MODELING OF SURFACE HAIRLINE-CRACK DETECTION IN METALS UNDER COATINGSUSING AN OPEN-ENDED RECTANGULAR WAVE-GUIDE

A surface-breaking hairline crack or a narrow slot in a metallic speci men when scanned by an open-ended rectangular waveguide probe influenc es the reflection-coefficient properties of the incident dominant mode , Subsequent recording of a change in the standing wave pattern while scanning such a surface results in what is known as the crack characte ristic signal. Since microwave signals penetrate inside dielectric mat erials, this methodology is capable of detecting cracks under dielectr ic coatings of various electrical thicknesses as well. To electromagne tically model the interaction of an open-ended rectangular waveguide w ith a surface-breaking hairline crack under a dielectric coating, the dielectric-coating layer is modeled as a waveguide with a large cross section, Thus, the problem is reduced to a system of three waveguides interacting with each other while the location of the crack is continu ously changing relative to the probing waveguide aperture (a dynamic s canning problem). An analysis of modeling the dielectric-coating layer as a dielectric-filled waveguide with a large cross section is given, and its comparison with radiation into an unbounded medium is present ed, For obtaining the reflection coefficients of the dominant and high er order modes, the electromagnetic properties of the probing waveguid e-dielectric-coating layer junction and the dielectric-coating layer-c rack junction are separately analyzed. For each junction, a magnetic-c urrent density M is introduced over the common aperture. Subsequently, the junction formed by the two respective waveguide sections is separ ated into two systems, A numerical solution employing the method of mo ments is obtained, and the properties of the junctions are expressed b y their respective generalized scattering matrices, Consequently, the generalized scattering matrix for the total system can be evaluated, T he convergence behavior of the system is studied to determine am optim al set of basis functions and the optimal number of higher order modes for a fast and accurate solution, Finally, the theoretical and measur ed crack characteristic signals are compared.