Gradient-field pulsed eddy current probes for imaging of hidden corrosion in conductive structures

Pulsed eddy current testing (PEC) has been found advantageous over other non-destructive evaluation (NDE) techniques particularly in detection and characterization of subsurface defects in conductive structures. The measurement of net magnetic field for acquisition of transient signals is normally employed in traditional PEC during inspection of conductors. In this paper, PEC in conjunction with gradient field measurement is investigated in an effort to enhance the inspection sensitivity to hidden corrosion in conductors and accuracy of corrosion imaging. Closed-form expressions of gradient field and its sensitivity to hidden corrosion are formulated via the extended truncated region eigenfunction expansion (ETREE) modeling. A series of simulations are subsequently conducted to analyze the characteristics of gradient field signals and inspection sensitivity to hidden corrosion. Following this, experiments of Gradient-field PEC (GPEC) for evaluation and imaging of hidden corrosion are carried out. Through theoretical and experimental investigation, it has been found that the GPEC probe is advantageous over that based on traditional PEC in terms of inspection sensitivity and accuracy of corrosion imaging.