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Sub-MHz EMAR for Non-Contact Thickness Measurement: How Ultrasonic Wave Directivity Affects Accuracy.

Alexander Siegl¹, David Auer¹, Bernhard Schweighofer¹

¹Christian Doppler Laboratory for Measurement Systems for Harsh Operating Conditions, Graz University of Technology, Inffeldgasse 23/2, 8010 Graz, Austria.

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Summary

This study demonstrates safe and cost-effective ultrasonic thickness measurement using electromagnetic acoustic resonance (EMAR) at sub-MHz frequencies. Accurate results were achieved by maintaining a specific wavelength-to-transducer ratio, ensuring reliable industrial applications.

Keywords:

EMAR limitations EMAR simulation electromagnetic acoustic resonance electromagnetic acoustic transducer thickness gauging

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Area of Science:

Materials Science
Non-Destructive Testing
Acoustics

Background:

Electromagnetic acoustic resonance (EMAR) is a standard non-contact ultrasonic thickness measurement technique.
Traditional EMAR operates at high frequencies (>1 MHz) requiring high voltages.

Purpose of the Study:

To extend EMAR into the sub-MHz frequency range for safer and more economical operation.
To investigate the critical design parameters for accurate sub-MHz EMAR.
To validate simulation models for sub-MHz EMAR.

Main Methods:

Experiments conducted on 20 mm thick copper blocks using sub-MHz EMAR.
Development of a system-based model to simulate EMAR wave propagation.
Analysis of the ratio of ultrasonic wavelength to EMAT track width (λ/w).

Main Results:

Achieved relative thickness accuracy better than 0.2% at sub-MHz frequencies.
Identified λ/w < 1 as critical for stable resonances and accurate measurements.
Experimental results closely matched simulation predictions.

Conclusions:

Sub-MHz EMAR is feasible with high precision, offering a safer and cost-effective alternative.
The λ/w < 1 principle ensures reliable thickness measurements in the low-frequency regime.
Low-voltage EMAR shows significant potential for industrial ultrasonic thickness gauging.