Material Characterization and Nde Using Focused Slanted Transmission Mode of AIr-Coupled Ultrasound

Results are reported of an experimental study and NDE applications for the focused slanted transmission mode (FSTM) of air-coupled ultrasound. The FSTM is based on the “resonance” transmission of an acoustic wave through the sample due to Lamb mode excitation and re-radiation. For oblique incidence, the azimuth rotation of the sample changes the direction and phase velocity of the plate wave propagating in anisotropic material. It was shown that to discern the in-plain elastic anisotropy of the plate-like sample one could use the measurements of amplitude and phase of the FSTM-output signal as functions of the azimuth angle. Such an interference technique is shown to be extremely sensitive in discerning the directions of reinforcement in composites and orientation of crystalline Si-wafers. The mechanisms of substantial contrast enhancement demonstrated for the FSTM C-scan imaging of the cracked defects and delaminations in polymers and composites also are discussed.