INVITED TALK: MULTI-DISCIPLINARY APPLICATIONS OF PIEZO-SENSORS: STRUCTURAL HEALTH MONITORING, BIO-MECHANICS AND ENERGY HARVESTING

Abstract

Piezoelectric materials, especially the ceramic version lead zirconate titanate (PZT), are the among the most widely used smart materials today. This paper presents the state-of-the art in the application of PZT patches in various disciplines of science and technology. The most striking application of the PZT patches in the field of non-destructive evaluation (NDE) is in the form of the electro-mechanical impedance (EMI) technique, and its several variants, the notable being the metal wire based approach. The paper presents the recently developed modelling approaches involving the EMI technique taking into account the PZT-bond-structure interaction. It also covers very recent applications of the PZT patches for foot pressure monitoring (bio-medical application), rebar corrosion assessment (civil engineering application) and energy harvesting (civil/ mechanical/ aerospace application). These striking applications undoubtedly establish the true potential of PZT patches in multi-disciplinary fields.

Full text

MULTI-DISCIPLINARY APPLICATIONS OF PIEZO-SENSORS: STRUCTURAL
HEALTH MONITORING, BIO-MECHANICS AND ENERGY HARVESTING
S. Bhalla1, R. Suresh, S. Moharana, T. Visalakshi, N. Kaur and S. Naskar
ABSTRACT
Piezoelectric materials, especially the ceramic version lead zirconate titanate (PZT), are the
among the most widely used smart materials today. This paper presents the state-of-the art in
the application of PZT patches in various disciplines of science and technology. The most
striking application of the PZT patches in the field of non-destructive evaluation (NDE) is in
the form of the electro-mechanical impedance (EMI) technique, and its several variants, the
notable being the metal wire based approach. The paper presents the recently developed
modelling approaches involving the EMI technique taking into account the PZT-bond-
structure interaction. It also covers very recent applications of the PZT patches for foot
pressure monitoring (bio-medical application), rebar corrosion assessment (civil engineering
application) and energy harvesting (civil/ mechanical/ aerospace application). These striking
applications undoubtedly establish the true potential of PZT patches in multi-disciplinary
fields.
Keywords: Structural health monitoring (SHM), Lead zirconate titanate (PZT), Electro
mechanical impedance (EMI) technique, Bio-mechanics, Energy harvesting.
1AssociateProfessor(CorrespondingAuthor),DepartmentofCivilEngineering,IndianInstituteofTechnology
(IIT)Delhi,HauzKhas,NewDelhi‐110016,(India).Email:sbhalla@civil.iitd.ac.in,Phone:(91)‐11‐2659‐1040,
Fax:(91)‐11‐2658‐1117

INTRODUCTION
The phenomenon of piezoelectricity occurs in certain classes of noncentro-symmetric
crystals, such as quartz, in which electric dipoles (and hence surface charges) are generated
due to mechanical deformations. The same crystals also exhibit the converse effect; that is,
they undergo mechanical deformations when subjected to electric fields. The constitutive
relations for piezoelectric materials for 1D interaction, such as for a piezoelectric plate shown
in Fig. 1, are (Shanker et al., 2011)
1313333 TdED T+=
ε
(1)
331
1
1Ed
Y
T
SE+= (2)
where S1 is the strain in direction ‘1’, D3 the electric displacement over the PZT patch, d31 the
piezoelectric strain coefficient and T1 the axial stress in direction ‘1’. )1( jYY EE
η
+= is the
complex Young’s modulus of elasticity of the PZT patch at constant electric field and
)1(
3333 j
TT
δεε
−= the complex electric permittivity (in direction ‘3’) at constant stress, with
1−=j. In these expressions,
η
and
δ
respectively denote the mechanical loss factor and
the dielectric loss factor of the PZT material. Equation(1) is used in sensing applications and
Equation (2) in actuation applications of the piezo-electric materials.
T1
l
h
w
E3
1
3 2
Fig. 1. A piezoelectric plate under the action of stress and electric field (1D interaction).

The EMI technique makes use of both the direct and the converse effects simultaneously. The
governing equation of the EMI technique is (Bhalla and Soh, 2004)
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+−
+
−
−=+= T
ZZ
Z
YdYd
h
l
jBjGY
effaeffs
effa
EE
T
,,
,
2
31
2
31
33
2
)1(
2
)1(
2
4
νν
εω
(3)
where Za,eff is the effecive mechanical impedance of the PZT patch and Zs,eff that of the host
structure.
T
is the complex tangent ratio, ideally equal to {tan(
κ
l)/
κ
l}, with
E
Y/)1( 2
νρωκ
−= being the wave number. Any damage to the host structure will alter Zs,eff.
Assuming that other parameters in the equation are not influence, the damage will get
detected easily as change in the admittance signature. The forthcoming sections describe
applications of PZT patches in multiple engineering and technological domains.
BIO-MEDICAL APPLICATIONS
Bhalla and Suresh (2013) successfully demonstrated the potential of the EMI technique
(which is conventionally employed for SHM of structural systems) for monitoring condition
of bones, covering detection of fracture as well as the healing process that follows. In
addition to detecting cracks and fracture, the conductance signatures of the PZT patches
could suitably detect changes occurring in bone density, both increase as well as decrease.
Suresh et al. (2015) demonstrated the proof-of-concept experimentation to measure foot
pressure distribution using PZT patches and fibre-Bragg grating (FBG) sensors (Fig. 2). The
PZT sensors carried out the measurement using d33 coupling, acting as sensors. A pressure
resolution up to 0.89 kPa was achieved with the PZT sensors. The FBG sensors employed a
special arch type configuration for higher sensitivity. The pressure values measured by the
two sensors were comparable in nature. The pressures at two locations (forefoot/heel) were
found to increase with the walking speed. The PZT patches, which can measure the pressure
with a high sampling rate (typically few kHz per second), can provide near real time pressure

measurement and are most suitable for fast walking speeds, typically higher than 3 kmph.
The FBG sensors, on the other hand, are suitable for both static as well as low frequency
dynamic measurements typically less than 3 Kmph. Acting in synergy, both the PZT and the
FBG sensors enabled measuring the foot pressure in wide speed range, starting from static
case to high speed walking.
MODELLING PZT-BOND-STRUCTURE INTERACTION
Bhalla and Moharana (2013) developed a refined model to take into account the interaction
between a PZT patch and the host structure through the medium of an adhesive bond layer.
This model duly considered the elastodynamic aspects of problem incorporating both shear
lag as well as inertia terms in the framework of lumped impedance. Later, Moharana and
Bhalla (2014) further improved the model by considering the variation in continuum rather
Fig. 2 Foot pressure measurement using a combination of PZT and FBG sensors.
(a) Shoes instrumented with both sensors (b) Test under progress
(a) (b)
PZT
p
atch FBG sensor

than a lumped impedance approach. The results of the continuum approach are so far the best
reported analytical results as far as closeness to the experimental results is concerned (Fig. 3).
REBAR CORROSION MONITORING
Talakokula et al. (2014) presented the first ever comprehensive monitoring of rebar corrosion
using the EMI technique (Fig. 4). Through accelerated corrosion testes on RC specimens, it
was demonstrated that PZT patches bonded to rebars could provide information regarding the
extent of corrosion (initiating stage, propagation stage or splitting stage) in terms of piezo
identified stiffness. The approach has several advantages in comparison with conventional
corrosion detection techniques.
Figure 3 Comparison of susceptance of experimental result with continuum shear lag model
(a) Normalized analytical susceptance (continuum model) for hs/hp=0.417 and hs/hp=0.834
(b) Normalized experimental susceptance for hs/hp=0.417 and hs/hp=0.834
(
b
) (a)
0.00E+00
2.00E-02
4.00E-02
6.00E-02
8.00E-02
0 50 100 150 200 250
Susceptance (S/m)
Frequency (kHz)
h
s
/h
p
= 0.417
h
s
/h
p
= 0.834
0.00E+00
2.00E-02
4.00E-02
6.00E-02
8.00E-02
1.00E-01
0 50 100 150 200 250
Suscep tance (S /m)
Frequ ency (k Hz)
h
s
/h
p
= 0.417
h
s
/h
p
= 0.834

ENERGY HARVESTING APPLICATIONS
Energy harvesting is another fascinating multi-disciplinary application of PZT patches.
Conventionally, built up configurations, such as stack arrangement or secondary structures
are employed for energy harvesting purposes. These are not only cumbersome, but also
expensive and interfere with functional service of the structure. Kaur and Bhalla (2014a, b)
evaluated the possibility of employing normal thin PZT patches for this purpose and
developed a coupled electro-mechanical model for this purpose. The model was first
extended to a typical real-life city flyover, modelled through FEM, whereby it was estimated
that two day operation of the PZT patch for energy extraction is sufficient enough to enable
one time operation of AD5933 for acquisition of structural health monitoring (SHM) related
data in surface-bonded configuration. The coupled electro-mechanical model for surface-
bonded and embedded patches were extended to eight real-life bridges across the world. It
has been estimated that Low power consuming circuits like typical A/D convertor, such as
TMP 112 (Texas Instruments, 2014) would warrant energy harvesting for 1 second and 2.5
minutes when powered by a surface bonded PZT patch on steel bridge and an embedded CVS

+
-
Brine solution Co nstan t pow er
supply device
Con cre te
+
Steel bar (Anode)
Copper rod (Cathode)
PZ T patc h
Brine
sBBrimnnfion
-
+
Brine
solution
Fig. 4 Rebar corrosion detection using EMI technique
(a) Experimental set-up
(b) Condition of a specimen after 120 days of accelerated corrosion exposure
(
b
) (a)

in a RC bridge, respectively. Hence, using the PZT patch both for SHM and energy
harvesting in real-life structures is certainly feasible. The same patch, which scavenges
energy during the idle period, could be used for SHM as and when warranted (Fig. 5). With
the ongoing developments in electronics, as lesser power consuming circuits are emerging, it
is believed that the energy scavenging time will drastically come down.
NEW VARIANTS OF EMI TECHNIQUE
Naskar and Bhalla (2014) successfully demonstrated the feasibility of indirect excitation of
structure using PZT patches bonded to metal wires (Fig. 6). This enables monitoring of
structures under inaccessible and hazardous conditions. In addition, the proposed approach
warrants minimum number of sensors as compared to conventional array configuration.
Fig. 5 Concept of dual use of PZT patch for energy harvesting as well as SHM
(Kaur and Bhalla, 2014a)
Fig. 5 Metal wire based variant of EMI Technique (Naskar and Bhalla, 2014)

CONCLUSIONS
This paper has presented the state-of-the-art in the multi-disciplinary applications of PZT
patches covering wide ranging fields such as SHM, bio-mechanics and energy harvesting.
These highlight the high potential of piezo materials in modern scientific and technical
applications.
REFERENCES
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