Broadband Microwave Imaging for Foam Insulation Diagnostics

Microwave imaging is one of the promising nondestructive testing (NDT) methods for porous radio-transparent composites when other testing methods are ineffective or not applicable. In this paper, the results of experiments on the microwave sounding of thermal insulation coatings for cryogenic fuel tanks provided by several aerospace companies are presented. The insulation samples represent a layer of radio-transparent porous polyurethane foam (PUF) glued onto a metal substrate and contain pre-produced defects imitating real PUF delamination. For the experiments, a setup designed for nondestructive testing of radio-transparent composites was used. The setup includes a compact radar emitting a continuous stepped-frequency signal in the range from 22.2 to 26.2 GHz, a two-coordinate electromechanical scanner with a platform for placing the samples under study, a microcontroller unit to synchronize the scanning and sounding procedures and a personal computer with the software for managing the experiments. Registering of the microwave holograms was performed by moving the sample under-study in the vicinity of a fixed antenna while measuring the complex amplitude of the electromagnetic field scattered by the sample. For the reconstruction of microwave holograms, the back propagation method based on the fast Fourier transform was used. To improve the contrast of defects against the background on the reconstructed images, the Butterworth bandpass filter was applied to the Fourier spectrum of the registered microwave hologram. The results of the PUF samples inner structure reconstruction in cases of single-frequency and broadband measurements are presented. It is shown that using broadband measurements allows reconstructing microwave images of the PUF defects with range resolution, which is unattainable when performing measurements at a single frequency. Obtaining range resolution enabled automatic focusing of the broadband microwave holograms: as the focusing criterion the depth corresponding to the maximum of holograms integral absolute amplitude was used.