A typical MR fluid consists of 20-40 percent by volume of relatively pure, 3-10 micron diameter iron particles, suspended in a carrier liquid such as mineral oil, synthetic oil, water or glycol. A variety of proprietary additives, similar to those found in commercial lubricants to discourage gravitational setting and promote particle suspension, are commonly added to LORD Corporation’s state-of-the-art MR fluids to enhance lubricity, modify viscosity and inhibit wear. For most engineering applications, a simple Bingham plastic model is effective in describing the essential, field-dependent fluid characteristics.
MR fluids made from iron particles exhibit maximum yield strengths of 50-100 kPa for applied magnetic fields of 150-250 kA/m. MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and usage. Further, because the magnetic polarization mechanism is unaffected by temperature, the performance of MR-based devices is relatively insensitive to temperature over a broad temperature range (including the range for automotive use).
MR fluids are usually applied in one of two modes. MR fluid operating in valve mode, with fixed magnetic poles, may be appropriate for hydraulic controls, servo valves, dampers, and shock absorbers. The direct-shear mode with a moving pole, in turn, would be suitable for clutches and brakes, chucking/locking devices, dampers, breakaway devices and structural composites.