Design and performance evaluation of magnetorheological fluids under mixed mode

Magnetorheological (MR) fluids are capable of providing continuously variable damping forces in response to a magnetic stimulus. Most MR devices have exploited the variable shear or flow characteristics of MR fluids. Greater potential application in vibration control/isolation may be sought if these fluids are utilised in the squeeze flow mode in which displacement levels are limited to few millimetres, but large force levels are available. In this paper, the results of an experimental assessment of the comparative performance of MR fluids in dynamic squeeze, shear-flow, and mixed modes are presented.

A mixed mode MR cell comprising a cylinder which provides the reservoir for the fluid and a piston was designed and fabricated. The cylinder is subjected to an oscillatory sinusoidal motion while the piston is fixed. This motion simultaneously subjects the fluid sandwiched between the parallel circular-planes of the cell to a compressive and tensile loading, and the fluid contained in the cylindrical gap to a combined shear and flow loading. The magnetic field required to energise the fluid is provided by a pair of toroidally shaped coils, disposed symmetrically about the cylinder/piston centreline. This arrangement permits either a separate excitation of the fluid contained in the circular and cylindrical gaps to simulate a single squeeze mode and a combined shear-flow mode, respectively or a simultaneous excitation of the fluid to simulate the mixed shear-flow and squeeze mode.

The cell was tested under various loading and magnetic field intensity conditions. Comparisons between the different modes of operation were carried out and the transmitted force level was seen to be greatly enhanced when the fluid was employed under the mixed mode. In addition, the implications of the results to vibration control, where MR fluids are utilised in short-stroke damping applications, are discussed.