H. H. Huang

Wave attenuation mechanism in an acoustic metamaterial with negative effective mass density

The wave attenuation and energy transfer mechanisms of a metamaterial having a negative effective mass density are studied. The metamaterial considered is represented by a lattice system consisting of mass-in-mass units. The attenuation of wave amplitude for frequencies in the stop band is studied from the energy transfer point of view. It is found that most of the work done by the external force on the lattice system is stored by the internal mass if the forcing frequency is close to the local resonance frequency. However, the energy stored in the internal mass is only temporary; it is taken out by the external force in the form of negative work in a cyclic manner. This behavior is utilized to design metamaterials for preventing stress waves from passing them.

Optimizing the band gap of effective mass negativity in acoustic metamaterials

A dual-resonator microstructure design is proposed for acoustic metamaterials to achieve broadband effective mass negativity. We demonstrate the advantage of acoustic wave attenuation over a wider frequency spectrum as compared to the narrow band gap of a single-resonator design. We explicitly confirm the effect of negative effective mass density by analysis of wave propagation using finite element simulations. Examples of practical application like vibration isolation and blast wave mitigation are presented and discussed.

Anomalous wave propagation in a one-dimensional acoustic metamaterial having simultaneously negative mass density and Young's modulus.

A mechanical model representing an acoustic metamaterial that exhibits simultaneously negative mass density and negative Youngs modulus was proposed. Wave propagation was studied in the frequency range of double negativity. In view of positive energy flow, it was found that the phase velocity in this range is negative. This phenomenon was also observed using transient wave propagation finite-element analyses of a transient sinusoidal wave and a transient wave packet. In contrast to wave propagation in the region of positive mass and modulus, the peculiar backward wave motion in the region of double negativity was clearly displayed.

Locally resonant acoustic metamaterials with 2D anisotropic effective mass density

A two-dimensional (2D) lattice model with anisotropic resonant microstructures is found to provide an anisotropic band gap structure. A 2D continuum with anisotropic effective mass density is introduced to represent this lattice system. Two methods are proposed to derive the equivalent continuum. In the first method, the effective mass density of the equivalent continuum is obtained by matching the dispersion relations for harmonic waves propagating in the principal directions. The second approach employs an approximate estimation of the effective mass density by volume-averaging an effective mass that represents the resonant microstructure. For both equivalent continuum models, the effective mass density is frequency-dependent and may become negative in certain frequency ranges. Subsequently, the effective mass density of the equivalent continuum assumes the form of a second-order tensor. Thus, it suffices to determine the effective mass density tensor with respect to the principle directions. It is shown numerically that the local-resonance effect is accurately described by the equivalent continuum model. In addition, the effect of anisotropic mass density on wave propagation is numerically illustrated and discussed.

A study of band-gap phenomena of two locally resonant acoustic metamaterials

The dynamic characteristics of two acoustic metamaterials were investigated using their representative mass-spring lattice models. Attention was focused on the band-gap behaviour of wave propagation in the two models. The study was carried out by performing finite element analyses. From the simulation results, it was found that both models have demonstrated the capability of filtering or blocking dynamic disturbances. However, in spite of having similar local resonance, the two models display different dynamic responses in the band gap. In addition, parametric studies of both metamaterials were carried out with the aim of providing a guide for the design of band gaps in acoustic metamaterials for desired functions.

Attenuation of transverse waves by using a metamaterial beam with lateral local resonators

This study numerically and experimentally investigated the wave propagation and vibrational behavior of a metamaterial beam with lateral local resonators. A two-dimensional simplified analytical model was proposed for feasibly and accurately capturing the in-plane dispersion behavior, which can be used for the initial design. The out-of-plane wave motions, however, required advanced three-dimensional (3D) modeling. Through experimental validations, 3D finite element simulations were demonstrated to be suitable for advanced design and analysis. This study provided a basis for designing metabeams for transverse wave mitigation. The proposed concept can be further extended to 3D metamaterial plates for wave and vibrational mitigation applications.

Behavior of Wave Motion in an Acoustic Metamaterial with Anisotropic Mass Density

An elastic solid with frequency-dependent anisotropic mass density was developed to represent an acoustic metamaterial to study wave propagation. The band gaps of the material were related to the frequencies for which the mass density of the equivalent elastic solid becomes negative. Reflection and transmission of a pressure wave impinging on the metamaterial with a finite width were investigated using the equivalent elastic solid. It was found that the impinging pressure wave can induce a strong shear-dominated wave mode accompanied by a weak extension-dominated wave in the metamaterial oriented in some directions. Since the fluid-like medium cannot transmit shear waves, the shear-dominated mode in the metamaterial may be trapped inside the metamaterial, and, thus, the fluid-like material behind the metamaterial can remain basically undisturbed.

Realization of a thermal cloak–concentrator using a metamaterial transformer

By combining rotating squares with auxetic properties, we developed a metamaterial transformer capable of realizing metamaterials with tunable functionalities. We investigated the use of a metamaterial transformer-based thermal cloak–concentrator that can change from a cloak to a concentrator when the device configuration is transformed. We established that the proposed dual-functional metamaterial can either thermally protect a region (cloak) or focus heat flux in a small region (concentrator). The dual functionality was verified by finite element simulations and validated by experiments with a specimen composed of copper, epoxy, and rotating squares. This work provides an effective and efficient method for controlling the gradient of heat, in addition to providing a reference for other thermal metamaterials to possess such controllable functionalities by adapting the concept of a metamaterial transformer.

Design and Static Analysis of Improved Grout Joint of Offshore Monopile Foundation

A wind turbine monopile usually consists of a “grout joint” that connects the pile with the sleeve and supports the wind turbine. The pile is misaligned frequently when driven into the seabed. The grout corrects the alignment of the sleeve and pile when the sleeve is installed. Nevertheless, the destruction of the concrete remains critical. This article focuses on the damage behaviour of the grout joints and proposes concepts for strengthening joints. The failure location and the maximum stress determined from a static numerical analysis are first discussed. To strengthen the traditional grout joint, two modified joints were designed, constructed, and compared with the traditional joint. Both numerical and experimental three-point bending tests of a small-scale monopile foundation were performed. Crack opening distances at the joint location of the specimens and the ultimate forces of the tests were measured and compared. The proposed modified grout joint demonstrates improved static load-carrying capability and lower stress in grout materials.

Prospect of Structural Health Monitoring Application for Offshore Wind Farm in Taiwan

For the recent surge of interests in Taiwan’s offshore wind farm program, the government takes the initiatives promoting pilot wind farm projects and developing strategy for 600 wind turbines off Taiwan’s west coast by 2030. Together with 450 land based turbines at the time, it anticipates a total power capacity of 4.2 GW. The announcement of this ambitious offshore energy program has stimulated significant interests among various industrial sectors, as well as academic and R&D institutions in Taiwan. The offshore wind turbines are exposed to extreme environmental conditions and high dynamic stresses. Incipient damage must be detected at the earliest stage possible in order to plan and take reasonable repair measures in due time. This can prevent the development of severe damage and thus lower the repair costs. Periodic inspections are not sufficient for an early detection of damage. Continuous monitoring ensures a higher level of safety. Wireless structural health monitoring (SHM) and risk-based reliability management (RRM) are cost-effective, infrastructural solutions to stable and increased energy yield. This paper presents an overview and prospect on the development of indigenous SHM/RRM for Taiwan offshore wind turbines to be erected in the coastal regions west of Taiwan. A strategic plan is discussed to use the demonstration wind turbines as a test platform for further technology enhancement for field applications, and to consolidate SHM/RRM into wind farm technology core R&D programs for academic research and industrial applications. doi: 10.12783/SHM2015/54