Near-field microwave image formation of defective composites utilizing open-ended waveguides with arbitrary cross sections

Many nondestructive testing techniques fail to detect the presence of defects inside composite materials due to their complex nature. Microwave near-field nondestructive testing utilizing open-ended rectangular waveguides showed promising results in detecting defects inside these composite structures. Carbon-loaded composites are lossy materials, which, in general, reduce the range of frequency that can be used to inspect these kinds of composites due to the high attenuation. Thus far, open-ended rectangular waveguide based probes have been used to inspect the structural integrity and homogeneity of these structures. These probes were found to be suitable for detecting defects in these composites. However, due to the linear polarization of the fields radiated out of open-ended rectangular waveguide probes and the linear nature of the carbon fibers, high attenuation of the radiated signal is inevitable. Consequently, only close to the surface defects were detected. Circular waveguides have not been used in near-field NDT. Circular waveguides offer many attractive features such as the circular polarization of the radiated waves. This fact might be used in penetrating deeper in high-loss composites. In this paper, the potential of utilizing rectangular and circular waveguides in inspecting defects in carbon-loaded composites will be theoretically investigated. An air void will be introduced inside both types of composites and theoretical images will be formed. Interpretation of the captured images will be done through power patterns in different planes.