Multiband wave filtering and waveguiding in bio-inspired hierarchical composites

We investigate the elastic wave propagation in bio-inspired hierarchical composites with nacre-like and biocalcite-like architectures. These two types of architectures consist of hard mineral and soft organic phases, which are hierarchically assembled to develop multilevel of hierarchy. We numerically demonstrate that multiple band gaps and passbands, covering an ultrawide frequency range, arise in the proposed hierarchical composites with two levels of hierarchy. We further reveal that the multilevel structural hierarchy itself is responsible for this multiband characteristic. Specifically, the low frequency band gaps in the composites with two levels of hierarchy are attributed to Bragg scattering, which are intrinsically governed by the hierarchical and periodic modulation of constituent phases at the second hierarchical level. By contrast, the multiple band gaps and passbands in high frequency ranges correspond to waveguide modes, enabling the incident wave to be either trapped inside the waveguides or efficiently transmitted through the waveguides. The findings in this paper not only shed light on the mechanisms responsible for the multiband features of bio-inspired hierarchical composites, but also offer new opportunities towards the design of compact and mechanically robust phononic crystals with the capability to effectively manipulate wave propagation.