Evaluation of the flaw depth using high-Tc SQUID

doi:10.1016/S0921-4534(01)01021-8

Physica C: Superconductivity

Volume 367, Issues 1–4, 15 February 2002, Pages 303-307

https://doi.org/10.1016/S0921-4534(01)01021-8 Get rights and content

Abstract

Using high-T_c SQUID, we investigated quantitative nondestructive evaluation for flaws in conducting samples. The spatial derivative of the defect field was found to provide valuable information about the position and the depth of the flaw. By analyzing the spatial derivative of the defect field, the quantitative flaw evaluation was demonstrated.

Introduction

In last decade, many researchers have made efforts to develop the SQUID-based nondestructive evaluation (NDE), which is a powerful tool for detecting deep flaws [1], [2]. Among many remarkable works [3], [4], [5], [6], [7], [8], [9], the method utilizing the sheet inducer was report to be useful for performing the phase-sensitive flaw detection with SQUIDs. The uniform excitation generated by the sheet inducer is advantageous for analyzing the eddy-current problem, but the spatial resolution is limited to the SQUID-to-sample distance because of the uniform excitation field. In contrast, the method using the differential coil exciter has achieved a better spatial resolution [4], [5], [6], [7], [8], [9]. But it is a challenge to use the differential coil exciter for the quantitative flaw evaluation because the anti-symmetric excitation field introduces a complex eddy-current problem. To avoid solving the complex eddy-current problem, we investigated the empirical relation between the defect field, which is the magnetic field due to the eddy current around a flaw, and the depth of the flaw in this work. The differential defect field was proposed to correlate the detected field with the depth and the position of flaws.

Section snippets

Experimental details

The electronics used for the SQUID NDE system was described elsewhere [8], [9]. The SQUID used in the system was a commercialized dc SQUID fabricated by Conductus Inc. The SQUID was mounted at 18 mm above the center of the differential coil exciter, where the excitation field generated by the differential coil exciter was zero. The distance from the sample to the differential coil exciter was 2 mm. To operate the system in the unshielded environment, we utilized a small μ-metal cylinder to

Results and discussion

The vertical component of the excitation field generated by the differential coil exciter was calculated according to the Biot–Savart law as shown in Fig. 1. The excitation current in the coil was 1 A, and the diameter of the coil exciters is D=28 mm. The symbol d shown in Fig. 1 represents the vertical distance measured from the plane of the coil to the SQUID, or to the sample beneath the coil. The excitation field, B_z, is found to be anti-symmetric with the zero amplitude at x=0 for various d

Conclusion

In conclusion, the evaluation of flaw depth was investigated by using the differential coil exciter. The magnitude and the phase of the differential defect field were correlated directly to the defect field. For the crack-like flaw with the width and the height much less than D, the phase of the differential defect field is useful in indicating the flaw depth at least for the flaw depth smaller than D. Using the differential defect field instead of the defect field is beneficial because the

Acknowledgements

This work is supported by the National Science Council of ROC under grant no. NSC89-2112-M-003-040, and NSC89-2112-M-002-071.

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Evaluation of the flaw depth using high-Tc SQUID

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgements

Physica C

Physica C

High-transition-temperature superconducting quantum interference devices

Rev. Mod. Phys.

Review article: SQUIDs for nondestructive evaluation

J. Phys. D: Appl. Phys.

Evaluation of the flaw depth using high-T_c SQUID