Fast inversion method for crack identification in eddy current testing

Abstract

Eddy current testing (ECT) is exclusively used for in-service inspection of steam generator tubes in pressurized water reactor-type nuclear power plants because of its attractive features, such as very high detectability, high scanning speeds, and no coupling medium with test specimens. Although, the cracks are characterized in magnitude and phase of their eddy current signals, they do not exactly tell us the details: how large cracks are, and what profiles they are. This problem is considered as an inverse problem. A method for the inversion was proposed, following the eddy current signal predictions, for which edge-based finite elements were used requiring much less computational memory in comparison to nodal finite elements. The trust region method, a kind of the quasi-Newton method, was used to find the optimal crack profiles, and the cracks were formed there by finite elements. It was found that while estimated results from laboratory data were in good agreement with the true profiles, this was fairly time-consuming; although, the cracks locally exist only in the test specimens, large algebraic equations derived from finite elements were repeatedly solved to obtain the sensitivity of the crack profiles to the signals because of its matrices, depending on the crack profiles.