli comparable in magnitude to conventional viscoelastic solids [96; 99].  
The reader can find a comprehensive review on different fabrication techniques, experimental methods, mathematical modeling, and methods of solution in the work of [PERSON] et al. [100]. Nevertheless, to the best knowledge of the author, there is a lack of studies providing a quantitative comparison between the damping performance of active sandwich structures using ERFs and MRFs and those more classical devices using a piezoelectric control.