A pulsed eddy current testing method for relative thickness of ferromagnetic components considering local hysteresis characteristics

A pulsed eddy current testing method is proposed for the assessment of the relative thickness of ferromagnetic components with uneven external insulation layer and remanence. By establishing a least squares problem of theoretical and measured value of induced voltage based on the time-domain analytical expression to pulsed eddy current testing model of a ferromagnetic plate, unknown parameters including the thickness, conductivity and relative permeability are determined by inversion algorithm. Then, the coupling relationship among parameters in the inverse problem is verified by experiments on a steel plate, and the detection method for relative thickness is proposed. Furthermore, the function relationship between the inversion results of the thickness and the relative permeability under different amplitudes of the pulsed current is obtained through the linear fitting. Finally, the suppression method is discussed for the influence of the variation of permeability. It is suggested that the reference value of the thickness should be dynamically corrected by this function, as to improve the accuracy and reliability. This paper provides a reliable non-destructive testing method for wall-thinning corrosion detection of coated ferromagnetic tubes or tanks in industry.