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

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.