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.


Electrical and Computer Engineering


Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio)
Missouri University of Science and Technology. Center for Aerospace Manufacturing Technologies


This work was supported in part by the Air Force Research Laboratory (AFRL) under Contract FA8650-04-C-5704 in conjunction with the Center for Aerospace Manufacturing Technologies (CAMT).

Keywords and Phrases

Crack Dimensions; Electromagnetic Simulation; Filled Crack; Function Of Frequency; Metal Plates; Millimeter Wave Frequencies; Non Destructive Testing; Operating Wavelength; Rectangular Slots; Reflection Coefficients; Resonant Characteristics; Shallow Cracks; Surface Cracks; Surface Damages; Damage Detection; Dielectric Materials; Dielectric Properties; Elastic Constants; Millimeter Wave Devices; Millimeter Waves; Nondestructive Examination; Plate Metal; Rectangular Waveguides; Reflection; Surface Defects; Surfaces; Crack Detection; Crack Sizing; Millimeter Wave; Nondestructive Testing; Resonant Technique

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