Applications of the Zero-Group-Velocity Lamb Mode for Air-Coupled Ultrasonic Imaging

Airborne ultrasound couples particularly well into plates at the zero‐group‐velocity point of the first order symmetric (S1) Lamb mode. Applications of this mode to ultrasonic imaging of plate‐like structures are discussed. The sensitivity and high Q of this mode makes it ideal for imaging. Images from a wide variety of materials and samples, including composites and honeycomb structures are presented. Transmission at the zero‐group‐velocity frequency is shown to be particularly sensitive to nearby flaws and discontinuities, and is therefore suitable for wide‐area scanning for cracks or manufacturing flaws.