MATERIALS THAT SENSE AND RESPOND:AN INTRODUCTION TO SMART MATERIALS

#1Shreshtha Verma #2 Megha Salve# Government College of Engg.Aurangabad

1 shr1.2009@rediffmail.com * Government College of Engg.Aurangabad

Abstract— This paper will provide an overview of the smart materials. Over the past century, we have learned how to create specialized materials that meet our specific needs for strength, durability, weight, flexibility, and cost. However, with the advent of smart materials, components may be able to modify themselves, independently, and in each of these dimensions. Smart materials can come in a variety of sizes, shapes, compounds, and functions. But what they all share- indeed what makes them "smart"-is their ability to adapt to changing conditions. Smart materials are the ultimate shape shifters. Smart materials are materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields. They can also do all of this while intelligently interacting with the objects and people around them. More boldly, it is highly likely that once smart materials become truly ubiquitous-once they are seamlessly integrated into a webbed, wireless, and pervasive network -smart materials will challenge our basic assumptions about, and definitions of "living matter."The various types of smart material are also presented in the paper. To get the clear idea about the smart materials, its definition and types are explained briefly. Some of the types of these include piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory alloys etc. Varieties of smart materials already exist, and research is being carried out extensively to device new materials. Applications of various types of smart materials are clearly explained. Some of applications of already existing smart materials are studied. The expectations of the smart materials and the predictions of future applications have been presented on the later part of the paper. And it is concluded that the application of smart material in future becomes a trend in various fields in engineering.

Key words smart materials, adapt , stimuli, types, applications

I. INTRODUCTION

. In certain respects, smart materials are an answer to many

contemporary problems. In a world of diminishing resources,

they promise increased sustainability of goods through

improved efficiency and preventive maintenance. In a world

of health and safety threats, they offer early detection,

automated diagnosis, and even self-repair. In a world of

political terrorism, they may offer sophisticated bio warfare

countermeasures, or provide targeted scanning and

intelligence- gathering in particularly sensitive environments.

In general, smart materials come in three distinct flavours:

passively smart materials that respond directly and uniformly

to stimuli without any signal processing; actively smart

materials that can, with the help of a remote controller, sense a

signal, analyse it, and then "decide" to respond in a particular

way; and finally, the more powerful and autonomous

intelligent materials that carry internal, fully integrated

controllers, sensors, and actuators.

II. SMART MATERIALS

They are the materials which have the capability to respond to

changes in their condition or the environment to which they

are exposed, in a useful and usually repetitive manner. They

are called by other names such as, intelligent materials, active

materials and adoptive materials. The devises that are made

using smart materials are called Smart Devices. Similarly the

systems and structures that have incorporated smart materials

are called Smart Systems and Smart Structures.. A smart

material or an active material gives a unique output for a well

defined input. The input may be in the form of mechanical

stress / strain, electrical / magnetic field or changes in

temperature.

Figure1: Smart Fluid

The picture above is that of a smart fluid developed in the

Michigan Institute of Technology.

III. TYPES OF SMART MATERIALS

Based on input and output, the smart materials are classified

as follows.

1. Shape Memory Alloys (SMAs): They are the smart

materials which have the ability to return to some previously

defined shape or size when subjected to appropriate thermal

changes.

Eg.: Titanium-Nickel Alloys.

2. Self-healing materials: Materials that have the structurally

incorporated ability to repair damage caused by mechanical

usage over time.

3. Piezoelectric Materials: These are the materials which have

capability to produce a voltage when surface strain is

introduced. Conversely, the materials undergo deformation

(stress) when an electric field is applied across it.

4. Electro-rheostatic (ER) and magneto-rheostatic (MR)

materials: They are materials fluids, which can experience a

dramatic change in their viscosity. These fluids can change

from a thick fluid (similar to motor oil) to nearly a solid

substance within the span of a millisecond when exposed to a

magnetic or electric field; the effect can be completely

reversed just as quickly when the field is removed.

5. Halochromic materials: They are materials which change

colour when pH changes occur. The term ‘chromic’ is defined

as materials that can change colour reversibly with the

presence of a factor. In this case, the factor is pH. The pH

indicators have this property .Halochromic substances are

suited for use in environments where pH changes occur

frequently, or places where changes in pH are extreme.

Halochromic substances detect alterations in the acidity of

substances, like detection of corrosion in metals

6. Dielectric elastomers (DEs): They are smart material

systems which produce large strains (up to 300%) and belong

to the group of electro active polymers (EAP). Based on their

simple working principle dielectric elastomer actuators (DEA)

transform electric energy directly into mechanical work

7.Ferrofluids: (compound of Latin ferrum, meaning iron, and

fluid) is a liquid which becomes strongly magnetized in the

presence of a magnetic field.Ferrofluids are colloidal mixtures

composed of nanoscale ferromagnetic, or ferrimagnetic,

particles suspended in a carrier fluid, usually an organic

solvent or water. The ferromagnetic nanoparticles are coated

with a surfactant to prevent their agglomeration (because of

van der Waals forces and magnetic forces). Ferro fluids

usually do not retain magnetization in the absence of an

externally applied field and thus are often classified as "super

paramagnets" rather than Ferro magnets.

A. SHAPE MEMORY ALLOYS:

Shape memory alloys (SMA's) are metals, which exhibit two

very unique properties, pseudo-elasticity, and the shape

memory effect

The term shape memory refers to the ability of certain alloys

(Ni – Ti, Cu – Al – Zn etc.) to undergo large strains, while

recovering their initial configuration at the end of the

deformation process spontaneously or by heating without any

residual deformation .The particular properties of SMA’s are

strictly associated to a solid-solid phase transformation which

can be thermal or stress induced. Currently, SMAs are mainly

applied in medical sciences, electrical, aerospace and

mechanical engineering and also can open new applications in

civil engineering specifically in seismic protection of

buildings.

Its properties which enable them for engineering applications

are

1. Repeated absorption of large amounts of strain energy

under loading without permanent deformation

2. Usable strain range of 70%

3. Extraordinary fatigue resistance under large strain cycles

4. Their great durability and reliability in the long run.

B. SELF-HEALING MATERIALS:

Self-healing materials are a class of smart materials that

have the structurally incorporated ability to repair damage

caused by mechanical usage over time. The inspiration comes

from biological systems, which have the ability to heal after

being wounded. Initiation of cracks and other types of damage

on a microscopic level has been shown to change thermal,

electrical, and acoustical properties, and eventually lead to

whole scale failure of the material. Usually, cracks are

mended by hand, which is difficult because cracks are often

hard to detect. A material (polymers, ceramics, etc.) that can

intrinsically correct damage caused by normal usage could

lower production costs of a number of different industrial

processes through longer part lifetime, reduction of

inefficiency over time caused by degradation, as well as

prevent costs incurred by material failure.

C. SELF HEALING IN POLYMERS AND FIBRE REINFORCED POLYMER COMPOSITES

1) LIQUID BASED HEALING AGENTS:

The first report of a completely autonomous man-made self

healing material was by the group of Prof. Scott White of the

University of Illinois at Urbana-Champaign. They reported an

epoxy system containing microcapsules. These microcapsules

were filled with a (liquid) monomer. If a microcrack occurs in

this system, the microcapsule will rupture and the monomer

will fill the crack. Subsequently it will polymerise, initiated by

catalyst particles (Grubbs catalyst) that are also dispersed

through the system. This model system of a self healing

particle proved to work very well in pure polymers and

polymer coatings

2) SOLID STATE HEALING AGENTS:

In addition to the sequestered healing agent strategies

described above, research into "intrinsically" self-healing

materials is also being performed. For example,

supramolecular polymers are materials formed by reversibly

connected non-covalent bonds (i.e. hydrogen bond), which

will disassociate at elevated temperatures. Healing of these

supramolecullary based materials is accomplished by heating

them and allowing the non-covalent bonds to break. Upon

cooling new bonds will be formed and the material will

potentially heal any damage. An advantage of this method is

that no reactive chemicals or (toxic) catalysts are needed.

However, these materials are not "autonomic" as they require

the intervention of an outside agent to initiate a healing

response.

D. PIEZOELECTRIC MATERIALS

Piezoelectric materials have two unique properties which are

interrelated. When a piezoelectric material is deformed, it

gives off a small but measurable electrical material it

experiences a significant increase in size (up to a 4% change

in volume)

Piezoelectric materials are most widely used as sensors in

different environments. They are often used to measure fluid

compositions, fluid density, fluid viscosity, or the force of an

impact. An example of a piezoelectric material in everyday

life is the airbag sensor in your car. The material senses the

force of an impact on the car and sends and electric charge

deploying the airbag.

Figure2: Piezo-electric material

E. ELECTRO-RHEOSTATIC AND MAGNETO-RHEOSTATIC

Electro-rheostatic (ER) and magneto-rheostatic (MR)

materials are fluids, which can experience a dramatic change

in their viscosity. These fluids can change from a thick fluid

(similar to motor oil) to nearly a solid substance within the

span of a millisecond when exposed to a magnetic or electric

field; the effect can be completely reversed just as quickly

when the field is removed. MR fluids experience a viscosity

change when exposed to a magnetic field, while ER fluids

experience similar changes in an electric field. The

composition of each type of smart fluid varies widely. The

most common form of MR fluid consists of tiny iron particles

suspended in oil, while ER fluids can be as simple as milk

chocolate or cornstarch and oil.

Figure3: MR Fluid

On the left is the MR fluid when no magnetic field is applied

& on the right is MR fluid which is solidified due to

application of magnetic field

MR fluids are being developed for use in car shocks,

damping washing machine vibration, prosthetic limbs,

exercise equipment, and surface polishing of machine parts.

ER fluids have mainly been developed for use in clutches and

valves, as well as engine mounts designed to reduce noise and

vibration in vehicles.

F. HALO CHROMIC MATERIALS:

Halo chromic material is a material which changes colour

when pH changes occur. The term ‘chromic’ is defined as

materials that can change colour reversibly with the presence

of a factor. In this case, the factor is pH. The pH indicators

have this property. Halo chromic substances are suited for use

in environments where pH changes occur frequently, or places

where changes in pH are extreme. These substances detect

alterations in the acidity of substances, like detection of

corrosion in metals.Halochromic substances may be used as

indicators to determine the pH of solutions of unknown pH.

The colour obtained is compared with the colour obtained

when the indicator is mixed with solutions of known pH. The

pH of the unknown solution can then be estimated. Obvious

disadvantages of this method include its dependency on the

colour sensitivity of the human eye, and that unknown

solutions that are already colored cannot be used.

The colour change of halo chromic substances occur when

the chemical binds to existing hydrogen and hydroxide ions in

solution. Such bonds result in changes in the conjugate

systems of the molecule, or the range of electron flow. This

alters the amount of light absorbed, which in turn results in a

visible change of colour. Halochromic substances do not

display a full range of colour for a full range of pH because,

after certain acidities, the conjugate system will not change.

The various shades result from different concentrations of

halochromic molecules with different conjugate systems

G. DIELECTRIC ELASTOMERS

Dielectric Elastomers (DEs) are smart material systems

which produce large strains (up to 300%) and belong to the

group of electro active polymers (EAP). Dielectric Elastomer

actuators (DEA) transform electric energy directly into

mechanical work. They offer a wide variety of potential

applications as a novel actuator technology that can replace

many electromagnetic actuators, pneumatics, and piezo

actuators. And dielectric elastomers can enable actuators to be

integrated into applications that were previously infeasible.

H. FERROFLUIDS:

Ferrofluids are composed of nanoscale particles (diameter

usually 10 nanometers or less) of magnetite, hematite or some

other compound containing iron. This is small enough for

thermal agitation to disperse them evenly within a carrier

fluid, and for them to contribute to the overall magnetic

response of the fluid. This is analogous to the way that the

ions in an aqueous paramagnetic salt solution (such as an

aqueous solution of copper(II) sulfate or manganese(II)

chloride) make the solution paramagnetic.Particles in

ferrofluids are dispersed in a liquid, often using a surfactant,

and thus ferrofluids are colloidal suspensions - materials with

properties of more than one state of matter. In this case, the

two states of matter are the solid metal and liquid it is in. This

ability to change phases with the application of a magnetic

field allows them to be used as seals, lubricants, and may open

up further applications in future nanoelectromechanical

systems. True ferrofluids are stable. This means that the solid

particles do not agglomerate or phase separate even in

extremely strong magnetic fields.

Figure4: Ferrofluid on glass with magnet below it

These surfactants prevent the nanoparticles from clumping

together, ensuring that the particles do not form aggregates

that become too heavy to be held in suspension by Brownian

motion. The magnetic particles in an ideal ferrofluid do not

settle out, even when exposed to a strong magnetic or

gravitational field.

IV. USES & APPLICATIONS:

There are many possibilities for such materials and structures

in the man made world. Engineering structures could operate

at the very limit of their performance envelopes and to their

structural limits without fear of exceeding either. These

structures could also give maintenance engineers a full report

on performance history, as well as the location of defects,

whilst having the ability to counteract unwanted or potentially

dangerous conditions such as excessive vibration, and effect

self repair The components of the smart materials revolution

have been finding their way out of the labs and into industrial

applications for the past decade. For instance, "smart

concrete"-under development at the State University of New

York at Buffalo-would be programmed to sense and detect

internal hairline fissures. If these conditions are detected, the

smart material would alert other systems to avoid a structural

failure. Smart materials are currently used for a growing range

of commercial applications, including noise and vibration

suppression (noise-cancelling headphones); strain sensing

(seismic monitoring of bridges and buildings); and sensors

and actuators (such as accelerometers for airbags). A number

of companies, including The Electric Shoe Company and

Compaq, are also exploring the use of smart materials. The

Electric Shoe Company is currently producing piezoelectric

power systems that generate electric power from the body's

motion while walking. Compaq is investigating the production

of special keyboards that generate power by the action of

typing. Descriptions of applications for the smart materials

mentioned above suggest that their impact will be broadly felt

across industries.

I. SMART MATERIALS IN AEROSPACE

Some materials and structures can be termed ‘sensual’

devices. These are structures that can sense their environment

and generate data for use in health and usage monitoring

systems (HUMS). To date the most well established

application of HUMS are in the field of aerospace, in areas

such as aircraft checking.

An airline such as British Airways requires over 1000

employees to service their 747s with extensive routine, ramp,

intermediate and major checks to monitor the health and usage

of the fleet. Routine checks involve literally dozens of tasks

carried out under approximately 12 pages of densely typed

check headings. Ramp checks increase in thoroughness every

10 days to 1 month, hanger checks occur every 3 months,

‘interchecks’ every 15 months, and major checks every 24000

flying hours. In addition to the manpower resources, hanger

checks require the aircraft to be out of service for 24 hours,

interchecks require 10 days and major checks 5 weeks. The

overheads of such safety monitoring are enormous.

An aircraft constructed from a ‘sensual structure’ could self-

monitor its performance to a level beyond that of current data

recording, and provide ground crews with enhanced health

and usage monitoring. This would minimise the overheads

associated with HUMS and allow such aircraft to fly for more

hours before human intervention is required

J. SMART MATERIALS IN CIVIL ENGINEERING APPLICATIONS

However, ‘sensual structures’ need not be restricted to hi-tech

applications such as aircraft. They could be used in the

monitoring of civil engineering structures to assess durability.

Monitoring of the current and long term behaviour of a bridge

would lead to enhanced safety during its life since it would

provide early warning of structural problems at a stage where

minor repairs would enhance durability, and when used in

conjunction with structural rehabilitation could be used to

safety monitor the structure beyond its original design life.

This would influence the life costs of such structures by

reducing upfront construction costs (since smart structures

would allow reduced safety factors in initial design), and by

extending the safe life of the structure. ‘Sensual’ materials and

structures also have a wide range of potential domestic

applications, as in food packaging for monitoring safe storage

and cooking.

The above examples address only ‘sensual’ structures.

However, smart materials and structures offer the possibility

of structures which not only sense but also adapt to their

environment. Such adaptive materials and structures have the

capability to move, vibrate, and exhibit a multitude of other

real time responses.

Potential applications of such adaptive materials and

structures range from the ability to control the aero elastic

form of an aircraft wing, thus minimising drag and improving

operational efficiency, to vibration control of lightweight

structures such as satellites, and power pick-up pantographs

on trains. The domestic environment is also a potential market

for such materials and structures, with the possibility of touch

sensitive materials for seating, domestic appliances, and other

products. These concepts may seem ‘blue sky’, but some may

be nearing commercial readiness as you read this.

K. SMART MATERIALS IN MECHANICAL ENGINEERING APPLICATIONS (MECHATRONICS)

Approaches vary from the use of mechatronics (essentially

hybrid mechanical/electronic systems) to the development of

truly smart materials, where sensing and actuation occurs at

the atomic or molecular level. The mechatronic approach is

familiar from systems already in existence such as ABS and

active ride control in road vehicles, and such an approach has

already been employed in the vibration control of high rise

Japanese buildings. However, in truly smart structures the

integration of sensing and actuation is generally greater than

that in pure mechatronic systems, with the required function

integrated within the structural material itself. Such structures

have been compared to Frankenstein's monster since separate

sensors and actuators are integrated (or bolted) together into a

structural material, but without the materials themselves being

smart.

L. APPLICATIONS OF SMART NANOMATERIALS:

Smart nanomaterials are expected to make their presence

strongly felt in areas like:

         Healthcare, with smart materials that respond to injuries

by delivering drugs and antibiotics or by hardening to produce

a cast on a broken limb.

         Implants and prostheses made from materials that

modify surfaces and biofunctionality to increase

biocompatibility

         Energy generation and conservation with highly

efficient batteries and energy generating materials.

         Security and Terrorism Defence with smart materials

that can detect toxins and either render them neutral, warn

people nearby or protect them from it.

         Smart textiles that can change colour, such as

camouflage materials that change colour and pattern

depending upon the appearance of the surrounding

environment. These materials may even project an image of

what is behind the person in order to render them invisible.

         Surveillance using “Smartdust” and “Smartdust Motes”

that are nanosized machines housing a range of sensors and

wireless communication devices. Individually they can float

undetected in a room with other dust particles. By combining

the information gathered from hundreds, thousands or millions

of these tiny specs can give a full report on what is occurring

with the area including sound and images.

M. NANOTECHNOLOGY ENABLED SMART MATERIALS:

Initial nanotechnology influenced improvements to smart

materials will be relatively simple changes to existing

technologies. The future however holds possibilities for

extremely complex solutions for producing not only smart

materials but ones that are highly intelligent.

These new materials may incorporate nanosensors,

nanocomputers and nanomachines into their structure. This

will enable them to respond directly to their environment

rather than make simple changes caused by the environment.

As an example materials may be able to shape shift –

comfortable, flexible clothing for motorcyclists could go rock

hard if it detects an impact, or similar material worn by a

police office could detect an approaching projectile and turn

itself bullet proof. The current emerging technology of surface

treatments for a wall that allows it to change colour might be

impressive now, but what if the wall material could change

itself to produce a window where and when required.

V. THE FUTURE

The development of true smart materials at the atomic scale is

still some way off, although the enabling technologies are

under development. These require novel aspects of

nanotechnology (technologies associated with materials and

processes at the nanometer scale, 10-9m) and the newly

developing science of shape chemistry.

Worldwide, considerable effort is being deployed to develop

smart materials and structures. The technological benefits of

such systems have begun to be identified and, demonstrators

are under construction for a wide range of applications from

space and aerospace, to civil engineering and domestic

products. In many of these applications, the cost benefit

analyses of such systems have yet to be fully demonstrated.

The concept of engineering materials and structures which

respond to their environment, including their human owners,

is a somewhat alien concept. It is therefore not only important

that the technological and financial implications of these

materials and structures are addressed, but also issues

associated with public understanding and acceptance

VI. CONCLUSIONS

The technologies using smart materials are useful for both

new and existing constructions. Of the many emerging

technologies available the few described here need further

research to evolve the design guidelines of systems. Codes,

standards and practices are of crucial importance for the

further development.

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