Controlling elastic/acoustic wave-front via compact designs has attracted increasing research interest in past decades. The emerging of metasurface concept provides an unconventional and attractive approach in this filed. A metasurface generally consists of a single array of subwavelength-scaled patterns in the host medium, introducing an abrupt phase shift in the wave propagation path. In this paper, we explored an elastic metasurface concept to control the propagation of low-frequency flexural Lamb waves. The phase modulation based on the Snell’s law was achieved by tailoring the thickness of thin beam resonators connecting two elastic host medium. Depending on the design of the phase-modulated structure (a.k.a. metasurface), elastic waves could be steered or focused which was verified through analytical and numerical models.

This paper presents a numerical study on multi-mode focusing of guided elastic waves in pipe-like structures over a range of frequencies using a single metamaterial lens design. We explore focusing of the first two longitudinal (L(0,1) and L(0,2)) and the first torsional (T(0,1)) wave modes in steel pipe integrated with a metamaterial lens made of cylindrical steel stubs of varying heights attached to outer surface of the pipe. Proposed metamaterial lens design is based on gradient index (GRIN) theory with hyperbolic secant distribution of refractive index in circumferential direction. Amplification of multi-mode guided wave signals are achieved at focal points of the lens which is verified through numerical simulations. The focusing performance of proposed lens is studied at multiple frequencies for all the three modes and first two focal positions are verified with theoretical predictions of GRIN theory.

Elastic lens and mirror concepts that have been explored to date for enhanced structure-borne wave energy harvesting are suitable for relatively high-frequency waves (e.g. tens of kHz), which are very much outside the typical ambient structural frequency energy spectrum. One direct way of reducing the design frequency of such phononic crystal-based lens and reflector/mirror designs is to increase their size, which would yield very large dimensions to operate at ambient vibration frequencies (∼hundreds of Hz). In this work, we exploit locally resonant (LR) metamaterials to enable low-frequency elastic wave focusing via LR lens and mirror concepts with practical size limitations. LR lens is designed in a similar way to its phononic crystal counterpart by tailoring the refractive index profile of the LR unit cell distribution. However, LR approach enables altering the dispersion characteristics, and thereby the phase velocity distribution, at much lower frequencies right below the local resonance frequency. Other than the local resonance frequency of the unit cells, the key factor in design is the mass ratio of the resonators to achieve a desired refractive index profile and focusing. LR mirror uses the low-frequency bandgap which is right above the resonance frequency of the unit cells. LR unit cells arranged in the form of a parabola, for instance, makes a low-frequency LR mirror that operates in the bandgap for plane wave focusing. These LR focusing concepts can be used in vibration civil, aerospace, and mechanical systems to localize and harvest structure-borne wave energy.

In this paper, we explore structure-borne elastic wave energy harvesting, both numerically and experimentally, by exploiting a Gradient-Index Phononic Crystal Lens (GRIN-PCL) structure. The proposed GRIN-PCL is formed by an array of blind holes with different diameters on an aluminum plate where the orientation and size of the blind holes are tailored to obtain a hyperbolic secant gradient distribution of refractive index guided by finite-element simulations of the lowest asymmetric mode Lamb wave band diagrams. Under plane wave excitation from a line source, experimentally measured wave field successfully validates the numerical simulation of wave focusing within the GRIN-PCL domain. A piezoelectric energy harvester disk located at the first focus of the GRIN-PCL yields an order of magnitude larger power output as compared to the baseline case of energy harvesting without the GRIN-PCL on the uniform plate counterpart for the same incident plane wave excitation. The power output is further improved by a factor of five using complex electrical load impedance matching through resistive-inductive loading as compared to purely resistive loading case.

Vibration-to-electricity conversion has been heavily researched over the last decade with the ultimate goal of enabling self-powered small electronic components to use in wireless applications ranging from medical implants to structural health monitoring sensors. Regardless of the transduction mechanism used in transforming vibrational energy into electricity, the existing research efforts have mostly focused on deterministic or stochastic harvesting of direct vibrational energy available at a fixed location in space. Such an approach is convenient to design and employ linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates. Although the harvesting of local vibrations using linear and nonlinear devices has been well studied, there has been little effort to investigate power extraction from elastic waves propagating in host structures to gain a fundamental understanding of power flow and to best exploit not only standing but also traveling wave energy. This paper explores the problem of piezoelectric energy harvesting from one-dimensional bending waves involving propagating and evanescent components with a focus on infinitely long thin beams. A pair of electroded piezoelectric patches is implemented as the energy harvesting interface connected to a complex electrical load. An analytical modeling framework is given in order to relate the harvested power to incoming wave in the presence of a generalized resistive-reactive circuit. Effects of energy harvesting on the global wave dynamics as well as individual propagating and evanescent wave components are investigated with an emphasis on the wavelength matching concept. The electrical loading conditions for maximum power and efficiency are identified for several special cases in the low frequency range.