APPLICATION OF FIBER BRAGG GRATING SENSORS IN MONITORING
CIVIL ENGINEERING STRUCTURES
CHIN KAI SAN
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Civil-Structure)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
JUNE 2006
v
ABSTRACT
Fiber Bragg grating (FBG) is finding increasing usage in aerospace guidance,
marine structures and civil engineering applications. This is due to their lightweight,
non-obtrusive, immunity to electromagnetic interference, high bandwidth and
sensitivity, and solid-state properties. Analysis of case studies is carried out to study
the applications of Fiber Bragg grating sensors in civil engineering structures. In this
research, a fiber Bragg grating sensing system for strain measurement is being
described. Low cost and simple grating-based FBG has been used to produce the
strain and induce Bragg wavelength shifts. A brief experimental testing on an
instrumented metal plate showed that the proposed system is able to perform strain
measurements with linear response. A general guideline is proposed on the
fabrication and installation of fiber Bragg gratings for structural monitoring in local
conditions.
vi
ABSTRAK
Gentian optik parutan Bragg semakin banyak digunakan dalam bidang
aerospace, struktur marine dan juga bidang kejuruteraan awam. Ini adalah
disebabkan oleh ciri gentian optik yang ringan, kurang kelihatan, terjamin selamat
daripada gangguan elektromagnet dan mempunyai sensitiviti yang tinggi. Analisa ke
atas beberapa kes telah dijalankan untuk menyelidik aplikasi gentian optik parutan
Bragg dalam struktur kejuruteraan awam. Dalam penyelidikan ini, sistem pengesan
gentian optik parutan Bragg untuk mengukur terikan telah digambarkan. Gentian
optik Bragg harga rendah telah digunakan untuk menghasilkan terikan dan
menyebabkan perubahan panjang gelombang Bragg. Kajian yang ringkas telah
dijalankan ke atas plat aluminium dan menunjukkan sistem pengesan terikan yang
dicadangkan berupaya mungukur terikan. Panduan umum mengenai pemprosesan
dan pemasangan gentian optik parutan Bragg telah dicadangkan untuk penggunaan
dalam keadaan tempatan.
vii
TABLE OF CONTENTS
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENT vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF GRAPHS xvii
LIST OF SYMBOLS xviii
CHAPTER TITLE PAGE
1 INTRODUCTION
1.1 General 1
1.2 Significance of Research 2
1.3 Objectives and Scope of Study 3
1.4 Methodology 4
2 LITERATURE REVIEW
2.1 Introduction to Fiber Optic Sensors 5
2.1.1 General 5
2.1.2 Fiber Optic Sensors vs. Conventional
viii
Strain Gauges 6
2.1.3 Applications of Fiber Bragg Gratings
sensors 7
2.1.3.1 Applications to Civil Engineering
Structures 7
2.1.3.2 Applications to Marine Structures 8
2.1.3.3 Aerospace Applications 9
2.2 Types of Fiber Bragg Gratings 10
2.2.1 Common Bragg Reflector 11
2.2.2 Blazed Bragg Grating 12
2.2.3 Chirped Bragg Grating 14
2.3 Photosensitivity Types of Fiber Bragg Gratings 15
2.4 Measurement Parameters of Fiber Bragg Gratings 16
2.4.1 Strain Measurements 17
2.4.2 Temperature Measurements 17
2.4.3 Damage Detection 18
2.4.4 Acoustic Detection 18
2.5 Advantages of Fiber Bragg Gratings 19
2.6 Limitations of Fiber Bragg Gratings 20
2.6.1 Temperature Compensation and Thermal
Strain 20
2.6.2 Signal Conditioning 21
2.6.3 Referencing Errors 21
2.6.4 Packaging 22
2.7 Present State of Art 23
2.7.1 Meissner et. al. (1997) 23
2.7.2 Storoy and Saether (1997) 25
2.7.3 Davis et. al. (1997) 27
3 MATERIAL, FABRICATION AND OPERATION OF FIBER
BRAGG GRATING SENSORS
3.1 Optical Fiber Material 29
3.2 Types of Optic Sensors 30
ix
3.3 Fabrication of Fiber Bragg Grating Sensors 32
3.3.1 Splicing of Optical Fiber 33
3.3.2 Strain-Temperature Discrimination 34
3.4 Fiber Bragg Grating Interrogation Techniques 37
3.5 Multiplexing of Optic Sensors 38
3.5.1 Time Division Multiplexing-TDM 39
3.5.2 Frequency Division Multiplexing–FDM 40
3.5.3 Wave Length Division Multiplexing–WDM 41
4 CASE STUDIES
4.1 General 43
4.2 Beddington Trail Bridge, Calgary, Canada 44
4.2.1 Additional Findings 53
4.3 Taylor Bridge, Winnipeg, Canada 55
4.3.1 Additional Findings 59
4.4 Storck’s Bridge, Winterthur, Switzerland 60
5 EXPERIMENTAL STUDY OF STRAIN MEASUREMENT
5.1 Objectives 66
5.2 Experimental Programme 67
5.2.1 Fiber Optic Cable (FO) 67
5.2.2 Optical Spectrum Analyzer (OSA) 68
5.2.3 Tunable Laser Source 69
5.2.4 Fiber Bragg Grating 70
5.2.5 Test procedure 70
5.3 Experimental Test Result 72
5.4 Theoretical Result 74
5.5 Discussion of the Results 75
x
6 PROPOSED GENERAL GUIDELINE FOR THE
APPLICATION OF FIBER BRAGG GRATING SENSOR
6.1 General 79
6.2 Selection Criteria of Fiber Optic Structural Sensors 80
6.3 Comparative Advantages of Fiber Optic Structural
Sensors 81
6.4 Temperature Compensation of Fiber Optic Sensors 82
6.5 Installation of Fiber Bragg grating Sensors 84
6.5.1 Surface installation on metal structures 85
6.5.2 Installation on or within concrete structures 88
6.5.2.1 Installation on existing concrete
structure 88
6.5.2.2 Installation in new concrete
structure 88
6.6 Workmanship 89
7 CONCLUSION AND RECOMMENDATIONS
7.1 Conclusion 91
7.2 Recommendations 92
REFERENCES 93
xi
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Key differences between Type I, IIA and II Bragg gratings 16
2.2 Major Load States in Destructive Bridge Test 26
5.1 Calculation of Bending Strain 73
5.2 Results of the experiment 73
5.3 Results of calculations for theoretical values 75
5.4 Discrepancies between theoretical and experimental results 76
xii
LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1 Illustration of common Bragg reflector 11
2.2 Band-pass filters 12
2.3 Blazed Bragg grating 13
2.4 Vector diagram of the Bragg condition for the blazed grating 13
2.5 Chirped Bragg grating 14
2.6 Special FBG sensor frame used for monitoring the traffic response
of a bridge crossing the A4 motorway near Dresden 23
2.7 Photograph of the FBG sensor frame shown in Figure 7, at its site
location on the bridge 24
2.8 Demodulation system for the FBG sensor employing a long-period
grating as the wavelength-dependent filter for the passive spectral
ratiometric system 24
2.9 Scanning DFB laser demodulation system for one of the FBG
sensors installed on the Smedstua Bridge 26
2.10 Fiber laser sensor demodulation scheme used for two of the three
FBG sensors 26
2.11 Variation of the strain measured by the FBG sensor read by the
tunable DFB laser, and a reference foil strain gauge, during various
load states of the Smedstua Bridge 27
xiii
FIGURE NO. TITLE PAGE
2.12 Schematic illustration of the FBG sensor demodulation system
used for monitoring the sensor array installed on the bridge 28
2.13 Dynamic response of the sensors in response o the traffic shown
in the video clip insets 28
3.1 Structure of an optical fiber 30
3.2(a) Extrinsic fiber optic sensors 31
3.2(b) Intrinsic fiber optic sensors 31
3.3 Partial listing of extrinsic fiber optic sensors application 31
3.4 Partial listing of intrinsic fiber optic sensors and typical
environments sensing applications 32
3.5 Point-by-point technique for fabricating Bragg gratings 33
3.6 Optical splice connection 34
3.7 A temperature-compensated FBG strain sensor in which half of
the FBG is strain free and respond only to the local temperature 35
3.8 Double-peaked reflection spectrum from the FBG that is partially
bonded to host structure as indicated in Figure 3.7. 36
3.9 A fiber optic temperature-compensated strain measuring approach
using two collocated FBGs operating at two different wavelengths,
850 and 1300 nm 36
3.10 Basic “broadband illumination” fiber Bragg grating-sensor system 37
3.11 Basic filter approach to grating wavelength shift detection 38
3.12 Basic time division multiplexing sensor array 40
3.13 Forward-coupled array concept 40
3.14 FMCW interferometric sensor multiplexing 41
3.15 Wavelength division multiplexed sensor array 42
3.16 FBG reflectometric WDM 42
4.1 Beddington Trail Bridge in Calgary during construction and when
open to the public in 1993 45
4.2 Representative samples of (a) Tokyo Rope’s CFCC and
(b) Mitsubishi Kasei’s Leadline 45
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FIGURE NO. TITLE PAGE
4.3 Variation of the Bragg wavelength for an FBG sensor attached to
a concrete beam with the load applied to that beam in a failure test 46
4.4 Variation of the Bragg wavelength of an FBG sensor attached to
a carbon fiber/PEEK specimen as it is loaded through 320,000
cycles of 0 to -2000 microstrain. The two small insets compare the
FBG transmission spectrumbefore and after fatigue cyclic loading 47
4.5 Careful routing of an optical fiber cable in one of the Beddington
Trail Bridge girders prior to pouring of the concrete 48
4.6 Diagram showing the placement of FBG sensors and prestressing
tendons in one of the tee-bulb precast girders used in the
Beddington Trail Bridge 48
4.7 One of the recessed metal outlet boxes used to house and protect
the optical fiber leads during concrete pour for one of the
Beddington Trail Bridge girders 49
4.8 Testing of the embedded FBG sensor after fabrication of the hot
girder for the Beddingotn Trail Bridge 50
4.9 Two views of the junction box for the fiber optic sensor array of
the Beddington Trail Bridge 50
4.10 Placement of FBG sensor in the Beddington Trail Bridge 51
4.11 Thermally corrected strain relief for the steel(SS), Tokyo Rope
(TR), and Leadline (LL) prestressing tendons used in the
Beddington Trail Bridge and observed over a 18 months period
by the embedded FBG sensors 52
4.12 Temporal variation of the strain measurements presented in
Figure 4.11 53
4.13 Dynamic response of one of the FBG sensors embedded within
a girder of the Beddington Trail Bridge to the passage of a truck
over the bridge 54
4.14 Precast Taylor Bridge I-beam girder, (below) schematic of the
girder and its reinforcement design 55
xv
FIGURE NO. TITLE PAGE
4.15 Schematic of the fiber optic sensing system incorporated into
the Taylor Bridge 56
4.16 Installation of FBG sensors into the CFRP shear reinforcement
frame for one of the precast concrete girders for Taylor Bridge 57
4.17 Installation of FBG sensors into Taylor Bridge barrier wall
Reinforcement frames 57
4.18 Schematic of the 66-FBG sensing system installed in the
Taylor Bridge, its demodulation system, and the communication
system linking it to the ISIS Central Office 58
4.19 Taylor Bridge in Winnepig 59
4.20 Temporal response of one FBG sensor embedded within the
Taylor Bridge to several passes of a truck over the bridge 60
4.21 Storck’s Bridge in Winterthur, Switzerland 61
4.22 Cross section of one of the two 35m CFRP stay cables used on
the Storck’s Bridge 62
4.23 Variation in the strain as measured by several of the FBG sensors
attached to one of the CFRP stay cables of the Storck’s Bridge
during different phases of construction and loading of the bridge 64
4.24 Variation in the strain experienced by one of the CFRP stay cables
of the Storck’s Bridge as measured by a number of sensor during
a period of the day where there is a gradual rise in temperature 65
4.25 Variation in the strain measured by a number of the FBG sensors
attached to one of the CFRP stay cables of the Storck’s Bridge
during hydraulic loading and unloading 65
5.1 Schematic of the experiment setup 67
5.2 Fiber Optic Cable with Connectors to FBG 68
5.3 Optical Spectrum Analyzer 69
5.4 Tunable Laser Source 69
5.5 Fiber Bragg Grating attached on metal plate 70
5.6 Test setup used for the experiment 71
xvi
FIGURE NO. TITLE PAGE
6.1 Temperature compensation FBG strain sensor installed on the
surface of a metal 83
6.2 Environment parameters of storage and operating temperature
range 84
6.3 Fiber optic cable 85
6.4 Weldable fiber optic Bragg grating strain sensor 86
6.5 FBG sensors mounted onto small metal plate welded to a steel rod 86
6.6 Schematic illustration of the six steps described in the text for
Surface installation of a fiber optic strain sensor 87
xvii
LIST OF GRAPHS
GRAPH NO. TITLE PAGE
1 Wavelength, λB vs Strain, ε 77
2 ∆λ/ λB vs Strain, ε 78
xviii
LIST OF SYMBOLS
C - Celcius
cm - centimeters
CW - continuous wave
DFB - Distributed FeedBack
FBG - Fiber Bragg Gratings
FRC - Fiber-reinforced concrete
J - Joule
K - wave vector
kN - kilo Newton
m - meters
MCMV - Mine Counters Measures Vessel
MHz - Mega Hertz
nm - nanometers
OTDR - Optical Time Domain Reflector
RNN - Royal Norwegian Navy
T - temperature
UV - ultraviolet
ε - strain
π - Pie
Λ - period of the grating
λB - Bragg wavelength
Λ0 - starting period
Λ1 - linear change (slope) along the length of the grating
Δn - Ctb ( C = 4.3x10-5, b = 0.32, t is in minutes)
xix
με - micro strain
° - degree
vi - magnitudes of the incident
vs - magnitudes of the scattered
θb - angle with respect to the fiber axis
CHAPTER 1
INTRODUCTION
1.1 General
Strain gauges are important for their ability to measure mechanical strain in
materials and structures. A wide variety of existing methods are used and being
developed for measuring strain and in many situations there is a need to make
contact strain measurements at high temperature.
A number of researchers have developed fiber-optic strain measurement
methods using a wide variety of optical sensor concepts since the late 1970s which
includes fiber Bragg gratings. Fiber Bragg gratings have a potential to operate in
high temperature environments, provided the fiber is suitably protected. Several
different optical sensing techniques have found their way into the market place but
fiber Bragg gratings (FBGs) are commercially one of the most successful. It is
finding increase usage in aerospace guidance, marine structures and civil engineering
applications. This is due to their lightweight, non-obtrusive, immunity to
electromagnetic interference, high bandwidth and sensitivity, and solid-state.
The usage of fiber Bragg gratings in civil engineering applications include vibrations
sensors, damage detection, and strain, temperature and electromagnetic field sensors.
2
The development of modern optical-based sensors like fiber Bragg gratings
sensors and its usage can lead to real time measurements, monitoring the formation
and growth of defects. Optical fiber sensors also allow for data to be transmitted over
long distances to a central monitoring location. The main advantage of optical fiber
in civil engineering applications is that they may either be attached to an existing
structure or embedded into concrete decks and supports.
There are several types of fiber Bragg gratings which includes common
Bragg reflector, the blazed Bragg grating, and the chirped Bragg grating. Different
types of Bragg gratings show different characteristics and provide different
applications to civil engineering structures.
1.2 Significance of Research
There is a growing concern over the state of civil infrastructure in countries
over the world. High fraction of infrastructure is deficient and deteriorating which
leads to a high demand of maintenance and rehabilitation in civil engineering
structure. Owing to the harsh environment found in the construction industry and the
large size of civil structures, conventional methods like electrical strain gauges have
found to be less efficient in real time measurement. Therefore, techniques have
recently been researched that allow the benefits of optical precision while
minimizing the disadvantages of conventional instruments. Fiber optic based sensors
especially fiber Bragg gratings offers excellent means of a stable strain measurement
for long term monitoring. However, there are little knowledge and applications of
fiber Bragg grating sensors in civil engineering in this region. Fiber Bragg gratings
base sensors are expected to be of significant influence on the development of
structural health monitoring techniques.
3
1.3 Objectives and Scope of Study
The objectives and scope of the study are:
1. To study the material properties, working principles and
instrumentation requirements of fiber Bragg gratings sensors.
2. To study the applications of fiber Bragg gratings in structural
monitoring.
3. To carry out comparison study of different application of fiber
Bragg gratings in bridge structures.
4. To propose a general guidelines on application of fiber Bragg
gratings for structural monitoring in local conditions.
4
1.4 Methodology
Step 1:
Search for literature review to study on fiber Bragg
grating in details.
Step 2:
Identify the suitable case studies which involved the
applications of fiber Bragg grating sensors in civil
engineering structures.
Step 3:
Analyze the selected case studies on the advantages,
technical issues and selection criteria of the use of fiber
Bragg grating sensors.
Step 4:
Compare each selected cases and evaluate the results
obtained.
Step 5:
Propose a guideline on application of fiber Bragg
gratings for structural monitoring in local conditions.
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