A Single-Transducer Strategy for Dual-Mode Ultrasonic Excitation Utilizing Edge-Induced Longitudinal Waves in d 15 Thickness-Shear Mode Transducer

The dual-mode ultrasonic technique, integrating longitudinal and shear waves, offers enhanced diagnostic performance through their complementary propagation characteristics and sensitivities. The generation of both wave modes typically requires separate transducers, thereby increasing system complexity, especially in scenarios demanding simultaneous co-located assessments. This work explores a practical and tunable approach for dual-mode ultrasonic excitation utilizingd15thickness-shear piezoelectric transducers. Coupling conditions are systematically evaluated by varying couplant type and coupling pressure (0-300N), revealing their influences on wave amplitudes. Among the four couplants, the well-designed cycloaliphatic epoxy resin produces the best-balanced dual-mode response, achieving a high signal-to-noise ratio (SNR) exceeding 30 dB for both wave modes. The longitudinal wave amplitude displays a non-monotonic trend with coupling pressure, initially increasing and then gradually plateauing, whereas the shear wave amplitude increases steadily. An optimal pressure range is experimentally determined to be 35–110 N (± 5–10 N), with modal amplitude balance achieved near 62 N and high SNR (> 30 dB). The proposed strategy enables high-fidelity response of both wave modes using a widely available transducer, without requiring structural modification or polarization redesign. Simple structure, small size, and manufacturability enable this approach a strong potential for practical deployment of dual-mode ultrasonic technique in nondestructive testing and structural health monitoring, particularly in spatially constrained or integrated sensing environments.