Evaluation of Coating Thickness Using Lift-Off Insensitivity of Eddy Current Sensor

Area of Science:

• Materials Science
• Non-Destructive Testing
• Electromagnetism

Background:

• Eddy current testing (ECT) faces challenges in measuring nonmagnetic coating thickness on ferromagnetic substrates due to lift-off variations.
• Lift-off distance significantly impacts the accuracy of conventional ECT methods for thickness retrieval.

Purpose of the Study:

• To develop a simplified, lift-off insensitive algorithm for accurate nonmagnetic coating thickness measurement on ferromagnetic substrates.
• To reduce computational burden and eliminate empirical calibration in eddy current testing.

Main Methods:

• Utilized the eddy current thin-skin effect and lift-off insensitive inductance (LII).
• Proposed a simplified iterative algorithm employing a multifrequency sensor.
• Developed embedded algorithms for online lift-off retrieval via frequency channels.

Main Results:

• Achieved coating thickness measurement error within 3% over a 10 mm lift-off range.
• The LII approach is sensor-dependent, negating the need for empirical calibration.
• Validated experimentally on dual-phase alloy substrates with varying coating materials and thicknesses.

Conclusions:

• The simplified inductance algorithm effectively compensates for lift-off variations in eddy current testing.
• This method offers a computationally efficient and accurate solution for non-destructive coating thickness measurement.
• The technique enhances defect detection capabilities on ferromagnetic materials.