High Temperature Electromagnetic Acoustic Transducer for Guided Wave Testing

183 Pages·2017·7.45 MB·English

by  Adrian Waka

High Temperature Electromagnetic
Acoustic Transducer for Guided Wave
Testing

A thesis submitted for the degree of Doctor of
Philosophy

by
Maria Kogia

School of Electronic and Computer Engineering
Brunel University
January 2017
Abstract
This research focuses on the theoretical analysis, development and experimental evaluation of a
water cooled Electromagnetic Acoustic Transducer (EMAT) specifically designed for high-
temperature Guided Wave Testing (GWT). Its novel design and detailed calculation of its optimum
operating conditions resulted in its effectiveness at high temperatures for both short (500oC) and
long-term inspection (250oC).
All the steps followed for the theoretical and experimental investigation of the limitations of the
existing technology and the development of a probe that can overcome these boundaries are
presented. Finite Element Analysis (FEA) was performed for the optimization of the EMAT design
and estimation of its ultrasonic and thermal properties at room and high temperatures over time.
The wave mode purity profile of the Periodic Permanent Magnet (PPM) EMAT was theoretically
studied as well as the effect of temperature rise on its ultrasonic performance. Thermal and
Computational Fluid Dynamics (CFD) analysis was accomplished for the EMAT design
optimization and calculation of its optimum operating conditions.
The experimental validation of the theoretical study was also accomplished. The novel water
cooled EMAT was developed and experimentally evaluated regarding its ultrasonic and thermal
response at room and high temperatures. An empirical method for the enhancement of EMAT
performance and its SNR was established. The wave mode purity characteristics of PPM EMAT
were experimentally investigated via Laser vibrometry tests, which agreed with FEA results. The
impedance analysis and ultrasonic evaluation of the EMAT at both room and high temperatures
against various operating conditions were linked and compared to the results obtained from the
ultrasonic, thermal and fluid FEA. In all cases, the experimental study is in good agreement with
the theoretical results.

Abstract 2
Contents
High Temperature Electromagnetic Acoustic Transducer for Guided Wave Testing ........................ 1
Abstract ............................................................................................................................................... 2
List of Figures ..................................................................................................................................... 6
List of Tables .................................................................................................................................... 10
Acknowledgements ........................................................................................................................... 11
Declarations ...................................................................................................................................... 12
Nomenclature .................................................................................................................................... 13
Chapter 1 ........................................................................................................................................... 17
Introduction ....................................................................................................................................... 17
1.1 Motivation ......................................................................................................................... 17
1.2. Aim ................................................................................................................................... 19
1.3. Specific Objectives ........................................................................................................... 19
1.4. Summary of Methodology Used ....................................................................................... 20
1.5. Thesis Outline ................................................................................................................... 21
1.6. Contributions to Knowledge ............................................................................................. 22
1.7. List of Publications ........................................................................................................... 23
Chapter 2 ........................................................................................................................................... 25
Literature Review .............................................................................................................................. 25
2.1 Introduction ............................................................................................................................. 25
2.2 NDT Techniques for High-Temperature Inspection ............................................................... 25
2.3 Guided Wave Testing ............................................................................................................. 29
2.4 Piezoelectric Transducers for GWT ........................................................................................ 30
2.5 EMATs .................................................................................................................................... 32
2.6 Summary ................................................................................................................................. 37
Chapter 3 ........................................................................................................................................... 38
Theoretical Background for the Design of High-Temperature EMAT ............................................. 38
3.1 Introduction ............................................................................................................................. 38
3.2 Guided Waves ......................................................................................................................... 38
3.2.1 Introduction ...................................................................................................................... 38
3.2.2 Guided Waves Physics ..................................................................................................... 40
Abstract 3
3.2.3 Temperature Rise Effect .................................................................................................. 45
3.3 EMAT Transduction ............................................................................................................... 46
3.3.1 Lorentz Force ................................................................................................................... 47
3.3.2 Magnetization Force ........................................................................................................ 49
3.3.3 Magnetostriction .............................................................................................................. 50
3.4 EMAT Reception .................................................................................................................... 51
3.5 EMAT Designs ....................................................................................................................... 52
3.6 Electromagnetic Losses .......................................................................................................... 56
3.6.1 Skin Effect ....................................................................................................................... 56
3.6.2 Impedance – Electrical Circuit ......................................................................................... 58
3.6.3 Lift-off .............................................................................................................................. 63
3.7 Temperature Effect ................................................................................................................. 64
3.8 Summary ................................................................................................................................. 67
Chapter 4 ........................................................................................................................................... 68
Numerical Simulation of High Temperature EMAT ........................................................................ 68
4.1 Introduction ............................................................................................................................. 68
4.2 Wave Propagation FEA .......................................................................................................... 68
4.2.1 FEA Model Overview ...................................................................................................... 68
4.2.2 FEA Model Implementation ............................................................................................ 70
4.2.3 FEA Results and Discussion ............................................................................................ 76
4.3 Transient Thermal Simulations ............................................................................................... 84
4.4 CFD Analysis .......................................................................................................................... 89
4.4.1 Water Cooled EMAT for Inspection ................................................................................ 89
4.4.2 Oil Cooled EMAT for Inspection .................................................................................... 92
4.4.3 Water Cooled EMAT for Long Term Inspection ............................................................. 94
4.4.4 Summary .......................................................................................................................... 96
Chapter 5 ........................................................................................................................................... 98
Design and Characterization of Water Cooled EMAT ..................................................................... 98
5.1 Introduction ............................................................................................................................. 98
5.2 Existing Technology ............................................................................................................... 98
5.2.1 Room Temperature Ultrasonic Evaluation....................................................................... 98
5.2.2 High-Temperature Ultrasonic Evaluation ...................................................................... 103
5.3 Water Cooled EMAT for High-Temperature GWT .............................................................. 106
5.3.1 EMAT Design ................................................................................................................ 106
5.3.2 Impedance Analysis ....................................................................................................... 107
5.3.3 Wave Mode Purity Characterization .............................................................................. 112
Abstract 4
5.3.4 Room Temperature Ultrasonic Evaluation..................................................................... 126
5.3.5 Summary ........................................................................................................................ 129
Chapter 6 ......................................................................................................................................... 131
High Temperature Experiments ...................................................................................................... 131
6.1 Introduction ........................................................................................................................... 131
6.2 High-Temperature Experiments – Short Term Exposure ..................................................... 131
6.2.1 Experimental Set-up ....................................................................................................... 131
6.2.2 EMAT Performance against Material of Specimen ....................................................... 133
6.2.3 Lift-off Limitations ........................................................................................................ 137
6.2.4 Power Requirements ...................................................................................................... 140
6.3 High-Temperature Experiments – Long Term Exposure ...................................................... 142
6.4 Summary ............................................................................................................................... 147
Chapter 7 ......................................................................................................................................... 149
Conclusions and Recommendation for Future Work ...................................................................... 149
7.1 Thesis Review ....................................................................................................................... 149
7.2 Thesis Findings ..................................................................................................................... 151
7.2.1 EMAT model for room and high temperatures .............................................................. 151
7.2.2 EMAT thermal and CFD model ..................................................................................... 153
7.2.3 Limitations of existing GWT EMATs - Mechanism for SNR enhancement ................. 154
7.2.4 Experimental validation of model - Wave mode purity of PPM EMAT ....................... 155
7.2.5 Water cooled PPM EMAT – Room temperature characterization ................................. 155
7.2.6 Water cooled PPM EMAT – High-temperature performance ....................................... 156
7.3 Suggestions for Future Work ................................................................................................ 157
References ....................................................................................................................................... 159

Abstract 5
List of Figures
3.1 Schematic of Lamb wave propagation in an isotropic plate 42
3.2 Dispersion curves of a 3mm thick steel plate 44
3.3 EMAT coil (a) spiral (b) racetrack (c) meander (d) butterfly 52
3.4 EMAT designs (a) spiral EMAT (b) racetrack EMAT (c) meander 53
EMAT d) butterfly EMAT
3.5 PPM EMAT 54
3.6 Equivalent electric circuit of EMAT coil 58
3.7 Equivalent electric circuit of (a) EMAT transmitter and sample (b) 60
sample and EMAT receiver
3.8 Electric circuit (a) EMAT/specimen transformer connection (b) 62
autotransformer connection (c) autotransformer connection without
parasitic capacitance
4.1 Dispersion curves of a 3mm thick 316L stainless steel plate 72
4.2 (a) Comsol FEA model configuration (b) EMAT configuration 75
4.3 (a) Excitation/eddy current distribution at 11μsec (b) Magnetic flux 77
distribution (MSP)
4.4 Schematic of coordinate system and displacement orientation
4.5 Y and Z displacement at 0o angle 78
4.6 Displacement of S wave mode at a) x, b) y and c) z axis 79
0
4.7 Displacement measured at 97μsec a) x, b) y and c) z axis 79
4.8 Eddy current density against temperature rise 80
4.9 (a) Y displacement against temperature rise (b) Displacement at 0o 80
angle at 500oC (c) Out – of – plane displacement at room temperature
and 500oC (zoom)
4.10 Displacement of S wave mode at a) x, b) y and c) z axis 82
0
List of Figures 6
4.11 Displacement measured at 97μsec a) x, b) y and c) z axis 82
4.12 Thermal EMAT model (a) entire EMAT design (b) cooling chamber 85
(c) coil structure
4.13 Temperature gradient of alumina encapsulated coil (a) 0.75mm 86
alumina thickness (b) 1mm alumina thickness
4.14 Optimum ceramic thickness graph 87
4.15 Temperature gradient of Kapton and alumina encapsulated coil (a) 88
0.75mm alumina thickness (b) 1mm alumina thickness
4.16 (a) EMAT temperature against coolant flow velocity (b) EMAT 90
temperature against coolant temperature
4.17 Water cooled EMAT (a) temperature gradient of EMAT components 91
at 500oC (b) temperature of EMAT components against temperature
rise
4.18 Temperature gradient of EMAT at 250oC after (a) 1min (b) 15mins 95
(c) 30mins (d) coil and magnets temperature against operating time
5.1 (a) Signal received from defect-free area without shielding (b) Signal 99
received from defect without shielding
5.2 (a) Signal received from defect-free area with shielding (b) Signal 100
received from defect with shielding
5.3 Room temperature PPM EMAT: Amplitude against lift-off 100
5.4 Room temperature PPM EMAT: Amplitude against power 101
5.5 High temperature experimental set-up (a) EMATs attached upon 102
specimen (b) electrical instrumentation – RITEC (c) thermal
instrumentation
5.6 Signal received at (a) room temperature (b) 60oC (c) 100oC (d) 180oC 103
5.7 Room temperature PPM EMAT: Amplitude against temperature rise 104
5.8 (a) EMAT design (b) EMAT prototype 105
5.9 (a) Constantan coil (b) Alumina & Kapton encapsulated coil (in mm) 106
5.10 Resistance against temperature rise 111
5.11 Inductance against temperature rise 111
List of Figures 7
5.12 Capacitance against temperature rise 111
5.13 Dispersion curves (a) phase velocity of 1.5 mm thick stainless steel 113
plate (b) group velocity of 1.5 mm thick stainless steel plate (c) phase
velocity of 0.5 mm thick stainless steel plate (d) group velocity of 0.5
mm thick stainless steel plate
5.14 (a) Experimental setup of vibrometry tests (b) zoom in EMAT and 114
scanning area
5.15 Wave mode purity for 3mm stainless steel plate. Displacement in (a) 115
x axis (b) y axis (c) z axis at 59 μsec (d) x axis (e) y axis (f) z axis at
97 μsec
5.16 Displacement at 180o in 3mm thick stainless steel plate in (a) all axis 116
(b) x-axis (c) y-axis (d) z axis
5.17 Displacement generated in 1.5mm thick stainless steel plate in (a) x 118
axis (b) y axis (c) z axis received at 58 μsec (d) x axis (e) y axis (f) z
axis received 97 μsec (g) x axis (h) y axis (i) z axis received 111 μsec
5.18 Displacement generated in 0.5mm thick stainless steel plate in (a) x 119
axis (b) y axis (c) z axis received at 58 μsec (d) x axis (e) y axis (f) z
axis received 97 μsec (g) x axis (h) y axis (i) z axis received 111 μsec
5.19 Displacement at 0o in 1.5mm thick stainless steel plate in (a) all axis 120
(b) x axis (c) y axis (d) z axis
5.20 Displacement at 0o in 0.5mm thick stainless steel plate in (a) all axis 120
(b) x axis (c) y axis (d) z axis
5.21 FEA results for wave propagation in 3mm thick mild steel plate in (a) 122
x-axis (b) y-axis (c) z-axis received at 56 μsec and (d) x-axis (e) y-
axis (f) z-axis received at 93 μsec
5.22 Displacement at 0o in 3mm thick mild steel plate in (a) all axis (b) x- 123
axis (c) y-axis (d) z axis
5.23 Vibrometry results. Displacement generated in 3mm thick mild steel 124
plate in (a) x-axis (b) y-axis (c) z-axis received at 56 μsec (d) x-axis
(e) y-axis (f) z-axis received at 93 μsec
5.24 Signal received from (a) room temperature EMAT on stainless steel 126
(b) water cooled EMAT on stainless steel (c) water cooled EMAT on
mild steel (d) amplitude difference
5.25 Water cooled EMAT – Amplitude against lift-off 127
5.26 Water cooled EMAT – Amplitude against power 128
List of Figures 8
6.1 Experimental setup for high-temperature experiments 131
6.2 Thermal picture of the experimental setup used for the heating up of 132
the specimen (500oC)
6.3 Signal received from both EMATs on mild steel at (a) room 133
temperature (b) 250oC (c) 500oC (d) amplitude against temperature
6.4 Signal received from both EMATs on stainless steel at (a) room 134
temperature (b) 250oC (c) 500oC (d) amplitude against temperature
6.5 Temperature of EMAT components: measured and estimated values 136
6.6 Nd-Fe-B EMAT: Amplitude against lift-off at high temperatures 137
6.7 SmCo EMAT: Amplitude against lift-off at high temperatures 138
6.8 Nd-Fe-B EMAT: Amplitude against power input at high temperatures 140
on (a) mild steel (b) stainless steel
6.9 SmCo EMAT: Amplitude against power input at high temperatures on 141
(a) mild steel (b) stainless steel
6.10 Thermal photo of experimental setup for the EMAT evaluation about 142
long-term inspection at 250oC
6.11 Nd-Fe-B EMAT: Amplitude over time at 250oC 142
6.12 Nd-Fe-B EMAT: Impedance over time at 250oC (a) Magnitude (b) 143
Phase
6.13 Nd-Fe-B EMAT: Signal received at 250oC after (a) 1min (b) 3 hours 144
operation
6.14 SmCo EMAT: Amplitude over time at 250oC 144
6.15 SmCo EMAT: Amplitude over time at 250oC and 350oC 145
6.16 SmCo EMAT: Impedance over time at 250oC 145
6.17 SmCo EMAT: Signal received at 350oC after (a) 1min (b) 3 hours 145
operation

List of Figures 9
List of Tables
4.1 Dispersion curves – Wave modes at 256kHz 72
4.2 Dispersion curves – Phase Velocity 73
4.3 Dispersion curves – Group Velocity 73
4.4 Material properties over temperature rise 74
4.5 Thermal properties of EMAT based on transient thermal 89
analysis
4.6 Temperature of oil cooled EMAT against flow velocity and oil 93
temperature
5.1 Preliminary EMAT impedance analysis 107
5.2 EMAT impedance against material properties of specimen 107
5.3 EMAT impedance against lift-off 107
5.4 Parasitic capacitance between EMAT and specimen 109
5.5 Phase and group velocity against plate thickness at 256kHz 117

List of Tables 10