A practical non-contact layer thickness measurement method for two-layer clad plates using EMAR

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Accurate determination of individual layer thicknesses in multilayer clad plates is essential for quality control and structural integrity assessment. This study proposes an electromagnetic acoustic resonance (EMAR)-based method for layer-thickness measurement of two-layer clad plates. A two-layer acoustic resonance model was validated experimentally using EMAT frequency-sweep measurements. Based on the model, an inversion algorithm was developed to determine individual layer thicknesses from measured resonance frequencies, given the material densities, total thickness, and shear-wave velocities. To address the non-uniqueness inherent in the transcendental resonance equation for layer thickness measurement, a method was developed to identify resonance frequencies yielding unique thickness solutions, together with a web-based computational tool (LayerThicknessCalculator-EMAT-https://wuligongshideg.github.io/clad-calculator-web/). Experiments were conducted on 316SS/Al, Cu/Al, and 304SS/Q235B clad plates, representing high, moderate, and nearly zero acoustic-impedance mismatches, respectively. The mean deviations between predicted and measured resonance frequencies were 0.014, 0.006, and 0.018 MHz, respectively. Accurate layer-thickness inversion was achieved for 316SS/Al and Cu/Al. For 304SS/Q235B, direct inversion was impractical because of the extremely high frequency resolution required; however, the mean resonance-frequency shift across multiple modes provided an effective indicator of layer-thickness variations.