Detection and Orientation Estimation of Short Cracks Using Circularly Polarized Microwave SAR Imaging

A microwave crack detection method using circularly polarized synthetic aperture radar (SAR) imaging is described. The advantages of circular over linear polarization, mainly that the crack detection is not dependent on the relative orientation of the crack, is discussed. A technique to extract the orientation of a detected crack using SAR images is presented, where the phase difference between the left- and right-hand circular polarizations (LHCP and RHCP) images is used for this purpose. The orientation estimation error introduced by a nonzero crack width is discussed and demonstrated through simulation, and a technique to overcome this error is proposed and demonstrated. The effect of the axial ratio on orientation error is also studied. Subsequently, a dual-circularly polarized antenna was designed, fabricated, and used to image several metal samples containing narrow electrical discharge machining (EDM) notches and real cracks, by which the detection and orientation estimation techniques are successfully demonstrated.