An experimental comparison of complex excitation sequences for eddy current testing

Abstract

Eddy Current Testing (ECT) is a Non Destructive technique widely used in many industrial application fields in which it is very important to detect the presence of thin defects (generally called cracks) in conductive materials. Features of this technique are the cost-effective implementation and the kind of retrieved measured data that make possible to estimate the geometrical characteristics of a crack as position, length, width and depth. The analysis of these characteristics allows the user to accept or discard realized components then improving the production chain. To accomplish for this task some aspects have to be taken into account during the measurement process. They mainly concern the realization of suitable measurement setup and post processing stages. As far as the measurement setup is concerned, crucial aspects are the choice of measurement and excitation devices. The choice of optimized excitation devices and strategies is of interest for research on Non Destructive ECT (ND-ECT): together with common aspects as the amplitude and the frequency of the exciting signal, the attention has been paid to issues as the type of signal to be adopted. In particular it has been found as the use of complex excitation signals, meant as signals different from the sinusoidal ones and with wide frequency content, might raise eddy current responses trying to support the measurement, detection and characterization stages when "difficult cases" are explored (i.e. very small and/or annealed cracks). In this paper the authors propose an experimental comparison of different excitation signals designed to improve the quality of experimental data when difficult cases are experienced and, consequently, to obtain a more reliable extraction of defects geometrical features.

An experimental comparison of multi-frequency and chirp excitations for eddy current testing on thin defects

“…To this aim information gathered by numerical FEM simulations and previous experimental results [6,12,17] has been taken into account. For the present configuration of the electromagnet and of the GMR sensor, it is found that a small defect produces an image very similar to the spatial profile of a Hermite-Gaussian mode of order (1,1) [34], characterized by 4 main lobes with alternate polarities symmetrically distributed with respect to the defect center [35].…”

“…The former has important representatives in pulsed (PEC) [2][3][4][5] or pseudo-noise excitations [6,7], while the latter proposes signals as the Multi-Frequency (MF-ECT) [8][9][10] and chirp one [11,12]. Pulsed signals are typically analyzed in the time-domain signal processing, while MF-ECT and chirp signals are instead analyzed by the use of transformed domains as the Fourier and the Chirplet.…”

“…The authors are investigating the field of Non-Destructive ECT for many years, proposing probes, measurement methods and excitation strategies to optimize the defect characterization by ECT methods [6,7,9,10,12,[17][18][19]. Currently their research activity focuses on the use of signals in the frequency domain with the aim to improve the sensitivity of defect detection in some particular application fields, and especially for small and sub-superficial cracks.…”