Ultrasonic synthetic aperture imaging with interposed transducer–medium coupling path

An interposed coupling material between an ultrasonic transducer and the test medium can be present in various non-destructive inspections and structural health monitoring imaging applications. One example is the wedge medium often used to direct ultrasonic beams into the test material for optimal interaction with internal defects. Another example is the ultrasonic imaging of multilayered structures. This article discusses the ways to perform synthetic aperture focus ultrasonic imaging in these cases where signal losses and complicated refractions at the coupling material/medium interface take place. Three main steps are proposed to maximize image quality. The first step is the delay-multiply-and-sum algorithm that increases the number of independent terms in the beamforming equation compared to the delay-and-sum algorithm. The second step is the utilization of a ray tracing algorithm to properly account for the refraction of the waves in both transmission and reflection paths, and accounting for both L-waves and S-waves that can potentially propagate. The compounding of multiple wave mode combinations is the third step proposed to significantly improve image quality. Validation experiments are presented for a transducer array on a wedge to image two closely spaced holes in an aluminum block. The delay-multiply-and-sum algorithm and wave mode compounding algorithm are also in principle applicable to other structural health monitoring imaging approaches that use, for example, sparse transducer arrays and guided-wave probing.