Intelligent structures and materials

Adjustable digital metamaterials

Metamaterials are a kind of artificial composite materials with characteristics that natural materials do not possess. It can be used in many important engineering fields, such as high efficiency radar produced, stealth structure design, nondestructive testing, vibration isolation and anti-seismic protection and so on. Traditional metamaterials are only effective in a specific frequency range, which greatly limits their application. In order to break the limitation of traditional metamaterials, a new modularized digital elastic metamaterial is designed, which can adjust the propagation path of waves by arbitrary shape waveguides. It has the ability of multi-degree-of-freedom adjustment which can not be realized by traditional metamaterials.

Wang Z, Zhang Q, Zhang K.*, and Hu G, Tunable Digital Metamaterial for Broadband Vibration Isolation at Low Frequency. Advanced Materials, 2016, 28, 9857–9861, doi.org/ 10.1002/adma.201604009.

Mass distribution adjustable metamaterials

At present, tunable local resonance metamaterials are realized based on stiffness modulation, but there is a disadvantage by this method, that is the equivalent density of materials is constant at high frequency. Thus, a new method to regulate the mass distribution within the cell is proposed. The local resonant metamaterials with an adjustable operating frequency band are designed by introducing liquid inclusions.

Zhang Q, Zhang K, Hu G, Tunable fluid-solid metamaterials for manipulation of elastic wave propagation in broad frequency range. Applied Physics Letters112, 221906 (2018); doi.org/10.1063/1.5023307

Acoustic metamaterial and wave control

Negative refraction of elastic waves at deep-subwave length scale in a single-phase metamaterial

When the elastic wave metamaterials have both negative equivalent mass and elastic modulus, the material will have negative refraction to the elastic wave.Through cell design in thin plate material, negative equivalent modulus is generated by mass rotation resonance in cell, and negative equivalent mass is generated by mass translational resonance, subwavelength elastic wave double negative metamaterial is successfully designed, and its negative refraction is verified by experiment.The results of negative refraction test and finite element prediction of the prepared metamaterials are compared.

R Zhu, XN Liu, GK Hu, CT Sun, GL Huang, Negative refraction of elastic waves at deep-subwave length scale in a single-phase metamaterial, Nature Communication, 5, 5510, 2014.

Effective medium theory of thin-plate acoustic metamaterials

Thin plate structure is widely used in engineering because of its light weight and bearing function.The effect of sound and vibration isolation can be effectively improved by introducing microstructure resonance on the thin plate.Based on the theoretical model, we analyze and calculate the acoustic response of the multilayer thin plate structure with periodic resonant element, and calculate the equivalent parameters of the structure by using the modified transfer matrix method.The analytical expressions of equivalent density and equivalent modulus are obtained for the single-layer thin plate structure.According to the parameter analysis, the negative equivalent mass of thin plate metamaterial with different vibrator spacing can be described by Lorentz model or Drude model.In the negative equivalent mass region, the equivalent parameters do not change with the number of layers.When the acoustic wave is inclined to incident, when the acoustic wave is longer than the cell periodic constant, the equivalent parameters do not change with the incident Angle; on the contrary, the equivalent parameters will change with both the acoustic frequency and the incident Angle, resulting in the spatial dispersion effect of the equivalent density.

P Li, SS Yao, XM Zhou, GL Huang and GK Hu, Effective medium theory of thin-plate acoustic metamaterials, J. Acoust. Soc. Am., 135, 1686, 2014

Sound reduction by metamaterial-based acoustic enclosure

In free space, acoustic metamaterials can effectively block the propagation of sound waves in the negative mass frequency band.For structures with closed cavities, there is a lack of relevant exploration on whether metamaterials have an inhibitory effect on acoustic radiation.For this reason, we studied the acoustic radiation suppression effect of the circular closed cavity structure composed of thin metamaterials on the built-in sound source.Study found that the sound source is located in the center, monopole, dipole and quaternary source can only inspire corresponding cavity resonance modes, and in a negative quality band can cause strong radiation, when the thickness of the metamaterial increases, the acoustic radiation suppression effect is obviously enhanced, and finally can be in the negative quality within the range of effective acoustic attenuation.When the sound source is placed eccentrically, the acoustic radiation resonance modes of similar frequencies are excited for all kinds of sub-level sound sources, so that multiple strong radiation frequencies appear in the negative mass frequency band. Similar to the situation of the sound source center placement, the acoustic radiation suppression effect can be effectively improved by increasing the thickness of the metamaterials.

SS Yao, P Li, XM Zhou and GK Hu, Sound reduction by metamaterial-based acoustic enclosure, AIP Advances, 4, 124306, 2014

An analytical model for the interaction between subwavelength thin film structures and acoustic waves

The experimental results show that this kind of structure has good low frequency sound insulation and sound absorption function.Characterization of metal point matching method based on the influence of the deformation of thin film, we established a precise calculation of this kind of structure acoustic response theory model, and verified with the results of finite element are identical, thus the weight of the sheet metal can be used for system research, the parameters such as shape, size, number and location of the influence law of acoustic reflection and absorption.In the analytical model, the tensioned film is characterized by a film equation containing prestress, and the corresponding structure is mainly used for low-frequency sound insulation design.For the analysis of sound absorption characteristics, we use the thin plate equation to describe the sound absorption mechanism of strain energy dissipation in the film surface.It is worth mentioning that the theoretical limit of 50% of the acoustic absorption coefficient of the film structure can be obtained through the analytical model, which has nothing to do with any microstructure parameters or the film itself loss factor, correcting some incorrect conclusions of relevant experimental results.

YY Chen, XM Zhou, GK Hu and GL Huang, Analytical coupled vibroacoustic modeling of membrane-type acoustic metamaterials: membrane model, J. Acoust. Soc. Am., 136, 969, 2014

YY Chen, GL Huang, XM Zhou, GK Hu and CT Sun, Analytical coupled vibroacoustic modeling of membrane-type acoustic metamaterials: plate model, J. Acoust. Soc. Am., 136, 2926, 2014

Low frequency ultra-thin sound absorption structure design

When the propagation distance of sound wave in the cavity structure meets the condition of 1/4 wavelength, the efficient absorption of sound wave can be realized. However, for low-frequency sound wave, thick materials are needed to achieve the absorption effect.In order to reduce the thickness of sound absorption material, the originally flat sound cavity was curled into a spiral shape, and the sample was prepared by using 3D printing technology. Through numerical simulation and experimental testing, it was found that effective sound absorption could be achieved near the frequency that met the condition of 1/4 wavelength, and the thickness of the improved structure was only 1/50 of the wavelength.Furthermore, the structure of Helmholtz resonator is designed. The experimental and simulation results show that this structure can effectively absorb sound waves at a lower frequency, when the thickness of the structure is only 1/102 of the wavelength.

XB Cai, QQ Guo, GK Hu and J Yang, Ultrathin low-frequency sound absorbing panels based on coplanar spiral tubes or coplanar Helmholtz resonators, Applied Physics Letters, 105, 121901, 2014.

Study on broadband vibration suppression of chiral lattice materials

Chiral lattice materials have the characteristics of light structure and high frequency forbidden band, but it is difficult to restrain the low frequency vibration.By introducing the resonant element into the chiral lattice material, the low frequency vibration of the chiral lattice structure can be suppressed by adjusting the resonant frequency of the resonant element.In addition, theoretical and experimental analysis shows that the effective band of vibration suppression can be widened by introducing resonance units with different resonant frequencies.

R.Zhu, XN Liu, GK Hu, CT Sun and GL Huang, A chiral elastic metamaterial beam for broadband vibration suppression, Journal of Sound and Vibration, 333:2759 , 2014.

Anisotropic chiral lattice materials are homogenized

The general form of the two-dimensional micropolar elastic tensor is derived by using the tensor irreducible orthogonal decomposition method.This general form can describe the chiral characteristics and various symmetries, such as hexagonal symmetry and quadrilateral symmetry.According to the derived form of the micropolar elastic tensor, the chiral lattice structures connected by rigid ring beams are homogenized, and the macroscopic equivalent elastic constants related to geometric parameters are obtained analytically.The variation of the elastic constant with the geometrical parameters and the propagation characteristics of the anisotropic structure are studied.

Y Chen, XN Liu, GK Hu, QP Sun, QS Zheng, Micropolar continuum modelling of bi-dimensional tetrachiral lattices, Proceedings of the Royal Society A, 470(2165), 20130734, 2014.

An elastic wave complex space transform control method is proposed and an elastic wave absorbing boundary layer is designed

Elastic wave absorbing boundary is of great significance in both calculation and experiment. It is mainly used to eliminate the boundary reflection and simulate the infinite space problem with finite space. In addition, the elastic wave measurement experiment also needs to eliminate the reflection from the boundary wave.At present, elastic wave absorbing boundary layer is only in the mathematical sense and mainly used for numerical calculation.By using the elastic wave transform method, the elastic wave propagation and dissipation can be controlled by controlling the imaginary part of the material constant.The elastic wave absorbing boundary layer is designed.

Z. Chang, DK Guo, XQ Feng and GK Hu, A facile method to realize perfectly matched layers for elastic waves, Wave Motion, 11,1170, 2014

Experimental study on acoustic superresolution imaging based on resonant tunneling effect

A super-resolution lens with porous structure based on resonant tunneling is designed.The lens is composed of aluminum plate drilled with periodic cylindrical holes, and the aperture changes periodically along the axis of the hole.Considering that there is a huge impedance mismatch between the aluminum plate and the air, the mass of the lens in the direction parallel to the interface can be considered as infinite, and the propagation of waves along the axis of the hole can satisfy the complete transmission condition caused by the resonant tunneling effect.Because the microstructure of the designed metamaterial lens is of subwavelength scale, it overcomes the problem of the large size of the material.Using sound field test system, test the frequency of resonant tunneling (acoustic wavelength is about 38 cm) to put on the surface of the lens two sound source (about 11 cm) imaging experimental test results, can be seen from the diagram of sound source spacing in a subwavelength scale, still can be distinguished clearly, prove that the designed lens has break through the diffraction limit imaging function.

HJ Su, XM Zhou, XC Xu and GK Hu, Experimental study on acoustic subwavelength imaging of holey-structuredmetamaterials by resonant tunneling, J. Acoust. Soc. Am., 135, 1686, 2014.

Vibration Control of Massive Structure

Study of heat-driven vibration of spacecraft based on absolute coordinate

We studied the heat-driven vibration of spacecraft under the load of sun’s heat radiation. For large circular antenna and spin stabilized spacecraft with hangar rod along axis, we applied absolute node coordinate method and natural coordinate method to heat-structural dynamics coupling analysis, and found out that orbiting spacecraft may suffer heat vibration under heat load. Besides, in order to depict cross section deformation of beam structure in motion, we introduced beam unit considering cross section distortion effect based on absolute node coordinate method.

Z. X. Shen, P. Li, C. Liu, and G. K. Hu, A finite element beam model including cross-section distortion in the absolute nodal coordinate formulation, Nonlinear Dynamics,77, 1019-1033, 2014

Vibration control of flexible rope based on wave motion control

With the development of space engineering, many massive flexible structures have been used in space, such as mechanical arms of space station and solar panels. Flexible structures are massive and lightweight with low damping feature, thus in motion they will have severe residual vibration which may last long. This will have huge impact on flexible parts efficiency, and may cause failure.

Wave motion control is effective in flexible structures. Based on previous control method of homogeneous structure, we set double pendulum and distributed mass as our researching targets. And by applying reflection absorbing method, we introduced open loop control law of inhomogeneous structure, then validated this law through numerical simulation and experiment. Results showed that, comparing with no control condition, residual vibration amplitude is 10 times less.

Xu X. W, Zuo S, Zhang K, Hu G, Wave-based transfer matrix method for dynamic response of large net structures. Journal of Sound and Vibration 433 (2018) 265-286;

Zhou J, Zhang K. and Hu G, Wave-Based Control of a Crane System With Complex Loads. Journal of Dynamic Systems, Measurement, and Control, 139(8), p.081016.

Pattern Control of Microstructure

Study of wrinkle mechanism and features of membrane with microstructure

Introducing microstructure into homogeneous membrane will change wrinkle patterns. We studied the mechanism and features of wrinkle mechanism of membrane. We made holes on the membrane with two ends fixed. Changing the position, radius and distribution of these holes, we found different wrinkle patterns. And through characteristic bulking analysis, post bulking analysis and stress analysis, explanation to mechanical mechanism is given. We got the wrinkle mechanism of membrane with microstructure and the relationship between wrinkle pattern and position, radius, distribution of holes. This helped us control the wrinkle pattern.

D Yan, K Zhang, F Peng, G Hu. Tailoring the wrinkle pattern of a microstructured membrane, Applied Physics Letters, 105, 071905, 2014.

Study of structure transformation based on 3D printing

According to research, 3D printed high polymeric periodic tube bundles will contract and have pattern transformation phenomenon under heating condition. The reason is that residual strain in structure imported by 3D printing precess will be released during heating. Experiment shows that the residual strain will increase as printing speed increases. Combining 3D printing high polymer with other materials, like paper, we can make various intelligent structures. This kind of intelligent structures will self folding to 3D lightweight structures in condition of heating and cooling down. Comparing with other self-folding structure fabrication technology, this method is simple and low cost.

Zhang Q, Yan D, Zhang K. and Hu G. , Pattern transformation of Heat-Shrinkable polymer by Three-Dimensional (3D) printing technique. Scientific Reports, 5, p. 8936.