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

Non-destructive evaluation of materials and structures is still a key issue in some industrial scenarios as the production process and the quality inspection. In the case of metallic materials, economic and implementation reasons push for the use of eddy current testing techniques. In the last years, the effort of the research activity is been focused on the development of eddy current measurement procedures capable of providing as much information as possible about the presence, the location and the geometrical characteristics of defects. To this aim, newer signals characterized by a wide spectral content able to penetrate in the different layers of the material under test are substituting the older sinusoidal excitation. Among these, multi-frequency and chirp represent two optimal candidates within the class of frequency domain-based signals. The former is characterized by the simultaneous presence of many sinusoidal tones, while the latter exhibits a constant envelope and an instantaneous frequency that increases or decreases with time. In literature many interesting papers dealing with both excitation types are reported but an experimental performance comparison on a number of real defects is missing. Moreover the comparisons are usually executed on single measurements collected in presence of a defect in the location corresponding to the highest defect signal. Even if this strategy allows the analysis of the defect signature in time and in frequency domain, from both experimental and practical point of view, this approach is extremely sensitive to noise and it could be also difficult to be applied in on-line or in-situ inspections. In this paper, the proposed comparison aims at highlighting the suitability of each considered excitation method with respect to the extraction of defects geometrical features. It is proposed to combine the various excitation signals with image processing: indeed by developing a proper 2D image procedure from 1D eddy-current data it is possible to improve the defect detection capability when difficult cases are experienced (such as annealed and small cracks) and to extract more accurate information about the defect's geometric characteristics. After the image processing application, the multi-tone and the chirp approaches are quantitatively compared by using an ad-hoc figure of merit.