A Match Coefficient Approach for Damage Imaging in Structural Components by Ultrasonic Synthetic Aperture Focus

Synthetic Aperture Focus (SAF) is a common technique for ultrasonic imaging of defects in structural components. Traditional SAF techniques use several transducers operated in a time backpropagation mode (“Delay-And-Sum”) that essentially locates the reflector by intersecting either ellipses (by tracking travel time between a transmission and a reception) or hyperbolae (by tracking travel time differences between two receptions). This approach results in notorious sidelobes and, at times, artifacts. We here propose a different SAF image reconstruction scheme that derives from the technique of Matched Field Processing (MFP) utilized for acoustic source localization in underwater acoustics and seismology. In MFP, the source is located by a matched filter between the measurements (“data vector”) and the expected responses at each point of the inspection volume (“replica vector”). In this paper, however, instead of considering the full waveforms, only selected features from the waveforms will be utilized to form the data vector and the replica vector. These features, that include time-of-flight and amplitudes, will also account for multiple wave modes that can naturally propagate in the solid (L-wave and S-wave for a 3D geometry, and multiple guided waves for a waveguide geometry). This allows to combine, or compound, information from several sources to increase the contrast and focus of the resulting image, in analogy with multiple frequency compounding or multiple excitation compounding in biomedical imaging. The compounding will generate “supervectors” for the measurements and the expectations. Various correlation metrics for the “data supervector” and the “replica supervector” will be considered for applications including defects in bulk solids and in plate-like solids. The fact that the supervectors are one-dimensional entities containing only scalar features leads to a computationally simple image reconstruction process.