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

xiv

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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