Şerife Tol

This work presents the absolute electromechanical characterization of a novel MEMS-based multifrequency piezoelectric sensor array using a non-contact Laser Doppler Vibrometer (LDV). In receive mode, the sensor's transfer function and absolute sensitivity were determined by correlating its electrical output to a precisely measured incident mechanical wavefield. In transmit mode, the array was electrically driven, and the resulting surface velocity was directly measured to quantify its actuation performance, achieving sensitivity up to 2.50 (m/s)/V. The results validate the sensor's out-of-plane, multi-resonant operational principle and establish its broadband performance, including its performance at off-resonance frequencies. This study demonstrates a robust methodology for the quantitative calibration of MEMS acoustic sensors.

Additive manufacturing plays an essential role in fabricating complex geometries and provides unlimited potential. While some defects may occur during any manufacturing technology, the frequency and the size of the defects produced in metal AM can negatively influence the integrity of load-bearing components. Since this manufacturing method inherently introduces defects such as pores, inclusions, and unfused powder particles, inspection methods must achieve resolutions tailored to the target usage and application. In this study, 3D printed metal calibration samples for evaluating bulk wave ultrasonics were developed. Two methodologies were used: (a) printing a fully dense stainless-steel plate and subsequently introducing defects as small as 150μm, and (b) printing with embedded, subsurface defects as small as 100μm at varying depths. Samples were manufactured with simulated voids by selectively leaving regions of unmelted powder within the print representative of common additive defects, such as lack of fusion porosity. Before primary ultrasonic testing, we evaluated how surface roughness, from an unpolished surface to a polished surface at a range of grit sizes, affects the reflectivity of ultrasonic signals. Samples were prepared accordingly and scanned using immersion ultrasonics with a 10MHz transducer, clarifying the optimal surface preparation for further tests. Results from the ultrasonic test show that the subsurface features of the 3D-printed part microstructure act as a scattering surface for frequencies above 10MHz. A resolution of 10MHz with a 259μm spot size enabled the detection of artificially introduced defects down to 150μm. These findings show the detection limits of ultrasonic testing for qualifying additive-manufactured metal components.

Acoustic metamaterials are a class of architected materials with dynamic properties that are designed at the sub-wavelength scale to achieve exotic or unique macroscopic response. Although early concepts of acoustic metamaterials relied on static configurations, recent research has further expanded the limits of acoustic customization by incorporating active or tunable responses. This article provides an introduction to the special issues of The Journal of the Acoustical Society of America and JASA Express Letters on active and tunable acoustic metamaterials and begins with a brief description of the general categories of active control and tunable response included in the contributions to the special issue and, then, provides a brief description of the articles in this special issue, grouped by general category, and how the research presented in these works contribute to the advancement of acoustic metamaterial research.

Willis metamaterials are periodic structures with engineered asymmetries that exhibit Willis coupling, a non-classical constitutive behavior linking stress to velocity and momentum to strain. This coupling gives rise to bianisotropy, a property enabling directional dependence of the reflected wave characteristics, which allows for asymmetric wave control and expanded design flexibility toward compact and tunable systems. In this work, we design, fabricate, and experimentally validate a viscoelastic metasurface that exhibits bianisotropic behavior resulting in direction-dependent wavefront shaping. The metasurface achieves arbitrary wave tailoring in the reflected field for waves incident in a particular direction. This bianisotropic behavior is demonstrated through a dual-focus metasurface, which focuses energy in the transmitted field independent of direction of incidence, and in the reflected field only for incidence in a particular direction. To effectively reduce the size of the large design parameter space, we introduce a digital coding-based design strategy for the bianisotropic metasurface. A 2-bit digitally coded version of the metasurface, utilizing only four discrete unit cell states, is shown to achieve focusing performance comparable to that of a conventional gradient metasurface. Experimental validation confirms strong qualitative agreement with numerical simulations. The proposed digitally coded metasurface architecture offers a promising platform for broadband, directionally selective wavefront control, with potential applications in energy harvesting, signal processing, and beyond.

Abstract In this paper, we present a hierarchical electromechanical metastructure (HEM) with piezoelectric shunts for tunable wave propagation control. By adjusting the electrical impedance in the shunt circuit with negative capacitance and resistance (NC-R), we tailor the transmission and reflection ratios of the HEM. The hierarchical architecture enhances wave control, resulting in broader band gaps, increased wave attenuation, and substantial asymmetric reflection. We develop fully coupled electromechanical models in transfer matrix form to analyze dispersion relations and wave propagation behavior across different hierarchical levels. Our analytical and numerical results demonstrate that the hierarchical arrangement expands the band gap and increases the maximum wave attenuation constant by more than threefold compared to a non-hierarchical design. Additionally, an HEM with graded NC-R shunts enables asymmetric reflection while maintaining strong wave attenuation performance. By leveraging hierarchical architectures, the proposed HEM achieves lightweight, tunable wave control with fewer piezoelectric materials and enhanced design flexibility, eliminating the need for structural modifications. This concept can be seamlessly integrated into mechanical and aerospace systems, enabling programmable wave manipulation for advanced applications.

Abstract This study leverages Willis asymmetry and the existence of a special condition called unidirectional zero reflection, where the scattering in the reflected field is dependent on the direction of the incident propagating wave and becomes zero in one direction if there is appropriate loss present in the system. It is shown that the system changes behavior rapidly near this point in the parameter space. This behavior is utilized to develop a defect sensor that shows very high sensitivity to small perturbations. The highly sensitive behavior is shown to be realizable in multiple physical domains through the development of concepts using longitudinal waves in the acoustic domain and flexural waves in the solid elastic domain. Further, comparison with the commonly used resonance-shift defect sensor shows much higher sensitivity for small perturbations. The proposed sensing mechanism exhibits high sensitivity and a relatively simple sensing mechanism — based on the scattering of waves by the designed inclusion — and therefore shows good potential for an alternative dynamics-based sensing method for applications requiring the detection of small variations.

Topological mechanical metamaterials offer platforms to control the propagation of mechanical waves, but are challenging to integrate into physical systems, because of their complex fabrication requirements. This study uses theoretical arguments and numerical simulations to design a self-supporting topological mechanical metamaterial that can isolate vibrations from sensitive payloads. Improving the ability to isolate vibration, however, comes at the cost of the ability to support an external load. The architecture described here shows promise for integrating topological mechanical metamaterials into engineered solutions to vibration isolation and impact mitigation.

Gradient index phononic crystal (GRIN-PC) lenses have been widely recognized for their effectiveness in focusing or localizing elastic waves at specific target locations. This wave-focusing capability enhances the energy-harvesting performance of piezoelectric transducers and improves defect detection sensitivity in non-destructive evaluation (NDE) applications. While GRIN-PC lenses have been extensively studied for planar structures, their application to curved geometries remains limited, primarily due to the lack of a comprehensive theoretical framework for understanding wave behavior in non-planar phononic crystal structures. In this work, we develop a conformal GRIN-PC theory to analyze elastic wave focusing in curved structures and propose a systematic design framework for implementing GRIN-PC lenses on non-planar surfaces. The proposed theory models wave propagation within conformal GRIN-PC lenses using ray trajectory analysis, accurately predicting the focal region. We validate this framework through numerical simulations of a conformal GRIN-PC lens applied to a steel pipe and demonstrate its accuracy in predicting focal points. Furthermore, the design framework is applied to fabricate a 3D-printed conical GRIN-PC lens, with numerical simulations and experimental results confirming its wave-focusing performance. This work establishes a foundation for expanding GRIN-PC applications to non-planar structural components widely found in mechanical, aerospace, and civil engineering structures. • Conformal gradient index phononic crystal lens theory for focusing elastic waves in curved structures. • Ray tracing in a curved gradient index phononic crystal lenses for accurate estimation of focal regions. • Design and implementation of cylindrical and conical gradient index phononic crystal lenses for multimode wave focusing. • Experimental validation of wave focusing in a 3D printed conical gradient index phononic crystal lens.

This work introduces novel structural design concepts for large aperture RF antennas that can be manufactured in space, overcoming the limitations of traditional launch loads and deployability. The spacecraft is designed to be mass-efficient, stable, and resilient with high precision. To this end, a form-finding approach is deployed to obtain the optimal design of the netband that supports the reflective mesh. The netband is tensioned and supported via eight truss spokes optimized to endow the coupled truss-netband system with the required precision, mass efficiency, and resiliency. The RF antenna design and its substructures are prototyped and statically and dynamically tested to validate the design concepts and metrics.

The precision of space structures is sensitive to high-temperature variation during on-orbit operations. This work develops zero thermal expansion (ZTE) metamaterials that can be integrated into space structures to restrict large thermal deformation under precision requirements. The proposed bi-material ZTE metamaterial beams show extremely low coefficients of thermal expansion (CTE) for a wide temperature variation. Static and dynamic analyses are conducted on the designed metamaterials for performance evaluation. The proposed ZTE beam elements are integrated into a 48-meter rib of a space RF antenna to enhance its thermal precision over a wide temperature range. Further, experimental validation of the ZTE truss beam design is conducted in a thermal chamber to demonstrate its low thermal deformation compared to a reference aluminum truss beam.

Highly precise space structures are sensitive to vibration during on-orbit operation. This work develops metamaterials that can be integrated into space structures, namely dissipative metamaterial beams for vibration mitigation. The proposed dissipative phononic crystal beam with twisted viscoelastic inclusions presents high structural damping over a large space-level temperature range. The developed novel metamaterials can be implemented in the next-generation space structures, which can be manufactured and assembled in space to enhance their spatiotemporal precision.

In elastic metamaterials, local microstructural differences lead to non-local macroscopic interactions between stress-strain and momentum-velocity, known as Willis coupling, resulting in exotic wave phenomena such as asymmetric wave reflection and unidirectional transmission. Recently, it was discovered that the structural asymmetry induces additional macroscopic cross-coupling between the electrical field and momentum in piezoelectric metamaterials, known as electro-momentum coupling. This new form of coupling offers external control over elastic wave propagation in metamaterials via external electrical stimulus. In this work, we introduce a mechanism for tuning Willis and electro-momentum couplings in 1D piezoelectric metamaterials using a resistor-inductor-capacitor (RLC) shunt circuit by simply tuning the shunt resistance and inductance. Using analytical derivations, we demonstrate a considerable variation in cross-coupling coefficients near the resonant bandgap frequencies, which is controlled by the RLC circuit resonance and electrical damping. We then showcase tunable asymmetric wave propagation in 1D piezoelectric metamaterials with resonant shunts.

This work introduces innovative design concepts for solar arrays that can be manufactured in space, overcoming the limitations of conventional launch loads and deployability. The spacecraft is designed to be mass-efficient, stable, precise, and highly resilient. To achieve this, thin plate structures with creases are integrated into the design to support the solar cells while enhancing stiffness and improving the spacecraft's resilience to damage. Numerical analysis demonstrates that creasing the plate that supports the solar cells significantly increases its stiffness with minimal increase in total mass, thereby improving the precision of the solar array. The 1 MW solar array design is optimized for mass efficiency and precision. A scaled-down model is fabricated using additive manufacturing and validated experimentally using modal analysis. The comparison between the eigenmodes and eigenfrequencies obtained from experiments and numerical simulations shows a high level of agreement.

In-space manufacturing is an advantage to the designer by liberating them from the constraints of launch loads and limitations on stowed volumes. Additionally, a structure manufactured entirely in space obviates the need for design checks based on the influence of gravity and eliminates parasitic mass resulting from deploying mechanisms. As a consequence, the primary concern for the designer shifts towards addressing accelerations during maneuvering, thermal gradients (including the effect of partial shadowing), and other vibrations during service, including those of actuators. This paper focuses on the design and optimization of a de-novo 1MW solar array structure intended for full in-space manufacturing. Specifically, a design optimization problem is introduced for a hexagonal solar array spanning 66 meters in width, comprised of a creased, thin, elastic plate on a Kagome bi-layer network. Six design variables have been identified, consisting of one discrete variable and five continuous variables. The present work proposes frameworks for global structural optimization in designing in-space manufacturable structures.

The study presented in this article examines a solar array structure that is intended to be fully manufactured and assembled in space. Consequently, the design of such a structure need not consider the launch loads, usually up to ten times Earth’s gravity. The solar array design is optimized to be highly mass-efficient and ensures high precision even under space-level acceleration loading. This paper focuses on the thermomechanical analysis of the structure and the mechanical response of the array while it goes through eclipses in orbit. The study considers large temperature differences to simulate a full eclipse and investigates the resulting von Mises stresses and displacements. The research found that the solar array’s optimized design maintains the required structural precision under extreme thermal loading. The numerical models for the thermo-mechanical behavior of the solar array were validated by experimentally testing a 3D-printed scaled-down model of a section of the array. The simulations show very good agreement with the experimental results.

Crumpling, folding and introducing creases will increase the transverse stiffness of thin, elastic plates. In this study, fundamental frequency is used as a measure of the stiffness of the plate. A comprehensive study is introduced to show the effect of ordered, pyramidal crumples on the fundamental frequencies of these plates. It is observed that by introducing nine ordered creases in a square sheet of side 6in, an increase of 124% in the fundamental frequency is achieved with only a 0.5% increase in the total mass when compared to a flat plate. A structural optimization formulation is introduced to show that, under the constraints of the problem, a unique and unordered creasing in a thin plate can be obtained that maximizes the fundamental frequency of the structure. The results show a 176% increase in frequency with only 0.84% increase in mass compared to a flat sheet.

We present a reconfigurable elastic metasurface design composed of an array of zigzag-folded sheets with parallel corrugations to control the wavefront of the refracted A 0 Lamb mode wave. The performance of this origami-inspired metasurface can be tuned by tailoring the thickness and folding angles of the sheets. Zigzag-folded sheets exhibit dynamic properties depending on their thicknesses and folding angles, yielding different phase profiles required for wavefront control via the metasurface. The transmission characteristics and phase modulation capability of the metasurface units are studied through numerical models and utilized to inform the metasurface design reconfiguring for different wave functions, such as wave focusing and deflecting at different frequencies. The design frameworks and the applicability of the reconfigurable metasurface are validated using a full-scale experimental setup. Overall, the proposed metasurface can accomplish distinct wavefront controls at adjustable geometrical parameters, developing new potentials for designing intelligent systems adaptable to different environments.

Local microstructural heterogeneities of elastic metamaterials give rise to non-local macroscopic cross coupling between stress–strain and momentum–velocity, known as Willis coupling. Recent advances have revealed that symmetry breaking in piezoelectric metamaterials introduces an additional macroscopic cross coupling effect, termed electro-momentum coupling, linking electrical stimulus and momentum and enabling the emergence of exotic wave phenomena characteristic of Willis materials. The electro-momentum coupling provides an extra degree of freedom for controlling elastic wave propagation in piezoelectric composites through external electrical stimuli. In this study, we present how to tune the electro-momentum coupling arising in 1D periodic piezoelectric metamaterials with broken inversion symmetry through shunting the inherent capacitance of the individual piezoelectric layers with a resistor and an inductor in series forming a resistor–inductor–capacitor circuit. Guided by the effective elastodynamic theory and homogenization method for piezoelectric metamaterials, we derived a closed-form expression of the electro-momentum coupling in shunted piezoelectric metamaterials. Moreover, we demonstrate the ability to tailor the electro-momentum coupling coefficient and control the amplitudes and phases of the forward and backward propagating waves, yielding tunable asymmetric wave responses. The results of our study hold promising implications for applications involving asymmetric wave phenomena and programmable metamaterials.

We numerically and experimentally demonstrate super-resolution focusing of the lowest anti-symmetric (A0) mode Lamb waves in a thin aluminum plate. The subwavelength focusing/imaging is achieved by exploiting the anisotropy in phononic crystal (PC) lattices and amplification of evanescent waves. To this end, we embedded a PC flat lens in the aluminum plate, consisting of holes arranged in a square lattice formation. We revealed that the bound slab phonon modes amplify evanescent waves, as previously observed for electromagnetic and acoustic waves. Hence, the slab mode helps propagate subwavelength information through the PC lens to reach the near-field image formed due to negative refraction and result in the high resolution image.

The gradient index phononic crystal (GRIN-PC) lens concept has been proven very effective for focusing elastic waves at a desired location. Although well-studied for planar structures, GRIN-PC lenses for elastic wave focusing in curved structures are scarce and lack the theoretical framework for studying the wave focusing mechanism. In this work, we develop conformal GRIN-PC theory to analyze wave focusing in non-planar geometries and present a design framework for conformal GRIN-PC lenses to be implemented over curved structures. The proposed conformal GRIN-PC theory studies the wave propagation in a curved GRIN-PC lens using ray trajectories that meet at the focal spot of the lens. We apply the conformal GRIN-PC theory to accurately predict the focal region of the GRIN-PC lens implemented over a steel pipe and validate the results with numerical simulations. Further, the design framework is utilized to design a 3D-printed conical GRIN-PC lens. The elastic wave focusing in the conical lens is demonstrated using numerical simulations and is further validated with experiments.

Local microstructural heterogeneities of elastic metamaterials give rise to non-local macroscopic cross-coupling between stress-strain and momentum-velocity, known as Willis coupling. Recent advances have revealed that symmetry breaking in piezoelectric metamaterials introduces an additional macroscopic cross-coupling effect, termed electromomentum coupling, linking electrical stimulus and momentum and enabling the emergence of exotic wave phenomena characteristic of Willis materials. The electro-momentum coupling provides an extra degree of freedom for controlling elastic wave propagation in piezoelectric composites through external electrical stimuli. In this study, we present how to tune the electro-momentum coupling arising in 1-D periodic piezoelectric metamaterials with broken inversion symmetry through shunting the inherent capacitance of the individual piezoelectric layers with a resistor and inductor in series forming an RLC (resistor-inductor-capacitor) circuit. Guided by the effective elastodynamic theory and homogenization method for piezoelectric metamaterials, we derived a closed-form expression of the electro-momentum coupling in shunted piezoelectric metamaterials. Moreover, we demonstrate the ability to tailor the electro-momentum coupling coefficient and control the amplitudes and phases of the forward and backward propagating waves, yielding tunable asymmetric wave responses. The results of our study hold promising implications for applications involving nonreciprocal wave phenomena and programmable metamaterials.

Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

Abstract In this paper, we design a tunable phase-modulated metasurface composed of periodically distributed piezoelectric patches with resonant-type shunt circuits. The electroelastic metasurface can control the wavefront of the lowest antisymmetric mode Lamb wave ( A 0 mode) in a small footprint due to its subwavelength features. The fully coupled electromechanical model is established to study the transmission characteristics of the metasurface unit and validated through numerical and experimental studies. Based on the analysis of the metasurface unit, we first explore the performance of electroelastic metasurface with single-resonant shunts and then extend its capability with multi-resonant shunts. By only tuning the electric loads in the shunt circuits, we utilize the proposed metasurface to accomplish wave deflection and wave focusing of A 0 mode Lamb waves at different angles and focal points, respectively. Numerical simulations show that the metasurface with single-resonant shunts can deflect the wavefront of 5 kHz and 6 kHz flexural waves by desired angles with less than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:math> deviation. In addition, it can be tuned to achieve nearly three times displacement amplification at the designed focal point for a wide range of angles from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>−</mml:mo> <mml:msup> <mml:mn>75</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> to 75 ∘ . Furthermore, with multi-resonant shunts, the piezoelectric-based metasurface can accomplish anomalous wave control over flexural waves at multiple frequencies (i.e. simultaneously at 5 kHz and 10 kHz), developing new potentials toward a broad range of engineering applications such as demultiplexing various frequency components or guiding and focusing them at different positions.