Measurement and Calculation of Transient Eddy-Currents in Layered Structures

In transient eddy-current inspection, an electromagnetic field pulse is excited in a conductor by causing a step change in the current through a coil. As this pulse propagates into the material, it is broadened by dispersion and scattered by discontinuities in the conductivity and permeability of the conductor. Subsurface defects cause part of the pulse to be scattered back to the surface of the conductor where it can be observed as a transient signal in the time domain, either as an EMF across the coil, or by direct measurement of the magnetic field using, for example, a Hall sensor. The observed transient is essentially the response function of the specimen with the transducer at a particular position. It contains information over a broad spectrum in contrast to time-harmonic excitation which yields information only at a single frequency