Yan-Feng Wang

Coupling of evanescent and propagating guided modes in locally resonant phononic crystals

In this paper, we present a combined theoretical, numerical and experimental study of acoustic wave propagation in 1D locally resonant phononic crystals made of acoustic resonators grafted onto a waveguide. The case of one single resonator grafted onto the waveguide is first investigated and a model of transmission cancellation at resonant frequencies is obtained. The model includes the excitation of evanescent guided waves attached to the grafting points. When extended to periodical arrays of grafted resonators, the model provides us with a definite theoretical expression for the complex band structure. Comparison with experimental results and complex band structures obtained by numerical simulation suggests a strong dependence of transmission through the crystal on the lattice constant of the grafted resonators. It is found that evanescent waves in the waveguide play a key role when the lattice constant is in the sub-wavelength range.

Reducing symmetry in topology optimization of two-dimensional porous phononic crystals

In this paper we present a comprehensive study on the multi-objective optimization of two-dimensional porous phononic crystals (PnCs) in both square and triangular lattices with the reduced topology symmetry of the unit-cell. The fast non-dominated sorting-based genetic algorithm II is used to perform the optimization, and the Pareto-optimal solutions are obtained. The results demonstrate that the symmetry reduction significantly influences the optimized structures. The physical mechanism of the optimized structures is analyzed. Topology optimization combined with the symmetry reduction can discover new structures and offer new degrees of freedom to design PnC-based devices. Especially, the rotationally symmetrical structures presented here can be utilized to explore and design new chiral metamaterials.

Wave propagation in a sandwich plate with a periodic composite core

By using the finite element method, the propagation of Lamb waves in a sandwich plate with a periodic composite core is investigated. The periodic composite core is constituted by a square array of elastic cylinders embedded in a solid matrix. The dispersion relations and transmission responses are calculated. The results show that both stop and pass bands are involved in the dispersion relations. The influences of the thickness and material properties of the facesheets and the filling fraction of the inclusions in the core on the stop band are discussed. It is noted that the thickness of either the core or the facesheets, the material properties as well as the filling fraction play key roles in tuning the dispersion relations of the sandwich plates. The analysis of the displacement fields shows that the eigenmodes located at the edges of the stop bands are influenced by the thickness of the facesheets. Besides, by removing one or an array of cylinders, a point or line defect is introduced into the sandwich plate. The dispersion relations and the displacement fields of the eigenmodes for such defected systems are calculated. The localization and/or propagation behaviors of the defect modes appearing in the frequency ranges of the stop bands are discussed in detail. The study is relevant to understanding the vibration and the wave motion of the sandwich plate with a periodic composite core and manipulating the propagation of the vibration and the wave motion in this structure.

Two-dimensional ternary locally resonant phononic crystals with a comblike coating

Two-dimensional ternary locally resonant phononic crystals can be used for vibration control and noise insulation in the low (even audible) frequency range. They traditionally consist of cylindrical scatterers with uniform coatings in their exterior. An alternative coating profile with a comblike profile is proposed and investigated in this paper. The band structures are calculated using the finite element method. We find that a complete bandgap can be induced at a significantly low frequency, the wavelength of which is more than 20 times the lattice constant. The mechanism for such a change is suggested using an equivalent spring–mass model and analyzing the eigenmodes at the bandgap edges. Numerical results and the results predicted by the spring–mass model are coherent.

Bandgaps and directional propagation of elastic waves in 2D square zigzag lattice structures

In this paper we propose various types of two-dimensional (2D) square zigzag lattice structures, and we study their bandgaps and directional propagation of elastic waves. The band structures and the transmission spectra of the systems are calculated by using the finite element method. The effects of the geometry parameters of the 2D-zigzag lattices on the bandgaps are investigated and discussed. The mechanism of the bandgap generation is analyzed by studying the vibration modes at the bandgap edges. Multiple wide complete bandgaps are found in a wide porosity range owing to the separation of the degeneracy by introducing bending arms. The bandgaps are sensitive to the geometry parameters of the systems. The deformed displacement fields of the transient response of finite structures subjected to time-harmonic loads are presented to show the directional wave propagation. The research in this paper is relevant to the practical design of cellular structures with enhanced vibro-acoustics performance.

Tunable fluid-filled phononic metastrip

We study the propagation of Lamb waves in a one-dimensional tunable phononic metastrip composed of a periodic sequence of hollow pillars that can be selectively filled with water. Band structures and transmission properties are computed numerically for metastrips with different fluid fillings by using the finite element method. Good agreement is observed with experimental results obtained with an aluminum metastrip. In particular, it is found that the frequency range of bandgaps and passbands can be controlled through fluid filling. Our results imply that Lamb waves in the solid metastrip can be harnessed through changing the properties of the pillars via fluid-solid interaction. The work in this paper is relevant to practical design of tunable acoustic devices.

Two-dimensional locally resonant elastic metamaterials with chiral comb-like interlayers: Bandgap and simultaneously double negative properties.

In this paper, bandgap and dynamic effective properties of two-dimensional elastic metamaterials with a chiral comb-like interlayer are studied by using the finite element method. The effects of the geometrical parameters of the chiral comb-like interlayer on the band edges are investigated and discussed. Combined with the analysis of the vibration modes at the band edges, equivalent spring-mass/pendulum models are developed to investigate the mechanisms of the bandgap generation. The analytically predicted results of the band edges, including the frequency where the double negative properties appear, and the numerical ones are generally in good agreement. The research findings in this paper have relevant engineering applications of the elastic metamaterials in the low frequency range.

Bandgaps and directional properties of two-dimensional square beam-like zigzag lattices

In this paper we propose four kinds of two-dimensional square beam-like zigzag lattice structures and study their bandgaps and directional propagation of elastic waves. The band structures are calculated by using the finite element method. Both the in-plane and out-of-plane waves are investigated simultaneously via the three-dimensional Euler beam elements. The mechanism of the bandgap generation is analyzed by studying the vibration modes at the bandgap edges. The effects of the geometry parameters of the xy- and z-zigzag lattices on the bandgaps are investigated and discussed. Multiple complete bandgaps are found owing to the separation of the degeneracy by introducing bending arms. The bandgaps are sensitive to the geometry parameters of the periodic systems. The deformed displacement fields of the harmonic responses of a finite lattice structure subjected to harmonic loads at different positions are illustrated to show the directional wave propagation. An extension of the proposed concept to the hexagonal lattices is also presented. The research work in this paper is relevant to the practical design of cellular structures with enhanced vibro-acoustics performance.

The effect of Ti addition on the thermal stability of

(Fe,Ti) - N films containing phase were prepared on Si and NaCl single-crystal substrates by facing target sputtering (FTS). An analytical transmission electron microscope was used for the structural studies prior to and during in situ annealing of the samples. The as-deposited films with 3 - 5 at.% Ti contained and phases and films with 15 - 18 at.% Ti comprised phase and TiN phase. The phase was the dominant phase in the two samples. It existed stably during in situ annealing from room temperature to C, and remained stable when the samples were cooled down to room temperature. It is found that proper Ti addition can stabilize the phase to a temperature of C. The saturation magnetization of the two annealed samples was 2.68 and 2.46 T respectively, which is larger than that of pure iron.

Longitudinal Near-Field Coupling between Acoustic Resonators Grafted onto a Waveguide

We investigate longitudinal near-field coupling between acoustic resonators grafted along a waveguide. Experiments are performed in the audible range with a simple acoustic system composed of a finite aperiodic sequence of air resonators. Transmission typically shows a zero around a resonance frequency of a single resonator, as is well known. When two identical resonators are brought in close proximity, however, we observe that longitudinal near-field coupling strongly influences the acoustic transmission. When the separation between resonators is increased so that they can be considered in the far field of one another, we further observe the appearance of Fano-like transmission profiles. We explain this observation by the formation of locally resonant Fabry-Perot interferometers from every pair of resonators. All experimental results are compared to three-dimensional finite element analysis of the acoustic system.