With capability of achieving structural state awareness and facilitating preventative maintenance schemes, the global market of structural health monitoring (SHM) is growing at an expected compound annual growth rate of 14.5 percent from 2020 to 2027. From a long run, the increasing adoption of internet of things (IoT) and digital twins are transforming the physical world in a wider range into virtual representations. This will sustain the technical advancement and market growth of SHM in the future. As the most commonly applied non-destructive testing method without radioactive hazard, the application of ultrasonic technology in SHM is currently far from the same level of popularity. The important reasons include the bulky size and mass of ultrasonic transducers, and inconsistency in acoustic coupling between the host structure and individual transducers usually manually installed. To overcome the challenges, our group has developed piezoelectric polymer coatings and lead-free piezoelectric ceramic coatings, with processing scalability over large area and conformability on curved surface. Low profile ultrasonic transducers and transducer array have been designed and produced in-situ on the host structure using the piezoelectric coatings, such as via direct-write process. In collaboration with our research and industry collaborators, guided wave-based ultrasonic SHM functions are being demonstrated on planar and tubular structures, including detections of various defects from presence of cracks to plastic deformation. The features and opportunities of these piezoelectric coating transducers in-situ produced for ultrasonic SHM will be discussed.

Elastic wave propagation in anisotropic media is of great interest in various branches of engineering and applied sciences. In this study, an anisotropic wave propagation behavior in isotropic material with orthogonal surface perturbation is presented. The conventional method of estimating dispersion equations for isotropic material is to apply Helmholtz decomposition on the potential functions for Rayleigh-Lamb wave and Shear Horizontal (SH) waves. However, the presence of isotropic material with orthogonal surface perturbations in two coordinate directions, which is known as doubly corrugated wave guide, can significantly affect the wave propagation behavior. This is because of the direction dependency of the wave propagation in the doubly corrugated structure, and hence, the Helmholtz decomposition of the potential functions cannot be applied to derive the dispersion equations. To validate the direction dependency of the wave propagation in isotropic material with doubly corrugated geometry, a time domain simulation is performed by the Finite Element Method using a tone burst signal to excite the wave guide. A similar baseline time domain study is also performed for a flat wave guide using the same material property without corrugation in any directions. The displacements of the particles of these two studies are compared at multiple time steps and analyzed for the direction dependency of wave propagation. The preliminary results show that the wave propagation in doubly corrugated structure is highly dependent on the corrugation parameters.

Development of a universal ultrasonic scanner system for nondestructive evaluation (NDE) of material degradation will allow the NDE users to utilize various scanning methods with a one stop system. Some ultrasonic scanning requires, water e.g. C-scan, Guided wave scan, some requires gel, e.g. hand held Pulse-echo and Phased Array, and some performs scan in air using air-coupled ultrasound. Additionally, in recent years development of new theories of wave propagation in phononic crystal and metamaterials demand scans of the systems using acoustic waves for fundamental understanding. There are many NDE system that can perform individual scans with specific objective. However, in addition to ultrasonic NDE scanning, acoustic scanning, guided wave scanning, biological sample inspection and many other forms of scanning e.g. Raman spectroscopy scanning to find material composition at various special spots of a material, may demand different setup and different mechanism. In this research we developed an ability to have a system that can utilize these various mechanisms and methods and unify them under one umbrella that can serve many applications in both research and commercial use. In this article, the discussion of the development and success of such a system will be elaborated on further as testing for various ultrasonic and acoustic methods on a variety of different materials (metal, composite, phononic crystals, metamaterials) have been proven successful. Tests using ultrasonic in both air and water and dry coupled scan was performed on various materials, yielding data to create ultrasonic images. These tests show the ease of use with the system and how simple it is to convert the scanner to utilize a different method as module of ultrasonic scanning. This scanner supports the use of various transducers at any frequency, Transmit and receive (T/R) mode and through mode with signal conditioning, and the ability to configure the resolution and size of the scan. This universal scanner has the protentional to be an all-around solution for NDE and acoustic research.

A passive solid cannot do work on its surroundings through any quasistatic cycle of deformations. This property places strong constraints on the allowed elastic moduli. In this talk, we show that static elastic moduli altogether absent in passive elasticity can arise from active, non-conservative microscopic interactions. These active moduli enter the antisymmetric (or odd) part of the static elastic modulus tensor and quantify the amount of work extracted along quasistatic strain cycles. In two-dimensional isotropic media, two chiral odd-elastic moduli emerge in addition to the bulk and shear moduli. We discuss microscopic realizations that include networks of Hookean springs augmented with active transverse forces and non-reciprocal active hinges. Using coarse-grained microscopic models, numerical simulations and continuum equations, we uncover phenomena ranging from auxetic behavior induced by odd moduli to elastic wave propagation in overdamped media enabled by self-sustained active strain cycles. We discuss active metamaterial realizations that conserve linear momentum but exhibit a non-reciprocal linear response. Next we ask: what happens to the well-established framework of phase transitions in these non-reciprocal systems far from equilibrium? Simple demonstrations with robots will be presented along with naturally occurring phenomena from various domains of science. In all these cases, the emergence of unique time-dependent many-body phases can be captured by combining insights from non-Hermitian quantum mechanics and bifurcation theory. This mathematical approach lays the foundation for a general theory of critical phenomena in systems whose collective dynamics is not governed by an optimization principle.

We demonstrate the vibrational edge mode transfer on the dimer mechanical lattice consists of the Triangulated Cylindrical Origami. The configuration of our dimer lattice can be altered only by twisting the chain, and therefore it does not require any replacement of constituent unit cells. Such in-situ tunable lattice opens the bandgap in the wave dispersion relationships with emerging boundary mode. By twisting the chain excited at boundary mode frequency, our numerical simulation shows high transfer fidelity of the edge state through the lattice. The simple and efficient state transfer can be leveraged for energy manipulation in engineering applications.

The Z2 invariant associated with quantum spin Hall topological insulators is connected to the unique property of fermionic systems whose wavefunction acquires a negative sign upon two consecutive applications of the time-reversal operator. However, this property is not acquired by the classical-wave systems. Instead, a combination of spatial and temporal symmetry is required to synthesize the Kramers degeneracy. In this study, we propose an elastic phononic plate engraved with a honeycomb lattice whose depth varies according to a Kekule pattern. The “local topological charge” can be defined based on the difference of integrated pseudospin-resolved Berry curvature profiles, as an alternative to the Z2 invariant. Such “local topological charge” is not a topological invariant in a rigorous sense, since it depends on the position of the reference frame. This condition also leads to the result that the same phononic structure can be in different topological phases based on different reference frames. It follows that edge states can exist on a dislocation interface connecting two pieces of the same phononic structure with relative shifting. Two pseudospin-polarized edge-state bands crossing at zero-k point (synthetic Kramers pair) are achieved without repulsion, hence indicating decoupling and robustness of the counter-propagating states.

We investigate the dynamics of one-dimensional elastic lattices with disorder in the form of random non-local connectivites according to the “small-world” model of network connections. We present preliminary investigations on their dynamics, and illustrate the formation of spectral gaps which are formed for increasing degrees of disorder. These gaps are shown to persist across multiple lattice realizations and different sizes, signaling a robust property of the disorder systems. We also discuss possible experimental realizations on acoustic waveguides. These preliminary findings illustrate intriguing possibilities for wave manipulation and transport properties that are enabled via disorder in elastic and acoustic systems with random network connections.

Owing to smooth surface transitions and open-cell connectivity, minimal surfaces can be conveniently designed to form singly, doubly, triply periodic, and quasi-periodic structures, that provide new opportunities for novel metamaterial design of potential engineering relevance. Prior studies have demonstrated band gaps and engineered mechanical properties in minimal surface structures. However, their potential for the exploitation of topological phenomena through elastic waves remains largely unexplored. In this work, we design periodic and quasi-periodic minimal surface metamaterials and explore their topological properties. We start with the construction of 1D/2D periodic minimal surfaces with dimerized-like parametrizations. The designs facilitate the opening of band gaps upon breaking the inversion symmetry of the unit cells through a band inversion process that defines distinct topological phases. Simulations demonstrate the existence of 0D localized modes in 1D interfaced structures, and robust waveguiding along a valley-type 1D interface separating distinct 2D domains. These designs are fabricated with additive manufacturing technologies and tested with laser vibrometry, confirming the presence of the predicted topological states. We then investigate 1D quasi-periodic minimal surfaces through a quasi-periodic modulation of the dimerization parameter. Such structures support topological gaps forming a fractal spectrum that resemble the Hofstadter butterfly. The existence of non-trivial gaps and localized modes in quasiperiodic systems extend the avenues for wave localization and transport exploring higher dimensional topologies. With the growth of additive manufacturing techniques, the presented framework of minimal surfaces provides remarkable design freedom to explore a variety of symmetries in 1D, 2D, and 3D domains, enabling a variety of other wave physics and topological phenomena to be explored.

Phased arrays have been a cornerstone of non-destructive evaluation, structural health monitoring and medical imaging for years due to their unique beam steering and focusing capabilities. Despite the recent advances in parallel beamforming and nonlinear imaging, such arrays are bounded by reciprocal symmetry which significantly limits the scope of their operation and applicability. Unlike band gap structures where nonreciprocity is often associated with a unidirectional diode-like behavior, a breakage of reciprocity in phased arrays manifests itself in the form different and independently tunable wave transmission (TX) and reception (RX) patterns. In this work, we present a combined analytical and experimental realization of an elastic phased array which operates within multiple frequency channels and is capable of simultaneous steering of multiple beams. To achieve this, we devise a class of phase shifters which augment a dynamic phase modulation on top of the conventional static phase gradient along the array transducers. As a result, the emergent array exhibits non-identical TX and RX profiles. The system’s performance is fully demonstrated via scanner laser vibrometer measurements of the displacement field and confirms the array’s ability to guide incident waves within frequency channels which are commensurate with the modulation rate and along the intended directional channels.

Topological edge state phenomena in acoustics and ultrasonics have been widely studied and are treated as an active area of research in the field of physics. Design methodologies capable of synthesizing materials and structures having better control on the energy propagation to attain topological effects can be applied to countless real-world applications. And phononic crystals, capable of such fascinating phenomena, can be utilized better. Another major topic that may help to strengthen pre-modeled wave-guiding can be Dirac cone and Dirac-like cones. Here, we investigate different topological features and states of acoustic energy inside Phononic Crystals (PnC), utilizing Dirac-like phenomena. Moreover, the formation of dual Dirac-like cones at the center of the Brillouin zone, at different frequencies, has been reported here. Generation of multiple Dirac-like cones at the center and the edge of a Brillouin zone, which is rare and, usually, non-manipulative is demonstrated in this article. By deploying variable angular position of the square PVC resonator as a unit cell in a PnC system, the locations of the degenerated double Dirac cones have been manipulated at various frequency points. A gradual change in dispersion behavior, as well as striking topological acoustic phenomena, have been demonstrated. The proposed exploitation of the metamaterial will have important applications in acoustic computing, ultrasonic imaging, and waveguiding.

An exceptional point (EP) is a branch singularity where eigen-modes coalesce. Using a discrete metamaterial model, this work studies the eigenfrequency band structure and the scattering response in the vicinity of an EP. Specific phenomena associated with EPs in the eigenfrequency band structure, including level repulsion, mode switching, and self-orthogonality are presented. The effects of reciprocity and fundamental symmetries are addressed in the 1D scattering analysis. By enabling complex stiffness in frequency domain, a MM specimen may be tuned to become completely undetectable from both directions at the EP frequency, thus having potential for novel wave filtering and cloaking applications.

This work introduces the source localization application using a phononic crystal (PC) array. The PC band structure and the eigen-modes are analyzed and utilized for detecting the angle of arrival. The eigen-modes, as the basis functions of the scattering wave, possess strong angle-dependent features, naturally suitable for developing source localization algorithms. An artificial neural network is trained with randomly weighted eigen-modes to achieve deep learning of the modal features and angle dependence. The trained neural network can then accurately identify the incident angle of an unknown scattering signal, with minimal side lobe levels and suppressed main lobe width.

Piezoelectric composites play an important role in the field of sensors. However, composites with randomly dispersed fillers are not efficient, especially when compared to electroactive polymers. Via dielectrophoresis technique we developed composites with fillers, spherical and fiber-shaped, arranged in columnar structures through the thickness of the material. The chosen matrix is thermosetting polydimethylsiloxane, known for its flexibility and biocompatibility, while the fillers are NaNbO3 and BaTiO3. The designed sensors are extremely efficient and comparable to electroactive polymers, and are suitable to be integrated in medical devices, such as smart catheters and bio-scaffolds.

Ultrasonics is a popular technique for active monitoring of structural materials and systems. A wide range of methods exist for ultrasonic testing, but all of them are affected by attenuation in inspected material. The focus of this study is to demonstrate that similar ultrasonic hardware can be used for both temporal and spectral attenuation studies. Attenuation experiments conducted in 1D beams and 2D plates samples demonstrated the validity of temporal and spectral approaches utilizing the same hardware. Recommendations are provided on the applications in which these approaches are more suitable to use.

Collimated beams have attained substantial attention for the past few decades. It mentions to a focused beam that propagates in a medium with little or no angular spread. The generation of such diffraction-free propagation-invariant solutions to the Helmholtz equation was pointed out earlier, demonstrating beams with spatial and temporal invariance traveling over significant distances with no or minimal diffraction. Apart from two unique properties of large non-diffracting range and self-healing/self-reconstruction ability, Bessel beams have shown great prospects in numerous applications like atom guiding, optical tweezers, laser ablation, and laser machining. Alike optical Bessel beams, an acoustic analog named acoustic Bessel beams has also projected significant attention for the past decade. However, Bessel beams in acoustics are still not so broadly applied as in optics, which is related to the lack of convenient techniques of formation of such acoustic waves. Here, we propose a successful generation of a zeroth-order acoustic quasi-Bessel beam using an acoustic axicon. In this article, diffraction-free, self-healing, and scattering resilience properties of a propagating zero-order acoustic Bessel beam under different material properties have been generated and shown by the proposed modified acoustic axicon. After elucidating and validating the Bessel beam generation for different materials, we approach towards achieving the best possible Bessel beam comprising higher amplitude and longer depth-of-field. We portray that our propagating acoustic field preserves an invariant Bessel profile across the transverse XY plane for several hundreds of acoustic wavelengths and provides the analogous self-reconstructions capability manifested by the optical counterpart.