optimization of electromagnetic acoustic transducers for nondestructive evaluation of concrete ...

100 Pages·2008·1.07 MB·English

OPTIMIZATION OF ELECTROMAGNETIC ACOUSTIC TRANSDUCERS FOR
NONDESTRUCTIVE EVALUATION OF CONCRETE STRUCTURES
_______________________________________________________________________

A Thesis presented to the Faculty of the Graduate school at the

University of Missouri-Columbia

_______________________________________________________________________

In Partial Fulfillment of the Requirements for the Degree

Master of Science

________________________________________________________________________

By

SIVA KRISHNA CHAITANYA PENAMAKURU
Dr. Glenn A. Washer and Dr. Steven P. Neal, Thesis Supervisors

MAY 2008
The undersigned, appointed by the dean of the Graduate School, have examined the
thesis entitled
OPTIMIZATION OF ELECTROMAGNETIC ACOUSTIC TRANSDUCERS
FOR NONDESTRUCTIVE EVALUATION OF CONCRETE STRUCTURES

presented by Siva Krishna Chaitanya Penamakuru,
a candidate for the degree of Master of Science,
and hereby certify that, in their opinion, it is worthy of acceptance.

Dr. Glenn A. Washer

Dr. Steven P. Neal

Dr. Roger Fales
ACKNOWLEDGEMENTS

The support and help of many people made my thesis possible. I would like to
take this opportunity to thank each of them. Firstly, I would like to thank University of
Missouri-Columbia and the department of Mechanical and Aerospace Engineering for
giving me the opportunity for pursuing Masters Degree. I wish to express my deepest
gratitude to Dr. Glenn Washer for his valuable guidance, encouragement and support
throughout my research, without which it wouldn’t have been possible for me to complete
this work. I would like to sincerely thank Dr. Steven Neal for his immense help and
valuable suggestions in completing my thesis. I would also like to extend my thanks to
Dr. Roger Fales for serving as a member of my defense committee.
I am also grateful to all my friends who made this arduous Masters process an
enjoyable and memorable one.
Lastly, I would like to thank my parents, brother and my grandparents for their
unconditional love and support along the path of my academic pursuits.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS .......................................................................................... ii
TABLE OF CONTENTS ............................................................................................ iii
LIST OF FIGURES ..................................................................................................... vi
LIST OF TABLES ....................................................................................................... ix
ABSTRACT ...................................................................................................................x

1 INTRODUCTION ......................................................................................................1
1.1 Goal/Approach .......................................................................................................1
1.2 Prestressed Concrete ..............................................................................................4
1.2.1 Prestressing strands .........................................................................................6
1.2.2 Problems of Prestressing strands .....................................................................6
1.3 Nondestructive Evaluation .....................................................................................8
1.3.1 Presently available inspection technologies (NDE techniques) for concrete
structures .................................................................................................................9
1.3.2 Need for new inspection technologies ........................................................... 14

2 ELECTROMAGNETIC ACOUSTIC TRANSDUCERS (EMATs) ....................... 16
2.1 Literature Review ................................................................................................ 16
2.1.1 Working Principle of EMATs........................................................................ 17
2.1.1.2 Magnetostriction Phenomenon ................................................................... 18
2.1.2 EMAT configurations.................................................................................... 20
2.1.2.2 EMAT geometries .......................................................................................... 21
2.1.3 Advantages and applications of an EMAT ..................................................... 24
2.2 Theoretical Background ....................................................................................... 25
2.3 Design of EMATs ................................................................................................ 29
2.3.1 Core Design .................................................................................................. 30
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2.3.2 Coil Windings ............................................................................................... 33
2.3.3 Magnets ........................................................................................................ 33

3 EXPERIMENTAL .................................................................................................... 35
3.1 Preliminary testing ............................................................................................... 35
3.2 Experimental Setup .............................................................................................. 37
3.3 Pulse propagation in strands ................................................................................. 40
3.4 Magnetic field tests .............................................................................................. 43
3.4.1 Test set up for cylindrical magnets ................................................................ 43
3.4.2 Magnetic circuit setup ................................................................................... 46
3.4.3 Magnetic field test matrix .............................................................................. 48
3.5 Testing for Number of coils ................................................................................. 49
3.6 Testing for different number of coil turns ............................................................. 50
3.7Summary of the test matrix ................................................................................... 50
3.8 Power and peak amplitude measurements ............................................................ 52

4 RESULTS/DISCUSSION ......................................................................................... 55
4.1 Magnetic Field Tests ............................................................................................ 55
4.1.1 Magnetic field tests results when the sensors of different number of coils
were placed as receivers ......................................................................................... 55
4.1.2 Magnetic field tests results for the sensor of different number of coils placed
as transmitters ........................................................................................................ 61
4.2 Number of coils tests............................................................................................ 63
4.2.1 Test results for sensors of different number of coils placed as receivers ......... 63
4.2.2 Test results for sensors of different number of coils placed as transmitters..... 67
4.3 Number of windings tests ..................................................................................... 68
4.4 Discussion ........................................................................................................... 76

5 CONCLUSIONS/FUTURE WORK ........................................................................ 84
5.1 Recommended sensor design ............................................................................... 85
5.2 Future work/Recommendations ............................................................................ 85
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REFERENCES ............................................................................................................ 87

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LIST OF FIGURES

Figure 1: Failure of Prestressing strands in a post-tensioned tendon……………..……....6

Figure 2: Photograph of a UPV system applied to a concrete beam in the laboratory ….11

Figure 3: Collapse of a pedestrian bridge outside Lowe’s Motor Speedway North
Carolina...............................................................................................................15

Figure 4: Schematic diagram of a Basic EMAT………………………………………....17

Figure 5: Diagram showing plate displacements for a) Asymmetric Lamb wave mode
b) Symmetric Lamb wave mode....................................................................... 21

Figure 6: Picture showing the EMAT coil configuration of a) Meander coil
b) Racetrack coil c) Spiral coil……………………………………...………...23

Figure 7: Solenoid coil…………………………………………………………………...24

Figure 8: Diagram of the sensors with dimensions a) 1-coil sensor b) 2-coil sensor
c) 4-coil sensor d) 8-coil sensor.........................................................................32

Figure 9: Periodicity of an EMAT.................................................................................... 32

Figure 10: Pictures showing the 1-coil, 2-coil, 4-coil and 8-coil sensors with winding...33

Figure 11: Photographs showing the EMAT cores with magnets.....................................34

Figure 12: Prestressing strand with windings...................................................................36

Figure 13: Picture showing the amplitude of the signal generated when a prestressing
strand (having 4 windings of 50 turns each) was hit by an iron material........36

Figure 14: Prestressing strand with the transmitting and receiving sensors.....................37

Figure 15: Schematic diagram of the experimental setup...............................................39

Figure 16: Photograph showing the experimental setup including the prestressing-
-strand, EMAT transducers and RAM 10000 instrument...............................40

Figure 17: Pulse transmission in a prestressing strand.....................................................41

Figure 18: Pulses detected by the 4-coil magnetostrictive EMAT................................... 42
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Figure 19: A 4-coil sensor with magnets placed at both ends of the sensor.....................43

Figure 20: Magnetic field measurement of a 4-coil sensor...............................................44

Figure 21: Graph showing the maximum magnetic field values recorded with an
increase in the number of magnets for a 2-coil, 4-coil and an 8-coil sensor...45

Figure 22: Magnetic setup.................................................................................................46

Figure 23: Graph denoting the amplitude peaks of a pulse...............................................53

Figure 24: Graph showing the area of the pulse under the curve......................................54

Figure 25: Graph showing the trend between the Magnetic field and power of the pulse
detected by a) 2-coil sensor b) 4-coil and 8-coil sensor..................................57

Figure 26: Graph showing the trend between the Magnetic field and the peak
Amplitude of the pulse detected by the a) 2-coil sensor b) 4-coil and an
8-coil sensor.....................................................................................................58

Figure 27: Trend between the power of the main pulse detected and the magnetic field
applied for a) 8-coil sensor b) 2-coil and 4-coil sensors..................................60

Figure 28: Trend between the amplitude peak of the main signal detected and the
applied magnetic field for a) 8-coil sensor b) 2-coil and 4-coil sensor...........61

Figure 29: Trend between the magnetic field and the power of the pulse detected by
the receiver when acted by a) 8-coil transmitter and b) 4-coil transmitter
and 2-coiltransmitter....................................................................................... 62

Figure 30: Amplitude of the signal detected by a) 2-coil sensor b) 4-coil sensor
c) 8-coil sensor at a magnetic field of 240 Gauss............................................65

Figure 31: Graph showing the power of the signal detected by sensors of different
number of coils................................................................................................66

Figure 32: Graph showing the Peak amplitude of the signal detected by the sensors of
different number of coils.................................................................................66

Figure 33: Trend between the power of the pulse detected by the receiver and the
number of coils of the transmitter ...................................................................67

Figure 34: Trend between the Amplitude peak of the main signal detected by the
receiver and the number of coils of the transmitter.........................................68

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Figure 35: Amplitude of the main signal detected by a) 1-coil sensor for 150turns per
coil, b) 2-coil sensor for 100turns per coil, c) 4-coil sensor for 75 turns per
coil d) 8-coil sensor for 50turns per coil..........................................................71

Figure 36: Amplitude of the main signal detected by an 8-coil sensor for a) 400turns,
b) 500 turns, and c) 600 turns..........................................................................73

Figure 37: Graph showing the power of the pulse detected by the receivers for
different number of coil turns..........................................................................74

Figure 38: Graph showing the amplitude peak of the signal detected by the receivers
for different number of coil turns....................................................................75

Figure 39: Trend between the amplitude peak detected by the sensors and the magnetic
field for a) Setup for cylindrical magnets and b) Magnetic circuit setup........77

Figure 40: Magnetostriction curve for steel (upper) and the efficiency of an EMAT
(below) as a function of magnetic field [31]....................................................79

Figure 41: Trend between the power of the signal produced and the number of coils.....82

Figure 42: Trend between the peak amplitude of the signal produced and the number
of coils.............................................................................................................82

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LIST OF TABLES

Table 1: Table showing the tests conducted as a part of this study……………………...51

Table 2: Table showing the type of sensor, number of coil turns made on the sensor
and the minimum and the maximum values of the magnetic fields applied……59
Table 3: Table showing the type of sensor, the magnetic field applied and the
different number of coil turns for which each sensor was tested…….…………69

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