Temperature Dependency of Electromagnetic Acoustic Resonance for Thickness Gauging

This work investigates the temperature dependency of electromagnetic acoustic resonance (EMAR) as a non-contact ultrasonic method for thickness gauging. Using a racetrack electromagnetic acoustic transducer (EMAT) on a 20.08 mm thick copper block, resonance frequencies were measured across temperatures from 30 °C to 50 °C. The study reveals that even moderate temperature variations lead to significant shifts in resonance frequencies- approximately 200 Hz/K - due to thermal expansion and reduced shear wave velocity. Without compensation, this results in thickness errors up to 57 µm (≈ 2800 ppm) for a 20 K increase, which could mask critical changes such as coating losses. To address this, a compensation model incorporating a temperature coefficient for wave velocity (κ ≈ 141 ppm/K) was developed, reducing the relative error to below 50 ppm. This approach enables reliable µm-scale thickness monitoring under varying thermal conditions, making EMAR a robust technique for structural health applications in environments subject to temperature fluctuations.