Abstract
Defect detection in ferromagnetic substrates is often hampered by nonmagnetic coating thickness variation when using conventional eddy current testing technique. The lift-off distance between the sample and the sensor is one of the main obstacles for the thickness measurement of nonmagnetic coatings on ferromagnetic substrates when using the eddy current testing technique. Based on the eddy current thin-skin effect and the lift-off insensitive inductance (LII), a simplified iterative algorithm is proposed for reducing the lift-off variation effect using a multifrequency sensor. Compared to the previous techniques on compensating the lift-off error (e.g., the lift-off point of intersection) while retrieving the thickness, the simplified inductance algorithms avoid the computation burden of integration, which are used as embedded algorithms for the online retrieval of lift-offs via each frequency channel. The LII is determined by the dimension and geometry of the sensor, thus eliminating the need for empirical calibration. The method is validated by means of experimental measurements of the inductance of coatings with different materials and thicknesses on ferrous substrates (dual-phase alloy). The error of the calculated coating thickness has been controlled to within 3% for an extended lift-off range of up to 10 mm.
Highlights
Coatings serve as protective barriers for substrate materials in industrial applications.In order to investigate their characteristics, various nondestructive techniques, eddy current (EC) sensing, have been used to directly measure the thickness of coating on a conductive substrate in a noncontact manner [1,2,3,4].Diverse methods using EC sensors have been proposed for the measurement of coating thickness
Considering the ferrous substrate, Yang and Tai have used the swept-frequency eddy-current (SFEC) for the determination of the substrate permeability, which serve as the input for subsequent measurements of conductivity and thickness of coatings using the pulsed eddy current (PEC) method [7,8,9,10,11]
Such measurement is caused by various quencythe, and Barkhausen noise high effectfre(where the ferrofactors, including resonant/proximity/parasitic effect of coil windings under frequency, and Barkhausen noise effect
Summary
Coatings serve as protective barriers for substrate materials in industrial applications. For the MEC, previous works have been proposed to reduce the error (caused by the lift-off distance variation) to derive important parameters such as the thickness (single layer), magnetic permeability, and electrical conductivity of samples [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. The ferromagnetic substrate is permeable (and even can be magnetized under large driving current or restrained eddy current under high-frequency skin effect) and affect the measured inductance and its sensitivities to different parameters [22] under different lift-offs and frequencies).
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