Local-resonance bandgap and Bragg bandgap can coexist in a metamaterial beam, and their coupling effect can be used to realize ultra-wide bandgap, which has great application potential in the field of wide-band vibration reduction. Previous studies usually consider the single-order coupling of local-resonance and Bragg bandgaps in metamaterial beams with a single array of local resonators, so that only a single-order ultra-wide coupling bandgap can be achieved, which cannot meet the needs of wide-band vibration reduction of double/multiple target frequency bands. In this paper, metamaterial beams with double arrays of local resonators are considered, and the regulation design and analysis of double-order coupling of local-resonance and Bragg bandgaps are carried out based on an analytical model of bending wave dispersion relation. Moreover, the vibration reduction characteristics of the double-frequency-resonator metamaterial beams with double-order coupling bandgaps are studied by using spectral element method and the finite element method. The main conclusions are as follows
1) A design method for realizing double-order coupling wide bandgap in a metamaterial beam with double arrays of local resonators is proposed. By using this method, the resonance frequencies of the local resonators can be quickly designed under the conditions of given host beam parameters, lattice constant and added mass ratio of the local resonators.
2) The double-order coupling bandgaps in a metamaterial beam carrying double arrays of local resonators are compared with the single-order coupling bandgaps in metamaterial beams with a single array of local resonators. It is found that, through proper design, the total normalized width of the double-order coupling bandgaps can be much broader than that of the single-order coupling bandgaps, so the double-order coupling bandgaps are more beneficial to wide-band vibration reduction.
3) It is found that for a given total added mass ratio of the double arrays of local resonators, it is necessary to optimize the mass distribution ratio of the double resonators to achieve a maximization of the total normalized width of double-order coupling bandgaps. An approximate formula for designing the optimal mass distribution ratio of the double resonators is further established.
4) The spectral element method is used to study the vibration reduction characteristics of the metamaterial beams carrying double arrays of local resonators designed based on double-order bandgap coupling. The accuracy of the spectral element method is verified by comparing with the finite element method. The results show that significant vibration reduction can be achieved in two wide frequency bands corresponding to the double-order coupling bandgaps. The influence of number of unit cells and resonator damping on the vibration reduction characteristics of the metamaterial beam is further analyzed. It is shown that the increase of number of unit cells can enhance the reduction performance in the bandgaps, and the increase of resonator damping can effectively broaden the vibration reduction frequency band.