Application of piezoelectrics to smart structures

Abstract

This paper describes piezoelectric materials, actuators and their use in smart structures. The paper provides criteria for the evaluation and selection of piezoelectric materials and actuator configurations. Typical applications using piezoelectrics in smart structures are also presented, with particular emphasis on shape and vibration control.

Full text

1
APPLICATION OF PIEZOELECTRICS TO SMART STRUCTURES
S. Eswar Prasad*, David F. Waechter*, Richard G. Blacow*,
Hubert W. King†, and Yavuz Yaman††
*Sensor Technology Limited
PO Box 97
Collingwood, Ontario, L9Y 3Z4 Canada
e-mail: eprasad@sensortech.ca, web page: http://www.sensortech.ca
†Department of Mechanical Engineering
University of Victoria
Victoria, B.C. V8W3P6 Canada
web page: http://www.me.uvic.ca
††Middle East Technical University
Department of Aerospace Engineering
Ankara, Turkey
web page: http://www.ae.metu.edu.tr
Keywords: Smart Structures, Smart Materials, Piezoelectric
Abstract. This paper describes piezoelectric materials, actuators and their use in smart
structures. The paper provides criteria for the evaluation and selection of piezoelectric
materials and actuator configurations. Typical applications using piezoelectrics in smart
structures are also presented, with particular emphasis on shape and vibration control.
1 INTRODUCTION
There is an increasing awareness of the benefits to be derived from the development
and exploitation of smart materials and structures in applications ranging from hydrospace
to aerospace. With the ability to respond autonomously to changes in their environment,
smart systems can offer a simplified approach to the control of various material and
system characteristics such as noise, shape and vibration, etc., depending on the smart
materials used.
II ECCOMAS THEMATIC CONFERENCE ON SMART STRUCTURES AND MATERIALS
C.A. Mota Soares et al. (Eds.)
Lisbon, Portugal, July 18-21, 2005

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
2
The benefits that can be derived from the use of smart materials and composites are
discussed. With the ability to develop high strains and to act with small or suitably
modeled hysteresis, these materials offer engineers the opportunity to micomanipulate
optical devices, small robots, and other system components1,2. Examples of material
properties that can be engineered into smart structures are presented based on recent
developments in the field.
This paper starts with a discussion of the piezoelectric effect and piezoelectric
materials. Focus will be on ceramic materials that offer efficient, reliable, and low-cost
sensor and actuator configurations. This will include some materials that have been used
for may years, and others that were recently developed3,4. Criteria for the selection of a
piezoelectric material will be discussed.
Sensor and actuator configurations will play an important role in smart structures. The
operating principles of such devices will be discussed and selection criteria based on
sensitivity, displacement, force, frequency response and bandwidth will be presented.
Trade-off considerations for such devices will also be discussed.
Application of piezoelectrics in smart structures will be reviewed. The paper will
identify four broad areas of application involving active control of vibration, shape, noise
as well as active health monitoring. Several case studies involving these applications will
be presented with a discussion of problems and issues. For example, vibration can cause
damage or compromise precision instruments and/or human health, especially for
operators of heavy equipment. Stray vibration may cause misreading and reduce
sensitivity in navigational gyroscopes; in precision machining devices chatter vibrations
present in a machine tool structure can severely reduce machining tolerances and mar
surface finishes. Active control techniques using embedded piezoelectric actuators can
substantially reduce the vibration amplitude and settling time5. Active control of shape can
be used in receiver filters in satellites to provided optimum response at all times6.
2.1 The Piezoelectric Effect
Piezoelectricity is a property exhibited by materials that become electrically charged
when subjected to mechanical stress. The converse effect, in which a mechanical
deformation is induced by an applied electric field, also occurs. Experimental measure-
ment of the piezoelectric effect was first reported by Pierre and Jacques Curie in 18807.
The Curies studied naturally occurring crystals such as quartz, Rochelle salt and
tourmaline. They later observed the converse effect8, which was predicted from
thermodynamic principles by Lippmann in 18819. The direct effect may be used in sensing
applications, while the indirect effect may be used in actuation and acoustic transduction.
The piezoelectric effect is caused by an asymmetry in the unit cell and the resulting
relationship between mechanical distortion and electric dipole separation. The effect may
be quantified through the use of suitable piezoelectric coefficients, the measurement of
which has been described in various standards on piezoelectricity10,11. The piezoelectric

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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charge coefficient is the ratio of the electric charge to the applied force that induced the
charge. It is expressed in Coulomb/Newton (C/N) and may be written as
In the converse effect, the same constant has the alternate expression
The coupling coefficient is defined by the ratio of the mechanical energy accumulated
in response to an electrical input or vice versa. The square of the coupling coefficient is
given by
Conversely,
The d and k constants above are usually represented as matrix quantities dij and kij,
where the subscript i refers to the direction of the applied or induced electric field and j to
the direction of stress or strain. By convention i,j = 3 corresponds to the poling direction
of the material, and the 1, 2 and 3-directions are mutually orthogonal. The subscripts j = 4,
5 and 6 are used for shear stresses about the 1, 2 and 3-axes respectively.
Lettered subscripts, as in dh and kp, are used to handle special cases2. Here, h is used for
hydrostatic stress that is equal in all directions with applied electric field in the
3-direction. The letter p is used for stress or strain that is equal in all directions perpen-
dicular to the 3-axis with applied field in the 3-direction.
Finally, complex components are sometimes used, particularly for materials with high
dielectric loss, to represent the lag between electrical or mechanical stimulus and
response12,13. These imaginary components are often neglected, unless otherwise stated.
StressApplied
circuit)(openDensityChargeGenerated
d=(1)
FieldApplied
circuit)(openDevelopedStrain
d=(2)
AppliedEnergy Mechanical
StoredEnergy Electrical
k2=(3)
AppliedEnergy Electrical
StoredEnergy Mechanical
k2=(4)

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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2.2 Currently Available Materials
Single crystal materials like α-quartz are still used in applications such as precision
frequency control and surface-acoustic-wave (SAW) devices14,15. However their use in
other applications rapidly declined following the development of polycrystalline ceramics
such as barium titanate in the 1940’s16 and lead zirconate titanate (PZT) in the 1950’s17.
Since the 1950s PZT has largely replaced barium titanate because of its larger Curie
temperature and higher efficiency. The polycrystalline ceramics are less expensive and
easier to machine in a wide range of shapes and sizes than single crystals. In this section
we review the properties of lead zirconate titanate, which has become the most commonly
used material composition in sonar transducers, actuators and smart structures. Lead
titanate (PT) and Lead Magnesium Niobate – Lead Titanate (PMN-PT) compositions are
also discussed.
Lead Zirconate Titanate (PZT)
PZT materials are available in a wide variety of compositions that are optimized for
different applications. PZT is a mixture of lead zirconate (PbZrO3) and lead titanate
(PbTiO3) and has the perovskite structure16. Various additives and Ti/Zr ratios may be
used to yield material that has one or more desired properties such as high piezoelectric
activity, low loss or temperature and time stability. Trade-offs are generally required to
obtain the best values of the most important properties at the expense of some degradation
of others.
PZT ceramics are often classified as “hard” or “soft”, according to the characteristics
shown in table 1. Those impurities that cause a hardening effect are acceptors while those
that cause softening are donors18. Charge imbalances induced by donors or acceptors are
made up for by vacancies at Pb or Zr-Ti sites in the case of donors and at oxygen sites in
the case of acceptors.
Harder ⇔ PZT ⇔ Softer
⇓piezoelectric d constants ⇑
⇓dielectric constant ⇑
⇓dielectric loss ⇑
⇓hysteresis ⇑
⇑mechanical Q ⇓
⇓coupling factor ⇑
⇓resistivity ⇑
⇑coercive field ⇓
⇓elastic compliance ⇑
⇑aging effects ⇓
Table 1: Comparison of hard and soft PZT ceramic properties

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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The high coercive field of hard PZT is due in part to the presence of an internal bias
field. The internal field allows higher externally applied electric field amplitude to be used
without depolarization-induced losses. In field-limited devices, such as high-power sonar
transducers, this can offset the disadvantage of lower piezoelectric d constants. A new
internally biased PZT composition (b-PZT) has recently been developed that has a high
internal bias field but with less degradation of the piezoelectric d constants than occurs in
other hard PZT compositions4.
Lead Titanate
Lead titanate compositions have been developed to achieve very high anisotropy in
piezoelectric properties. These compositions are most often used in applications where it is
necessary to eliminate interference from radial modes. While pure lead titanate has the
highest Curie temperature (TC) known for perovskite feroelectrics (490oC)19, the impurity
additions used in most commercial high anisotropy compositions lower TC to values
comparable to soft PZT.
Lead Magnesium Niobate – Lead Titanate (PMN-PT)
PMN-PT is a relaxor perovskite that exhibits low hysteresis and high strain near a diffuse
ferroelectric to paraelectric transition3. In the most commonly used composition, PMN-0.1PT,
it is an electrostrictive material that has a square-law dependence between strain and applied
electric field.
3 CRITERIA FOR SELECTING PIEZOELECTRIC MATERIALS
In this section we first review the main actuator configurations that may be realized using
piezoelectric materials, with emphasis on the force and displacement ranges that can be
achieved. This is followed by a discussion of piezoelectric material properties and the
material selection criterion for the actuation devices.
It is worth noting that while piezoelectric materials can be used for sensing as well as
actuation, it is more common at the present time to use alternative sensing technologies such
as strain gages, capacitive sensors or optical sensors in smart structure systems. This is due to
mechanical loading, nonlinearity and other considerations that currently favor the latter
technologies. However this situation may change in the future by implementing self-sensing
techniques in which the sensing signal is extracted from the piezoelectric actuation signal
using a suitable nonlinear model20. But for the purposes of this paper, we focus on the
actuation component of smart structure technology and merely assume that a feedback signal
for control purposes will be available by the most appropriate means.

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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3.1 Actuator Configurations
Piezoelectric actuators may be implemented in a wide range of configurations depending
on the force and displacement requirements. In this section we focus first on two actuator
typesmulti-layer stack actuators and bending-mode actuatorsthat bracket extreme ends of
the force-displacement range that is attainable with single-stroke piezoelectric actuators. We
then discuss other actuator types that provide intermediate force and displacement
capabilities. Finally, we discuss compound devices that use step-and-repeat or traveling-wave
motions to achieve long-range displacement.
For multi-layer stack actuators the zero-load displacement and blocked force at frequencies
substantially below resonance may be expressed by the relations21
and
where N is the number of layers in the stack, d33 is the piezoelectric charge constant, V is the
applied voltage, Y is average Young’s modulus of the stack, A is the stack area, and h is the
stack height. Maximum displacements are typically in the range of tens of microns, while
blocked-force values of thousands of Newtons can be readily achieved.
It can be seen from the above equations that displacement of multi-layer actuators
increases with number of layers, while the blocked force is independent of the number of
layers because of the N/h dependence (h=N*layer thickness). Increasing the stack area is the
primary means to increase the blocked force of a multi-layer stack. When estimating the
blocked force using eq. 6, the Young’s modulus, YE33, of the piezoelectric ceramic can be
used to provide an upper bound. However when epoxy or other intermediate layers are used
in the stack construction, the Young’s modulus of these materials may lower the average
Young’s modulus of the overall stack, resulting in a reduced blocked force. Nevertheless, it is
possible to come reasonably close to the theoretical blocked force, appropriate to the ceramic
modulus, by using sufficient pre-stress to compress any epoxy layers used in the stack
construction.
Bending mode actuators typically consist of two piezoelectric plates that are bonded
together, with one plate biased to expand and the other biased to contract. If one end of the
plate is fixed in a cantilever configuration and the other is free to deflect, the zero load
displacement and blocked force at frequencies well below resonance may be expressed as21
and
VNdL 33
=
∆
(5)
,
33 h
A
VYNdF =(6)
E
t
L
d
2
31
2
3
=
δ
(7)

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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where * is the cantilever deflection, d31 is the piezoelectric d constant, L is the cantilever
length, w is the actuator width, t is the total ceramic thickness and Y is the effective Young’s
modulus of the structure, approximately equal to YE11 of the ceramic. E is the electric field,
which is equal to V/t for series-connected plates and 2V/t for parallel-connected plates.
Bending-mode actuators provide larger displacement, up to the mm range, but have
smaller force capability than multi-layer actuators, typically no more than 1 or 2N. Length
and width scaling according to equations 7 and 8 can be used to arrive at suitable geometry
for given displacement and force requirements.
Bending mode actuators can also be realized by bonding a single piezoelectric plate to a
thin metal foil. Rainbow (Reduced and Internally Biased Oxide Wafer) actuators are a
variation of this design that have somewhat higher displacement and force capability. In these
devices a reduction process forms an integral electrode with internal compressive stress and
yields a bowed shape suitable for point loading22.
Flextensional devices can be used to amplify the displacement of piezoelectric actuators,
but typically achieve a strain amplification of no more than five times. This amplification
level, however, is usually accompanied by a decrease in load carrying capability by up to
1000 times21. Lever systems can provide similar or greater displacement amplification with
relatively less degradation of the force capability21, but both lever and flextensional systems
have limited temperature ranges due to thermal mismatch between components.
Tubular actuators are a versatile technology that can provide either linear extension or
multi-directional bending motion in a simple compact device. When a piezoelectric tube has
uniform metal on the inside and outside surfaces, an applied voltage causes the length to
increase or decrease, depending on polarity. Displacements of up to several microns can be
achieved21. On the other hand, if the contact electrodes are segmented with segment
boundaries parallel to the tube axis, it is possible to achieve a bending motion by biasing
opposing segments in opposite directions. By using a four-segment tube, two-dimensional x-y
bending can be achieved. By further adding an unsegmented section on top of the four-
segment portion, full x-y-z control can be obtained. Devices of this type are ideally suited for
probe control in scanning tunneling microscopes23.
High displacement actuators can be achieved by using a step-and-repeat approach in
Inchworm actuators. These devices typically use two clamps and an extender section, which
are activated in a properly timed sequence, to achieve long range motion. A fine-positioning
mode can also be realized by activating the extender section only while one clamp is on and
the other is released. It has recently been shown that by using a complementary design, where
one clamp releases with low voltage and the other with high voltage, the fine-positioning
mode can be realized with both clamps unpowered. Also in this case, the two clamps can
share a common drive signal in the coarse-positioning mode24,25. Step-and-repeat motors can
,
8
32
31 E
L
wt
YdF =(8)

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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achieve high stiffness, with up to 200N restraining force and nanometer resolution26. With
optimal clamp timing, speeds as high as 50cm/s may be achieved27.
Ultrasonic motors are another piezoelectric technology that can achieve long-range
motion. These devices use an elliptical motion that results from waves that are produced by a
piezoelectric actuator attached to a vibrator28. The motion is transmitted by friction to the
moving part. These motors can be very compact but are not suitable for low-speed
positioning.
Fig. 1 shows the force versus displacement characteristics of various piezoelectric
actuators. Single-stroke actuators are limited to no more about a millimeter displacement and
the force becomes small near the higher end of the displacement range. Nevertheless, the
force-displacement range for single-stroke devices is useful for many smart structure
applications such as those described in section 4. The linear-motor portion of the figure
includes the inchworm and ultrasonic-type motors discussed above.
Figure 1: Force and displacement ranges for piezoelectric actuators
3.2 Material Selection
Table 2 summarizes key properties of selected piezoelectric ceramics. For low-frequency
operation, equations 5-8 suggest that ceramics having large piezoelectric d constants, such as
Navy Type VI, are preferred for actuator applications in smart structures. However, when
temperature stability is also required, Navy Type II, with its larger Curie temperature, is a
better choice.
Electrostrictive PMN-0.1PT may also be considered for low-frequency applications.
Properties of this material are not shown in Table 2, because the quadratic strain versus field
characteristic of electrostrictive materials makes it possible to define only differential
properties at specified fields. The apparent d33 obtained by differentiating the strain versus
field curve is small at low fields but reaches a peak as high as 1800pC/N at a field of
1
10
100
1000
10000
1.E-01 1.E+ 00 1.E+ 01 1.E+02 1.E +03 1.E +04 1.E+05
Force (N)
Displaceme nt (10-6m)
10-1 100101102103104105
Bending
Mode Multi-layer
Stack
Mechanically A m plified
Tube
Linear
Motors

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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0.4MV/m3. However, this material is also temperature-sensitive, even more so than Navy
Type VI.
For higher frequencies, the higher dielectric constant of softer PZT ceramics may pose
difficulties for current-limited power supplies. This is also true for PMN-0.1PT, which has a
temperature-dependent relative dielectric constant that can exceed 30,0003. Harder PZT
ceramics can be driven with higher frequency and electric field amplitude, both from the point
of view of power supply limitations and from the point of view of dielectric loss tangent and
internal heating4.
The smaller dielectric loss tangent of hard PZT ceramics is connected with smaller
hysteresis in the polarization vs. field loops. This is in turn is reflected in smaller hysteresis in
strain versus field loops which is beneficial for high positioning accuracy, under both open-
loop and closed-loop control29. However, hysteresis modeling can be integrated into a closed-
loop controller to achieve high positioning accuracy even with soft PZT ceramics30.
LT b-PZT Navy
Type I
Navy
Type II
Navy
Type III
Navy
Type V
Navy
Type VI
Lead
Titanate
“hard”
biased
PZT
“hard”
PZT
“soft”
PZT
“hard”
PZT
“soft”
PZT
“soft”
PZT
KT
33 --- 200 1080 1350 1750 100 2750 3250
Tan *% 2.0 0.3 0.4 1.6 0.3 2.0 2.0
QM--- 800 1000 500 80 1000 70 70
k31 --- 0.03 0.31 0.35 0.37 0.3 0.37 0.39
k33 --- 0.51 0.64 0.7 0.72 0.64 0.72 0.75
d 31 10-12C/N -3.0 -100 -125 -175 -60 -215 -270
d 33 10-12C/N 70 250 300 365 225 500 590
YE
11 1010N/m214 9.3 8.0 6.5 9.1 6.9 7.1
YE
33 1010N/m211 6.5 6.7 5.3 7.4 5.1 5.0
TC
oC 225 320 350 360 325 225 210
Table 2: Comparison of key properties of piezoelectric ceramics31
4 APPLICATIONS TO SMART STRUCTURES
4.1 Active Vibration Control
Smart structures that use discrete piezoelectric patches to control the vibration of thin
plates have been of considerable interest in recent years. The development of finite
element codes, such as ANSYSTM, makes it possible to fully model coupled thermo-
mechanical-electrical systems of one or more dimensions and obtain reciprocal relations
between piezoelectric actuator voltages and system response. By integrating such models
into a closed-loop control system, very effective active vibration suppression can be
achieved. Active vibration control strategies such as H∞ have been applied to one-
dimensional beams32 as well as two-dimensional plates33 and a fin that emulates an aircraft
tail32,5. These studies used multiple Navy Type II piezoelectric patches of area

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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25mmx25mm and thickness 0.5mm, bonded to the aluminum structure whose vibration is
to be controlled. In the case of the two-dimensional fin, shown in Fig. 2, it was found that
a single sensor as control input was inadequate and it is believed that a multi-input multi-
output system model is needed to suppress multiple vibrational modes5.
Figure 2: Smart fin with attached piezoelectric patch actuators.
4.2 Active Noise Control
The sources for noise are numerous. For example, noise can radiated from engines,
exhaust systems, fans, and blowers. Active noise control (ANC) was developed as a way to
reduce if not eliminate some of these different types of noise. Noise is usually defined as a
sound due to irregular vibration or any sound that causes discomfort. The importance of
active noise control in the workplace is becoming increasingly important primarily due to
hearing loss resulting from long term exposure to workplace noise34.
Active control of sound is very similar in nature to ANC and is often described in the
same terms. ANC works on the basic principle of destructive interference, where the
undesirable sound wave is countered with a sound wave of equal amplitude, but 180 degrees
out of phase. The result is that the sound waves cancel each other, and the undesirable sound
is reduced or eliminated. This principle is implemented in smart structures, including noise
cancellation headsets, transformer quieting systems, and interior noise reduction in
automobiles and aircraft35,36.
Such a smart system will consist of a sensor(s), electronics and a projector or
transmitter of sound. This system may be configured differently depending on the acoustic
objectives of the system. Control system strategies applied to active noise or sound control
are similar to those employed in active vibration control. The control strategy involves the
noise source being measured in the primary sound field and electronically conditioned before
creating and passing the conditioned signal through a transmitter. In this system no prior
knowledge of the noise is necessary.

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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Piezoelectrics are used extensively to monitor the noise associated with panel
vibrations such as those present in transformers. In underwater acoustics, piezoelectrics
generally far outperform other types of sensors and actuators for the generation and reception
of sound. It is generally very difficult to produce broadband sound sources for underwater due
to difficulties associated with the design and manufacture of acoustic transducers. Active
control of sound principles can be used to create such a source. The Broadband Acoustic
Transmission System37 uses such technology. A hydrophone is generally used as a sensor for
these applications. It is relatively simple to build a broadband hydrophone with standard
piezoelectric ceramic materials. However, broadband high-power acoustic projectors are
difficult to build. Fig. 3 shows the response characteristics of such a transducer (SX01 from
Sensor Technology) and also shows the radiating characteristics of the same transducer after
conditioning the signal. Such broadband systems have a number of applications including the
study of marine mammals, sonar communication, underwater pagers, diver recall systems and
seismic surveys.
Figure 3: Response characteristics of the BATS smart system

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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4.3 Active Shape Control
Active shape control is of substantial interest for reflectors and antennas that must
maintain precise dimensions for optimal performance. This is of particular importance for
structures in space that are made of lightweight materials and are exposed to thermal
distortion. Instruments such as millimeter-wave and sub-millimeter-wave passive
instruments and those operating up to the infrared spectrum for space radio astronomy
need to maintain micrometer accuracy, with overall dimensions of up to a few meters38.
Yet thermal distortions of space structures of this sizeeven those using low co-efficient
of thermal expansion materials such as carbon fiber reinforced plasticcan result in RMS
surface errors on the order of 0.8mm39. For these applications, smart structures involving
attached piezoelectric actuators have been proposed38.
Active shape control has also been proposed for use in RF filters for communications
satellites. A typical filter design uses cascaded resonators that have adjustable posts for
frequency tuning. The posts are adjusted prior to launch but exhibit thermal expansion in
space, resulting in frequency shifts. By integrating piezoelectric stack actuators into the
posts and applying active control, a three-times reduction in frequency shifting with
temperature has been reported6.
4.4 Active Health Monitoring
Civil, industrial and aerospace structures can benefit from the smart structure approach as
the basis for active health monitoring. Structural panels embedded with a series of sensors and
actuators can be used in civil, industrial and aerospace structures. These panels can actively
monitor the structural integrity and detect faults at early stages, thereby providing precise
information on structural failure and life expectancy. This will be very useful for the
aerospace sector40 where in the absence of active health monitoring, expensive aircraft or
their systems are prematurely taken out of service or allowed to operate when ‘unsafe’, which
could result in loss of life in addition to the aircraft. If the health of these structures is known
with good reliability, considerable cost savings could be realized by extending the useful life
of these aircraft or their systems. Other promising applications of health monitoring are in
heavy machinery (such as turbines) applications, early and accurate detection of earthquakes
in seismic regions , and a smart bridges with the ability to monitor and readjust its structure to
decrease stress levels in the bridge41.
Acoustic emissions from dislocation movements, phase transformations, friction
mechanisms, and crack formation and extension was proposed as a method of monitoring the
health of structures for several decades. Piezoelectrics are by far the most widely used device
materials for the monitoring of such acoustic emissions. However, since these emissions are
usually surface waves, special measuring techniques need to be applied. Once a surface wave
has been detected, the structure can process that information in order to take an appropriate
action, which may involve grounding an aircraft, suggesting maintenance, or the activation of
mechanical stress relief in the specific part of the structure40.

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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Sun et al.42 used piezoelectric sensor/actuator patches on a truss structure to monitor
the integrity of the truss. The technique used the impedance-signature technique to assess the
structural integrity of the truss, as any damage to the structure would effect the mechanical
impedance of the structure. Piezoelectric ceramics have also been bonded to the outside of
structures to monitor delaminations43,44 and damage45 in composites. In both cases, piezo-
electrics monitor the natural vibration frequencies of the composite. If the composite begins
to delaminate, its vibrational frequency will be effected and the piezoelectric sensor will be
able to detect those changes. By exciting and monitoring the frequency response of the
composite and being able to monitor the structure’s integrity, more cost effective maintenance
could be completed before the structural element failed during operation.
In acoustic emission techniques, it is important that the area of contact with the structure is
small and the sensor itself should have a broadband response. Fig. 4 shows such a sensor
manufactured by SensorTech and its frequency response transfer function characteristic.
Figure 4: Acoustic sensor response transfer function

S. Eswar Prasad, David F. Waechter, Richard G. Blacow, Hubert W. King and Yavuz Yaman
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Inchworm is a registered trademark of Burleigh Instruments, Inc.
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