Development of an Accurate and Robust Air-Coupled Ultrasonic Time-of-Flight Measurement Technique

Area of Science:

• Non-destructive testing
• Acoustics
• Materials science

Background:

• Ultrasonic time-of-flight (ToF) is crucial for non-destructive material characterization.
• Traditional methods require liquid couplants, which are time-consuming and potentially damaging.
• Air-coupled ultrasound offers a couplant-free alternative but requires accounting for air transmission variations.

Purpose of the Study:

• To develop a novel, non-invasive method for in-air ranging and ToF determination.
• To overcome limitations of traditional ultrasonic testing, including couplant use and sensitivity to ambient conditions.
• To enable robust and accurate ToF measurements independent of transducer characteristics or environmental factors.

Main Methods:

• Utilized refracto-vibrometry (RV) mode of a laser Doppler vibrometer to measure in-air travel time via azimuthal directivity.
• Determined acoustic signal entry time using autocorrelation of the RV signal.
• Employed cross-correlation with the RV signal as a reference for transmission ToF measurement through the specimen.

Main Results:

• Achieved a ranging accuracy of less than 0.1 mm.
• Obtained a transmission ToF accuracy of 0.3 μs in experiments on a polyamide specimen.
• Demonstrated a non-invasive method robust to changing ambient conditions and waveform variations.

Conclusions:

• The proposed refracto-vibrometry-based method enables fast and accurate non-invasive ToF measurements.
• The technique eliminates the need for liquid couplants and prior knowledge of ambient conditions.
• This advancement simplifies ultrasonic testing procedures and broadens its applicability in material characterization.