Multiple mode conversions of elastic waves via a one-dimensional gradient elastic metamaterial

Existing elastic meta -structures capable of mode conversion are typically limited to a single function, that is, converting one specific mode into another. However, the modal richness inherent in elastic waveguides presents a broader opportunity. Motivated by this, in this work, we present a gradient elastic metamaterial composed of a graded array of cylindrical pillars with varying heights, which is designed to convert flexural waves with different polarizations into distinct elastic wave modes at the same working frequency. Specifically, converting out-of-plane flexural waves (polarization along the thickness direction) into longitudinal waves, and in-plane flexural waves (polarization along the width direction) into torsional waves. Transient simulations and experiments are implemented to validate the multifunctional mode conversion of the gradient metamaterial. Notably, the frequency component of the converted waves is more concentrated than that of the incident waves, indicating an inherent filtering effect. Our work presents a clear and systematic design strategy for a gradient metamaterial capable of multiple mode conversions. This metamaterial not only can filter out non-target frequency components and convert out-of-plane vibrations that cause damage to the structure into in-plane vibrations that cause less damage to the structure. In particular, we envision extending this concept to the development of high-frequency acoustic filtering devices and to the suppression of low-frequency vibrations in bridge structures.