1. INTRODUCTION
A transducer is anything which converts one form of energy to other.
In piezoelectric transducer, piezoelectricity is the key characteristic. When a
piezoelectric material is squeezed or stretched, an electric charge is
generated across the material, which is called ‘direct piezoelectricity.’
Conversely, a piezoelectric material mechanically deforms when subjected to
electric voltage, which is called ‘converse piezoelectricity’.
Figure 1: Direct and Converse
Piezoelectricity
Piezoelectric transducers have been mostly used for local damage
detection, and there is increasing interest in integrating these local
nondestructive testing (NDT) techniques with global vibration monitoring
techniques for improved structural health monitoring of civil infrastructures.
2. PIEZOELECTRIC MATERIALS
Natural piezoelectric materials such as quartz (SiO2) and
Rochelle salt (NaKC4H4O6–4H2O) have
been widely used for piezoelectric transducers. However, its applications are
often limited due to its vulnerability to liquid and high temperature. To
overcome the limitations of these natural piezoelectric materials and improve
the piezoelectric performance, synthesized piezoelectric materials have been
developed.
One of the widely used piezoelectric material is piezoelectric
ceramics such as barium titanate (BaTiO3), lead titanate (PbTiO3),
and lead zirconate titanate (PZT) (PbZrTiO3). Macro-fiber composite
(MFC) is an innovative flexible transducer offering high-performance at a
competitive cost. MFC was first developed at NASA Langley Research Center in
1996 to enhance the flexibility of piezoelectric transducers. Another widely
used flexible piezoelectric transducer is active fiber composite (AFC)
developed by Massachusetts Institute of Technology. Polyvinylidene fluoride
(PVDF) is another popular piezoelectric polymer because of its flexibility. Smart
aggregate is a new piezoceramic device developed for concrete structure
monitoring as shown in Figure 2. The smart aggregate is composed of a
waterproof piezoelectric patch with lead wires embedded in a small concrete
block. The devices are then embedded in concrete structures during casting. One
smart aggregate is used as an actuator to generate a desired input signal,
while the other smart aggregates are used as sensors to detect the
corresponding responses. They are used for early-age strength monitoring
Figure 2:
Smart Aggregate
3. BONDING OF PIEZOELECTRIC
MATERIALS TO THE STRUCTURE
Figure 3 shows the bonding layer
between the piezoelectric transducer and the host structure. In typical SHM
applications, the piezoelectric transducers are assumed to be perfectly bonded
with a host structure via an adhesive. In reality, however, the adhesive forms
an interfacial layer of finite thickness between the piezoelectric element and
the host structure, and this adhesive layer significantly affects the shear
stress.
Figure 3: Piezoelectric material bonded
to a structure
4. STRUCTURAL
HEALTH MONITORING (SHM) TECHNIQUES
4.1 Guided Wave Techniques
It is one of the most popular SHM techniques. These techniques are
attractive because guided waves, defined as elastic waves confined by the
boundaries of a structure, can travel a long distance with little signal
attenuation and high sensitivity to small structural damages. Figure 4 depicts
two typical modes of guided wave measurement. When an electrical voltage is
applied to PZT mounted on a plate-like target structure, guided waves are
generated and propagate along the target structure. Then, the corresponding
responses can be measured by the same PZT in a pulse–echo mode or by the other
PZT in a pitch–catch mode. The guided waves traveling through a structural
discontinuity produce scattering, reflection, and mode conversion, making it
possible to identify structural damage. Guided waves are, however, also
sensitive to environmental and operational variation, often resulting in false
alarms. To minimize these effects on the guided wave techniques, reference-free
guided wave techniques have been proposed. In conventional guided wave
techniques, structural damage is often identified by simple comparison between
baseline data obtained from the pristine condition of the target structure and
the current data measured from current state of the target structure. On the
other hand, the reference-free techniques utilize only current data for damage
diagnosis, thus making them less sensitive to environmental and operational
variations
Figure 4: Guided Wave Technique
4.2 Impedance Techniques
Impedance techniques using piezoelectric transducers have been
developed to detect local damages in complex structures. In the impedance
technique, an electromechanical impedance signal is measured by applying an
electric voltage to PZT and measuring the corresponding output current when the
PZT is attached to a host structure, as shown in Figure 5. Since the
electrical impedance of the PZT is coupled with the mechanical impedance of the
host structure, potential damage can be manifested by monitoring the change of
the measured impedance signal.
.
Figure 5: Scheme of the impedance
technique.
The impedance technique is attractive for local damage detection because
it is sensitive to even small damage and can be applied to complex structures. However,
impedance measurements become difficult with highly damped materials such as
carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer
(GFRP) or large-scale structures with high mechanical impedance, because PZT
transducers cannot produce excitation forces large enough to create standing
waves, which are a requisite to obtain impedance signal. In the impedance
technique, one of the most challenging issues is that the impedance signals are
also sensitive to environmental variations, such as temperature and loading
changes as well as structural damages.
4.3 Acoustic Emission
Techniques
Acoustic emission (AE) is defined as ‘transient elastic stress waves
produced by a release of energy from a localized source’. An AE sensor composed
of a thick piezoelectric element shown in Figure 6 converts the
mechanical energy caused by elastic waves into an electrical signal. When a
load applied to a structure gradually increases, some microscopic deformations
may occur, resulting in elastic waves propagating through the target surface.
Then, these elastic waves are detected and converted to voltage signals by an
AE sensor mounted on the structure’s surface. In addition, the location of
damage can be identified using multiple AE sensors based on the differences in
the arrival times of the AE signals. The AE techniques have been used to detect
damage in metallic and composite structures.
Figure 6: AE Sensor
Figure 7: Scheme of Acoustic Emission
Technique for Damage Detection.
4.4 Piezoelectric
Transducer Self-Diagnosis Technique
Piezoelectric transducers
used for SHM systems themselves often become the weakest link within the entire
SHM system due to harsh environments. To tackle this issue, a number of
self-diagnosis techniques have been developed. Figure 8 shows an
overview of the Time Reversal Process (TRP)-based PZT debonding detection
procedure. First, a symmetric toneburst input signal is applied to a PZT, and
the response reflected off from the boundaries is measured at the same PZT.
Then, the measured response is scaled and reversed in the time domain, and
re-emitted to the PZT. Finally, the corresponding response, which is named as
the reconstructed signal, is measured again at the same PZT.
Figure 8: Piezoelectric Transducer Self-Diagnosis Technique Based on TRP
5. APPLICATIONS
5.1 Bridge
Structures
The demands for bridge monitoring are triggered by past historical
bridge incidents. To meet these demands, global bridge monitoring techniques
have been widely investigated. However, the global monitoring techniques are
often insensitive to local incipient damage. To overcome this limitation, local
bridge monitoring techniques using piezoelectric transducers have been studied The
piezoelectric transducer-based bridge monitoring, however, still has a number
of challenges to be overcome. First, the durability issue of piezoelectric
transducer itself is critical. In general, piezoelectric transducers embedded
for local bridge monitoring may deteriorate faster than the target bridge
structure. Figure 9 shows a bridge in Germany which was monitored using
piezoelectric transducers.
Figure 9: Fixing Piezoelectric
Transducers to a Bridge in Germany
5.2 Pipeline
Structures
Guided wave imaging
technique can be effectively used for pipeline monitoring using circumferential
array of piezoelectric shear transducers, and the effectiveness of this method
was numerically and experimentally validated. The uniqueness of pipeline SHM
applications is that the conformability of piezoelectric transducers, guided
waves can travel relatively longer distances than other applications since the
energy is confined within the pipe, and often a long range data and power
transmission is possible.
5.3 Nuclear
Power Plants
Nuclear energy is seen as
one of the most promising alternative energy sources to oil, and monitoring of
nuclear power plants (NPPs) is another area where piezoelectric transducers can
be potentially exploited. In response to this interest, there have been several
preliminary studies where the applicability of piezoelectric transducers to NPP
monitoring has been investigated. The biggest challenge for NNP applications is
that sensors often need to be embedded for online monitoring, and should be
designed to withstand high temperature and radiation. Currently there are no
commercially available piezoelectric transducers that can meet these stringent
requirements imposed by NNPs.
6. CONCLUSION
The field of structural health monitoring is a vast developing area and
new monitoring methodologies are continuously experimented using newly
fabricated piezoelectric materials. When it comes to
permanent installation and embedded sensing, future research should focus on
addressing the long-term ruggedness, miniaturization, increased flexibility,
and applications under high-temperature, high-strain, and high-radiation
environments. After all, the monitoring using piezoelectric transducers will
become as common as it can be wisely used in the important structures like
bridges.
