Couplant-free and high-purity excitation of unidirectional shear waves via a meta-exciter

Traditional shear wave excitation faces a fundamental trade-off: high modal purity requires rigid tangential coupling (bonding or couplant), limiting scanning flexibility and throughput—critical for inline inspection. Portable methods (EMATs, laser ultrasound) suffer from low efficiency and multimodal interference. This work proposes a normal-encoding paradigm that enables high-purity directional shear wave excitation using only normal surface contact, eliminating the need for tangential stress transmission. The principle exploits standing wave nodes formed by SV-Rayleigh interference in a half-space: at these nodes, tangential displacement vanishes while normal displacement exhibits a prescribed phase distribution. Placing a normally oriented point-source array at these nodes with the designed phase gradient synthesizes a directional SV wavefield without any tangential traction—shifting excitation from tangential-pushing to normal-encoding. This inherently supports rapid, couplant-free scanning. An analytical relation linking SV angle to array spacing and phase gradient enables inverse design; a plane wave expansion model allows rapid wavefield computation. Finite-element simulations indicate an SV energy proportion exceeding 90%; experiments on a 1 mm aluminum plate (plane stress approximation) further demonstrate beam steering within 1° This work establishes a rigorous framework for high-efficiency, couplant-free ultrasonic inspection with strong potential for automated inline NDT and elastography.