E-Journal of Advanced Maintenance Vol.2 (2010/2011) 25-33
Japan Society of Maintenology

Quantitative Evaluation of Pipe Wall Thinning by Electromagnetic
Acoustic Resonance
Ryoichi URAYAMA1, Tetsuya UCHIMOTO1,*, Toshiyuki TAKAGI1, and Shigeru
KANEMOTO212

Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
School of Computer Science and Engineering, The University of Aizu, Japan(Received; April 21st, 2010)
Abstract. Electromagnetic acoustic transducer (EMAT) provides non-contacting measurements and is often
applied to monitoring in high temperature environment. One of drawbacks of EMAT is its low sensitivity andS/N ratio. One way to overcome the drawback is to introduce an Electromagnetic acoustic resonance method(EMAR); it has a high capability of evaluating thickness or ultrasonic velocity, and it is applied to themeasurement of thickness, elastic constants, damping properties and so on. Since the principle of the method isbased on the through-thickness resonances of bulk waves, that is, thickness oscillations, targets are usuallylimited to ones with simple geometry such as plates. In this study, pipe wall thinning, where the thickness
changes on the curved surface, is evaluated by EMAR. For the purpose, several data processing methods areapplied to extract thickness information from spectral responses. Measured spectra are obtained by anexperiment using a carbon steel pipe with two-dimensional thinning machined by milling, and results of the dataprocessing are compared in view of accuracy and stability. Finally, the data processing method is applied to theEMAR spectra of a pipe specimen cut from a mock-up test loop.
KEYWORDS: EMAT, EMAR, Pipe wall thinning, Thickness measurement, Signal processing

1.

Introduction

In nuclear and thermal power plants, it is highly required to find reasonable methods of pipe wall
thinning of management by flow accelerated corrosion (FAC) and liquid droplet impingement erosion(LDI). Currently, ultrasonic thickness gauges are applied for inspection and monitoring of pipe wall
thinning[1]. The probes of ultrasonic thickness gauges are usually transducers with piezoelectricelements, and thickness is measured by a pulse-echo method. The drawback of the current systems isthe fact that they require coupling media between the targets and the probes; it is especially critical inapplying them in high temperature environment since it is difficult to find heat-resistant coupling
media with long-term reliability.
Electromagnetic acoustic transducer (EMAT), which is one of ultrasonic measurement method,
provides non-contacting measurements, since ultrasonic waves are transmitted and received inelectromagnetic way[2]. Due to the advantage, this method is often applied to measurement in hightemperature environment[3,4]. The drawback of the EMAT is its low electromechanical conversion
efficiency, which leads to low sensitivity and low S/N ratio. One of the ways to overcome thedrawback of EMAT is to introduce an electromagnetic acoustic resonance method (EMAR)[5]. It has ahigh capability of evaluating thickness or ultrasonic velocity, and it is applied to the measurement ofthickness, elastic constants, damping properties and so on. Since the principle of the method is basedon the through-thickness resonances of bulk waves, that is, thickness oscillations, targets are usuallylimited to ones with simple geometry such as plates.

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

ISSN – 1883 – 9894/10/11 © 2010/2011 – JSM and the authors. All rights reserved.

25

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic ResonanceIn this study, the signal processing of EMAR spectra is discussed to evaluate thickness of piping
by EMAR, where the thickness changes on the curved surface. For the purpose, two data processing
methods are applied to extract thickness information from spectral responses. Measured spectra are
obtained by an experiment using a carbon steel pipe with two-dimensional thinning, and results of thedata processing are compared in view of accuracy and stability. Finally, the signal processing method
is applied to a measurement of piping cut from the mock-up test loop to simulate pipe wall-thinning in
nuclear power stations.
2.

Principal of EMAR

Figure 1 schematically illustrates the structure and principal of a typical EMAT transducer fortransmission and reception of shear waves[2]. The transducer is usually consists of magnets andtransmitter and receiver coils. When the coil is driven by a current at the desired ultrasonic frequency,
eddy currents will be induced in a near surface of the target of electrically conducting object. Since astatic magnetic field is provided by the magnet, these currents will experience Lorentz forces, and ashear wave propagates in the target in the case of the configuration shown in Fig. 1. Reception of theround-trip waves can be made in the reverse process described above.

NS

MagnetSN

CoilEddy current
Lorentz force

Shear wave
Fig.1. Schematic drawing of structure of EMAT transducer for emission and reception of shear waves.

The principal of EMAR is the through-thickness resonances of bulk waves[5]. When a transducer isplaced on the surface of a plate of thickness d, and transmits a ultrasonic burst wave propagating
normally to the surface, whose wave length is λ, the wave reflects repeatedly at the both surfaces andis received by the same transducer each time. Then, the resonance is observed when the wave length
satisfies n λ= 2 d. The resonance frequency of the nth order is
fn = n v / 2 d(1)where v is the ultrasonic velocity. Sweeping the ultrasonic frequency, the resonance frequency can be
identified.
3.

Preliminary Experiment

3.1 Experimental
Preliminary experiment was carried out to discuss the feasibility of signal processing methods ofthickness measurement based on EMAR. A specimen used in the preliminary experiment is carbon
steel piping (JIS STPT370) of 50A Sch80, which has an outer diameter of 60.5 mm, a nominal
thickness of 5.5 mm and length of 120 mm. The material and dimensions of the pipe specimen are the26

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonancesame as that of mock-up test loop which is discussed later. The simulated wall thinning wasintroduced by milling as shown in Fig. 2. The profile of the wall thinning has a curvature of circle
with diameter of 40 mm, and the resulting width and maximum depth of thinning are 20 mm and 0.5mm, respectively. The EMAT transducer employed in this study is schematically illustrated in Fig. 3.

Fig. 2. Cross section of the pipe specimen with a simulated wall thinning for preliminary experiment.

(a) Photo of transducer and configuration of exciting and pickup coils.

MagnetCoilsNSSN(b) Arrangement of the EMAT transducer on the pipe specimen
Fig. 3. Structure and arrangement of EMAT transducer used in this study.

27

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic ResonanceIts structure is basically same as that shown in Fig. 1, and the transducer consists of two Nd basepermanent magnets, a exciting coil and a pickup coil. Each of the rectangular shaped magnets has awidth of 10 mm, length of 20 mm and height of 20 mm. One face of combining magnets was grinded
so as to fit the surface of the pipe specimen as shown in Fig. 3 (b). Diameter and number of turns of
the coils are 10 mm and 40, for the exciting one, and 20 mm and 80, for the pickup one, respectively,
and they are combined together as shown in Fig. 3(a). A pulser/receiver (RITEC RPR-4000), a wide
range decade filter (NF Corporation FV-628B) as the filter in the detection frequency, an oscilloscope(TEKTRONIX DPO4104), and a PC for data collection were used in the measurements. The
transducer was driven by burst signals with period of 100 μs. The driving frequency was swept from
0.5 to 4.0 MHz at the interval of 10 kHz. Signal amplitude of each frequency was computed by thesuperheterodyne processing at the interval of 1 kHz. The thickness measurement was carried out
scanning circumferentially over the wall thinning at the interval of 2.5 mm. The ultrasonic velocity
was determined by the reference test piece of RB-E( JIS standard) made of STPT370, and the value of3270 m/s was employed for the velocity of shear wave.
3.2 Results
Figure 4 shows the thickness-resonance spectrum measured at (a) the center of wall thinning (Point Ain Fig.2), and (b) the point 5mm away from the center (Point B in Fig.2). Spectrum of Fig. 4(a) shows
sharp peaks at resonance frequencies, though many peaks are observed for each resonance frequency inthat of Fig. 4(b). In the case of Fig. 4(b), the curvature of the wall thinning, that plays the role of reflector,
is inclined with respect to the outer surface, which means the reflected waves have the information ofvarious thickness and many peaks are observed. In the next section, two signal processing methods areapplied to identify the resonance frequency from the complicated spectrum like Fig. 4(b).
ch=2, d(mm)=4.954545, fr(MHz)=0.330000, max=0.847213, HW(KHz)=45.212565

Org. EMAR Signal

0.8
0.6
0.4
0.2
00

0.5

1

1.5

2
2.5
Frequency(MHz)3

3.5

4(a) Center of the wall thinning.
ch=4, d(mm)=4.924699, fr(MHz)=0.332000, max=0.569737, HW(KHz)=76.011228

Org. EMAR Signal

0.2
0.15
0.1
0.05
000.5

1

1.5

2
2.5
Frequency(MHz)

33.5(b) 5mm from center of wall thinning.
Fig.4. EMAR spectra of pipe specimen with simulated wall thinning284

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonance4.

Signal Processing Methods

4.1 Details of each processing
In this study, two signal processing methods described below are applied, and their feasibility isdiscussed.
(a) Auto-correlation method
This method is based on the correlation function between the original spectrum and one shifted infrequency. The correlation function is supposed to be the maximum when the spectrum is shifted by
the fundamental resonance frequency, Δ f. Finally, the thickness is evaluated as(2)

where x(f) is the spectra, fs the frequency shift, argmax the function giving the argument of the maximum.
Figure 5 shows the profile of correlation function based on the spectra at (a) the center ofwall-thinning, and (b) the point 5mm away from the center. The circles and bars in the figure indicatethe maximum position of the correlation function and half maximum full-width (HMFW),respectively. HMFW indicates the variation of thickness in the sound field of the transducer, andHMFW of Fig. 5(a) and (b) are small and large, respectively. This is consistent with the discussion inthe section 3.2.
Auto-Correlation

10.5

0-0.5

00.05

0.1

0.15

0.2
0.25
0.3
Frequency(MHz)

0.35

0.4

0.45

0.5

(a) Center of wall thinning.

Auto-Correlation

10.5

HMFW

0-0.5

00.05

0.1

0.15

0.2
0.25
0.3
Frequency(MHz)0.35

0.4

0.45

0.5(b) 5mm from center of wall thinning.
Fig.5. Auto-correlation function as a function of shifted frequency. The peaks areobserved around the fundamental resonance frequency.

(b) Superposition of nth compression
In the ideal spectrum, the resonance peaks appear at every fundamental resonance frequency. If thefrequency axis of the original spectrum is compressed into nth, the nth peak moves to the fundamental
resonance frequency. Summing up the spectra of nth compression, largest peak is supposed to appear
around the fundamental resonance frequency. Finally, the thickness is evaluated as29R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonance(3)

ch=2, d(mm)=4.954545, fr(MHz)=0.330000, max=3.234823, HW(KHz)=3.257231

1
0.5
00.5

1

1.5

2
2.5
Frequency(MHz)

33.5

4

0

0.5

1

1.5

2
2.5
Frequency(MHz)

3

3.5

4

00.2

0.22

0.38

0.4

Sum of N Signals

04

Sum of N Signals

Org. EMAR Signal

Figure 6 shows the original spectra, superposition of nth compression (SNC) and magnification ofSNC at the peak where the peak position and HMFW are shown with the circles and bars, respectively.
The measurement points are the same as before. As the auto-correlation method, the peak in Fig. 6(b)is wider than that of Fig. 6(a).

4202

0.24

0.26
0.28
0.3
0.32
0.34
Frequency(MHz) Extended Scale

0.36

Org. EMAR Signal

1

Sum of N Signals

0.2

Sum of N Signals(a) Center of wall thinning.
ch=4, d(mm)=4.989827, fr(MHz)=0.327667, max=0.692730, HW(KHz)=14.024743

0.1
000.5

1

1.5

2
2.5
Frequency(MHz)

33.5

400.5

11.5

22.5
Frequency(MHz)

3

3.5

4

00.2

0.22

0.38

0.4

0.5
0

1
0.5

0.24

0.26
0.28
0.3
0.32
0.34
Frequency(MHz) Extended Scale

0.36(b) 5mm from center of wall thinning.
Fig. 6. The original spectra, superposition of nth compression (SNC) and magnification of SNC at the peak.

(c) Discussion
Figure 7 shows the profiles of the thickness of wall thinning evaluated by the two signal processing
methods together with real thickness. Evaluated thickness varies from 5.4 to 4.9 mm smoothly, whichindicates the validity of the two methods. Comparing between two methods, SNC method provide morestable and conservative results than auto-correlation method. Figure 8 shows the HMFW and peakvalues as the function of transducer position. The HMFW correlates with the inclination of the innersurface to the outer one, and can be the index of the range of thickness change at the transducer position.

30

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonance
5.5

Wall thickness [mm]

5.4
5.3
5.2
5.1
SNC method
Auto-correlation method
Real thickness

54.902.55

7.5

10

12.5

15

17.5

Position [mm]Fig. 7. Comparison of thickness profiles evaluated by the present signal processing methods

Peak value and half-value width

16
peak value
half-value width141210864
2
00

2.55

7.5

10

12.5

15

17.5

Position [mm]Fig. 8. Profiles of half-value width and peak value in the SNC method.

5.

Measurement of Mock-up Specimen

5. 1 Details of specimen and measurement
The pipe specimen was cut from the mock test loop aiming at simulation of pipe wall thinning in
nuclear power plants. It is carbon steel piping (JIS STPT370) of 50A Sch80, which has outer diameterof 60.5 mm, nominal thickness of 5.5 mm. A pipe elbow and an orifice were put above the specimen
in the loop to induce eccentric orifice flow and to accelerate corrosion. The test duration andtemperature are 1420 hrs. and 165 degrees C, respectively. The 1350 mm long specimen section wascut from the loop after the corrosion test.
EMAR measurement was carried out, using the same EMAT transducer and measurement system
as preliminary experiment. The ultrasonic velocity was determined by the reference carbon steel pipeof STPT370 without any corrosion, and the value of 3241 m/s was employed for the velocity of shear
wave. Here, the SNC method which provides stable and conservative evaluation compared with theAuto-correlation method was applied for the signal processing of EMAR spectra. After themeasurement, the specimen was cut in the long direction, and the real thickness was measured byobserving the cross section with a microscope.

31

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonance

5. 2 Results
Figure 9 shows the measured thickness as a function of position along two lines lying in the long
direction of the pipe specimens. The real thickness was also plotted in solid lines. In the region ofposition between 50 mm and 150 mm, the wall-thinning of around 0.5 mm was observed. The
measured thickness by EMAR agrees well with real one. In Fig. 10, the estimated thickness iscompared with real one. In the figure, the results of ultrasonic thickness gauge are also plotted forreference. Two transducers of 9 mm and 14 mm in diameters were used. The smaller transducer gives
more accurate measurement because of the curved surface as shown in Fig. 10. Accuracy of theEMAR is almost the same as that of the smaller transducer, and the error is between – 0.1 mm to + 0.2mm.
5.5
Line-a microscope
Line-e microscope
5.4

Line-a EMAR

Wall thickness [mm]5.3Line-e EMAR

5.2
5.1
5.0
4.9
4.8
4.7
4.64.530507090110130150170190

Position [mm]Fig. 9. Profiles of wall thickness evaluated by EMAR with the SNC method.

5.55.4

Estimated thickness [mm]

5.3
5.2
5.1
5.0
4.9
4.8
UT(φ14mm)
UT(φ9mm)
EMAR

4.7
4.6
4.5

4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5
Real thickness [mm]Fig. 10. Accuracy of thickness evaluation by EMAR with SNC method and conventional ultrasonicthickness gauge.

32

R. URAYAMA et al./ Quantitative Evaluation of Pipe Wall Thinning
by Electromagnetic Acoustic Resonance6.

Summary

In this study, pipe wall thinning, where the thickness changes on the curved surface, is evaluatedby EMAR. To indentify the resonance frequency of EMAR spectra, two data processing methods,
Auto-correlation method and SNC method are applied and their feasibility was discussed. Results ofthickness measurement of a carbon steel pipe with simulated wall thinning show that SNC method
gives stable and conservative evaluation. Finally, the SNC method is applied to a measurement ofpiping cut from the mock-up test loop, and it turned out that accuracy of the EMAR is almost thesame as optimal results of ultrasonic thickness gauge, and the errors is between – 0.1 mm to + 0.2mm.

Acknowledgement
This study was performed as part of “project on enhancement of aging management andmaintenance of NPPs” supported by nuclear and industrial safety agency, METI, JAPAN, This study
was also supported by Tohoku University Global COE Program, “world center of education andresearch for trans-disciplinary flow dynamics”. The mock specimen was provided by the Tokyo
Electric Power Company. The authors would like to thank Mr. Takeshi Sato and Mr. Tsutomu
Watanabe of the Institute of Fluid Science, Tohoku University, for the preparation of the specimens
and the fabrication of the probe.
References[1] The Japan Society of Mechanical Engineers ed., Codes for Nuclear Power Generation Facilities:
Rules on Pipe Wall Thinning Management for BWR Power Plants, (2006), 7-49, The Society ofMechanical Engineers．
[2] R. B. Thompson, Physical principles of measurements with EMAT Transducer, PhysicalAcoustics Vol.XIX, Academic Press Inc., (1990), 157-200.
[3] N. Yamagata, M. Takahashi and N. Ahiko: Thickness Measuring Technology for Pipes ofThermal Power Plants, Toshiba Review (in Japanese), 63 (4), (2008), 46-49.
[4] R. Urayama, T. Uchimoto, T. Takagi, Application of EMAT/EC Dual Probe to Monitoring ofWall Thinning in High Temperature Environment, International Journal of Applied
Electromagnetics and Mechanics, (2010) in press.
[5] M. Hirao and M. Ogi, EMATS for science and industry: noncontacting ultrasonic measurements,
Kluwer Academic Publishers, (2003).

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