Magnetorheological Fluids Employing Substitution of Nonmagnetic for Magnetic Particles to Increase Yield Stress

Magnetorheological fluids (MRFs) were synthesized to investigate potential enhancements in magnetorheology when replacing magnetic particles with nonmagnetic micro-scale glass beads, that is, to increase yield stress, while reducing density and particle settling rate. Two MRF samples having 40 volume percent (vol%) of particles were synthesized: MRF-40 and MRF-37. MRF-40 had 40 vol% of carbonyl iron (CI) particles, and MRF-37 contained 35.7 vol% of CI particles and 4.3 vol% of glass beads. A comparative study of MRF characteristics was conducted to determine the impact of the nonmagnetic glass beads on yield stress, as well as viscosity, and settling rate. Both MRFs were characterized as follows: 1) magnetorheology as a function of magnetic field, 2) sedimentation rate in a fluid column measured using an inductance-based sensor, and 3) cycling of a small-scale damper undergoing sinusoidal excitations at frequencies of 1 Hz for characterization and 4 Hz for endurance tests. Optical micrographs of the glass beads were taken before and after damper cycling to assess durability. The MRF with glass beads initially doubled the damper yield force (relative to the MRF with no glass beads) at high field strengths in damper tests. This increase in yield force did not persist as the damper was cycled in an endurance test and the glass beads eroded. Fe particle sedimentation rate was reduced by about 4% in the MRF with glass beads.