Inversion of eddy-current data and the reconstruction of three-dimensional flaws

Inverse scattering models of the type that is often used to invert eddy-current data are inherently nonlinear, because they involve the product of two unknowns, the flaw conductivity and the true electric field within the flaw. Computational inverse models, therefore, often linearize the problem by assuming that the electric field within the flaw is known a priori. In the present work, the authors describe such a linearized model; it is fully three-dimensional and applies to metals, such as stainless-steel, or to advanced composites, such as graphite-epoxy. The model is based on an integral equation that is then discretized by means of the method of moments. The measured data are inverted by means of the conjugate gradient algorithm. an example is shown in which a linear classifier algorithm is used to improve convergence of the conjugate gradient algorithm.>