Small-strain dynamic mechanical behavior of magnetorheological fluids entrained in foams

An oscillatory double-lap-shear configuration was used to study the dynamic mechanical properties in shear of magnetorheological (MR) fluids entrained in open-cell polyurethane foams. The dynamic viscoelastic mechanical properties of the fluid were studied by varying strain amplitude, frequency, and magnetic field strength. We find that these samples exhibit linear viscoelastic behavior below a threshold level of strain and only in zero applied magnetic field. Above the critical strain, and in applied magnetic fields, they behave as nonlinear viscoelastic materials with significant damping capacity that is strongly dependent on the field applied perpendicular to the direction of strain. For example, the energy dissipated by the material increases by over an order of magnitude when the flux density is increased from 0 to 0.6 Tesla. In addition, the magnitude of the complex modulus increases from 170 kPa in 0 Tesla to over 2 MPa with an applied flux density of 0.6 Tesla. We also found that the overall dynamic mechanical material properties changed very little with applied mechanical frequency in the range studied, 0.1 to 10 Hz.