Structural health monitoring (SHM) is a technology used for the safety assurance of mechanical, aerospace, building structure and human life. By periodical inspection using embedded sensors like piezorestive sensor, the optical fiber sensor a SHM system can provide advanced warnings that prevent structural failures or damage. Strain is one of the most important mechanical parameters acquired by SHM systems. The factors that could potentially cause structural failures or excessive loading, vibration, foundation damages, crack development and environmental aging, etc. By monitoring strain changes in load-bearing structures the failure can be avoided. Various strains sensors have been developed. A detailed review of different strain sensing mechanisms can be found in this paper. The most commonly used strain sensor is the piezoresistive thin-film strain gauge, made using semiconductor. On the other hand, Optical fiber- based strain sensors are an attractive choice of sensor for SHM systems. OFS has characteristics like small size, light in weight, remote monitoring, ability to multiplex and immune to electromagnetic interferences.

1. Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
NITTTR, Chandigarh EDIT -2015 144
Strain sensors for strain measurement:
A Review
1
Shivendra , 2
Garima Saini
1
ME scholar, 2
Assistant Professor
1,2
Department of Electronics, NITTTR, Chandigarh, India
Shivendra.986@gmail.com
Abstract— Structural health monitoring (SHM) is a
technology used for the safety assurance of mechanical,
aerospace, building structure and human life. By periodical
inspection using embedded sensors like piezorestive sensor,
the optical fiber sensor a SHM system can provide advanced
warnings that prevent structural failures or damage. Strain is
one of the most important mechanical parameters acquired
by SHM systems. The factors that could potentially cause
structural failures or excessive loading, vibration, foundation
damages, crack development and environmental aging, etc.
By monitoring strain changes in load-bearing structures the
failure can be avoided. Various strains sensors have been
developed. A detailed review of different strain sensing
mechanisms can be found in this paper. The most commonly
used strain sensor is the piezoresistive thin-film strain gauge,
made using semiconductor. On the other hand, Optical fiber-
based strain sensors are an attractive choice of sensor for
SHM systems. OFS has characteristics like small size, light in
weight, remote monitoring, ability to multiplex and immune
to electromagnetic interferences.
Index Terms—SHM, OFS, Piezoresistive, Multiplex,
Electromagnetic interference.
INTRODUCTION.
SHM can be understood as the system that have the
capability of sensing, intelligence and possibly also
actuation devices to allow the loading and damage-causing
conditions of a structure to be stored, calculate, identified,
and predicted in such a way that autonomous testing
becomes an essential part of the structure. According to the
function and degree of complexity, SHM systems can be
classified in different levels.these levels are
To detect the damage and fatigue periodically.
To estimate the effect of external load on the structure.
To estimate the remaining life.
To make the structure more reliable, cost effective, long
life time.
The higher the level, the higher will be the complexity and
functionalities [1]. SHM is associated with self-working,
maintenance, optimized technical structures in addition to
the minimization of the potential social, economic
impacts. With SHM systems, unusual structural behavior
like vibration, fatigue can be detected at an early stage,
decreasing the risks of sudden damage and conserving
nature, goods and even human lives. In addition, these
systems enable in-time refurbishment intervention, the
extension of their life-time guaranteeing fewer direct
economic losses (repair, maintenance, and reconstruction)
and also helping to avoid losses for users due to structural
failures. Using SHM systems, hidden structural issues can
be detected early, enabling better exploitation of the
materials and components of the current structures. A key
issue in the SHM systems is the measurement of
Chemicals (pH, oxidation, corrosion, penetration, and
timber decay); Mechanical (strain, deformation,
displacement, crack opening, stress, and load); and
Physical (temperature, humidity, pore pressure, etc.) [1].
Several types of sensors, embedded or attached to a
structure, can be used for this task, but only those based on
fiber technology offer the capability to perform integrated,
quasi-distributed, and distributed measurements on or even
within the structure, in addition to other advantages. As
main Challenges for SHM systems, two fundamental
technical Challenges are identified: the development of
reliable and Sensitive techniques to detect early structural
malfunction or unusual structural behavior and the
development of data selection, storage and processing
models, and robust algorithms to detect structural
malfunctions. In addition, user-friendly and simple
interfaces with the infrastructure are needed [2].
A system of sensors allows the detection and
characterization of damages that could have a significant
effect on the operational capability of the structure. Ideally,
this will provide warnings, prevent complete failure, and
facilitate countermeasures. SHM sensors detect various
parameters such as temperature, humidity, or strain. The
characterization of strain gives information on cracks,
deformations, or vibrations in the structure [2]. It is
therefore an essential parameter for the conclusion on
Operability. However, in the course of their lifetimes,
structures are subject to adverse changes in their structural
health conditions due to potential damage or deterioration
induced by environment, wear, falts in design and
manufacturing, overloads and some unexpected events like
earthquakes or impacts or, simply, through their normal
working life [1].
Structural degradation can be induced by a wide set of
factors.
1) Unsatisfactory inspection and monitoring of existing
infrastructure mean problems become apparent only when
Structures are in dire need of repair and then, repair costs
can be comparable to replacement costs.
2) Corrosion of conventional steel reinforcement within
Concrete can provoke expansion of steel, which leads to
cracking, fragmentation or further deterioration. It leads to
a reduction in strength and serviceability resulting in the
need for repair and/or replacement.
3) Increased loads or design requirements over time like
heavier trucks, overload on ships, planes, etc. induces
deterioration due to overloads or to structural inadequacies
resulting from design. Then the structures are deemed
unsafe or unserviceable and strengthening or replacement
is required.
4) The overall deterioration and aging can induce
detrimental effects on structural performance, safety and

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145 NITTTR, Chandigarh EDIT-2015
serviceability, and then repair, rehabilitation, strengthening
or replacement may be needed.
. II. PIEZORESISTIVE STRAIN SENSOR
Strain is the relative change in shape or size of an object
due to externally-applied force. When external forces are
applied to objects made of elastic materials, they produce
changes in shape and size of the object. In other word
strain is defined as extension per unit length. Strain is
dimensionless and has no units.
Strain = extension / original length
ɛ = …………
(1)
Where,
ɛ = strain experienced by the structure,
L = original length of the structure,
ΔL = change in length of structure.
The basic function of the strain gauge is based on
transforming the strain in certain directions as to change its
electrical resistance. It allows measuring plenty of
nonelectrical quantities such as deformation, bending, force,
acceleration, etc. Piezoresistivity of a material is the
dependence of electrical resistivity on strain and normally is
quantified by the gauge factor [3]. Strain gauges are fairly
straight forward devices that output a voltage signal based
on a change in resistance when the object to which they are
attached to undergoes tension or compression. For
example, the piezoresistive strain gauge is a semiconductor
device whose characteristic is resistance varies non linearly
with strain. The most widely used gauge, however, is the
bonded metallic strain gauge [3]. Gauge factor is defined
as the ratio of the fractional change in electrical resistance
to the fractional change in length (strain)
GF =
∆ ⁄
ɛ
……………….
(2)
Where,
R – Change in resistance caused by strain,
Rg – Resistance of the unformed gauge,
ɛ - Applied strain.
Piezoresistive Sensors operate on a sensor principle
whereby an electrical resistor will change its resistance
when it is subjected to a strain or deformation.
Semiconductor strain gauge called the piezoresistor, based
on the piezoresistive effect is the commonly used strain
gauge [4]. Several piezoresisitive strain sensors have been
designed to be used for various strain measurements with
an excellent gauge factor. Semiconductor’s conductivity
depends on temperature change. Piezoresistive sensors are
fabricated with semiconductor, so it is very sensitive to
temperature changes. It is proved that by increasing the
doping level, piezoresistive sensors which are less
sensitive to temperature variation can be obtained, but
gauge factor of piezoresistive strain sensor get reduced and
strain sensor required high gauge factor [4].
III. OPTICAL FIBER SENSORS
An OFS can be understood as the device in which the
measurand, introduces modifications or modulates
characteristics of light in some part of an optical fiber
system, reproducing it faithfully in the electric domain [5].
In general terms, an OFS is usually made up of a
transducer device, a communication channel and an
optoelectronic unit.
A fiber optic sensor is a sensor that uses optical fiber either
as the sensing element, or as a relaying signals from a
remote sensor to the electronics that process the signals. In
an OFS is usually made up of a transducer device, a
communication channel and an optoelectronic unit. It is
found that OFS technology is attractive in those cases
where it offers superior performance compared to the more
proven conventional sensors. Its other benefits are [5]
1) Improved quality in the measurements.
2) Better reliability.
3) Manual readings, and automatic measurements.
4) Easier installation and maintenance.
An optical fiber sensor with a linear relation between
transmitted light intensity and applied stress. Optical fibers
have the advantage of high bandwidth and free from
electromagnetic interference, but the high cost of
implementation has made it prohibitive for usage along
with the robustness of the sensor system, transduction, data
interpretation, stability and reliability. In present time OFS
technology is attractive in those cases where it offers
superior performance compared to the more proven
conventional sensors offering, in addition to improved
quality in the measurements, better reliability, and the
possibility of replacing manual readings and operator
judgment with automatic measurements, easier installation
and maintenance or a lower lifetime cost [6].
A fiber Bragg grating (FBG) is a sensor of distributed
Bragg reflector constructed in a short segment of optical
fiber that reflects particular wavelengths of light and
transmits to all remaining wavelengths. This is achieved by
creating a periodic variation in the refractive index of the
fiber core, which generates a wavelength-specific dielectric
mirror [7]. A fiber Bragg grating can therefore be used as
an inline optical filter to block certain wavelengths, or as a
wavelength-specific reflector.
Although fiber-optic sensors are apparently expensive for
widespread use in health monitoring however, they are
better approaches for applications where reliability in
challenging environments is essential. When reliability is a
key problem in certain critical health monitoring
applications, price is often no longer an issue [8]. The
application potential for OFSs in structural monitoring is
vast, including civil or industrial structure monitoring like
concrete beam tests, bridge girders, ore mines, nuclear
containers, tunnels, and hydroelectric dams, composite
materials spacecraft, aircraft tail spars, helicopter and
windmill rotor blades, ship and submarine hulls, composite
cure monitoring, and composite girders for bridges,
acoustic sensing in-plant or distribution of electric power
utilities, gas pipelines, and industrial control, monitoring
and processes [8].
IV. CONCLUSION
SHM sensors detect various parameters such as
temperature, humidity, or strain. The characterization of
strain gives information on cracks, deformations, or
vibrations in the structure. It is therefore an essential
parameter for the conclusion on operability. Piezoresistive
strain sensors are Very sensitive to temperature changes,
difficult to implement them in a commercial environment
considering the complexity of the network involved.

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Optical fibers based strain sensors: have the advantage of
high bandwidth and freedom from electromagnetic
interference. For strain measurement of a building
structure, need to drill the building at many points, which
is called weak point, the drilling affects the quality of
building.
FUTURE WORKS
In order for the installation of a permanently installed
sensing system in buildings to be economically viable, the
sensor modules must be wireless to reduce installation
costs, must operate with a low power consumption to
reduce servicing costs of replacing batteries, and use low
cost sensors that can be a microstrip patch antenna.
ACKNOWLEDGEMENT
The authors would like to thank Dr, Umesh Tiwari
Scientist, CSIO,Chandigarh, India for their continuous and
valuable guidelines during work.
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