High contrast air-coupled acoustic imaging with zero group velocity lamb modes

Area of Science:

• Materials Science
• Acoustics
• Ultrasonics

Background:

• The first-order symmetric (S1) plate wave mode exhibits a zero group velocity point.
• This phenomenon offers potential for enhanced sensitivity in ultrasonic imaging.
• Conventional air-coupled scanning has limitations in sensitivity.

Purpose of the Study:

• To exploit the zero group velocity of the S1 plate wave mode for improved air-coupled ultrasonic imaging sensitivity.
• To demonstrate the efficient coupling and localization of energy at this specific frequency.
• To showcase the capability of this technique for imaging thin layers and defects.

Main Methods:

• Utilizing a broadband, focusing, air-coupled transducer.
• Exciting the S1 plate wave mode at its zero group velocity frequency.
• Analyzing the enhanced transmission and localized energy coupling.

Main Results:

• Achieved significantly higher sensitivity compared to conventional air-coupled scanning.
• Demonstrated efficient coupling of a broad range of wavenumbers at a constant frequency.
• Successfully imaged a single layer of Scotch tape on Plexiglas and Teflon inserts in a composite laminate.

Conclusions:

• Exploiting the S1 mode's zero group velocity frequency significantly enhances air-coupled ultrasonic imaging sensitivity.
• The technique allows for efficient energy coupling and localization, enabling high-resolution imaging.
• This method provides a practical approach for non-destructive evaluation of thin layers and internal structures.