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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.
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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
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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
27
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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