Sensors and Actuators A 147 (2008) 150–164

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Sensors and Actuators A: Physical

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Review

Fibre Bragg gratings in structural health monitoring—Present

status and applications

Mousumi Majumder ∗, Tarun Kumar Gangopadhyay, Ashim Kumar Chakraborty,

Kamal Dasgupta, D.K. BhattacharyaCentral Glass & Ceramic Research Institute (CSIR), 196 Raja S.C. Mullick Road,PO: Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 23 August 2007Received in revised form 15 April 2008Accepted 15 April 2008Available online 22 April 2008

Keywords:Fibre-optic sensorFibre Bragg gratingsFBG strain sensorSensor for structural monitoringSensor for composite

a b s t r a c t

In-service structural health monitoring (SHM) of engineering structures has assumed a significant rolein assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, insitu, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBGsensors represents a key advantage over other technologies in the structural sensing field. In this article,the recent research and development activities in structural health monitoring using FBG sensors havebeen critically reviewed, highlighting the areas where further work is needed. A few packaging schemesfor FBG strain sensors are also discussed. Finally a few limitations and market barriers associated withthe use of these sensors have been addressed.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Principle of operation of FBG sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Strain measurement using FBG sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2. Strain–temperature cross-sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. FBG interrogation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. FBG encapsulation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. Applications of FBG strain sensors in structural sensing . . . . . . . . . . . . . . . . . .

5.1. Strain monitoring in civil infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1.1. Strain monitoring in reinforced concrete beams. . . . . . . . . .5.1.2. Strain monitoring in smart beams . . . . . . . . . . . . . . . . . . . . . . . .5.1.3. Pile load monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1.4. Early-age cement shrinkage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1.5. Moisture/humidity measurement in civil applications . . .5.1.6. FBGs in geodynamic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1.7. Ultrasonic non-destructive testing of structural health us

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Fax: +91 33 24730957.E-mail address: mousumi@cgcri.res.in (M. Majumder).

0924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.sna.2008.04.008

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159ing FBG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

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1. Introduction

Structural integration of fibre-optic sensing systems representsa new interdisciplinary branch of engineering which involves theunique combination of laser-optics, fibre-optics, optoelectronics,microelectronics, artificial intelligence, composite material scienceand structural engineering. Fibre-optic sensors have a number ofadvantages over their electrical counterparts and are the primarycandidates for complete sensing systems.

Fibre Bragg grating (FBG) sensors have undergone a rapiddevelopment in the recent years following the observation ofvery-narrow-band reflection in the photosensitive core-region ofGe-doped silica optical fibres [1] and its first successful fabricationon fibre-core by exposure of a coherent two-beam UV interferencepattern in 1989 [2]. FBGs are immune to electromagnetic inter-ference (EMI) and ground loops. They are lightweight and havesmall physical dimensions, suitable for being embedded into, orattached to a structure. No wires are required to connect sensors tothe control system as the fibres themselves act as both the sensingelements and the signal propagation conduit. FBG sensors offer aunique advantage of single ended connection to control systemsbecause only reflected signals from the FBGs are important fordemodulation. FBGs possess excellent resolution and range, waterand corrosion resistance, ability to be multiplexed, immunity toharsh weather conditions, compact sensor and harness size, andreasonable cost per channel. Besides, wavelength encoded infor-mation is given by FBGs. Since wavelength is an absolute parameter,signal from FBG may be processed such that its information remainsimmune to power fluctuations along the optical path. Thus, theyoffer a self-referencing, absolute measurement scheme. The FBGsensing technology shows great potential for applications within avariety of industries [3]. FBG sensors have attracted interest fromthe civil structure communities over the past decade for structuralhealth, vibration and seismic response monitoring. FBG sensorshave been embedded in concrete for deformation monitoring andtraffic load assessment in bridges and buildings.

FBG sensors have been established as a major leading technologyas compared to other competing fibre-optic sensor technologies.A major share of the papers presented at the 17th Optical FibreSensors Conference (OFS-18) held at Cancun, Mexico in 2006 wereon fibre Bragg gratings. The main advantages of FBGs over otherfibre sensor schemes are its low cost, good linearity, wavelengthmultiplexing capacity, resistance to harsh environments and thetransduction mechanism, which eliminates the need for referenc-

ing as in interferometric sensors. FBG sensor technology is now onthe verge of maturity after almost two decades of active researchand development in this field. Efforts are now concentrating ondelivering complete FBG sensor systems including front-end elec-tronics.

Strain studies in civil structures are pivotal in avoiding unex-pected catastrophic failures. Long-term strain study of structuresalso helps in freezing the design limits of similar structures. Con-ventionally, most structures rely on maintenance schedules, visualinspection and a few conventional sensors for the purpose of dam-age monitoring. But the high cost of maintenance, lack of precisionin visual inspection and susceptibility of sensors to harsh environ-mental conditions has made structural health monitoring (SHM)a necessity. Over the past few decades, Fibre Bragg grating sen-sors have emerged as a suitable, accurate and cost-effective tool inSHM of civil structures like high-rise buildings, bridges, tunnels anddams. For existing structures, FBG sensors can be attached onto thestructure surface, whereas for new structures, these sensors can beembedded into the structure during the construction phase with-out any serious effect on the structural integrity. The informationfrom such SHM systems can provide early warning for compro-

uators A 147 (2008) 150–164 151

Fig. 1. Transmission and reflection spectra from an FBG.

mised integrity of structures and thus help avoid severe losses. Suchinformation is also helpful to adapt and update newer designs ofsimilar structures.

Several review papers on applications of FBGs have beenpublished [4–11]. Strain and temperature have so far been thedominating measurands of interest. These reviews have primarilyfocused on the various usages of the FBG sensors in different sens-ing areas. This paper aims to provide an overview of the applicationof FBG sensors for strain measurement, particularly in the field ofstructural sensing. Various applications as in structural health mon-itoring for bridges and concrete structures, moisture sensing, strainsensing of smart structures using FBG–FRP bars and ultrasonic non-destructive testing using FBG sensors have been discussed. Practicalaspects like packaging of the sensors and demodulation techniquessuitable for the use of FBG sensors in structural health monitoringhave been discussed in detail. This review is expected to provideuseful insight to researchers in the field of structural sensing usinga reliable and effective strain sensing platform.

2. Principle of operation of FBG sensors

FBGs are obtained by creating periodic variations in the refrac-tive index of the core of an optical fibre [2]. Fig. 1 shows the internalstructure of an optical fibre with an FBG written in it.

When light is made to pass through the grating, at a particularwavelength, called the Bragg wavelength, the light reflected by thevarying zones of refractive indices will be in phase and amplified.

The Bragg wavelength is expressed as

�B = 2neff� (1)

where �B is the Bragg wavelength, neff is the effective refractiveindex of the FBG and � is the grating period.

2.1. Strain measurement using FBG sensors

When strain is induced in an FBG, the relative change in Braggwavelength is expressed as

��B

�B= (1 − �e)ε (2)

where ε is the longitudinal strain on the FBG and �e is the effectivephoto-elastic constant of the fibre core material.

�e = n2eff2

[p12 − v(p11 + p12)] (3)

where pij are the silica photo-elastic tensor components and � isthe Poisson’s ratio. For an FBG of central wavelength of 1550 nm,typical strain sensitivity is approximately 1.2 pm/microstrain [9].

152 M. Majumder et al. / Sensors and Actuators A 147 (2008) 150–164

Table 1Temperature compensation techniques in FBG interrogation

Schemes of temperature compensation Remarks

Two separate FBGs (one for strain measurement The most simple and straightforward technique for temperature compensation. However, resolution of the techniqueo fibre

asure

ct thas. Res0 microurceingle120 ◦

issioadingreadinspecti00 mic

efringtemp

f the mure chto 60

monitoring system is a key issue. Bare FBGs being very fragilein nature, require suitable encapsulation before being put intoregular monitoring service. Various encapsulation techniques forFBG strain sensors have been proposed in literatures [61–67].Fig. 2(a)–(e) shows some of the encapsulated FBGs from literatureas referenced and used for strain measurement on concrete surfacesand mental surfaces, respectively.

Fig. 2(a) shows a capillary encapsulated FBG sensor for use inconcrete structures. The mental holder ring is attached on thesurface of the concrete structure and it faithfully transmits thedeformation of the structure to the FBG sensor. Fig. 2(b) shows anFBG sensor encapsulated in slice base by gluing. This type of encap-sulated FBG sensors can be very well used in mental and concretestructures. Fig. 2(c) shows smart FRP–FBG sensors. The FRP barsare embedded with FBG sensors thus utilizing the dual properties

and the other for temperature measurement)[12,13]

is low and interrogation of tw

Two closely spaced gratings of differentwavelengths inscribed in the same fibre [14]

Interrogation is easier. In a me

One LPG and two FBG scheme [15] This technique relies on the facompared to short period FBG

Two FBGs with entirely different wavelengths[16]

In a measurement range of 60technique requires two light s

Two FBGs of varying diameter spliced together[17,18]

System is illuminated from a srange of 2500 microstrain and

Single FBG with an EDFA [19] The amplified spontaneous emmeasure temperature. This rewavelength to get pure strain18.2 microstrain and 0.7 ◦C, re

Single FBG inscribed in erbium–ytterbium fibre[20]

In a measurement range of 11

FBG embedded in composite material [21,22] This technique utilizes the birpeaks provides an estimate ofvalue

Athermal packaging of the fibre using metalcoating of the FBG [23]

By controlling the thickness oare controlled. For a temperat

FBG glued to a bimetal alloy strip used as acantilever beam [24]

In a temperature range of −20

2.2. Strain–temperature cross-sensitivity

The Bragg wavelength �B is also affected by temperaturechanges. The relative change in the Bragg wavelength due to tem-perature change is expressed as

��B

�B= (˛ + �) �T (4)

where �T is the change in temperature experienced at the FBGlocation, ˛ is the thermal expansion and � is the thermo-opticcoefficient. For an FBG of central wavelength of 1550 nm, typicaltemperature sensitivity is approximately 13 pm/◦C [9]. CombiningEqs. (2) and (4), we get the effective Bragg wavelength shift due tostrain and temperature and are expressed as

��B

�B= (˛ + �) �T + (1 − �e)ε (5)

For pure strain measurements, effects of temperature changeon the Bragg wavelength has to be suitably compensated. Severaltechniques to offset this behavior are available in literature. Table 1shows a few competing techniques to achieve the purpose of tem-perature compensation of FBGs.

3. FBG interrogation techniques

Various interrogation techniques of FBGs have been proposedin literatures [25–60]. The interrogation units are responsible forreading the Bragg wavelength shift of the FBGs induced by variousphysical parameters like strain, temperature, etc. Optical spectrumanalyzers are unsuitable for this purpose due to their high cost andlow scanning speed.

The choice of the interrogation method depends upon severalfactors like type and range of strain being measured, accuracy andsensitivity required, number of sensors being interrogated and costof the instrumentation.

The FBG interrogation schemes commonly used are tabulated(Table 2).

s is cumbersome

ment range of 0–900 microstrain and 25–120 ◦C, error reported is 5%

t LPGs have a much higher temperature response but lower strain responseolution reported is ±9 microstrain and ±1.5 ◦Costrain and 50 ◦C, error reported is 10 microstrain and ±5 ◦C. However, this

s and two interrogation units, thereby increasing the overall cost of the systembroadband source and interrogated using a single interrogator. In a measurementC, maximum error reported is 17 microstrain and 1 ◦Cn power of the EDFA source has a linear relation with temperature and is used tois subtracted from the combined strain temperature response of the Bragggs. Experimental r.m.s. deviation values of strain and temperature reported were

velyrostrain and 50–180 ◦C, error reported is 55.8 microstrain and 3 ◦C

ence property of the gratings when embedded. Wavelength separation of the twoerature, whereas average wavelength between such separations provide the strain

etal coating, the effective thermal expansion coefficient and the Young’s Modulusange 30–80 ◦C, Bragg wavelength shifts by about 50 pm◦C, thermo-opto coefficient reported is −0.4 pm/◦C

4. FBG encapsulation techniques

Owing to the large span and long service period of civilstructures, the durability, reliability and robustness of its health

of FRP’s mechanical strength and the FBG’s sensing ability. Fig. 2(d)shows another encapsulated FBG for use in cement structures. Thefibre is placed inside a steel tube, which is further enclosed ina concrete-proof plastic hose. This protects the fibre from strongalkaline environment of the cement as well is effective in faith-ful transmission of strain to the sensor. Fig. 2(e) shows a packaged

Table 2FBG interrogation schemes

Types Technologies Reference

Passivedetec-tionscheme

Linearly wavelength-dependent device [25–29]CCD spectrometer [30–33]Power detection [34–37]Identical chirped-grating pair [38]

Activedetec-tionscheme

Fabry-perot filter [39–41]Unbalanced Mach-Zehnder interferometer [42–49]Fibre Fourier transform spectrometer [50,51]Acoustic-optic tunable filter [52–54]Matched FBG pair [55–58]Michelson interferometer [59]LPG pair interferometer [60]

M. Majumder et al. / Sensors and Actuators A 147 (2008) 150–164 153

Fig. 2. (a) Mental capillary encapsulated FBG sensor [62]. (b) Mental slice encapsulated F(e) Long-gage FBG sensor with surface mounting facility [61]. (f) Schematic of FBG embestrain sensor.

FBG for civil structure applications where we require to monitor aconsiderably macrostrain value. The FBG is encapsulated in a tubewhere the distance between the tie points define its effective gage.Optional brackets are also provided to enable surface mounting ofthe sensors. This technique thus, is effective in increasing the overallgage of the sensor. Another packaging technique proposed by Ngoiet al. [63] is to embed the FBG coaxially in a cylindrical siliconerubber tubing as shown in Fig. 2(f). This packaging is specificallyuseful for sensing lateral loading. FBGs have been known to sufferfrom a phenomenon called peak splitting under the influence of alateral load. Peak splitting occurs due to the effect of birefringenceof the FBGs when subjected to lateral loading, i.e. unequal load-ing along the two perpendicular axes of the fibre. This issue hasbeen taken care of by packaging the FBG in silicone rubber which

BG sensor [62]. (c) FRP–FBG sensors [62]. (d) FBG encapsulated in a steel tube [92].dded coaxially in a cylindrical polymer package [63]. (g) Athermally packaged FBG

is known to have a low elastic modulus and high Poisson’s ratio.It is also thermally stable in the temperature range from −100 to320 ◦C. Experimental data obtained from FBG sensors reveal agree-ment with the finite element simulation results. This packaging hasserved to increase the lateral pressure sensitivity of the FBG sensorwithout inducing birefringence. However, the system suffers froman inherent time lag due to the typical viscoelastic nature of siliconerubber. Upon applying a load, the packaged FBG sensor shows anabrupt rise in wavelength, which gradually settles down to a stable,lower value within a few seconds. Authors propose readings to betaken once this spurious response settles down, approximately 5 safter the application of the load.

Leng et al. in 2005 have proposed various designs of FBG sensorprotection depending upon the usage area. These include sensors

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154 M. Majumder et al. / Sensors a

for metallic surfaces, CFRP composites and concrete structures [64].Designs include FBGs embedded inside steel tubes, steel rebars,CFRP prepegs, etc. The packaged sensors have been evaluated foroptimum strain transfer between the sensor and test specimen byusing non-linear finite element analysis. Concrete cylinders instru-mented with FBG sensors and electrical resistance strain gaugeshave been subjected to compressive loading and the results foundfrom both type of sensors to be in proper agreement. Authors alsoclaim that due to higher resolution, FBG strain sensors would beable to detect the initiation of failure of structures earlier than thestrain gauges.

Dawood et al. have described in detail a procedure to embedFBG sensors between the foam core and cross-ply laminate ofGFRP sandwich material using vacuum infusion technique [65].The sandwich structure consisted of a single layer of polymer foamsandwiched between two layers of GFRP skins. An array of six mul-tiplexed FBG sensors was used. The area of the gratings was leftuncoated to provide better mechanical coupling to the GFRP. TheFBGs were to be embedded between the core and the skin of thesandwich. The optical fibre was then aligned and laid up along thecenter line of the foam core. The prepared sandwich specimen wasthen placed in a vacuum bag and resin/hardener mixture infusedinside. The specimen was cured for 15 h at room temperature. Thistype of packaging is useful in sensing microscopic localized defectslike debonding of the GFRP material. The FBG being fully embeddedin the test specimen is able to detect the internal defects of the spec-imen at an early stage. However, the embedding process is involvedand requires a greater degree of precision and care. Accuracy andrepeatability have been found to be satisfactory under static anddynamic loading conditions.

Lu and Xia have used FBG sensors directly embedded into CFRPsheets for real-time monitoring of RC beams [66]. The authors claimthat in this case there is no need of a protective coating or adhe-sive layer between the bare FBG and the CFRP sheets. Thus, themeasured strain from the FBG–CFRP composite provides the actualstrain measured without incurring any dampening effect. This is adistinct advantage over other encapsulation technique. RCC beamsinstrumented with FBG-embedded CFRP sheets were subjected tocompressive load. A theoretical calculation of strain at differentloading conditions using the dimensional values and Young’s Mod-ulus of the RCC beams was done. The measured values were ingood agreement with the theoretical strain values. Another verycommonly used and simple packaging technique of FBGs for strainmonitoring in concrete structures is to install the sensors in a steel

rebar and then use the rebar at the site of measurement.

Chung and Kang [67] have used such a technique where theyhave placed a six-FBG multiplexed fibre in a groove cut in a steelrebar and used a fast curing adhesive to bond the fibre to the rebar.This FBG embedded rebar has been used at the site of strain mon-itoring. However, before using this type of packaging it is essentialto study the strain transfer characteristics of the adhesive and therebar material for accurate strain measurement.

Two major issues associated with the use of FBG sensors ashealth monitoring tool in civil structures are their high fragilityand cross-sensitivity to more than one measurand. Special ather-mal encapsulated FBGs that take care of the strain–temperaturecross-sensitivity are available [23,68,69]. Lo and Kuo have proposedathermal packaging of FBGs using a metal coating that serves asthermal compensator [23]. An FBG 1 cm in length is written using aphase mask. It has a central wavelength of 1532.93 nm at 30 ◦C. Thefibre substrate is quartz and a copper coating of 5-�m thicknessis deposited onto the substrate using electroless plating technique.Quartz having a much lower thermal expansion coefficient thancopper, any rise in ambient temperature results in a greater expan-sion of the copper than that of the FBG. This compresses the FBG

uators A 147 (2008) 150–164

and creates a negative strain on it, thereby compensating for thetemperature-induced wavelength shift of the FBG. This proposedtechnique of temperature compensation thus involves a simple bi-material that is reliable and feasible for mass production.

Moyo et al. have reported a packaged FBG that is suitable for usein the harsh conditions of the construction industry and also takescare of the temperature compensation of the sensors [69]. The pack-aged sensor is dumb-bell shaped and consists of two FBGs placedclosely. One FBG, sandwiched between two layers of carbon com-posite material, is epoxied on the dumb-bell surface and is proneto both strain and temperature changes. Another FBG, encased ina metal tube is prone only to temperature perturbations. Severaltests were performed on these packaged FBG sensors and the datacompared against conventional foil strain gauges. Tensile tests werereported on steel rebars and the sensor response was found to belinear and closely correlated to those of foil gauges. Static test onsimply supported reinforced concrete beams instrumented withthe sensors also showed approximately linear response, thus jus-tifying the packaging and installation procedures of the sensors.Dynamic tests on the beam were carried out using an impulsehammer and the maximum strain thus recorded by the FBG andfoil gauges were respectively 55 and 58 microstrain. The packagedsensor was also embedded inside a concrete cylinder, which wassubjected to compressive load. Only the strain sensor showed ahigh sensitivity whereas the temperature-monitoring sensor wasalmost unaffected.

It may be noted that in most cases, the strain sensitivity of anencapsulated FBG is significantly different from that of the bare FBG.Hence calibration of the encapsulated FBG sensor must be carriedout before it is put into real-world application.

5. Applications of FBG strain sensors in structural sensing

5.1. Strain monitoring in civil infrastructure

A major application of FBG strain sensors is in the field ofreal-time online health monitoring of bridges and civil structures[70–73]. FBG sensors have a major advantage over conventionalnon-destructive techniques in that they are capable of remotelymonitoring the condition of the test structure. The interrogatinginstrumentation being located off-site results in higher efficiencyof the system and better safety of the personnel.

Twenty-six FBG strain sensors have been reported to be moni-

toring the Horsetail Falls Bridge in Oregom successfully for 2 years[74]. The bridge was originally built in 1914 and in 1998 it wasstrengthened by placing composite wraps over the concrete beams.Long-gage FBG strain sensors are placed in grooves cut into the con-crete and in the wraps as well. This is done with a view to assessthe actual strengthening provided by the wraps to the bridge. Thelong-gage sensors provide a relatively macroscopic strain value thatcan measure several kHz of dynamic strain with a resolution as lowas 0.1 microstrain and hence are very useful in structural monitor-ing. A similar test is reported on the Sylvan Bridge with fourteenlong-gage sensors.

Saouma et al. [75] have used FBG sensors on aluminum as wellas concrete specimens to monitor the strain and have validatedthe results against electrical strain gauges. They have extended thelaboratory work by instrumenting six beams and six columns of anewly constructed building with FBG sensors that can be used tomonitor the strain of the beams and columns online.

Monitoring of the prestressing tendons of the Beddington TrailBridge, Canada using Bragg grating strain sensor array has beendone by Maaskant et al. [76]. Three types of prestressing tendonshave been used in this bridge, namely steel strand, carbon fibre

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last 3 years.

M. Majumder et al. / Sensors a

Fig. 3. Packaged FBG-based strain sensor used in the health monitoring of West MillBridge [78].

composite cable and leadline rod. The main objective behind thiswork was to study the long-term losses in the tendons due to stressrelaxation and creep. FBG sensors were bonded to each type of ten-don and then embedded in concrete girders. A total of 18 sensorswere placed strategically on the girder in order to monitor the pointof maximum strain generation. Proper cabling of the sensors min-imizes the effect of moisture and alkalinity of the medium on thesensors and also reduces pinching and microbending phenomena ofthe fibre. Static strain measurements of the girders with a precisionof ±40 microstrain have been done. A comparison of data collectedover 19 months reveal that the loss of prestress in CFRP tendons isalmost 25% lesser compared to that in steel tendons thereby jus-tifying some merit in the use of CFRP material in bridges. Besides,dynamic strain monitoring on various positions of the girder werestudied by passing vehicles with known loads. Strain resolution of1 microstrain over a range of 10,000 microstrain had been achieved.This information can be useful from the end of traffic monitoring,bridge designing and its maintenance.

FBGs have been used in the structural health monitoring of theTsing Ma Bridge in China [77]. At 1377 m, this is the longest sus-pension bridge in the world. It has a double deck configuration, onefor highway traffic and the other for railway. The deck is of hybridarrangement using both truss and box forms. This bridge is in ser-vice from 1997 and a structural health monitoring system, windand structural health monitoring (WASHM) has been monitoringits health from inception. In this report, the response of FBG sensorshas been checked against the WASHM system under specific load-ing conditions. FBGs were fabricated in-house, suitably packagedand installed on the bridge. Tests were carried out at three specificlocations viz., hanger cables, rocker bearing and supporting struc-

ture on a section of the lower deck using 21 FBG sensors. SeparateFBGs were used to monitor the instantaneous temperature of thestructure and compensate accordingly in strain readings. When-ever a heavy traffic load was subjected on the bridge, the responseof the FBG monitoring system peaked which was in close agree-ment to the response of the resistive strain gauge sensors of theWASHM system.

Gebremichael et al. in 2005 have reported the use of 40 FBGsensors to remotely monitor the real-time strain on Europe’s firstall-fibre reinforced composite bridge, The West Mill Bridge [78].Fig. 3 shows the packaged FBG sensor used in the structural healthmonitoring of this bridge. The main objective behind this studywas to collect real-time, in situ strain data from the bridge and toanalyze this data for assessment of its structural integrity, main-tenance scheduling and validation of design codes. The authorshave developed a dedicated FBG interrogation system based on theWDM technique for use in this work. Cost per channel of the instru-mentation is reported to be comparable to those of conventionalstrain sensors, if used in multiplexed sensor scheme. The moni-toring system was first tested inside the laboratory on structural

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test elements instrumented with FBG and strain gauge sensors.The structural elements were subjected to both quasi-static anddynamic loading. The tests reveal that response of the FBG sen-sors was linear, repeatable and without any significant hysteresis.Finally the two-lane bridge, which is fully made of glass and carbonfibre reinforced polymer, is carefully and strategically instrumentedwith 40 FBG and 11 strain gauge sensors. The FBG sensors werecoated with moisture-proofing silicon compound and consecu-tively with composite stripes that minimize birefringence effect infibres due to transverse loading. On-site testing was done with a30-ton lorry positioned at different points on the bridge. The strainmonitored in the different channels conforms to the relative posi-tioning between the sensors and the loading point. The strain dataobtained from FBGs and resistive gauges were found to be closelycorrelated and the technique has been established as a long-termcondition monitoring of the all-fibre composite bridge.

As a very significant continuation of this work, Kister et al. haveevaluated the performance of the adhesive and protection systemused with the FBG sensors installed on the West Mill Bridge [79].Unpackaged FBG sensors were installed on the bridge structureusing cynoacrylate glue, which is the primary adhesive. Beads ofepoxy adhesive were also deposited on top of the fibres to ensureadded anchorage. Strips of glass fibre composite material werebonded on both sides of the fibres using the two adhesives. Siliconesealant was then applied from top to seal the package from mois-ture. Composite covers were then used to envelope the completesensor package. The dimensions of the final packaged sensor were1.8 mm in thickness and maximum 1 m in length. The interfacialbond strength developed between the adhesive layers and the opti-cal fibre were evaluated by the modified fibre pull-out test [80]. Itwas reported that whereas unstripped fibres failed due to debond-ing and sliding of cladding, stripped fibres failed due to fracture ofthe cladding close to the glue edge or due to rupture of the gluelength. It was also observed that cynoacrylate-glued fibres couldwithstand a higher failure load than the epoxy-glued fibres. Hencecynoacrylate had been chosen as the primary adhesive for bond-ing the optical sensors on to the bridge structure. The durability ofthe sensor protection system was assessed by immersing couponsembedded with packaged sensors in water for a duration of 90 days.Results show negligible influence of water absorption on the sen-sors. Sensor survival rate while bonding on the bridge structure hasbeen reported as 100%. Integrity of adhesives and durability of thesensors has been assured and the sensors on the West Mill Bridgehave been providing continuous satisfactory performance over the

5.1.1. Strain monitoring in reinforced concrete beamsNumber of strain studies of reinforced concrete beams

instrumented with FBG sensors have been reported in liter-atures [81–83,64,67]. Maher and Nawy [82] have comparedthe response of FBG strain sensors and conventional resistivestrain gauges on reinforcing bars. The bars having dimension of305 cm × 25.4 cm × 30.5 cm were subjected to three-point bendingtests. Few FBG sensors were carefully embedded into V-groovescut into the reinforcing bars and a few others were simply epox-ied on the back surface of the bars alongside the conventionalstrain gauges. For test duration of 7 days, the nominal compressivestrength of the concrete was found to be 76 MPa and data collectedfrom FBG sensors and strain gauges were in agreement.

Davis et al. have reported the use of embedded wavelength divi-sion multiplexed FBG sensors in monitoring the strain on reinforcedconcrete beams and decks till their failure [83]. An 8-ft long beamwas instrumented with the Bragg sensors at different strategic loca-tions and subjected to four-point bending. The fibre sensors werebonded to rebars using ordinary foil gauge adhesives and coated

156 M. Majumder et al. / Sensors and Act

CFRP has been reported [66] (Fig. 5). Beams were 2 m in lengthand have cross-sectional dimension of 12 cm × 25 cm. Internal rein-forcement was provided by 2 bars and 19 stirrups. The concretesurface was washed with water jet, vacuumed and a layer of primerapplied. CFRP–FBG was bonded to the concrete using a resin coat-ing. The beams were tested using a 500 kN hydraulic actuator andboth FBGs and strain gauges were used as strain sensors. At the ini-tial cracking of the beams, the calculated load and the measuredload by the FBGs are comparable. However, after the yield point,the theoretical and calculated values differ. This is explained bythe slipping at the steel–concrete interface, and debonding at theCFRP–concrete interface.

Fig. 4. Schematic of testing of CFRP wrapped concrete cylinder with FBGs and straingauges [64].

with adhesive for protection. The sensor at the center showed amaximum strain at 47,500 lbs. Similar results were obtained forthe decks at a maximum load of 48,000 lbs. Interestingly, all FBGssurvived till the failure of the test specimens.

FBG sensors and strain gauges were surface mounted on CFRPwrapped concrete cylinders along both axes and subjected to com-pressive loading [64] (Fig. 4). At a compressive force of 56 MPa, thefailure strain in longitudinal direction is 4600 microstrain, whichis considerably higher than that along the hoop direction. Belowa strain value of 43 MPa, both strain gauge and FBG sensors in thehoop direction show a close match. However, along the longitudi-nal direction, FBG sensors show higher strain values than the straingauge sensor.

A prestressed concrete box girder of dimensions 205 cm ×140 cm was subjected to four-point bending and loaded till failure[67]. FBG sensors embedded on steel rebars have been used to mon-itor the failure behavior of the girder. The initiation of crack occursat the bottom of the girder at a load of 1500 kN at midspan and theinitial yielding is observed at 2200 kN. Yielding measured by strain

gauge was 2000 kN, which is comparable to the FBG sensor.

Laboratory trials of measuring the internal strain generatedinside concrete structural components have been reported [75].The fibre was embedded inside a prismatic specimen while a straingauge was mounted on the surface. Strain readings obtained fromthe two sensors show close agreement with each other.

5.1.2. Strain monitoring in smart beamsSandwich composite materials like glass fibre reinforced poly-

mer (GFRP) and carbon fibre reinforced polymer (CFRP) haveemerged as a promising load-bearing material in the civil engi-neering industry. FRP materials are non-metallic and thus have adistinct advantage over steel and other metallic building materials,which are highly susceptible to corrosion. They have high strength,high stiffness and low weight. However, one drawback of the FRPmaterials is their weak shear strength. Creep and brittle natureof the FRP materials have made it necessary to study their inter-nal failure at an early stage by embedding suitable strain sensorsin them. Towards this end, FBGs have come up as an interestingstrain sensing tool due to its small dimensions, light weight and

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ability to be multiplexed. Kalamkarov et al. have described the useof pultrusion technique to manufacture smart carbon and glass FRPcomposites embedded with FBG sensors [84]. The technique com-bines the strengthening properties of fibre reinforced plastic andthe sensing properties of optical fibre Bragg grating. It is shownthat polymide coating; instead of acrylate coating on the fibre, pro-vides better bonding with the host material. The pultruded smarttendons thus produced were subjected to quasi-static and cyclictensile tests whereby they showed similar results to those of anextensometer.

Another interesting work reported the application of smartFRP–FBG bars in reinforced concrete beams [85]. The FRP–FBG barsare embedded in 12 concrete beams to monitor the strain of FRPbars and cracking of the concrete. The beams are subjected to three-point bending tests. The monitored strain provides information onthe status of cracking of the concrete and also the slip between FRPbars and concrete. The strain range monitored and resolution of thesensor are 1200 microstrain and 1–2 microstrain, respectively. TheFRP–FBG bars can hence be conveniently used in reinforced cementconcrete structures both as sensors as well as reinforcing bars.

Health monitoring of smart alumina-fibre reinforced plas-tic embedded with FBG sensors under tensile loading has beenreported [86]. In a test coupon, one FBG sensor was located at thecenter and coated with polymide. The resolution of the interroga-tion setup was 1 microstrain. On observing the wavelength pattern,it is found that the spectrum pattern changes with the initiationof the cracks. It is seen that the FBG sensors were able to detectthe cracks adjacent to the sensing region of the fibre. However, fordetection of exact location of failure in the entire specimen, an arrayof several Bragg grating sensors is suggested.

Real-time strain monitoring of RC beams using FBG-embedded

5.1.3. Pile load monitoringOne of the fundamental issues in designing any sound civil

infrastructure is proper designing and load monitoring of piles atthe construction stage. Since piles carry the weight of the foun-dation and transmit the load of the structure to the subsoil, it isessential to monitor the strain generated on them to prevent catas-trophic failure of the structure. A few of the important pile designconsiderations are compression, tension and bending moment of

Fig. 5. RC beam instrumented with smart FRP–FBG sensors [66].

M. Majumder et al. / Sensors and Act

Fig. 6. FBG-based sensor for strain monitoring in concrete piles [79].

pile material, nature and magnitude of expected load to which thestructure will be exposed during its lifetime, soil characteristics andground water level. Bragg grating sensors have recently been usedsuccessfully by few researchers in the civil engineering communityto monitor the strain in concrete piles [79,87–89]. Fig. 6 shows thesensor protection scheme employed in concrete pile monitoring.

Li et al. [87] have reviewed the use of optical fibre sensors in thestrain monitoring of piles for composite marine applications [90]and for a concrete pile [91].

A 13-storey building in Bankside, London was instrumentedwith FBG strain sensors and monitored for pile loading under theconditions of pouring of concrete into borehole, curing of the con-crete and construction of the building floors [88]. The aim of thisstudy was to provide a realistic assessment of integrity of thefoundation piles for future use. The building was supported on 67foundation piles, out of which two were instrumented with theFBG sensors. The piles were 46 m in depth and 1.5 m in diameter.A circular, three section steel cage was used to reinforce the con-crete piles. The pile type used was bored augered, which is suitablefor clayey soil of London. The piles were designed with a factor ofsafety of 3. The optical fibres were glued to rebars using cynoacry-late adhesive and the rebars were then welded on to the three cage

sections. The fibres were protected using a carbon fibre compos-ite packaging. The different cage sections were lowered one by oneinto the pile bore and clamped to each other. After the clampingprocedure, the lead-out optical fibres were connected to the inter-rogation unit. Concrete was then poured into the borehole and thestrain of the piles monitored as a function of the height of concretein the borehole. It was observed that initial compressive strainschanged into tensile strains. Assuming a value of 11.5 ◦C−1 as thethermal expansion coefficient of the steel rebars, the average ten-sile strain in the bottom, middle and top sections were found tobe 123, 51 and 67 microstrain, respectively. The effect of concretecuring was also studied by monitoring the online strain of the FBGsevery 7 days over a period of 4 weeks. As expected, the sensorsshowed an initial rise in tensile strain, which was attributed to thecuring of the concrete, and the resultant rise in temperature of thesteel rebars due to the exothermic reaction. Once the reaction sub-sides, the sensors also detected a gradual decrease in the strainvalues. Another effect to be studied was that of the construction ofthe building floors. The construction of the ground floor was under-taken after a period of 48 days from the pouring of concrete in theborehole. The strain monitored by the FBG sensors differed with the

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increasing number of floors and results showed a similar trend ofthe vibrating wire strain gauges installed on the pile cages. Authorshave provided explanations for all observed phenomena from thestructural engineering view. To overcome the problem of measure-ment of localized versions of structural strain by the FBG sensors,use of long-gage Bragg grating sensors has been suggested.

5.1.4. Early-age cement shrinkageThe curing process of cement is affected by the water to cement

ratio, the curing temperature, humidity and type of cement used.Hydration is responsible for the hardening (strength) of the con-crete. For concrete, the gain in strength continues for a long time,and theoretically for an indefinite period of time. However, thestrength of the concrete reaches a peak within 7 days of cast-ing. During this process water in the concrete mixture evaporates,resulting in a decrease in the volume of the concrete. The volumeof concrete also decreases due to re-arrangement of finer parti-cles within the larger ones. The result of the volume change isstrain, also known as shrinkage strain, and this is responsible forthe small cracks that may appear after the curing process. At times,the thermal stresses induced during the curing process due to theexothermic nature of the curing reaction may cause cracks withinthe structure, thus weakening it. These cracks can grow in sizeand penetrate the structure at a later stage. However, the effectof how the temperature, inner pressure and strain changes withinthe concrete affect the structure is still not very clear.

FBG strain sensors have been used to study early-age cementpaste shrinkage (between 0 and 12 h after mixing) [92]. A majorpractical problem in laying concrete floors is associated to early-age cracking due to shrinkage of young concrete. Capillary watertrapped inside finds a way out of the hardening concrete therebygiving rise to fine cracks within the first 24 h of casting, i.e. the plas-tic stage. In this stage major reactions accompany the transitionof cement water suspension phase to cement paste phase. Mea-suring early shrinkage of cement using conventional strain gaugesis difficult because most gauges can be attached to the concreteonly after it has attained a minimum strength. FBG strain sen-sors resolve this problem and are embedded in the cement pastespecimens right from the beginning of casting. FBG strain sensorshave revealed early-age shrinkage of cement paste under variousdrying conditions. Prismatic specimens of Portland cement of size9 cm × 3 cm × 3 cm were cast into a form. The form chosen had alow stiffness in order to allow maximum shrinkage of the sample.An impermeable layer of plastic placed between the form and the

cement, prevents any transport of water to the form. The FBG sen-sors, protected with an acrylate coating were embedded parallel tothe longitudinal axis of the specimen along the center line. Ther-mocouples were also embedded in the cement specimens to takecare of the strain–temperature discrimination. As a test of bondstrength between the cement paste and the optical fibres, the fibreswere subjected to fibre pull-out test at different ages. A typicalbond slipping of the fibres was observed at an age of 6 h. FBG sen-sors show that for a w/c ratio of 0.45, significant shrinkage occursbetween 3 and 6 h after mixing. This high shrinkage in the early ageis attributed to the capillary transport of trapped water towards thespecimen surface. After the high shrinkage phenomenon occurs arise in internal temperature due to the exothermic nature of thehydration reaction. Total strain observed by the early-age detectionscheme after 12 days was 2700 �m/m as compared to 2000 �m/mas measured by the conventional tests, i.e. monitoring beginningafter 24 h of mixing. The total water loss at early age was seen tobe almost linear. One of the specimens was re-saturated with waterafter 16 days of initial drying. As expected, the strain increased oncethe drying action set in. It was proved that phenomenon of dry-ing shrinkage of cement is reversible. Differing shrinkage results

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for different material composition of cement and different dryingconditions showed that the early-age shrinkage is a function ofcomposition and environmental factors. Early-age shrinkage wasalso shown to be dependent on the geometry of the specimen.This report provides a comprehensive insight into the early-ageshrinkage of cement under various influences.

Camara et al. have studied the temperature change associatedwith the early-age curing of Portland cement [93]. A sample holderof size 9 cm × 3 cm × 3 cm was filled with Portland cement paste inthe w/c ratio of 0.5. A FBG sensor was used to monitor the tem-perature of the paste for a period of 55 h. Noticeable temperaturevariation occurs between 10 and 25 h after mixing of the cementpaste. This temperature change is ascribed to the exothermic hydra-tion reaction between the components of cement and water. Thisstudy reveals that appreciable temperature rise of the cement pasteinitiates 10 h after the mixing. However, the study does not includedirect measurement of shrinkage property of the cement in its earlyage.

Wong et al. have studied the shrinkage and temperature changebehavior of reactive powder concrete (RPC) in the early age usingFBG sensors [94]. RPC as well as normal concrete have been used inthe construction of road bridges [95,96]. RPC as construction mate-rial has certain advantages like ultra-high compression strength(200–800 MPa), high flexural strength (40 MPa), lightweight andbetter homogeneity. In monitoring the early-age shrinkage ofRPC, similar problems were encountered as with normal concretewhereby conventional strain gauge sensors could not be attached tothe specimens before hardening. As a result, FBG sensors were usedas suitable candidates for monitoring the shrinkage and tempera-ture. In the early age, RPC shrinkages are mainly due to autogenous,thermal and drying effects. The FBG sensors were suitably placedinside the casting moulds before the pouring of RPC mixture. Themixture prepared had a w/c ratio of 0.31. Moulds of two sizes wereplaced on vibrating table and the mixture poured inside. The vibra-tion enabled even distribution and compaction of the RPC. Theshrinkage and temperature monitoring was started immediatelyafter the pouring of the mix into the mould. The total shrinkagefor 7 days was 488 microstrain, out of which 377 microstrain wasrecorded in the first 24 h. This shows that the maximum shrinkageof RPC (approximately 77%) occurred in the early-age, i.e. within thefirst 24 h. Temperature monitored showed a double peak behaviorand the maximum temperature rise (4.7 ◦C) was observed 7 h aftercasting. This was concluded as the setting time of RPC. Also RPCspecimens of two different sizes were evaluated for shrinkage and

temperature and it was observed that smaller dimension prismshave a higher overall shrinkage and lower temperature change. Thisstudy provides useful data of the plastic shrinkage of RPC and couldbe helpful in improving the mix proportion of RPC in future work.

5.1.5. Moisture/humidity measurement in civil applicationsRelative humidity (RH) detection is an important criterion

in many civil engineering applications. For example, it providesinformation about soil moisture content or water ingress in civilstructures like seepage in walls. It may also help to detect leaks inconcrete tanks or water pipes. Recently fibre Bragg grating sensorshave been reported for RH measurement [97–100]. These sensorsessentially rely on a measurand-specific material that produceseither a strain or temperature variation effect on the FBG. Corro-sion resistance, small dimensions and insensitivity to EMI are theprimary advantages of using FBG-based sensors for humidity detec-tion in civil applications. Fig. 7 shows the schematic of a FBG-basedmoisture sensor reviewed by Leung [11].

FBG-based RH sensors were first reported by Giaccari et al. in2001 [97]. Bare silica fibres are unperturbed by change in RH. How-ever, a chemical polymer called polymide is hygroscopic in nature

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Fig. 7. A FBG-based distributed water/moisture sensor [11].

and swell with the absorption of water molecules. The effect ofRH sensitivity of polymide was also observed to be reversible innature. This property of polymide had been made use of in thesesensors to coat them such that they swell in the presence of aqueousmedia and thereby cause a shift of the characteristic Bragg wave-length. The group has studied the effects of RH and temperaturevariation on these sensors and results showed a linear dependenceof FBG center wavelength on these two measurands. The relativeBragg wavelength shift ��/� of the sensor is given by the followingequation:

��

�= SRH �RH + ST �T

where SRH and ST are the RH and temperature sensitivities of theFBG sensor, respectively.

In extension of this work in 2002, Kronenberg et al. have studiedthe effects of coating thickness of the polymide layer on the FBG-based sensor’s RH and temperature sensitivities [98]. Eight FBGscoated with varying thickness of polymide (ranging from 3.6 to29.3 �m) were tested in controlled climatic chamber under varyingconditions of RH and temperature. The RH was increased in stepsfrom 10 to 90% at five temperature values ranging from 13 to 60 ◦C.Data has been presented for the eighth sensor (one with maximumcoating thickness) and values for SRH and ST have been reportedas (2.21 ± 0.10) × 10−6/%RH and (7.79 ± 0.08) × 10−6/K, respectively.Sensors also showed an almost linear relationship of their SRH andST with the coating thickness and saturated asymptotically. Val-ues of thermal and hygroscopic expansion coefficient of polymidehave been calculated as 5.5 × 10−5/K and 8.3 × 10−5/%RH, respec-tively. Authors have suggested a separate FBG temperature sensorto take care of the strain–temperature cross-sensitivity and havealso pointed out on the possible use of such sensors as multipoint

RH sensors capable of distributed sensing in real-world cases.

Relative humidity measurement of a climatic chamber usingpolymer-coated FBG strain sensors have also been reported byYeo et al. [99]. FBGs used in this work were inscribed onboron–germanium co-doped photosensitive optical fibres using thephase mask technique. They were then dip-coated with a polymidesolution to a desired thickness. PI is chosen as the coating mate-rial because of its sensitivity, linearity and reversibility to humiditychange.

The sensing principle relies on the swelling of the moisture sen-sitive polymer coating, which in turn produces a strain on the FBG.The Bragg wavelength shift due to relative humidity and tempera-ture is expressed by

��B

��= (1 − �e)εRH + (1 − �e)εT + � �T (6)

where εRH and εT are the humidity and thermal expansion experi-enced by the fibre which leads to inducing a strain in it.

εx =[

ApEp

ApEp + Af Ef

](˛p(x) − ˛f (x)) �x (7)

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M. Majumder et al. / Sensors a

where x denotes either RH or T, p and f represent the two materialsviz., polymer and fibre, A is the area of cross-section of the material,E is the Young’s Modulus of the material, ˛ is the coefficient ofmoisture expansion (CME) or the coefficient of thermal expansion(CTE).

FBG sensors were exposed to an airtight climatic chamber hav-ing controlled RH and temperature conditions. Six different coatingthickness of the polymide have been reported and the sensor wastested at different humidity levels whereby the maximum sensitiv-ity achieved was 5.6 pm/%RH for a coating thickness of 42 �m. TheCME value reported was in the range between 82 to 104 ppm/%RH.The response time and recovery time of the sensors with differentcoating thickness when subjected to a variation of the RH between33 and 75% in the chamber were seen to be in the range of 18–45 minand 4–28 min, respectively. Hysteresis was reported to be within 5%of RH. These sensors could be well used in civil engineering appli-cations for RH measurement where slightly lower resolution andresponse time are not very critical.

Huang et al. have proposed a low-cost RH sensor by coating FBGsensors with inexpensive thermoplastic polymide solution [100].The sensors were dip-coated in the solution for 5–10 min and driedin a drying cabinet. The coating thickness was reported to be 8 �m.A copper sleeving was used to protect the sensors from exter-nal mechanical damages. Coarse wavelength division multiplexing(CWDM) technique was used to interrogate the sensors. RH wasmeasured in the range from 11 to 98% at constant room temperatureand the results from the FBG sensors were found to be linear, repro-ducible and reversible. RH sensitivity and response time measuredwere 0.000266 V/%RH and 5 s respectively, which is an apprecia-ble improvement over similar previous sensors. However, sensorswere perturbed by temperature variation and some temperaturediscrimination scheme should be built-in.

Yeo et al. have reported an effort to optimize the response ofthe RH sensors [101]. In order to ensure repeatability of the sensorresponse, authors have used a silicane coupling agent between thesilica fibre and polymide coating layer. This is claimed to ensureuniform adhesion of the coating on to the fibre surface and therebyallow for identical amounts of strain to be transferred from thecoating to the FBG sensor under exposure to similar moisture levels.

5.1.6. FBGs in geodynamic studiesFBG strain sensors also have an important application in the field

of geodynamical monitoring [102]. Typical cases could be the studyof rock deformation, fibre-optic geophone, vertical seismic profil-

ing, etc. Ground stress and strain regime study in seismic areas hasbeen proved to be an effective aid in forecasting major hazards. Adense array of sensors is required in such studies to provide not onlylocalized measurements of ground strain, but also some data aboutthe origin of the disturbance. The main advantage of FBG sensorsin the geodynamical field is its low cost, ruggedness in harsh envi-ronments and distributive nature of sensing, enabling monitoringof a wide frequency range. However, the major limitations of thissensor are its low sensitivity (10−7 strains) in quasi-static domainas compared to volumetric strain meters (10−12 strains) and itstemperature-strain cross-sensitivity. Several approaches [103–105]have been suggested to overcome these limitations.

FBG strain sensors have been used to measure dynamic defor-mations in rock masses and the results thus obtained are validatedagainst conventional geophone data [106]. Rock bolts made of steelare used in structures inside underground cavities. Rock bolts arecapable of 20% elongation whereas the FBG sensors reported in thisstudy can monitor a maximum strain of 1%.

Application of FBG strain sensors for long-term surveillance oftunnels has been presented [107]. The FBGs were embedded in glassfibre reinforced polymers (GFRPs) of two different diameters. The

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Fig. 8. Positioning of FBG sensors in critical strain points in a prototype of offshoreplatform [112].

rockbolts were made of this GFRP and tested in a tunnel in Switzer-land for a period of 1 year. The strain generated on them was studiedand a comparison made for the two sensors. Maximum range ofstrain that could be measured was 1.6%.

Ground strain conditions during an earthquake have been stud-ied using FBG sensor systems [108]. A patent for the use of FBGsensors for vertical seismic profiling in earth boreholes has beengranted [109].

Liu et al. have reported the use of FBG sensor system to moni-tor the underground seismic signals in rock masses with a dynamicstrain resolution of 10−9 at 1.7 kHz [110]. The sensor is epoxied ontothe glass fibre reinforced polymer (GFRP) made rockbolt. Severaltests, both in laboratory conditions as well as in actual test siteshave been conducted. Seismic signal was generated on the labora-tory prototype by using metallic ball excitation, whereas in the fieldtests by using a hammer and then by exploding 50 g explosive mate-rial. Seismic signals thus obtained are found to be prone to noise dueto instability in the interrogating laser wavelength, disturbances inthe optical cables, connectors, etc. The data is adaptive filtered usinga combination of windowed discrete Fourier series (DFS) and dis-crete autocorrelation function (DACF) and the filtered data is almostnoise-free and in good agreement with the data obtained froma conventional geophone. For maximum sensitivity of the fibre,wavelength is optimized at 1549.87 nm and 1550.07 nm. However,for better sensitivity of the FBG-based geodynamic sensors, opti-mization of wavelength of the laser source as well as that of thesensors is required.

Wu et al. have reported a temperature-controlled FBG sensor

system for use in monitoring the dynamic strain on geotechnicalstructures [111]. The grating is glued to the sensor head, which ismade of composite material. For testing purpose the sensor headand a conventional geophone are installed along the axial direc-tion in a rock mass of 1.5 m length. Seismic signals in the range of100–2500 Hz are generated on the rock mass using a metallic ballimpact. The grating placed inside the sensor head, namely FBG1 isilluminated from a broadband light source and the reflected signalfrom it is fed directly into a log ratio amplifier. The other input of theratio amplifier comes from the light reflected by the second grat-ing namely, FBG2, which is glued onto a peltier chip. The peltierchip is maintained in a closed loop whereby the drift in workingtemperature is constantly being monitored and compensated bymeans of a PID programmer. The ratiometric approach leads toreduction in undesired background noise. The results obtained fromthe FBG sensor system and the geophone are within acceptableagreement. 15 dB and 620 microstrain are respectively the signal-to-noise (SNR) and resolution of the system reported.

FBG sensors have also been reported to be used in the healthmonitoring of offshore platform models [112]. Fig. 8 shows a

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prototype of an offshore platform instrumented with FBG sensorsfor its health monitoring. In comparison to other civil structure, off-shore platforms are subjected to mechanical shocks from sea waves,ice, storms and accidental collisions from vessels. Other aggravat-ing factors include continuous corrosion, erosion and scour fromthe seawater. FBG strain sensors having the advantages of abilityto function in harsh environment, to be multiplexed and remotesensing are suited in such situations. Authors have prepared a pro-totype of the offshore platform of Shengli oil field in Yellow Sea,China in the scale of 1:14. FBG sensors packaged in a steel tubewere mounted on the prototype and used to monitor the straingenerated on it. An underwater shaking table was used to simu-late the vibration on the prototype. An extra FBG sensor served asa temperature sensor and readings from it used for temperaturecompensation of the FBG strain sensor. The maximum working fre-quency of the FBG sensor was calculated to be 5.625 kHz, while thatof the shaking table was 50 Hz. Thus, the sensors were judged to becapable of measuring the dynamic strain response of the prototype.The prototype was subjected to sine wave, El centro wave and Tian-jin wave and in all cases it was observed that readings obtainedfrom the FBG sensors and conventional strain gauges showed ahigh correlation. Besides, FBG sensors were much less affected bynoise (1 microstrain) as compared to strain gauges (10 microstrain).Since FBG sensors have been able to monitor the strains on the plat-form prototype satisfactorily, the work may be suitably extendedfor similar real-world applications.

5.1.7. Ultrasonic non-destructive testing of structural health usingFBG

Ultrasonic inspection of structures is one important methodfor monitoring the health of structures. Propagation of ultrasonicwave is affected while passing through damaged area of structures.So far piezoelectric type transducers are used for generation anddetection of ultrasonic wave in structures. Recent studies showedthat fibre Bragg grating sensors can be effectively used as ultra-sound sensors for monitoring the health of metallic (stainless steel)[113–115], composite (CFRP) [116–118] structures. Fig. 9 shows theexperimental setup for ultrasonic non-destructive testing usingFBG sensors for the health monitoring of cross-ply CFRP. In compos-ite structures FBG ultrasound sensors can effectively distinguish theultrasonic wave passing through the undamaged and damaged areawhere as the conventional piezoelectric type ultrasound sensorscannot distinguish the same. Moreover, piezoelectric type ultra-sonic detectors are influenced by electromagnetic interferences

and therefore the use of such type of sensors is inconvenient inareas of high EMI. FBG being immune towards EMI can be a bet-ter alternative to piezoelectric type ultrasonic sensors. As FBGs arecharacterized by very low insertion loss they are suitable for multi-plexing in series along a fibre and are thus ideal for multiple sensorapplications, where time and wavelength division multiplexingcan be applied. FBG sensors are characterized by great spatial andtemporal resolution. Unlike other optical interferometric sensors,FBG sensors are wavelength encoding type self referencing sensorsand so its operation is immune to fluctuating light levels, sourcepower and connector losses. Further FBG sensors are environmen-tally more stable and durable with high corrosion resistance andused in unapproachable and dangerous environments (in explosivezones). Due to these advantages FBGs are considered to be promis-ing ultrasound sensors in structural health monitoring. However,FBGs have limitations in the temporal bandwidth due to radial res-onances in the fibre which will constrain its sensitivity (over a fewtens of MHz).

Several studies have already been done in recent years toinvestigate the possibility of using FBG as ultrasonic sensorsduring ultrasonic inspection of structural health monitoring

Fig. 9. Experimental setup in the ultrasonic non-destructive testing of structuralhealth in cross ply CFRP and schematic illustrating the variation in reflectivity ofthe FBG sensor with applied strain (a) using broadband light source and (b) tunablelaser source.

[113,114,116–118]. Tsuda studied ultrasonic inspection methodusing small diameter FBG sensors for determining impact damagein CFRP plate. The FBG sensors so used have typical gage lengthof 5–10 mm and typical grating period of 0.5 �m. In his work, theauthor has described two types of FBG-based ultrasonic sensingsystem using broadband light source and tunable laser source fordamage detection in cross-ply CFRP [116]. A piezoelectric deviceis used as an ultrasonic transmitter to generate shear waves (fre-quency 250 Hz). In the first technique (Fig. 9) light from a broad

band light source travels to FBG1 via an optical circulator and thereflected light is detected by a photo detector after being filteredby FBG 2. The Bragg’s wavelength of the sensor FBG1 (1550.28 nm)is slightly longer than that of the filter FBG2 (1550.18 nm). Dur-ing varying strain, the change in reflectivity of FBG1 with respect totransmissivity of FBG2 is detected by the photo detector, the outputof which increases during tension and reduces during compression(Fig. 9a). In the second technique the broad band light source isreplaced by a tunable laser source (1550.38 nm) with the laser emis-sion wavelength set to �out in such a way that at this wavelengththe reflectivity of the sensor FBG at strain free is reduced to half asshown in Fig. 9(b). In this case also the intensity of light reflectedfrom the FBG sensor directly corresponds to ultrasonic response.The author have further compared the techniques with that usingthe S mode waves of piezoelectric shear wave transmitter (centralfrequency 250 kHz) and found that the FBG sensing systems pro-duced better detection of damage in the composite structures thanpiezoelectric sensors.

In another work, Takeda et al. have used FBG sensing system fordetection of lamb waves in CFRP laminate using high speed opti-cal wave length interrogation system [117]. They had shown that

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in order to receive the lamb wave, the gage length of the FBG sen-sor should be shorter than 1/7th of the ultrasonic wavelength. Thetechnique was used by the authors for detection of delaminationin the CFRP cross-ply laminate and debonding in the CFRP bondingstructure using small diameter FBG sensor embedded in double laptype coupon specimen and was found to be very effective.

In their work, Lee et al. investigated the birefringence effects ofFBG sensor during ultrasonic measurements in both surface mount-ing and embedded configurations [118]. They had shown that glueinduced low-birefringence results in loss of sensitivity in ultrasonicmeasurement by FBG sensors and proposed simple and effectivesolutions with respect to respective installation configurations forremoving the birefringence effect.

6. Conclusion

This paper presents a review of recent research and devel-opment activities in structural health monitoring using FBGs.High-quality FBG interrogation systems; practical encapsulation(packaging) techniques and practical applications are the cores forFBGs to be widely popularized in infrastructures. Due to the inher-ent properties of FBG sensors such as lightweight, immunity toelectromagnetic interference and harsh environment and ability tobe multiplexed for distributive measurement, these sensors haveemerged as a suitable solution in longitudinal strain measurementin static and dynamic strain sensing and acousto-ultrasonic sensingin a number of application areas.

In this paper, firstly, the principle of FBG sensor in strainmeasurement and the effect of temperature in strain measure-ment are briefly discussed. Several temperature compensationtechniques to correct the effect of temperature during strainmeasurement are reviewed form the literature and their advan-tages/disadvantages are pointed out. Commonly used schemes ofFBG interrogation are reviewed. Secondly, several practical FBGencapsulation (packaging) techniques published in the literaturesare reviewed highlighting the advantages/disadvantages of thetechniques. Thirdly some of the salient areas in structural healthmonitoring for the application of these sensors have been reviewedin this paper. Finally few techniques for ultrasonic non-destructivetesting of damage in composite materials using FBG are reviewedexplaining the ultrasonic sensing methods.

This report has presented the state-of-the-art in strain monitor-ing of different application areas related to civil structures usingfibre Bragg grating sensors. Works of various researchers have

undoubtedly proved the suitability and reliability of these sensorsin strain monitoring of both laboratory specimens and real struc-tures. From these studies some unique advantages of Bragg gratingsensors in strain monitoring of structures have emerged. Theseinclude immunity from emi and harsh environment, capability ofdistributed sensing and small sensor and harness size, etc.

Yet in real-world applications of SHM, conventional sensorsystem still outnumber their FBG-based competitors. A possibleexplanation for the FBG sensors not reaching their full marketpotential is perhaps due to the non-standardization of these sen-sors. Even after much active research in this field, no standards forthe sensor packaging and their usage in SHM have been arrived atby any international governing body. Another disadvantage associ-ated with the use of FBG sensors is the high cost of the interrogationsystem currently available in the market. As such, there seems tobe some hesitancy of the civil engineering community to entirelyreplace conventional strain sensors with FBG sensors for a criticalissue like structural health monitoring.

Besides, a few remaining issues in the application of FBG sensorsneed to be addressed. In composites, where these sensors are to beembedded, wavelength shift compensation due to the temperature

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effect is still not possible without compromising the fibre’s resolu-tion. Besides, the multi axis measurement of strain in structuresis still an active area of research [119]. During the service-life ofa structure, several types of defects are generated on it (warping,cracks, delamination, etc.). In order to be a useful tool in SHM, itis necessary to carefully analyze the FBG sensor response to thesedifferent types of defects. Moreover, for all practical situations, dueto their brittle nature, it is necessary to encapsulate the bare FBGsensors before mounting them on any structure. However, the pro-tective layer and the adhesive layer absorb a part of the strain andthe indication given by the FBG is not the true strain on the struc-ture. Lau et al. have done a theoretical modeling of the bondingcharacteristics at the interface of bare fibre and coating, coatingand adhesive layer, adhesive layer and host material and validatedthe results using FEM tools [120]. The study concludes that for bet-ter strain transfer from the host material to the FBG sensors, a thinlayer of adhesive, a high modulus coating material and a sufficientembedding length of the sensor is necessary.

A recent market overview of FBG sensors by Mendez estimatesthe present global FBG market size at 15–35 million USD per yearwith an annual growth rate of 15–25% [121]. However, to achievethe full market potential, technologists have to ensure reliability,cost-effectiveness and standardization of the FBG-based sensors.It is expected that in the near future the demand for these sen-sors will increase both in conventional and niche markets. On onehand there will be applications concerning distributed sensing inmonitoring of bridges, tunnels, pipelines, etc. On the other, newerapplications of these sensors in chemical and biomedical sensingis also envisaged.

After almost three decades of research in FBGs, technology forSHM using FBG sensors is on the verge of maturity. The main thrustof technology development at present should be focused on thevarious application areas of civil infrastructure monitoring usingFBGs and the standardization of the procedure. Hence, the opticalfibre community should collaborate more with domain specialistslike civil engineers and architects to extend the use of FBG strainsensors into newer avenues in structural health monitoring.

Acknowledgements

The authors would like to acknowledge the support and guid-ance provided by the Director, Central Glass & Ceramic ResearchInstitute, Kolkata, India. The work has been carried out under the

project Technology for Engineering Critical Assessment (TECA),CORR 0022.

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Biographies

Mousumi Majumder obtained her B. Tech in Electronics and TelecommunicationEngineering from the North-Eastern Hill University, India in 1997. She had joinedthe Council for Scientific and Industrial Research, India in 1998. Presently she isemployed as a Scientist in the Instrumentation Division at Central Glass & CeramicResearch Institute, Kolkata, India. She has participated in several research projectslike the “Technology for Engineering Critical Analysis” and “Technology for Assess-ment and Refurbishment of Engineering Materials and Components”. Her currentprofessional interests lie in material characterization, and usage of various strainsensing platforms in Structural Health Monitoring and Refurbishment.

Tarun Kumar Gangopadhyay graduated Bachelor of Electrical Engineering in 1989and Master of Electrical Engineering in 1991, both from the Jadavpur University,

164 M. Majumder et al. / Sensors and Act

Calcutta, India. He completed his Ph.D. in December 2005 in the field of fibre-opticsensor from the University of Sydney, Australia. He has authored several journalpapers and international conference papers. He is currently employed as a Scientistin the Optical Fibre Division at Central Glass & Ceramic Research Institute, Kolkata,India. His area of expertise lies in optical fibre sensors, FBG sensors for smart struc-tures, FBG sensors for power line application, fibre fabrication and characterization,fibre-optic amplifier and fibre-optic components such as bi-directional coupler,WDM coupler, etc. His current research interests are development of Fibre BraggGrating sensors, bio-medical sensors and PM fibre coupler for Gyro application. Heis a member of IEEE, and Optical Society of America (OSA), USA.

Ashim K. Chakraborty has obtained his M.E. (Hons.) degrees in Instrumentationand Electronics from Jadavpur University, Kolkata, India in 1990. He served SimonCarves Limited, India before joining as a scientist in the Instrumentation Section ofCentral Glass and Ceramic Research Institute (CSIR), Kolkata, India in 1987 wherehe is presently a senior scientist and the head of the section. He has active inter-est in technology development and has participated in a number of sponsoredresearch projects. He has served as the research project leader for development ofFBG strain sensors under Technology for Engineering Critical Analysis (2004–2007),

uators A 147 (2008) 150–164

a network project among CSIR laboratories, India. His current research interestsare in the area of technology development for specialty glasses and optic fibresensors.

Kamal Dasgupta is a senior Scientist and is presently heading the Optical Commu-nication Fibre Division at Central Glass & Ceramic Research Institute, Kolkata, India.His professional interests lie in different kinds of specialty fibre sensors.

Dipak K. Bhattacharya, a graduate in Metallurgy obtained his doctoral degree fromthe Indian Institute of Science, Bangalore, India in 1995 working on the subjectof correlation of Barkhausen Signal and magnetic hysteresis loop parameters withmicrostructures in steels. His professional interests lie in materials processing &characterization, and R&D project management. He has worked in various NDT tech-niques for the evaluation of remaining life assessment of engineering materials andstructures in the power and petrochemical plants. He is presently heading the Ana-lytical Facility Division and Programme Management Division in Central Glass &Ceramic Research Institute, Kolkata, India. He has more than 60 publications in Inter-national Journals. He is the Chief Editor of the Journal of Nondestructive Evaluationpublished by the Indian Society for NDT.