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Eddy Current Depth of Penetration

by Don Hagemaier*

Figure 1
Table 1
Table 2

Introduction

The depth of penetration of eddy currents is a function of the:

• conductivity of the material being tested
• magnetic permeability of the material being tested
• frequency of the alternating current driving the probe.

An increase in these three factors produces a decrease in the penetration of the eddy currents. Since the decrease of the strength of the eddy currents is approximately exponential with depth, a factor called standard depth of penetration, designated by , is generally used to obtain the best results on testing of a specimen. When performing conductivity testing, the minimum thickness of the test part should be 3

The following is a brief discussion on how to obtain the standard depth of penetration or the operating frequency of the probe for the desired eddy current test.

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There are a number of equations for calculating the standard depth of penetration. 

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Determination of Standard Depth of Penetration
There are a number of equations for calculating the standard depth of penetration. These equations can also be used to obtain an operating frequency, as is shown in Table 1. Calculating the proper operating frequency based on the part thickness and conductivity can take time and may cause errors.

Another method for determining the operating frequency is to refer to a graph similar to Figure 1. However, a more useful way to determine the proper operating frequency based on the part thickness and conductivity of nonmagnetic alloys is shown in Table 2. Determining the proper depth of penetration is necessary when trying to detect cracks or corrosion thinning on the backside of the part. It is also necessary when trying to detect second or third layer cracks or corrosion thinning.

To use Table 2, determine the conductivity and thickness of the part. Locate the conductivity value of the part on the horizontal scale. Then move down vertically until the value of the part thickness is obtained. Finally, move horizontally left to obtain the proper operating frequency. For example, a part having a conductivity of 17.4 MS/m (30% IACS) - a pure aluminum - and a 1.27 mm (0.05 in.) thickness would show an operating frequency for detecting backside discontinuities of 9 kHz.

 

Acknowledgments
Table 2 is based on a chart compiled by the NDT Engineering Group, Boeing, Long Beach, California.

 

References
ASM International, Metals Handbook, eighth edition: Vol. 11, Nondestructive Inspection and Quality Control, Metals Park, Ohio, ASM International, 1976, pp. 79-85.

ASNT, Nondestructive Testing Handbook, second edition: Vol. 4, Electromagnetic Testing, Columbus, Ohio, ASNT, 1986.

Brown, Russell L., "The Eddy Current Slide Rule," Materials Evaluation, Vol. 26, No. 6 (June 1968), pp. 120-123.

Franklin, E.M., "Eddy Current Inspection," Materials Evaluation, Vol. 40, No. 10 (September 1982), pp. 1008-1010.

Hochschild, R., "Electromagnetic Methods of Testing Metals," Progress in NDT, Vol. 1, New York, McMillan, 1959.

Hocking Electronics, "Brochure on Phasec D4A," Halo Instruments, Inc., reprinted in Nondestructive Testing Handbook, second edition: Vol. 4, Electromagnetic Testing, Columbus, Ohio, ASNT, 1986, p. 381.

Libby, H., Introduction to Electromagnetic Nondestructive Test Methods, New York, Wiley Interscience, 1971.

MIL-STD-1537: Electrical Conductivity Test for Measurement of Heat Treatment of Aluminum Alloys, Eddy Current Method, Washington, DC, Department of Defense, 1981.

PS 21207: Eddy Current Determination of Electrical Conductivity for Aluminum Alloys, Revision D, St. Louis, McDonnell Douglas, 1972.

 

* 8566 Colusa Cr., Apt. 903E, Huntington Beach, CA 92646; (714) 960-6687.

 

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