Nabeel Ahamed

SEMINAR REPORT

ON

MAGNETORHEOLOGICAL FLUIDS AND ITS

APPLICATION IN INDUSTRIAL SHOCK

ABSORBERS

Submitted by

Mr. NABEEL AHAMED

In partial fulfilment for the award of the degree

Of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

LBS COLLEGE OF ENGINEERING KASARAGOD

KERALA, INDIA - 671542

MARCH 2013

1

CERTIFICATE

This is to certify that the Seminar Report entitled “MAGNETORHEOLOGICAL

FLUIDS AND ITS APPLICATION IN INDUSTRIAL SHOCK ABSORBERS”

submitted by ‘NABEEL AHAMED’ to the University of Kannur in partial fulfilment of

the requirements for the award of the Degree of Bachelor of Technology in Mechanical

Engineering is a bonafide record of work carried out by him under my guidance and

supervision. The contents of this report, in full or in parts, have not been submitted to

any other Institute or University for the award of any Degree.

Place: Signed by

Date: Mr. Anil Kumar B.C.

Assistant Professor, MED

Signature of Head of the Department

2

ACKNOWLEDGEMENTS

Sometimes words cannot express the feelings in its fullness. I express sincere gratitude to my

HOD Prof. MOHAMMED SHEKOOR and my tutor Mr. SREEJITH.M for their valuable

suggestions and instructions. I express my deep gratitude to my guide, Mr. ANIL

KUMAR.B.C for his valuable guidance. Also I remember my friends who helped me a lot. I

am thankful to my parents for giving help and support throughout the seminar. Above all I

am thankful to the almighty lord for making this seminar a success.

3

ABSTRACT

Magnetorheological fluids are suspensions of solids in liquid whose properties changes

drastically when exposed to magnetic field. They are micron sized, magnetisable particles

suspended in an appropriate carrier liquid such as mineral oil, synthetic oil, water or ethylene

glycol. When magnetic field is applied the stress required to make the fluid flow called the

“yield stress” of the fluid increases in a matter of milliseconds. Due to these special

characteristics it has got wide application in the field of mechanical engineering.

Magnetorheological fluids are now used in automobile clutches, machineries and some

researchers are going on. The activation of Magnetorheological fluid clutch’s built in

magnetic field causes a fast and dramatic change in the apparent viscosity of the

Magnetorheological fluid contained in the clutch. The fluid changes state from liquid to semi-

solid in about 6 milliseconds. The result is a clutch with an infinitely variable torque output.

In this presentation a brief introduction of the topic, physical and chemical properties of

Magnetorheological fluid, equations and working and various applications are listed out. Also

the application of Magnetorheological fluid in clutches are explained and highlighted in

detail. Advantages, limitations and future scopes are also discussed.

4

List of contents:

CHAPTER 1: INTRODUCTION 8

CHAPTER 2: FIELD RESPONSIVE FLUID 10

CHAPTER 3: A GOOD MR FLUID 13

3.1. Chemical composition 13

3.2. Physical properties 16

3.3. Magnetic properties 17

3.4. Rheological properties 18

CHAPTER 4: ADVANTAGES AND DISADVANTAGES 21

CHAPTER 5: WORKING 22

CHAPTER 6: APPLICATIONS OF MR FLUIDS

6.1. Industrial Shock Absorbers 24

6.2. Clutches 27

6.3. Automotive Industries 28

6.4.Optics 29

6.5. Human Prosthesis 29

6.6. Military and Defence 29

CONCLUSION 30

REFERENCE 31

5

LIST OF TABLES

Table 2.1.Comparison the Properties of MR Fluids, ER Fluids and Ferrofluids 11

Table.3.2.1.Properties of MR Fluid 17

6

LIST OF FIGURES

Fig.1.1. MR fluid particle distribution: a) no magnetic field b) with magnetic field. 11

Fig.3.1.1.Composition of MR fluids 15

Fig.3.3.1.Schematic representation of the affine deformation of a chain of spheres 17

Fig.3.4.1.Classification types of the behaviour of the fluid 19

Fig.5.1. MR Fluids in the absence of magnetic field 22

Fig.5.2. Alignment of MR particles under the action of magnetic field 22

Fig.6.1.1.Examples of typical hydraulic shockabsorbers(a) with a by passvalve and (b)

With an orifice between the piston and cylinder 24

Fig.6.1.2.Design of MR ShockAbsorber 25

Fig.6.1.3.Different braking characteristics 26

Fig.6.1.4.Diagram of shock absorber with MR fluid 27

Fig.6.1.5.Static characteristics of a built shock absorber with the gap height equal to

(a) 0.5mm and (b) 0.25mm. 27

Fig.6.3.1.schematic and photo of fluid damper 28

7

CHAPTER 1: INTRODUCTION

Science and technology have made amazing developments in the design of and machinery

using standard materials, which do not have particularly special properties (i.e. steel,

aluminium, gold). Imagine the range of possibilities, which exist for special materials that

have properties scientists can manipulate. Some such materials have the ability to change

shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost

instantly when near a magnet. Magnetorheological fluid falls under this category.

Amongst these smart fluids, MR fluids gain more attention since they can produce the

highest stress, which can be applied into many applications. An MR fluid is a suspension of

micron-sized magnetically soft particles in a carrier liquid, which can exhibit dramatic

changes in rheological properties. The change from a free-flowing liquid state to a solid-like

state is reversible and is dependent on the presence of a magnetic field. Iron powder is the

most popular material used as particle inclusion due to its high saturation magnetization.

Under the influence of a magnetic field, these iron particles are arranged to form very strong

chains of “fluxes” with the pole of one particle being attracted to the opposite pole of another

particle. Once aligned in this manner, the particles are restrained from moving away from

their respective flux lines and act as a barrier preventing the flow of the carrier fluid.

Magnetorheological fluids are magnetically polarisable particles suspended in viscous

fluids. They have the ability to change their rheological properties as shear modulus and

viscosity reversibly in milliseconds when subjected to magnetic fields. While the magnetic

particles are randomly distributed in the liquid when no magnetic field is applied, they form

chains in the presence of a magnetic field, and as a result rheological properties of the fluid

increase. Typically, the magnetisable particles are metal or metal oxide particles with size of

on the order of a few microns. The viscous fluid can be a non-magnetic liquid, usually oils.

Additionally, surfactants are used to allow for high particle volume fractions of the

Magnetorheological fluids that can yield higher variations in the rheological properties, and

increase the fluid’s stability against sedimentation. Depending on the type of magnetic

particles, viscous fluids and their volume rate, the rheological properties of

Magnetorheological fluids vary. The typical shear strength could vary from 2-3 kPa with no

magnetic field to 50-100 kPa with an applied magnetic field of 3000 Oersted. They can

operate in a temperature range of -40 0C to +150 0C. The viscosity of Magnetorheological

fluids can vary between 0.20 to 0.30 Pa-s at 25 0C.

MR fluids can be operated in three working modes depending on the type of deformation

8

employed such as shear mode, valve mode and squeeze mode. In the case of the shear mode,

the MR fluid is located between surfaces moving in relation to each other with the magnetic

field flowing perpendicularly to the direction of motion of these shear surfaces. In the valve

mode, the MR fluid is forced to flow directly between static plates, while in the squeeze

mode, the MR fluid is squeezed by a normal pressure in the direction of the magnetic field

under dynamic or static (compression or tension) loadings.

Fig.1.1: MR fluid particle distribution: a) no magnetic field, b) with magnetic field.

Advances in the application of MR materials are parallel to the development of new, more

sophisticated MR materials with better properties and stability. Many smart systems and

structures would benefit from the change in viscosity or other material properties of MR.

Nowadays, these applications include brakes, dampers, clutches and shock absorbers systems.

Applications of Magnetorheological fluids in torque transmission clutches are discussed in

this seminar. Quick time response and variable rheological properties of Magnetorheological

fluids in response to an applied magnetic field are utilized in generating the variable torque

transmission. Magnitude of the transmitted torque is adjusted by the level of the magnetic

field applied over the Magnetorheological fluids in the clutch mechanism.

9

CHAPTER 2: FIELD RESPONSIVE FLUIDS

Field responsive fluids are typical smart fluids, whose rheological properties depends on the

external applied field. Field responsive fluids are materials that undergo significant responses

leading to consequent rheological changes upon the influence of an external field. There are

three main classes of field responsive fluids. They are Magnetorheological fluids,

Electrorheological fluids and Ferrofluids. Magnetorheological fluids and Electrorheological

fluids works under the influence of applied magnetic and electric fields, respectively. The

fluids comprise a carrier liquid, such as a dielectric medium, including mineral oil or

hydrocarbon oil, and solid particles. Magnetorheological fluids require the use of solid

particles that are magnetisable, and ER fluids make use of solid particles responsive to an

electric field. In addition, Ferrofluids (magnetic liquid) also can be categorized as smart

materials. In the presence of a magnetic field, colloidal magnetic fluids retain their liquid

properties. They do not generally exhibit the ability to form particle chains or develop a yield

stress. However, Ferrofluids experience a body force on the entire fluid, and this force causes

the fluids to be attracted to regions of high magnetic field strength. Table 2 shows the

comparison of some of the properties between them. In a general manner,

Magnetorheological and Electrorheological fluids demonstrate specific advantages or

disadvantages which can be considered as complementary rather than competitive. They have

their own markets and applications in different fields. For instance, one of the advantages of

Magnetorheological fluids is higher stresses that they can withstand, while the major

advantage of Electrorheological fluids is a smaller size of the systems that they can be

developed with them.

The utilization of Magnetorheological or Electrorheological fluids can work to rapidly

respond in active interface between sensors or controls and mechanical outputs. The fluids

can be employed in vibration isolation systems as an example of precision surface

shaping/polishing machines, mechanical clutches, brakes damping devices, building seismic

isolators, torque/tension controllers, gripping/latching devices and fluid flow controllers.

MAGNETORHEOLOGICAL FLUID

The discovery of Magnetorheological fluids is credited to Jacob Rabinow at the US National

Bureau of Standard in 1948. MR fluids can be described as magnetic field responsive fluids

which are part of a group of relatives known as smart or actively controllable fluids.

10

Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic

change in their rheological behaviour. The essential characteristic of these fluids is their

ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with

controllable yield strength in milliseconds when exposed to a magnetic field.

Magnetorheological fluids consist of magnetically permeable micron-sized particles

dispersed throughout the carrier medium either a polar or non-polar fluid, which then

influence the viscosity of the Magnetorheological fluids.

Items MR fluids ER fluids ferrofluids

Particulate

Material

Ferromagnetic

Ferrimagnetic

Polymers,

Zeolite,etc

Magnetite,

Heamatite,etc

Particle Size 0.1-10µm 0.1-10 µm <10nm

Carrier Fluid Non-Polar And Polar

Liquids,Etc

Gel And Other

Polymers

Paramagnetic Salt

Solution

Density(G/Cc) 3-5 1-2 1-2

Off Viscosity

(Pa/S @25C

0.1-0.3 0.1-0.3 0.002-0.5

Required Field ~3koe ~3kv/Mm ~1koe

Device Excitation Electromagnets/

Permanent Magnets

High Voltage Permanent Magnet

Yield

Strength(Field)

100kpa 10kpa (B)/ (0)2

Table 2.1: Comparison the properties of MR fluids, ER fluids and Ferrofluids

Magnetorheological fluids are controllable fluids that exhibit dramatic reversible change in

rheological properties like elasticity, plasticity or viscosity either in solid-like state or free-

flowing liquid state depending on the presence or absence of a magnetic field. In the presence

of an applied magnetic field, the suspended particles appear to align or cluster and the fluid

drastically thickens or gels. The flow resistance i.e. apparent viscosity of the fluid is

intensified by the particle chain. When the magnetic field is removed, the particles are

returned to their original condition, which lowers the viscosity of the fluid. The fluid structure

is dependent on many factors such as volume fraction, magnetic field strength and carrier

fluid. The fluid structure is also responsible for a rapid formation and is reversible either in

solid-like state or free-flowing liquid state. The changes of solid-liquid state or the

consistency or yield strength of the Magnetorheological fluids can be precisely and

11

proportionally controlled by altering the strength of the applied magnetic field. These

characteristics provide simple, quiet and rapid response interfaces between electronic control

and mechanical systems.

Most of the researchers used carbonyl iron as particles scatter in oil medium, for instance

silicone oil, hydrocarbon oil, mineral oil and hydraulic oil. The material also can be produced

at a relatively lower cost as compared to Magnetorheological fluids that include hydrophobic-

oil type fluids as a carrier fluid. Iron powder is the most popular material used as particle due

to its high saturation magnetization about 2.1 T. Those particles are arranged in a proper

order from one pole to another pole of a magnet to form very strong chains or fluxes.

Initially, in the absence of the magnetic field, the iron particles in the space between two

walls move unrestrained. In the presence of the magnetic field, the iron particles are

organized along the direction of the applied magnetic field. These particles are constructed

into a uniform polarity and connected to the walls. Once aligned in this manner, the iron

particles are refrained from moving out of their respective flux lines and act as a barrier to an

external force. The yield stress, in this case, symbolizes the maximum of the stress-strain

relationship, and the chains will break when the stress has reached its maximum which allows

the fluid to flow even if the magnetic field is still applied.

Magnetorheological and Electrorheological fluids use feedback information such as rapid

response interfaces between electric controls and mechanical systems to vigorously change

the material behaviour. By changing the material behaviour, the performance of the devices is

intensified to a certain level that unattainable using conventional materials and devices.

Magnetorheological fluids can be considered as unique smart materials because they produce

milliseconds response time. The fluids may be used in both small and large displacement

devices in order to generate very large forces and torques without reliance on the velocity of

the working systems. The performance of the fluids depends on the fluids’ structure in

connection with many factors such as volume fraction, carrier fluid and particle size.

Research studies done by industries such as Lord Corporation and Liquids Research Limited

and all over the world have contributed to the Magnetorheological technologies in order to be

used in a wide variety of applications.

12

CHAPTER 3: A GOOD MR FLUID

The most common response to the question of what makes a good MR fluid is likely to be

"high yield strength" or "non-settling" for the purpose of effective working condition.

However, those particular features are perhaps not the most critical when it comes to ultimate

success of a magnetorheological fluids. As anyone who has made MR fluids knows, it is not

hard to make a strong MR fluid. Over fifty years ago both Rabinow and Winslow described

basic MR fluid formulations that were every bit as strong as fluids today. A typical MR fluid

used by rabinow consisted of 9 parts by weight of carbonyl iron to one part of silicone oil,

petroleum oil or kerosene.1 to this suspension he would optionally add grease or other

thixotropic additive to improve settling stability. The strength of rabinow’s MR fluid can be

estimated from the result of a simple demonstration that he performed. Rabinow was able to

suspend the weight of a young woman from a simple direct shear MR fluid device. He

described the device as having a total shear area of 8 square inches and the weight of the

woman as 117 pounds. For this demonstration to be successful it was thus necessary for the

MR fluid to have yield strength of at least 100 kpa.

MR fluids made by Winslow were likely to have been equally as strong. A typical fluid

described by Winslow consisted of 10 parts by weight of carbonyl iron suspended in mineral

oil.2 to this suspension Winslow would add ferrous naphthenate or ferrous oleateas a

dispersant and a metal soap such as lithium stearate or sodium stearate as thixotropic

additive. The formulations described by Rabinow and Winslow are relatively easy to make.

The yield strength of the resulting MR fluids is entirely adequate for most applications.

Additionally, the stability of these suspensions is remarkably good. It is certainly adequate

for most common types of MR fluid application. As early as 1950 rabinow pointed out that

complete suspension stability, i.e. no supernatant clear layer formation, was not necessary for

most mr fluid devices. Mr fluid dampers and rotary brakes are in general highly efficient

mixing devices.

3.1. CHEMICAL COMPOSITION

magnetorheological fluids consist of non-colloidal suspensions, magnetically soft

ferromagnetic, ferrimagnetic or paramagnetic elements and compounds in a non-magnetic

medium. However, magnetorheological fluids consist of suitable magnetizable particles like

iron, iron alloys, iron oxides, iron nitride, iron carbide, carbonyl iron, nickel and cobalt. A

13

preferred magnetic responsive particle that is commonly used to prepare magnetorheological

fluids is carbonyl iron. The possible maximum yield stress induced by magnetorheological

effect is mainly determined by the lowest coercivity and the highest magnitude of saturation

magnetization of the dispersed particles. Therefore, soft magnetic material with high purity

such as carbonyl iron powder appears to be the main magnetic phase for most of the practical

magnetorheological fluids composition. Other than carbonyl iron, fe-co alloys and fe-ni

alloys can also be used as magnetorheological materials, whereby, fe contributes to the high

saturation magnetization. However some of the ferrimagnetic materials such as mn-zn ferrite,

ni-zn ferrite and ceramic ferrites have low saturation magnetizations and are therefore

suitable to be applied in low yield stress applications.

Iron powder magnet can be prepared by hydrogen reduction of ferric oxide or by chemical

vapour deposition from iron pentacarbonyl,fe(co)5. Once the particles are magnetized, the

oriented domains can grow with the magnetization persisted and simultaneously increased

permeability. Saturation magnetization of the iron can be obtained when all of the domains

are properly oriented. The domain walls can easily move, ideally making the magnetization a

single-valued function of the magnetizing field, so that there is no hysteresis loss when the

field reverses repeatedly. The particle size should be meticulously selected, so that it can

exhibit multi-domain characteristics when subjected to an external magnetic field.

Magnetorheological particles are typically in the range of 0.1 to 10 μm, which are about 1000

times bigger than those particles in the ferrofluids. In the magnetorheological fluids, magnetic

particles within a certain size distribution can give a maximum volume fraction without

causing unacceptable increasing in zero-field viscosity. For instance, fluid composition that

consists of 50 % volume of carbonyl iron powder was used in the application of

electromechanically controllable torque-applying device.

The carrier liquid forms the continuous phase of the magnetorheological fluids. Examples

of appropriate fluids include silicone oils, mineral oils, paraffin oils, silicone copolymers,

white oils, hydraulic oils, transformer oils, halogenated organic liquids, diesters,

polyoxyalkylenes, fluorinated silicones, cyanoalkyl siloxanes, glycols, water and synthetic

hydrocarbon oils. A combination of these fluids may also be used as the carrier component of

the magnetorheological fluids. In the earlier patents and findings, inventors were using

magnetizable particles dispersed in a light weight hydrocarbon oil, either a liquid, coolant,

antioxidant gas or a semi-solid grease and either a silicone oil or a chlorinated or fluorinated

14

suspension fluid. However, when the particles settled down, the field-induced particle chains

formed incompletely at best in which magnetorheological response was critically degraded.

Later, in order to prevent further sedimentation, new compositions of magnetorheological

fluids with consideration on viscoplastic and viscoelastic continuous phases were formulated,

so that the stability could be improved immensely. In addition, a composite MR fluid has

been prepared by panetal. With a combination of iron particles powder, gelatine and carrier

fluids. They showed that the MR effects were superior under low magnetic field strength, and

had a better stability compared to pure iron carbonyl powder alone.

Surfactants, nanoparticles, nanomagnetizable or coating magnetizable particles can be

added to reduce the sedimentation of the heavy particles in the liquid phase. The

sedimentation phenomenon can cause a shear-thinning behaviour of the suspension. With

further sedimentation, with magnetorheological fluids under the influence of high stress and

high shear rate over a long period of time, the fluid will thicken. Sedimentation phenomenon

will reduce the magnetorheological effect where the particles in the mr fluids are settled

down and form a hard “cake” that consists of firmly bound primary particles due to

incomplete chain formation. Magnetorheological particles such as carbonyl iron can be

described as the particle erosions and similar to onion like structure where they can be easily

peeled by jolt or frictions. Anti-settling agent such as organo clay can provide soft

sedimentation. When the composition of magnetorheological fluids has relatively low

viscosity, it does not settle hard and can easily re-disperse. Coating of the polymer layer also

influences magnetic properties of the particles and cause them to easily re-disperse after the

magnetic field is removed. However, specific properties of MR fluids such as shear and yield

stresses under the same conditions were enormously degraded inevitably by addition of the

coating layer. This is due to the shielding of the polymer layer that affects the magnetic

properties of the particles. In addition, some additives can improve the secondary properties

like oxidation stability or abrasion resistance.

Fig.3.1.1.composition of MR fluids

15

3.2. PHYSICAL PROPERTIES

Typical magnetorheological fluids are the suspensions of micron sized, magnetizable

particles (mainly iron) suspended in an appropriate carrier liquid such as mineral oil,

synthetic oil, water or ethylene glycol. The carrier fluid serves as a dispersed medium and

ensures the homogeneity of particles in the fluid. A variety of additives (stabilizers and

surfactants) are used to prevent gravitational settling and promote stable particles suspension,

enhance lubricity and change initial viscosity of the magnetorheological fluids. The

stabilizers serve to keep the particles suspended in the fluid, whilst the surfactants are

adsorbed on the surface of the magnetic particles to enhance the polarization induced in the

suspended particles upon the application of a magnetic field.

magnetorheological fluids made from iron particles exhibit maximum yield strengths of 30–

90 kPa for applied magnetic fields of 150–250 ka/m (1kOe). Magnetorheological fluids are

not highly sensitive to moisture or other contaminants that might be encountered during

manufacture and use. Further, because the magnetic polarization mechanism is not affected

by the surface chemistry of surfactants and additives, it is a relatively straightforward matter

to stabilize magnetorheological fluids against particle-liquid separation in spite of the large

density mismatch. The ultimate strength of the magnetorheological fluid depends on the

square of the saturation magnetization of the suspended particles.

Typically, the diameter of the magnetizable particles range from 3 to 5 microns. Functional

magnetorheological fluids may be made with larger particles, however, stable suspension of

particles becomes increasingly more difficult as the size increases. Commercial quantities of

relatively inexpensive carbonyl iron are generally limited to sizes greater than 1 or 2 microns.

Smaller particles that are easier to suspend could be used, but the manufacture of such

particles is difficult. Significantly smaller ferromagnetic particles are generally only available

as oxides, such as pigments commonly found in magnetic recording media.

Magnetorheological fluids made from such pigment particles are quite stable because the

particles are typically only 30 nm in diameter. However, because of their lower saturation

magnetization, fluids made from these particles are generally limited in strength to about 5

kPa and have a large plastic viscosity due to the large surface area. In the absence of an

applied field, magnetorheological fluids are reasonably well approximated as newtonian

16

liquids. For most engineering applications a simple Bingham plastic model is effective at

describing the essential, field-dependent fluid characteristics. A Bingham plastic is a non-

Newtonian fluid whose yield stress must be exceeded before flow can begin. Thereafter, the

rate-of-shear vs. Shear stress curve is linear.

Property Typical value

Initial viscosity 0.2 – 0.3 [Pa/s] (at 25C)

Density 3 – 4 [g/cm3]

Magnetic field strength 150 – 250 [kA/m]

Yield point o 50 – 100 [kPa]

Reaction time few milliseconds

Typical supply voltage and

current intensity

2 – 25 V, 1–2 A

Work temperature -50 do 150 [C]

Table.3.2.1. Properties of MR fluid

3.3. MAGNETIC PROPERTIES

It is the special magnetic properties and the effect of magnetism on the rheology of the fluid

that made magnetorheological fluid one of the best among the smart fluids. By properly

controlling the magnetic field applied, the yield stress and the amount of torque or power

transmitted by using MR fluids can be effectively controlled.

The static magnetic properties of magnetorheological fluids are important to design any

magnetorheological fluid-based devices and can be characterized by b-h and m-h hysteresis.

Through the magnetic properties, the dependence of the magnetorheological fluid response

on the applied current in the device can be predicted. There are many methods to measure the

hysteresis loops for the fluid under different fields such as vibrating sample magnetometer

(vsm), alternating gradient magnetometer (agm) and other induction techniques

Under the influence of the magnetic field, a standard model for the structure is used to

predict the behaviour of the particle of magnetorheological fluid. The model is based on a

cubic network of infinite chains of the particles arranged in a line with respect to the direction

of the magnetic field as shown in figure 6.1. The chains are considered to deform with the

same distance between any pair of neighbours in the chains and increase at the same rate with

the strain when the magnetorheological fluid is strained. This model is seems quite simple

17

since the chains, in actual case, are formed into some more compact aggregates of spheres in

which can be constituted in the form of cylinders. Under shear stress, these aggregates might

deform and eventually break. Even though the particles develop into different complicated

structures under different conditions, the standard model still can be used in order to give a

valid prediction of the yieldstress.

Fig.3.3.1: schematic representation of the affine deformation of a chain of spheres.

3.4. RHEOLOGICAL PROPERTIES

Rheology is the study of deformation and flow of matter under the influence of an applied

stress. The term was coined by Eugene Bingham, a Professor at Lehigh University, in 1920,

from a suggestion by a colleague, Markus Reiner. The term was inspired by Heraclitus’s

famous expression panta rei, “everything flows”. Rheology is defined as a study of the flow

properties and the behaviour of materials or the response of materials to applied stress.

Rheology is an interdisciplinary field and is used to describe the properties of a wide variety

of materials such as oil, food, ink, polymers, clay, concrete, asphalt and others. A rheometer

is the instrument used to measure a material’s rheological properties for which the equipment

uses the working principal of a viscometer. There are many types of rheometers with very

versatile control such as the stress and strain rheometers and capillary rheometers. The

measurement of rheological properties of suspension, colloidal dispersion and emulsion

provides critical information for product and process performance in many industrial

applications. The materials must be stable in order to be performed properly or to process

efficiently. These are often complex formulation of solvents or fluids; suspended particles of

18

varying sizes and shapes, and various additives used that affect stability.

Many factors affect the stability such as hydrodynamic forces, Brownian motion,

strength of the antiparticles interaction, volume fraction, electrostatic forces, size and shape

of particles, and steric repulsion. these factors are responsible for properties of fluids. for

example, a quick formation of a network in response to an external field creates a rapid

liquid-to-solid transition. Measuring the rheology of a formulation gives an indication on the

colloidal state and the interactions that are occurring. Rheology measurements can help

predict which formulation exhibit flocculation, coagulation or coalescence, resulting in

undesired effects such as settling, creaming, separation and others. Flocculation is referred to

the process by which particles are caused to stick together in floc (formation of loose or open

aggregates), while coagulation is a process in which dispersed colloidal particles agglomerate

(formation of compact aggregates) and coalescence is the disappearance of the boundary

between two particles in contact, or is the process by which particles merge and pull each

other to make the slightest contact. Rheology measurements and parameters can be used to

determine the processing behaviour of non-Newtonian materials, viscoelastic behaviour as a

function of time, the degree of stability of a formulation at rest condition or during transport,

and zero shear viscosity or the maximum viscosity of the fluid phase to prevent

sedimentation.

Fig.3.4.1: Classification types of the behaviour of the fluid.

The viscosity of a Newtonian fluid is independent of time and shear rate. In addition,

the deviation of the behaviour of Newtonian fluid is known as a non-Newtonian fluid which

the viscosity change is dependent on the applied shear rate. As shown in figure 7.1, the

19

behaviour of the fluids can be classified into Newtonian fluids and non-Newtonian fluids

such as plastic, Bingham plastic, pseudo-plastic and dilatants fluids. Fluids are said to be

plastic when the shear stress must reach a certain minimum value before it begins to flow.

For the pseudo-plastic or shear-thinning fluid, the dynamic viscosity decreases as the shear

rate increases. On the other hand, a shear-thickening or dilatants fluid exhibits the converse

property of pseudo-plastic for which the dynamic viscosity increases as the shear rate

increases.

Additional non-Newtonian behaviour or time dependent properties are rheopecty and

thixotropy. In principle, shear thickening proceeds from the rheopecty and shear thinning

proceeds from the thixotropy. As stress is applied, the apparent viscosity increases with the

duration of the stress, the fluid is then called rheopectic. If the apparent viscosity decreases

with the duration of stress, the fluid is then called thixotropic. Rheopectic behaviour occurs as

a result of temporary aggregation due to interaction between the particles rather than

breakdown due to collision of the attractive particles. On the other hand, the decrease in the

viscosity of the thixotropic fluid occurs because of the breakdown of the microstructure and

behaves like a liquid. These time-dependent behaviour are reversible, which is, when the

stress is removed the structure that was disturbed by shearing builds up in the thixotropic

material and breaks down in the rheopectic material. Thus, the material settles back into its

original consistency.

20

CHAPTER:4 ADVANTAGES

Primary advantage stems from their high dynamic yield strength due to high

magnetic energy density this stress allows for small device size and high dynamic

range.

MR fluids can operate at high temperatures from -40 to 150 degree centigrade with

only slight variation in yield stress so magnetic polarization is not influenced by

temperature.

MR fluids are not sensitive to impurities commonly encountered during

manufacturing and usage.

Antiwear and lubricity additives can generally be included in MR fluids to enhance

stability, real life and bearing life.

Can be easily driven by common low voltage, current driven power sources

outputting only 1-2 Amps.

Inherent system stability.

Quick response time.

Simple design.

Continuous variable control of damping, motion and position control.

Long service life.

Fast response in the order of milliseconds.

Lower power requirement.

Little sedimentation.

Controllable rheological properties.

DISADVANTAGES

High cost-owing to seals, electromagnet assembly, control electronics and volume of

MR fluid.

High density, due to presence of iron, makes them heavy. However, operating

volumes are small, so while this is a problem, it is not insurmountable.

High-quality fluids are expensive. High-quality fluids are expensive.

Fluids are subject to thickening after prolonged use and need replacing.

Settling of Ferro-particles can be a problem for some applications. (i.e. particle

sedimentation over time due to the inherent density difference between the particles

and their carrier fluid.

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CHAPTER:5 WORKING

Magnetorheological fluids are material which changes its rheological properties under the

application of an applied magnetic field. Magnetorheological fluids display Newtonian like

behaviour in the absence of magnetic field. When exposed to a magnetic field the ferrous

particles that are dispersed throughout the fluid form magnetic dipoles. These dipoles align

themselves along lines of magnetic flux.

The magnetic particles, which are typically micrometre or nanometre scale spheres or

ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension

under normal circumstances.

When a magnetic field is applied, however, the microscopic particles (usually in the

0.1–10 µm range) align themselves along the lines of magnetic flux. When the fluid is

contained between two poles (typically of separation 0.5–2 mm in the majority of devices),

the resulting chains of particles restrict the movement of the fluid, perpendicular to the

direction of flux, effectively increasing its viscosity. Thus in designing a Magnetorheological

device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the

motion to be restricted.

Figure.5.1.MR fluids in the absence of magnetic field.

Figure.5.2. Alignment of MR particles under the action of magnetic field.

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On a large scale, this reordering of ferrous dipole particles can be visualised as a very

large number of microscopic beads that are threaded on to a very thin string. In this analogy

the spherical beads represent iron particles and each string represents a single flux line. One

can picture many of these strings of beads placed closely together much like the bristles of

tooth brush. Once aligned in this fashion, the ferrous particles resist being moved out of their

respective flux lines and the amount of resistance is proportional to the intensity of the

applied magnetic field and act as a barrier to fluid flow. Typically, MR fluids can be used in

three different ways, all of which can be applied to MR clutch design depending on its

intended use. These modes are referred to as valve mode, shear mode and squeeze mode.

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CHAPTER: 6 APPLICATIONS

MR fluids find a variety of applications in almost all the vibration control systems. It is now

widely used in automobile suspensions, seat suspensions, clutches, robotics, design of

buildings and bridges, home appliances like washing machines etc.

6.1. INDUSTRIAL SHOCK ABSORBERS

6.1.1 Passive Industrial Shock Absorbers

Typical shock absorbers are based on a hydraulic cylinder with a spring. Fig.6.1 shows two

typical solutions. In the first solution the cylinder chambers are connected by a valve with

orifices (Fig.6.1a). In the second case there is a gap between the cylinder and the piston (Fig.

6.1b). When a load hits the shock absorber piston rod, the movement of the piston forces the

hydraulic fluid to flow through orifices or gaps.

Fig6.1.1. Examples of typical hydraulic shockabsorbers(a) with a by passvalve and (b) with

an orifice between the piston and cylinder

6.1.2. Industrial Shockabsorber with Magnetorheological Fluid

Magnetorheological fluids were discovered and developed in the late 1940s. In the last 20

years many attempts have been made to apply MR fluids in dampers, brakes, clutches and

other energy dissipating devices. An MR damper is one of the more promising devices used

for oscillation reduction in structures. Such a damper is a semi-active control device which can

generate a force according to applied electric current. The electrical energy required by such a

damper is minuscule (a few Watts) while the dissipated energy can reach hundreds of Watts.

The speed of its response is in the range of milliseconds.

Magnetorheological fluid is a suspension of ferromagnetic particles in a carrier liquid, usually

mineral oil, synthetic oil, water or glycol. Ferromagnetic particles are soft iron particles, e.g.

carbonyl iron (sometimes cobalt or nickel) with a m. The percentage of ferromagnetic particles

in the liquid is usually in the range of 20–50% (max. 85%). Proprietary additives similar to

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those applied in commercial lubricants are commonly added to discourage gravitational

settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear.

Normally, MR fluids are free owing liquids with a consistency similar to motor oil. However,

in the presence of an applied magnetic field, the iron particles acquire a dipole moment

aligned with the magnetic field, which causes particles to form linear chains parallel to the

field. The fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic

solid. Examples of devices with the MR fluid include not only linear but also rotary dampers

other potential applications include the absorption of shocks of off-road motorcycle systems

and seismic response reduction in buildings and bridges.

MR damper designs typically place the coil in the piston head. In the design developed

in the study the coil is moved off the piston to either end of the damper. The active areas are

stacked on both sides of the damper inner cylinder. The piston rides in this cylinder and forces

fluid flow from one chamber through two bifold MR valves to the second chamber. Thus, four

active volumes are created using only two coils. Two design goals are achieved: high force

and compactness. The tests demonstrate that this novel MR damper was able to provide a high

damping force at a high frequency (up to 12 Hz). Study provides an experimental analysis of

magnetorheological dampers subjected to impact and shock loading. A drop- tower is

developed to apply impulse loads to the dampers. The results show that at large impact

velocities, the peak force does not depend on the current supplied to the damper, as is

commonly the case at low velocities. This phenomenon is hypothesised to be the result of the

fluid inertia preventing the fluid from accelerating fast enough to accommodate the rapid

piston displacement. Thus, the peak force is primarily attributed to compression of the MR

Fluid.

Fig.6.1.2.Design of MR ShockAbsorber

In order to have the possibility of controlling the braking force during the stopping

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process,we propose an application of MR fluid in an industrial shock absorber (Fig.6.1.2).

The absorber proposed by us is based on a double rod hydraulic cylinder, in which chambers

are connected by a by-pass cylindrical MR valve placed outside the cylinder. When the piston

moves, the MR fluid flows from one chamber to the other through the MR valve. The

effective fluid orifice is the entire space between the coil outside diameter and inside

diameter of the valve housing. The design with a double-ended piston rod has an advantage:

no rod volume compensator needs to be incorporated into the device.

A properly adjusted shock absorber should safely dissipate energy, reducing

damaging shock loads and noise levels. At beginning of the braking process, as shown in Fig.

6.1.3, the breaking force increases rapidly, due to the impact of the moving mass on the

absorber piston rod, which is not moving. The braking force then reduces gradually. As

Fig.6.1.3 illustrates, if the classical, passive shock absorber is used (curves 1, 2, 3), the force

drops as the piston speed decreases. If the kinetic energy of the moving mass is too high, the

mass is stopped by hard impact and bounces at the absorber bottom (curve 1). If this energy is

too small, the mass is stopped before reaching the end position (curve 2). The proper

matching of the braking force and the kinetic energy of the mass is shown by curve 3. For the

case of using the MR fluid in the absorber, the force can be maintained at a more or less

constant level, until the mass is stopped at the end position (curve 4). The value of the

braking force is established by the electronic controller, which enables the adaptation of the

braking force to the element kinetic energy. Summarising, we can hypothesise that the best

stopping process can be obtained when using a shock absorber with MR fluid.

Fig.6.1.3.different braking characteristics

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Fig.6.1.4.Diagram of shock absorber with MR fluid

To maximise the effectiveness of the MR shock absorber, the controllable force should be as

large as possible. To obtain it, a small gap size is required. On the other hand, a small gap

size decreases the controllable range, because the viscous force increases much faster (1/h3)

with the gap magnitude increase than the MR controllable force (1/h).Therefore a

compromise is necessary.

Fig.6.1.5.Static characteristics of a built shock absorber with the gap height equal to

(a) 0.5mm and (b) 0.25mm.

6.2. CLUTCHES

Magnetorheological fluids are increasingly being considered in clutches. The activation of

MRF clutch’s built-in magnetic field causes a fast and dramatic change in the apparent

viscosity of the MR fluid contained in the clutch. The fluid changes state from liquid to semi-

solid in about 6 milliseconds. The result is a clutch with an infinitely variable torque output.

Magnetorheological fluids are used in clutches for variable transfer of motion and power

between driver and driven shafts. Bansbach, proposed a double-plate and a multi-plate MRF

torque transfer apparatus with a controller that adjusts the input current. The apparatus is

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proposed to be placed between the engine of a car and its differential. Gopalswamy also

studied a controllable multi-plate Magnetorheological transmission clutch. This clutch was

also designed to be placed between the engine and differential.

Magnetorheological clutch operates in a direct-shear mode and transfers torque between

input and output shaft. There are two main types constructions of MR clutch: cylindrical and

frontal. In the cylindrical model MR fluid works between two cylindrical surfaces and in

frontal MR fluid fills gap between two discs. During work magnetic field produced by coils

increases viscosity of fluid and causes transfer of torque form input to output shaft. Useful

torque is available after 2-3 milliseconds from stimulation. Magnetorheological dampers of

various applications have been and continue to be developed. These dampers are mainly used

in heavy industry with applications such as heavy motor damping, operator seat/cab damping

in construction vehicles, and more. Materials scientists and mechanical engineers are

collaborating to develop stand-alone seismic dampers which, when positioned anywhere

within a building, will operate within the building's resonance frequency, absorbing

detrimental shock waves and oscillations within the structure, giving these dampers the

ability to make any building earthquake-proof, or at least earthquake-resistant.

6.3. AUTOMOTIVE INDUSTRIES

In automotive industry currently the most lucrative application for MRFs is in automotive

suspension technology.Fig.6.3.below shows a fluid damper used in automobile suspension

system

Fig.6.3.1.schematic and photo of fluid damper

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In MR damper system, valves and magnetic circuit are fully contained in piston. Valves are

magnetically controlled within he damper. Current is carried to the electromagnetic coil via

the leads through the hollow shaft, causes the fluid to change to solid state. Thus damping is

increased. By changing the current, damping can be varied.Benefits include a 40% reduction

in mechanical parts, mostly valves; elimination of the traditional shock-absorber fluid; and

the capability of adapting to changing levels of shock and motion 500 times per second.

Several applications are emerging for MRFs-beginning with industrial fork lift in the area of

steer-by-wire, in which no mechanical connection exist between the steering wheel and the

drive wheels. Carlson envisions ultimately extending the technology to brake-by-wire, clutch-

by-wire, and shift-by-wire. Replacing mechanical and hydraulic component with simple wire

connections enables manufactures to reduce vehicle weight. Active MRF engine mounts may

further reduce vibration and quiet noise before it enters a vehicle.

6.4. OPTICS

Magnetorheological finishing, a Magnetorheological fluid-based optical polishing method,

has proven to be highly precise. It was used in the construction of the Hubbles Telescopes

corrective lens.

6.5. HUMAN PROSTHESIS

Magnetorheological dampers are utilized in semi-active human prosthetic legs. Much like

those used in military and commercial helicopters, a damper in the prosthetic leg decreases

the shock delivered to the patients’ leg when jumping, for example. This results in an

increased mobility and agility for the patient.

6.6. MILITARY AND DEFENCE

The U.S. Army Research Office is currently funding research into using MR fluid to

enhance body armor. In 2003, researchers stated they were five to ten years away from

making the fluid bullet resistant. In addition, Humvees, and various other all-terrain vehicles

employ dynamic MR shock absorbers and/or dampers.

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CONCLUSION

Atoms combine to form molecules. Molecules combine to form matter. It is with this matter

that the entire universe is made of. So material development plays a crucial role in the

development of mankind. First man invented wooden weapons in the pre historic age. Then

he invented stone as weapon. After that man invented fire. Then a revolutionary discovery

was made in the form of wheels. Then the growth was fast. Now he conquered the Everest of

knowledge and standard of life. All this development was made possible by the development

of variety of materials with the help of an intellectual brain that god had gifted to man. The

development of smart materials will undoubtedly be an essential task in many fields of

science and technology such as information science, microelectronics, computer science,

medical treatment, life science, energy, transportation and safety engineering and military

technologies. Materials development in the future, therefore, should be directed toward

creation of hyper functional materials which surpass even biological organ in some aspects.

The current materials research is to develop various pathways that will lead the modern

technology toward the smart system. These fluids can reversibly and instantaneously change

from a free-flowing liquid to a semi-solid with controllable yield strength when exposed to a

magnetic field. In the absence of an applied field, MR fluids are reasonably well

approximated as Newtonian liquids.MR technology has moved out of the laboratory and into

viable commercial applications for a diverse spectrum of products. Applications include

automotive primary suspensions, truck seat systems, control-by-wire/tactile-feedback

devices, pneumatic control, seismic mitigation and human prosthetics and in more reliable

and effective power transmitting clutches with the enhancement of variable power

transmission. This clutch has got more reliability and faster response than conventional

friction clutches. Also this is not the maximum this is just the development stage of MR

technology. These achievements like automotive primary suspensions, truck seat systems,

control-by-wire/tactile-feedback devices, pneumatic control, seismic mitigation and human

prosthetics and in more reliable and effective power transmitting clutches are not the

maximum success of the MR technology, because success is a journey not a destiny. Thus

from this study it is observed that MR technology is an area of wide scope and hope it will

develop far better in future.

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REFERENCE

“Application of magnetorheological fluid in industrial shock absorbers”Andrzej

Milecki,Miko"aj,HaukePoznan,UniversityofTechnology,60965Poznan,ul.Piotrowo3,

Poland

Kciuk. M, R. Turczyn (2006) “Properties and application of Magnetorheological

fluids” AMME Journal of Achievements in Material and Manufacturing Engineering

Volume-18.

Melek Yalcintas (1999) “Magnetorheological Fluid Based Torque Transmission

Clutches” Proceedings of Ninth International Offshore and Polar Engineering

Conference, Brest, France.

Saiful Amri Bin Mazlan (2008) “The Behaviour of Magnetorheological Fluids in

Squeeze mode” A Thesis Submitted For The Degree Of Doctor Of Philosophy Dublin

City University.

J. David Carlson, (July 9-13, 2001) “What Makes a Good MR Fluid?,” 8th

International Conference on ER Fluids and MR Fluids Suspensions, Nice.

Naoyuki TAKESUE, Junji FURUSHO, Masamichi SAKAGUCHI (2001)

“Improvement of Response Properties of MR-Fluid Actuator by Torque Feedback

Control” Proceedings of the 2001 IEEE International Conference on Robotics &

Automation, Seoul, Korea.

M.R. Jolly (1999) “Properties and Applications of Magnetorheological Fluids,”

(Invited) Proc. of MRS Fall Meeting, Vol. 604, Boston, MA, Nov. 29-Dec. 3, 1999.

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