Magnetorheological fluid paper

MR fluid, foam and elastomer devices

J. David Carlson*, Mark R. Jolly

Lord Corporation, Materials Division, 110 Lord Drive, Cary, NC, 27511-7900, USA

Abstract

Magnetorheological (MR) fluids, foams and elastomers comprise a class of smartmaterials whose rheological properties may be controlled by the application of an external

magnetic field. MR fluids are liquids whose flow or shear properties are easily controlled toenable a variety of unique torque transfer or vibration control devices. MR foams, in whichthe controllable fluid is contained in an absorptive matrix, provide a convenient way of

realizing the benefits of MR fluids in highly cost sensitive applications. MR elastomers aresolid, rubber-like materials whose stiness may be controlled to provide tunable oradjustable mounts and suspension devices. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Materials whose rheological properties can be varied by application of magnetic

fields belong to a specific class of so-called smart materials because they can

respond, via solid-state electronics and modern control algorithms, to changes in

their environment. In this paper, consideration is given to materials consisting of a

suspension of non-colloidal, magnetically-polarizable particles in a non-magnetic

medium. These materials respond to applied magnetic fields and are thus referred

to as magnetorheological (MR) materials. Such materials can be utilized in devices

or can be incorporated in traditional composites to form advanced intelligent

composite structures, whose continuum magneto-rheological response can be

0957-4158/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.PII: S0957 -4158 (99)00064 -1

Mechatronics 10 (2000) 555569

* Corresponding author.

E-mail address: [email protected] (J.D. Carlson).

actively controlled in real-time. Applications that can benefit from materialswhose rheology can be continuously, rapidly and reversibly varied are numerous.The most common MR materials are of liquid state. The controllable

rheological response of such fluids results from the polarization induced in thesuspended particles by application of an external magnetic field. The interactionbetween the resulting induced dipoles causes the particles to form columnarstructures, parallel to the applied field. These chain-like structures restrict the flowof the fluid, thereby increasing the viscous characteristics of the suspension. Themechanical energy needed to yield these chain-like structures increases as theapplied magnetic field increases resulting in a field dependent yield stress. In theabsence of an applied field, the controllable fluids exhibit Newtonian-likebehavior.The initial discovery and development of MR fluids and devices can be credited

to Jacob Rabinow at the US National Bureau of Standards [36,40]. Interestingly,this work was almost concurrent with Willis Winslows work on electrorheological(ER) fluids [53]. Except for a flurry of interest after their initial discovery, therehas been scant information published about MR fluids. Only recently has aresurgence in interest in MR fluids been seen [46,7,17,19,21,25,31,41,46,51].The field-responsive behavior of MR fluids is often represented as a Bingham

plastic having a variable yield strength (e.g. [38]). For stresses t above the fielddependent yield stress ty, the flow is governed by Binghams equation:

t ty Z_g, t > ty 1Below the yield stress (at strains of order 103), the material behavesviscoelastically:

t Gg, t < ty 2where G is the complex material modulus. It has been observed in the literaturethat the complex modulus is also field dependent [35,52]. While the Binghamplastic model has proved useful in the design and characterization of controllablefluid-based devices, true controllable fluid behavior exhibits some significantdepartures from this simple model. Perhaps the most significant of thesedepartures involves the non-Newtonian behavior of controllable fluids in theabsence of a field [25,32].

2. MR materials

2.1. Composition

The composition of MR fluids is similar to their ferrofluid cousins: a highconcentration of magnetizable particles in a non-magnetic medium. Dierences inparticle size and composition however result in distinct behavioral dierences. Inparticular, MR fluid particle sizes typically range from 107 to 105 m one to

J.D. Carlson, M.R. Jolly /Mechatronics 10 (2000) 555569556

three orders of magnitude larger than colloidal ferrofluid particles. The larger MRfluid particles allow for stable, highly magnetizable materials and reversibleparticle aggregation. Typical micron-sized MR particles will support hundreds ofmagnetic domains. Domain dipole rotation in the presence of a field causesinterparticle attraction. Maximum interparticle attraction and thus maximummagnetorheological eect is increased by choosing a particle material with highsaturation magnetization Js. Iron has the highest saturation magnetization ofknown elements with Js=2.1 Tesla. Iron particles with spherical shape obtainedfrom the thermal decomposition of iron pentacarbonyl are commonly used. Alloysof iron and cobalt are known to have slightly higher saturation magnetization (upto Js=2.4 Tesla) and have also been used in MR fluids [11]. Typical particlevolume fractions are between 0.1 and 0.5.Researchers at BASF [32] have created MR fluids using ferrite-based particles

on the order of 30 nm in diameter coated with long chain molecules. These fluids,which are very similar to ferrofluids, are reported to have excellent stability andabrasion properties. They, however, exhibit an order of magnitude less yieldstrength than iron-based MR fluids resulting from inferior magnetic properties offerrite and the predominance of thermal particle forces.Carrier liquids are typically chosen based upon their rheological and

tribological properties and on their temperature stability. Typically, petroleumbased oils, silicone, mineral oils, polyesters, polyethers, water, synthetichydrocarbon oils and others are used. Ashour et al. used a synthetic EAL arcticseries lubricant produced by Mobil [2]. Kormann et al. used polar liquids such as:triethylene glycol, diethylene glycol methyl ether, hexyl and cyclohexyl acetate,methyl propionate, and others [32]. MR fluids often contain other additives toprovide additional lubricating properties, as well as additives that inhibitsedimentation and agglomeration. Sedimentation is typically controlled by the useof thixotropic agents and surfactants such as xantham gum, silica gel [41],stearates and carboxylic acids [53]. The thixotropic networks disrupt flow atultralow shear rates (the viscosity becomes nearly infinite) but thins as the shearrate is increased. Stearates form a network of swollen strands when used inconjunction with mineral oil and synthetic esters that serve to entrap particles andimmobilize them. Fine carbon fibers have also been used for this purpose [42,43].The fibers build viscosity through physical entanglement but exhibit shear thinningdue to shear-induced alignment.

2.2. Basic physical properties

A summary of the basic properties of typical MR fluids is given in Table 1. MRfluids routinely exhibit dynamic yield strengths in excess of 50 kPa for appliedmagnetic fields of 150250 kA/m [10,25]. The o-state viscosity for MR fluids isgenerally in the range of 0.101.0 Pas at 258C. The ultimate strength of MR fluidsis limited by magnetic saturation. Operational temperatures for MR fluids easilyrange from 408C to +1508C and are generally limited by the volatilityproperties of the carrier fluid rather than the details of the polarization

J.D. Carlson, M.R. Jolly /Mechatronics 10 (2000) 555569 557

mechanism. Unlike ER fluids, dissipative electric currents and joule energy loss inMR fluids are not a concern. One is able to eectively use permanent magnets toenergize MR fluids with no steady-state power requirement at all.MR fluids are not highly sensitive to contaminants or impurities such as are

commonly encountered during manufacture and usage. Further, because themagnetic polarization mechanism is not aected by the surface chemistry ofsurfactants and additives, it is relatively straightforward to stabilize MR fluidsagainst particle-liquid separation in spite of the large density mismatch. Most MRfluids are quite dense with specific gravity in the range of 34 due to their highcontent of dense iron particles.The factor Zp=t

2y field is a figure of merit useful in estimating how large a given

MR fluid device must be in order to achieve a specified level of performance[10,26]. The minimum volume of active fluid in a device is proportional to thisfactor. Typical MR devices require 250 watts of input power. Severalcommercially available MR fluids are given in Table 2.

2.3. Field-responsive eect

The field responsive eect of the two commercial MR fluids is shown in Fig. 1.This shear stress data was taken at relatively low shear rates and thusapproximates the fluid yield stress as defined in Eq. (1). At low fields, MR fluids

Table 1

Typical MR fluid properties [10]

Property Typical value

Maximum yield strength, ty(field) 50100 kPaMaximum field 0250 kA/mPlastic viscosity, Zp 0.11.0 Pa sOperable temperature range 40 to 1508C (limited by carrier fluid)Contaminants unaected by most impurities

Response time

are seen to exhibit sub-quadratic behavior. Indeed the MR fluids exhibit anapproximate power law index of 1.75 at low and intermediate fields. Thissubquadratic behavior is attributed to gradual particle saturation with increasingfield and is, in part, predicted by contemporary models of magnetorheology[22,28]. Beyond fields of about 0.1 Tesla, the eects of bulk magnetic saturationare revealed as a departure from power law behavior. The stress responseultimately plateaus as the MR fluids approach complete magnetic saturation.Simple theory predicts that the ultimate yield stress of MR fluids is proportionalto fJ 2s where f is the particle volume fraction and Js is the particle saturationmagnetization. Fig. 2 demonstrates the quadratic dependency of MR fluid yieldstress on particle saturation magnetization.

Fig. 1. Shear stress versus applied field for two commercial MR fluids.

Fig. 2. The quadratic dependency of MR fluid stress on particle saturation magnetization. Each data

point corresponds to an MR fluid made from a dierent iron-based alloy.


2.4. Zero-field rheology

The viscosity of controllable fluids in the absence of a field is most significantlya function of the carrier oil, suspension agents, and particle loading. Rheologicalfigures-of-merit for controllable fluids [25,26] benefit from low fluid viscosity, butmust be balanced with other fluid requirements such as temperature range andparticle resuspendability. Because of the inclusion of suspension agents andchanges in particle microstructure during shear, most MR fluids exhibit significantshear thinning.

2.5. Magnetic properties

Magnetic induction curves, or BH curves, of the MR fluids are shown inFig. 3. As can be seen, the MR fluids exhibit approximately linear magneticproperties up to an applied field of about 0.02/mo A/m (mo=4pe

7 Tm/A is thepermeability of a vacuum). In this region, the permeabilities are relatively constantat approximately 59 times that of a vacuum. MR fluids begin to exhibit gradualmagnetic saturation beyond the linear regime. Complete saturation typicallyoccurs at fields beyond 0.4/mo A/m. The intrinsic induction or polarization density(BmoH ) of MR fluids at complete saturation is fJs Tesla, where f is the volumepercent of particles in the fluid and Js is the saturation polarization of theparticulate material [28]. Little or no hysteresis can be observed in the inductioncurves. This superparamagnetic behavior is a consequence of the magnetically softproperties of the iron used as particulate material in these fluids and the mobilityof this particulate phase.

Fig. 3. Magnetic properties of two commercial MR fluids.


2.6. Magnetorheological elastomers

Structurally, field responsive elastomers can be thought of as solid analogs of

field responsive fluids. Like many field responsive fluids, field responsiveelastomers are composed of polarizable particles dispersed in a polymer mediumand the physical phenomena responsible for the field sensitivity of theseelastomers is very similar. There are however some distinct dierences in the wayin which these two classes of materials are typically intended to operate. The mostnoteworthy is that the particle chains within the elastomer composite are intended

to always operate in the pre-yield regime while field responsive fluids typicallyoperate within a post-yield continuous shear or flow regime. Indeed the strengthof field responsive fluids is characterized by their field dependent yield stress whilethe strength of field responsive elastomers is typically characterized by their fielddependent modulus.

Typically, magnetic fields are applied to the polymer composite duringcrosslinking such that particle chain (columnar) structures form and becomelocked in place upon final cure. Such processing has been used for some time toimpart special anisotropic properties on viscoelastic materials. Only recently hasthe field responsiveness of the viscoelastic properties of these elastomers beenexplored. The formation of columnar particle structures within elastomers

corresponds to a low dipolar energy state. Shearing of the cured composite in thepresence of the field causes particle displacement from this low energy state,thereby, requiring additional work. In principle, this required additional workrises monotonically with applied field, thus resulting in a field dependent shearmodulus.

Experiments on double lap shear specimens of MR elastomers were reported byJolly et al. [29]. Testing involved recording the complex modulus of various

Fig. 4. The eect of average composite flux density on the elastic modulus for MR elastomers

containing 10% (D), 20% (o) and 30% (x) iron by volume (adapted from Jolly et al. [29]).


specimens at various frequencies, strains and applied magnetic fields. The eect ofaverage composite flux density on the elastic modulus is shown in Fig. 4 for threetest specimens of 10, 20 and 30% carbonyl iron by volume. As can be seen, thechange in modulus increases monotonically with increasing volume percentage ofiron. While the maximum change in modulus increases to nearly 0.6 MPa as ironvolume concentration increases to 30%, the percentage of maximum increase inmodulus for the three samples remains relatively constant between 3040%. Thesame researchers observed a pronounced drop o in the magnetorheological eectand a corresponding increase in field dependent energy dissipation (tand) at strainsabove 12%. This strain dependency was attributed to the onset of magneticyielding of the particle chains.

2.7. Magnetorheological foams

MR fluid foam devices contain MR fluid that is constrained by capillary actionin an absorbent matrix such as a sponge, open-celled foam, felt or fabric [7,8].The absorbent matrix serves to keep the MR fluid located in the active region ofthe device between the poles where the magnetic field is applied. The absorbentmatrix requires only a minimum volume of MR fluid that is operated in a directshear mode without the need for seals, bearings or precision mechanicaltolerances. The absorbent matrix is normally attached to one of the poles.Application of the magnetic field causes the MR fluid in the matrix to developyield strength and resist shear motion. This basic arrangement may be applied inboth linear and rotary devices wherever a direct shear mode would normally beused.Because of their open structure, the shape of a MR fluid foam device is much

less constrained than that of a normal controllable MR fluid device. Multipledegrees of freedom are easily accommodated. Linear devices such as dampers maybe tubular, flat or planar while rotary brakes may take on the form of a localizedmagnetic caliper operating on a thin, un-housed disc. MR fluid foam devices arehighly robust and exhibit very low o-state forces. They are particularly suitablefor low to medium force applications where a high dynamic range is desired.Fluids in these devices are resistant to gravitational settling because of the wickingaction of the matrix.

3. Engineering with MR materials

3.1. Typical modes of use

Virtually all devices that use MR fluids can be classified as having either: (a) avalve mode (flow mode); (b) a direct shear mode (clutch mode); (c) a squeeze filmcompression mode; or (d) a combination of these modes. Diagrams of these basicmodes of operation are shown in Fig. 5. Examples of valve mode devices includeservo-valves, dampers, shock absorbers and actuators. Examples of shear mode


devices include clutches, brakes, chucking and locking devices, dampers andstructural composites. While less well understood than the other modes, thesqueeze mode has been explored for use in small amplitude vibration and impactdampers [12,27].

3.2. Active material volume

Eq. (3) gives the minimum active fluid volume, V=Lwg, necessary to achieve adesired control ratio Fon/Fo at a given speed S and maximum force Fon. This isthe amount of MR fluid that must be energized at any given instant in order toachieve a specified mechanical performance. (For an MR damper this is theamount of fluid that is actually in the valve, not the total amount of fluid that fillsthe damper.) This minimum fluid volume is proportional to fluid viscosity andinversely proportional to the square of the yield stress.

Vrk

Zpt2y field

!FonFoff

Fon S 3

where k=1 for shear mode devices and k02 for flow mode devices.The above equation often provides a simple means of assessing the feasibility of

a given application. Most successful MR fluid devices require only a very few cm3

of active fluid. For a representative MR fluid having a maximum yield strength of50 kPa and a plastic viscosity of 0.25 Pa s the factor Zp=t

2y field equals 10

10 s/Pa.Since the maximum energy density that needs to be established in the fluid isapproximately 0.1 J/cm3, the minimum electrical power requirement in watts isapproximately 0.1 times the fluid volume (in cm3) divided by the time required toinput the energy. (For rotary applications one can simply use torque in N-m andangular speed in rad/s).

3.3. Other practical considerations

The bandwidth of controllable fluid devices is largely determined by factorsextrinsic to the fluid such as the dynamics associated with field generation. These

Fig. 5. Basic operation modes for MR fluids.


include coil dynamics and eddy current eects in MR fluid devices. An advantageof MR fluids is the ancillary power supply needed to control the fluid. MRdevices can be powered directly from common, low voltage sources. Further,standard electrical connectors, wires and feedthroughs can be reliably used. Thisaspect is particularly important in cost sensitive applications and is one of the keyadvantages of MR fluids versus ER fluids.Because of the high loading of dense iron, MR fluids are heavy. In weight

sensitive applications this fact needs to be considered. While the active volume ofthe MR fluid may be quite small, the total fluid volume may be significantly largerdepending on the actual application, e.g. long stroke shock absorbers. Of concernin many rotary applications, e.g. clutches, are centrifugal eects. Because of thelarge density dierence between particles and liquid, centrifugal separation canoccur at high rotational speeds. However, for brakes in which the housing isstationary, centrifugation is generally less of a concern because of the continualshear induced remixing. Particle and fluid density mismatch are a concern forgravitational settling. However, because of the great flexibility one has in choosingsurfactants and additives, this concern can usually be addressed successfully. MRfluids exhibiting long-term stability with little or no sedimentation are achievable[25,32,34].

4. Applications

In parallel to increasing theoretical understanding of these materials, there hasbeen considerable eort over the past decade to improve the practicality ofcontrollable materials. MR fluid-based devices have recently enjoyed commercialsuccess in exercise equipment [1,14], for vehicle seat vibration control [11,34] andfor primary automotive suspensions [3,13,45]. Although there is currently littlepublished on applications of elastomers with controllable rheology, there is littledoubt that there are numerous applications that can make use of controllablestiness and the unique anisotropic characteristics of these elastomers. In thissection some of the main application areas of MR fluids are reviewed. The readeris also referred to several review articles that discuss applications in more detail[11,26].

4.1. Applications of MR fluids

A main application area for MR fluids is in devices for torque transfer whichinclude brakes and clutches [1,9,20,39]. Fig. 6a shows a schematic of an MR fluid-based disk-type brake (or clutches, if the housing is allowed to rotate). Other basicMR fluid-based brake/clutch geometries, including the so-called concentriccylinder-type, are disclosed by Rabinow [39]. MR fluid-based brakes are currentlycommercially available from Lord Corporation and are being used in variousexercise equipment [1,11,14]. Controllable fluid-based brakes and clutches may


also soon find commercial success in various automotive applications andtensioning applications [30].Another main application area for controllable fluids is in dampers and mounts

for use in semi-active or adaptive vibration control and snubbing. There has beeninterest in applying this technology to automotive applications, such as primarysuspension [13,45], secondary suspensions [11,34,48] and engine mounts [16,44].Fig. 6b shows a schematic of an MR fluid damper. It can be seen that theoperation of this device is fundamentally dierent from that of brakes andclutches, in that MR fluid is forced through annular orifices rather than beingdirectly sheared. Lord Corporation currently sells the MR damper in Fig. 6bwithin a system for use in vehicle seat vibration control. Other applications ofvibration control using controllable fluid dampers include seismic damping [18,46]and helicopter rotor damping augmentation [24].

4.2. Applications of MR elastomers

Elastomers with field responsive rheology hold promise in enabling simplevariable stiness devices. Although there are few applications appearing in theliterature for controllable elastomers, there are countless applications for systems

Fig. 6. Rotary MR fluid brake (a) and linear MR fluid damper (b).


that employ a variable stiness. Among these are adaptive tuned vibrationabsorbers (TVAs) [33,49], stiness tunable mounts and suspensions [15,23], andvariable impedance surfaces [37]. Ford Motor Company has patented anautomotive bushing employing a magnetorheological elastomer [47,50]. Thestiness of the bushing is adjusted based on the state of the automobiles powertrain to reduce suspension deflection and improve passenger comfort. For TVAsemploying MR elastomers, the fractional change in natural frequency can becalculated in terms of the fractional change in the modulus. In particular, it iseasy to show that:

Dooo

1 DG

Go

s 1 4

where DG/Go is the fractional change in modulus and Do/oo is the correspondingfractional change in natural frequency.In addition to the field dependent rheological response of these elastomer

composites, utility may also be found in their inherent anisotropic properties. Thisanisotropy is a result of the unique structure of the particles within the matrix.Indeed, it has been observed that elastomer composite materials are anisotropic interms of mechanical, magnetic, electrical, and thermal properties. Mechanicalanisotropy, for example, may be used to reduce the complexity of elastomerbearings and other laminated systems. Flexible materials with electrical andthermal anisotropy can find abundant usage in electronics packaging applications.Elastomeric materials with magnetic anisotropy may find usage in magnetic fluxfocusing in electromagnetic devices.

4.3. Application of MR foams

The basic elements of a simple, linear, MR fluid foam damper are shown inFig. 7a. No seals or bearings are required and only about 3 ml of MR fluid areneeded. A layer of open-celled, polyurethane foam saturated with MR fluidsurrounds the steel bobbin and coil. Together, these elements form a piston on theend of the shaft that is free to move axially relative to the tubular housing. Thesteel tube provides the magnetic flux return path. Since MR foam dampers stressthe MR fluid in a direct shear mode, maximum force is proportional to the areaof active MR fluid foam. Control currents of l amp or less and correspondingoperating voltages of 12 volts or less are typical. Typical performance curves for aMR sponge damper as described above are shown in Fig. 7b. The low-o stateforce and large dynamic range possible with this type of damper is readilyapparent.MR fluid foam dampers exhibit long life. Little wear of the foam matrix occurs

as the stresses are carried by the field induced iron particle structure in the MRfluid. Further, performance is largely unaected by wear of the foam. The fit ofthe foam in the gap between the poles is not critical; successful devices have been


constructed in which the precompression of the foam ranges from 0 to 70%. Theabsence of seals, bearings and gas accumulators found in normal fluid dampersmeans that the achievable stroke length is virtually unlimited.Fig. 7c shows a caliper type of brake geometry. Rather than a housing that

fully encloses the rotor, the MR fluid and magnetic circuit are localized in asimple, magnetic caliper arrangement. The absorbent foam filled with MR fluid isattached to the pole faces of the steel yoke. Again, the containment of the MRfluid in the absorbent foam eliminates the need for a fluid seal. MR foam brakesof this sort can provide a very large controlled torque simply by using a largediameter rotor. If the rotor is very thin it is not even necessary that it be madefrom a highly magnetically permeable material. Partial arc versions in which therotor is a pie shaped sector are another possibility.

5. Conclusion

The technology of materials with field responsive rheology is currently enjoyingrenewed interest within the technical community in terms of fundamental andapplied research. Research eorts of the past decade in field responsive materials

Fig. 7. Simple, low cost MR foam devices: (a) vibration damper; (b) damper performance; and (c)

rotary caliper brake.


are beginning to pay o. There are now several commercial MR fluids available.Recently, MR fluid-based devices have enjoyed commercialization within theexercise industry and transportation industry. The emergence of new applicationsfor controllable materials and the ongoing commercialization of both materialsand devices provide an impetus for continued research in this area.

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Magnetorheological fluid paper