Diffraction aperture non-ideal behaviour of air coupled transducers array elements designed for NDT

Area of Science:

• Ultrasonic Transducer Technology
• Non-Destructive Testing
• Acoustic Materials

Background:

• Air-coupled piezoelectric ultrasonic array transducers offer potential for non-contact material testing using Lamb waves.
• A significant challenge is the impedance mismatch between piezoelectric materials and air, requiring adaptation layers.
• These matching layers can alter transducer surface behavior and radiation characteristics, complicating simulations.

Purpose of the Study:

• To investigate the acoustic aperture and surface vibration of an air-coupled piezoelectric array transducer with matching layers.
• To characterize the traveling wave phenomena on the matching layer surface.
• To compare experimental radiation patterns with simulations based on idealized apertures.

Main Methods:

• Utilized a concave linear-array transducer (64 elements, 0.8 MHz) with polyurethane and porous cellulose matching layers.
• Employed sub-millimeter vibrometer laser scanning to measure acoustic aperture and surface vibration.
• Applied the Impulse Response Method (IRM) for experimental and simulation comparisons.

Main Results:

• Experimental measurements revealed an increased acoustic aperture beyond the physical element dimensions.
• A non-uniform, symmetric apodized velocity profile with a concave delay contour indicated traveling wave phenomena.
• Propagation velocities of approximately 2500 m/s and attenuation of 15-20 dB/mm were determined for these waves.
• Good agreement was found between experimental radiation diagrams and simulations of an idealized rectangular aperture.

Conclusions:

• Matching layers significantly influence the surface behavior and radiation characteristics of air-coupled ultrasonic transducers.
• Traveling wave phenomena on the matching layer surface are present and affect transducer performance.
• The Impulse Response Method provides a viable approach for validating simulations against experimental data in such systems.