Nonlinear modulation technique for NDE with air-coupled ultrasound

Area of Science:

• Nonlinear Acoustics
• Non-Destructive Testing
• Ultrasonic Wave Propagation

Background:

• Traditional nonlinear acoustic modulation (NAM) techniques offer high sensitivity for defect detection.
• Existing methods often require direct contact, limiting application flexibility.
• Noncontact ultrasound excitation provides remote defect localization and imaging capabilities.

Purpose of the Study:

• To enhance the flexibility and application scope of NAM by developing an air-coupled version.
• To combine noncontact ultrasound excitation with nonlinear methods for improved defect characterization.
• To investigate the nonlinear acoustic response of cracks and material inhomogeneities using air-coupled transducers.

Main Methods:

• Utilized a pair of focused air-coupled transducers for generating and receiving high-frequency waves (longitudinal or flexural) in plate-like samples.
• Excited low-frequency (LF) vibrations using a shaker or loudspeaker.
• Performed temporal and spectral analysis of the received signals to identify nonlinear modulation and frequency side-bands.

Main Results:

• Demonstrated highly efficient nonlinear amplitude modulation and multiple frequency side-bands for sound transmission and flexural wave propagation through cracked defects.
• Observed negligible nonlinear modulation for large/medium inclusions and material inhomogeneities, distinguishing them from cracks.
• Observed a new subharmonic NAM mode at high excitation levels and the radiation of very high-order harmonics (>>100) from crack vibrations.

Conclusions:

• The developed air-coupled NAM technique effectively detects and images cracks remotely through their unique nonlinear acoustic emissions.
• This noncontact approach significantly expands the applicability of NAM for defect detection in various materials.
• The high-order harmonic radiation from cracks provides a distinct signature for sensitive and specific crack imaging.