Time-Gating of Pulsed Eddy Current Signals for Defect Characterization and Discrimination in Aircraft Lap-Joints

Abstract

Pulsed eddy current (PEC) nondestructive testing differs from conventional eddy current techniques in that the probe coil is excited by a pulse, rather than continuous excitation at a single frequency. Reviews of early work on pulsed eddy currents are given by Waidelich1 and by Renkin.2 Pulsed excitation causes the propagation of a highly attenuated traveling wave, which is governed by the diffusion equation.3 The diffusive propagation of the eddy current pulse results in spatial broadening and a delay, or travel time, proportional to the square of the distance traveled. It was realized in early work on pulsed eddy current systems that this time dependence offered certain advantages over conventional eddy currents.4 In the current study we demonstrate the ability of a prototype pulsed eddy current instrument, described elsewhere,5,6 to take advantage of this time dependence to discriminate flaws from such interfering signals as probe liftoff, air gaps, and fasteners.