A review on seismic metamaterials: From natural toartificial structures

Abstract

<p indent="0mm">In the recent three decades, the breath-taking discoveries of phononic crystals in conjunction with research advances in acoustic metamaterials have far-reaching implications in many science and engineering branches. These new discoveries can be traced to the fundamentals of wave manipulation and wave attenuation, etc., within a very wide frequency spectrum from ultrahigh electromagnetic wave frequency to very low seismic frequency ranges. Inspired by electromagnetic and photonic crystals, these newly designed artificial structures are able to control acoustic/elastic waves at relatively low frequency ranges. In periodic systems, Bloch waves are formed by reflection, refraction and scattering of De Broglie, electromagnetic/light and acoustic/elastic waves. These waves contain marvelous band structures which allow transmission or lead to attenuation. Frequency domains for transmission and attenuation correspond to passbands and stopbands, respectively. There exist many studies that materialize the idea of designing metadevices at will by utilizing wave band behaviors. Generally, the mechanism for forming stopbands contains two parts, i.e., Bragg scattering and local resonance, that can also be used to identify phononic crystals and acoustic metamaterials. Especially, a Bragg scattering type metamaterial prohibits waves with a wavelength of the same order as the lattice constant from propagating through the structure. It results in a large-scale structural dimension for a low frequency bandgap. Comparatively, local resonance type metamaterials allow the generation of bandgaps at low frequency ranges with relatively small dimensions as their properties rely primarily on the local resonance. The study of seismic wave propagation in civil engineering has been reinvigorated due to the new, exciting concept of seismic metamaterials. The spectacular designs provide new approaches to control seismic surface (Rayleigh and Love) and bulk (pressure and shear) waves for civil and structural engineers. This review traces the state-of-the-art developments of seismic metamaterials and establishes a link between photonic crystals at nano/micro scale and seismic metamaterials at macro scale. We present a survey of more recent developments in seismic metamaterials in terms of geometry, physics (i.e., the mechanism for forming stopbands) and experimental approaches which range from natural sources to artificial structures. A variety of mathematical and physics tools can be extended from photonic crystals to seismic megastructures, such as effective media theory, Brillouin zone, reciprocal space, rainbow trapping, etc. The development of finite element methods provides the possibility to simulate and analyze these complex artificial designs numerically. A school of forest trees as natural seismic metamaterials is a focal point in this review, which enables attenuation of seismic waves using natural resources and also inspires the discovery of seismic metasurfaces. For artificial structures, designs are generally made based on locations (i.e., metabarriers and metafoundations) or mechanisms (i.e., Bragg scattering and local resonance mechanism). Some metamaterial-like transformed urbanism or cities are also verified to work as seismic shielding or cloaking. Further, similar to experimental methods in other disciplines, scale model experiments and full-scale experiments have been explored to validate the efficiency of seismic metamaterials in wave attenuation. Although research in phononic crystals is moving towards extreme frontiers, metastructures working at low frequencies with stability, high bearing capacity, high ductility, wide bandgap and efficient attenuation zones remain substantial challenges. Besides soil-structure interactions, the combination of various components of seismic waves, ground conditions, soil properties, effects of groundwater table, structural nonlinearity or other ineluctable factors is yet to be fully considered. Ultimately, the emergence of seismic metamaterials broadens the horizons for seismic isolation and seismic vibration control. This review will help future researchers witness the continuous development of seismic metamaterials and comprehend the current challenges in this promising, potential and exciting research.