Removing Eddy-Current probe wobble noise from steam generator tubes testing using Wavelet Transform

Abstract

One of the most important nondestructive evaluation (NDE) applied to steam generator tubes inspection is the electromagnetic Eddy-Current testing (ECT). The signals generated in this NDE, in general, contain many noises which make difficult the interpretation and analysis of ECT signals. One of the noises present in the signals is the probe wobble noise, which is caused by the existing slack between the probe and the tube walls. In this work, Wavelet Transform (WT) is used in the probe wobble de-noising. WT is a relatively recent mathematical tool, which allows local analysis of non-stationary signals such as ECT signals. This is a great advantage of WT when compared with other analysis tools such as Fourier Transform. However, using WT involves wavelets and coefficients' selection as well as choosing the number of decomposition level needed. This work presents a probe wobble de-noising method when used in conjunction with the traditional ECT evaluation. Comparative results using several WT applied to Eddy-Current signals are presented in a reliable way, in other words, without loss of inherent defect information. A stainless steel tube, with two artificial defects generated by electro-erosion, was inspected by a ZETEC MIZ-17ET ECT equipment. The signals were de-noised through several different WT and the results are presented. The method offers good results and is a promising method because it allows for the removal of Eddy-Current signals' probe wobble effect without loss of essential signal information. © 2007 Elsevier Ltd. All rights reserved.