三維螺旋晶體之聲學可調式能隙分析

Abstract

機械超穎材料由人造的週期性微結構組成，可具有許多自然界當中不存在的性質，例如負蒲松比、負楊式模數、負等效質量密度等。以往的超穎材料大多為固定的工作頻域，近年來有越來越多由變形驅動的可調式能隙超穎材料，但因發展尚未成熟，故有許多改進空間，例如，可調式能隙超穎材料之微結構大多都在線彈性段外產生變形，長期使用容易造成材料損傷，且現有的可調式能隙超穎材料幾乎都是由二維的微結構組成，應用上有其限制。本文討論由三維反四邊掌形結構組成之超穎材料，此種材料在變形時具有負蒲松比性質，且掌形微結構可在線彈性內產生變形，使結構勁度改變，進而產生能隙。由有限元素模擬與實驗量測結果相互印證後可得知，三維反四邊掌形結構組成之超穎材料產生變形時，材料可以依據變形的方向產生兩種不同區段的能隙，且材料會依據應變方向產生對應方向的能隙，並藉由此材料非等向性的性質操控欲產生能隙的方向，此性質未來可應用在可控制方向之聲學開關的相關設計。

Mechanical metamaterials, composed of artificial periodic microstructures, exhibit unusual properties such as negative Poisson’s ratio, negative Young’s modulus, negative effective mass density and other properties not found in nature. Most of the metamaterials are set to work on fixed frequency. Recently, much work has been done on studying the deformation-driven tunable metamaterials. In the previous work little has the deformation of the microstructures in the linear-elastic region and most of the microstructures are two-dimensional. In this study, a three-dimensional anti-tetrachiral structure was used as the deformation-driven tunable metamaterial that can deform in linear-elastic region and then cause the change of stiffness and band structure. The finite element simulations were validated and found to be in good agreement with experimental observations. The three-dimensional anti-tetrachiral structure could be exploited to turn on or off the bandgap directionally in the linear-elastic region. There were two frequency ranges of the bandgap that could be manipulated by applying deformation in different direction. Moreover, the bandgap appeared in specific direction according to the deformed direction meaning that change of the directional bandgap is possible by deforming the material. This open avenues for the design of acoustic switches making a new possibility to manipulate the wave propagation.