Special Issue Article Journal of Intelligent Material Systems and Structures 1–12 Ó The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1045389X15571386 jim.sagepub.com Enhanced properties of magnetorheological fluids: Effect of pressure Andrea Spaggiari and Eugenio Dragoni Abstract Magnetorheological fluids are extensively used in the industrial world to produce dissipative systems in an easily adjusta- ble or even self-adaptive way. Sometimes their intrinsic rheological properties fail to meet system requirements in terms of available forces or yield stress for a given design space. In technical literature, previous works show a dependencyof the shear strength of magnetorheological fluids on the internal pressure of the fluid, called squeeze strengthen effect. This work aims at the experimental validation of the behaviourof the magnetorheological fluids in both flow and shear modes under a given compressive state. Two specific ad hoc experimental test rigs are used for the campaign. The sys- tems are designed in order to apply the magnetic field and the pressure at the same time and the tests are carried out following a design of experiment method. The magnetic parts of the system are designed with the help of a magnetic finite element simulation software, then the experiments are performed and the results are collected. The output is ana- lysed through an analysis of variance approach, a statistical procedure that shows the influence of multiple variables on the system outputs. The outcome of the experimental tests confirms the beneficial effect of the pressure in both flow and shear modes, with performances up to three times compared with the datasheet values, where no pressure is considered. Keywords Magnetorheological fluids, pressure, experimental test, flow mode, shear mode Introduction Magnetorheological (MR) fluids are smart and control- lable materials that are widely used in several devices, such as dampers and clutches (Carlson, 2000; Jolly et al., 1999). MR fluids are suspensions of magnetizable particles in a carrier fluid; when a magnetic field is applied, solid MR particle chains are formed along the field lines. In order to move the fluid and break these chains, a threshold stress, called the yield stress (t y ), should be achieved. This is the key parameter for the MR fluids, since it is the maximum stress the fluid can withstand before starting to flow. This value is funda- mental in MR fluid design, because it is directly linked to the maximum force that can be dissipated by an MR device. Unfortunately, since t y is affected by a magnetic field, saturation of the ferromagnetic particles occurs at high induction fields and limits the maximum yield stress of the material. In literature, we can trace feasible ways to enhance the yield stress of MR fluids: that is, selecting a particle material with a larger magnetic saturation or increasing the volume fraction of ferromagnetic materials. Ginder and Davis (1994) used a finite element analysis to study the effect of magnetic nonlinearity and saturation of magnetic particles, show- ing that high yield stress (over 200 kPa) could be reached only at volume fraction of 50% of iron, which is too much for industrial applications. The limitation is due to the paste-like behaviour of the MR even without magnetic field and the very high density of the fluid. Tang et al. (2000) and Zhang et al. (2004) reported that the yield stress of MR fluids compressed along the direction of the magnetic field gave an increment in terms of t y of 10 times. They surmised the formation of thicker and thus stronger columns of particles that are able to sustain the load. This effect was previously Department of Engineering Sciences and Methods, University of Modena and Reggio Emilia, Reggio Emilia, Italy Corresponding author: Andrea Spaggiari, Department of Engineering Sciences and Methods, University of Modena and Reggio Emilia, Via Amendola, 2 – Campus S. Lazzaro, 42122 Reggio Emilia, Italy. Email: andrea.spaggiari@unimore.it at GEORGIAN COURT UNIV on February 27, 2015 jim.sagepub.com Downloaded from