Meta-excitation of shear waves in three-dimensional elastic solids

Shear-wave (S-wave) excitation in elastic solids typically relies on tangential actuation to achieve high modal purity, thereby limiting flexibility and efficiency in practical applications. In this work, a surface-normal encoding paradigm is proposed to enable directional S-wave generation using only normal surface excitation. By spatially encoding the phase (0 and π) of an array of normally oriented point sources, a directional shear wavefield can be synthesized without introducing tangential displacement or traction, thereby transforming the excitation mechanism from conventional tangential driving to phase-controlled wavefield construction based on normal actuation. To efficiently predict three-dimensional elastic wavefields, a machine learning-assisted semi-analytical framework is developed, allowing rapid evaluation under both single- and multi-source excitations. Based on this framework, square and circular meta-exciters are designed and optimized. The square configuration generates highly directional S-waves concentrated at horizontal angles of 0° and 180°, with dominant radiation in the vertical range of 25°–50°, whereas the circular configuration enables S-wave excitation over the entire horizontal plane (0°–360°). These results provide an efficient and flexible strategy for controllable S-wave excitation, with strong potential for high-throughput, couplant-free nondestructive testing (NDT) applications.