E-Journal of Advanced Maintenance Vol.9-1 (2017) 15-25
Japan Society of Maintenology

Effect of Scaly Structure on the Measurement of Pipe Wall Thickness using
EMAT
Hongjun SUN1, Ryoichi URAYAMA2, Tetsuya UCHIMOTO2, Fumio KOJIMA2,
Toshiyuki TAKAGI2,*, Hiroshi ABE3, Kunihiro KOBAYASHI4
1

Graduate School of Engineering, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi
980-8577, Japan
2
Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980-8577,
Japan
3
Graduate School of Engineering, Tohoku University, 6-6-01-2 Aoba, Aramaki, Aoba, Sendai, Miyagi
980-8579, Japan
4
Tohoku Electric Power Co., Inc., 1-7-1 Honcho, Aoba, Sendai, Miyagi 980-8550, Japan
ABSTRACT
To use an electromagnetic acoustic transducer (EMAT) to measure the thickness of a carbon steel pipe wall, theelectromagnetic acoustic resonance (EMAR) and pulse-EMAR methods have been proposed previously. In thispaper, the EMAR and pulse-EMAR methods are used to measure the wall thickness of the dorsum part of a bent
carbon steel pipe that has experienced flow accelerated corrosion (FAC). The received signals are processedusing superposition of nth compression (SNC). The measurement results obtained with the EMAR andpulse-EMAR methods are compared with equivalent measurements of minimum thickness using a caliper gauge,
and the RMSE discrepancies are 0.24 mm and 0.36 mm, respectively. The results show that these discrepancies
can be attributed to the scaly structures created by FAC. Further investigation shows that even larger
discrepancies exist between the pulse-EMAR measurements and the minimum-thickness caliper measurements,
again mainly in the parts affected by FAC. Typical parts of the pipe wall are observed using a three dimensional
confocal laser scanning microscope, which reveals differently shaped scaly structures in different parts of thespecimen. The pulse-EMAR signals are simulated numerically. These results show that the pulse-EMAR
method measures thickness accurately in the initial stages of FAC, but that discrepancies gradually increase. The
pulse-EMAR results include all the surface conditions in the measurement region under the probe. According to
the numerical analysis, combining the pulse-EMAR and SNC methods might lead to the thickness of only thepristine part of the measurement area being registered, which might explain the discrepancies obtained in themeasurements.
KEYWORDS

ARTICLE INFORMATIONElectromagnetic Acoustic Transducer, Electromagnetic Acoustic Resonance,
Pulse-EMAR, Flow Accelerated Corrosion, scaly structure, Ultrasonic

Article history:
Received 13 January 2017
Accepted 11 May 2017

1.

Introduction

Carbon steel pipes are used extensively in nuclear power plants and fossil fuel power plants, etc[1, 2]. The pipe wall thinning due to the flow accelerated corrosion (FAC) is an important form of
age-related degradation [2-7]. A characteristic of FAC is a scaly structure that is produced on the innersurface of the pipe [3–7]. Therefore, operational safety depends crucially on being able to measure thewall thickness accurately. At present, this is usually done by ultrasonic testing (UT) using an
ultrasonic thickness gauge. However, such UT thickness measurements using the ultrasonic thickness
gauge require some form of couplant liquid between the sample and the transducer. Not only is thisvery often practically inconvenient in the field, but also it makes it difficult to perform reproducible
measurements [8]. In contrast, an electromagnetic acoustic transducer (EMAT) can generate elastic
waves in an electrically conductive material without requiring any mechanical contact with thespecimen [9, 10]. As such, EMAT has been applied in the field of non-destructive testing because of*

Corresponding author, E-mail: takagi@ifs.tohoku.ac.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

its contactless characteristic. Therefore, it doesn't need couplant liquid and suits for measurement athigh temperature.
The electromagnetic acoustic resonance (EMAR) method uses an EMAT to generate ultrasoundin a specimen. If the total propagation distance is an integer multiple of the ultrasonic wavelength, theultrasound is strengthened by resonance phenomena. The pulse-EMAR method uses an EMAT togenerate a pulse wave in a specimen. The thickness of the specimen is obtained by analyzing thefrequency components of the echo wave [11, 12].
The basic signal processing method for ultrasonic thickness measurement is the echo time
method, which uses the time difference between the two echo signals form bottom surface to calculate
the distance of ultrasonic wave propagation in the specimen. The thickness d of the specimen is equal
to half the propagation distance, namely

d

vt
,
2

(1)

where v is the wave velocity of ultrasound and t is the time difference of the two echoes signal.
Because the transduction efficiency of an EMAT is very low [8-10], a low signal to noise ratio is
obtained when using the EMAR and pulse-EMAR methods [11]. To resolve this problem, Urayama et
al. proposed processing the received signal using a method known as superposition of nth compression(SNC) [11, 13], and they used the EMAR method combined with SNC signal processing to measure
the thickness of pipes in a nuclear power plant. The SNC method superimposes the peak value of thefrequency of each order of resonance on the peak value of the fundamental resonance frequency. It
can be expressed as
f1  arg max  xnf  ,
f
n


(2)

where f1 is the fundamental resonance frequency, x(f) is the SNC spectrum intensity, “arg max” is thefrequency at which the SNC spectrum intensity is maximum, n is the superimposed resonance order.
The peak value of the fundamental resonance frequency is SNC peak value. The SNC method has
obtained good signal to noise ratio [11, 13]. Hence, according to the theory of ultrasonic resonance,
the specimen thickness d can be calculated by

d

v
.
2 f1

(3)

In previous studies, scaly structures as one of factors that may affect the measurement results
were briefly discussed [13]. However, it only treated a special specimen to simulate artificial scaly
structures. There has been no discussion about the scaly structures of an actual pipeline, and there wasno numerical simulation of the effect of scaly structures. The purpose of this paper is to study theeffect on EMAT measurements of pipe-wall thickness of the scaly structures that form on the innersurface of the pipe because of FAC. Therefore, in this study, the wall thickness of the dorsum part of abent carbon steel pipe was measured using the EMAR and pulse-EMAR methods. As a comparison,measurements were performed in the same place using a caliper gauge. We then observed typicalfeatures of the scaly structure using a three-dimensional (3D) confocal laser scanning microscope(CLSM), as well as simulating the pulse-EMAR method numerically. All received signals were
processed using the SNC method. Finally, the experimental and numerical results are discussed inrelation to the scaly structures.

2.

Experiments

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

2.1 Wall thickness measurement for an actual pipe
The specimen is the dorsum part of a bent carbon steel pipe from a fossil fuel power plant [3],and it is shown schematically in Fig. 1 (a). The dorsum part is cut into 12 parts. Part of the innersurface of the dorsum is covered in a scaly structure associated with FAC. The outer and innerdiameters of the bent pipe are 139.8 mm and 126.6 mm, respectively. The nominal thickness of thebent pipe wall is 6.6 mm, and the radius of curvature of the bent pipe is 420 mm. The dorsum of thebent pipe has five measurement points in the circumferential direction and 23 measurement points inthe pipe (i.e., flow) direction, as shown in Fig. 1(b).

(a) Bent pipe shape(b) Measurement points of dorsum

Fig. 1. Specimen and measurement points

The diameter of the transmitter EMAT coil is 10 mm (40 turns) and the diameter of the receiver
EMAT coil is 20 mm (80 turns). They are placed under a samarium–cobalt (SmCo) permanent magnet(10 × 20 × 20 mm3). Figure 2 shows the EMAT probe schematically. The complete thickness
measurement system is shown in Fig. 3. In the EMAR method, the transmitter frequency range is
1–2.5 MHz. In the pulse-EMAR method, the center frequency of the pulse wave is 2 MHz. For
comparison, the same measurements were performed using a caliper gauge with which the minimum
thickness of the measurement point area could easily be measured [3]. As for the maximum thickness,
we measured at numerous points close to the measurement point and took the maximum value as the
maximum thickness of the measurement point area.

Fig. 2. EMAT probe

Fig. 3. System of EMAT thickness measurement

2.2 Thickness measurement results
The EMAR and pulse-EMAR received signals were processed using the SNC method, theanalytical range of which is 1–3.5 MHz. Figure 4 shows separate comparisons of the EMAR andpulse-EMAR measurement results with the minimum measurement results made using the caliper
gauge. These comparisons show that there was good agreement between the EMAR/pulse-EMAR andcaliper measurements for some thicknesses but discrepancies for others. The largest of these
discrepancies are 0.79±0.24 mm (EMAR) and 1.32±0.36 mm (pulse-EMAR), where each error is the17

H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

respective root-mean-square error (RMSE).
We select four specimens sequentially from the middle part of the pipe and refer to them as
specimens A, B, C, and D. Specimen A is in the initial stages of FAC and has almost no scaly
structure. Specimen B has a little wall thinning and a large number of scattered scaly structures.
Specimen C has obvious wall thinning and is covered with large scaly structures. Specimen D has
severe wall thinning and is covered with dense small scaly structures.
We select three representative measurement points on each specimen and number them as 1–3.
The measurement results for all points are listed in Table 1. For specimens B and C, the measurement
results made using the pulse-EMAR method are almost the same as the maximum thicknesses
determined using the caliper gauge. For specimen D, the measurement results made using thepulse-EMAR method fall between the maximum and minimum thicknesses determined using thecaliper gauge.

Fig. 4. Comparison of thickness measurement results
Table 1 Thickness measurement results for specimens A, B, C, and D (unit: mm)

Point
A-1
A-2
A-3
B-1
B-2
B-3
C-1
C-2
C-3
D-1
D-2
D-3

Pulse-EMAR
4.96
4.62
4.92
4.11
4.75
3.95
4.14
4.13
3.17
3.11
3.19
2.81

Caliper gauge (min)
5.00
4.65
4.90
3.36
3.58
3.33
3.15
3.30
2.65
2.63
3.10
2.35

Caliper gauge (max)
*
*
*
4.20
4.80
3.95
4.23
4.25
3.58
3.38
3.95
3.15

* There was no obvious concavity at the measurement points of specimen A. Therefore, the thickness ateach of those three measurement points is listed as the minimum thickness only.

2.3 Discussion and micro-observation of the scaly structure parts
A transducer measures over an area of a specimen, not at a point. The measurement resultscorrespond to the overall situation within this area. The reverberation surface of a part with a scaly
structure is not smooth, such that the ultrasound experiences oblique incidence. This may lead todifferent mode conversion and energy partitioning for longitudinal and shear waves [14, 15], which inturn can lead to poor repeatability of the measurement results. Therefore, a scaly internal structure isoften the main cause of discrepancies in the measurement results.

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

As can be seen in Fig. 4, the data points associated with the pulse-EMAR method are distributed
mainly above and to the left of the isoline. This means that the thickness measured using thepulse-EMAR method tends to be larger than the minimum thickness measured using the caliper gauge.
The largest such discrepancies are mainly in the thickness range of 3–5 mm by the pulse-EMAR
method. From Table 1, the measurement results for specimen A are the most accurate, which isunsurprising because specimen A is similar to a flat plate. Specimens B and C have more scaly
structures than flat parts. The pulse-EMAR measurement results for specimens B and C are closer tothe maximum caliper thicknesses than to the minimum ones. This means that the reflected signal isdominated by the more coherent component from the flat parts. For specimen D, the pulse-EMAR
measurement results fall between the minimum and maximum caliper thicknesses. Any part of ameasurement area that is found to be pitted is deemed to be a scaly structure [3]. Synthesizing theabove analyses, it is clear that the scaly area ratio has an appreciable influence on the measurement of
thickness.
The actual surface topographies in the scaly parts of specimens A, B, C, and D were observed
using a 3D CLSM (VK-X 100; Keyence Corp.). Surface modification was used to process the original
results because the specimens were from a bent pipe. A single scan took about 1 h. Figure 5 shows 3D
images of the measured areas, the dimensions of which are 11,240.6 μm × 800.0 μm (points A-1, B-1,
and C-1) and 8,780.5 μm × 7,199.5 μm (point D-1). For comparison, the surface profile of the middle
section of each specimen is shown in Fig. 6. The ordinate is the depth from the highest point of themeasurement area. In the measurement areas of specimens A, B, C, and D, the scaly area ratios are 0,
71.2%, 88.2%, and 100%, respectively. The maximum depths of the surface pits are 0, 1,090 μm,
1,416 μm, and 1,176 μm, respectively. In the measurement area of point D-1, small dense scaly
structures due to a large number of large block scaly pits mutual interference. The maximum depth ofthe pits in the region in which mutual interference occurs is 774 μm. Based on the above observations,
we now proceed to numerical simulations of the effect of various scaly area ratios on the EMAT
thickness measurements.

Point A - 1

Point B – 1

Point C - 1

Point D – 1

Fig. 5. 3D images of measured areas

3.

Numerical simulations of Pulse-EMAR

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

In order to confirm that the EMAR and pulse-EMAR methods give larger thickness values
compared to the caliper gauge results, we performed numerical simulations. To evaluate thepulse-EMAR method of measuring the wall thickness of a carbon-steel pipe, we developed a 3D finite
element (FE) model, the geometry of which is shown in Fig. 7. The FE model consists of two
simulation parts: the electromagnetic field and the ultrasonic propagation. These are simulated using
the commercial software GiD and ComWAVE (Itochu Techno-Solutions Corp.), respectively, with thetwo parts coupled via the electromagnetic force.

Point A - 1

Point B – 1

Point C - 1

Point D - 1

Fig. 6. Surface profiles of middle sections in Fig. 5(a) Top view(b) Elevation view(c) Local enlarged drawing

Fig. 7. Geometries of finite element (FE) model

3.1 Coupling of electromagnetic and mechanical mechanisms
In ferromagnetic materials, the transformation mechanism between electromagnetic field andelastic wave includes Lorentz force, magnetization force and magnetostrictive force. They can be
expressed as [8],FL  J B,(4)20

H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMATF M  H   M ,F MS    e  H  ,

(5)
(6)

where FL is the Lorentz force, J is the eddy current, B is the magnetic flux density, FM is themagnetization force, H is the magnetizing field, M is the magnetization, FMS is the magnetostrictive
force, and e is the tensor of the magnetostriction.
The magnetization force is very weak, and hence we choose to ignore it. In this study, themagnetostrictive force is also neglected because the Lorentz force is the dominant transduction
mechanism in a strong static magnetic field [8, 9].
3.2 Electromagnetic field simulation implementation
The electromagnetic model was implemented using the software GiD by discretizing the domain
into 136,800 hexahedron elements as shown in Fig. 8 (elevation view). The strength of the field
generated with the permanent (SmCo) magnet employed is 1 T. The coil carries a 10 A current that
oscillates at a frequency of 2 MHz. The specimen material is low carbon steel with an electricalconductivity of 5.26×106 S/m and a relative permeability of 100 [16]. The FE mesh is refined near the
specimen surface because of the skin effect of the eddy current.
The distribution of the Lorentz force is calculated using the Lorentz-force calculation module
because of the distributions of the static magnetic field and the eddy current. The Lorentz force is usedsubsequently to generate ultrasound in the ultrasonic propagation simulation. The calculation time of
the FE model was approximately 20 min (Intel Core i7 4790, 3.6 GHz, 8 GB).

Fig. 8. Meshes of FE model

3.3 Ultrasonic propagation simulation implementation
The ultrasonic propagation model was set up using the software ComWAVE. To simulate the pitsof the scaly structures due to FAC, the lower surface of the specimen was indented with partial
spheres. In this study, according to the measurement results in Sect. 2.3, the four specimens shown in
Fig. 9 were calculated. Model I simulates specimen A, which has a smooth reflection surface.
Model II simulates specimen B, by indenting the lower surface with partial spheres of diameter
4.74 mm and height (depth) 1.10 mm. In order to realize a scaly area ratio of about 71.2%, we set the
gap between adjacent rows of partial spheres to 0.5 mm; this gave a scaly area ratio of 69.8% and a
thickness range in the pitted regions of 3.90–5.00 mm. Model III simulates specimen C and is similar
to model II. However, the diameter and depth of the partially spherical indentations are now 7.82 mm
and 1.40 mm, respectively. The intentions are also now more closely arranged such that the scaly area
ratio is 90.7%, and the thickness range in the pitted regions is 3.60–5.00 mm. Model IV simulates
specimen D and is similar to models II and III. However, the diameter and depth of the partiallyspherical indentations are now 9.20 mm and 1.20 mm, respectively. There is also overlap between
neighboring indentations, and the thickness range in the pitted regions is 3.80–4.60 mm. All thesimulation parameters are listed in Table 2.
Each of the four models is divided into a different number of hexahedron elements: 4,300,800

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

(model I), 4,161,512 (model II), 4,055,504 (model III), and 4,045,628 (model IV). In order to reduce
the effect of reflection, absorption bands (3 mm thick) are set on the side surfaces of the specimens.
The density of the material is 7,850 kg/m3. The shear wave and longitudinal wave velocities are
3,230 m/s and 5,950 m/s, respectively. Near the specimen upper surface, the meshes are refined
because of the skin effect of the eddy current. The ultrasonic propagation process is calculated overthe first 1.1×10-4 s. The calculation time of the FE model is about 46 h (Intel Core i7 4790, 3.6 GHz,
8 GB).

Model I

Model II

Model III

Model IV

Fig. 9. Specimen models (unit: mm)Table 2 Simulation parameters of the specimen models

Model I
Model II
Model III
Model IV

Scaly area
ratio
0
69.8%
90.7%
100%

Thickness of
scaly area
5.00 mm
3.90–5.00 mm
3.60–5.00 mm
3.80–4.60 mm

Maximum depthsof the pits01.10 mm
1.40 mm1.20 mmDepth of mutualinterference parts
0.80 mm

3.4 Results and discussion
In order to compare with experiments, the analytical range of the SNC method was set to 1000–
3500 kHz. Figure 10 shows the FFT spectrum on the analytical range over the SNC for the fourmodels. All data have been normalized on the maximum SNC value of the specimen without scaly
structure (model I). Figure 11 shows the SNC spectrum for the four models. For comparative purposes,
we set the peak value for model I to unity and normalize the other data accordingly. All the results arelisted in Table 3. The thickness values of models I–IV obtained by the SNC method are almost within
the true thickness range, which demonstrates the validity of the SNC method. When the reflection
surface is uneven, such as for models II–IV, the SNC peak values decrease rapidly. The thicknesses ofmodels II and III obtained by the SNC method are close to the maximum thickness. This is because
30.2% and 9.3%, respectively, of the measurement area is not scaly. For model IV, the thickness
obtained by the SNC method is between the minimum and maximum thicknesses. These results are inagreement with the experimental results. Therefore, the proportion of the measurement area that isscaly may affect the measurement results.
The effects of a scaly structure on the measurement of pipe wall thickness can be divided roughly
into four cases: specimens A–D or models I–IV. In model I, the SNC-calculated thickness accuratelyreflects the actual thickness of the pipe wall. In models II and III, the SNC-calculated thickness isapproximately that of the flat part (i.e., the maximum thickness). In this case, the result of thepulse-EMAR method is larger than the minimum thickness. In model IV, although the wall thinning ismore serious, the measurement results are close to the minimum wall thickness. There are somediscrepancies in the EMAR and pulse-EMAR measurement results and the minimum thicknesses
obtained with the caliper gauge. This is essentially because the EMAR and pulse-EMAR methods
measure over an area, not at a point. Hence, the EMAR and pulse-EMAR results include surface

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

information from the whole measurement region under the probe. According to the numerical results,
the spectrum includes several peaks, which might reflect the surface conditions. However, the SNC
result might give the thickness of the flat surface. The Pulse-EMAR method gives accurate thickness
measurement results. From the numerical results, we see for models II and III that the method might
give the maximum thickness. This might explain the discrepancies obtained in the measurements. In
order to obtain better results, the thickness of a smaller measurement area could be assessed by
reducing the transducer size or focusing the ultrasound, hence better approximating the conditions at
the actual measurement point. Therefore, in future work, we intend to improve the design of thetransducer so that it generates and receives stronger ultrasonic signals over a smaller area.

4.

Conclusions

In this study, the wall thickness of the dorsum part of a corroded bent carbon steel pipe wasmeasured using the EMAR and pulse-EMAR methods and with a caliper gauge, and the results werecompared. The caliper/EMAR and caliper/pulse-EMAR RMSE discrepancies were 0.24 mm and0.36 mm, respectively. The results show that the scaly structure owing to FAC leads to discrepancies
between the various measurement results. To reveal the effects of this scaly structure on themeasurement of pipe wall thickness, typical parts of the pipe wall were observed using 3D CLSM.
This led on to numerical simulations of the effects of various scaly structures on the pulse-EMAR
thickness measurements. The received signal was processed using the SNC method, which showedthat the proportion of scaly structure in the measurement area can affect the measurement results. Thisagrees with the results of the experiments mentioned above.
Overall, the results show that the pulse-EMAR method measures thickness accurately in theinitial stage of FAC, after which the discrepancies gradually increase. It is even possible for thepulse-EMAR method to return the thickness of the flat regions in the measurement area under theprobe, which might explain the discrepancies obtained in the measurements.
Shrinking the transducer or focusing its waves would allow thickness to be measured over asmaller area. This would bring the measurement results closer to the actual thickness at themeasurement point.

Model I

Model II

Model III

Model IV

Fig. 10. FFT spectrum of specimens

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H. Sun, R. Urayama, T. Uchimoto, F. Kojima, T. Takagi, H. Abe, K. Kobayashi.
Effect of Scaly Structure on Pipe Wall Thickness Measurement with EMAT

Model I

Model II

Model III

Model IV

Fig. 11. SNC spectrum of specimens (analytical range of SNC method: 1000 – 3500 kHz)
Table 3 simulation results (analytical range of SNC method: 1000 – 3500 kHz)

Model I
Model II
Model III
Model IV

Real thickness of
scaly structure area
5.00 mm3.90 - 5.00 mm
3.60 - 5.00 mm
3.80 - 4.60 mm

Fundamental resonant
frequency of SNC
320 kHz322 kHz323 kHz366 kHz

Thickness by
SNC method
5.05 mm5.02 mm5.00 mm
4.41 mm

Normalized SNC
peak value
1.000
0.104
0.065
0.050

Acknowledgements
This work was partly supported by 1) the JSPS Core-to-Core Program, A. Advanced Research
Networks, “International research core on smart layered materials and structures for energy saving”,
2) the “Fundamental Research and Human Resources Development Program for Nuclear
Decommissioning related to Integrity Management of Critical Structures including PrimaryContainment Vessel and Reactor Building, and Fuel Debris Processing and Radioactive Waste
Disposal” by the Ministry of Education, Culture, Sports, Science and Technology of Japan, and 3) byan IFS Research Assistant Award for International Students. The authors are grateful to the
Hashizume Laboratory of Tohoku University for offering the use of the 3D CLSM.

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