NDTnet 1998 Aug, Vol.3 No.8

Abstract

Progress in mean of modelisation and inverse problem solving let us hope to dispose soon of these tools for flaws 3D imaging in Non Destructive Control with eddy current sensors. This will achieve a real improvement of the actual methods, mainly based upon signature analysis. But the actual eddy current probes used for steam generators tubes inspection in nuclear industry do not produce the adequate measurements and/or are not modelisable.

Fig.

Therefore we present here a new eddy current probe designed with imaging objectives. The elements of this probe have been choosen after a relevant analysis of electromagnetic interactions :

• axisymetric excitation coils, with different excitation modes, large enough to induce currents in the whole thickness of the tube.
  
• micro-sensors to sample accurately the radial component of the electomagnetic field, wich is null in absence of a flaw.

The amount of computation needed for 3D image reconstruction is too large to be applicated to the whole tubes length. It is necessary to dispose of a simple and efficient way to detect suspicious regions, for which the heavy computation will be done. The flaws we hope to detect are so small that their signals disappear among perturbations ( surface noise, miscentring and so on). We propose two methods to detect rapidly flaws ( possibly in realtime ), one based upon principal components analysis ( only statisticals properties of signals ), and the other using a limited bank of wavelets( time-frequency properties). These two processes can be chained.

developped maps, before and after preprocessing

The association of the new probe and the pre-processings give very satisfying results, as we can see in the above figure, obtained from circonferential external flaws with 100 gm width and 10, 20, 40, 60 and 100% deepness. So we now dispose of a set which can be used on its own, but also produces convenient measurements for imaging.

[1] D. MILLER, DYLACKO "Pre-processing of signals delivered by a new eddy current sensor for non ferromagnetic tubes testing" in IMTC 96

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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