A new class of materials termed magnetorheological elastomers (MREs) are developed that respond to externally imposed magnetic fields. Magnetic particles are embedded in viscoelastic solids or liquid elastomeric precursors. This kind of composite demonstrates a unique combination of good magnetic controllability and elastic properties. Polybutadiene (hydrocarbon based) based polyurethane MREs are developed because of their thermooxidative, hydrolytic, and chemical resistance. The structure-property relationships of polyurethaneMREs are investigated using several characterization techniques. Morphological features such as interdomains of soft and hard segments are identified with tappingmode atomic force microscopy. The thermal and mechanical behavior is evaluated with dynamic mechanical analysis, differential scanning calorimetry, and stressstrain tests.

This study presents a modular, large-scale, magneto-rheological (MRF) by-pass valve to be used in seismic damper retrofits for energy mitigation. The by-pass valve is designed, constructed and tested. The MR valve can be used to retrofit a commercial passive seismic damper as a semi-active device. The performance of the MRF valve was characterized by means of quasi-static characterizations. A new MR fluid is also developed for the seismic by-pass MRF damper application. This MR fluid has low off-state viscosity and high field-dependent yield strength. The fielddependent rheology of the MR fluid is evaluated with a MR shear rheometer. In addition, a theoretical model is developed taking into account geometric dimensions, fluid properties and applied magnetic field strength. Threedimensional electromagnetic finite element analysis is used to determine and maximize the magnetic field strength inside the by-pass MRF valving region. Both experimental and theoretical results show that the modular large-scale by-pass MRF damper can generate sufficient dynamic force range which meets the high-force requirements of large-scale structures subjected to seismic or other significant hazards.