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]
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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
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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
Structural integrity monitoring with fibre Bragg grating sensors
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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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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.
Structural integrity monitoring with fibre Bragg grating sensors
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