Magnetorheological (MR) fluids are a class of adaptive materials that reversibly change from a liquid to a weak solids state through an externally applied magnetic field. The rheology of magnetorheological fluids as a function of magnetic field strength, shear rate, and temperature have been widely characterized in the steady shear regime. However, the dynamic behavior of MR fluids at small oscillatory strain amplitudes (e.g., γ = 10-4 – 10-1) and high frequencies (f = 5 – 50 Hz) has not been extensively reported in the open literature. Better characterization and a fundamental understanding of the dependence of the MR fluid storage and loss moduli on the control factors are critical for developing dynamic models to enable design of dampers and other electrically controllable devices. In this paper, we describe a shear-lap test method for measuring the shear moduli as a function of applied strain, magnetic field strength, and frequency. We find that magnetized MR fluids are viscoelastic below a threshold strain. Above this critical strain, they behave as nonlinear viscoplastic materials with significant damping capacity.
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