Comparison of time and frequency domain features’ immunity against lift-off in pulse-compression eddy current imaging

This paper reports a comparison between various imaging procedures based on time- and frequencydomain features applied to eddy current data collected on an Aluminium alloy benchmark sample containing a set of small notches with various depths. Historically, the pulsed eddy current method was introduced to improve the performance of eddy current testing in terms of increased penetration depth and lift-off invariance by exploiting time-domain features analysis precluded when using single- or multi-tone excitations. In this work, such analysis is instead accomplished by using a swept-frequency excitation signal along with an optimized pulse-compression procedure. This approach allows the user to deliver more energy to the system with respect to the pulsed approach, thus improving the resultant signal-to-noise-ratio (SNR) without losing the capability of time-domain feature extraction. The imaging procedure makes use of amplitude and phase features of both frequency- and time-domain data where time-phase is defined through the Hilbert transform of the pulse-compression output. Detection capability of various imaging strategies, namely A-, B- and C-scans, are compared in terms of inspection depth and lift-off robustness by using the SNR merit factor. It is shown that time-domain features outperform frequency-based ones in terms of SNR for the case of deeper defects and that phase features are robust against lift-off variations for both time and frequency domains. In addition, the analysis of time-amplitude images clearly evidences the presence of liftoff invariant points. To our knowledge, this is the first experimental evidence of the lift-off invariance points retrieved after applying pulse-compression in combination with coded excitation instead of using directly pulsed, multi-tone or single-tone sinusoidal signals. This not only confirms previous results achieved by the authors, but also demonstrates that pulse-compression eddy current can represent a valid solution to combine the advantages of pulsed and sinusoidal excitation strategies.