Depth evaluation of shallow surface cracks in metals using rectangular waveguides at millimeter-wave frequencies

This paper presents a resonant technique, which is founded on previous extensive work on millimeter-wave surface crack detection and sizing, for the accurate depth evaluation of long and shallow surface damages (scratches or cracks), which are represented as rectangular slots, in metal plates. A crack in a metal plate may be considered a short-circuited rectangular waveguide, which presents certain resonant characteristics when its electrical depth coincides with a quarter of the operating wavelength. Furthermore, a shallow crack may be filled with a dielectric material to electromagnetically make it appear deeper and hence facilitate its depth evaluation. The resonant properties of a crack depend on the dielectric properties of the material filling the crack and the crack dimensions. It is shown that a slight amount of loss, which is associated with the dielectric material, causes a relatively significant and characteristic change in the reflection coefficient measured using a probing rectangular waveguide aperture. In particular, this change affects the magnitude of the reflection coefficient, which is an easier parameter to measure than the phase. This information, as a function of frequency, may then be used to determine the shallow crack depth. This paper presents the foundation of this technique at millimeter-wave frequencies, along with supporting electromagnetic simulations and experimental results.