Inversion Method in Pulsed Eddy Current Testing for Wall Thickness of Ferromagnetic Pipes

Based on time-domain analytical expression to pulsed eddy current field of a ferromagnetic pipe, a least squares inverse problem, between the calculated and measured values of induced voltage, is established to determine the wall thickness, conductivity, and relative permeability. And then, the coupling relationship among parameters in the inverse problem is investigated. Furthermore, the inverse problem is reestablished and the method for calculating the relative variation of wall thickness is proposed via taking the ratio of the inversion results between two testing points. Finally, the experiment results on a steel tube indicate that inversion results are independent on the initial value, and can effectively suppress the influence of the magnetic permeability variation, thus the reliability of the thickness testing get improved. Based on the above, a noncontact pulsed eddy current testing method is presented for measuring the relative wall thickness of ferromagnetic pipes. It can be used for nondestructive evaluation of wall-thinning corrosion of in-service coated ferromagnetic tube.