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Air-Coupled Ultrasonics

Air-Coupled Ultrasonics. Principles and Applications. Jinying Zhu. Contents. Introduction Benefits and Limitations Transducer Design Applications Reflection Plate Wave Through Transmission Products Information. Introduction - Theory. Use air as couplant Non-contact measurement

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Air-Coupled Ultrasonics

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  1. Air-Coupled Ultrasonics Principles and Applications Jinying Zhu

  2. Contents • Introduction • Benefits and Limitations • Transducer Design • Applications • Reflection • Plate Wave • Through Transmission • Products Information

  3. Introduction - Theory • Use air as couplant • Non-contact measurement • Detect air pressure change • Detect leaky waves leaked from materials surface Reflection Inspection Guided wave Inspection Through-transmission Inspection

  4. Benefits and Limitations • Non-contact Measurement • Allow for rapid inspection of large structures • Avoid contamination of materials • Foam and paper-based products which are not suitable for water coupling detection • Inexpensive and less complexity compared with laser technique • Large acoustic impedance mismatch between air and solid • Increasing absorption with frequency • Velocity depends on temperature

  5. Transducer Solutions • Maximize acoustic output • Minimize path losses • Maximize sensitivity of the receiver • Signal processing Challenges • Large sound impedance mismatch between air and detected materials • Energy transmission • Several air-solid interfaces for a complete test system • Energy loss >100dB compared to water couplant (i.e. energy transmission about 10-8)

  6. Piezoelectric transducer • Un-damped Resonant Piezoelectric transducer • Use Paired receiver • Use an intermediate acoustic matching layer (lightweight polymer) • Use toneburst excitation QMI AS400C Air-coupled probe*

  7. Piezoelectric transducer • using an acoustic lens to focus the sound energy • Limitation • Narrow frequency range • Ring effect curved piezoelectric element flat piezoelectric element and external optics plastic lens bonded to a flat piezoelectric

  8. Capacitance transducer • Much better match to air compared to piezoelectric transducer • Broadband frequency response (0kHz – 2MHz) • High sensitivity • No ‘ring’ effect, allow for high-frequency imaging at small distance Structure of capacitance transducer*

  9. Applications -- Reflection Inspection • Automation of manufactuering processes and control, quality assurance • Thickness determination of paint coating • Automation of wool shearing process • Guidance and positioning in robotics Topographic imaging of a PCB board with 4mm diameter center hole and 2mm diameter holes

  10. Applications-- Through transmission Inspection • Defect detection in composite materials • Detect disbonds in multi-layer structures • Limitation: need to access both sides Detect delaminations in aerospace materials using piezoelectric transducer *Grandia, et al. ‘Airscan Transducers, Terchnique and Applications’

  11. Laser generation and air coupled detection Using surface acoustic signals for non-contact inspection of composites Applications -- Lamb/Rayleigh wave • Low attenuation in long distance • Suitable for large structures and sheet materials • Suitable for surface/subsurface defects (surface wave)

  12. Application-- Concrete Detection

  13. Application-- Concrete Detection • Concrete wall • Thickness: 915mm • P-wave Velocity : 3812m/s • R-wave velocity : 2100 m/s The Non-dispersive wave velocity is Between 2000 to 2100 m/s

  14. Conclusions • Non-contact NDE technique • Increasing application in composite material detection (Aerospace Industry) • Inexpensive Instrument • Possible extend application to civil engineering

  15. Product Information • Sonatest Plc • AirStar Inc. • QMI Inc • MicroAcoustic Instruments Inc • Useful Links • http://www.parteeq.queensu.ca/techs/physsci/aircoupl.html • http://www.ndt.net

  16. References • Buckley J., 15th WCNDT, Roma 2000. • Castings M., Hosten B., NDT&E International, 2001; 34:249-258 • Lanza di Scalea, J. Comp. Mater., 2000;34:1860 • Schindel D.W., et al., IEEE trans. Ultrason.Ferroelect. Freq. Cont., 1995;42:42-58 • Zhu J., Popovics J., 28th QNDE, 2001

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