Issue 23

A. Spaggiari et alii, Frattura ed Integrità Strutturale, 23 (2013) 75-86; DOI: 10.3221/IGF-ESIS.23.08

Scilla 2012 - The Italian research on smart materials and MEMS

Effect of pressure on the physical properties of magnetorheological fluids

A. Spaggiari, E. Dragoni Dept. of Engineering Sciences and Methods, University of Modena and Reggio Emilia, Italy andrea.spaggiari@unimore.it

A BSTRACT . To date, several applications of magnetorheological (MR) fluids are present in the industrial world, nonetheless system requirements often needs better material properties. In technical literature a previous work shows that MR fluids exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluid based devices are rotary devices, this paper investigates the behaviour of MR fluids under pressure when a rotation is applied to shear the fluid. The system is designed in order to apply both the magnetic field and the pressure and follows a Design of Experiment approach. The experimental apparatus comprises a cylinder in which a piston is used both to apply the pressure and to shear the fluid. The magnetic circuit is designed to provide a nearly constant induction field in the MR fluid. The experimental apparatus measures the torque as a function of the variables considered and the yield shear stress is computed. The analysis of the results shows that there is a positive interaction between magnetic field and pressure, which enhances the MR fluid performances more than twice. K EYWORDS . Magnetorheological fluids; Shear mode; Pressure; Design of experiment. agnetorheological (MR) fluids are smart materials that are increasingly used in many applications, such as controllable clutches and dampers [1]. An external magnetic induction field reversibly changes the apparent viscosity of MR fluids. MR fluids are capable to switch from a free-flow liquid at no induction, to a quasi solid state when a strong magnetization is present. The quickness of change (5-10 milliseconds) when a magnetic field is applied, makes this material interesting for adaptive damping and dissipative applications. MR fluids can be used to build silent, fast and tunable mechanical devices, which are enhanced by the ease of integration with the electronic control unit. The MR effect is obtained by a 30-40% in volume of ferromagnetic particles dispersed in the carrier fluid (silicon or hydrocarbon oil). When a magnetic field is applied to MR fluids, the particles are subjected to a dipole magnetic moment and align with the flux lines. Consequently there is a formation of linear chains of particles (at a ), which is the maximum stress the fluid can withstand before shear occurs. This value is fundamental in the design of any MR fluid devices, because the higher is the stress the higher is the dissipated power of the system. Since many MRF devices are dissipative (e.g. dampers) the higher the power the better the performance. The yield shear stress, τ y is controlled by the magnetic field, as shown in the technical datasheet supplied by the producer ( Lord Corporation [2]) for the commercial MR fluid 140-CG. There is a magnetic saturation of the magnetic particles in MR fluids at high magnetic flux levels [2] which causes a typical sigmoid shape of the B-H curve of the material as reported in the producers datasheets. This saturation leads to a limitation of τ y , and unfortunately the dissipated power reaches a maximum no matter how high the magnetic field is. It is possible to increase the value of τy by changing the micro-scale) which means that, at the macro-scale, the MR fluid becomes a solid-like material. One of the most important design parameter of MR fluids is the yield shear stress ( τ y M I NTRODUCTION

75