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RESEARCH PAPERS

A Nondestructive Strategy for the Distinction of Natural Fatigue and Stress Corrosion Cracks Based on Signals From Eddy Current Testing

[+] Author and Article Information
Zhenmao Chen

MOE Key Laboratory for Strength and Vibration, School of Aerospace, Xi’an Jiaotong University, 28 West Xianning Road, Xi’an 710049, Chinachenzm@mail.xjtu.edu.cn

Ladislav Janousek, Noritaka Yusa, Kenzo Miya

 International Institute of Universality, 7F Imon Building, 2-7-17 Ikenohata, Taitou-ku, Tokyo 110-0008, Japan

J. Pressure Vessel Technol 129(4), 719-728 (Sep 07, 2006) (10 pages) doi:10.1115/1.2767365 History: Received May 31, 2006; Revised September 07, 2006

In this paper, a novel nondestructive strategy is proposed for distinguishing differences between a stress corrosion crack (SCC) and a fatigue crack (FC) based on signals from eddy current testing (ECT). The strategy consists of measurement procedures with a special ECT probe and crack type judgment scheme based on an index parameter that is defined as the amplitude ratio of the measured signals. An ECT probe, which can induce eddy current flowing mainly in a selected direction, is proposed and applied to detect crack signals by scanning along the crack with different probe orientations. It is clear that the ratio of the amplitudes of signals detected for parallel and perpendicular probe orientations is sensitive to the microstructure of the crack, i.e., the parameter is much bigger for a fatigue crack than that of a SCC. Therefore, whether a crack is a SCC or a FC can be recognized nondestructively by comparing the index parameter with a threshold value that can be previously determined. In order to verify the validity of the proposed strategy, many artificial SCC and FC test pieces were fabricated and ECT inspections were performed to measure the corresponding crack signals. Numerical simulations were also conducted to investigate the physical principles of the new methodology. From both the numerical and experimental results, it is demonstrated that the strategy is very promising for the distinction of artificial SCC and FC; there is also good possibility that this method can be applied to natural cracks if the threshold value can be properly determined.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Cross section of a test piece of Inconel 600 material with a FC that is introduced into the test piece by tension-tension fatigue testing using a three-point bending jig

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Figure 2

An example of SCC in SUS304 stainless steel material: (a) overview, (b) at the surface, (c) inside the crack, and (d) at crack tip

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Figure 3

Conceptual models for a SCC and a FC. The model of a SCC has a wider width and the crack region is conductive while the model of the FC has a narrow width and the crack region is almost not conductive. (a) Cross section of a SCC and (b) cross section of a FC.

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Figure 4

Dimensions and configuration of the testing ECT probe (uniform eddy current probe)

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Figure 5

Distribution of eddy current flowing perpendicular to the crack: (a) case of SCC and (b) case of FC

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Figure 6

Distribution of eddy current flowing parallel to the crack: (a) case of SCC and (b) case of FC

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Figure 7

Numerical model of the inspection target and the crack (unit: mm)

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Figure 8

Scanning path of the probe considered in the simulation and experiments

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Figure 9

The distribution of absolute value for the C scan eddy current signals of the uniform eddy current probe and a crack of wc=0.1mm, σc=0%, lc=10mm, and dc=4mm, the center of crack is located at (0,0) and with orientation along the y axis; (a) case of eddy currents perpendicular to the crack and (b) case of eddy currents parallel to the crack

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Figure 10

The distribution of absolute value for the C scan eddy current signals of the uniform eddy current probe and a crack of wc=0.5mm, σc=50%, lc=10mm, and dc=4mm, the center of crack is located at (0,0) and with orientation along the y axis: (a) case of eddy currents perpendicular to the crack, (b) case of eddy currents parallel to the crack

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Figure 11

The correlation of signal amplitude with the crack width and conductivity: (a) case of eddy currents perpendicular to the crack and (b) case of eddy currents parallel to the crack

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Figure 12

The ratio α(Aper∕Apar) versus crack width and conductivity

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Figure 13

Basic design of the artificial SCC test piece

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Figure 14

A PT image of a SCC test piece

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Figure 15

Example of the cross section of a SCC after destruction testing

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Figure 16

Example of the cross section of a FC after destruction testing

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Figure 17

The absolute value of the signal versus probe position (case FC, 10kHz, the center of crack is located at (0,0) and with orientation along the y axis); (a) the case of eddy currents perpendicular to the crack and (b) the case of eddy currents parallel to the crack

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Figure 18

The absolute value of the signal versus probe position (case SCC, 10kHz, the center of crack is located at (0,0) and with orientation along the y axis); (a) the case of eddy currents perpendicular to the crack and (b) the case of eddy currents parallel to the crack

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Figure 19

α versus type of a crack, 10kHz, Inconel 600 alloy

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Figure 20

α versus type of a crack, 20kHz, Inconel 600 alloy

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Figure 21

α versus type of a crack, 50kHz, Inconel 600 alloy

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Figure 22

α versus type of crack, 10kHz, SUS304 austenitic stainless steel

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