THROUGH-THICKNESS CHARACTERIZATION OF SOLIDS BY WIDE-BAND AIR-COUPLED ULTRASOUND

A non-contact inspection system employing wideband air-coupled ultrasound has been used for the characterization of thin solid material. Generation and detection of pulsed ultrasound were accomplished on opposite sides of the sample (in the through-thickness mode) using micromachined air-coupled capacitance transducers. The resulting bandwidth allowed absolute velocities and thickness changes to be estimated with reasonable accuracy (greater-than-or-equal-to 1%) for polymers, wood products, glass, and carbon-fibre reinforced polymer composites. An appreciable increase in sensitivity is shown to result for toneburst operation when the frequency matches the sample's through-thickness resonance, and this was employed for the imaging of defects in composite materials