Analysis of the influence of lift-off variation on Eddy-Current images

The development of lift-off invariant strategies is one of the main goals in Eddy Current Non-Destructive Testing research. In this paper various imaging procedures for Eddy Current Testing are analyzed with respect to change of lift-off and tested by varying randomly the lift-off value at every point in a measurement grid. The experimental data, collected with both chirp and multi-frequency excitation, shows that imaging procedures relying on phase analysis are quite insensitive to liftoff variation and then more robust with respect to imaging procedures based on the visualization of amplitude-derived features.