E-Journal of Advanced Maintenance Vol.1 (2009) 83-86
Japan Society of Maintenology

Electromagnetic Nondestructive Evaluation of Rotating Blades
Daigo KOSAKA1,*, Haoyu HUANG2, Noritaka YUSA3, Kenzo MIYA2
1

Polytecnic University 4-1-1, Hashimotodai, Sagamihara, Kanagawa 229-1196, JapanIIU Corp. 2-7-17, Ikenohata, Taito, Tokyo 110-0008, Japan3
Department of Nuclear Engineering and Management, Graduate School of Engineering The
University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-8656 Japan2(Received; July 12th, 2008)

Abstract. This study proposes a method to nondestructively detect and evaluate tiny defects appearing at the tipof a rotating blade. The method utilizes a permanent magnet to generate a static magnetic field and detects theeddy currents induced by the blade moving inside the magnetic field. The eddy current is detected by the coil ofthe sensor. Experimental validations are carried out using a commercial pump. Artificial slits fabricated into theimpeller were well detected in spite of the presence of a metallic case and a large lift-off.

KEYWORDS: rotary machine; pumps; electromagnetic field; vibration

1.

Introduction

Rotating machines such as rotors, pumps, turbines, and fans are among the most commonly
utilized as industrial components. Current vibration-based methods used to monitor Rotating Bladesof rotating machines such as pumps can well predict many possible problems [1-4]; however, not all
degradations can be satisfactorily detected before problems arise, as has been shown [5]. Somedegradations such as tiny cracks or corrosion appearing at the tip of rotating blades, do not influence
the mechanical vibration of the machine itself and thus cannot be detected by vibration-based methods
until they become so large that the integrity of the machine is actually threatened.
Against the background above, this study proposes a novel electromagnetic nondestructive
testing method to detect and evaluate tiny defects appearing on rotating blades. The method utilizes apermanent magnet to produce a static magnetic field and detects the disturbance of the field caused by
eddy currents flowing inside a rotating blade. The eddy current is detected by the coil of the sensor.
The method is quite simple but detects defects well even through a metallic case containing therotating blades.
2.

Experimental validation of the method

For the experiment we used a commercial water pump. The revolutions per minute of the pumpare 3600. For the test a defect was introduced into the pump’s impeller. Using vibrometer made inShowa Sokki. The model of the vibrometer is Model-1332A. Measurement Ranges of the acceleration
are from 0.01m/s2〜199.9m/s2.
A picture of the pump used in the experiment is shown in Fig. 1(a). A cross-sectional profile isshown in Fig. 1(b). An approximately 1.5 mm thick case made from type 304 stainless steel contains
an aluminum impeller measuring 120 mm in diameter and 15 mm in thickness.
The sensor we developed is shown in Fig. 1(c). The sensor consists of a Nd2Fe14B magnet 30
mm in diameter and a 500 turn coil winding over the magnet. The magnet generates a static magnetic
field penetrating the case that induces eddy currents inside the impeller. The coil then detects the*Corresponding author, E-mail: kosaka@uitec.ac.jp

ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

83./D. KOSAKA et al
Electromagnetic Nondestructive Evaluation of Rotating Blades

magnetic field generated by the disturbance of the eddy currents caused by the presence of theintroduced slits or cut out wastage.
Two kinds of test piece were made. One had slits inserted as demonstrated in the two slit test
pieces shown in Fig. 2(a, b). The other had pieces cut out, a wastage test piece is shown in Fig. 2(c).
Comparisons were made by measuring with a vibrometer.

case

unit: mm20
magnetsensor

19120 146 149
30coilimpeller(b) cross-section(a) photograph(c) sensor

Fig. 1. Experimental setup

10unit: mm

d0.50.5

15 degreed = 3, 5, 10, 205

13(a) 4 slits modeld120120t = 10t = 10(b) 1 slit model120
d = 0.5, 1.0, 3.0t = 10(c) wastage modelFig. 2. Test impellers

3.

Experiment results

The measurements of our developed sensor and the vibrometer were compared. This experimentused the test piece with 4 slits as shown in Fig. 2(a). The results of the vibrometer are presented in Fig.
3. The vibrometer isn’t able to detect the number of slits. The results of developed sensor arepresented in Fig. 4. The developed sensor was able to detect the number and size of the slits
individually.
The signal to noise ratio (S/N) of the developed sensor was evaluated. The signal is the Vpp of
the sensor output with slit or wastage. The noise is the Vpp of the sensor output without slit andwastage. This experiment used the test piece shown in Fig. 2(b) and (c). The results are presented inFig. 5 (a) and (b). The S/N grows by large the slit or the wastage. The developed sensor was able todetect the size of the slit and the wastage quantitatively by using Fig. 5.

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./D. KOSAKA et al
Electromagnetic Nondestructive Evaluation of Rotating Blades
15
voltage (V)voltage (V)

15
1050

10500100

200
300
400
Frequency (Hz)

5000100(a) without slit
Fig. 3. Measured signals by the vibrometer

200300Frequency (Hz)(b) with slit

400

500

10.50.5

voltage (V)voltage (V)10mm 3mm
10-0.50-0.5-1-1-1.5-1.50102030time (ms)405mm 0.5mm050

1010108866S/N

S/N(a) without slit
Fig. 4. Measured signals by the sensor

422030time (ms)(c) with slit

4050

4200051015slit depth (mm)200(a) slit

1234wastage depth (mm)5(b) wastage

Fig. 5. Measured results with slit and wastage by the sensor

4.

Results of numerical analysis

The signals measured and the signals from the numerical analysis using 3D FEM were compared.
The FEM was used A-φ method. The results are presented in Fig. 6 (a) and (b). The time is 2ms when
the sensor position is at the center of the slit or wastage.
The measured and the analytical signals in the slit correspond well in Fig. 6(a). The measured
and analytical signals in the wastage do not correspond, as shown in Fig. 6(b). This was because thepresence of the wastage made the specimen unbalanced and the simulations did not take this intoconsideration.

85

D. KOSAKA et al
Electromagnetic Nondestructive Evaluation of Rotating Blades./1exp. 0.5mmexp. 1.0mm
exp. 3.0mm
num. 0.5mm
num. 1.0mm
num. 3.0mm

0.5
voltage (V)0.5
voltage (V)1exp. 3mm
exp. 5mm
exp. 10mm
num. 3mm
num. 5mm
num. 10mm00-0.5-0.5-1-10

1

2time (ms)304(a) slit

12time (ms)34(b) wastage

Fig. 6. Numerical analysis signals with the slit and the wastage

5.

Conclusion

This study proposed a novel but rather simple method to nondestructively detect tiny defects
appearing at the tips of rotating blades. Its effectiveness was validated through experiments using acommercial pump. The features of this method are as follows.
1. It is more reliable than vibration methods when there are two or more defects.
2. It can measure the defect without any contact.
3. It can examine and optimize the sensor using numerical analysis.
Improvement of the S/N and optimization of the sensor will be examined in the future by numericalanalysis.

Acknowledgement
This study was supported in part by the Japan Society of Maintenology.

References[1] D. Dyer, Detection of rolling element bearing damage by statistical vibration analysis, Trans. ofASME Mechanical Design, Vol. 100 (1978), pp. 229-235[2] T. Toyota and S. Nakashima, Machine diagnosis by sound and vibration analysis, The Journal of
the Acoustical Society of Japan, Vol. 39, No. 8 (1983), pp. 550-555[3] M. Tsuchiya and M. Takagi, Application of wavelets to diagnostic system for rotational
Machinery, Transactions of the Japan Society of Mechanical Engineers. Series C, Vol. 64, No.
618 (1998), pp. 465-472[4] S. Kobayashi, Vibration diagnosis technology and its application to rotational machines, Journalof the Japanese society for non-destructive inspection, Vol.58, No. 1 (1978), pp. 6-9[5] Japan Nuclear Energy Safety Organization, 2007/16/15 Press Release(available at http://www2.jnes.go.jp/atom-db/jp/trouble/2006fy/news-20060615/01.html)

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