Characteristics of magnetorheological fluids under single and mixed modes

Rheological properties of magnetorheological (MR) fluids can be changed by application of external magnetic fields. These dramatic and reversible field-induced rheological changes permit the construction of many novel electromechanical devices having potential utility in the automotive, aerospace, medical, and other fields. Vibration control is regarded as one of the most successful engineering applications of MR devices, most of which have exploited the variable shear, flow, or squeeze characteristics of MR fluids. These fluids may have even greater potential for applications in vibration control if utilized under a mixed-mode operation. This paper presents results of an experimental investigation conducted using MR fluids operated under dynamic squeeze, shear-flow, and mixed modes. A special MR fluid cell comprising a cylinder, which served as a reservoir for the fluid, and a piston was designed and tested under constant input displacement using a high-strength tensile machine for various magnetic field intensities. Under vertical piston motions, the MR fluid sandwiched between the parallel circular planes of the cell was subjected to compressive and tensile stresses, whereas the fluid contained within the annular gap was subjected to shear flow stresses. The magnetic field required to energize the fluid was provided by a pair of toroidally shaped coils, located symmetrically about the centerline of the piston and cylinder. This arrangement allows individual and simultaneous control of the fluid contained in the circular and cylindrical fluid gaps; consequently, the squeeze mode, shear-flow mode, or mixed-mode operation of the fluid could be activated separately. The performance of these fluids was found to depend on the strain direction. Additionally, the level of transmitted force was found to improve significantly under mixed-mode operation of the fluid.