Advanced eddy-current methods for quantitative NDE
The objectives of this dissertation were to devise and develop advanced eddy-current methods for quantitative NDE. The techniques used include time-domain methods (pulsed eddy current), frequency-domain methods (swept-frequency eddy current), and the photoinductive imaging method that combines eddy-current and laser-based thermal-wave techniques. We first developed theoretical models to predict the pulsed eddy current signal and showed this technique can be used to characterize metallic coatings on metal substrates. A feature-based rapid inversion method was developed to determine the conductivity and thickness of the coating simultaneously. In the second work, we studied the fundamentals of eddy current interactions with magnetic metals using swept-frequency eddy current method. We have found that the eddy current response of well-annealed, demagnetized commercially-pure nickel is dominated by a thin region at the sample's surface that has a very significantly reduced permeability--i.e., a surface dead-layer. This dead layer may be due to the presence of surface damage. We calculated the impedance of the coil based on the hypothesized single layer structure and found excellent quantitative agreement between the model and experiment. These results may have important consequences for many aspects of the interaction of low frequency electromagnetic fields with magnetically soft metals. In the third work, we developed theoretical calculations and practical measurement methods using both swept-frequency eddy current and pulsed eddy current methods for determining the thickness, conductivity, and permeability of metallic coatings on metal substrates for the case when either coating, metal, or both are ferromagnetic. This work paves the way for development of new, quantitative methods to characterize surface layers on ferrous materials, such as depth of case hardening. In the fourth work, we applied the photoinductive imaging technique to characterize corner cracks on the surface around a bolt hole. The photoinductive signals reflect the geometrical shape of the triangular and rectangular electrical-discharge-machined (EDM) notches as well as real fatigue cracks. The results show promise for using this technique to characterize the shape, depth, and length of corner cracks. The capability of the photoinductive imaging technique is demonstrated in this work.