Microwave imaging of thick composite structures using circular aperture probe

Thick multilayer composite structures based on sandwiched dielectric honeycomb and foam cores are extensively utilized in the construction of many structural and protective components in the aerospace industry. These structures are susceptible to various subsurface flaws which could be induced due to in-service stresses and manufacturing faults. Non-destructive testing (NDT) techniques are utilized on these structures to detect and assess the severity of the induced flaws. Microwave NDT methods have been shown to be particularly efficient for thick composite inspection. Conventionally, these methods are based on rectangular aperture probes. In this paper, inspection of thick composite samples is performed based on near-field microwave imaging utilizing a custom-made circular waveguide probe operating at 24 GHz. To highlight the utility of the proposed probe, practical multilayer composite samples are imaged using the proposed circular probe and a conventional waveguide rectangular probe. To further benchmark the obtained results, three of the samples are imaged using a phased array ultrasonic testing (PAUT) system and the images are compared with the images obtained using the proposed circular probe. It is demonstrated that microwave imaging using a circular probe yields images of higher fidelity when compared to a conventional rectangular probe as well as the PAUT system.