Application of air-coupled ultrasonics for the characterization of polymer and polymer-matrix composite samples

This study investigates the application of air-coupled ultrasonics for nondestructive, non-contact characterization of polymer and polymer-matrix composite samples. A lot of progress has been made with this technique which does not require any liquid couplant, hence water sensitive samples can be studied without deterioration. We demonstrate the power of highly focused transducers for topography imaging of structured polymer surfaces such as a craquelure, fingerprint or an acoustic lens with a lateral resolution up to 0.1 mm and an elevation resolution of 2 μm. With a further technique, transmission resonance, the thickness of plates can be imaged with a resolution of 3 μm. Air-coupling is also used for checking the integrity of a polymer-matrix composite plate, revealing invisible internal impact damage. Finally, a single-sided Lamb-wave technique is used to detect sub-wavelength artificial defects such as notches and through-holes in plates.