volume | PIER Journals

Abstract

Eddy current testing (ECT) is known as an effective technology for inspecting surface and near surface defects in metallic components. It is well known that the amplitude of eddy current (EC) density decreases with increasing depth, which is referred to as skin effect. Skin depth is an important parameter that quantifies the speed of attenuation of EC in the depth direction and is closely related to the capability of ECT for detecting deeply hidden defects. It is found that the traditional formula for calculating skin depth derived under the assumption of uniform plane field excitation is not applicable to the cases of ECT with coils. The skin effect in component with flat surface excited by pancake coil has been investigated by the authors. The skin effect in conductive tube tested by bobbin coil and that in conductive bar tested by encircling coil are more complex. The paper studies the skin effect in these two cases. Finite element analysis shows that the attenuation of EC is not only due to the ohmic loss, but also influenced by the diffusion effects, the aggregation effect, and the combined cancellation/diffusion effect of EC. The skin depth of EC associated with bobbin coil is always smaller than that associated with uniform plane field excitation, whereas the skin depth of EC associated with encircling coil can be greater than that associated with uniform plane field excitation under certain conditions.

1. Dahia, A., E. Berthelot, Y. L. B. And, and L. Daniel, "A model-based method for the characterisation of stress in magnetic materials using eddy current non-destructive evaluation," Journal of Physics D: Applied Physics, Vol. 48, 195002-195011, 2015.
doi:10.1088/0022-3727/48/19/195002

2. Pereira, D. and T. G. R., "Clarke modeling and design optimization of an eddy current sensor for superficial and subsuperficial crack detection in inconel claddings," IEEE Sensors Journal, Vol. 15, 1287-1292, 2015.
doi:10.1109/JSEN.2014.2362072

3. Jesenik, M., M. Beković, A. Hamler, and M. Trlep, "Searching for hidden cracks and estimations of their depths by using the database," NDT & E International, Vol. 86, 44-52, 2017.
doi:10.1016/j.ndteint.2016.11.007

4. Zhou, D., M. Pan, Y. He, and B. Du, "Stress detection and measurement in ferromagnetic metals using pulse electromagnetic method with U-shaped sensor," Measurement, Vol. 105, 136-145, 2017.
doi:10.1016/j.measurement.2017.04.001

5. Cheng, J., J. Qiu, H. Ji, E. Wang, T. Takagi, and T. Uchimoto, "Application of low frequency ECT method in noncontact detection and visualization of CFRP material," Composites Part B, Vol. 110, 141-152, 2017.
doi:10.1016/j.compositesb.2016.11.018

6. Mottl, Z., "The quantitative relations between true and standard depth of penetration for air-cored probe coils in eddy current testing," NDT & E International, Vol. 23, 11-18, 1990.

7. Mook, G., O. Hesse, and V. Uchanin, "Deep penetrating eddy currents and probes," Materials Testing, Vol. 49, 258-264, 2007.
doi:10.3139/120.100810

8. Hoshikawa, H. and K. Koyama, "Eddy current distribution using parameters normalized by standard penetration depth," Materials Evaluation, Vol. 57, 587-593, 1999.

9. Hagemaier, D. J., "Eddy current standard depth of penetration," Materials Evaluation, Vol. 43, 1438-1442, 1985.

10. Cacciola, M., S. Calcagno, G. Megali, F. C. Morabito, D. Pellicanó, and M. Versaci, "FEA design and misfit minimization for in-depth flaw characterization in metallic plates with eddy current nondestructive testing," IEEE Transactions on Magnetics, Vol. 45, 1506-1509, 2009.
doi:10.1109/TMAG.2009.2012691

11. Jiao, S., X. Liu, and Z. Zeng, "Intensive study of skin effect in eddy current testing with pancake coil," IEEE Transactions on Magnetics, Vol. 53, 6201608, 2017.
doi:10.1109/TMAG.2017.2669181

12. Reboud, C., D. Prémel, D. Lesselier, and B. Bisiaux, "Recent advances in simulation of eddy current testing of tubes and experimental validations," Review of Progress in Quantitative Nondestructive Evaluation, Vol. 894, 241-248, 2007.
doi:10.1063/1.2717979

13. Lai, S., D. Chen, H. Chen, and Y. Fu, "Pulsed eddy current testing of inner wall flaws in pipe under insulation," Procedia Engineering, Vol. 130, 1658-1664, 2015.
doi:10.1016/j.proeng.2015.12.334

14. Majidnia, S., J. Rudlin, and R. Nilavalan, "Investigations on a pulsed eddy current system for flaw detection using an encircling coil on a steel pipe," Non-Destructive Testing and Condition Monitoring, Vol. 56, 560-565, 2014.
doi:10.1784/insi.2014.56.10.560

15. Zhang, J., M. Yuan, S. Song, and H. Kim, "Precision measurement of coating thickness on ferromagnetic tube using pulsed eddy current technique," International Journal of Precision Engineering and Manufacturing, Vol. 16, 1723-1728, 2015.
doi:10.1007/s12541-015-0226-7

16. Ma, X., A. J. Peyton, and Y. Zhao, "Eddy current measurements of electrical conductivity and magnetic permeability of porous metals," NDT & E International, Vol. 39, 562-568, 2006.
doi:10.1016/j.ndteint.2006.03.008

17. Nateq, M. H., S. Kahrobaee, and M. Kashefi, "Use of eddy-current method for determining the thickness of induction-hardened layer in cast iron," Metal Science and Heat Treatment, Vol. 55, 370-374, 2013.
doi:10.1007/s11041-013-9638-0

18. Bowler, J. R. and T. P. Theodoulidis, "Eddy currents induced in a conducting rod of finite length by a coaxial encircling coil," Journal of Physics D: Applied Physics, Vol. 38, 2861-2868, 2005.
doi:10.1088/0022-3727/38/16/019

Dr. Jianwei Yang

Dr. Shaoni Jiao

Prof. Zhiwei Zeng

Mr. Junming Lin

Dr. Jincheng Zhao