Response of excited magnetorheological fluid along the field direction

Abstract

Magnetorheological (MR) fluid that is classified as a kind of smart material is a dispersion of magnetic particles in an inert carrier liquid. This material shows a dramatic but reversible increase in shear yield stress in the presence of an external magnetic field. The dramatic change in shear yield stress is due to the magnetic polarization induced in particles, resulting in the dipole-dipole interaction forces between particles, which, in turn, lead to the formation of particle chains along the direction of magnetic field. There are many researches and investigations on the field dependent shear properties of MR fluids, including experimental determination of the yield shear stresses, steady shear flow, stress relaxation under simple step shear and large amplitude oscillatory shear flow. They have given a framework to the rheological behaviour of MR fluids especially in shearing. A lot of their applications have been identified in the engineering field. For example, MR fluids are being used in the semi-active control damper for automotives, polishing/grinding technology and flexible fixture. Probably, it can also be used in medical applications. Nevertheless, the mechanical/rheological response of excited MR fluids along the field direction, which is referred to as the normal force, has not been explained clearly though researches on this issue have been conducted. Theoretical study based on the energy method was employed by Shkel and Klingenberg [57] to find that the relationship between the normal stress (33) and the magnetic field intensity (H) could be expressed as 33 H2. De Vicente et al. [8] carried out studies on the normal force generated by an excited MR fluid. They claimed that the normal force could only be generated where two criteria are satisfied: (a) the magnetic field exceeded a critical value; and (b) the MR fluid was under shear strain. The normal force increased with the shear strain up to a maximum and then deceased with any further increase in the shear strain. See and Tanner [9] reported that the normal force of a MR fluid could be generated by a magnetic field even where there was no shear action. The normal force increased with a magnetic field of B2.6 and if a shear was imposed, the normal force decreased. The drop of the normal force continued with an increase in the shear strain until it eventually settled at a steady value. The value at which it settled depended on the magnitude of the shear rate. The higher the shear rate applied, the lower the settled value was. These phenomena, whereby the normal force was increased by magnetic field and reduced by a shear, were respectively attributed to stronger elongated aggregates and the breaking of MR particle chains under shear strain. Laun et al. [10] have recently proposed that the relationship between the normal force and the magnetic field could be expressed as FN B2.4, and also found that the normal force could be increased with shear. These different observations inspire the research on the characteristics of the field dependent normal force of MR fluids. Furthermore, the understanding of the normal force characteristics can optimize the design of the MR devices and extend the engineering applications of MR fluids, such as the active axial force actuator and pattern polishing. The major objective of this thesis is to carry out experimental studies on the characteristics of the normal force of an excited MR fluid using a purposely built parallel-plate tester. All experiments were carried out with the tested MR fluid being placed between parallel plates under a magnetic field. The first experiment examined the change of the normal force of the MR fluid with the magnetic field. It was found that the relationship between the normal force (FN) and the magnetic field (B) is similar to the findings of other researchers as FN B2.006. The significant increase in the normal force of the excited MR fluid under no shearing is genuine. The second experiment was to investigate the change of the normal force of the excited MR fluid under steady shearing. Experimental results indicated that the normal force of the excited MR fluid increases with shearing as the same of Laun et al.’s results [10]. A model of the field dependent normal characteristics that was originally proposed for electrorheological fluids by Klingenberg and Zukoski [69] and used by Martin and Anderson [52] was employed here to explain the phenomenon of shearing enhanced normal force of the MR fluid. The results of this thesis can broaden the application potentials of MR fluids by employing the features of the field dependent normal force.

Date of Award	15 Jul 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pat Lam Patrick WONG (Supervisor)