Non-Contact Delamination Detection of Cu/Al Clad Sheets Using Electromagnetic Acoustic Resonance with Nonlinear Frequency Modulation Excitation

Accurate and efficient detection of interfacial delamination in Cu/Al clad sheets is essential for evaluating bonding quality in multilayer metallic structures. This study proposes an electromagnetic acoustic transducer (EMAT)-based nonlinear frequency modulation (NLFM) resonance method for non-destructive delamination characterization of Cu/Al clad strips. A two-layer resonance frequency model was implemented and validated through theoretical analysis, finite element simulations, and EMAT experiments, showing good agreement among the three approaches. To overcome the long acquisition time associated with conventional frequency-sweep methods, an NLFM excitation waveform incorporating frequency-dependent pre-emphasis was proposed to achieve broadband excitation with an optimized energy distribution over the target resonance frequency range. Compared to linear frequency modulation (LFM), the proposed NLFM method provided enhanced resonance amplitudes, reduced out-of-band energy leakage, and improved signal-to-noise ratio (SNR). An NLFM-based B-scan inspection method was further developed for delamination detection using artificial T-slot defects in Cu/Al clad sheets. The proposed method successfully identified defects with widths of 1.5–9.5 mm through resonance frequency variations. The detected defect distributions showed good agreement with the actual defect dimensions, demonstrating the capability of NLFM resonance analysis for non-contact and high-sensitivity delamination detection in multilayer metallic structures.