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Lissajou curve

In eddy current testing, signals are often displayed as Lissajous curves, where the in-phase component (I) is plotted against the quadrature component (Q). The shape and orientation of the curve carry essential diagnostic meaning.

Lift-off or probe positioning effects

Variations in the distance between the probe and the surface mainly produce a straight line or elongated ellipse. This corresponds to a gradual change dominated by geometry rather than a flaw.

Defects and discontinuities

When the probe passes over a crack, corrosion, or other anomaly, the Lissajous curve deviates from its baseline trajectory, often forming a loop, kink, or sharp excursion. The amplitude of the deviation is related to the severity of the defect, while its orientation reflects whether the response is dominated by resistive (conductivity-related) or reactive (permeability-related) effects.

Frequency-based discrimination

Using multiple frequencies, different curves can be compared: higher frequencies emphasize surface flaws, while lower frequencies are more sensitive to deeper defects. The relative rotation and spread of the Lissajous figures help separate these contributions.

Overall, the Lissajous curve provides an intuitive visual fingerprint of how eddy currents interact with the inspected material, making it easier to distinguish lift-off, material changes, and real defects.

Sources

  1. Leclerc, R., & Samson, R. (2000). Eddy current array probe for corrosion mapping on ageing aircraft. In Review of Progress in Quantitative Nondestructive Evaluation (Vol. 19, AIP Conference Proceedings 509), pp. 489–496. AIP Publishing.

  2. Reboud, C., Prémél, D., Pichenot, G., Lesselier, D., & Bisiaux, B. (2005). Development and validation of a 3D model dedicated to eddy current non-destructive testing of tubes by encircling probes. Proceedings of ISEM 2005, pp. 278–279.

  3. Demaldent, E., Réboud, C., Sollier, T., & Cattiaux, G. (2014). Efficient simulation of steam generator tube inspection with the PlusPoint eddy-current probe. 11th European Conference on Non-Destructive Testing (ECNDT 2014) / e-Journal of Nondestructive Testing, Vol. 19(12), article id.