Comparison of time and frequency domain processing in pulse-compression eddy current imaging

In the last decades, many research efforts have been devoted to increase the performances of Eddy Current Testing (ECT) mainly by means of Multi-Frequency (MF) or Pulsed Eddy-current (PEC) signals, or by exploiting electrical resonance phenomena. By exciting the Sample Under Test (SUT) at several frequencies simultaneously or through time-multiplexing, the defect detection can be enhanced and at the same time useful information can be inferred about detects’ depth and height through the analysis of the response at different frequencies. This is because the skin effect makes high frequencies less penetrating in the sample than low frequencies. Various strategies have been reported in literature to optimize the MF-ECT relying on both hardware and software solutions. In this framework, the goal of the present paper is o quantitatively compare the performance of imaging procedures proposed for frequency- and time-domain features in terms of SNR, and qualitatively in terms of defect detection capability of each feature. For the fair comparison in both time- and frequency-domain, an imaging procedure is implemented relying on the theory of Eddy Current (EC) diffusion. In addition, as a peculiar result of the present paper, instead of starting from PEC data, The time- and frequency- domain analysis is implemented by using a Swept Frequency (SF) “Chirp” excitation in combination with Pulse-Compression (PuC), henceforth referred as PuC-ECT. This approach allows the typical PEC output signals to be retrieved after the application of the PuC while assuring contextually the typically high SNR values of MF analysis, thus combining the positive characteristics of both techniques. A further goal is to evaluate the robustness of the various features proposed in this paper, against LO noise and verifying if LOI points are retrieved on the impulse responses estimated trough PuC.