This paper presents two types of elastic meta-structural functional units with symmetric and antisymmetric layouts, synthetically collaborating for realizing one-way, nonreciprocal transmission of Lamb waves. The elastic meta-structural functional unit cells are comprised of Locally Resonant (LR) cylinders arranged in a periodic pattern bonded on an aluminum plate. The mechanism behind the metamaterial guided wave diode resides in the wave mode manipulation, including mode conversion and selective mode transmission. This study explores with a systematic parametric study on the antisymmetric elastic meta-structural unit cell with various shapes. The optimal configuration is determined from the parametric study to identify the most effective mode conversion performance. Highly effective mode conversion from S0 mode into A0 mode can be achieved by the antisymmetric units. The height of the unit cell stub and the amount of unit cells are respectively explored with a finite element model (FEM) using harmonic analysis. The performance of the proposed ultrasound mode convertor is further substantiated via the harmonic analysis of a metamaterial chain model, via showcasing the frequency spectrum of the transmitted wave modes. Subsequently, filtering of symmetric Lamb modes can be achieved by symmetric resonant elastic metamaterial. Modal analysis with Bloch-Floquet boundary condition is performed to obtain the dispersion features of the metamaterial system. By investigating the band structure and the resonant motions, a complete antisymmetric Lamb modes transmission band within the symmetric Lamb modes bandgap can be established in a wide frequency range. Afterwards, an elastic wave diode is constructed by combining the proposed unit cells for the nonreciprocal transmission manipulation of symmetric Lamb waves. The proposed nonreciprocal transmission capability may possess great potential for the purpose of one-way wave control and manipulation.

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