Air-coupled ultrasound is a well-established tool for acoustic NDT and material characterization. Its major shortcoming is concerned with a weak penetration into material due to a severe impedance mismatch at the air-solid interface. A strong rise in acoustic coupling to solids is obtained by using acoustic mode conversion in slanted configurations. In our experiments, a substantial increase of the ultrasound amplitude was observed in various solid materials (metals, wood, concrete, composites) under phase matching conditions for plate and surface acoustic waves. On this basis, fully air-coupled and hybrid air-coupled-optic configurations are developed and applied for non-contact NDT. New opportunities of the mode conversion approach are demonstrated in remote mapping of elastic anisotropy, drying and thickness measurements of paint films, and ultrasonic imaging of defects.
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