Neelakanta, P.S. “Smart Materials”
The Electrical Engineering Handbook
Ed. Richard C. Dorf
Boca Raton: CRC Press LLC, 2000
58
Smart Materials
58.1 Introduction
58.2 Smart/Intelligent Structures
58.3 Objective-Based Classification of Smart/Intelligent
Materials
Smart Structural Materials?Smart Thermal Materials?Smart
Acoustical Materials?Smart Electromagnetic
Materials?Pyrosensitive Smart Materials
58.4 Material Properties Conducive for Smart Material
Applications
Piezoelectric Effect?Magnetostrictive Effect?Electroplastic
Effect?Shape-Memory Effects?Electrorheological
Property?Nonlinear Electro-optic Properties?Nonlinear
Electroacoustic Properties?Pyrosensitive Properties?Nonlinear
Electromagnetic Properties
58.5 State-of-the-Art Smart Materials
Piezoelectric Smart Materials?Magnetostrictive Smart
Materials?Electroplastic Smart Materials?Shape-Memory Smart
Materials?Electrorheological Smart Fluids?Electro-optic Smart
Materials?Electroacoustic Smart Materials?Electromagnetic
Smart Materials?Pyrosensitive Smart Materials
58.6 Smart Sensors
Fiber-Optic-Based Sensors?Piezoelectric-Based
Sensors?Magnetostriction-Based Sensors?Shape-Memory
Effects-Based Sensors?Electromagnetics-Based
Sensors?Electroacoustic Smart Sensors
58.7 Examples of Smart/Intelligent Systems
Structural Engineering Applications?Electromagnetic Applications
58.8 High-Tech Application Potentials
58.9 Conclusions
58.1 Introduction
Smart materials are a class of materials and/or composite media having inherent intelligence together with
self-adaptive capabilities to external stimuli. Also known as intelligent materials, they constitute a few subsets
of the material family that “manifest their own functions intelligently depending on environmental changes”
[Rogers and Rogers, 1992].
Classically, such intelligent material systems have been conceived in the development of mechanical structures
that contain their own sensors, actuators and self-assessing computational feasibilities in order to modify their
structural (elastic) behavior via feedback control capabilities. The relevant concepts have stemmed from intel-
ligent forms of natural (material) systems, namely, living organisms; hence, in modern concepts smart or
P. S. Neelakanta
Florida Atlantic University
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intelligent materials and systems are conceived as those that mimic the life functions of sensing, actuation,
control, and intelligence.
The inherent intelligence and self-adaptable control of artificial smart materials should be programmable in
terms of the constituent processing, microstructural characteristics, and defects to permit the self-conditionings
to adapt in a controlled manner to various types of stimuli. The dividing line between smart materials and the
so-called intelligent structures is not, however, distinct. In simple terms, intelligent material systems are
constructed of smart materials with a dedicated, discrete set of integrated actuators, sensors, and so on, and
smart materials contain largely a built-in or embedded set of distributed sensors. In general, the term smart
materials usually connotes the structural constituent in which the discrete functions of sensing, actuation, signal
processing and control are tangibly integrated. Intelligent structures, as an extension, are constructed with
smart materials to respond to the environment around them in a predetermined, desired manner.
Intelligent or smart materials that manifest their own functions intelligently vis-à-vis the changes in their
surroundings are capable of performing, in general (Chong et al., 1990):
?Primary functions specifying the adaptive roles of the sensor, the effector and processor capabilities
(including the memory functions)
?Macroscopic functions that enclave the extensive or global aspects of the intelligence inherent in the
materials
?Built-in social utility aspects with an instilled human-like intelligence with hyper-performance capabilities
58.2 Smart/Intelligent Structures
The framework of intelligent structures as a subset in the gamut of conventional material-based systems is
illustrated in Fig. 58.1. This general classification of material structures refer to [Chong et al., 1990]:
?Sensory structures, “which possess sensors that enable the determination or monitoring of system states
or characteristics” [Chong et al., 1990]
?Adaptive structures, which possess actuators that facilitate the alteration of system-states or character-
istics in a controlled manner
?Sensory systems, which may contain sensors, but no actuators
?Adaptive systems, which contain actuators, but no sensors
Referring to Fig. 58.1, the intersection of sensory versus adaptive structures depicts the controlled structures
with a feedback architecture. That is, the active structure has an integrated controlled unit with sensors and/or
actuators that have structural as well as control functionality. Hence, the logical subset that defines an intelligent
structure is a highly integrated unit (with controlled logic, electronics, etc.) that provides the cognitive element
of a distributed or a hierarchic controlled structure.
FIGURE 58.1 Set of structures. (Adapted from B. K. Wada, J. L. Fanson, and E.F. Crawley, “Adaptive structures,” J. Intell.
Mat. Syst. Struct., 1, 1990.)
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58.3 Objective-Based Classification of Smart/Intelligent Materials
Smart Structural Materials
Intelligent structural engineering materials are the classical versions of smart systems in which the mechanical
(elastic) properties of a structure can be modified adaptively by means of an imbedded distribution of smart
material(s), and an associated (integral) set of sensors and actuators together with an external control system
to facilitate adaptive changes in the elastic behavior of structures so that motion, vibration, strength, stiffness,
redistribution of load path in response to damage, etc. are controlled.
Smart Thermal Materials
A smart thermal material, in response to environmental demands, can self-adaptively influence its thermal
states (temperature or such thermal properties as conductivity, diffusivity, absorptivity), by means of an
integrated conglomeration of thermal sensors, heaters, or actuators with an associated control system.
Smart Acoustical Materials
Smart acoustical materials can be classified as those that have self-adaptive characteristics on their acoustical behavior
(such as transmission, reflection, and absorption of acoustical energy) by means of sensors that assess the acoustical
states (intensity, frequency, response, etc.), along with a set of actuators (dampers, exciters) with an associated
control system. Again, the self-adaptive behavior of these materials is in response to ambient acoustical changes.
Smart Electromagnetic Materials
Smart Magnetic Shielding Materials
As warranted by the surroundings, the self-adaptive shielding effectiveness to magnetic fields at low frequencies
(power frequencies such as 60 or 50 Hz) can be achieved by means of an integrated set of magnetic field sensors
and actuators (magnetic biasing, current elements, etc.) plus a control system arrangement [Neelakanta and
Subramaniam, 1992].
High-Frequency Smart Shielding Materials
Corresponding to radio and higher frequency environments, the shielding requirement warrants curtailing
both electric and magnetic fields. Hence, the relevant self-adaptive intelligent shielding system would consist
of an array of distributed electromagnetic sensors with appropriate elements (actuators) and a control system.
Smart Radar-Absorbing Materials
Absorption of microwave/millimeter wave energy at radar frequency is useful in radar stealth applications. Adap-
tively controllable smart radar-absorbing materials (smart RAMs) can be synthesized with integrated distribution
of electromagnetic detectors (sensors) with appropriate actuators and control system [Neelakanta et al., 1992].
Smart Optical Surface Materials
Smart optical surface materials can be envisioned as those in which the surface optical properties (hue, intensity,
etc.) can be adaptively controlled by means of an intelligent sensor/actuator combinational control system.
Pyrosensitive Smart Materials
Electromagnetic active surfaces constituted by pyrosensitive inclusions have been successfully developed to manage
the electromagnetic reflection and/or absorption characteristics from the active surface by means of thermal actu-
ation of the pyrosensitive nodes imbedded in the medium [Neelakanta et al., 1992]. With the inclusion of a feedback
systems, smart operation in adaptively manipulating the active surface characteristics can be achieved.
58.4 Material Properties Conducive for Smart Material Applications
Certain specific characteristics of materials make them suitable for smart material applications. These properties are:
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1.Piezoelectric effect
2.Magnetostrictive effect
3.Electroplastic effect
4.Shape-memory effects
5.Electrorheological properties
6.Nonlinear electro-optic properties
7.Nonlinear electroacoustic properties
8.Nonlinear electromagnetic properties
9.Pyrosensitive properties
Piezoelectric Effect
Piezoelectric property of a material refers to the ability to induce opposite charges at two faces (correspondingly,
to exhibit a voltage difference between the faces) of the material as a result of the strain due to mechanical
force (either tension or compression) applied across the surfaces. This process is also reversible in the sense
that a mechanical strain would be experienced in the material when subjected to opposite electric charging at
the two faces by means of an applied potential.
In the event of such an applied voltage being alternating, the material specimen will experience vibrations.
Likewise, an applied vibration on the specimen would induce an alternating potential change between the two
faces. The most commonly known materials that exhibit piezoelectric properties are natural materials like
quartz and a number of crystalline and polycrystalline compounds.
The strain versus the electric phenomenon perceived in piezoelectric materials is dictated by a coefficient
that has components referred to a set of orthogonal coordinate axes (which are correlated to standard crystal-
lographic axes). For example, denoting the piezoelectric coefficient (ratio between piezoelectric strain compo-
nent to applied electric field component at a constant mechanical stress or vice versa) as d
mn
, the subscript n
(1 to 3) refers to the three euclidian orthogonal axes, and m = 1 to 6 specifies the mechanical stress-strain
components. The unit for d
mn
is meter/volt which is the same as coulomb/newton.
In the piezoelectric phenomenon, there is an electromechanical synergism expressed as a coupling factor K
defined by K
2
, which quantifies the ratio of mechanical energy converted into electric charges to the mechanical
energy impressed on the material. Being a reversible process, a relevant inverse ratio is also applicable.
Magnetostrictive Effect
Magnetostrictive effect refers to the structural strain experienced in a material subjected to a polarizing
magnetic flux. A static strain of Dl/l is produced by a dc polarizing magnetic flux density B
o
such that Dl/l =
CB
o
2
, where C is a material constant expressed in (meter
4
/weber
2
) taking the units for B
o
as weber/meter
2
.
The magnetic stress constant (L) in (newton/weber) is given by L = 2CB
o
Y
o
where Y
o
refers to the Young’s
modulus of a linearly strained free bar. The coefficient (L) could be both positive or negative. For example,
nickel contracts with increasing B
o
, whereas magnetic alloys such as 45 Permalloy (45% Ni + 55% Fe), Alfer
(13% Al, 87% Fe) exhibit positive magnetostrictive coefficient [Reed, 1988].
Electroplastic Effect
The electroplastic effect (EPE) refers to the plastic deformation of metals with the application of high-density
electric current with an enhanced deformation rate (that persists in addition to that caused by the side effects
of the current such as joule-heating and the magnetic pinch effect). The plastic strain rate resulting from a
current pulse is given by e
I
/e
A
= a J
2
exp(bJ) where e
I
is the strain rate occurring during the current pulse, e
A
is the strain rate in the absence of the current pulse, J is the current density and a and b are material constants.
Typically the EPE has been observed in zinc, niobium, titanium, etc.
Shape-Memory Effects
The mechanism by which a plastically deformed object in the low-temperature martensitic condition regains
its original shape when the external stress is removed and heat is applied is referred to as the shape-memory
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effect (SME) [Jackson et al., 1972]. It is a memory mechanism that is the result of a martensitic transformation
taking place during heating.
Alhough the exact mechanism by which the shape-memory effect occurs is still under study, the process by which
the original shape is regained is associated with a reverse transformation of the deformed martensitic phase to the
higher temperature austenite phase. A group of nickel-titanium alloys (referred to as Nitinol) of proper composition
exhibit the shape-memory property and are widely used in smart material applications [Jackson et al., 1972].
Electrorheological Property
Electrorheological property is the property exhibited by certain fluids that are capable of altering their flow
characteristics depending on an external applied electric field. These fluids have a fast response time, only a
few milliseconds. Once the external field is applied, there is a form of progressive gelling of the fluid proportional
to the applied field strength. Without the applied field, the fluid flows freely. If the electrified electrorheological
(ER) fluid is sheared by an applied force larger than a certain critical value, it flows. Below this critical value
of applied shear force, the electrified fluid remains in the gel phase [Gandhi and Thompson, 1989].
An electrorheological fluid requires particles (1 to 100 mm in diameter) dispersed in a carrier fluid. Sometimes
a surfactant is also added to help the dispersion of particles in the fluid. The surfactant is used to prevent particle
interaction that could otherwise result in a tendency for the particulates to clump together when the fluid is allowed
to stand still over a stretch of time. The tendency of the particles to clump together is referred to as settling.
The applied electric field to perceive the electrorheological phenomenon is usually in the order of 4 kV/mm.
When the electric field is applied, the positive and negative charges on the suspended particles are separated,
forming a dipole of charges. These dipoles then align (polarize) themselves by mutual forces of attraction and
repulsion to other similar dipoles, resulting in unique flow characteristics. In the absence of an electric field,
there is no dipole separation of charges, and hence the fluid returns to its normal flow.
An ideal electrorheological fluid is one that has a low viscosity in the absence of an applied field and that
which transforms into a high-viscosity gel capable of withstanding high shear stresses when the field is on.
Further, it must also have a low power consumption. The first reported ER fluid consisted of finely dispersed
suspensions of starch or silica gel in mineral oil nearly 40 years ago.
Nonlinear Electro-optic Properties
In certain materials that are optically transparent when subjected to an external electric field, the refractive
index of the material changes. Invariably the electric field versus optical effect thus experienced is nonlinear,
FIGURE 58.2 Application-specific classification of smart/intelligent materials.
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with the result that a time-varying electric field will modulate the refractive index, and hence a phase shift is
experienced by the light passing through the medium. In materials that have a central symmetry, this phenomenon
is called the Kerr effect; in noncentrosymmetric materials, it is referred to as Pockel’s effect [Kaminow, 1965].
Nonlinear Electroacoustic Properties
Electroacoustic synergism is experienced in certain classes of materials in which the mechanical atomic vibrations
are influenced by the electronic polarizability, with the result that nonlinear interaction between the atomic dis-
placements versus the electric field causes modulation effects resulting in the generation of new sideband frequencies.
Such sidebands (labeled Raman frequencies) and the response function of a Raman active medium have the form
H(v) = A
1
E(v) + A
2
E
2
(v) + A
3
E
3
(v) + · · ·
Pyrosensitive Properties
The pyrosensitive property is governed by a class of materials known as solid electrolytes. On thermally
energizing such materials, they exhibit superionic electric conduction (also known as fast ion conduction),
with the result that the medium, which is dielectric under cold conditions, becomes conducting at elevated
temperatures. Correspondingly, the media that are embedded with solid electrolytes show different extents of
electromagnetic reflection/transmission characteristics at low and high temperatures and hence can be manip-
ulated thermally [Neelakanta et al., 1992].
Typical solid electrolytes that can be adopted for such pyrosensitive applications are, for example, AgI and RbAg
4
I
5
.
The materials like b-AgI and b-alumina show increasing conductivity with increasing temperature. The compound
b-AgI exhibits superionic conductivity, with an abrupt transition at a temperature close to 147°C. This transition
is known as the b- to a-phase transition, and there are a host of other materials that exhibit this phenomenon. For
example, the material RbAg
4
I
5
has a high electrical conductivity even at room temperature. It has also been observed
that solid electrolytes provide sufficiently high electrical conductivity in the a-phase even when included in low
volume fractions in a mixture with a nonsolid-electrolyte host [Neelakanta et al., 1992].
Nonlinear Electromagnetic Properties
Basically, the nonlinear electromagnetic properties can manifest as two subsets of material characteristics,
namely, nonlinear dielectric properties and nonlinear magnetic properties.
Nonlinear Dielectric Properties
Dielectric materials whose permittivity has a distinct dependence on the intensity of the applied electric field
are referred to as active or nonlinear dielectrics. Such materials demonstrate very high values of permittivity
(in the order of several thousand), pronounced dependence of dielectric parameters on the temperature, and
a loop of electric hysteresis under the action of an alternating voltage.
Ferroelectrics are the most typical example of nonlinear dielectrics. Rochelle’s salt (potassium sodium
tartrate) was the first substance in which nonlinearity was discovered. All ferroelectrics, however, possess
nonlinear properties only within a definite temperature range. The temperature transition points over which
the ferroelectric materials gain or lose their ferroelectric properties are referred to as Curie points. The arsenates
and dihydrogen phosphates of alkali metals are also examples of ferroelectric materials.
Piezoelectrics also fall under the category of active dielectrics. Electrets, which are capable of preserving an
electric charge for a long period of time (hence regarded analogous to permanent magnets), exhibit highly
nonlinear dielectric properties.
Nonlinear Magnetic Properties
Ferromagnetic materials are materials in which the permanent magnetic dipoles align themselves parallel to
each other. These materials have a characteristic temperature below and above which their properties differ
greatly. This temperature is referred to as the Curie temperature. Above the Curie temperature they behave as
paramagnetic materials, while below it they exhibit the well known hysteresis B versus H curves. Examples of
such ferromagnetic materials are iron, Mu-metal, and Supermalloy. Ferrimagnetic materials are similar in their
hysteresis properties to ferromagnetic materials but differ from them in that their magnetic dipoles align
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themselves antiparallel to each other. Ferrites are the most popular ferrimagnetic materials, and they are of the
greatest interest in electrical engineering applications.
58.5 State-of-the-Art Smart Materials
Piezoelectric Smart Materials
Piezoelectric smart materials find applications primarily in intelligent structures deploying electroelastic syn-
ergism, and a class of ceramics (popularly known as ferroelectric ceramics) have emerged in recent times for
such applications. Typically, such ceramics include the base polycrystalline piezoelectrics such as BaTiO
3
,
CdTiO
3
, PbZrO
3
, and PbTiO
3
, formulated with various stoichiometric proportions. Another class of piezoelec-
tric flexible composite that has the potential for smart applications is a compound consisting of PbTiO
3
and
chloroprene rubber. A set of glass ceramic composites containing the crystalline phases of Li
2
SiO
3
, Li
2
Si
2
O
5
,
Ba
2
TiSi
2
O
8
, Ba
2
TiGe
2
O
8
, Li
2
B
4
O
7
, etc. are also emerging samples in smart material engineering [Chong et al., 1990].
Piezoelectric smart materials can also be made from the family of polymers, namely, polyvinylidene fluoride
(PVDF). The main advantages of using this polymer are that it can be formed into very thin sheets and has
excellent mechanical strength combined with high sensitivity to pressure changes.
Another piezoelectric material recently developed in the NTK Research facility in Japan is a kind of rubber-
based material referred to as piezoelectric rubber. This material is composed of a base material of synthetic
rubber, namely, chloroben, dispersed with fine particles of a popular piezoelectric ceramic, called PZT (lead
zirconium titanate). Piezoelectric rubber combines the favorable properties of PZT, namely, high sensitivity,
chemical inertness, linearity, and simplicity, with that of the rubber base, namely, flexibility. The main drawback
with the piezoelectric rubber is in making an electrical contact with it. This problem has been circumvented
by the development of a coaxial cable connection that is easier to use [Ting, 1990].
Magnetostrictive Smart Materials
Materials with a high degree of magnetostriction are deployed in modern intelligent structures. Typically, the
amount of strain inducible with intelligent materials in the current state of the art is 2000 ppm. These are alloys
made with iron and rare earth materials such as terbium (Te), dysprosium (Dy), and niobium (Nb). A
commercially known material of this category is Terfenol [Reed, 1988]. Magnetostrictive transducers for smart
applications have also been developed with a certain class of metallic glass materials.
Electroplastic Smart Materials
Electroplastic materials are useful as smart elastic media inasmuch as the stimulus that modifies the elastic
deformation is the electric current that can be controlled externally. The usefulness of these materials for smart
systems under room temperature conditions is still under investigation.
Shape-Memory Smart Materials
Shape-memory smart materials include three categories, namely shape-memory alloys (SMA), shape-memory
hybrid composites (SMHC), and shape-memory polymers (SMP).
Nickel–titanium (Nitinol) alloys of proper composition exhibit unique memory, or shape-restoration force
characteristics, and are the most popular shape-memory alloys. When the material is plastically deformed in
its low-temperature phase and then heated above its characteristic transition temperature, the original config-
uration or shape is restored. Deformations up to 6–8% can be completely restored by heating the material. It
is this property that is used in smart electromechanical actuations.
Shape-memory hybrid composites are composite materials that contain SMA fibers or films in such a way
that they can be mechanically controlled by heat. These materials can be heated by passing a current through
the fibers. SMHCs offer a wide scope of applications in material–structure interaction. The fibers used in these
composites are also made of Nitinol alloys.
The third form of shape-memory materials are the shape-memory polymers. These materials have an
elastic memory, meaning that a large reversible change in the elastic modulus exists across the glass-transition
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temperature. In other words, across the glass-transition temperature, the material can change from a glass to
rubbery state, allowing significant deformation in response to temperature changes. Shape-memory polymers,
in general, are durable, lightweight, and transparent. Nippon Zeon Company and Mitsubishi Company have
developed high-performance SMPs in the recent past [Chong et al., 1990]. While the SMP of Nippon Zeon
Company is polynorborene based, Mitsubishi’s SMP is polyurethane based, which overcomes crucial weaknesses
such as poor processability and limited-temperature operating range. In their applications SMPs can be used
either as an elastic memory material or a shape-memory material. Depending on which of these possibilities
are used, the range of applications differs.
Electrorheological Smart Fluids
Current research on electrorheological fluids is focused toward development of carrier–particle combinations
that result in the desirable characteristics to achieve smart elastic behavior [Gandhi and Thompson, 1989]. The
earlier versions of electrorheological fluids contained adsorbed water, which limited their operating temperature
change (up to 80°C). Particles in the newer electrorheological fluids are, however, based on polymers, minerals,
and ceramics, which have a higher operating range (200°C). Also, the increase in power consumption is less
with temperature increments in the recent anhydrous systems. The most commonly used carrier fluids are silicone
oil, mineral oil, and chlorinated paraffin, which offer good insulation and compatibility for particulate dispersion.
Electro-optic Smart Materials
Typically potassium dihydrogen phosphate (KDP) exhibits electro-optic behavior. Synthetic materials that have
the ability to alter their refractive index (and hence the optical transmission and reflection characteristics) in
the presence of an electric stimulus can be comprehended as viable smart sensor applications.
Electroacoustic Smart Materials
Although classically the nonlinear interaction of a vibrational (acoustic) wave and an electromagnetic wave has
been studied in reference to Raman active media, relevant concepts can be exercised for smart engineering
applications using those materials that exhibit strong vibrational versus piezoelectric characteristics. The NTK
piezorubber, PZT ceramics, LiNBO
3
, PZT with donor additives, insolvent additives, etc. are viable candidates
for smart applications in addition to piezoelectric polymers.
Electromagnetic Smart Materials
In recent times a number of materials that possess ferroelectric properties have been discovered, the most
popular of which is barium titanate (BaTiO
3
). Barium titanate has an excellent prospect as a smart material
because of the several advantages it offers, such as high mechanical strength, resistance to heat and moisture,
and ease of manufacturing. BaTiO
3
and other similar materials are frequently referred to as ferroelectric
ceramics. Also, electrets such as polymethylmethacrylate offer promise for smart applications.
Among the nonlinear magnetic materials, ferromagnetic materials such as Alnico V, platinum– cobalt, and
a variety of ferrites are possible smart materials.
Pyrosensitive Smart Materials
Pyrosensitive smart materials are useful in realizing intelligent electromagnetic active surfaces, radar-absorbing
materials, electromagnetic shielding, and so on. For example, it has been demonstrated [Neelakanta et al., 1992]
that the microwave reflection characteristics at a surface of a composite medium comprised of thermally
controllable, solid-electrolytic zones (made of AgI pellets) show broadband microwave absorption/reflection
characteristics under elevated temperatures. This principle can be adopted in conjunction with an electromag-
netic sensor to provide a controllable feedback for thermal activation of fast-ion zones reconfigurably in order
to acheive smart active-surface characteristics. Exclusive for this application, depending on the temperature
limited conditions, the solid electrolyte can be chosen on the basis of its a- to b-phase transition characteristics.
In order to keep the cost of the system low, a mixture phase can also be adopted, in which, commensurate with
the elevated temperature operation, the host medium of the mixture could be a ceramic (dielectric).
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58.6 Smart Sensors
Fiber-Optic-Based Sensors
The field of sensing technology has been revolutionized in the past decade by the entry of fiber optics. The
properties of fiber optics that have made the technology suitable for communications are responsible for it
being successful as a sensor as well. Fiber-optic sensors are of two types, namely, extrinsic and intrinsic. In the
extrinsic type, the fiber itself acts only as a transmitter and does no part of the sensing. In an intrinsic type,
however, the fiber acts as a sensor by using one of its intrinsic properties, such as induced birefringence or
electrochromatism, to detect a phenomenon or quantify a measurement. Relevant to smart systems, the use of
fiber optics in conjunction with optical (sensors) is based on changes in optical effects such as refractive index,
optical absorption, luminescence, and chromic properties due to alterations in the environment in which the
fiber is imbedded. Such alterations refer to strain or other elastic characteristics and thermal and/or electro-
magnetic properties [Claus, 1991]. Surfaces located with smart fiber sensors are known as smart skins.
Piezoelectric-Based Sensors
The most conventional form of sensing technology is that of piezoelectric materials, which generate an electrical
response to a stimulus. In recent times piezoelectric materials have been greatly improved in mechanical strength
and sensitivity. Pressure and vibration can be directly sensed as a one-to-one transduction effect resulting from
the elastic-to-piezoelectric effect. Bending, on the other hand, can be sensed via piezoabsorption characteristics.
Magnetostriction-Based Sensors
The use of metallic glass as a distributive magnetostrictive sensor has been studied. Typically, in the imbedded
smart sensing applications using the magnetostrictive property, the magnetic field is in the submicrogauss
regime, and the nonlinearity associated with the hysteresis of magnetostriction provides a detectable sensor
signal. Pressure and force, which cause static or quasi-static magnetic fields, as well as vibrations, which induce
alternating magnetic fields, can be regarded as direct magnetostrictive sensor responses. In the bending mode,
corresponding magnetostrictive absorption can also be sensed via reduction in the Q-factor due to absorption
losses in a magnetostrictively tunable system.
Shape-Memory Effects-Based Sensors
The latest form of sensing technology utilizes shape-memory materials, namely, Nitinol alloys. The Nitinol
sensors are used to measure strain and consist of superelastic Nitinol wires. The basic concept is to measure
the change in resistance of a Nitinol wire used as an unbalanced arm of a Wheatstone bridge as a function of
the strain. The desirable properties of Nitinol in such a sensing application are its high sensitivity and super-
elastic nature (which permits strains up to 6% to be accurately and repeatedly measured). The piezoelectric
and Nitinol sensing materials can also be used for actuation applications.
Electromagnetics-Based Sensors
Smart electromagnetic sensors are simple deviations of classic electric/magnetic probes, more properly known
as antennas or pickups. Depending on changes in the surroundings vis-à-vis the electromagnetic characteristics,
these sensors respond and yield a corresponding signal. Again, the environmental changes refer to possible
alterations caused by elastic, thermal, optical, magnetic, electric, and/or chemical influences.
Electroacoustic Smart Sensors
Electroacoustic smart sensors are embedded acoustic (vibration) sensors (similiar to a microphone) that
adaptively yield a signal proportional to the acoustic input. Such inputs could result from changes in the
alterations in the surroundings caused by elastic or thermal effects.
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As far as smart sensor technology is concerned, in fact, all the synergistic responses and effects between the
electric and nonelectric phenomena just discussed can be judiciously adopted. Considering the state-of-the-
art technology and practical considerations, however, the existing smart sensors are limited to the aforesaid
versions. Future trends could, however, include other possible electric to nonelectric synergistic responses.
58.7 Examples of Smart/Intelligent Systems
The method of synthesizing a smart/intelligent system is illustrated in Fig. 58.3. The output response under a
given set of input condition(s) of a parent test system is normally decided by the properties of the constituent
(conventional) materials. If the system-states (changes) under the influence of external inputs are sensed,
however, an appropriate feedback control can be used to actuate an imbedded smart material in the parent unit,
so that output will track adaptively a desired response. The feedback path may include relevant electronic hardware
(such as microprocessors) for on-line processing of the feedback signal to optimize the system performance.
Essentially, the smart materials can be adopted in two regimes of the system shown in Fig. 58.3. The sensing unit
can be zones of an integrated set of smart material that senses the response of the parent system on a real time
basis. (Sometimes, conventional sensors/tranducers can serve this purpose, as well.) The actuating unit, built-in as
a part of the parent structure, consists of a smart material, which upon receiving the electric signal from the feedback
loop modifies the response of the parent system, as dictated by the input signal. Thus, the actuation is based on
the synergism between the electric input to the corresponding material property of the parent structure being altered.
The feedback control unit may consist of decision logic, which can relatively modify the error signal being
fed to the actuator. The decision logic refers to, for example, response linearization, time-averaged smoothing,
amplitude-limiting, and bandwidth control. On the basis of the general schematic depicted in Fig. 58.3, the
following discusses a few examples of application-specific intelligent systems using smart materials.
Structural Engineering Applications
Figure 58.4 illustrates a smart vibration control strategy in structural beams. Normally, the parent beam is
made of conventional materials, and its vibrational characteristics are decided by the elastic behavior of the
constituent materials. Suppose a smart material is imbedded in the test beam. This material could be one of
the types indicated in Fig. 58.2. A vibration sensor yields an electric output proportional to the vibration.
Suppose the dynamic response of the beam (as observed at the output of the sensor) deviates from the desired
characteristics. Then an error signal can be generated, which in turn can be used to develop an optimal control
signal and this control signal can be fed back to the smart material whose elastic behavior is then altered as a
function of the control input. As a result, the vibration characteristics of the entire (parent) structure are
modified, or the system is dynamically tuned in an adaptive manner.
FIGURE 58.3Schematic of a smart system.
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The vibration sensor used can be either a conventional transducer (such as resistive, capacitive, inductive,
or optical displacement versions) or it can be a smart sensor by itself. For example, an optical fiber with a leaky
sheath (which permits the light energy to leak from the core to the outside surface) can be imbedded in the
parent structure. When the structure is deformed, the extent of light leakage from the fiber to the surrounding
material will modify proportionately. Hence, the detected light signal from the fiber optics, when detected,
delivers information on the deformation or the dynamic structural characteristics of the test beam. This sensor
can be made smart by integrating a distributed set of fibers that can sense strain, vibration, temperature, if
needed, and so on, so that the network implemented with appropriate algorithms will provide exhaustive data
for a comprehensive adaptive feedback control strategy.
Although the scheme illustrated in Fig. 58.4 refers to vibration control (or damping) in structures, judicious
choice of subsystems and materials will permit adaptive control over other structural aspects also, namely,
strain, bending moment, and redistribution of load path in response to failures.
Electromagnetic Applications
Smart material/structural techniques can be adopted in electromagnetic systems. The following are possible
applications:
?Smart low-frequency magnetic shields
?Smart high-frequency electromagnetic shields
?Smart electrostatic dissipative/conductive surfaces
?Smart radar-absorbing materials (smart RAMs)
?Smart linear and aperture antennas
In all the preceding applications, the basic consideration is that the relevant structure can smartly and
adaptively change its electromagnetic properties (normally specified via dielectric permittivity, magnetic per-
meability, and electrical conductivity parameters) so that the desired electromagnetic performance is acheived.
Two typical systems are detailed next.
Electromagnetic Active Surface Embedded with Ferroelectric Inclusions
Figure 58.5 illustrates the concept of a smart electromagnetic active surface. The surface is made of a mixture
of polyacrylamide, ferrite, and barium titanate on a ceramic substrate. This skin material, which represents a
lossy, nonlinear electromagnetic medium with anisotropic ferroelectric and ferromagnetic properties, offers
different extents of surface impedence, in the presence and absence of an electric voltage stimulus applied to
FIGURE 58.4Active control of vibrating beams.
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it. Hence, the reflection coefficient of this material to electromagnetic energy can be altered via electric stimulus.
Relevant feedback can facilitate adaptive smart responsiveness of the system as illustrated [Neelakanta et al., 1992].
Smart Electromagnetic Aperture
The aperture-radiation of microwaves can be smartly controlled by using a pyrosensitive material as illustrated
in Fig. 58.6. A set of solid-electrolyte (AgI) pellets interconnected via nichrome heating elements is placed at
the aperture of a microwave horn. At room temperature, the pellets behave as dielectrics (b-phase AgI). When
heated, however, the b-phase AgI changes to a highly conducting medium (a-phase), which would mask a part
of the aperture, thus modifying the radiation pattern of the horn antenna. Again, an appropriate feedback loop
would render the functioning of this system intelligent [Neelakanta et al., 1992].
58.8 High-Tech Application Potentials
Although smart material technology is in its infancy pending significant efforts to make it usable on a wide
scale, the existing results and ongoing research have confirmed the usability of these materials in several avenues
of modern high-technology systems.
Currently imaginable enclaves for the use of intelligent materials not only include structural engineering but
also such areas as electromagnetics and biomedical, optical, and biological techniques. Relevant research has
also been focused heavily in aerospace, aeronautics, marine vessel, and robotic applications.
Adaptive, self-monitoring of well-being by a system that has an integrated set of smart devices to self-assess
its performance, diagnosing any malfunctions and failures and able to change its system characteristics vis-à-
vis the environment, has been the objective of the relevant seed research pursued until now. For example, self-
health checks by aircraft via a network of smart-skin sensors offer real-time monitoring of the structural well-
being of tomorrow’s aircraft [Claus, 1991]. The protocols in such efforts include self-diagnosis, prediction and
notification, and self-repair strategies relevant to mechanical structures (such as aircraft bodies).
Another domain of smart material application is in self-induced morphologies in the infrastructure of the
material with self-adaptive adjustments to the surroundings. Examples of this category include materials usable
over a wide range of temperatures (as in space shuttles), with a smart adaptibility to transform according to
the environment. Similarly, in radar stealth applications, the target skin could offer variable electromagnetic
absorption over a broad band of radar frequencies.
FIGURE 58.5Smart electromagnetic active surface.
? 2000 by CRC Press LLC
Extensions of smart material concepts can cover selective acoustical absorptions and adaptive chromic
controls in glasses, mirrors, etc. In short, viable smart systems can be conceived with various combinations of
material characteristics discussed earlier together with the advent of new conventional materials, innovative
sensors, advances in microcomputers, artificial intelligence, neural networking, and other upcoming technol-
ogies. Currently imaginable outlets for smart materials are summarized in the following list.
1. Structural/mechanical engineering
? Airborne/space-borne systems with smart skins for adaptive self-health check feasibilities
? Earthquake-resistant intelligent buildings
? Large deployable space structures
? Nondestructive evaluation of large structures
2. Thermal engineering
? Adaptive heat transfer and heat-resistant structures (space shuttles, etc.)
3. Optical engineering
? Adaptive hue, optical transparency, reflection, opaqueness control in glasses and mirrors
4. Electromagnetic engineering
? Magnetic and electrostatic shielding
? High-frequency shielding
? Radar-absorbing materials
? Active surfaces
? Adaptive scattering/radiation control
5. Acoustical engineering
? Active absorption/reflection of sonar radiation
? Adaptive anechoic chambers
6. Chemical engineering
? Materials with adaptive adsorption characteristics
? Adaptive corrosion-resistant materials
7. Biomedical engineering
? Materials with smart structural properties usable as artificial limbs
? Materials with adaptive biochemical properties
8. Warfare systems
? Smart shelters
? Shock-resistant structures
FIGURE 58.6 Smart electromagnetic aperture radiation control.
? 2000 by CRC Press LLC
58.9 Conclusions
The quest for new materials in scientific endeavors and engineering applications is everlasting. The emergence of
the smart material concept has set a trend that science and technology in the coming years will rely to a large extent
on the development of exotic materials, with intelligent materials being the leading candidates. Such materials will
be hyper-functional with unstereotyped purposiveness responses to novel and changing situations.
Defining Terms
Electroacoustic smart materials: Materials that have self-adaptive characteristics in their acoustical behavior
(such as transmission, reflection, and absorption of acoustical energy) in response to an external stimulus
applied as a function of the sensed acoustical response.
Electromagnetic smart materials: Materials such as shielding materials, radar-absorbing materials (RAMs),
and electromagnetic surface materials, in all of which some electromagnetic properties can be adaptively
controlled by means of an external stimulus dictated by the sensed electromagnetic response.
Electro-optic smart materials: Materials in which optical properties are changed self-adaptively with an
external electric stimulus proportional to the sensed optical characteristics.
Electroplastic effect: Plastic deformation of metals with the application of high-density electric current.
Electroplastic smart materials: Materials with smart properties of elastic deformation changes proportional
to a controlled electric current applied in proportion to the sensed deformation.
Electrorheological property: Property exhibited by some fluids that are capable of altering their flow char-
acteristics depending on an externally applied electric field.
Electrorheological smart fluids: Fluids with smart flow characteristics dictated to change self-adaptively by
means of an electric field applied in proportion to the sensed flow parameters.
Intelligent structures: Structures constructed of smart materials with a dedicated, discrete set of integrated
actuators, sensors, etc., in order to respond to the environment around them in a predetermined (desired)
manner.
Magnetostrictive effect: Structural strain experienced in a material subjected to a polarizing magnetic flux,
or reversibly, experiencing magnetic property changes to external mechanical stresses.
Magnetostrictive smart materials: A class of materials with self-adaptively modifiable elastic properties in
response to a magnetic field applied in proportion to sensed stress–strain information.
Nonlinear dielectric property: The distinct dependence of the electric permittivity of certain dielectric
materials on the intensity of an applied electric field.
Nonlinear electroacoustic property: Nonlinear interaction between the atomic displacement and the electric
field experienced in certain materials that would cause modulation effects resulting in the generation of
new sideband frequencies (called Raman frequencies).
Nonlinear electro-optic property: Nonlinear changes in the refractive index of certain optically transparent
materials with change(s) in the externally applied electric field.
Nonlinear magnetic property: Nonlinear dependence of the magnetic susceptibility of certain materials on
the intensity of an applied magnetic field.
Piezoelectric property: Ability of a material to induce opposite charges at two faces (correspondingly to exhibit
a voltage difference between the faces) of the material as a result of the strain due to a mechanical force
applied across the faces; reversibly, application of a potential across the faces would induce a mechanical strain.
Piezoelectric smart materials: Materials capable of changing their elastic characteristics (by virtue of their
piezoelectric property) self-adaptively in response to an externally applied electric potential proportional
to the observed elastic behavior.
Pyrosensitive properties: Exhibited by materials known as solid electrolytes whose electromagnetic properties
can be altered by temperature.
Pyrosensitive smart materials: Materials that self-adaptively (smartly) manage the electromagnetic surface
characteristics of active surfaces constituted by pyrosensitive inclusions, in response to an external
temperature-inducing stimulus applied per the feedback information on electromagnetic characteristics.
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Shape-memory effects: Mechanism by which a plastically deformed object in the low-temperature martensitic
condition regains its original shape when the external stress is removed and heat is applied.
Shape-memory smart materials: Materials that smartly change their elastic characteristics by virtue of their
shape-restoration characteristics achieved by means of an external stimulus in proportion to the magni-
tude of sensed shape changes.
Smart, or intelligent, materials: A class of materials and/or composite media having inherent intelligence together
with self-adaptive capabilities to external stimuli applied in proportion to a sensed material response.
Smart sensors: Sensors with inherent intelligence via bulit-in electronics.
Smart structural materials: Materials in which the mechanical (elastic) properties can be modified adaptively
through the application of external stimuli.
Smart thermal materials: Materials that can influence their thermal states (temperature or thermal properties
such as conductivity) self-adaptively by means of an external control in response to environmental demands.
Related Topics
49.1 Introduction ? 49.2 Mechanical Characteristics ? 50.2 Equation of State ? 55.5 Dielectric Materials ?
56.2 Physical Sensors
References
K.P. Chong, S.C. Liu, and J.C. Li (Eds.), Intelligent Structures, London and New York: Elsevier, 1990.
R.O. Claus, “Fiber sensors as nerves for smart materials,” Photonics Spectra, vol. 25, no. 4, p. 75, 1991.
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M.V. Gandhi and B.S. Thompson, “A new generation of revolutionary ultra-advanced intelligent materials
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conductors,” IEEE Trans. Microwave Theory Tech., vol. MTT-40, no. 5, pp. 28–30, 1992.
P.S. Neelakanta and K. Subramaniam, “Controlling the properties of electromagnetic composites,” Adv. Materials
and Process, vol. 141, no. 3, pp. 20–25, 1992.
R.S. Reed, “Shock isolation using an active magnetostrictive element,” in Proc. 59th Shock and Vibration Symp.
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Further Information
Intelligent Structures, edited by K.P Chong, S.C. Liu, and J.C. Li, contains the papers presented in an international
workshop on intelligent structures held on 23–26 July 1990 in Taipei, Taiwan, Elsevier Science Publishers,
1990.
Another source is the Proceedings of the International Workshop on Intelligent Materials, The Society of Non-
Traditional Technology, 1989.
Recent Advances in Adaptive and Sensory Materials and Their Applications, by C.A. Rogers and R.C. Rogers,
Lancaster, Pa.: Technomic Publishing Co., Inc., 1992.
The author’s publication with K. Subramaniam, “Controlling the Properties of Electromagnetic Composites”
in Advanced Materials and Processes, vol. 141, no. 3, pp. 20–25, 1992.
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