Seminar report 2011-2012 MAGNETO RHEOLOGICAL FLUIDS

Introduction

Magneto rheological fluids commonly known as MR fluids are suspensions of solid in liquid whose properties changes drastically when exposed to magnetic field. It is this property which makes it desirable to use in different vibration controlling systems.

A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.

What are MR fluids?

Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic change in their rheological behavior. 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.

PROPERTIES

Chemical composition

A typical MR fluid consists of 20%40% by volume of relatively pure, soft iron particles, typically 35 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear.Physical properties

MR fluids made from iron particles exhibit maximum yield strengths of 3090 kPa for applied magnetic fields of 150250 kA/m (1 Oe . 80 A/m). MR 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 MR fluids against particle-liquid separation in spite of the large density mismatch. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles.

Viscoelasticity

The distinction between solid and liquid materials is not as clear as the preceding would indicate. Most materials have some solid like character as well as some viscous properties. It is easy to visualize the appearance differences between viscous liquids and gelled or cross-linked solids. These fluids are characterized by taking the viscosity of liquids and the gel strength of solids. A problem arises when one tries to characterize a fluid that behaves like both a viscous liquid and a gel. Such fluids are called viscoelastic fluids. They can be pumped easily (although they have the appearance of a viscous liquid), but they are able to suspend small solid particles indefinitely (so have the characteristic of an elastic solid). In fact, molten polymers are viscoelastic to different degrees.

Shear strengthLow shear strength has been the primary reason for limited range of applications. In the absence of external pressure the maximum shear strength is about 100 kPa. If the fluid is compressed in the magnetic field direction and the compressive stress is 2 MPa, the shear strength is raised to 1100 kPa.If the standard magnetic particles are replaced with elongated magnetic particles, the shear strength is also improved.

Particle sedimentationFerroparticles settle out of the suspension over time due to the inherent density difference between the particles and their carrier fluid. The rate and degree to which this occurs is one of the primary attributes considered in industry when implementing or designing an MR device. Surfactants are typically used to offset this effect, but at a cost of the fluid's magnetic saturation, and thus the maximum yield stress exhibited in its activated state.

Material behaviorTo understand and predict the behavior of the MR fluid it is necessary to model the fluid mathematically, a task slightly complicated by the varying material properties (such as yield stress). As mentioned above, smart fluids are such that they have a low viscosity in the absence of an applied magnetic field, but become quasi-solid with the application of such a field. In the case of MR fluids (and ER), the fluid actually assumes properties comparable to a solid when in the activated ("on") state, up until a point of yield (the shear stress above which shearing occurs). This yield stress (commonly referred to as apparent yield stress) is dependent on the magnetic field applied to the fluid, but will reach a maximum point after which increases in magnetic flux density have no further effect, as the fluid is then magnetically saturated. The behavior of a MR fluid can thus be considered similar to a Bingham plastic, a material model which has been well-investigated.

However, a MR fluid does not exactly follow the characteristics of a Bingham plastic. For example, below the yield stress (in the activated or "on" state), the fluid behaves as a viscoelastic material, with a complex modulus that is also known to be dependent on the magnetic field intensity. MR fluids are also known to be subject to shear thinning, whereby the viscosity above yield decreases with increased shear rate. Furthermore, the behavior of MR fluids when in the "off" state is also non-Newtonian and temperature dependent, however it deviates little enough for the fluid to be ultimately considered as a Bingham plastic for a simple analysis.

Thus our model of MR fluid behavior becomes:

Where = shear stress; y = yield stress; H = Magnetic field intensity = Newtonian viscosity; is the velocity gradient in the z-direction.

Common MR fluid surfactantsMR fluids often contain surfactants including, but not limited to:

oleic acid

tetramethylammonium hydroxide

citric acid

soy lecithin

These surfactants serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of MR fluids. The ideal MR fluid would never settle, but developing this ideal fluid is as highly improbable as developing a perpetual motion machine according to our current understanding of the laws of physics. Surfactant-aided prolonged settling is typically achieved in one of two ways: by addition of surfactants, and by addition of spherical ferromagnetic nanoparticles. Addition of the nanoparticles results in the larger particles staying suspended longer since to the non-settling nanoparticles interfere with the settling of the larger micrometre-scale particles due to Brownian motion. Addition of a surfactant allows micelles to form around the ferroparticles. A surfactant has a polar head and non-polar tail (or vice versa), one of which adsorbs to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular micelle,respectively, around the particle. This increases the effective particle diameter. Steric repulsion then prevents heavy agglomeration of the particles in their settled state, which makes fluid remixing (particle redispersion) occur far faster and with less effort. For example, magnetorheological dampers will remix within one cycle with a surfactant additive, but are nearly impossible to remix without them.

While surfactants are useful in prolonging the settling rate in MR fluids, they also prove detrimental to the fluid's magnetic properties (specifically, the magnetic saturation), which is commonly a parameter which users wish to maximize in order to increase the maximum apparent yield stress. Whether the anti-settling additive is nanosphere-based or surfactant-based, their addition decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids on-state/activated viscosity, resulting in a "softer" activated fluid with a lower maximum apparent yield stress. While the on-state viscosity (the "hardness" of the activated fluid) is also a primary concern for many MR fluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity, maximum apparent yields stress, and settling rate of an MR fluid.

Theory In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian 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. In this model, the total yield stress is given by:

where:

= yield stress caused by applied magnetic field

= magnitude of magnetic field

= shear rate

= field-independent plastic viscosity defined as the slope of the measured shear stress vs. shear strain rate relationship, i.e., at H=0.

How it works?

Applying a magnetic field to magnetorheological fluids causes particles in the fluid to align into chains.

When some low-density MR fluids are exposed to rapidly alternating magnetic fields, their internal particles clump together. Over time they settle into a pattern of shapes that look a bit like fish viewed from the top of a tank. Such clumpy MR fluids don't stiffen as they should when magnetized. The fish tank pattern is fragile and takes about an hour to fully develop. It doesn't occur in MR fluids that are constantly mixed and agitated, as in a car's suspension, but it could prove troublesome in other situations.The magnetic particles, which are typically micrometer or nanometer scale spheres or ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances, as below.

When a magnetic field is applied, however, the microscopic particles (usually in the 0.110m range) align themselves along the lines of magnetic flux, see below. When the fluid is contained between two poles (typically of separation 0.52mm 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. Importantly, mechanical properties of the fluid in its on state are anisotropic. Thus in designing a magnetorheological (or MR) device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the motion to be restricted.

What makes 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". However, those particular features are perhaps not the most critical when it comes to ultimate success of a magnetorheological fluid. The most challenging barriers to the successful commercialization of MR fluids and devices have actually been less academic concerns.

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 weightof 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 Rabinows 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 Rabinowpointed 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.

Advancements in MR fluid technology

In addition to cost-sensitive applications such as washing machines, MR fluid dampers are being used in rotary brakes for exercise equipment and pneumatic systems; in complete semi active damper systems for heavy-duty truck seat suspensions; in adjustable linear shock absorbers for racing cars; and in semi active suspensions for passenger cars.

Now under commercial development are very large MR fluid dampers designed for seismic damage mitigation in civil engineering structures such as buildings and bridges.

Finally, the technology is being investigated for applications in vehicular steer-by-wire devices and medical equipment such as the joints of prosthetic limbs.

The nervous systems of future robots might use MR fluids to move joints and limbs in lifelike fashion. There are many potential applications that make these fluids very exciting." For example, MR fluids flowing in the veins of robots might one day animate hands and limbs that move as naturally as any humans. Book makers could publish rippling magnetic texts in Braille that blind readers could actually scroll and edit. It might even be possible to train student surgeons using synthetic patients with MR organs that flex and slices like the real thing.

New developments in MR fluid technology allow the use of permanent magnets which has lots of advantages. The question often arises asking if it is possible to use a permanent magnet to bias a MR fluid valve or device at a mid-range condition. Current could then be applied to the accompanying electromagnetic coil to cancel the magnetic field and open the valve. Alternatively, a reverse current could be applied to the coil to add to the magnetic field taking the device to a higherrange condition. One motivation for creating such a system is to provide a fail-safe mode of operation wherein the device remains in a locked condition when power is lost. Another motivation may be energy conservation in systems intended to remain closed or locked for extended periods of time and then only open momentarily.

Modes of operation and applicationsAn MR fluid is used in one of three main modes of operation, these being flow mode, shear mode and squeeze-flow mode. These modes involve, respectively, fluid flowing as a result of pressure gradient between two stationary plates; fluid between two plates moving relative to one another; and fluid between two plates moving in the direction perpendicular to their planes. In all cases the magnetic field is perpendicular to the planes of the plates, so as to restrict fluid in the direction parallel to the plates.

Flow mode

Shear Mode

Squeeze-Flow Mode

The applications of these various modes are numerous. Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied. Shear mode is particularly useful in clutches and brakes - in places where rotational motion must be controlled. Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces. This particular flow mode has seen the least investigation so far. Overall, between these three modes of operation, MR fluids can be applied successfully to a wide range of applications. However, some limitations exist which are necessary to mention here.

Magnetorheological fluid before and after exposure to a magnetic fieldWithout current flowing through the wires, the armor would remain soft and flexible. But at the flip of the switch, electrons would begin to move through the circuits, creating a magnetic field in the process. This field would cause the armor to stiffen and harden instantly. Flipping the switch back to the off position would stop the current, and the armor would become flexible again.

In addition to making stronger, lighter, more flexible armor, fabrics treated with shear-thickening and magnetorheological fluids could have other uses as well. For example, such materials could create bomb blankets that are easy to fold and carry and can still protect bystanders from explosion and shrapnel. Treated jump boots could harden on impact or when activated, protecting paratroopers' boots. Prison guards' uniforms could make extensive use of liquid armor technology, especially since the weapons guards are most likely to encounter are blunt objects and homemade blades.

However, the technologies do have a few pros and cons. Here's a rundown:

Applications of MR fluids

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. Before discussing the above said applications in detail it is desirable to go through the behavior of MR fluids on different types of loading and what are the design considerations provided to compensate this.

MR fluid in dampers

As motion control systems become more refined, vibration characteristics become more important to a systems overall design and functionality. Engineers, however, have tended to look at motion control and vibration as separate issues. Motion control, it might be said, presents fairly familiar design engineering problems while vibration suggests more subtle problems. Few design engineers have either the hands-on experience or the training to address both sets of problems in a single design solution.

Most devices use MR fluids in a valve mode, direct-shear mode, or combination of these two modes. Examples of valve mode devices include servo valves, dampers, and shock absorbers. Examples of direct-shear mode devices include clutches, brakes, and variable friction dampers

. In valve mode When the piston in a MR fluid damper moves, theMR fluid jets through the orifices quite rapidly causing it to swirl and eddy vigorously even for low piston speed. Similarly, the shear motion that occurs in a MR brake causes vigorous fluid motion. As long as the MR fluid does not settle into a hard sediment, normal motion of the device is generally sufficient to cause sufficient flow to quickly remix any stratified MR fluid back to a homogeneous state. For a small MR fluid damper two or three strokes of a damper that has sat motionless for several months are sufficient to return it to a completely remixed condition.

Except for very special cases such as seismic dampers, lack of complete suspension stability is not a necessity. It is sufficient for most applications to have a MR fluid that soft settles upon standing a clear layer may form at the top of the fluid but the sediment remains soft and easily remixed. Attempting to make these MR fluids absolutely stable may actually compromise their performance in a device. One of the areas where MR fluids find their greatest application is in linear dampers that effect semi-active control. These include small MR fluid dampers for controlling the motion of suspended seats in heavydutytrucks, larger MR fluid dampers for use as primary suspension shock absorbers and struts in passenger automobiles and special purpose MR fluid dampers for use in prosthetic devices.

In all of these devices one of the most important fluid properties is a low-off state viscosity. While in all of these examples having a MR fluid with a high yield strength in the on-state is important, it is equally important that the fluid also have a very low offstate.The very ability of an MR fluid device to be effective at enabling a semi-active control strategy such as sky -hook damping depends on being able to achieve a sufficiently low off-state. Care must be taken in choosing fluid stabilizing additives so that they do not adversely affect the off-state viscosity.

Earthquake dampers and other some other special applications in which the device will sit quiescent for very long periods of time represent special cases where fluid stability issues may have overriding importance. Because of the transient nature of seismic events these dampers never see regular motion, which can be relied on to keep the fluid mixed. This lack of motion also has it benefit. Unlike dampers used in highly dynamic environments, seismic dampers do not need to sustain millions of cycles. The fact that durability and wear are not issues gives the fluid designer grater latitude to formulate a highly stable fluid. MR fluids for these applications are typically formulated as shearing thinning thixotropic gels.

Advantages of MR dampers MR technology makes it possible forengineers to design a wide range of devices and systems with far greater flexibility. The result:

Improved Performance

Reduced Part Count and Complexity

Smaller Package Size

Less Weight

The MR fluid sponge damper requires neither seals nor bearings, and uses the same inexpensive components found in existing passive dampers, but with a few important modifications. The damper consists of a layer of open-celled, polyurethane foam, or other suitable absorbent matrix materials, saturated with ~3 ml of MR fluid surrounding a steel bobbin and coil.

During passage through resonance, these controllable dampers may be energized to provide a high level of damping which protects the associated machine.

At high speed, the MR sponge dampers are turned off to enable a high level of vibration isolation. Ideally, each of a pair of controllable dampers would have to provide 50150 N of damping force when energized and a low residual force of