  Phononic crystals (PCs) and metamaterials are artificially structured
materials with the unprecedented property of the existence of complete
stop-bands. However, the generally narrow width of the stop-bands in such
materials severely limits their applicability. Here, a particular kind of
gradient zigzag metamaterial beams exclusively with flat bands within a broad
frequency range is proposed based upon our previously proposed uniform
counterparts. The calculated band structures by the Finite Element Method (FEM)
and the Spectral Element Method (SEM) show the distinctly different
characteristics of the gradient zigzag metamaterial beams from the conventional
PCs and metamaterials, where the band structures are totally filled with flat
bands within a broad frequency range without the normal pass bands. These flat
bands are caused by the local resonance of certain parts in the gradient zigzag
metamaterial beams, and the elastic waves with the frequencies at these flat
bands are trapped in the resonant parts and cannot propagate. As a result, our
proposed gradient zigzag metamaterial beams can be used as broadband vibration
isolators for beam-like structures, which is verified by both numerical
simulations and experiments conducted on the 3D-printed samples. This design
strategy opens a new avenue for designing and constructing novel broadband
vibration isolators.
