R. Zhu

Negative refraction of elastic waves at the deep-subwavelength scale in a single-phase metamaterial

Negative refraction of elastic waves has been studied and experimentally demonstrated in three- and two-dimensional phononic crystals, but Bragg scattering is impractical for low-frequency wave control because of the need to scale the structures to manageable sizes. Here we present an elastic metamaterial with chiral microstructure made of a single-phase solid material that aims to achieve subwavelength negative refraction of elastic waves. Both negative effective mass density and modulus are observed owing to simultaneous translational and rotational resonances. We experimentally demonstrate negative refraction of the longitudinal elastic wave at the deep-subwavelength scale in the metamaterial fabricated in a stainless steel plate. The experimental measurements are in good agreement with numerical simulations. Moreover, wave mode conversion related with negative refraction is revealed and discussed. The proposed elastic metamaterial may thus be used as a flat lens for elastic wave focusing.

Focusing guided waves using surface bonded elastic metamaterials

Bonding a two-dimensional planar array of small lead discs on an aluminum plate with silicone rubber is shown numerically to focus low-frequency flexural guided waves. The “effective mass density profile” of this type of elastic metamaterials (EMMs), perpendicular to wave propagation direction, is carefully tailored and designed, which allows rays of flexural A0 mode Lamb waves to bend in succession and then focus through a 7 × 9 planar array. Numerical simulations show that Lamb waves can be focused beyond EMMs region with amplified displacement and yet largely retained narrow banded waveform, which may have potential application in structural health monitoring.

Experimental study of an adaptive elastic metamaterial controlled by electric circuits

The ability to control elastic wave propagation at a deep subwavelength scale makes locally resonant elastic metamaterials very relevant. A number of abilities have been demonstrated such as frequency filtering, wave guiding, and negative refraction. Unfortunately, few metamaterials develop into practical devices due to their lack of tunability for specific frequencies. With the help of multi-physics numerical modeling, experimental validation of an adaptive elastic metamaterial integrated with shunted piezoelectric patches has been performed in a deep subwavelength scale. The tunable bandgap capacity, as high as 45%, is physically realized by using both hardening and softening shunted circuits. It is also demonstrated that the effective mass density of the metamaterial can be fully tailored by adjusting parameters of the shunted electric circuits. Finally, to illustrate a practical application, transient wave propagation tests of the adaptive metamaterial subjected to impact loads are conducted to valida...

Fast damage imaging using the time-reversal technique in the frequency–wavenumber domain

The time-reversal technique has been successfully used in structural health monitoring (SHM) for quantitative imaging of damage. However, the technique is very time-consuming when it is implemented in the time domain. In this paper, we study the technique in the frequency‐wavenumber (f‐k) domain for fast real-time imaging of multiple damage sites in plates using scattered flexural plate waves. Based on Mindlin plate theory, the time reversibility of dispersive flexural waves in an isotropic plate is theoretically investigated in the f‐k domain. A fast damage imaging technique is developed by using the cross-correlation between the back-propagated scattered wavefield and the incident wavefield in the frequency domain. Numerical simulations demonstrate that the proposed technique cannot only localize multiple damage sites but also potentially identify their sizes. Moreover, the time-reversal technique in the f‐k domain is about two orders of magnitude faster than the method in the time domain. Finally, experimental testing of an on-line SHM system with a sparse piezoelectric sensor array is conducted for fast multiple damage identification using the proposed technique. (Some figures may appear in colour only in the online journal)

Effective Dynamic Properties and Multi-Resonant Design of Acoustic Metamaterials

In the study, a retrieval approach is extended to determine the effective dynamic properties of a finite multilayered acoustic metamaterial based on the theoretical reflection and transmission analysis. The accuracy of the method is verified through a comparison of wave dispersion curve predictions from the homogeneous effective medium and the exact solution. A multiresonant design is then suggested for the desirable multiple wave band gaps by using a finite acoustic metamaterial slab. Finally, the band gap behavior and kinetic energy transfer mechanism in a multilayered composite with a periodic microstructure are studied to demonstrate the difference between the Bragg scattering mechanism and the locally resonant mechanism.

Enhanced flexural wave sensing by adaptive gradient-index metamaterials

Increasing sensitivity and signal to noise ratios of conventional wave sensors is an interesting topic in structural health monitoring, medical imaging, aerospace and nuclear instrumentation. Here, we report the concept of a gradient piezoelectric self-sensing system by integrating shunting circuitry into conventional sensors. By tuning circuit elements properly, both the quality and quantity of the flexural wave measurement data can be significantly increased for new adaptive sensing applications. Through analytical, numerical and experimental studies, we demonstrate that a metamaterial-based sensing system (MBSS) with gradient bending stiffness can be designed by connecting gradient negative capacitance circuits to an array of piezoelectric patches (sensors). Furthermore, we demonstrate that the proposed system can achieve more than two orders of magnitude amplification of flexural wave signals to overcome the detection limit. This research encompasses fundamental advancements in the MBSS with improved performance and functionalities, and will yield significant advances for a range of applications.

A single-phase elastic hyperbolic metamaterial with anisotropic mass density

Wave propagation can be manipulated at a deep subwavelength scale through the locally resonant metamaterial that possesses unusual effective material properties. Hyperlens due to metamaterials anomalous anisotropy can lead to superior-resolution imaging. In this paper, a single-phase elastic metamaterial with strongly anisotropic effective mass density has been designed. The proposed metamaterial utilizes the independently adjustable locally resonant motions of the subwavelength-scale microstructures along the two principal directions. High anisotropy in the effective mass densities obtained by the numerical-based effective medium theory can be found and even have opposite signs. For practical applications, shunted piezoelectric elements are introduced into the microstructure to tailor the effective mass density in a broad frequency range. Finally, to validate the design, an elastic hyperlens made of the single-phase hyperbolic metamaterial is proposed with subwavelength longitudinal wave imaging illustrated numerically. The proposed single-phase hyperbolic metamaterial has many promising applications for high resolution damage imaging in nondestructive evaluation and structural health monitoring.

Biomechanical Strain Analysis at the Interface of Brain and Nanowire Electrodes on a Neural Probe

The viability of neural probes with microelectrodes for neural recording and stimulation in the brain is important for the development of neuroprosthetic devices. Vertically aligned nanowire microelectrode arrays can significantly enhance the capabilities of neuroprosthetic devices. However, when they are implanted into the brain, micromotion and mechanical stress around the neural probe may cause tissue damage and reactive immune response, which may degrade recording signals from neurons. In this research, a finite-element model of the nanowire microelectrode and brain tissue was developed. A rigid body method was provided, and the simulation efficiency was significantly increased. The interface between the microelectrode and brain tissue was modeled by contact elements. Brain micromotion was mimicked by applying a displacement load to the electrode and fixing the boundaries of the brain region. It was observed that the vertically aligned nanostructures on the electrode of the neural probe do increase the cellular sheath area. The strain field distributions under various physical coupling cases at the interface were analyzed along with different loading effects on the neural electrode.

Focusing flexural Lamb waves by designing elastic metamaterials bonded on a plate

In this paper, a method to focus flexural Lamb waves to a local area by mounting elastic metamaterials (EMMs) on the surface of the plate is proposed. The EMM consists of silicon rubber and lead connected in series bonded vertically on an aluminum plate. A simplified effective mass-“spring”-mass model is used to study the EMM plate. The frequency-dependent effective mass density of the EMM plate is determined with the aid of the numerically based effective medium method. By making use of the low locally resonant frequency of the EMM plate, the EMM plate is carefully designed with different dimensions to attain high effective mass densities. The effective mass density can be assumed to dominate the change of wave velocity and propagation direction in the EMM plate. An effective mass density profile is then employed along the transverse direction of wave propagation to achieve focusing. Finally, numerical simulation with finite element method (FEM) is utilized to investigate the focusing phenomenon of the A0 mode Lamb waves at 30 kHz and the out-of-plane displacement response beyond the EMM region. Numerical simulation results have shown that focusing the low frequency A0 mode Lamb waves using EMMs is feasible. The focusing may have potential applications in structural health monitoring by manipulating Lamb waves through controlling and focusing Lamb waves to any arbitrary location of the plate with amplified displacement and yet largely retained five-peaked toneburst waveform.

Numerical effective formulation for guided wave propagation in a metamaterial plate with anisotropic mass density

A numerical method for obtaining the effective anisotropic mass density of elastic composite with arbitrary periodic microstructure is presented and the effective anisotropic mass density is proved to be a second-order tensor. Using the proposed method, a new metamaterial plate with strong anisotropicity in mass density is obtained. Using 3-D elasticity theory, the metamaterial plate is modeled as a continuum medium with obtained effective material properties. The accuracy of the continuum model was evaluated by comparing the dispersion curves with those obtained by exact finite element analysis. Moreover, mode coupling and level repulsion in the anisotropic metamaterial plate are discussed. Finally, preferential directions of wave propagation and energy flow are studied through the comparison of the difference between the phase velocity and group velocity directions.