Generalized Analytical Expressions of Liftoff Intersection in PEC and a Liftoff-Intersection-Based Fast Inverse Model

Pulsed eddy current (PEC) inspection is widely used in nondestructive evaluation of the integrity of conductive structures employed in such engineering fields as aerospace, energy, nuclear, and transportation. In contrast to conventional eddy current (EC) inspection, in PEC magnetic field signals in lieu of impedance signals are obtained during the course of inspection. It is noted that liftoff intersection (LOI) can be found also in PEC with variant liftoffs, whose analytical expressions are beneficial to forward and inverse problems of PEC. In this paper, the generalized closed-form expression of LOI occurring in magnetic field signals, which are acquired from the solid-state magnetic field sensors placed in arbitrary locations within the excitation coil, is formulated based on extended truncated region eigenfunction expansion (ETREE) modeling of PEC. Following this, regarding a coated thick plate, an inverse model based on the analytical expressions of LOI is established in a bid to inversely retrieve the thickness of the nonconductive coating and the conductivity of the base material simultaneously. The inverse model is verified by experiments. It has been found that the estimated values of the two unknown parameters have good agreement with the true values, which indicates that the inverse model is valid and applicable to evaluation of coated materials.