Applied microwave imaging of composite structures using open-ended circular waveguide

Thick multi-layer dielectric composite materials are integral part of many industries like aerospace, marine, petrochemical, etc. It is very important to ensure that these materials are of good quality and free from any kind of defects since these materials are used for critical applications. Several Non-destructive testing (NDT) methods have been applied to test and evaluate any flaws in these materials. In this paper, a waveguide based microwave NDT technique is used to interrogate the composite materials. A custom made Teflon loaded circular waveguide is employed for the inspection of defects in a test sample. The multi-layer composite structure and the waveguide are modeled in a numerical electromagnetic simulation software. The sensitivity of the circular probe to air inclusions in the composite is checked at 24 GHz by analysing the electric field variation at the defected layers. In addition to this, the response of the circular waveguide to the composite structure defects is compared to that of a conventional air filled rectangular waveguide at the same frequency point. The numerical simulation results are validated by imaging a practical five-layer honeycomb composite sample. The measured results are compared with the images produced using both the waveguide probes. It is observed that the circular probe is better in terms of resolution compared to the conventional rectangular waveguide.