nondestructive testing of bridge and railroad …wvuscholar.wvu.edu/reports/nandam_krishna.pdf ·...

Nondestructive Testing of Bridge and Railroad

Components Using Ultrasonic and Infrared

Thermography Techniques

Krishna Chandra Nandam

Problem Report submitted to the

College of Engineering and Mineral Resources

at West Virginia University

in partial fulfillment of the requirements

for the degree of

Master of Science

in

Civil Engineering

Udaya B. Halabe, Ph.D., P.E., Chair

Hema J. Siriwardane, Ph.D., P.E.

Powsiri Klinkhachorn, Ph.D.

Department of Civil and Environmental Engineering

Morgantown, West Virginia

2010

Keywords: Ultrasonics, Infrared Thermography, FRP, Composites, Bridges

ABSTRACT

Nondestructive Testing of Bridge and Railroad Components using Ultrasonic

and Infrared Thermography Techniques

Krishna Chandra Nandam

This study focuses on the review and application of ultrasonic and infrared

thermography techniques for detecting subsurface defects in bridge structural components.

The study has also been extended to include application of infrared thermography for

detecting subsurface defects in timber railroad ties encased in Fiber Reinforced Polymer

(FRP) composites.

Ultrasonic technique is a very effective nondestructive technique for detecting defects

in metallic bridge components since ultrasonic waves propagate very effectively through

metals. The ultrasonic technique has also been used for condition assessment of concrete

bridge decks. Infrared thermography has proved to be an effective nondestructive technique

for the detection of internal voids, delaminations and cracks in concrete and composite

structures such as bridge decks, highway pavements, and FRP composite railroad ties.

This problem report presents a comprehensive review of recent advances in the

application of ultrasonic and infrared thermography techniques for condition assessment

bridge components. In addition, the report investigates the use of digital infrared

thermography on FRP composite encased timber railroad ties under the laboratory conditions

before and after conducting static and fatigue (rupture) loading tests. The infrared

thermography test before loading was conducted to evaluate the presence of subsurface

defects in the ties just after the manufacturing process. Subsequent infrared thermography

testing of the railroad ties was conducted after load testing of the ties to assess if any defects

were formed during the loading tests. In addition, field infrared thermography testing was

conducted to assess the condition of in-place composite ties subjected to railroad loading.

The infrared tests were conducted with the help of a commercially available infrared camera,

which was a light weight model with low cost. This low cost model performed very well and

produced good data quality for subsurface defect detection. This shows that the infrared

thermography based field inspection is now quite affordable.

iii

ACKNOWLEDGEMENTS

I would like to gratefully acknowledge the support, encouragement and assistance

given by my advisor and a great teacher, Dr. Udaya B. Halabe, in completing this

research. Working under his supervision was one of the best things that happened to me

in this research. His knowledge in the field of nondestructive testing was invaluable.

I sincerely thank Dr. Hema J. Siriwardane and Dr. Powsiri Klinkhachorn for

serving as members of my Advisory and Examining Committee (AEC). I would also like

to thank Dr. P.V. Vijay and Dr. Hota GangaRao for their help and support in getting the

field data for my research.

I gratefully acknowledge the educational experience and financial support given

by the Department of Civil Engineering at West Virginia University and the U.S.

Department of Transportation - Federal Railroad Administration, during my Master's

degree program and research.

I would like to dedicate this problem report to my family for their love, support

and encouragement provided by them.

iv

TABLE OF CONTENTS

ABSTRACT .................................................................................................................. ii

ACKNOWLEDGEMENTS......................................................................................... iii

TABLE OF CONTENTS .............................................................................................iv

LIST OF FIGURES ................................................................................................... viii

LIST OF TABLES .....................................................................................................xvii

1 INTRODUCTION .................................................................................................. 1

1.1 Background .................................................................................................................... 1

1.2 Research objectives ........................................................................................................ 3

1.3 Scope ............................................................................................................................. 3

1.4 Report organization ........................................................................................................ 3

2 LITERATURE REVIEW: ULTRASONICS ........................................................ 5

2.1 A fundamental study on detection of defects in the web gap region of steel plate girder

bridges by the plate wave ultrasonic technique [Shirhata et al. 2004] ..................................... 5

2.1.1 Experiment .......................................................................................................... 6

2.1.2 Experimental results ............................................................................................ 7

2.1.3 Numerical analysis ............................................................................................. 10

2.1.4 Summary ........................................................................................................... 12

2.2 Ultrasonic inspection of bridge hanger pins [Benjamin et al. 2000] ............................... 12

2.2.1 Bridge description .............................................................................................. 13

2.2.2 Field ultrasonic inspection.................................................................................. 14

2.2.3 Immersion tank ultrasonic testing ...................................................................... 15

2.2.4 Comparison of field and immersion tank ultrasonic results ................................ 18

2.2.5 Conclusions ........................................................................................................ 18

2.3 Ultrasonic underside inspection for fatigue cracks in the deck plate of a steel orthotropic

bridge deck [Bakker et al. 2003] ............................................................................................ 18

2.3.1 Bridge deck specimen and fatigue set-up ........................................................... 19

2.3.2 Optimized ultrasonic measurement methods ..................................................... 21

2.3.3 Visual and ultrasonic inspection results .............................................................. 22

2.3.4 Conclusions ........................................................................................................ 28

v

2.4 Automatic ultrasonic testing (AUT) [Miki et al. 2005] .................................................... 29

2.4.1 First round robin test ......................................................................................... 29

2.4.2 Second round robin test ..................................................................................... 33

2.4.3 Summary: .......................................................................................................... 35

2.5 Ultrasonic wireless health monitoring system for near real-time damage identification of

structural components [Banerjee 2008] ................................................................................ 35

2.5.1 Near real-time wireless health monitoring platform ........................................... 35

2.5.2 Results from the new SHM platform .................................................................. 36

2.5.3 Statistical damage index (SDI) approach ............................................................. 37

2.5.4 Experimental results .......................................................................................... 38

2.5.5 Conclusions ........................................................................................................ 41

2.6 Ultrasonic guided waves for NDE of sign support structures [Xuan et al. 2009] ............. 41

2.6.1 Experimental setup ............................................................................................ 41

2.6.2 Experimental results .......................................................................................... 44

2.6.3 Conclusions ........................................................................................................ 45

2.7 Defect detection and imaging using focused ultrasonic guided waves [Sicard et al. 2007]

45

2.7.1 Conventional phased array theory ..................................................................... 45

2.7.2 Lamb wave phased array.................................................................................... 46

2.7.3 Wedge considerations for lamb waves ............................................................... 46

2.7.4 Effects of the type of excitation.......................................................................... 48

2.7.5 Multiple reflectors ............................................................................................. 49

2.7.6 Conclusion ......................................................................................................... 50

2.8 Ultrasonic wave velocity signal interpretation of simulated concrete bridge decks

[Toutanji 2000] ..................................................................................................................... 50

2.8.1 Experimental procedure..................................................................................... 50

2.8.2 Results ............................................................................................................... 52

2.8.3 Conclusions ........................................................................................................ 57

2.9 Nonlinear ultrasonic testing on a laboratory concrete bridge deck [Shannon et al. 2006]

57

2.9.1 Experiments ....................................................................................................... 58

2.9.2 Results ............................................................................................................... 59

2.9.3 Conclusions ........................................................................................................ 60

vi

2.10 Application of ultrasonic methods for early age concrete characterization [Mikulic et

al. 2005] ................................................................................................................................ 61

2.10.1 Measurements based on velocity of longitudinal waves ..................................... 61

2.10.2 Measurements based on velocity of transversal waves ...................................... 63

2.10.3 Reflection of elastic waves measurement .......................................................... 63

2.10.4 Conclusions ........................................................................................................ 66

2.11 Fatigue crack detection in metallic members using ultrasonic rayleigh waves with time

and frequency analyses [Halabe and Franklin 1997] .............................................................. 67

2.11.1 Analysis of ultrasonic signals .............................................................................. 67

2.11.2 Measurement conditions ................................................................................... 68

2.11.3 Results and discussions ...................................................................................... 70

2.11.4 Conclusions ........................................................................................................ 77

2.12 Ultrasonic diagnostic load testing of steel highway bridges [Mandracchia 1996] ........ 77

2.12.1 Measurement Strategy ...................................................................................... 78

2.12.2 Productization.................................................................................................... 79

2.12.3 Laboratory test results ....................................................................................... 79

2.12.4 Diagnostic load test results ................................................................................ 80

2.12.5 Cyclic loading ..................................................................................................... 82

2.12.6 Conclusion ......................................................................................................... 83

2.13 Ultrasonic instrumentation for measuring applied stress on bridges [Fuchs et al. 1998]

83

2.14 Summary ................................................................................................................... 85

3 LITERATURE REVIEW: INFRARED THERMOGRAPHY ........................... 86

3.1 Effects of solar loading on infrared imaging of subsurface features in concrete [Washer

et al. 2010] ............................................................................................................................ 86

3.2 Detection of subsurface defects in fiber reinforced polymer composite bridge decks

using digital infrared thermography [Halabe et al. 2007] ....................................................... 88

3.3 Introducing infrared thermography in dynamic testing on reinforced concrete structures

[Luong and Dang-Van 2005] .................................................................................................. 92

3.4 Infrared thermography and void detection in GRP laminates [Allinson 2007] ................ 94

3.5 Use of infrared thermography for quantitative non-destructive evaluation in FRP

strengthened bridge systems [Kumar and Karbhari 2002] ..................................................... 95

3.6 Conclusions .................................................................................................................. 98

vii

4 INFRARED THERMOGRAPHY TESTING ................................................... 100

4.1 Laboratory testing, analysis and results ...................................................................... 100

4.1.1 Infrared tests of the railroad tie subjected to static loading .............................. 103

4.1.2 Infrared tests of the railroad tie subjected to fatigue loading ........................... 113

4.2 Field testing, analysis and results (May 14, 2010) ....................................................... 127

4.3 Conclusions ................................................................................................................ 138

5 CONCLUSIONS AND RECOMMENDATIONS ............................................. 140

5.1 Conclusions ................................................................................................................ 140

5.2 Recommendations for future research ....................................................................... 141

REFERENCES .......................................................................................................... 142

viii

LIST OF FIGURES

Figure 2.1 Fatigue cracking and monitoring system [Shirhata et al. 2004] ................................................ 5

Figure 2.2 Dispersion curves and excitation zone [Shirhata et al. 2004] ................................................... 5

Figure 2.3 Girder specimen [Shirhata et al. 2004] ..................................................................................... 6

Figure 2.4 Experimental setup [Shirhata et al. 2004] ................................................................................ 7

Figure 2.5 Wave forms of steps 1 and 2 [Shirhata et al. 2004] .................................................................. 8

Figure 2.6 Definition of deviation [Shirhata et al. 2004] ........................................................................... 8

Figure 2.7 Deviation of steps 1 and 2 [Shirhata et al. 2004] ...................................................................... 9

Figure 2.8 Wave forms of steps 3 and 4 [Shirhata et al. 2004] .................................................................. 9

Figure 2.9 Wave forms of steps 5 and 6 [Shirhata et al. 2004] ................................................................ 10

Figure 2.10 Three dimensional Finite Element Models [Shirhata et al. 2004] ......................................... 11

Figure 2.11 Wave forms of numerical models (a) through (e) [Shirhata et al. 2004] ............................... 11

Figure 2.12 Defect echo height [Shirhata et al. 2004] ............................................................................. 11

Figure 2.13 Schematic diagram of an in-service pin-and hanger connection prior to pin removal

[Benjamin et al. 2000] ............................................................................................................................ 12

Figure 2.14 Photographs of hanger pin prior to and following removal from bridge [Benjamin et al.

2000] ...................................................................................................................................................... 13

Figure 2.15 Cracked section results from field ultrasonic inspection [Benjamin et al. 2000] ................... 14

Figure 2.16 Immersion tank ultrasonic inspection of a hanger pin [Benjamin et al. 2000] ...................... 15

Figure 2.17 Ultrasonic pulse showing beam spread around the pin shoulder and defects under pin

shoulders at the shear plane level [Benjamin et al. 2000] ...................................................................... 16

Figure 2.18 Ultrasonic pulse showing beam spread around the pin shoulder and defects under the pin

exterior face at the shear plane level [Benjamin et al. 2000] ................................................................. 17

Figure 2.19 Cscan images of defect indications in hanger pins S106 and S108 [Benjamin et al. 2000] .... 17

Figure 2.20 Cracked section results from immersion tank ultrasonic inspection [Benjamin et al. 2000] . 18

Figure 2.21 Left: Construction of a steel orthotropic bridge deck. Right: Bottom view on the bridge deck

specimen and its rib welds [Bakker et al. 2003]...................................................................................... 19

Figure 2.22 Vertical cross section of the crack initiation zone [Bakker et al. 2003] ................................. 19

Figure 2.23 The fatigue set-up with loading at ribs 1 and 3 [Bakker et al. 2003] ..................................... 20

Figure 2.24 Pitch-catch technique set-up for crack depth detection. One of these inspection areas is

indicated by a circle in Figure 2.21 [Bakker et al. 2003] .......................................................................... 21

Figure 2.25 Vertical cross section of the pitch-catch set-up [Bakker et al. 2003]..................................... 21

Figure 2.26 Pulse-echo set-up for crack length detection [Bakker et al. 2003] ........................................ 22

Figure 2.27 Vertical cross section of the pulse-echo set-up [Bakker et al. 2003] ..................................... 22

ix

Figure 2.28 Average peak amplitude in the pitch-catch scans as a crack indicator [Bakker et al. 2003] .. 24

Figure 2.29 Average, normalized wave count in the pitch-catch scans as a crack indicator [Bakker et al.

2003] ...................................................................................................................................................... 25

Figure 2.30 Peak amplitude in the 70 deg probe pulse-echo scans as a crack length indicator [Bakker et

al. 2003] ................................................................................................................................................. 26

Figure 2.31 Peak amplitude in the 45 deg probe pulse-echo scans as a crack length indicator [Bakker et

al. 2003] ................................................................................................................................................. 27

Figure 2.32 Specimens of the round robin test [Miki et al. 2005] ........................................................... 30

Figure 2.33 Data to be submitted (Miki et al. 2005) ............................................................................... 31

Figure 2.34 Destructive test (Miki et al. 2005) ........................................................................................ 31

Figure 2.35 Results of the first performance test (a) 40mm (b) 60mm (c) 80mm (d) 100mm and (e) all the

specimens, disregard level L/2 [Miki et al. 2005] .................................................................................... 32

Figure 2.37 Results of the performance of the second test, data submitted after one week, (a) 40 mm,

(b) 60mm, (c) 80 mm, (d) 100 mm, and (e) all the specimens [Miki et al. 2005]...................................... 34

Figure 2.36 Results of the performance of the second test, data submitted on the day of testing, (a) 40

mm, (b) 60mm, (c) 80 mm, (d) 100 mm, and (e) all the specimens (Miki et al. 2005) .............................. 34

Figure 2.38 The health monitoring system consisting of preamplifiers, amplifiers and data

sampling/processing unit [Banerjee 2008] ............................................................................................. 36

Figure 2.39 Photo of the bonded piezoelectric sensors used for the test. The function generator is used

to transmit a sine source [Banerjee 2008] .............................................................................................. 36

Figure 2.40 Sample acquired data showing the expected time delay as the receiver distance from the

transmitter increases [Banerjee 2008] ................................................................................................... 37

Figure 2.41 The experimental setup used to test the functionality of the SDI approach [Banerjee 2008]38

Figure 2.42 Layout of the test for establishing correlation between excitation frequency and the damage

size [Banerjee 2008] ............................................................................................................................... 39

Figure 2.43 (a) Layout of the test for the damage identification and localization in a composite plate; (b)

Display of measurement paths with top 5 statistic t values [Banerjee 2008].......................................... 40

Figure 2.44 Display of the measurement paths with top statistic t values showing the approximated

identified location of the notches [Banerjee 2008] ................................................................................ 40

Figure 2.45 (a) Photo of the sign support structure tested at the University of Pittsburgh. (b) Sketch of

the truss cross section. (c): Sketch and dimensions of the truss’ chords *Xuan et al. 2009+ .................... 42

Figure 2.46 Location and relative distance of PZTs C0 to C10. Dimensions are in millimeters [Xuan et al.

2009] ...................................................................................................................................................... 42

Figure 2.47 Time waveforms generated by actuator C0 and (a) detected by sensor C1 and (b) detected

by sensor C5 [Xuan et al. 2009] .............................................................................................................. 42

x

Figure 2.48 (a) Scheme of the truss and location of the joint under monitoring (b) Photo of the hydraulic

actuator and the steel beam that transfers the load onto the truss (c) Orientation of the crack with

respect to the longitudinal axis of the chord [Xuan et al. 2009] ............................................................. 43

Figure 2.49 Crack size as a function of the number of cycles [Xuan et al. 2009] ...................................... 43

Figure 2.50 (a): Time waveforms generated by C0 and detected by C5 at 0 and 100 000 cycles; (b): Close-

up view of (a); (c): Time waveforms generated by C5 and detected by C0 at 0 and 100 000 cycles; (d)

Close-up view of (c) [Xuan et al. 2009] ................................................................................................... 44

Figure 2.51 Correlation coefficients as a function of number of cycles (a) Actuator C10 and receivers C6,

C7 and C8; actuator C6 acted and receivers C8, C9 and C10. (b) Actuator C0 and receivers C4 and C5;

actuator C5 and receivers C0 and C1 [Xuan et al. 2009] .......................................................................... 44

Figure 2.52(a) Illustration of a conic wave beam projected on the plate and the subsequent lamb wave

beam for incidence angles of (a) 20° and (b) 40°. The filled section in the wedge corresponds to a

constant incidence angle within the incident beam, while the flat filled area corresponds to the Lamb

wave field [Sicard et al. 2007] ................................................................................................................ 47

Figure 2.53 (a) Illustration of the assumed wave propagation path for time-of-flight calculations in (a)

2D and (b) 3D [Sicard et al. 2007] ........................................................................................................... 47

Figure 2.54 Recorded echoes from the pulse-echo inspection of a 3 mm through hole in a 6.12 mm, 1018

steel plate using the A} mode for (a) experimental data and (b) simulated data. Dashed line: time-of-

flight curve of the center frequency component. Solid line: Phased array time delays curve plotted with

a temporal offset corresponding to the time-of-flight of the element closest to the defect [Sicard et al.

2007] ...................................................................................................................................................... 48

Figure 2.55 Results of Figure 2.54, time shifted according to the computed phased array delays for (a)

experimental data and (b) simulated data. Dashed line: time-of-flight corresponding to the focal spot

position using the center frequency component [Sicard et al. 2007] ...................................................... 48

Figure 2.56 Comparison of focusing using tone burst and spike excitation, (a) Experimental frequency

spectrum (solid line: 9 cycle tone burst at 810kHz ; dotted line: spike ; dashed lines: theoretical

frequencies computed from Snell's law), (b) Focusing result on a though hole using, from left to right,

tone burst excitation (S1), and spike excitation filtered to isolate S1 (same parameters as for the

toneburst), S1 (delays computed for the principal frequency component), and A1. The lateral scale of

each image is identical [Sicard et al. 2007] ............................................................................................. 49

Figure 2.57 Comparison of focusing of the Ai mode in a 1.82mm 302 stainless steel plate with 5

machined FBH (diameter of 1.5mm, 50% depth), (a) Configuration; (b) B-Scan ; (c) Depth focusing at 47

mm ; (d) Dynamic Depth Focusing (DDF) from 40mm to 60mm (step of 2mm) [Sicard et al. 2007] ........ 49

Figure 2.58 Details of test specimens [Toutanji 2000] ............................................................................ 51

xi

Figure 2.59 Time domain responses of concrete slabs with various types of artificial cracks (direct

method) [Toutanji 2000] ........................................................................................................................ 53

Figure 2.60 Frequency domain responses using the direct method of concrete specimens with different

artificial cracks [Toutanji 2000] .............................................................................................................. 54

Figure 2.61 Frequency domain responses of concrete slabs with Plexiglas simulated cracks, measured 15

cm in diameter and 0.9 cm in thickness, placed at different depth (direct depth) [Toutanji 2000] ......... 55

Figure 2.62 shows the amplitude spectrum of simulated crack with plexiglas discs placed at different

depths [Toutanji 2000] ........................................................................................................................... 55

Figure 2.63 Frequency domain responses of concrete slabs with no cracks (solid lines) and with vertical

cracks (dashed lines) at different depth, using the indirect method [Toutanji 2000] .............................. 56

Figure 2.64 Plan view of testing and cut girder locations [Shannon et al. 2006] ..................................... 58

Figure 2.65 View of transducer configurations from bottom of deck [Shannon et al. 2006] ................... 58

Figure 2.66 Average percent change in second harmonic ratio, A2/A12

[Shannon et al. 2006] ................. 59

Figure 2.67 Average percent change in third harmonic ratio A3/A13

[Shannon et al. 2006] ..................... 60

Figure 2.68 Ultrasound measurements and setting time determination by means of penetrometer

[Mikulic et al. 2005] ............................................................................................................................... 62

Figure 2.69 Velocity in time for (a) concrete without admixture (mixture I) (b) concrete with addition of

retarder (mixture II) [Mikulic et al. 2005] ............................................................................................... 62

Figure 2.70 Velocity of P waves dependent on setting and hardening process for (a) different admixtures

[Grosse et al. 2003] and (b) different w/c ratios [Herb et al. 1999] ........................................................ 63

Figure 2.71 Transmitted energy and velocity of P waves during cementitious mortar setting [Grosse et

al.2001] .................................................................................................................................................. 63

Figure 2.72 Schematic representation of device for determination of young concrete properties [Mikulic

et al. 2005] ............................................................................................................................................. 64

Figure 2.73 First and second reflection of measured signal [Mikulic et al. 2005] .................................... 64

Figure 2.74 Reflection loss and temperature of concrete during the hydration process development

[Shah et al. 1999] ................................................................................................................................... 65

Figure 2.75 Reflection loss as a function of concrete compressive strength [Akkaya et al. 2003] ........... 65

Figure 2.76 Decrease of a WRF factor with time [Voight et al. 2001] ...................................................... 66

Figure 2.77 Reflection loss as a function of degree of hydration for different w/c ratio samples [Sun

Zhihui et al. 2004] .................................................................................................................................. 66

Figure 2.78 Correlation between the reflection loss and decrease of capillary porosity [Sun Zhihui et al.

2004] ...................................................................................................................................................... 66

Figure 2.79 Power spectral density magnitude plot for various couplants [Halabe and Franklin 1997] .. 69

xii

Figure 2.80 Specimens for fatigue crack detection: (a) uncracked; (b) microfatigue crack 0.025 mm thick

(c) macrofatigue crack 1 mm thick (d) saw cut 1 mm thick [Halabe and Franklin 1997] .......................... 71

Figure 2.81 Time domain signals obtained (a) uncracked section; (b) microfatigue crack [Halabe and

Franklin 1997] ........................................................................................................................................ 71

Figure 2.82 Power spectral density plots (a) uncracked section; (b) microfatigue crack [Halabe and

Franklin 1997] ........................................................................................................................................ 72

Figure 2.83 Integral of power spectral density magnitude plots obtained from through transmission

using rayleigh waves produced by ten cycle sine pulse on fatigue specimens [Halabe and Franklin 1997]

............................................................................................................................................................... 73

Figure 2.84 Time domain signals obtained from pulse echo using rayleigh waves produced by five cycle

sine pulse on fatigue specimens: (a) uncracked section; (b) microfatigue crack [Halabe and Franklin

1997] ...................................................................................................................................................... 73

Figure 2.85 Integral of power spectral density magnitude plots for back echo obtained from pulse echo

using rayleigh waves produced by five cycle sine pulse on fatigue specimens [Halabe and Franklin 1997]

............................................................................................................................................................... 74

Figure 2.86 Long Beam Specimen [Halabe and Franklin 1997] ................................................................ 75

Figure 2.87 Time Domain signals obtained from pulse echo using rayleigh waves produced by five cycle

sine pulse on long beam specimen: (a) crack near sensor; (b) crack far from sensor [Halabe and Franklin

1997] ...................................................................................................................................................... 76

Figure 2.88 Integral of power spectral density magnitude plots for back echo obtained from pulse echo

using rayleigh waves produced by five cycle sine pulse on long beam specimen [Halabe and Franklin

1997] ...................................................................................................................................................... 77

Figure 2.89 Acoustic strain Gauge [Mandracchia 1996] .......................................................................... 79

Figure 2.90 Sensitivity Factor [Mandracchia 1996] ................................................................................. 80

Figure 2.91 Acoustic vs. Resistance Strain Gauge Correlation [Mandracchia 1996] ................................ 82

Figure 2.92 Time series strip chart (100 seconds) [Mandracchia 1996] ................................................... 82

Figure 2.93 Rainflow (20 minutes) [Mandracchia 1996] ......................................................................... 83

Figure 2.94 Test vehicle traveling at 8 Km/h (5 mph): EMAT (bottom), strain gage (top) [Fuchs et al.

1998] ...................................................................................................................................................... 84

Figure 2.95 Test vehicle traveling at 89 Km/h (55 mph): EMAT (bottom), strain gage (top) [Fuchs et al.

1998] ...................................................................................................................................................... 85

Figure 3.1 Diagram of concrete test block with embedded targets at depths of (A) 25mm, (B) 51mm, (C)

76mm, and (D) 127mm [Washer et al. 2010] .......................................................................................... 86

Figure 3.3 Thermal contrast at embedded targets for sunny day 12/25/07, indicating contrasts for 25,

51, 76, and 127mm targets relative to the solar load [Washer et al. 2010]............................................. 87

xiii

Figure 3.2 Thermal image of the test block showing embedded targets at depths of 25, 52 and 76mm

[Washer et al. 2010] ............................................................................................................................... 87

Figure 3.4 Experimental setup using digital infrared camera and close up view of the camera [Halabe et

al. 2007] ................................................................................................................................................. 89

Figure 3.5 Front and cross-sectional views of the GFRP bridge deck specimens (a) without wearing

surface overlay, and (b) with wearing surface overlay [Halabe et al. 2007] ............................................ 89

Figure 3.6 Photographs showing the location of the delaminations [Halabe et al.2007] ........................ 90

Figure 3.7 Infrared image of the specimen with air-filled delaminations at the flange-flange junction

[Halabe et al. 2007] ................................................................................................................................ 90

Figure 3.8 Digital photographs and corresponding infrared images of various debonded areas in the

GFRP Bridge deck [Halabe et al. 2007].................................................................................................... 91

Figure 3.9 Infrared thermography of a plain concrete specimen subject to compressive vibrations

(temperature changes are given in degrees Celsius) [Luong and Dang-Van 2005] .................................. 92

Figure 3.10 Graphical determination of the fatigue limit FL of a plain concrete (dissipation is given in

degrees Celsius proportional to energy) [Luong and Dang-Van 2005] .................................................... 93

Figure 3.11 Infrared thermographic determination of dissipation caused by plasticity of steel

reinforcements (temperature changes are given in degrees Celsius) [Luong and Dang-Van 2005] ......... 93

Figure 3.12 Infrared thermographic determination of dissipation by slippage of steel reinforcements

embedded in the concrete matrix (temperature changes are given in degrees Celsius) [Luong and Dang-

Van 2005] ............................................................................................................................................... 94

Figure 3.13 (a) Exterior surface of the gel coat is shiny and uniform. No suspicious signs of deformity or

voids in the Gel Coat as seen with unaided eye (b) Thermal pattern as displayed by the Infrared Camera

while the hull is being gently warmed by the electric hot air gun [Allinson 2007] .................................. 95

Figure 3.14 (a) Schematic of deck-girder assembly (b) Test setup [Kumar and Karbhari 2002] ............... 96

Figure 3.15 (a) Representation of defect type 1 (no progression) (b) Representation of type 2 defects (c)

Type 3 defect representative of debonding at the composite-concrete interface (d) Type 3 defect

representative of debonding initiating between the transverse and longitudinal FRP strips and

continuing to the concrete-compoite interface (e) Representation of a defect at a localized crack

opening (f) Defect representative of debonded area at the intersection of the longitudinal and

tansverse FRP strip (g) Thermographic representation of interlaminar debonding. Note: 846-0 kN

indicates that the inspection was carried out after the specimen was unloaded after loading to 846 kN

load cycle [Kumar and Karbhari 2002] .................................................................................................... 98

Figure 4.1 Schematic of the composite tie ............................................................................................ 100

Figure 4.2 FRP encased wooden railroad ties being tested under static and fatigue loading conditions101

Figure 4.3 Shop heater ......................................................................................................................... 101

xiv

Figure 4.4 InfraCAM SD™ infrared camera ........................................................................................... 102

Figure 4.5 FRP composite encased railroad tie subjected to static load [Chada 2011] .......................... 103

Figure 4.6 First part of the bottom surface of the composite railroad tie and its infrared image before

conducting the static load test ............................................................................................................. 104

Figure 4.7 Middle part of the bottom surface part of the composite railroad tie and its infrared image

before conducting the static load test .................................................................................................. 105

Figure 4.8 Last part of the bottom surface part of the composite railroad tie and its infrared image

before conducting the static load test .................................................................................................. 106

Figure 4.9 First part of the top surface of the composite railroad tie and its infrared image before


Figure 4.10 Middle part of the top surface of the composite railroad tie and its infrared image before


Figure 4.11 Last part of the top surface of the composite rail road tie and its infrared image before


Figure 4.12 First part of the bottom surface of the composite railroad tie after and its infrared image

after conducting the static loading test ................................................................................................ 109

Figure 4.13 Middle part of the bottom surface part of the composite railroad tie and its infrared image

after conducting the static loading test ................................................................................................ 109

Figure 4.14 Last part of the bottom surface of the composite railroad tie and its infrared image after

conducting the static loading test ........................................................................................................ 110

Figure 4.15 First part of the top surface of the composite railroad tie and its infrared image after


Figure 4.16 Middle part of the top surface of the composite railroad tie and its infrared image after


Figure 4.17 Last part of the top surface of the composite railroad tie and its infrared image after


Figure 4.18 FRP composite encased railroad tie subjected to fatigue load [Chada 2011] ...................... 113

Figure 4.19 First part of the top surface of the composite railroad tie and its infrared image before

conducting the fatigue test .................................................................................................................. 114

Figure 4.20 Middle part of the top surface of the composite railroad tie and its infrared image before


Figure 4.21 Last part of the top surface of the composite railroad tie and its infrared image before


Figure 4.22 First part of the bottom surface of the composite railroad tie and its infrared image before


xv

Figure 4.23 Middle part of the bottom surface of the composite railroad tie and its infrared image

before conducting the fatigue test ....................................................................................................... 117

Figure 4.24 Last part of the bottom surface of the composite railroad tie and its infrared image before



running the fatigue test for half million cycles ..................................................................................... 119

Figure 4.26 Second part of the top surface of the composite railroad tie and its infrared image after


Figure 4.27 Third part of the top surface of the composite railroad tie and its infrared image after


Figure 4.28 Last part of the top surface of the composite railroad tie and its infrared image after running

the fatigue test for half million cycles .................................................................................................. 121


completing one million loading cycles in the fatigue load test ............................................................. 122








completing one and half million loading cycles in the fatigue load test ................................................ 125







Figure 4.37 (a) #1 and #2 ties (b) #3, #4 and #5 ties (c) #6 and #7 ties [Srinivas 2010]........................... 128

Figure 4.38 Part of the #1 tie between the rails and its corresponding infrared image ......................... 128

Figure 4.39 Part of the #1 tie on the outer side of the right rail and its corresponding infrared image . 129



Figure 4.42 Part of the #2 tie on the outer side of the left rail and its corresponding infrared image ... 131


xvi










xvii

LIST OF TABLES

Table 2.1 Experiments [Shirhata et al. 2004] ............................................................................................ 7

Table 2.2 Number of Fatigue cycles before inspections 0 to 7 [Bakker et al. 2003] ................................. 22

Table 2.3 Ultrasonically determined crack lengths for inspections 4 to 7 [Bakker et al. 2003] ................ 28

Table 2.4 Testing conditions of AUT systems of the first round robin test [Miki et al. 2005] .................. 30

Table 2.5 Testing conditions of AUT systems of the second round robin test. [Miki et al. 2005]............. 33

Table 2.6(a) SDI calculations for 3mm hole (b) SDI calculations for 6 mm hole [Banerjee 2008] ............. 39

Table 2.7 Summary of specimen configurations [Toutanji 2000] ............................................................ 51

Table 2.8 Results of estimated crack sizes [Toutanji 2000] ..................................................................... 57

Table 2.9 Comparison of NLUT and pulse velocity [Shannon et al. 2006]................................................ 60

Table 2.10 Mixture composition [Mikulic et al. 2005] ............................................................................ 61

Table 2.11 Results from through transmission using rayleigh waves produced by ten cycle sine pulse on

fatigue specimens (2.25 MHz central frequency) [Halabe and Franklin 1997] ........................................ 72

Table 2.12 Results from pulse echo using rayleigh waves produced by five cycle sine pulse on fatigue

specimens (2.25 MHz central frequency) [Halabe and Franklin 1997] .................................................... 74

Table 2.13 Results from pulse echo using rayleigh waves produced by five cycle sine pulse on long beam

specimen (0.9 MHz central frequency) [Halabe and Franklin 1997] ........................................................ 76

Table 2.14 Acceptable liftoff for various frequencies [Mandracchia 1996] ............................................. 79

Table 2.15 Diagnostic Load Rating data [Mandracchia 1996].................................................................. 81

Table 2.16 Diagnostic Load Rating Test Results [Mandracchia 1996] ...................................................... 81

Table 3.1 Time of day each target reaches its maximum contrast and time lag relative to sunrise ......... 88

1

1 INTRODUCTION

1.1 Background

Nondestructive testing (NDT) has been practiced for many years in the fields of

medicine and mechanical/aerospace engineering. More recently, NDT has gained

popularity for quality evaluation and maintenance of civil infrastructure. For a safe

lifetime design of a structure, it is required that the structure should not develop any

subsurface defects during its life. To meet this requirement, many sophisticated and

efficient nondestructive techniques have emerged. This study focuses on two

nondestructive techniques, namely, ultrasonics and infrared thermography.

Ultrasonic testing has been practiced for many decades. It uses high frequency

elastic waves to conduct examination of a structural component and detect subsurface

flaws or characterize the material. Mostly, ultrasonic testing is performed on steel and

other metals and alloys, but it can also be used on concrete, wood and other composites,

but with a lower resolution. The basic procedure involved in the use of ultrasonic testing

is the interpretation of the signals received from a transducer on a diagnostic machine or

computer. The profile of the signal differs depending on the amount of signal attenuation

or the intensity of the reflected waves and the travel distance, representing the arrival

time of the reflection. Presence of imperfections in the space between the transmitter and

the receiver reduce the amount of wave energy transmitted, which affects the received

signal. The advantages of using ultrasonics are that the waves are highly penetrating,

especially in metals, permitting for the detection of deep flaws in a structural component;

high sensitivity, allowing for the detection of very small flaws; and non-hazardous to the

operator and the environment. In this study, a review of recent advances and application

of ultrasonic testing on various structural components of a bridge such as steel girders,

and concrete bridge decks was conducted.

Infrared thermography has proved to be an effective nondestructive technique to

detect internal voids, subsurface delaminations and cracks in concrete and composite

structures such as bridge decks, highway pavements, Fiber Reinforced Polymer (FRP)

composite wrapped railroad ties, etc. Infrared thermography uses the thermal energy

emitted by an object to characterize its subsurface conditions. There is usually a

2

temperature difference between the normal surfaces of a structure and the surface just

above the imperfections (such as subsurface cracks or defects present in the structure).

Infrared energy has a high wavelength which makes it invisible to human eye. In the

infrared domain, every object with a temperature above absolute zero emits heat energy.

Higher the temperature of an object more is the infrared radiation emitted. Infrared

thermography cameras generate visible images from invisible infrared radiation emitted

by an object and thus provide surface temperature measurements without any contact.

The most common advantages of using infrared thermography are that temperatures over

larger areas can be compared very quickly using the infrared images, deteriorating areas

in structures can be detected prior to their failure, and the technique can be used to

observe objects in inaccessible and dark areas. In this study, a review of recent advances

and application of infrared thermography testing on different components of a bridge was

conducted. The study also included application of infrared thermography for detecting

defects in timber railroad ties encased in FRP composites under laboratory and field

conditions. It should be noted that while ultrasonics has a big niche in application to

metallic components, infrared thermography has advantages in terms of application to

non-metallic components, especially if rapid scanning of large areas is needed.

In recent years, FRP composites have become popular construction material as

they offer many advantages such as light weight, excellent corrosion and fatigue

resistance, high strength and high impact resistance. FRP composite is one of those most

efficient and economical material for rehabilitation of deteriorating structures such as old

buildings and bridges, as well as for new construction. FRP composite can be bonded or

wrapped around structural members like beams and columns to enhance their strength

and ductility, even after these members have been severely damaged due to loading

conditions in the field. For example, columns in a building can be wrapped with FRP

composite fabric for achieving higher strength and ductility by restraining the lateral

expansion of the column. For an economical maintenance of a composite structure and to

ensure continued structural integrity, subsurface defects such as debonds between the

FRP composite and the underlying member must be detected in their earlier stages so that

timely repair can be carried out. Here comes the important role of nondestructive testing

for early detection of defects in a composite structure. In addition to literature review, this

3

study also evaluates the use of infrared thermography technique for detection of

subsurface defects in FRP composite encased timber railroad ties under laboratory and

field conditions.

1.2 Research objectives

The objectives of this research study are as follows:

To conduct a comprehensive literature review of the recent advances on the

application of ultrasonic technique for defect detection in bridge structural

components

To conduct a comprehensive literature review of the recent advances on the

application of infrared thermography for subsurface defect detection in concrete

and composite structural components

To demonstrate the use of infrared thermography for visualizing the subsurface

defect pattern in FRP composite encased timber railroad ties just after the

manufacturing process and after application of static and fatigue loads in the

laboratory and field setting.

1.3 Scope

A comprehensive literature review on ultrasonics and infrared thermography

techniques and its applications have been carried out. This review has placed more

emphasis on the recent publications, primarily covering the 2000-2010 period. A low-

cost and portable infrared camera was used in this research to produce infrared images in

radiometric JPEG format. Laboratory infrared testing was conducted on FRP composite

railroad ties just after the manufacturing process and after applying loads. In-place

composite ties subjected to real life railroad loading were also tested during this study.

These infrared tests revealed the presence of subsurface defects at various locations and

provided a comparison of the subsurface defect pattern before and after applying loads.

1.4 Report organization

This problem report is organized into five chapters. Chapter 1 presents the

background, objectives and scope of this study. Chapter 2 presents literature review

4

dealing with the ultrasonic technique, and recent developments and applications of

ultrasonic technique on various structural components of a bridge. Chapter 3 presents

literature review dealing with the infrared thermography technique, and recent

developments and applications of infrared thermography on various concrete and

composite bridges. Chapter 4 provides detailed description of the laboratory and field

testing work conducted on FRP composite encased railroad ties using infrared

thermography. Chapter 5 presents the conclusions of this research and recommendations

for future studies. Finally, a listing of all the references cited in this study is provided at

the end.

5

2 LITERATURE REVIEW: ULTRASONICS

This chapter presents the literature review on recent advances and applications of

ultrasonic testing on different structural components of a bridge. All the papers reviewed

here are from recent years which present the latest advances in the field of ultrasonics for

nondestructive evaluation of structural components.

2.1 A fundamental study on detection of defects in the web gap region

of steel plate girder bridges by the plate wave ultrasonic technique

[Shirhata et al. 2004]

Fatigue cracking in steel bridges is mostly induced by the Torsion. As a vehicle

passes on a bridge, relative displacement of main girders takes place causing secondary

force in the transverse members of the bridge, which in turn cause out of plane distortion

of the girder web. As the result, web gap area is subjected to significant stress

concentration, which leads to fatigue cracking as shown in Figure 2.1(a) and (b).

Figure 2.1 Fatigue cracking and monitoring system [Shirhata et al. 2004]

Figure 2.2 Dispersion curves and excitation zone [Shirhata et al. 2004]

6

This paper discussed the development of a long term monitoring system to detect

fatigue cracks and monitor the propagation quantitatively. Transducers are fixed on the

web plate of a bridge all the time. Detection and monitoring was achieved by observing

change of wave forms recorded periodically.

As the first step in this study, detectability of small defect in the web gap area of a

girder specimen was investigated experimentally and numerically. Because mode

conversion of the plate waves might be a difficult problem, the influence of a vertical

stiffener on the wave form was also investigated.

Since plate waves are dispersive, it was important to select an appropriate

excitation mode. In this study, S0 mode was chosen as it was most suitable for long range

inspection. Figure 2.2 shows dispersion curves of steel plate.

2.1.1 Experiment

The girder specimen shown in Figure 2.3 measured about 4100mm long and

thickness of the web plate was 8mm. The setup had two transducers, a pulser/receiver, an

oscilloscope, and a computer as shown in Figure 2.4. Considering S0 mode excitation, the

frequency of the transducers was set to 100 kHz. Two acrylic wedges were used at the

attachment. Water based couplant was used.

Figure 2.3 Girder specimen [Shirhata et al. 2004]

7

Figure 2.4 Experimental setup [Shirhata et al. 2004]

Experiments consisted of three series as shown in Table 2.1. The influence of a

vertical stiffener on the wave form was investigated in the first series, because a stiffener

was thought to be a wave reflector and mode convertor. Detectability of a hole was

investigated in the second series. Instead of real fatigue crack, as it is difficult to control

the initiation and propagation of fatigue cracks, a hole was made in this study. In the third

series, detectability of more than one hole was investigated. Each of the steps generated

two wave forms.

Table 2.1 Experiments [Shirhata et al. 2004]

2.1.2 Experimental results

Wave forms from step 1 and 2 were shown in Figure 2.5. Many small echoes

were seen because of the small holes on the web plate of the girder specimen and most

part of the girder was covered with rust. Before data acquisition of step 2, a steel plate

about 8 mm thick was welded on the web plate. The stiffener was placed about 2200 mm

away from the transducers. Transducers were fixed on the web plate during welding, in

order to keep the coupling condition same. When the waveforms were compared, around

0.7 msec change was noticed due to the stiffener.

8

Figure 2.5 Wave forms of steps 1 and 2 [Shirhata et al. 2004]

Figure 2.5 shows that change of wave forms was very small. To estimate the

change of wave forms quantitatively, deviation was proposed. As shown in Figure 2.6 the

amplitude of reference and target waves were assumed ai and bi, respectively and ai vs. bi

was plotted. If the difference between the two wave forms was bigger, the plot would

have scattered from the 45 degree line and if there is no difference, the plot would be on

the 45degree straight line. di is distance from the 45 degree line.

Figure 2.6 Definition of deviation [Shirhata et al. 2004]

Deviation D was represented by the equation (2.1). N is the number of data.

Deviation had information of difference between two wave forms.

D=

N

1i

2id

N

1 (2.1)

9

Figure 2.7 Deviation of steps 1 and 2 [Shirhata et al. 2004]

For the wave forms in Figure 2.7 deviation was calculated. It was difficult to

observe difference for further point from the transducer as attenuation by the distance

was strong. Deviation was calculated for each part of the time domain dividing it into 12

parts. Deviation of the 2nd

part was observed higher of any other part and this was

because that this part came from the stiffener.


10

Wave forms of series 2, wave forms of steps 3 and 4 are shown in Figure 2.8. A

drill hole with diameter 4mm was made in the web gap region before step 4. In series 2

and series 1, transducers were fixed on the web plate during drilling. In step 4, the echo

was a little higher than step 3.

In series 3, two more holes were made around the web gap area. Figure 2.9 shows

the wave forms of steps 5 and 6, and also the detail of the web gap area. An echo around

0.75 msec came from the drill holes. The other parts deviation was less than this part and

three holes were detected by this instrumentation.

The results showed that a wave form was very sensitive to the coupling condition

and defect echo was very small. The attachment of the transducers including coupling

had to be improved. Data acquisition should be improved too, for the improvement of

signal to noise ratio.

2.1.3 Numerical analysis

Numerical analyses by the finite element method were performed. Figure 2.10

shows the models; changing details of the web gap area and Figure 2.11 shows wave

forms of these models. As the defect was bigger, difference of wave forms were seen

more clearly. The defect echo was calculated by subtraction from no defect wave form.

Figure 2.12 shows defect echo levels of models (b), (c), (d), and (e). Defect echo of

model (c) was as high as model (e). From the experimental and numerical results, crack

such as model (e) can be detected by the instrumentation.


11

Figure 2.10 Three dimensional Finite Element Models [Shirhata et al. 2004]

Figure 2.11 Wave forms of numerical models (a) through (e) [Shirhata et al. 2004]

Figure 2.12 Defect echo height [Shirhata et al. 2004]

12

2.1.4 Summary

Experimental results of applicability of the plate wave ultrasonic technique for

fatigue crack detection in the web gap area showed that a stiffener might have some

influence on the wave form. Defect echo was very small, but it was possible to detect

three holes. Numerical analysis results exhibited that echo height of three hole model was

as high as a crack model turning around the edge of the vertical stiffener which makes it

possible to detect the crack propagating in vertical direction of the web plate.

2.2 Ultrasonic inspection of bridge hanger pins [Benjamin et al. 2000]

A study was conducted by the staff of the Federal Highway Administration's

(FHWA) Nondestructive Evaluation Validation Center (NDEVC) to determine the

reliability of contact ultrasonic techniques in the field to accurately locate defects in

hanger pins. The study examined and compared Contact Ultrasonic method which was

used during an in-service inspection, and noncontact ultrasonic method for testing

decommissioned pins through a noncontact ultrasonic method using an immersion tank.

Hanger pins are the structural elements connecting the suspended span of the

bridge to the fixed cantilever arm of that same bridge and its primary function is to allow

for longitudinal thermal expansion and contraction in the bridge superstructure. A

diagram of a pin-and-hanger connection is shown in Figure 2.13. Photograph of one of

the hanger pins examined for this study is shown in Figures 2.14. The exterior faces of

the pins are the only surfaces through which ultrasonic pulses can be transmitted while

the pin is in situ.

Figure 2.13 Schematic diagram of an in-service pin-and hanger connection prior to pin removal [Benjamin et al. 2000]

13

Figure 2.14 Photographs of hanger pin prior to and following removal from bridge [Benjamin et al. 2000]

Pin-and-hanger connections were typically placed directly beneath bridge deck

expansion joints and were frequently exposed to water and debris that falls through the

joint which lead to corrosion of the pin at the critical shear planes. This corrosion can

have two damaging effects on the pin. The cross-section of the pin can decrease due to

corrosive section loss and produce pitting that might act as crack-initiation sites. The

other one is, corrosion can effectively lock the pin within the connection so that no

rotation about the pin is permitted which lead to large torsional stresses within a reduced

section of the pin. The torsional stresses, combined with the shear stresses, provide a

likely location for the development and propagation of cracks.

Locating cracks that initiate on the pin barrel at the shear plane perimeter was a

difficult task. The shear plane was not visible unless the pin was removed from the

connection. Ultrasonic field inspection provided the requisite combination of simplicity

of inspection procedure and accuracy of results to make it a preferred method.

2.2.1 Bridge description

The bridge studied for this research had a deck which accommodated two lanes

of traffic and was supported by a superstructure consisting of four welded steel plate

girders. The bridge contained 12 pin-and-hanger connections, all of which occur at

expansion joints approximately 1.5 from an adjacent pier. The hanger pins in the bridge

all had a barrel diameter of 76 millimeters and an overall length of 216 mm. The distance

from the exterior face of the pin to the pin shoulder is 32 mm, and the distance from each

exterior face to the closer shear plane was 76 mm.

14

2.2.2 Field ultrasonic inspection

The FHWA NDEVC conducted ultrasonic testing of the 22 pins on the bridge. .

The ultrasonic testing was conducted with an intention of identifying cracked or failed

pins. The inspections were conducted from a boom lift positioned below the structure.

Before ultrasonic testing was conducted, the pin exterior faces were prepared by grinding

to remove paint and the surface was smoothened to facilitate sound transmission into the

pin.

Ultrasonic examinations were performed using a Krautkramer/Branson USN 52

model ultrasonic flaw detector. This instrument featured digital electronics, an electro-

luminescent display, and 70 data set storage registers. Preliminary scans were performed

using a straight-beam (0 degrees) transducer, with subsequent scans using an incident

angle of 15 degrees. Throughout the testing, a transducer frequency of 5 MHz and a 12.7-

mm element size were used. The transducer and flaw detector were calibrated to establish

sensitivity and time base, before the testing.

A straight-beam scan was initially used to confirm pin geometry and also used to

identify large cracks or complete failure. The angled-beam scans were then conducted to

more accurately resolve reflectors placed near the pin barrel surface and to test areas

shielded from the straight-beam scan, such as the pin shoulder region. Angled-beam

scans were concentrated on pin shear planes where the potential for wear grooves,

corrosion, and cracking indicators was high. All scanning was proceeded in a systematic

manner to ensure complete coverage of the pin interior.

The field inspection indicated that two of the 22 original pins were cracked and

five additional pins had wear-groove indications at the shear plane. All these seven pins

were removed and were shipped to NDEVC.

Figure 2.15 Cracked section results from field ultrasonic inspection [Benjamin et al. 2000]

15

This study primarily focused on the two cracked pins. Figure 2.15 shows the

results from field ultrasonic testing of the two cracked pins, labeled S106 and S108. The

approximate defect profile at the shear plane was shown in this figure for each pin.

2.2.3 Immersion tank ultrasonic testing

At NDEVC, Immersion Tank Ultrasonic Testing of the seven hanger pins was

performed. The purpose of this testing was to accurately detect and quantify crack-like

defects that might be present in any or all of the pins.

Figure 2.16 Immersion tank ultrasonic inspection of a hanger pin [Benjamin et al. 2000]

The immersion tank ultrasonic examinations were performed using a system

developed by Scanning Systems International Inc. The software used was TestPro,

release 6.0. Figure 2.16 shows the basic setup of the scans. The pin was placed on end

while immersed in a water-filled tank. The ultrasonic transducer was placed above the pin

with its face parallel to the end face of the pin. A 5-MHz, 12.7mm unfocused transducer

was used, with a 60-mm separation between the transducer face and the pin face. The

movement of the transducer for the scanning process was computer-controlled, allowing

the electric motors to systematically move the transducer within the plane parallel to the

exterior face of the pin.

A standard distance-area amplitude block was used for the system calibration.

This block consisted of a 51mm diameter stainless steel cylinder that is 89 mm tall. A

3.2mm diameter flat-bottomed hole that is 12.7 mm (0.5 in) deep was present in the

center of one end of the reference block. This reference block was chosen as it provided a

16

pulse path from transducer to defect that was similar to that used in hanger pin

specimens. This reference block was used to determine the beam spread and the required

threshold levels for various transducer gain and attenuation configurations. As a result, a

defect location and the extent on a particular plane within the pin could be determined.

Initial scans were performed on all seven pins. The pins were scanned from both

exterior faces to provide full coverage within the pin barrel. This setup allowed for the

detection of virtually all reflectors, including most defects shrouded from direct detection

by the shoulder of the pin.

These initial scans showed that only pins S106 and S108 contained cracks and

that the cracks in these pins occurred at the level of the shear plane. Detailed scans were

then performed on these two specimens to accurately locate the defects. This additional

testing was conducted in two phases. First, the testing apparatus was set up to generally

find small defects near the perimeter of the pin body, below the shoulder. Second, the

testing apparatus was set up to precisely locate any defect that occurred within the body

of the pin directly under the exterior face.

Figure 2.17 Ultrasonic pulse showing beam spread around the pin shoulder and defects under pin shoulders at the shear plane level [Benjamin et al. 2000]

Through the use of a calibration block, it was determined that these settings

allowed for the scan to receive a reflection from a defect at the level of the shear plane,

which was 6.4 mm outside of the direct path of the transducer. Figure 2.17 shows the

17

beam spread of the ultrasonic pulse that allows for these defects to be located.

Accordingly, the procedure could detect a defect that was near the perimeter of the pin

body at the level of the shear plane.

Figure 2.18 Ultrasonic pulse showing beam spread around the pin shoulder and defects under the pin exterior face at the shear plane level [Benjamin et al. 2000]

The results of these scans were presented in the form of a Cscan image as shown

in Figure 2.18. The Cscan image of the defects was superimposed over a darkened Cscan

image of the exterior face of the pin. This showed clear identification of the travel path of

the ultrasonic pulse. The outline of the pin barrel was also presented.

The second phase of the detailed investigation of the cracked shear plane regions

was then conducted. The goal of this set of scans was to accurately find the defects that

lay directly under the exterior face of the pin. The calibration block was used to

determine what settings were required so that only defects that were partially under the

transducer were reported. The Cscan results for these tests are shown in Figure 2.19.

Figure 2.19 Cscan images of defect indications in hanger pins S106 and S108 [Benjamin et al. 2000]

Using the results shown in Figures 2.18 and 2.19, a schematic representation of

the defects present at the shear plane level of S106 and S108 can be determined. These

results are shown in Figure 2.20. The cracked portion of the shear plane was shaded in

the figure.

18

2.2.4 Comparison of field and immersion tank ultrasonic results

Comparison between the field ultrasonic results (Figure 2.15) and the immersion

tank ultrasonic results (Figure 2.20) indicated that the two methods provided similar

conclusions as to the extent of cracking at the shear plane level in both specimens. In pin

S106, the immersion tank findings indicated that 79 percent of the cross-section was

cracked, and the field ultrasonic‟s indicated that 83 percent of the section was cracked.

For S108, the immersion tank and field ultrasonic findings indicated, that 30 and 22

percent of the section was cracked respectively,.

Figure 2.20 Cracked section results from immersion tank ultrasonic inspection [Benjamin et al. 2000]

2.2.5 Conclusions

To investigate possible defects within hanger pins removed from an in-service

bridge, Field and immersion tank ultrasonic inspection techniques were used. The field

inspections identified crack-like defects within two pins. The results from the both

immersion tank testing and the field ultrasonic testing were well correlated with each

other. Both the defect location and defect size findings showed a high level of

consistency between the two ultrasonic techniques.

2.3 Ultrasonic underside inspection for fatigue cracks in the deck plate

of a steel orthotropic bridge deck [Bakker et al. 2003]

Fatigue cracks appeared through the deck plate of orthotropic steel bridge decks

in the Netherlands due to an unexpected increase of heavy traffic. These cracks initiated

at the weld joins of the deck plate, a rib and a girder. Ultrasonic inspection of the deck

plate from the underside of the bridge deck was done using two special measurement

19

techniques, angled pitch-catch technique to detect the maximum crack depth and simpler

pulse-echo technique to detect the crack length along the rib weld.

The inspection was done by first investigating a suitable method of ultrasonic data

acquisition, possible crack locations were irradiated with high-frequency waves. The

waves that were reflected and scattered by a crack were detected and provided

information on its size and position. Then, a robust calibration method was applied to

determine the crack size from the ultrasonic data. The calibration was obtained by

performing a sequence of fatigue tests on bridge deck specimen until cracks were visibly

detectable at the roadside of the deck plate.

2.3.1 Bridge deck specimen and fatigue set-up

The main component of the fatigue set-up (shown in Figure 2.23) for the

inspection was the specimen itself, its geometrical construction, the material it was made

of and the type and location of the cracks that are to be found and characterized.

Figure 2.21 Left: Construction of a steel orthotropic bridge deck. Right: Bottom view on the bridge deck specimen and its rib welds [Bakker et al. 2003]

Figure 2.22 Vertical cross section of the crack initiation zone [Bakker et al. 2003]

20

The left panel in Figure 2.21 shows the structural elements of the 2x2 meter

bridge deck specimen which had three steel trapezoidal ribs that were first welded onto

the bottom of the steel deck plate. The steel girder was welded over the ribs and then onto

the deck plate. The right panel in Figure 2.21 shows a bottom view on the bridge deck,

where the deck plate surface in between the ribs was available for ultrasonic data

acquisition. The vertical cross section of the crack initiation zone is shown in Figure 2.22.

The fatigue loading was simulated by cyclic loading at 1 Hz on the roadside of the

bridge deck, applied in the middle of a rib and symmetrically over the girder. The loading

was imposed first simultaneously at the outer ribs and subsequently only at the middle

rib. The top-top amplitude of the cyclic force was 64 kN per loading area of 270 mm in

the length direction and 320 mm in the width direction of the deck, which area simulates

the tire impression of a big truck.

Figure 2.23 The fatigue set-up with loading at ribs 1 and 3 [Bakker et al. 2003]

The strain in the deck plate at the girder-rib intersections was monitored on the

roadside with strain gauges during the Fatigue testing. The measured strain served as an

indicator to suspend the fatigue test. After a suspension, the deck was first visually

inspected for signs of cracks and then the specimen was taken from the set-up and placed

upside down to allow for more convenient ultrasonic data acquisition at the underside of

the deck. The specimen was rebuild into the set-up to continue the cyclic loading, after

the ultrasonic measurements.

21

In the similar fashion, total 8 inspections were carried out before the cracks came

completely through the deck plate and had grown to a length of more than 125 mm.

Inspection 0 was conducted before any loading was applied, and inspection 1 was

conducted after a static pressure test to make sure that a static loading to a new specimen

does not cause some detectable damage to the welds. The static test was conducted only

on the middle rib, so that inspection 1 yielded data only for welds „C‟ and „D‟. It was

noted that due to the stiffness of the ribs, the applied levels of loading may have a

significant influence only on the welds nearest to the loading areas. Inspections 2 to 7

were conducted after the number of fatigue cycles as listed in Table 2.2 was reached

2.3.2 Optimized ultrasonic measurement methods

An angled pitch-catch technique [Krautkamer et al. 1990] was used to determine

the crack depth. This setup is shown in Figure 2.24 and Figure 2.25.

Figure 2.24 Pitch-catch technique set-up for crack depth detection. One of these inspection areas is indicated by a circle in Figure 2.21 [Bakker et al. 2003]

Figure 2.25 Vertical cross section of the pitch-catch set-up [Bakker et al. 2003]

22

Once the crack had grown to sufficient size it became detectable along the rib

weld by using a horizontally perpendicular irradiation of the crack surface. At this stage,

the crack length was of prime interest. For this purpose the simpler pulse-echo

[Krautkamer et al. 1990] technique may be employed, which required only one

transducer. This set-up is shown in Figure 2.26 and Figure 2.27.

Figure 2.26 Pulse-echo set-up for crack length detection [Bakker et al. 2003]

Figure 2.27 Vertical cross section of the pulse-echo set-up [Bakker et al. 2003]

2.3.3 Visual and ultrasonic inspection results

Visual Inspection

Cracks became visible in the deck plate on the roadside at inspection 6. The

shortest horizontal distance between the IP and observed crack on the roadside of the

deck plate gave information on the vertical angle. These measurements suggest that the

crack angles were always steeper than 20deg.

Table 2.2 Number of Fatigue cycles before inspections 0 to 7 [Bakker et al. 2003]

23

Peak amplitude as ultrasonic crack depth indicator

The crack indicator I was defined as the average over all 31 scan positions, i.e.

31

1n

A31

1I (2.2)

Where m=0, 1 …7. A indicates the absolute peak amplitude at the nth

receiver position in

the mth

inspection. The development of I through the subsequent inspections was plotted

in Figure 2.28 for rib welds „A‟ to „F‟. The curves for the three different pitch-catch scans

were indicated by the transmitter distances of 50, 60 and 70 mm.

From the trends in Figure 2.28 it was inferred that the first signs of fatigue

damage was in inspection 2, expect for welds „C‟ and „F‟ where it became detectable in

inspection 3. For weld „B‟ the crack appeared to be almost through the deck plate in

inspection 3, since the level was similar to that in inspection 7. On the other hand, the

crack in weld „C‟ did not reach this stage until inspection 6.

Wave count as ultrasonic crack depth indicator

Wnm indicates the number of detected waves at the nth receiver position in the mth

inspection, then the crack indicator I was defined as the average over all 31 scan

positions. The exact number of wave counts in the uncracked state was actually

irrelevant, and it may also vary to some degree from weld to weld. Therefore, all curves I

for m=1, 2 ...7 were normalized to the wave count in inspection 0, after which it was set

I=1. This lead to the definition

31

1nw

0

W31

11I

I (2.3)

Where m=1, 2 …7. The development of I through the subsequent inspections was plotted

in Figure 2.29 for rib welds „A‟ to „F‟. The curves for the three different pitch-catch scans

are indicated by the transmitter distances of 50, 60 and 70 mm.

24

Figure 2.28 Average peak amplitude in the pitch-catch scans as a crack indicator [Bakker et al. 2003]

25

Figure 2.29 Average, normalized wave count in the pitch-catch scans as a crack indicator [Bakker et al. 2003]

Peak amplitude as ultrasonic crack length indicator

The main purpose of the pulse-echo scans along the rib welds was to detect the

crack length. The peak amplitude was sufficiently accurate crack length indicator. The

peak amplitudes were plotted in Figure 2.30 for the pulse-echo scan with the 70 deg

probe and in Figure 2.31 for the pulse-echo scan with the 45 deg probe.

26

Figure 2.30 Peak amplitude in the 70 deg probe pulse-echo scans as a crack length indicator [Bakker et al. 2003]

27

Figure 2.31 Peak amplitude in the 45 deg probe pulse-echo scans as a crack length indicator [Bakker et al. 2003]

28

Table 2.3 Ultrasonically determined crack lengths for inspections 4 to 7 [Bakker et al. 2003]

The length of the crack at the bottom of the deck plate was larger than the visible

surface- breaking part at the road surface of the deck plate.

2.3.4 Conclusions

A new ultrasonic method was proposed for the inspection of a steel orthotropic

bridge deck from the underside of the deck plate. Using ultrasonic data from a fatigue test

on a bridge deck specimen, the calibration between the ultrasonic crack indicators and the

actual size was obtained.

The method proved quite sensitive to small cracks, while the actual crack depth in

the subsequent fatigue stages was estimated with %15 accuracy. The ultrasonic results

indicated two stages in its growth: first a predominant depth growth, which stopped

before through-cracking occurs, followed by a stage of predominant length growth. This

finding was substantiated by the ultrasonic crack length measurements, which pointed to

a long, shallow type of crack with a depth to length ratio between 1:7 and 1:9.

The ultrasonic method proved to be quite robust, which made it suitable for in-situ

bridge deck inspection, where the coating and the wear layer may cause additional

amplitude variations and damping.

29

2.4 Automatic ultrasonic testing (AUT) [Miki et al. 2005]

Automatic Ultrasonic Testing (AUT) systems are used for quality evaluation of

welded joints in steel members of a bridge. Japanese Ministry of Construction conducted

tests on applicability of AUT and fatigue performance of the field- welded joints to

determine the allowable flaw size and establish specifications for quality evaluation in

1999. In the study on AUT, two round robin tests were conducted. One was to determine

detectability of the current AUT systems used in the field and the other was to confirm

the applicability on site, considering the results from the first test.

2.4.1 First round robin test

This test was done in four steps. Test was announced and then the participating

companies tested the specimens one after another and submitted the testing results. Then,

the actual data of the welded flaws was obtained by conducting a destructive test on the

specimens. Lastly, comparing the actual flaws and AUT indications of different

companies, performance of the respective AUT systems were obtained.

Seventeen companies participated using more than one testing condition and a

total of 20 systems were used, some were being used before and the others were

developed for this test. Table 2.4 shows the testing conditions of the participating

companies.

30

Table 2.4 Testing conditions of AUT systems of the first round robin test [Miki et al. 2005]

Specimens were configured similar to the main girder flange plates of field-

welded joints of plate girder bridges, with a V-groove. Four thicknesses of specimens

were made, namely 40, 60, 80, and 100 mm thick. Three specimens for each thickness

were prepared. For 40 and 60 mm thick specimens, there were two types, depending on

the surface of the weld and the taper. Flaws such as cracks, lack of fusion, blow holes,

slag inclusions and incomplete penetration were made intentionally in the specimen and

each was given 2.5 days for inspection. Figure 2.32 shows the specimens of the first

round test.

Inspection data is required to be submitted within two weeks after testing by each

company. The data included location and length of weld flaws and location of peak echo.

A coordinate system with x, k and d axes were chosen for the location data as shown in

Figure 2.33, x axis is the weld direction, k is the distance from the center of the weld, and

d is depth in the thickness direction. xs, xe, xp, k and d were submitted regarding the

location of the flaw, where xs is the start of an indication, xe is the end of an indication

and xp is the location where the peak echo was obtained.

Figure 2.32 Specimens of the round robin test [Miki et al. 2005]

31

Once the tests are finished by the participate-ng companies, a destructive test of

all the specimens is conducted as shown in Figure 2.34, where the weld portion of the

specimen was cut out at 0.5mm pitch and pictures were taken. A total of 1200 pictures

were taken.

Figure 2.33 Data to be submitted (Miki et al. 2005)

Figure 2.34 Destructive test (Miki et al. 2005)

32

To evaluate the performance of AUT, two items were considered, ratio of

detection and ratio of over detection. If indication from an AUT and actual weld flaw are

coincided, we say it as detection and if there‟s an indication from an AUT system, even

though there is no flaw existed, then we say it as over detection. In the evaluation of AUT

systems, indications and actual flaws longer than t/5 mm were chosen and compared to

each other. The results of the first round robin test were shown in the Figure 2.35.

Figure 2.35 Results of the first performance test (a) 40mm (b) 60mm (c) 80mm (d) 100mm and (e) all the specimens, disregard level L/2 [Miki et al. 2005]

33

2.4.2 Second round robin test

Announcement for the second test was done to the participating companies,

stating that they are required to report the observation of the first test result and

improvement for the second test. All the results of evaluation and pictures taken of the

destructive test were sent to them.

The reason for over detection considered by most of the companies was the result

of a coupling between the transducer and the specimen, and wanted to change the

transducers to higher frequency and lower incident angle, so that surface waves would

not be transmitted.

In the second test, inspection data should be submitted on the same day of the test.

Combinations of testing conditions, as shown in the Table 2.5, were accepted from the

companies. Configuration of the specimens was same as the first test, except there were

only two specimens for each thickness. Data to be submitted was same as the first test xp,

xs, xe, l, k and d. After all the inspection is finished, a destructive test similar to the first

one was conducted, except the grinding pitch was increased to 1.0mm.

The results of the second round robin test are shown in the Figure 2.36, submitted

on the same day of testing and Figure 2.37, one week after the testing. The horizontal

axis in the figures is the AUT system and the vertical axis is the ratio of detection and

over detection.

Table 2.5 Testing conditions of AUT systems of the second round robin test. [Miki et al. 2005]

34

Figure 2.37 Results of the performance of the second test, data submitted after one week, (a) 40 mm, (b) 60mm, (c) 80 mm, (d) 100 mm, and (e) all the specimens [Miki et al. 2005]

Figure 2.36 Results of the performance of the second test, data submitted on the day of testing, (a) 40 mm, (b) 60mm, (c) 80 mm, (d) 100 mm, and (e) all the specimens (Miki et al. 2005)

35

2.4.3 Summary:

From first round robin test

Weld flaws became difficult to detect, as the specimens became thicker. Both

ratios of detection and over detection were higher. AUT systems using 2 or 3 MHz waves

achieved higher ratios of detection. Incident angles of 65o to 70

o were better, regarding

the ratio of detection and all echo recording systems showed better performance.

From second round robin test

Flaws were difficult to detect for thicker plates. Data submitted on the day of

testing showed less performance in detection when compared to the data submitted after

one week. Similarly, all echo recording systems were better as to results on the day of

testing. Calibrating flaw length estimation led to a performance improvement.

2.5 Ultrasonic wireless health monitoring system for near real-time

damage identification of structural components [Banerjee 2008]

This paper discussed about the development of a wireless-autonomous system that

acquired processes, and delivered waveform data for analysis and inferred structural

integrity on the condition of the monitored structure. The data processing tool for the

system used was an internally developed Generation 4 Smart Sensor Node (G4SSN). The

overall system was designed with a small factor size so it can be discretely placed in situ

for monitoring purposes. The data analysis algorithm was very fast and automatic. It was

named statistical damage index approach which computed damage parameter, namely

statistic t, with a high degree of accuracy.

2.5.1 Near real-time wireless health monitoring platform

The newly designed platform for this system shown in Figure 2.38 consisted of a

500K gate Xilinx Field Programmable Gate Array (FPGA), 4 Meg of RAM, a 2 channel

high speed A/D and 2 channels of high speed D/A. FPGAs acted as the core logic unit,

and combined the ability of non-specific logic to perform multiple calculations

simultaneously in either a synchronized or un-synchronized fashion with the ability to be

designed and re-programmed from a simulatable software environment.

36

Figure 2.38 The health monitoring system consisting of preamplifiers, amplifiers and data sampling/processing unit [Banerjee 2008]

The processor / sampler boards supported 2 transducers which were configured as

either a source or receiver. The G4SSN acted as a control unit sending configuration and

commands to the processor / samples. The G4SSN then collected results from the

processor / samplers and passed these results back to a remote monitoring station for

evaluation.

2.5.2 Results from the new SHM platform

The new hardware had been tested for the data gathering ability using a 4 channel

data collection setup. Five piezoelectric transducers were placed approximately 2 in apart

and bonded onto the surface of a 4 ft x 6 ft aluminum plate as shown in Figure 2.39. As

the receiver distance from the transmitter increased, the results (Figure 2.40) showed an

expected time delay.

Figure 2.39 Photo of the bonded piezoelectric sensors used for the test. The function generator is used to transmit a sine source [Banerjee 2008]

37

Figure 2.40 Sample acquired data showing the expected time delay as the receiver distance from the transmitter increases [Banerjee 2008]

2.5.3 Statistical damage index (SDI) approach

Damage Index (DI) was used to determine structural damage when compared

with the measured dynamical response of two successive states of the structure [Banerjee

et al. 2007, Mal et al. 2007, and Mal et al. 2005]. The measurements taken on an

undamaged or partially damaged structure served as a baseline, and were used to evaluate

the damage indices. The damage modified the elastic waves propagating between the

source and the receivers due to the reflection, scattering, and diffraction of the waves by

the damaged region. The frequency spectra (FS) of the recorded signals reflected such

variations, so ultrasonic wave propagation measurements were analyzed to detect them.

The DI is defined as follows: AI

ADAI2

(2.4)

38

AI and AD are the area of the frequency spectrum (FS) for the reference and damaged

structures, respectively.


A sample photograph of the experimental test setup is shown in Figure 2.41.

Figure 2.41 The experimental setup used to test the functionality of the SDI approach [Banerjee 2008]

Correlation between source frequency and damage size

The sensitivity of the source frequency on the damage size was investigated using

a simple test on a 1mm thick aluminum plate as shown in Figure 2.41. The layout of the

test is shown in Figure 2.42. First, the baseline signals corresponding to the actuator and

sensor paths (Table 2.6) are recorded at a known intact condition of the plate. With a

source central frequency of 1 MHz it was possible to detect a hole size of 6mm with a

high level of confidence, whereas the SDI failed to detect a hole size of 3mm. It can be

seen from Tables 2.6(a) and (b) that the statistical damage index, t, is quite high for hole

size of 6mm and that the means of the undamaged and damaged samples is rejected (i.e.

appearance of damage, D=1) in all cases.

39

Figure 2.42 Layout of the test for establishing correlation between excitation frequency and the damage size [Banerjee 2008]

Table 2.6(a) SDI calculations for 3mm hole (b) SDI calculations for 6 mm hole [Banerjee 2008]

Damage identification and localization in structural composites

A composite plate (12 X 12 in X 3/64 in) consisting of 8 layers carbon fiber/

epoxy was made out of resin infusion technique and was investigated with 10 control

points, which can be used as an actuator as well as a sensor location (Figure 2.43(a)). If

the sensor located in position 1 is used as a source then the sensors positioned at 2 to 10

are used as receivers. In this way, ultrasonic data for a total of 90 measurement paths was

obtained. The statistic damage index t, was evaluated for every measurement path.

Measurement with top 5 statistic t values are displayed Figure 2.43(b).

40

(a) (b)

Figure 2.43 (a) Layout of the test for the damage identification and localization in a composite plate; (b) Display of measurement paths with top 5 statistic t values [Banerjee 2008]

Damage identification and localization in steel beams

As shown in Figure 2.44, a 2 ft long steel beam was used for the test. It was found

that possible damage appearance in the web can be best interrogated if the sensors placed

at the web/flange intersection are used as actuators (location 1, 2, 5 and 6 in the figure).

Measurement paths with highest statistical damage index t, are shown by arrows, which

determine the approximate damage position with some confidence.

Figure 2.44 Display of the measurement paths with top statistic t values showing the approximated identified location of the notches [Banerjee 2008]

The two important observations from the tests conducted on composite structures

and steel beams were, the value of the statistic t was significant if the defect falls right on

41

the measurement paths; and a large number of sensor arrays were required to locate the

damage precisely.

2.5.5 Conclusions

The study clearly illustrated the effectiveness of the SDI to predict the

approximate location and severity of the damage from a large dataset collected by a

network of distributed sensors and actuators in complex structures. The approach

presented was very useful in the development of an automated continuous structural

health monitoring system because of its simplicity and minimal requirement for operator

involvement.

2.6 Ultrasonic guided waves for NDE of sign support structures [Xuan

et al. 2009]

This paper discussed a method based on Ultrasonic Guided Waves for the detection

of cracks in sign support structures, which can be found along any major highway across

the United States. This method had the advantages of UGWs and signal processing to

extract defect-sensitive features aimed at performing a multivariate diagnosis of damage.

The method exploited the waveguide (pipe-like) geometry of the horizontal chords.

UGWs were made to propagate along pipe‟s longitudinal direction and excite the entire

cross-section. This study focused on the detection of an artificial notch created at a joint,

near a weld toe, of dismantled overhead sign structure truss. An array of lead zirconate

titanate (PZT) transducers was attached to the structure. The transducers were controlled

through a National Instruments-PXI running under a Lab View program and they were

acted either as actuators or receivers.

2.6.1 Experimental setup

The four chord sign support structure shown in Figure 2.45 was tested. Figure

2.46 shows the relative position of the transducers with respect to the monitored joint and

are sequentially numbered as C0, C1 to C10.

42

Figure 2.45 (a) Photo of the sign support structure tested at the University of Pittsburgh. (b) Sketch of the truss

cross section. (c): Sketch and dimensions of the truss’ chords *Xuan et al. 2009]

A modulated five-cycle 10 V peak to-peak (ppk) toneburst with a Gauss window

was used as excitation signal. A sweep of frequencies ranging between 100 and 275 kHz

with a frequency step equal to 25 kHz was excited. Thus, a total of 8 frequencies were

considered. Once the signal was detected, it was amplified 20 times by a linear amplifier,

sampled at 10 MHz, and averaged ten times to increase the signal-to-noise ratio, and

stored for post-processing analysis. The time waveforms generated from these signals are

shown in Figure 2.47.

Figure 2.46 Location and relative distance of PZTs C0 to C10. Dimensions are in millimeters [Xuan et al. 2009]

Figure 2.47 Time waveforms generated by actuator C0 and (a) detected by sensor C1 and (b) detected by sensor C5 [Xuan et al. 2009]

43

Figure 2.48 (a) Scheme of the truss and location of the joint under monitoring (b) Photo of the hydraulic actuator and the steel beam that transfers the load onto the truss (c) Orientation of the crack with respect to the

longitudinal axis of the chord [Xuan et al. 2009]

An artificial notch was devised near the weld toe at the joint illustrated in Figure

2.48(a) to investigate the capability of UGWs to detect crack initiation and growth. In

order to induce crack growth, a cyclic loading was applied to the structure. The position

of the applied force on the truss was schematized in Figure 2.48(a). Figure 2.48(b) shows

the loading setup.

The load was cycled from 2 kips to 28 kips resulting in a load range of 26 kips.

Cycling was carried out at a rate of 1 Hz. The position and the orientation of the notch are

shown in Figure 2.48 (c). The load history of the crack, i.e. the crack size as a function of

the number of cycles is presented in Figure 2.49.

Figure 2.49 Crack size as a function of the number of cycles [Xuan et al. 2009]

44


Figure 2.50 shows the signals collected from PZT C0 as actuator and C5 acted as

a sensor and vice versa. The UGWs were recorded before, at 0 and at 100 000 cycles.

Figure 2.50 (a): Time waveforms generated by C0 and detected by C5 at 0 and 100 000 cycles; (b): Close-up view of (a); (c): Time waveforms generated by C5 and detected by C0 at 0 and 100 000 cycles; (d) Close-up view of (c) [Xuan

et al. 2009]

Figure 2.51 shows the correlation coefficients for different wave‟s path, i.e. for

different actuator-receiver pairs. The UGWs originated from PZT C6 showed less

sensitivity to the presence of defect than the UGWs traveling from PZT C10. The results

when compared to the results shown in Figure 2.51(a), it was evident that the sensitivity

of the correlation coefficient to the presence of defect was smaller. The scatter of the

UGWs associated with the presence of the angular members was more dominant, and

therefore the effect of damage might be less visible.

Figure 2.51 Correlation coefficients as a function of number of cycles (a) Actuator C10 and receivers C6, C7 and C8; actuator C6 acted and receivers C8, C9 and C10. (b) Actuator C0 and receivers C4 and C5; actuator C5 and receivers

C0 and C1 [Xuan et al. 2009]

45

2.6.3 Conclusions

The approach exploited the pipe-like geometry of the horizontal chords of the truss.

An array of PZT was used to generate and detect the waves. Ultrasonic data was

processed by using a correlation method that aimed at detecting changes in the

correlation coefficients between two waveforms. These changes were associated to the

presence of crack. The correlation coefficient was sensitive to the presence of damage but

it was also dependent upon the position of the actuator-receiver pair with respect to the

welded joint, and with respect to the circumferential coordinate.

2.7 Defect detection and imaging using focused ultrasonic guided

waves [Sicard et al. 2007]

An interesting solution for defect detection and localization was suggested with

the use of numerical and phased focusing to perform guided wave inspection. Also,

imaging of ultrasonic guided waves inspection using color coded B-Scans was considered

a highly valuable tool for data interpretation. This paper presented an experimental study

on using ultrasonic Lamb waves generated by the angle beam wedge method for the

implementation of focal law algorithms applied to the inspection of plate structures. To

demonstrate the feasibility of the Lamb wave inspection technique with both type of

excitations, a comparison between spike and tone burst excitation was also performed.

2.7.1 Conventional phased array theory

The principle of phased array imaging was based on the phase matching of waves

propagating through different paths by applying proper delays on the wave generation

and reception. The ability to focus at a certain point within a material using an array

resided in the application of individual delays on each element of the array in order to

create a constructive interference of the waves at the desired point. A simplified

representation of linear phased array focusing can be expressed by

(2.5)

Where Sh (t) is the waveform recorded by the hth

element of the array, F (x, t) is the

phased array response along the array axis x, N D is the number of focusing depths

46

considered (incremented by/), N A is the number of azimuthally focusing points

considered (incremented by g), N E is the number of elements of the array (incremented

by h), and and are respectively the focusing delay at the wave

transmission and reception that need to be applied to the hth

element for the focal law

defined by f, g.

2.7.2 Lamb wave phased array

Two methodologies were used to perform focusing in case of Lamb Waves:

synchronizing the time-of-flight of the wave packet and synchronizing the phase of the

wave packet at the defect position. A two-element array aimed at a focal spot located at a

distance r1/2 from the first element (closest element) and r2/2 from the second element

was considered. In the Fourier domain, the phased array process can be described as

∆tT,2+∆tR,2 ) d , (2.6)

Where S12 ( ) represents the Fourier amplitude of elements 1 and 2, k = /V ( ) is the

angular wave number as a function of the phase velocity V ( ), and where the time

delays and of the first element are zero since it is the closest to the focal spot.

The equation is maximal if the arguments of both exponential functions are identical.

Knowing that k= / V ( ) and defining r = {r2—r1}, this leads to the time delays for

the second element.

(2.7)

2.7.3 Wedge considerations for lamb waves

The main consideration in the definition of focal laws for Lamb waves arises with

the use of a wedge for the wave generation. Figure 2.52 illustrated the guided wave beam

spread resulting from different incidence angles for a limited incident beam divergence

angle (15° in this case, for a total aperture of 30°). Figure 2.53 illustrated the assumed

wave propagation path for time-of-flight calculations.

47

Figure 2.52(a) Illustration of a conic wave beam projected on the plate and the subsequent lamb wave beam for incidence angles of (a) 20° and (b) 40°. The filled section in the wedge corresponds to a constant incidence angle

within the incident beam, while the flat filled area corresponds to the Lamb wave field [Sicard et al. 2007]

Figure 2.53 (a) Illustration of the assumed wave propagation path for time-of-flight calculations in (a) 2D and (b) 3D

[Sicard et al. 2007]

To verify the assumptions previously made on the utilization of the main

frequency component as a reference for the calculation of the phased array delays,

experiments were conducted. A 3mm circular hole machined in a 6.12mm thick 1018

steel plate was inspected for this purpose using a 1MHz flat circular transducer (diameter

of 13mm) mounted on a 37° Perspex wedge. The inspection was performed in pulse-echo

with the A1 mode excited at 780 kHz using a 4 cycle tone burst signal and a spatial

sampling of 1.27mm, with the defect located at 225.5mm from the scanning line. Figure

2.54 presents the echo area of (a) the experimental B-Scan obtained on this sample and

(b) a simulation performed using the methodology presented in [Sicard et al. 2007], for

the A1 mode alone, free from interferences with back wall reflections and other modes.

The results shown in the Figure 2.54 and 2.55 confirmed the assumption that a unique

incidence angle can describe the phase and time-of-flight of the echoes.

48

Figure 2.54 Recorded echoes from the pulse-echo inspection of a 3 mm through hole in a 6.12 mm, 1018 steel plate using the A} mode for (a) experimental data and (b) simulated data. Dashed line: time-of-flight curve of the center frequency component. Solid line: Phased array time delays curve plotted with a temporal offset corresponding to

the time-of-flight of the element closest to the defect [Sicard et al. 2007]

Figure 2.55 Results of Figure 2.54, time shifted according to the computed phased array delays for (a) experimental

data and (b) simulated data. Dashed line: time-of-flight corresponding to the focal spot position using the center frequency component [Sicard et al. 2007]

2.7.4 Effects of the type of excitation

Tone burst excitation helps in the excitation of a given mode by a proper selection

of the frequency bandwidth of the pulse. With a tone burst pulser, the frequency tuning

was more efficient in terms of selecting the proper center frequency by tuning the number

of tone cycles. When a spike pulser was used, the center frequency and bandwidth were

mostly defined by the transducer spectrum and excitability of the mode [Rose 1999]

according to the wedge and plate properties. As an example, Figure 2.56(a) presents the

frequency content of echoes resulting from both pulser types, illustrating the variations in

the resulting frequency content of the modes. Figure 2.56(b) presents the depth focusing

inspection of a 3mm hole in the same plate as in Figure 2.54.

49

Figure 2.56 Comparison of focusing using tone burst and spike excitation, (a) Experimental frequency spectrum

(solid line: 9 cycle tone burst at 810kHz ; dotted line: spike ; dashed lines: theoretical frequencies computed from Snell's law), (b) Focusing result on a though hole using, from left to right, tone burst excitation (S1), and spike excitation filtered to isolate S1 (same parameters as for the toneburst), S1 (delays computed for the principal

frequency component), and A1. The lateral scale of each image is identical [Sicard et al. 2007]

2.7.5 Multiple reflectors

To evaluate the possibility to perform phased array focusing on realistic defects, a

1.82mm, 302 stainless steel plates with a cluster of machined FBH was investigated using

a 2MHz, 5x3mm piezoelectric element mounted on a 33° LOTEN wedge. Four tone

cycles excited at 2.38MHz were used to excite the A1 mode in the plate and the inspection

was performed with a scanning step of 0.5mm. Figure 2.57 shows the defect

configuration (a), B-Scan (b), Depth focusing (c) and DDF (d) results obtained for these

defects.

Figure 2.57 Comparison of focusing of the Ai mode in a 1.82mm 302 stainless steel plate with 5 machined FBH

(diameter of 1.5mm, 50% depth), (a) Configuration; (b) B-Scan ; (c) Depth focusing at 47 mm ; (d) Dynamic Depth Focusing (DDF) from 40mm to 60mm (step of 2mm) [Sicard et al. 2007]

The selection of a Lamb wave mode and array was carefully evaluated according

to the application in order to fulfill spatial sampling criteria while ensuring satisfactory

mode amplitude. It then arises that such a technique would be rather well suited for low

50

frequency inspection with the S0 mode due to the small dispersion effects and the

relatively large wavelength, which allows the use of larger transducer elements.

2.7.6 Conclusion

Focal laws were developed for phased array imaging using Lamb waves

generated using the angle wedge method. It was found that the formulation of the laws

does not differ from that of a typical bulk wave problem and that the phase matching can

be performed without considering the wedge properties. The study highlighted the

advantages of using a tone burst pulser to perform phased array imaging, although

common phased array systems based on spike pulser technology could also be used in

some situations.

2.8 Ultrasonic wave velocity signal interpretation of simulated concrete

bridge decks [Toutanji 2000]

This paper presented the results of an experimental and analytical study

conducted to interpret and characterize the ultrasonic wave signals received from

different kinds of anomalies in concrete bridge decks. Both the surface and direct

methods were investigated. In order to study the relationships between the types of

cracks, crack depths and the frequency spectra, specimens with differing artificial cracks

were tested. The predicted crack sizes matched well with the actual sizes. The results

presented in this study demonstrated that the ultrasonic pulse velocity technique was a

promising means of both providing information about the internal conditions of concrete

bridge decks and estimating crack sizes in these structural members.

2.8.1 Experimental procedure

Test specimens

Four batches of twelve concrete slabs were constructed with equal specifications.

The mix design ratio of the concrete slabs was cement: sand: gravel: water = 1:2:3:0.55

by weight. The concrete slab specimens measuring 610 x 610 x 152 mm were cast in

wood forms. The molds were filled in two layers, each layer was rodded 25 times with

the tamping rod and the simulated cracks were placed horizontally in the center of the

slabs. The coordinates of the simulated crack position were noted for later reference.

51

The varying internal conditions in the concrete slabs are summarized in Table 2.7

One slab was constructed as plain concrete with no artificial crack. Seven specimens

were cast with artificial cracks of Plexiglas. Plexiglas thin plates of the same size were

placed at different depths. In two of the slabs air crack and honeycombed concrete were

positioned. Fresh and saline-water delaminations at the center of two slabs were

introduced.

Two concrete slabs from the same batch were made to analyze and interpret the

waveforms at different vertical crack depths. Each concrete slab measured 30 cm long, 30

cm wide and 15 cm thick. Cuts were made vertically and parallel to the thickness of the

concrete slab, as shown in Figure 2.58.

Measurements of the waveform were registered by the transducers placed on the

surface of the concrete slabs. The depth of the crack ranged from 0 to 9 cm with an

interval of 1.5 cm.

Table 2.7 Summary of specimen configurations [Toutanji 2000]

Figure 2.58 Details of test specimens [Toutanji 2000]

52

Instrumentation

`A V-Meter Mark II ultrasonic test system was used in this research. This device

generated ultrasonic pulses and measured the time taken to pass from one transducer to

the other through the material interposed between them. The V-Meter Mark II gave a

direct reading of subtracted from all future readings. The V-Meter Mark II was connected

to a Personal Computer via a TP508| interface card. The TP508 was used as a digital

storage oscilloscope, a spectrum analyzer, a voltmeter, or a transient recorder. To get a

good coupling between the transducers and the slab, lithium complex grease was used as

a coupling agent.

Vibration of thin plate

When a wave propagated through a thin plate, longitudinal waves were generated

causing a compression force on one surface and tension force on the other surface of the

plate. The flexural motion of the thin plate was analyzed on the basis that every straight

line perpendicular to the middle surface of the plate remained straight after deformation

and perpendicular to the deflected middle surface; this surface was known as neutral axis.

This implied that the total longitudinal force on the neutral axis was equal to zero.

Nevertheless, a curvature moment and shear force produced the curvature effect on the

plate [Landau et al. 1986].

2.8.2 Results

2.8.2.1 Horizontal cracks

Time domain

In time-domain, to characterize the waveforms of different slabs was difficult.

Due to the presence of artificial cracks in the concrete slabs, the waveforms seemed to

spread out and their shapes became less uniform as compared to that of plain concrete, as

may be seen in Figure 2.59. This was because the device had send a longitudinal pulse

with a certain band-width and when this pulse traveled through a material and hit a

discontinuity (a crack), part of the energy (wave) was reflected, another was scattered,

and the remainder continues to travel through or around the crack. The superposition of

these waves was depicted in the time-domain responses (Figure 2.59).

53

Figure 2.59 Time domain responses of concrete slabs with various types of artificial cracks (direct method) [Toutanji

2000]

Frequency spectra domain

Data was transformed from time-domain to frequency spectra domain using the

Fast Fourier Transform technique in which it was possible to relate the maximum peak

frequencies of the waveforms to the types of cracks. Figure 2.60 shows the spectral

frequency responses of the same specimens analyzed in Figure 2.59, plain concrete

specimen and specimens with various types of artificial cracks. The shapes of the Fourier

frequency spectrum of the waveform were different in each specimen, and each specimen

showed a different maximum frequency peak. The specimen with Plexiglas exhibited a

maximum peak of around 16 kHz whereas the plain concrete specimen showed a

maximum peak of around 8 kHz. The specimen with fresh water-filled crack exhibited a

maximum frequency of around 7.5 kHz whereas the saline water-filled crack specimen

exhibited a maximum peak of 10 kHz. All graphs were normalized with respect with the

transducer resonance frequency of 54 kHz.

54

Figure 2.60 Frequency domain responses using the direct method of concrete specimens with different artificial cracks [Toutanji 2000]

The cracks were located at different depths, 3, 5, 7.5, 10, and 12 cm from the top

surface of the concrete slab. Results showed that, using the direct method, concrete slabs

with the same crack type and size located at different depth exhibited similar maximum

peak frequency. The maximum peak frequency was around 15 kHz, regardless of the

depth of the crack (Figure 2.61).

When the cracks were deeper inside the concrete, more than half the depth of the

slab, the amplitudes were much lower as seen in Figure 2.62. This was expected as the

surface method the ultrasonic pulse propagates the concrete in the layer near the surface

and, thus, does not give any information on deeper concrete; it only collects information

on the surface.

55

Figure 2.61 Frequency domain responses of concrete slabs with Plexiglas simulated cracks, measured 15 cm in diameter and 0.9 cm in thickness, placed at different depth (direct depth) [Toutanji 2000]

Figure 2.62 shows the amplitude spectrum of simulated crack with plexiglas discs placed at different depths

[Toutanji 2000]

56

2.8.2.2 Vertical cracks

The variation in the spectral responses of the concrete slabs with no cracks (solid

lines) and with vertical cracks at different depth (dashed lines) is shown in Figure 2.63.

The maximum peaks were analyzed. It was found that the amplitude of the frequency

peak decreases gradually with the increase of the crack depth. Thus, when the crack depth

was increased, the peak frequency shift left and lowered numerical values.

Figure 2.63 Frequency domain responses of concrete slabs with no cracks (solid lines) and with vertical cracks

(dashed lines) at different depth, using the indirect method [Toutanji 2000]

2.8.2.3 Crack size

The size of the cracks was estimated by modeling a crack either as a thin plate or

as a thin drum [Arfken et al. 1996]. The analytical values calculated using the vibration

of Thin Plate and the Drum models were compared to the experimental values. Summary

of the results is presented in Table 2.8. Results showed that the predicted radius size of

different cracks compared well with the actual radius sizes. The difference between the

predicted values and those of actual ranged between 2 and 30%.

57

Table 2.8 Results of estimated crack sizes [Toutanji 2000]

2.8.3 Conclusions

Time-domain waveform signals received using ultrasonic wave velocity technique

did not provide clear information on the internal conditions of concrete specimens,

whereas frequency spectra waveforms provided more information on the internal

conditions of concrete specimens. It was difficult to relate peak frequency to horizontal

crack depth using the surface method (indirect method). However, the amplitude or the

intensity of the waveforms was higher in specimens with cracks close to the surface.

Using the direct method, concrete slabs with the same crack type and size located at

different depth showed similar maximum peak frequency. Evaluation of the crack depth

was estimated from curves that relate the peak frequency ratios to the normalized crack

depth. The crack size in concrete slabs was estimated using the Thin Plate or the

Membrane Drum model. Results showed that the predicted radius sizes compared closely

to those of the actual radius sizes.

2.9 Nonlinear ultrasonic testing on a laboratory concrete bridge deck

[Shannon et al. 2006]

In this research, Non-Linear ultrasonic testing (NLUT) of a laboratory concrete

bridge deck was done to detect distributed damage along the deck. These results were

compared to the results of pulse velocity tests. NLUT was successful in detecting and

increase in damage in the concrete deck from areas of low stress near the supports, to

areas of high stress at mid-span and near the girder cut.

58

2.9.1 Experiments

The Bridge was simply supported with span 40ft and width 11ft. The deck

thickness was 6 in. and was supported by three W2lx62 girders spaced 4.5 ft. on center.

Tests were conducted on one of the sides of the bridge deck at six different locations that

ranged from areas of low damage level to high damage level. Figure 2.64 shows the

testing and cut girder locations. Three transducers were configured at each location with

three different power levels (30%, 60% and 90%). Figure 2.65 shows the three transducer

configurations. The transducers were configured in such a way that large volume of the

material would be tested.

Figure 2.64 Plan view of testing and cut girder locations [Shannon et al. 2006]

Figure 2.65 View of transducer configurations from bottom of deck [Shannon et al. 2006]

RITEC SNAP measuring system was used to conduct the NLUT tests. This used

two tone burst pursers to generate the signal. A frequency sweep of 50 to 75 KHz was

used to conduct each test. The data was stored in the computer and the amplitude vs.

frequency graphs were displayed for each test. The peak amplitude was used to calculate

the harmonic ratios.

59

Pulse velocity tests using both one-sided and two-sided transmission were

conducted at the same locations as the NLUT. For the two-sided pulse velocity test, the

three transducer configurations were same except that the receiver was located at the

center of the configuration on the top of the deck.

2.9.2 Results

Average of the harmonic ratios calculated at each location was taken. The

percentage change in harmonic ratio was calculated using the undamaged reference

points as locations 3 and 6, as shown in the Figure 2.64. Locations 3 was reference point

for locations 4 and 5. Location 6 was the reference point for locations 7 and 8.

Figure 2.66 shows the average percent change in the second harmonic ratio for

locations 3 through 5 and 6 through 8 which indicated that as damage increased, the

second harmonic ratio increased. The results showed that there were intermediate levels

of damage at location 4 and high levels of damage at location 5.

Figure 2.67 shows the average percentage change in third harmonic ratio which

indicates that the third harmonic ratio increased drastically from the undamaged reference

points.

Figure 2.66 Average percent change in second harmonic ratio, A2/A12


60

Figure 2.67 Average percent change in third harmonic ratio A3/A13


Table 2.9 Comparison of NLUT and pulse velocity [Shannon et al. 2006]

Generally with increase in damage, the pulse velocity should decrease. Locations

3 to 4 and 3 to 5 for the two sided transmission and locations 3 to 5 for one-sided

transmission were the only locations that showed this trend.

2.9.3 Conclusions

NLUT easily detected the intermediate levels of damage. Location 5, which was

closest to the cut girder and showed the most visible damage, had the largest percent

change in both harmonic ratios over all other locations. The second harmonic ratio is less

sensitive to damage than the third harmonic ratio.

61

2.10 Application of ultrasonic methods for early age concrete

characterization [Mikulic et al. 2005]

This Paper discussed about the ultrasound methods that have potential to be used

for the purpose of conducting the standardized characterization tests like Penetrometer,

pull out and Vicat needle methods for young concrete. Based at phase, velocity,

frequency, attenuation, relaxation and reflection measurements, material properties were

determined. With ultrasonic methods, it was possible to determine the kinetics and degree

of hydration, setting time, compressive strength and dynamic modulus of elasticity. The

longitudinal compressive wave velocity through concrete and mortar was measured

during hardening process. In the first few hours after preparation microstructure of fresh

concrete is very brittle. In contrast to conventional testing methods, ultrasound waves

with enough small energy can't disturb microstructure.

2.10.1 Measurements based on velocity of longitudinal waves

To study setting and hardening processes on two concrete mixtures: without

admixture and with addition of retarder (Table 2.10), ultrasound test method with short

pulses of longitudinal waves was used. To perform measurements, 10×10×30 cm

concrete prisms were prepared. Simultaneously with the measurements of ultrasound

velocity, setting time was determined by means of Penetrometer on separate specimens

(Figure 2.68).

Table 2.10 Mixture composition [Mikulic et al. 2005]

62

Figure 2.68 Ultrasound measurements and setting time determination by means of penetrometer [Mikulic et al. 2005]

Figure 2.69 Velocity in time for (a) concrete without admixture (mixture I) (b) concrete with addition of retarder (mixture II) [Mikulic et al. 2005]

Fresh concrete contained many air bubbles during the first two hours after mixing

which resulted with small and constant velocity of ultrasonic waves, for some extent

greater than ultrasonic velocity in air. As content of air decreased, velocity increased to

about 1500 m/s which corresponded to ultrasonic wave velocity in water, as shown in

Figure 2.69. With the hydration process development, concrete lose water and ultrasonic

wave velocity increase became slow, which is shown at the right side of graphs in Figure

2.69. Results represented in Figure 2.70 and Figure 2.71 shows that wave velocity

increased with age, dependent on used admixture and on w/c ratio.

63

Figure 2.70 Velocity of P waves dependent on setting and hardening process for (a) different admixtures [Grosse et al. 2003] and (b) different w/c ratios [Herb et al. 1999]

Figure 2.71 Transmitted energy and velocity of P waves during cementitious mortar setting [Grosse et al.2001]

2.10.2 Measurements based on velocity of transversal waves

It was well known that transversal ultrasonic waves can propagate in the rigid

bodies, but not in the fluids. This made possible determination of initial setting time for

concrete or mortar mixtures. In the point of initial setting, concrete suddenly started to

conduct transversal ultrasonic wave, so this moment can be determined with great

accuracy.

2.10.3 Reflection of elastic waves measurement

Method of elastic waves reflection was used for measurement of the reflection

coefficient between steel plates inserted into concrete and concrete. Figure 2.72 shows

experimental device [Shah et al. 1999].

64

Figure 2.72 Schematic representation of device for determination of young concrete properties [Mikulic et al. 2005]

Piezoelectric transducer transmitted ultrasonic wave pulse into steel and

transmission of acoustic waves into concrete changed dependent on phase of concrete

setting. The reflection coefficient expressed in decibels was used to represent the

measured results [Shah et al. 1999, Akkaya et al. 2003] usually named as a reflection loss

(RL(t)) or wave reflection factor (WRF). Reflection loss described relation (2.8), where

Ar1(t) and Ar2(t) are amplitudes of first and second signal reflected on the boundary

between steel plate and concrete. Figure 2.73 shows the first and second reflection of the

measured signal.

(2.8)

Figure 2.73 First and second reflection of measured signal [Mikulic et al. 2005]

Figure 2.74 illustrated experimental results of reflection loss measurements which

was dependent to concrete setting and hardening. RL(t) had value of 1 approximately until

starting of the hydration reaction. When the cement hydration temperature increased,

reflection loss decreased, and after end of exothermic reaction (induction period),

decrease of RL(t) became slow and linear. It seemed that reflection loss method can be

65

used for initial and final setting time determination which was marked in the Figure 2.74

with numbers 1 and 2.

Figure 2.74 Reflection loss and temperature of concrete during the hydration process development [Shah et al.

1999]

Experimental results [Akkaya et al. 2003] had linear dependence of RL(t) to

compressive strength, as shown in Figure 2.75.

Figure 2.75 Reflection loss as a function of concrete compressive strength [Akkaya et al. 2003]

Figure 2.76 illustrated experimental results which showed decrease of WRF with

concrete setting and hardening process for different used admixtures [Voight et al. 2001].

By comparing pastes of different water-cement ratio (Figure 2.76 and 2.77), it was noted

that the slope of the linear regression lines changed with water-cement ratio [Sun Zhihui

et al. 2004].

66

Figure 2.76 Decrease of a WRF factor with time [Voight et al. 2001]

Figure 2.77 Reflection loss as a function of degree of hydration for different w/c ratio samples [Sun Zhihui et al.

2004]

Figure 2.78 Correlation between the reflection loss and decrease of capillary porosity [Sun Zhihui et al. 2004]

Decrease of capillary porosity as seen in Figure 2.78 was defined as a difference

between the volume fraction of pore phase at initial stage (Pc0) and the capillary porosity

(Pc) for a given degree of hydration.

2.10.4 Conclusions

Ultrasonic testing methods can be based on the measurement of relaxation,

attenuation, velocity and reflection of longitudinal, transversal or Rayleigh ultrasonic

waves [Sekulic et al. 2004]. This Paper described only some of these approaches to the

measurement of setting and hardening of concrete. Results indicated that impulse velocity

67

was directly effected by moisture and air content, which was to a certain degree the weak

point of ultrasound testing of concrete setting time. Knowledge about time development

of material properties during hydration process and physical properties of ultrasonic

waves was crucial, for successful application of ultrasonic methods.

2.11 Fatigue crack detection in metallic members using ultrasonic

rayleigh waves with time and frequency analyses [Halabe and

Franklin 1997]

This paper developed a methodology to standardize several factors for producing

repeatable signals, and focused on the use of Rayleigh waves for detection of fatigue

cracks. It also included fatigue crack detection in metallic structural members using both

time and frequency domain analyses.

Through transmission and pulse echo modes were used to receive signals from

cracked and uncracked sections. Power spectral density was obtained by transforming the

time domain signals into frequency domain using fast Fourier transform. Changes in the

characteristics of power spectral density were observed to detect the cracks in several

aluminum plates and beam specimens.

2.11.1 Analysis of ultrasonic signals

Time domain analysis

Time domain signals were interpreted using the arrival time, peak to peak signal

amplitude and root mean square amplitude. All these parameters, signal arrival time, loss

in peak to peak amplitude and root mean square amplitude were used to detect cracks in

through transmission mode and pulse echo mode.

Power spectral density

Fourier transform was used to obtain amplitude versus frequency plots which

produced additional information for crack detection. Power spectral density was defined

as the distribution of energy in a signal as a function of frequency and was given by

Moharaz and Elgadamsi (1989) as:

(2.9)

Where the magnitude of the fast Fourier transform at frequency f

68

the corresponding energy.

The parameters considered to describe the characteristics of power spectral

density curve with respect to frequency were size, shape and location. Size was defined

by the area under the power spectral density magnitude curve and was mathematically

represented as

The shape parameter was defined as the rth moment of the power spectral density

magnitude curves by Horne and Duke (1993). The third moment (r=3) for testing in

wood was proposed by Lemaster et al. (1993) and mathematically represented as

The location parameter is taken as the central frequency for the power spectral density

magnitude plot and was mathematically represented as

(2.12)

These parameters were used to detect the presence of cracks as the cracks affect the

energy in the received signal.

2.11.2 Measurement conditions

The magnitude of and associated parameters depended on several

experimental factors such as type of transducers and couplant used and clamping force on

the transducers [Halabe et al. 1996]. These factors had to be carefully standardized for

specific application for optimal measurement conditions.

Ultrasonic equipment and transducers

A Maximum of 425 V peak to peak alternating current was produced using

Ultrasonic pulse generators. Ultrasonic longitudinal waves were produced and received

using dry coupled transducers. Shear waves and Rayleigh waves were produced by

69

coupling polystyrene wedges to the longitudinal transducers. Signals were received and

amplified using a broadband receiver. These signals were digitized using a digitizing

oscilloscope.

Clamping force and couplant

The optimal clamping force for P-wave, S-wave and R-wave transducers were

determined for the transducers used in their study [Franklin 1997]. Tests were conducted

under a constant clamping force of 20 lb on R-wave transducers using different

couplants. Figure 2.79 shows power spectral density magnitude plots corresponding to

various couplants.

Figure 2.79 Power spectral density magnitude plot for various couplants [Halabe and Franklin 1997]

Sonotech UT-X couplant was chosen for all the further experiments based on the above

considerations and the strength of the power spectral density plots in Figure 2.79.

Signal acquisition and data processing

The sampling rate at which the analog signal was digitized was 25 MHz and

transducer was excited at 2.25 MHz . The effect of random noises was reduced by taking

average of 100 waveforms. DADiSP software was used to perform time and frequency

domain analyses. To nullify the small variations produced by the pulser possibly due to

voltage fluctuations, the power spectral density curve was normalized with the energy of

the excitation pulse.

70

Excitation pulses and wave types

Rayleigh waves when compared to longitudinal and shear waves produced a

good signal to noise ratio and high sensitivity to fatigue cracks [Franklin 1997]. Rayleigh

wave transducers with five and ten cycle sine pulse excitations were therefore used for all

further experiments. The sine pulses were set to 425 V for five cycle pulse and 315 V for

ten cycle pulse peak to peak.

2.11.3 Results and discussions

Aluminum plates with dimensions 610 by 610 by 6.4 mm and with 25 mm long

simulated single and multiple fatigue cracks were tested. In single crack configurations,

the cracks were placed at a distance of 38 mm from the transducer and at a distance of 76

mm from the backwall. In multiple crack configurations, tow cracks were placed

symmetrically about the center and spaced at 25 mm and 254 mm apart, respectively.

Cracks were detected along the length of the plate in the pulse echo mode using

the crack echo signals. Loss in peak to peak and root mean square amplitude of the

backwall echo were observed in the time domain signals. There was also a significant

loss in the third moment of the power spectral density curve corresponding to the

backwall echo signal but the central frequency remained unchanged.

It was observed that there was no significant change in the time of flight in

through transmission mode. But it was observed that the peak to peak and root mean

square amplitude dropped by 80 percent and the area and the third moment dropped by

about 95 percent without any change of central frequency.

When the results from five and ten cycle excitations were compared it was

observed that a ten cycle sine pulse excitation produced a higher signal to noise ratio

which is very crucial in the case of long travel distances.

Fatigue crack specimens

Aluminum rectangular beams with dimensions 406 by 32 by 76 mm were tested.

Figure 2.80 shows the specimens used for the real and simulated fatigue cracks. T

denotes the transmitting transducer, R denotes the receiving transducer. Both transducers

were used in the through transmission mode. In pulse echo mode transmitting transducer

itself was used as receiving transducer.

71

Figure 2.80 Specimens for fatigue crack detection: (a) uncracked; (b) microfatigue crack 0.025 mm thick (c) macrofatigue crack 1 mm thick (d) saw cut 1 mm thick [Halabe and Franklin 1997]

Signals along these specimens in through transmission mode using the Rayleigh

waves produced by ten cycle sine pulses were acquired. As shown there was no

significant change in the time domain signals received from uncracked and microfatigue

cracked specimens.

Figure 2.81 Time domain signals obtained (a) uncracked section; (b) microfatigue crack [Halabe and Franklin 1997]

72

Power spectral density frequency curves of the two signals from the Figure 2.81

are shown in Figure 2.82. There was a significant difference observed between these

curves which were quantified using the area under the curves and plotted as shown in

Figure 2.83.

Figure 2.82 Power spectral density plots (a) uncracked section; (b) microfatigue crack [Halabe and Franklin 1997]

Table 2.11 Results from through transmission using rayleigh waves produced by ten cycle sine pulse on fatigue specimens (2.25 MHz central frequency) [Halabe and Franklin 1997]

73

Figure 2.83 Integral of power spectral density magnitude plots obtained from through transmission using rayleigh waves produced by ten cycle sine pulse on fatigue specimens [Halabe and Franklin 1997]

In pulse echo transmission mode, Rayleigh waves produced by five sine pulse

excitations were used to locate the fatigue cracks from the received echo signals. Figure

2.84 shows the time domain signals from the uncracked and microfatigue cracked

specimens. The second multiple echo from the crack was overlapped with the backwall

echo as the crack was located at the center.

Figure 2.84 Time domain signals obtained from pulse echo using rayleigh waves produced by five cycle sine pulse on fatigue specimens: (a) uncracked section; (b) microfatigue crack [Halabe and Franklin 1997]

74

Figure 2.85 shows the integral of power density curves for the backwall echo

which indicated a significant decrease due to the microfatigue crack. This decrease was

also noticed in other amplitude and energy parameters as shown in Table 2.12.

Table 2.12 Results from pulse echo using rayleigh waves produced by five cycle sine pulse on fatigue specimens (2.25 MHz central frequency) [Halabe and Franklin 1997]

Figure 2.85 Integral of power spectral density magnitude plots for back echo obtained from pulse echo using rayleigh waves produced by five cycle sine pulse on fatigue specimens [Halabe and Franklin 1997]

Long I-beam specimen

Tests were conducted on a 3 m long aluminum I-beam shown in Figure 2.86 in

order to assess the detectability of cracks on long specimens.

75

Figure 2.86 Long Beam Specimen [Halabe and Franklin 1997]

A simulated fatigue crack of 1 mm wide and 25 mm long was located at 610 mm

from one end. The specimen was tested in pulse echo mode using five cycle sine pulse

excitation at 0.9 MHz frequency and also tested in through transmission mode using 10

cycle sine pulse excitation at 2.25 MHz. Figure 2.87 shows the time domain signals

received from the two test configurations and Figure 2.88 shows the integral of power

spectral density magnitude plots. The bottom flange was tested for pulse echo mode for

uncracked case and the results are shown in Table 2.13. It was implied from Figure 2.87

that the cracks could easily be detected when the cracks are close to the transducer.

In through transmission mode, the long beam specimen showed a decrease of 10

to 20 percent in the root mean square amplitude, area under the power spectral density

and the third moment of power spectral density. The peak magnitude of power spectral

density was found to be increased by about 20 percent.

76

Table 2.13 Results from pulse echo using rayleigh waves produced by five cycle sine pulse on long beam specimen (0.9 MHz central frequency) [Halabe and Franklin 1997]

Figure 2.87 Time Domain signals obtained from pulse echo using rayleigh waves produced by five cycle sine pulse on long beam specimen: (a) crack near sensor; (b) crack far from sensor [Halabe and Franklin 1997]

77

Figure 2.88 Integral of power spectral density magnitude plots for back echo obtained from pulse echo using rayleigh waves produced by five cycle sine pulse on long beam specimen [Halabe and Franklin 1997]

2.11.4 Conclusions

Rayleigh waves produced by five to 10 cycle sine pulse excitations were found to

be very efficient in detecting fatigue cracks in metallic members. Longer duration pulse

excitation was found to be more desirable for through transmission tests and shorter

duration for pulse echo tests. Combination of frequency domain analysis with time

domain analysis provided more detailed information for detecting and locating fatigue

cracks in structural members. Root mean square amplitude, arrival time of echoes and

area under the power spectral density magnitude curve, all these parameters proved to be

simple yet robust methodology for detecting cracks using ultrasonics.

2.12 Ultrasonic diagnostic load testing of steel highway bridges

[Mandracchia 1996]

Acoustic Strain gauge (ASG) is a product that utilizes a non-contact ultrasonic

technology to measure applied strain requiring no paint removal and minimal surface

preparation. A diagnostic test under the supervision of Dr. Abba Lichtenstien was

conducted to validate the ASG as being functionally equivalent to the resistance strain

gauge, and to demonstrate a cost effective enabling technology to the civil and structural

engineering communities. For the purpose of this study the Rodeo Gulch bridge

superstructure was modeled and structural loading profiles were determined using both

78

resistive and acoustic strain measurement techniques, and were compared to theoretical

loads in order to determine if the structure was operating in a safe and reliable manner.

2.12.1 Measurement Strategy

The ultrasonic technique implemented in the ASG involved, measuring the

variation of a Rayleigh waves Time-of-Flight (TOF) between two acoustic transducers to

dynamically measure compressive and tensile strains. Allowing the transducers to move

freely relative to each other generated variations in the acoustic wave‟s TOF, as the

distance between the transducers change due to deformations caused by the strain. The

Rayleigh acoustic wave is geometrically bounded near the surface of the structural

member which makes it easy for the direct comparison of Rayleigh wave‟s TOF and

applied strain at the measurement surface.

Electromagnetic Acoustic Transducers (EMATs) were used to induce acoustic

energy into the structural member under test. It was required that most of acoustic energy

be focused in a specific direction, which was achieved by incorporating a periodic

structure into the EMAT coil geometry.

Distance between the EMAT coils and the measurement surface is called the

Liftoff, which influence the acoustic transducers coupling efficiency. The relationship

takes the form

Vr=Voe-2ΠG/D

(2.13)

where Vr is the received signal amplitude, Vo is the zero liftoff signal amplitude, G is the

liftoff distance between the measurement surface and the EMAT coils, and D is the

spacing between adjacent transducer coils. With increase in liftoff (G), the current

distributions were spread and began to overlap, leading to cancellation of current and a

decrease in the acoustic coupling efficiency.

79

Table 2.14 Acceptable liftoff for various frequencies [Mandracchia 1996]

2.12.2 Productization

To implement the ultrasonic measurement technology described above, Sonic

Force had chosen in the form of the Acoustic Strain Gauge as shown in the Figure 2.89

Figure 2.89 Acoustic strain Gauge [Mandracchia 1996]

ASG used was an electrically shielded hand held instrument with control and

measurement electronics, acoustic transducers and deployment mechanisms contained in

it. A laptop computer via a RS-485 communications interface was used for individual

ASG controls, data acquisition, display and storage. A prototype of the ASG shown

above was used for the purposes of the diagnostic test program described in this paper.

2.12.3 Laboratory test results

Various structural members were tested to verify linearity of the acoustic

measurement technique. Strain gauges were placed on the surface of each test specimen,

then placed in an Instron force application instrument capable of applying 66,000lbs and

then subjected to bending, compressive and tensile stresses. Figure 2.90 illustrated the

measured variation of the Rayleigh waves‟s TOF for various applied loads on a L6 x 4

mils steel angle beam.

80

Figure 2.90 Sensitivity Factor [Mandracchia 1996]

The data had shown a high degree of correlation between the acoustic wave‟s

TOF and the surface strain as measured by a resistive strain gauge. The slope of the best

fit trend line determines the calibration factor, m, used to translate TOF measurements

directly to strain. The measured calibration coefficients had been determined to be

approximately 0.06 nSec/με for all structural geometry‟s tested.

2.12.4 Diagnostic load test results

The Rodeo Gulch Bridge is located in Santa Cruz along sequel Drive, a four lane

highway built in two stages. The portion which was tested composed of structural steel

members with its first span from the west abutment, 30 feet long and has seven stringers

(20 I 65) with a reinforced concrete deck on top of these stringers. There are no shear

connectors between the slab and steel. The first interior beam and second interior beam

were instrumented at the 9 foot distance from the west abutment for tension

measurements in the bottom flange of the beams. The gauges were installed at the ends of

these two beams along the centerline of the bearing at the abutment to check on shear

values in the web. The weights of two trucks were measured (truck A 30 tons, truck B

22.8 tons). After recording the zero readings on all the gauges, truck A was placed at the

9 foot position and readings taken. Truck A then was moved to the 15 foot spot (middle

of the beam) and readings were taken. Then the whole procedure was repeated for truck

A; and truck B was entered onto the structure and the four-step test repeated twice and

readings were taken. As an addition, both trucks were placed on the bridge at the same

81

time, at the 9‟ and 15‟ locations, and readings were taken. The readings are shown in the

Table 2.15.

Table 2.15 Diagnostic Load Rating data [Mandracchia 1996]

The values obtained by the resistance strain gauge were expressed in psi and were

compared with ASG readings which were also converted to psi shown in Table 2.16.

Table 2.16 Diagnostic Load Rating Test Results [Mandracchia 1996]

Bridge members were found to be performing well as the actual stresses were

much smaller than calculated. Thus, the Rodeo Gulch Bridge, more than 60 years old, is

still in good condition.

The other purpose of the test was to validate the ASG as being functionally

equivalent to the resistance strain gauge. Figure 2.91 shows the regression analysis of all

the 48 resistive and acoustic strain measurements obtained

82

Figure 2.91 Acoustic vs. Resistance Strain Gauge Correlation [Mandracchia 1996]

A correlation of 0.998 was exhibited with an average deviation between the

conventional resistive ASG‟s to be 0.73 μin/in demonstrating considerable agreement

between measurement techniques. Furthermore, The ASG‟s standard error +/-2.7 μin/in

or approximately +/-1.5% relative to the maximum reading proved that the performance

of the ASG can be considered more reliable for determining live load stresses in the

bridge members tested.

2.12.5 Cyclic loading

Two ASGs were used to monitor accumulated cyclic loading on a very active

bridge located outside Milwaukee, Wisconsin.

Figure 2.92 Time series strip chart (100 seconds) [Mandracchia 1996]

Measurements were taken approximately midspan, on the bottom flanges of the

two adjacent girders, over a 20 minute period. ASG #7 was placed closet to right lane and

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ASG #5 on the girder to the left. Generally, the location of heavier traffic should be

weighted toward the right lane and this was confirmed by the large strain hits recorded by

ASG#7 as shown in the Figure 2.92.

Rainflow counting is a method of determining accumulated irregular loading

histories and normalizes into a series of constant amplitude loading events. A typical

Rainflow diagram illustrated the irregular load histories of ASG #5 and #7 in Figure 2.93.

Figure 2.93 Rainflow (20 minutes) [Mandracchia 1996]

2.12.6 Conclusion

Sonic Force Company had demonstrated cost effective strain measurements not

only on fractured critical bridges far beyond their design life, but also as part of regular

bridge inspections.

2.13 Ultrasonic instrumentation for measuring applied stress on bridges

[Fuchs et al. 1998]

This study focused on the advantages of ultrasonic technique as a possible method

for measuring applied stress compared to the conventional strain gauges. Acoustoelastic

effect is used in ultrasonic technique, where a change in stress results a change in

ultrasonic velocity.

Electromagnetic acoustic transducer (EMAT) was chosen for applied stress

measurement on bridges using ultrasonics. Since EMATs work as noncontact

transducers, they could be used easily on the rough, pitted and rusted surfaces of the

bridge. Also, these transducers can propagate waves through nonconductive and

conductive (lead-based) paint which gave an additional advantage over the strain gages as

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they required paint removal for the surface preparation. For measuring the maximum

bending stress along the length of the girder using EMATs, Rayleigh wave propagating in

the flange of the bridge girder was chosen as it travels near the surface of the test

specimen and its velocity is related to the stress in the extreme fiber of the girder.

To determine the effectiveness of the applied stress measurement system using

EMATs under field conditions, a series of field tests were conducted on girders of three

bridges (I-79, Rt. 119, and I-64). These girders were made of A36 steel painted with lead-

based paint. The field measurements were taken under static and dynamic loading on the

bridge using a test vehicle travelling at 8 km/h (5 mph) and at 89 km/h (55 mph),

respectively. Figures 2.94 and 2.95 show the test results for the static and dynamic

loading cases, respectively. In the figures, the strain gage measurements have been

shifted upward by a value of 2 ksi (~14 MPa) to ensure that the strain gage and EMAT

curves are distinctly visible without an overlap. Both cases (Figures 2.94 and 2.95) show

an excellent comparison between EMAT and strain gage measurements. It is very clear

from these figures that EMAT produced data as accurate as a strain gage.

Figure 2.94 Test vehicle traveling at 8 Km/h (5 mph): EMAT (bottom), strain gage (top) [Fuchs et al. 1998]

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Figure 2.95 Test vehicle traveling at 89 Km/h (55 mph): EMAT (bottom), strain gage (top) [Fuchs et al. 1998]

2.14 Summary

Different ultrasonic techniques (contact and non-contact) and their applicability

on bridge components to detect the defects were discussed as a part of this review. Also,

a review on recent advances in the field of ultrasonics was conducted.

Ultrasonic techniques such as plate wave ultrasonic technique, pulse-echo

technique and angled pitch-catch technique proved effective for detecting fatigue cracks

at the welded joints of bridge girder. Field ultrasonic inspection technique (contact

ultrasonic technique) and immersion tank ultrasonic technique (non-contact ultrasonic

technique) both provided similar results in finding the defects and the extent of cracking

in structural components. Both techniques showed a high level of consistency in finding

the defect size and location.

A new wireless-autonomous system was introduced that acquired processes and

delivered waveform data for analysis, and inferred structural integrity on the condition of

the structure. This proved efficient in predicting the location and severity of the damage

from a large dataset.

Ultrasonic technique such as pulse velocity technique also proved to be an

efficient means of providing information about the internal conditions of concrete bridge

decks and estimating crack sizes in these structural members. Ultrasonic technique also

proved to be efficient in conducting the standardized characterization tests for concrete as

it provides the advantage of conducting the tests without disturbing the microstructure of

concrete.

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3 LITERATURE REVIEW: INFRARED

THERMOGRAPHY

This chapter presents the literature review on recent advances and applications of

infrared thermography testing on different structural components of a bridge. All the

papers reviewed here are from recent years which present the latest advances in the field

of infrared thermography for nondestructive evaluation.

3.1 Effects of solar loading on infrared imaging of subsurface features

in concrete [Washer et al. 2010]

This paper focused on the effects of solar loading on thermal imaging conducted

using infrared thermography for the detection of subsurface defects in concrete structures

of bridges. A large concrete test block was constructed with built-in defects and was

monitored to measure the effect of solar loading on thermal imaging. Thermal contrast

seen in the thermal images of the concrete block was quantitatively shown and analyzed.

The effect of the depth of the embedded defect and time of inspection for conducting the

infrared thermography to get the best results with maximum contrast in thermal images

was discussed. The inspection parameters such as time of day and effect of radiant

heating from the sun were identified by quantitatively analyzing the effect of solar

loading on the detectability of subsurface defects in concrete.

Figure 3.1 Diagram of concrete test block with embedded targets at depths of (A) 25mm, (B) 51mm, (C) 76mm, and (D) 127mm [Washer et al. 2010]

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Figure 3.1 shows the diagram of the large concrete test block measured 2.4m x

2.4m with a thickness of 0.9m and supported on a concrete footing. Styrofoam sheets

300mm x 300mm in area and 13mm thick were used as embedded targets and they were

attached to the reinforcing steel cage before placing concrete. An IR camera with a

thermal sensitivity of 0.08 °C was mounted in a test house located 9m from the concrete

block was used to capture the thermal images at 10 minute intervals, 24 hours a day. To

record the ambient temperature of the environment, the average wind speeds, the relative

humidity and the solar loading, an on-site weather station was installed.

Figure 3.2 shows the thermal image of south side of the test block (placed in an

open field 12 miles northwest of Columbia, MO) which shows the three of the embedded

targets with a different color contrast. This contrast was calculated by measuring the

difference between temperatures at the target and the overall remaining surface of the

block. Figure 3.3 shows the effect of solar loading on the thermal contrast for a 24 hour

period on 25th December 2007. The solar loading shown in this figure was taken from the

solar radiation sensor mounted adjacent to the test block. At approximately 2:00 p.m. the

maximum contrast occurred and a maximum solar loading of 360 W/m2 occurred just

after 12:00 p.m.

Figure 3.3 Thermal image of the test block showing embedded targets at depths of 25, 52 and 76mm

[Washer et al. 2010]

Figure 3.2 Thermal contrast at embedded targets for sunny day 12/25/07, indicating contrasts for 25, 51, 76, and 127mm targets relative to the solar load [Washer et

al. 2010]

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Table 3.1 Time of day each target reaches its maximum contrast and time lag relative to sunrise

Table 3.1 gives data about the time of day each target reached its maximum

contrast and time lag relative to sunrise. It was observed from the data that as the depth of

the target is increased, the targets had maximum contrast later in the day which indicated

the time lag for heat transfer through the concrete block. It was also observed that the

thermal contrast was reduced significantly as the depth of the targets increased.

From the results it was observed that the optimum time of day for detecting

subsurface defects varied from 5:40 hrs after sunrise for a 25mm deep target to slightly

more than 9:00 hrs after sunrise for a target at 127mm deep. Standard deviations were

typically observed on the order of 1:00 hr. The results also indicated that there can be a

delay between the solar loading on a concrete structure and the observation of subsurface

defects and also the periodic cloud cover can reduce the thermal contrast in the IR

images. These results were useful for knowing the best inspection times for portions of a

highway bridge exposed to direct solar loading.

3.2 Detection of subsurface defects in fiber reinforced polymer

composite bridge decks using digital infrared thermography

[Halabe et al. 2007]

This paper focused on the use of infrared thermography for subsurface defect

detection in FRP bridge decks. Subsurface delaminations of different sizes and thickness

were created at the flange-to-flange junction between two FRP deck modules in the

laboratory. These embedded subsurface defects were identified by using the infrared

technique. A laboratory and field study was also conducted to study the debond patterns

between the wearing surface and the underlying FRP bridge deck.

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ThermaCAM S60 (FLIR Systems) digital infrared camera shown in Figure 3.4 was

used for detecting the defects in the laboratory. This camera had the capability of

acquiring continuous radiometric images at a 60Hz frame rate and in the spectral range of

7.5-13mm.

Figure 3.4 Experimental setup using digital infrared camera and close up view of the camera [Halabe et al. 2007]

Figure 3.5 Front and cross-sectional views of the GFRP bridge deck specimens (a) without wearing surface overlay, and (b) with wearing surface overlay [Halabe et al. 2007]

Figure 3.5 shows GFRP deck specimens with and without wearing surface overlay.

Air-filled delamination of size 3” x 3” (76 x 76 mm) in plan and 1/20” (1.27 mm)

thickness was created on one side of the specimen in the middle of flange joint area. On

the other side of the specimen two more delaminations of sizes 2” x 2” (51 x 51 mm) and

1” x 1” (25 x 25 mm), both with thicknesses 1/16” (1.6 mm) were created. Figure 3.6

shows these specimens before they were joined to flanges of adjacent defect-free module.

Figure 3.7 shows the infrared image of the these specimens with embedded

delaminations. These infrared images were obtained from the specimens prior to pouring

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the wearing surface. The temperature differences between these delaminations and the

surrounding defect free area were observed to be 3.8, 5.8 and 10.7 °C respectively.

Additional examples of infrared images from specimens with wearing surface can

be found in Halabe et al. (2007).

Figure 3.6 Photographs showing the location of the delaminations [Halabe et al.2007]

Figure 3.7 Infrared image of the specimen with air-filled delaminations at the flange-flange junction [Halabe et al. 2007]

Infrared tests were conducted on a two-lane bridge in the field which had a 3/8”

(9.5 mm) thick polymer concrete overlay and the wearing surface overlay covered an area

of 26‟ x 41‟ (7.9 x 12.5m) in plan. Infrared images shown in Figure 3.8 were taken by

dividing the bridge into several small grids. These images were taken when the bridge

deck temperature was as high as 50 °C and the ambient air temperature was around 28-30

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°C. This field testing helped to find many debonds of different sizes on the bridge deck

and it was observed that two-thirds of the bridge deck had defective areas.

From the laboratory and field testing results it was observed that infrared

thermography is a very useful tool for defect detection in FRP composite bridge decks.

Figure 3.8 Digital photographs and corresponding infrared images of various debonded areas in the GFRP Bridge deck [Halabe et al. 2007]

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3.3 Introducing infrared thermography in dynamic testing on

reinforced concrete structures [Luong and Dang-Van 2005]

This paper discusses the use of infrared thermography to detect, observe and

evaluate the evolution of temperature changes that occur due to diverse physical

processes of irreversible physical phenomena in reinforced concrete structures. This

paper also highlighted the advantages of infrared thermography technique in observing

the physical manifestation of damage, to detect the intrinsic dissipation localization and

to evaluate the fatigue strength of concrete.

The energy that causes the plastic deformations in concrete is dissipated as heat and

this heat generation can be easily observed using infrared thermography. Vibro-

thermography process was used for observing the damage process of concrete materials.

A vibratory loading at 100Hz when applied on a specimen subjected to a given static

compression, using a high frequency servo-hydraulic test machine in the laboratory,

exhibited the irreversible plastic strain concentrations around gaps and cracks. This was

shown in the infrared image taken as seen in Figure 3.9.

Figure 3.9 Infrared thermography of a plain concrete specimen subject to compressive vibrations (temperature changes are given in degrees Celsius) [Luong and Dang-Van 2005]

Analysis of thermal images was done by isolating the intrinsic dissipation from

thermal noises and this was achieved by subtracting the thermal image at reference time

from the thermal image at 1000 load cycles using computer aid thermography software.

The resultant image processing provided quantitative values of intrinsic dissipation. This

procedure was followed for each step and then the fatigue damage mechanism was

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revealed by a break of the intrinsic dissipation regime. These experimental results were

summarized in the Figure 3.10.

Two large scale specimens (36 tons) representing 1/3rd

scaled five-story buildings

were tested under dynamic seismic loading on the major shaking table. The specimens

were tested for two different boundary conditions. The first specimen composed of two

lightly reinforced walls anchored to the shaking table allowing a plastic hinge at the base.

Examinations done after tests had shown failure of steel reinforcements. The dissipation

caused due to the plasticity of the steel was easily observed in the infrared image shown

in Figure 3.11.

Figure 3.10 Graphical determination of the fatigue limit FL of a plain concrete (dissipation is given in degrees Celsius proportional to energy) [Luong and Dang-Van 2005]

Figure 3.11 Infrared thermographic determination of dissipation caused by plasticity of steel reinforcements (temperature changes are given in degrees Celsius) [Luong and Dang-Van 2005]

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The other specimen also composed of two lightly reinforced walls, but was

simply rested on a 40 cm thick sand layer. Figure 3.12 shows the infrared thermographic

image taken after the test which shows the slippage of steel reinforcements.

Figure 3.12 Infrared thermographic determination of dissipation by slippage of steel reinforcements embedded in the concrete matrix (temperature changes are given in degrees Celsius) [Luong and Dang-Van 2005]

These experiments have demonstrated that the infrared thermographic technique

can be used to observe the physical process of concrete degradation and to detect the

occurrence of its intrinsic dissipation and can also be used to provide the fatigue limit of

concrete within a few hours instead of several months.

3.4 Infrared thermography and void detection in GRP laminates

[Allinson 2007]

This paper discussed on the use of infrared thermography to detect voids in

Fiberglass Reinforced Plastic (GRP) laminates used in the construction of yachts. These

yachts constructed with GRP are composed of an exterior gel coat, followed by layers of

fiberglass cloth, a core material, and more layers of fiberglass cloth. Voids are formed in

the laminates when there are some air bubbles or pockets formed while trying to remove

the excess resin used to wet the layers of fiberglass cloth by hand rolling with metal

bubble rollers.

The process involved heating of the GRP surface in discrete sections with a gentle

stream of warm air supplied by an electric hot air heat gun, and simultaneously scanning

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the surface with an infrared camera. Void areas in the GRP laminate warm quickly and

can be detected easily with the thermal contrast shown in the infrared camera between the

voided areas and the surrounding cooler areas. Thus proper and procedural IR thermal

scanning of GRP laminates by factory Quality Assurance and Quality Control personnel

under the direction of a certified Infrared Thermographer offers a quick and effective way

to find voids in Gel coat and laminates.

Figure 3.13 (a) Exterior surface of the gel coat is shiny and uniform. No suspicious signs of deformity or voids in the Gel Coat as seen with unaided eye (b) Thermal pattern as displayed by the Infrared Camera while the hull is being

gently warmed by the electric hot air gun [Allinson 2007]

3.5 Use of infrared thermography for quantitative non-destructive

evaluation in FRP strengthened bridge systems [Kumar and

Karbhari 2002]

This paper discusses the appearance and progression of damage in the FRP systems

and at the FRP-concrete interface with an increase in the level of loading by

quantitatively monitoring using infrared thermography. A three-girder two bay reinforced

concrete bridge deck segment was loaded under field representative conditions and

subsequently they were strengthened with FRP composites. The damage appearance and

progression was then quantitatively monitored using infrared thermography. Figure 3.14

shows the schematic of the deck-girder assembly and the test setup. The test was

conducted in three phases gradually increasing the load. The girder was strengthened

(a) (b)

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with externally bonded FRP composite before beginning of phase three which involved

further loading of the test specimen until the ultimate failure of the specimen.

Figure 3.14 (a) Schematic of deck-girder assembly (b) Test setup [Kumar and Karbhari 2002]

A pulsed IR thermography technique was used where two xenon flashtubes with 5

ms flash duration, each powered by a 6.4 kJ capacitor. An IR camera with 2-5 µm

spectral range was used to continuously acquire data at a 60 Hz frame rate for 10 s after

flash heating for each shot. The post processing of the data involved generating linear and

2-D thermal intensity profiles along the length of the composite for each location at each

load level. These thermal intensity profiles were then compared with the baseline thermal

intensity for the same location to quantitatively determine the appearance of the new

defects or the growth of existing defect areas. Figure 3.15 shows the thermal intensity

profiles generated from the thermography test conducted at different locations of the slab.

The thermal intensity profile generated from the baseline thermography inspections

which were carried out after strengthening slab with prefabricated strips i.e. before phase

two and phase three load cycles were compared with the thermal intensity profiles

generated from the thermography inspections at the end of the load cycles. This

quantitatively gave the size and accurate location of the damage at these locations of the

slab.

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Figure 3.15 (a) Representation of defect type 1 (no progression) (b) Representation of type 2 defects (c) Type 3 defect representative of debonding at the composite-concrete interface (d) Type 3 defect representative of

debonding initiating between the transverse and longitudinal FRP strips and continuing to the concrete-compoite interface (e) Representation of a defect at a localized crack opening (f) Defect representative of debonded area at

the intersection of the longitudinal and tansverse FRP strip (g) Thermographic representation of interlaminar debonding. Note: 846-0 kN indicates that the inspection was carried out after the specimen was unloaded after

loading to 846 kN load cycle [Kumar and Karbhari 2002]

3.6 Conclusions

Infrared Thermography technique proved to be the most efficient non-destructive

technique when inspecting structures over larger areas in a non-contact fashion. Infrared

thermography technique can not only be used to detect defects in concrete structures but

can also be used to detect air-filled and water-filled debonds in composite structures

such as FRP bridge decks. Infrared thermography can also be used to observe the

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physical process of concrete degradation by detecting the occurrence of intrinsic heat

dissipation and can provide the fatigue limit of concrete under load beyond which the

material is susceptible to failure. Infrared thermography test conducted on structures

under the solar heat can show very good results, i.e., a good thermal contrast between

the defective area and the surrounding defect-free area of the structure. However,

uniform solar heating is achieved only for field structural components which are

horizontal (e.g., bridge decks). Also, good infrared images are obtained on a hot sunny

day, typically between 11 A.M. to 3 P.M., when the thermal gradient through the

structural component is a maximum.

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4 INFRARED THERMOGRAPHY TESTING

This chapter presents the results of infrared thermography testing carried out on

several FRP composite encased wooden railroad ties. The laboratory test results included

infrared thermography testing of the ties before and after application of static and fatigue

loads on the ties. In addition, field testing was conducted to evaluate the condition of the

ties subjected to service loads and environmental degradation.

4.1 Laboratory testing, analysis and results

FRP composite encased wooden railroad ties were tested in the laboratory under

static and fatigue loading conditions to assess the ultimate load capacity of the ties before

sending similar ties to the field. Infrared testing was conducted on these railroad ties

under laboratory conditions, before and after subjecting the ties to static and fatigue load

tests, in order to study the profile of cracks and defects present on these FRP composite

railroad ties before loading, and subsequent development or growth of the defects due to

loading. The size of the cast composite ties was 102” x 9‟‟ x 7.5‟‟ (2591 mm x 229 mm x

190.5 mm) as shown in Figure 4.1. This figure shows a schematic of the three-

dimensional cross-section of the composite tie with wooden core. The actual composite

ties are shown in the photographs in Figure 4.2.

Figure 4.1 Schematic of the composite tie

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Figure 4.2 FRP encased wooden railroad ties being tested under static and fatigue loading conditions

Infrared testing was conducted on the bottom and top surfaces of these composite

ties. Infrared testing on the entire surface of the tie was conducted by dividing the surface

into three equal parts and conducting on each part separately. These infrared tests were

conducted by first heating the surface of the tie with a shop heater shown in Figure 4.3

from a height of about 15” for about 90 seconds duration. The infrared images were then

captured during the cooling cycle using InfraCAM SD™ infrared camera shown in

Figure 4.4. The images with the best contrast were recorded and used for the analysis.

Figure 4.3 Shop heater

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InfraCAM SD™ infrared camera, manufactured by FLIR systems, was used for

acquiring the infrared images. The camera measures the infrared radiation emitted from

an object and converts it to an equivalent temperature value in accordance with the

Stefan-Boltzmann law. The temperature profile of an object's surface is depicted as a

visual thermal images by the infrared camera, and the images are directly saved on a SD

memory card which stores thousands of images in standard radiometric JPEG format.

This camera comes with a much lower price tag (~$3500), making it lot more affordable

than the $50,000 price tag for high-end infrared cameras (e.g., ThermaCAM™ S60).

Figure 4.4 InfraCAM SD™ infrared camera

The InfraCAM SD™ infrared camera is the lightest infrared thermal imaging

camera that is currently commercially available and weighs just 1.21 pounds. The camera

has a built in 24o lens and meets IP 54 standards and withstands harsh industrial

environments. It can detect infrared radiation in the spectral range of 7.5 to 13 microns.

The camera is capable of making temperature measurements in the range of -10°C to

+350°C (+14°F to +662°F). The thermal images of InfraCAM SD™ are clearly displayed

on the large 3.5” color LCD with 240 by 240 pixels (the actual detector is 120 by 120

pixels). The minimum focus distance of the infrared camera is 0.3m. Thermal sensitivity

of this infrared camera is 0.1oC. The images that the camera produces can be analyzed

either in the field by using the real-time spot measurement marker built into the camera

software, or in a computer using FLIR Systems QuickReport software. The spot

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temperature measurement option offered by the software enables temperature

measurement corresponding to any point. The area feature provides average temperature

over a small area and has the advantage of minimizing the random noise associated with

the various pixels.

4.1.1 Infrared tests of the railroad tie subjected to static loading

This section presents the pictures and infrared images of the composite railroad

ties taken before and after the tie was subjected to the static loading test [Chada 2011].

This section also discusses the bright spots (i.e. thermal contrast due to temperature

differences) observed in the infrared images of the tie before and after the static loading

test. The presence of any subsurface defects or cracks in the composite railroad tie

impedes the heat transfer into the tie which results in the temperature differences on the

surface of the tie.

Figure 4.5 shows the pictures of the static loading set up where the composite

railroad tie is subjected to failure. The load testing part was conducted by Chada (2011).

Figure 4.5 FRP composite encased railroad tie subjected to static load [Chada 2011]

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4.1.1.1 Before static loading test (April 24, 2010)

Infrared testing was conducted on the composite railroad ties before it was

subjected to static load test. The surface of the tie was heated with the shop heater shown

in Figure 4.3 to a temperature around 40 o

C – 50 oC. The surface was heated from a

height of around 15” from the surface of the tie for about 90 second duration till the

desired temperature is reached. The heater was then removed and the infrared images

were taken by using the InfraCAM SD™ infrared camera.

Figure 4.6 shows the picture of the first part of the bottom surface of the

composite railroad tie that was later subjected to static load test and also shows the

corresponding infrared image taken after heating the surface. The infrared image shows a

bright spot on the right edge of the railroad tie surface which has an average temperature

of about 47.4 oC when compared to the surrounding defect free region which has an

average temperature of about 41.8 oC. This temperature difference (brighter spot) is due

to the presence of a defect in the subsurface of the composite tie between the FRP and the

underlying wooden core.

Figure 4.6 First part of the bottom surface of the composite railroad tie and its infrared image before conducting the static load test

105

Figure 4.7 shows the picture of the middle part of the bottom surface of the

composite rail road tie and the corresponding infrared image before it was subjected to

static load test. The infrared image shows a brighter region where the noted temperature

was about 46.1 oC. The surrounding region has a surface temperature of about 41.0

oC.

This temperature difference indicates the presence of a defect at the bright spot. The

defect can be an air gap or void.

Figure 4.8 shows the picture of the last part of the bottom surface of the

composite railroad tie and its infrared image before it was subjected to static load test.

There are bright hot spots in the infrared image on the right edge of the tie which might

be due to presence of any possible defects in the subsurface of the tie. The temperatures

of the hot spots were around 39.4 o

C to 39.6 oC. The remaining surface of the tie had a

uniform temperature of about 36.6 oC.

Figure 4.9 shows the picture of the first part of the top surface of the composite tie

and its infrared image before it was subjected to static load test. A bright spot was

observed on the edge of the composite tie with temperature 52.6 °C. .The surrounding

surface of this part of the composite tie has a temperature of 48.7°C.

Figure 4.7 Middle part of the bottom surface part of the composite railroad tie and its infrared image before conducting the static load test

106

Figure 4.8 Last part of the bottom surface part of the composite railroad tie and its infrared image before conducting the static load test

Figure 4.9 First part of the top surface of the composite railroad tie and its infrared image before conducting the static load test

107

Figure 4.10 shows the picture of the middle part of the top surface of the

composite railroad tie and its infrared image before it was subjected to static load test.

The infrared image shows a narrow bright spot in the middle region of the composite

railroad tie which had a temperature of 45.0 °C. The surrounding temperature for this part

of the tie was about 41.5 °C. This temperature difference is due to presence of defect in

the subsurface of the tie.

Figure 4.11 shows the picture of the last part of the top surface of the composite

tie and its infrared image before it was subjected to static load test. There was a large

bright spot observed on the infrared image in the middle portion of this part of the tie

which had a temperature of around 45.5 °C – 45.8 °C. Also there was a bright spot just on

top of the large bright spot, with a temperature of about 44.2 °C. The surrounding

temperature for this part of the tie was observed to be 40.7 °C.

Figure 4.10 Middle part of the top surface of the composite railroad tie and its infrared image before conducting the static load test

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4.1.1.2 After static loading test (May 27, 2010)

The same procedure of infrared testing was conducted on the composite railroad

ties after it was subjected to static load test to failure.


composite railroad tie and its infrared image after it was subjected to static load test.

There was a bright spot observed on the right side of this part of the tie which was even

observed before conducting the static load test (Figure 4.6). It could be observed by

comparing the two infrared images (Figure 4.6 and Figure 4.12) that the size of the bright

spot increased after the static load test. This indicates that the size of the defect increased

after the static load test to failure. The temperature of this bright spot was 45.0 °C and the

temperature of surrounding defect free area for this part of the tie was 41.7 °C.

Figure 4.11 Last part of the top surface of the composite rail road tie and its infrared image before conducting the static load test

109

Figure 4.12 First part of the bottom surface of the composite railroad tie after and its infrared image after conducting the static loading test

Figure 4.13 Middle part of the bottom surface part of the composite railroad tie and its infrared image after conducting the static loading test

110



There were bright spots observed in the infrared image at the top portion of this part, i.e.

near to the crack where the beam has failed. The temperature for these spots was ranging

from 41.6 °C to 42.1 °C. The surrounding temperature for this part was observed to be

40.0 °C. These bright spots were even observed before the test (Figure 4.7), but the size

of these bright spots increased after the static loading test which indicates increase in

defect size.


composite railroad tie and its infrared image after it was subjected to static load test. The

infrared image shows a bright spot on the right side portion of this part with a

temperature 42.2 °C. The surrounding temperature for this part of the tie was observed to

be 39.9 °C. There was not any noticeable change in the infrared image for this part of the

composite tie before (Figure 4.8) and after (Figure 4.14) the static load test.

Figure 4.14 Last part of the bottom surface of the composite railroad tie and its infrared image after conducting the static loading test

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Figure 4.15 shows the picture of the first part of the top surface of the composite

railroad tie and its infrared image after it was subjected to static load test. When this

infrared image is compared to the infrared image of the same part of the composite tie

taken before the static load test, the size of the bright spot increased whereas the size of

the dark spot remained the same. The temperature of the larger spot was observed to be

45.1 °C and the temperature of the dark spot was observed to be 39.0 °C. The remaining

part of the tie had a temperature of 41.1 °C.



There was a narrow bright spot observed near to the crack where the composite tie failed

under the static load test. This was similar to the one observed in the infrared image taken

for the same part before (Figure 4.10) it was subjected to static load test. The temperature

for this bright spot in this infrared image is 40.0 °C. The remaining portion has a

temperature of 38.2 °C.

Figure 4.15 First part of the top surface of the composite railroad tie and its infrared image after conducting the static loading test

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Figure 4.16 Middle part of the top surface of the composite railroad tie and its infrared image after conducting the static load test

Figure 4.17 Last part of the top surface of the composite railroad tie and its infrared image after conducting the static loading test

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railroad tie and its infrared image after it was subjected to static load test. The bright

spots observed in this infrared image have temperature ranging from 38.3 °C to 38.7 °C.

The remaining portion of this part has a uniform temperature of 37.3 °C.

4.1.2 Infrared tests of the railroad tie subjected to fatigue loading

This section discusses the infrared tests conducted on the composite railroad ties

before and after conducting the fatigue load tests. The fatigue load test was conducted by

Chada (2011) with a minimum load of 8 kips and a maximum load of 80 kips and at a

frequency of 2 Hz. The infrared images were taken after completion of 516,100 loading

cycles. Figure 4.18 shows the fatigue test setup for the FRP composite encased railroad

tie.

Figure 4.18 FRP composite encased railroad tie subjected to fatigue load [Chada 2011]

4.1.2.1 Before fatigue test (April 24, 2010)

Figure 4.19 shows the picture of the first part of the top surface of the composite

railroad tie and its infrared image before it was subjected to fatigue load test. There was a

bright spot observed in the infrared image around the region where the strain gage was

attached. The temperature of this bright spot was observed to be 49.1 °C. The remaining

surface of this part has a uniform temperature of about 43.7 °C.

114


composite railroad tie and its infrared image before it was subjected to fatigue load test.

The infrared image shows a large bright spot in the middle portion of this part, just below

the attached strain gage. The temperature of this bright spot is 38.7 °C. The surrounding

remaining portion of this part of the tie has a temperature of about 36.0 °C. This

temperature difference may not really be a defect. Subsequent tests after fatigue loading

helped to confirm that the big patch was not a defect; instead smaller defects were formed

in this region as a result of fatigue loading.

Figure 4.19 First part of the top surface of the composite railroad tie and its infrared image before conducting the fatigue test

115


railroad tie and its infrared image before it was subjected to fatigue load test. There were

two distinct bright spots observed on this part of the composite tie, one above the strain

gage and the other below the strain gage, they have temperatures 37.4 °C and 36.9 °C

respectively. The surrounding defect free region has a temperature of about 35.5 °C.

Figure 4.20 Middle part of the top surface of the composite railroad tie and its infrared image before conducting the fatigue test

116

Figure 4.21 Last part of the top surface of the composite railroad tie and its infrared image before conducting the fatigue test

Figure 4.22 First part of the bottom surface of the composite railroad tie and its infrared image before conducting the fatigue test

117



There were three distinct bright spots at three different places on this part of the

composite tie. Their temperatures were observed to be 43.2 °C, 42.7 °C and 42.3 °C. The

surrounding defect free region has a temperature of about 39.8 °C.



There were bright spots observed on this part of the composite tie which have

temperatures ranging from 48.0 °C to 48.6 °C. The surrounding defect free region has a

temperature of about 45.8 °C.



Bright spots were observed with temperatures ranging from 42.8 °C to 43.5 °C. The

remaining portion of the composite tie has a uniform temperature of about 40.2 °C.

Figure 4.23 Middle part of the bottom surface of the composite railroad tie and its infrared image before conducting the fatigue test

118

4.1.2.2 After fatigue test

Infrared images of the FRP composite rail road tie which is subjected to fatigue

loading were taken at three different stages during the test. The first infrared

thermography testing was conducted after completion of just over half million loading

cycles (516,100 cycles), the second infrared testing was conducted at one million loading

cycles and the last infrared test was conducted after completion of 1.5 million loading

cycles. Infrared images were taken by dividing the surface of the tie in four parts without

disturbing the setup with the rails, in contrast to the images taken before the test (with no

rails) where the surface of the tie was divided into three parts. The infrared images for

"after fatigue test" could only be acquired from the top surface of the tie since the other

three sides were surrounded by ballast.

At 0.5 million loading cycles (July 14, 2010)

Figure 4.25 shows the picture of the first part of top surface of the FRP composite

railroad tie and its infrared image after running the fatigue test for more than half a

million cycles. There were two distinct bright spots observed on the infrared images

Figure 4.24 Last part of the bottom surface of the composite railroad tie and its infrared image before conducting the fatigue test

119

which can be possible defects present in the subsurface of the railroad tie. The

temperature readings for these two bright spots were 58.1 °C and 55.0 °C. Defect

indicated by the bright 'hot' spot in the infrared image with temperature of 58.1 °C can be

seen clearly on the surface of the tie in the photograph. The surrounding defect free area

of this part of the tie has a temperature of 46.9 °C.

Figure 4.25 First part of the top surface of the composite railroad tie and its infrared image after running the fatigue test for half million cycles

Figure 4.26 Second part of the top surface of the composite railroad tie and its infrared image after running the fatigue test for half million cycles

120

Figure 4.26 shows the picture of the second part of top surface of the composite

railroad tie and its infrared image after running the fatigue test for more than half million

cycles. There were two bright hot spots observed on the surface of the tie due to the

temperature differences with surrounding area of the tie. These temperature differences

are may be due to the presence of possible defects in the subsurface which delays the heat

transfer into the tie. These hot spots were close to the strain gages attached. These bright

spots had temperatures of 44.4 °C and 45.5 °C. The surrounding defect free area of this

part of the tie has temperature of 41.7 °C.

Figure 4.27 shows the picture of the third part of top surface of the composite

railroad tie and its infrared image after running the fatigue test for more than half million

cycles. The infrared image shows a bright hot spot on the right edge of the tie with

temperature 55.3 °C which can also be a possible defect in the subsurface of the tie

developed during the loading cycles. The surrounding surface of the tie has a temperature

of 51.4 °C.

Figure 4.27 Third part of the top surface of the composite railroad tie and its infrared image after running the fatigue test for half million cycles

121

Figure 4.28 shows the picture of last part of top surface of the composite railroad

tie and its infrared image after running the fatigue test for more than half million cycles.

A very bright spot was observed on the infrared image which read a temperature of 39.2

°C. This bright spot can possibly due to presence of a subsurface defect. The surrounding

area for this part of the tie has a temperature of 38.6 °C.

At 1.0 million loading cycles (July 19, 2010)

Figure 4.29 shows the picture of the first part of top surface of the composite

railroad tie and its infrared image after completing one-million loading cycles. Infrared

image showed bright hot spots similar to the ones observed in the infrared image taken at

half-million cycles. There was not any noticeable change in these spots. This confirms

that the defects observed at half million loading cycles had no change after completing

one million loading cycles for this part of the tie. The temperature for the bright spots

was around 60.6 °C to 63.6 °C. The surrounding defect free surface has a temperature of

53.6 °C.

Figure 4.28 Last part of the top surface of the composite railroad tie and its infrared image after running the fatigue test for half million cycles

122

Figure 4.29 First part of the top surface of the composite railroad tie and its infrared image after completing one million loading cycles in the fatigue load test

Figure 4.30 Second part of the top surface of the composite railroad tie and its infrared image after completing one million loading cycles in the fatigue load test

123


railroad tie and its infrared image after completing one-million loading cycles. Infrared

image shows a big bright spot on the left edge and also some smaller bright spots which

were not observed in the infrared image taken at half million cycles. This indicates that

there were small defects developed in this part of the railroad tie after completion of one

million loading cycles. The temperature for the bright spots was around 60.8 °C to 64.4

°C. The surrounding surface has a temperature of 50.7 °C.


railroad tie and its infrared image after completing one-million loading cycles. There is a

big patch of bright spot and small hot spots observed in the infrared image. The smaller

bright spots had temperatures of 53.7 °C and 54.9 °C. The big bright spot had an average

temperature of 52.8 °C. The surrounding surface has a temperature of 46.9 °C. The bright

spots observed in the middle portion of this part were not observed in the infrared image

taken at half-million loading cycles. This indicates that there are defects in the tie which

developed during the subsequent loading cycles.

Figure 4.31 Third part of the top surface of the composite railroad tie and its infrared image after completing one million loading cycles in the fatigue load test

124

Figure 4.32 shows the picture of the last part of top surface of the composite

railroad tie and its infrared image after completing one-million loading cycles. The

infrared image shows a bright spot at the right edge with a temperature of 44.0 °C. The

surrounding surface has a temperature of 41.4 °C. This bright spot might indicate the

growth of a defect in the subsurface region of the tie.

At 1.5 million loading cycles (September 9, 2010)

Figure 4.33 shows the picture of the first part of top surface of the composite

railroad tie and its infrared image after completing one and half million loading cycles.

There were three prominent bright spots on this infrared image with temperatures 69.3

°C, 66.4 °C and 61.8 °C respectively. The surrounding defect free region has a uniform

temperature of about 58.9 °C. The bright spots were observed on the infrared image at the

same places where they were observed on the infrared images taken previously at half

and one million loading cycles. But these bright spots were not as conspicuous as they

were previously. The reason for this can be that the infrared images for one and half

million loading cycles were taken six weeks after stopping of the fatigue test and there

Figure 4.32 Last part of the top surface of the composite railroad tie and its infrared image after completing one million loading cycles in the fatigue load test

125

was possible relaxation of stresses in the tie during this period, whereas infrared images

for the half and one million loading cycles were taken just after stopping the test.

Figure 4.33 First part of the top surface of the composite railroad tie and its infrared image after completing one and half million loading cycles in the fatigue load test

Figure 4.34 Second part of the top surface of the composite railroad tie and its infrared image after completing one and half million loading cycles in the fatigue load test

126



There was a big bright spot observed in the infrared image with a temperature of 47.5 °C

which was also observed in the infrared images of the earlier loading cycles. The

surrounding defect free surface has a temperature of about 43.3 °C.



This was a similar case to the first part where the bright spots were not as prominent as

they were observed in the earlier loading cycles. There is a bright spot observed with a

temperature of 44.2 °C and the surrounding defect free surface has a temperature of about

40.2 °C.

Figure 4.35 Third part of the top surface of the composite railroad tie and its infrared image after completing one and half million loading cycles in the fatigue load test

127

Figure 4.36 shows the picture of the last part of top surface of the composite

railroad tie and its infrared image after completing one and half million loading cycles. A

big bright spot was observed in the infrared image. This spot has a temperature of about

38.4 °C and the surrounding defect free surface with no bright spots has a temperature of

about 34.2 °C.

4.2 Field testing, analysis and results (May 14, 2010)

Infrared testing was conducted on seven FRP composite encased railroad ties which

were installed in the field at South Branch Valley Railroad (SBVR) in Moorefield, West

Virginia in early summer of 2009. The SBVR line is used to provide freight and

passenger service to the state‟s eastern panhandle by the West Virginia State DOT-State

Railway Authority (SRA). The testing was conducted on May 14, 2010 starting at 11:00

am and continued until 1:00 pm. The ambient air temperature during this period ranged

from 22.2 °C (72 °F) to 24.4 °C (76 °F). These composite railroad ties are numbered one

to seven starting from south end to north end. The #1 and #2 ties are placed at the

southern end, #3, #4 and #5 ties are placed at a different place in between, and #6 and #7

Figure 4.36 Last part of the top surface of the composite railroad tie and its infrared image after completing one and half million loading cycles in the fatigue load test

128

ties are placed at the Northern end. Figure 4.37 shows the pictures of the seven composite

railroad ties and their location in the field.

Figure 4.37 (a) #1 and #2 ties (b) #3, #4 and #5 ties (c) #6 and #7 ties [Srinivas 2010]

Figure 4.38 Part of the #1 tie between the rails and its corresponding infrared image

129

Figure 4.38 shows the picture of the part of the #1 composite railroad tie in

between the rails and its corresponding infrared image. Most of this part of the composite

tie had a uniform temperature of about 44.1 °C as seen from the infrared image. There

were as such no brighter spots noticed on this part of the tie. The dark spots seen in the

infrared image are due to the presence of ballast stones and also duct tape on the surface

of the tie which can be seen in the photograph.

Figure 4.39 shows the picture of part of the #1 composite railroad tie on the outer

side of the right rail and its corresponding infrared image. The Infrared image shows

bright spots located very close to the rail with high temperatures of about 47.4 °C. There

were also some hot spots observed with temperature of about 45.9 °C on the outer edge

of the composite tie. These hot spots on the infrared image indicate the presence of

defects in this portion of the tie. The remaining surface of this part of the tie had a

uniform temperature of about 42.8 °C.

Figure 4.40 shows the picture of part of the #2 composite railroad tie in between

the rails and its corresponding infrared image. There were two hot spots (bright spots)

observed as seen in the infrared image. These hot spots had high temperatures compared

Figure 4.39 Part of the #1 tie on the outer side of the right rail and its corresponding infrared image

130

to the remaining portion of this part of the tie. One of the hot spots located near to the rail

had temperature of about 47.6 °C and the other had temperature of about 47.8 °C. The

remaining surface had uniform temperature of about 45.2 °C.



131


side of the right rail and its corresponding infrared image. The infrared image showed hot

spots located near the rail and also on the outer edges of the composite tie. These bright

(hot) spots have high temperatures ranging from 45.2 °C to 45.6 °C. This is an indication

of presence of subsurface defects at these places. The remaining portion of this part of the

tie had uniform temperature of about 43.3 °C.


side of the left rail and its corresponding infrared image. The infrared image shows bright

hot spots with high temperatures located near the rail and the outer edges of this tie. The

temperatures for these observed hot spots were about 45.0 °C. This gives an indication

that there are defects being developed in places very close to the rail (loading zone) as

well as the outer edge of the tie (possibly due to friction with the ballast). The remaining

portion of the tie had a uniform temperature of about 41.1 °C.


the rails and its corresponding infrared image. In this infrared image there were hot bright

spots with high temperatures observed in the middle portion and places close to the rails.

Figure 4.42 Part of the #2 tie on the outer side of the left rail and its corresponding infrared image

132

This indicates the presence of some cracks or defects being developed in these regions.

The temperatures for the bright spots located in the middle portion were 49.1 °C. The

remaining part of the tie is defect free and has a uniform temperature of about 45.1 °C.



133


side of the right rail and its corresponding infrared image. The infrared image shows

bright spots with high temperatures located near the rail. The temperatures for these hot

spots were about 51.5 °C. The remaining portion of this part of the tie had uniform



side of the right rail and its corresponding infrared image. There was a bright spot

observed in the infrared image with a temperature of about 28.3 °C. The remaining

portion of this part of the tie had a temperature of about 26.5 °C. The lower temperature

for this particular tie was due to the shadow of a tree obstructing the sunlight from

directly falling over the tie.


the rails and its corresponding infrared image. The infrared image showed a large bright

spot close to the right rail with a temperature of about 36.6 °C. This indicates the


134

presence of a defect at that location. The remaining defect free region of this part of the

tie had a uniform temperature of about 30.6 °C.



135


side of the left rail and its corresponding infrared image. There were some bright spots

observed in the infrared image for this part of the tie. These hot spots have high

temperatures of about 47.7 °C. The remaining defect free surface of this part of the tie has



side of the right rail and its corresponding infrared image. The temperature for this part of

the tie was almost uniform, ranging from 24.1 °C to 26.2 °C. This tie had a low

temperature because it was under the shadow of a tree which obstructed the sunlight from

directly falling over the tie.


136


side of the left rail and its corresponding infrared image. The infrared image shows a

brighter region (hot spot) on the outer edge of this part of the tie. This bright spot had

high temperature of about 28.6 °C. This indicates the presence of a defect developed at

the outer edge of the tie, possibly due to friction with the ballast and environmental

degradation. The remaining portion of this part of the tie had a uniform temperature of

about 26.7 °C which indicates that it is a defect free surface.


the rails and its corresponding infrared image. It is observed from the infrared image that

most of this part had uniform temperature of about 24.7 °C except for one bright spot

close to the right rail which has a high temperature of about 26.1 °C. This can be a

possible defect in the subsurface of the composite railroad tie.


137



138


side of the left rail and its corresponding infrared image. The infrared image shows a

bright hot spot on the outer edge of the tie. This hot spot has a temperature of about 28.7

°C. This high temperature corresponds to the defect which can be clearly seen in the

digital image of this part of the tie. The remaining portion of this part of the tie has

uniform temperature of about 25.7 °C.


side of the right rail and its corresponding infrared image. The infrared image shows the

crack surface (also clearly seen in the digital image) with a temperature of about 24.4 °C.

The remaining portion of the tie has a uniform temperature of about 25.2 °C. The overall

temperature for this tie was lower since it was under the shadow of a tree which

obstructed the sunlight from directly falling over the tie.

4.3 Conclusions

From the infrared testing conducted on the FRP composite encased railroad ties in

the laboratory, it was noted that there were some embedded defects (manufacturing


139

defects) in the tie as observed from the bright spots in the infrared images taken prior to

the static loading test. Most of the bright spots increased in size after conducting the static

load test to failure. This shows that the defect size increased after the static loading of the

tie to failure.

Infrared testing of the composite tie which was subjected to fatigue loading showed

some defects prior to the fatigue test. Infrared testing conducted on the top surface of the

tie after the fatigue test showed that most of the defects present before the test remained

unchanged but there were some defects which developed under the loading. These

infrared tests could only be conducted on the top surface of the tie since the other three

sides were surrounded by ballast.

The field infrared thermography testing was conducted on the seven railroad ties

which were installed more than a year back. There were bright hot spots observed in the

infrared images due to the defects that developed in these ties in the field environment

over time. Most of the bright spots were observed close to the rails which is because the

train loads are concentrated on the rails causing high stress zones leading to defect

growth. There were also some bright spots observed on the infrared images of the ties in

between the rails and close to the edges of the tie. All the infrared images were taken

under direct solar loading. The heating was not adequate for some parts of the #4, #5, #6

and #7 ties as they were under the shadow of trees which led to poor thermal contrast

compared to the other parts of the tie.

The laboratory and field testing results presented in this chapter have served to

demonstrate the usefulness of infrared thermography testing for composite railroad ties.

140

5 CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

Based on the literature review conducted on the recent advances and application of

ultrasonic testing on various structural components of a bridge such as steel girders and

concrete bridge decks, the following conclusions can be drawn.

Ultrasonic technique proves to be quite robust and cost efficient, which makes it

quite suitable for in-situ inspection of bridge structural components.

Ultrasonic techniques like plate wave ultrasonic technique, angled pitch-catch

technique, and pulse-echo technique proved to be efficient for detecting and

monitoring fatigue cracks in the steel members of a bridge.

Contact ultrasonic testing methods like field ultrasonic testing and non-contact

ultrasonic testing methods like immersion tank testing technique showed a high

level of consistency in finding the defect size and defect location in steel

components. However, the immersion tank testing can only be carried out in the

laboratory.

Ultrasonic technique also proved efficient in finding the internal conditions of a

concrete bridge deck and also estimating the crack sizes.

Amplitude or intensity of the waveform signal received from the ultrasonic wave

velocity technique was higher in specimens with cracks close to the surface.

Ultrasonic technique also proved to be efficient for early age characterization of

concrete.

Ultrasonic technique can also be used as an efficient means for measuring applied

stress instead of using the conventional strain gauges in case of metallic members.

Based on the literature review conducted on recent advances and application of

infrared thermography testing on different structural components of a bridge, the

following conclusions were drawn.

Infrared thermography technique can be a very efficient method when trying to

rapidly detect defects in large areas of concrete structures in a non-contact

fashion.

141

When a concrete structure is subjected to infrared testing under solar heating,

maximum thermal contrast (i.e., the temperature difference for the defects in the

infrared images) increases later in the day with increasing defect depth.

Infrared thermography technique can also be successfully used to detect air-filled

and water-filled delaminations in a FRP composite deck.

Infrared thermography provides a nondestructive technique to detect the

occurrence of intrinsic heat dissipation in concrete using the temperature changes

caused due to the heat generated.

Infrared thermography can also be used to find the fatigue limit of concrete in a

very short time, compared to traditional testing techniques.

From the laboratory and field testing of the FRP composite railroad ties, following

remarks can be made.

Infrared thermography testing can be used efficiently in detecting the subsurface

defects in FRP composite encased railroad ties just after they are manufactured

and before they are installed in the field. Thus, infrared thermography can be used

as an effective tool for manufacturing quality control.

Infrared thermography can also be used in detecting the defects in field installed

FRP composite railroad ties under solar heating, which can help in deciding if the

ties are in good condition or need to be replaced.

5.2 Recommendations for future research

The following issues are recommended for future investigation:

Quantitative characterization of the data acquired from the infrared thermography

tests can provide more detailed information such as exact size and depth of the

subsurface defects in a structural component when compared to the qualitative

analysis which only shows the presence of defects.

Improved heating sources with higher intensity and uniformity can result in better

thermal contrast between the defective areas and the surrounding defect-free

region of the structural component. Hence, further research and development is

needed to come up with better heating sources which are also portable and easy to

use in the field.

142

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