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Eddy Current Depth of Penetration

by Don Hagemaier*

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.

Figure 1  - Eddy current standard depth of penetration for materials of different conductivity at varying frequencies. 

Table 1 Formulas used for determining the standard depth of penetration (ASNT, 1986)

	Hochschild (1959)	Libby (1971)	Hocking (1986)	Brown (1969)	PS 21207	ASM (1976)
		Franklin (1982)	MIL-STD-1537		MIL-STD-1537
Formula
	centimeters	meters	millimeters	inches	inches	inches
	3.14	3.14
	hertz	hertz	hertz	hertz	hertz	hertz
	% IACS	mhos per meter			% IACS
			1	1	1	1

 

Table 2  Chart for calculating the standard depth of penetration based on conductivity (given in megasiemens per meter or percent IACS) and frequency

Frequency	0.58 MS/m	2.9 MS/m	5.22 MS/m	7.54 MS/m	9.86 MS/m	12.18 MS/m	17.4 MS/m	23.3 MS/m
	(1% IACS)	(5% IACS)	(9% IACS)	(13% IACS)	(17% IACS)	(21% IACS)	(30% IACS)	(40% IACS)
100 Hz	66.04 mm	29.54 mm	22.02 mm	18.31 mm	16.03 mm	14.40 mm	12.07 mm	10.44 mm
	(2.600 in.)	(1.163 in.)	(0.867 in.)	(0.721 in.)	(0.631 in.)	(0.567 in.)	(0.475 in.)	(0.411 in.)
300 Hz	38.13 mm	17.04 mm	12.70 mm	10.57 mm	9.25 mm	8.33 mm	6.96 mm	6.02 mm
	(1.501 in.)	(0.671 in.)	(0.500 in.)	(0.416 in.)	(0.364 in.)	(0.328 in.)	(0.274 in.)	(0.237 in.)
500 Hz	29.54 mm	13.21 mm	9.86 mm	8.18 mm	7.16 mm	6.45 mm	5.38 mm	4.67 mm
	(1.163 in.)	(0.520 in.)	(0.388 in.)	(0.322 in.)	(0.282 in.)	(0.254 in.)	(0.212 in.)	(0.184 in.)
700 Hz	24.97 mm	11.15 mm	8.33 mm	6.93 mm	6.05 mm	5.44 mm	4.55 mm	3.94 mm
	(0.983 in.)	(0.439 in.)	(0.328 in.)	(0.273 in.)	(0.238 in.)	(0.214 in.)	(0.179 in.)	(0.155 in.)
900 Hz	22.02 mm	9.86 mm	7.34 mm	6.10 mm	5.33 mm	4.80 mm	4.01 mm	3.48 mm
	(0.867 in.)	(0.388 in.)	(0.289 in.)	(0.240 in.)	(0.210 in.)	(0.189 in.)	(0.158 in.)	(0.137 in.)
1 kHz	20.88 mm	9.35 mm	6.96 mm	5.79 mm	5.05 mm	4.55 mm	3.81 mm	3.30 mm
	(0.822 in.)	(0.368 in.)	(0.274 in.)	(0.228 in.)	(0.199 in.)	(0.179 in.)	(0.150 in.)	(0.130 in.)
3 kHz	12.07 mm	5.38 mm	4.01 mm	3.35 mm	2.92 mm	2.64 mm	2.21 mm	1.91 mm
	(0.475 in.)	(0.212 in.)	(0.158 in.)	(0.132 in.)	(0.115 in.)	(0.104 in.)	(0.087 in.)	(0.075 in.)
5 kHz	9.35 mm	4.17 mm	3.12 mm	2.59 mm	2.26 mm	2.03 mm	1.70 mm	1.47 mm
	(0.368 in.)	(0.164 in.)	(0.123 in.)	(0.102 in.)	(0.089 in.)	(0.080 in.)	(0.067 in.)	(0.058 in.)
7 kHz	7.90 mm	3.53 mm	2.64 mm	2.18 mm	1.90 mm	1.73 mm	1.45 mm	1.24 mm
	(0.311 in.)	(0.139 in.)	(0.104 in.)	(0.086 in.)	(0.075 in.)	(0.068 in.)	(0.057 in.)	(0.049 in.)
9 kHz	6.96 mm	3.12 mm	2.31 mm	1.93 mm	1.68 mm	1.52 mm	1.27 mm	1.09 mm
	(0.274 in.)	(0.123 in.)	(0.091 in.)	(0.076 in.)	(0.066 in.)	(0.060 in.)	(0.050 in.)	(0.043 in.)
10 kHz	6.60 mm	2.95 mm	2.21 mm	1.83 mm	1.60 mm	1.45 mm	1.19 mm	1.04 mm
	(0.260 in.)	(0.116 in.)	(0.087 in.)	(0.072 in.)	(0.063 in.)	(0.057 in.)	(0.047 in.)	(0.041 in.)
30 kHz	3.81 mm	1.70 mm	1.27 mm	1.07 mm	0.91 mm	0.84 mm	0.69 mm	0.61 mm
	(0.150 in.)	(0.067 in.)	(0.050 in.)	(0.042 in.)	(0.036 in.)	(0.033 in.)	(0.027 in.)	(0.024 in.)
50 kHz	2.95 mm	1.32 mm	0.99 mm	0.81 mm	0.71 mm	0.64 mm	0.53 mm	0.46 mm
	(0.116 in.)	(0.052 in.)	(0.039 in.)	(0.032 in.)	(0.028 in.)	(0.025 in.)	(0.021 in.)	(0.018 in.)
70 kHz	2.49 mm	1.12 mm	0.84 mm	0.69 mm	0.61 mm	0.53 mm	0.46 mm	0.41 mm
	(0.098 in.)	(0.044 in.)	(0.033 in.)	(0.027 in.)	(0.024 in.)	(0.021 in.)	(0.018 in.)	(0.016 in.)
90 kHz	2.21 mm	0.99 mm	0.74 mm	0.61 mm	0.53 mm	0.48 mm	0.41 mm	0.36 mm
	(0.087 in.)	(0.039 in.)	(0.029 in.)	(0.024 in.)	(0.021 in.)	(0.019 in.)	(0.016 in.)	(0.014 in.)
100 kHz	2.08 mm	0.94 mm	0.69 mm	0.58 mm	0.51 mm	0.46 mm	0.38 mm	0.33 mm
	(0.082 in.)	(0.037 in.)	(0.027 in.)	(0.023 in.)	(0.020 in.)	(0.018 in.)	(0.015 in.)	(0.013 in.)
300 kHz	1.19 mm	0.53 mm	0.41 mm	0.33 mm	0.30 mm	0.25 mm	0.23 mm	0.20 mm
	(0.047 in.)	(0.021 in.)	(0.016 in.)	(0.013 in.)	(0.012 in.)	(0.010 in.)	(0.009 in.)	(0.008 in.)
500 kHz	0.94 mm	0.41 mm	0.30 mm	0.25 mm	0.23 mm	0.20 mm	0.18 mm	0.15 mm
	(0.037 in.)	(0.016 in.)	(0.012 in.)	(0.010 in.)	(0.009 in.)	(0.008 in.)	(0.007 in.)	(0.006 in.)
700 kHz	0.79 mm	0.36 mm	0.25 mm	0.23 mm	0.20 mm	0.18 mm	0.15 mm	0.13 mm
	(0.031 in.)	(0.014 in.)	(0.010 in.)	(0.009 in.)	(0.008 in.)	(0.007 in.)	(0.006 in.)	(0.005 in.)
900 kHz	0.69 mm	0.30 mm	0.23 mm	0.20 mm	0.18 mm	0.15 mm	0.13 mm	0.10 mm
	(0.027 in.)	(0.012 in.)	(0.009 in.)	(0.008 in.)	(0.007 in.)	(0.006 in.)	(0.005 in.)	(0.004 in.)
1 MHz	0.66 mm	0.30 mm	0.23 mm	0.18 mm	0.15 mm	0.15 mm	0.13 mm	0.10 mm
	(0.026 in.)	(0.012 in.)	(0.009 in.)	(0.007 in.)	(0.006 in.)	(0.006 in.)	(0.005 in.)	(0.004 in.)
3 MHz	0.38 mm	0.18 mm	0.13 mm	0.10 mm	0.10 mm	0.08 mm	0.08 mm	0.05 mm
	(0.015 in.)	(0.007 in.)	(0.005 in.)	(0.004 in.)	(0.004 in.)	(0.003 in.)	(0.003 in.)	(0.002 in.)
5 MHz	0.30 mm	0.13 mm	0.10 mm	0.08 mm	0.08 mm	0.08 mm	0.05 mm	0.05 mm
	(0.012 in.)	(0.005 in.)	(0.004 in.)	(0.003 in.)	(0.003 in.)	(0.003 in.)	(0.002 in.)	(0.002 in.)

 

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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