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 magnetorheological devices, most of which have exploited the variable shear, flow or squeeze characteristics of magnetorheological fluids. These fluids may have even greater potential for applications in vibration control if utilised under a mixed-mode operation. This article presents results of an experimental investigation conducted using magnetorheological fluids operated under dynamic squeeze, shear-flow and mixed modes. A special magnetorheological 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 magnetorheological 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 energise 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.