Enhancement of microwave time reversal imaging using metallic reflectors

There is a growing interest in the use of composites in several industries such as aerospace, automotive and civil infrastructure due to their unique properties such as light-weight, corrosion resistance and excellent thermo-mechanical properties. However, it is critical to ensure that no defects, such as disbonds, voids and delaminations are introduced during fabrication or during service. A variety of nondestructive evaluation (NDE) techniques have been proposed and developed to detect such defects that can compromise the integrity of these structures. Microwave NDE techniques are well suited for inspection of dielectric materials such as composites because of the ability of electromagnetic waves to interact with these materials. Far field electromagnetic inspection systems have the capability of rapid, large area inspection at large stand-off distance. However the diffraction limits restrict the resolution of conventional far field imaging. This contribution focuses on enhancing the resolution of microwave far field imaging using metal reflectors to increase the virtual aperture of the receiver array. Model based studies demonstrate the feasibility and robustness of this approach and also determine the limits of this technique. Preliminary experimental results based on a time reversal cavity environment validates the approach for enhancing the resolution of microwave NDE imaging.