Dual-frequency EMAT for amplitude-modulated Rayleigh wave mixing: Theory, implementation, and thermal damage detection

Nonlinear ultrasonic techniques are highly sensitive to early-stage material degradation but are limited by attenuation and weak nonlinear accumulation. This study proposes a double-sideband suppressed-carrier excitation strategy that utilizes a low-frequency sideband with reduced attenuation. A unified theoretical framework demonstrates that this approach enables substantially more efficient nonlinear accumulation than conventional single-frequency excitation. Extending this concept to Rayleigh waves, this study demonstrates that the wave mixing enables phase matching and internal resonant interactions, which enhance surface nonlinear energy transfer. Based on this framework, a compact single-channel EMAT with a parallel meander-line coil is developed to suppress intrinsic EMAT nonlinearity while maintaining narrowband efficiency. Numerical and experimental results demonstrate a substantial enhancement in sensitivity to thermal damage compared with single-frequency approaches, establishing DSB-SC Rayleigh waves as a powerful modality for non-contact, high-temperature nondestructive evaluation and structural health monitoring.