Structural Integrity Monitoring With Fibre Bragg Grating Sensors

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Structural integrity monitoring with fibre

Bragg grating sensorsRobert Bogue

Associate Editor, Sensor Review

AbstractPurpose – This paper describes a recent collaborative project involving the development of a multiplexed fibre Bragg grating (FBG) sensor system forstructural integrity monitoring.Design/methodology/approach – The system is described and field trials on both conventional and novel composite bridges are discussed. A FBGsensor-based structural monitoring system was developed, based on a fluorescent fibre as the optical source. It used a tuneable, fibre-coupled, Fabry-Perot filter, actuated by piezoelectric transducers and operated over the bandwidth of the source at up to 250 scans/second. Light from the source wasfiltered and reflected back from the Bragg gratings, through optical couplers, to eight photodiode detectors. These detected the resulting time-domainspectra of the sensors in each of the serially connected sensor arrays. The system was tested at City University and then subjected to trials on the

Mjosund road bridge in Norway and on West Mill bridge in Oxfordshire, UK, which is the first bridge to be fabricated from a new type of compositematerial.Findings – During the Norwegian trials the system was arranged with four or five FBG sensors per channel giving a total of 32 measurement pointswith eight parallel channels. Twelve conventional foil strain gauges and a number of thermocouples were also installed. Different static and dynamicloads were applied over a period of 18 months and the results showed that the thermally compensated strain data obtained optically matched thosefrom the resistive gauges to within,5 m 1 . During the construction stage of the Oxfordshire bridge, sections of the decking and longitudinal compositesupport beams were instrumented with 40 FBG sensors with temperature compensation, placed at pre-selected sites of maximum strain. Theseexhibited a resolution of ^5 m 1 and an operating range of over ^2,000m 1 .Originality/value – This research has shown that multiplexed, multi-point FBG sensor systems can accurately and reliably monitor both static anddynamic strains in large structures over a range of temperatures and for extended periods of time.

Keywords Condition monitoring, Fibre optic sensors, Sensors

Paper type Technical paper

Integrity monitoring is an essential tool for ensuring the safety

and assessing the condition of critical concrete, steel and

other structures such as bridges, roads, offshore rigs, railways

and process plant. Traditionally, it has utilised a disparate

range of condition monitoring and NDT techniques such as

ultrasonics, acoustic emission, strain gauge measurements

and various optical inspection methods. However, to monitor

the strain in a large structure at several points simultaneously

and over an extended period of time is problematic, as

existing techniques suffer drawbacks such as electrical

interference or the inability to make multiple measurements

in real time.

In recognition of these limitations, a collaborative, three-year, £0.8 million research project was started in 2001. This

was funded by the UK’s Department of Trade and Industry

(DTI) and the Engineering and Physical Science Research

Council (EPSRC) under the Faraday/Intersect partnership.

Intersect and Faraday Partnerships

The UK government’s Intersect scheme provides expert advice based on theexperience of a network of academic and industrial partners in the sensor,

measurement and data analysis fields. It is supported by the DTI

(Department of Trade and Industry) and EPSRC (the Engineering andPhysical Sciences Research Council) and managed by SIRA and the NPL

(National Physical Laboratory).

A Faraday Partnership is an alliance which can include research andtechnology organisations, universities, professional institutes, trade

associations and companies, whose aim is to improve the competitiveness

of UK Industry through the research, development, transfer and exploitationof new and improved science and technology.

It involved a combination of UK universities, potential users

and technology providers and aimed to exploit state-of-the-art

optoelectronic, communications and sensor technology to

develop a system for the real time measurement of strain and

temperature in structures.Project partners

City University

Cranfield University

UK Highways Agency

CorusQinetiQ

National Physical Laboratory (NPL)

EM TechnologyBNFL

The Emerald Research Register for this journal is available at

www.emeraldinsight.com/researchregister

The current issue and full text archive of this journal is available at

www.emeraldinsight.com/0260-2288.htm

Sensor Review

25/2 (2005) 109–113

q Emerald Group Publishing Limited [ISSN 0260-2288]

[DOI 10.1108/02602280510585682]

109

http://www.emeraldinsight.com/researchregister

http://www.emeraldinsight.com/0260-2288.htm

http://www.emeraldinsight.com/0260-2288.htm

http://www.emeraldinsight.com/researchregister



Metronet

SIRA

Mouchel

Lumen Photonics

The system was based on multiplexed fibre Bragg grating

( FB G) sensors and needed to m eet certain critical

requirements, i.e.. multi-point sensing capability;. robust, for use in harsh environments, with effective

sensor protection;. repeatable, stable and accurate;. compensated for temperature effects;. ability to measure static and dynamic strain;. flexible, for periodic or continuous measurements;. remote instrument control and data management, via a

web interface and database system.

A FBG sensor is a section of an optical fibre with a Bragg

grating written into it, that is, a periodic perturbation of the

refractive index of the core of the fibre. The combined strain

and temperature sensing response of the grating is given by a

single combined Bragg wavelength shift which can be

represented by a linear relationship. This contains a numberof thermo-optic and strain-optic coefficients which, when

known for the fibre type used, yields a sensing technique that

is self-calibrating and which allows drift-free, long-term strain

and temperature measurements. Being independent of any

fluctuations in the power of the source, this type of sensor is

ideally suited to long-term monitoring applications. However,

the need to decouple the temperature and strain signals poses

something of a challenge and numerous different approaches

have been tried and described in the literature in recent years.

These include the use of two superimposed FBGs at two

different wavelengths from which the temperature and strain

components of the wavelength shift can be discriminated

simultaneously using two linear equations and using a fibre

Fabry-Perot cavity and an FBG grating in which the FP cavitysenses strain and the Bragg grating measures both the strain

and temperature. In this system a differential technique was

used, whereby an FBG sensor that is subject to the thermal

variations but positioned so as to be free from strain acted as a

compensation element.

A fluorescent fibre was used as the optical source with a

bandwidth of some 20-40 nm from 1,520 to 1,560 nm. The

tuneable filter is a fibre-coupled, free space, Fabry-Perot filter,

actuated by piezoelectric transducers and operated over the

bandwidth of the source at up to 250 scans per second. Light

from the source is filtered by the FP filter and reflected back

from the Bragg gratings in the array, through the optical

couplers, to eight photodiode detectors. These detect the

resulting time-domain spectra of the grating sensors in each of

the serially connected arrays. The overall measurement

scheme is shown in Figure 1 and comprises three sections.

The first is a PC-based computer system with graphical user

interface (GUI) software for real time data visualisation and a

fast serial interface connected to the digital sampling

processor (DSP) system. The synchronous DSP, in turn,

controls and takes data from the optoelectronic system which

is typically connected to the FBG sensor arrays in a

multiplexed sensor system with wavelength division

multiplexing (WDM) architecture.

Prior to field trials, prototype systems were tested

extensively at City University (Plate 1). Critical issues

addressed during this phase were testing the strain response

under both static and dynamic loadings, evaluating various

means of attaching the sensors to the test structures, sensor

protection and evaluating a range of thermal compensation

techniques. Plate 2 shows a strain-isolated temperature sensor

on a steel bridge box section.

A prototype system was tested on the Mjosund road bridge

in Norway (Plate 3). In addition to providing an opportunityto test the sensor system under harsh operating conditions,

the work aimed to assist the Norwegian Roads Authority in

monitoring the many bridges joining the island coastline of

Norway to the mainland. The bridge was a 346 m-long steel

box section structure with a concrete platform carrying the

road access to the bridge. The system was arranged with four

or five FBG sensors per channel giving a total of 32

measurement points with eight parallel channels. Twelve

c onve nt io nal f oi l s tr ai n g au ge s an d a n umb er o f

thermocouples were also installed. The sensor configuration

was such that for each foil gauge, there were two FBG sensors

placed at the same strain point for data verification. Different

static and dynamic loading conditions were applied at

different times over a test period of 18 months and the

results showed that the thermally compensated strain data

obtained optically closely matched the readings from the

resistive gauges to within ,5 m 1 (equivalent to the system

noise). Figure 2 shows the data from the FBG sensors and the

strain gauges when a 50T lorry was driven across the bridge at

30 km/h. The temperature variation on the location of this

bridge ranged from 2408C in the winter to þ258C in

summer, which demonstrated well both the importance and

effectiveness of the temperature compensation technique

used.

One of the most important tests for the system was

monitoring Europe’s first public highway bridge to be

constructed entirely from advanced composites. This is West

Mill bridge in Oxfordshire, UK, which was fabricated from a

new type of composite material of glass and carbon fibre-reinforced polymer. This is a very strong and light material,

which can replace reinforced concrete or steel bridge decks.

The bridge was developed under the four-year, £2.9 million

ASSET (Advanced Structural Systems for Tomorrow’s

Infrastructure) project and was part-funded by the EU and

seven European partners, led by Mouchel, the bridge’s

designers. It was constructed on four longitudinal polymer

beam supports with the composite transverse decking being

fabricated in large extruded profile sections and delivered to

the site for bonding. During the construction stage, prior to

bonding of the composite profiles sections, several sections of

the composite decking and longitudinal composite support

beams were instrumented with 40 FBG sensors, both as

single-axis and rosette gauges w ith tem perature

compensation, strategically placed at pre-selected sites of

maximum strain. These exhibited a strain resolution of ^5 m 1

and an operating range of .^2,000 m 1. As with the

Norwegian trials, a number of resistive strain gauges were

also attached for comparative measurements. The sensors and

the fibre cabling were protected with composite stripes from

transverse strain effects as well as construction site hazards

and silicon compound was applied for moisture protection.

The system was web-enabled so as to provide real time data

over the internet, as shown in Figure 3.

Prior to the bridge being opened to the public in late 2002,

madatary commissioning tests were carried out. These

Structural integrity monitoring with fibre Bragg grating sensors

Robert Bogue

Sensor Review

Volume 25 · Number 2 · 2005 · 109–113

110



controlled loading trials were conducted with a 30T lorry

positioned at various points on the bridge as specified under

the bridge design specifications and were conducted in three

stages. In the first, the lorry was located centrally on the

carriageway and proceeded from one end of the bridge to the

other, stopping momentarily at 1 m steps while continuous

measurements were taken by the FBG sensor system.Resistive strain gauge readings were also taken at every stop.

Two similar tests were then carried out with the lorry wheels

positioned 50 mm from the curb at the left lane, followed by

the same pattern on the right lane. As the lorry progressed

along the bridge, the strain readings were seen to increase

monotonically until the lorry reached the mid section, across

which all the sensors were located, this being followed by a

decrease in the strain as the lorry moved away from the mid

section. The difference in strain measured by each sensor

correlated with the relative position of the sensors and the

loading point. During late 2004 it is hoped to provide a

Figure 1 Schematic of overall measuring system

Plate 1 Prototype system undergoing testing at City University

Plate 3 The Mjosund bridge in Norway

Plate 2 Strain-isolated temperature sensor on a 10 m steel box modelbridge in the laboratory


Robert Bogue

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Volume 25 · Number 2 · 2005 · 109–113

111



Figure 2 Dynamic loading data when a lorry was driven across the Mjosund bridge at 30 km/h (Top trace: FBG sensor data; Lower trace: foil straingauge data)

Figure 3 Schematic of the web-based instrument control and data acquisition system


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Volume 25 · Number 2 · 2005 · 109–113

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constant source of power and a fixed phone line to the system

and start to monitor the bridge continuously. Real time data

will then be available on the web for both the project partners

and the public.

In addition to use on road bridges, it is hoped that the

availability of such a real time structural monitoring system

will benefit the European “Sustainable Bridges” project,

which assesses the suitability of railway bridges to meet thefuture demands of faster trains, increased capacity and heavier

loads. Other anticipated applications by the Intersect project

partners include monitoring the integrity of the concrete

around nuclear reactors by BNFL and detecting cracks in

girders and rail tracks by Corus. A further use of the system is

now being studied under the EC’s 5th Framework project

(RuFUS) Re-use of Foundations for Urban Sites. This

reflects the fact that buildings in major European cities have a

working life of about 25 years and in regional centres about 40

years. It is essential that redevelopment uses as much of the

existing buildings as possible, as by reusing the foundations,

the consumption of raw materials and energy for construction

is reduced, the volume of soil from foundation construction is

virtually eliminated and the construction time significantly

reduced with a consequent reduction in the whole life costing

of a building. Similarly, if a building can be redeveloped for a

change of use without the need for additional or upgraded

foundations, the savings in energy, raw materials and disposal

of spoil can be substantial. It is hoped that the sensor system

will aid the assessment of the integrity of foundations and thusallow their re-use, so speeding up the redevelopment of urban

sites. Trials are already underway in London.

FBG technology is seen by many as being the key to the

widespread, commercial use of optical sensors for multipoint

measurements. Perhaps more than any other, this research has

demonstrated that well-engineered FBG sensor systems can

offer an accurate and reliable means of monitoring strains in

large structures, suggesting a multitude of possible future

uses.

Contact: Dr William Boyle, City University, e-mail: w.j.o.

[email protected]


Robert Bogue

Sensor Review

Volume 25 · Number 2 · 2005 · 109–113

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Structural Integrity Monitoring With Fibre Bragg Grating Sensors

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