We propose a reconfigurable elastic metasurface composed of an array of zigzag-base folded sheets with parallel corrugations to control the wavefront of the refracted flexural wave. The desired phase gradient is achieved by tailoring the folding angle of sheets and the thickness of each metasurface unit. By exploiting the dynamic properties of zigzag-base folded sheets, the reconfigurable metasurface can achieve different wavefront functions depending on the folding angle. We present a wave-focusing metasurface that can localize the flexural wave in the transmitted region at different values of the folding angle. In addition, the refracted wave can be steered in different directions by only folding the metasurface.

We proposed a physics-guided machine-learning based inverse design approach for realizing multifunctional wave control in active metabeams connecting with negative capacitances. The transfer matrix method which relates the wave field and its derivative to carry the wave propagation information will be embedded in the ML network to construct the mapping between the input and output responses of the unit cell. After this network is well trained, global wave propagation behavior in the active metabeam can be accurately described by the concatenation of networks of each unit cells into a global stiffness matrix. We further apply the proposed network as a surrogate model for genetic algorithm on the inverse design of the metabeam for multifunctional wave control. Our proposed approach can not only be easily extended to design other types of active/passive metamaterials, but also provides some insights into optimization aided engineering in high-dimensional design space of metamaterials.

This conference presentation was prepared for the Active and Passive Smart Structures and Integrated Systems XVII conference at SPIE Smart Structures + Nondestructive Evaluation, 2023

In this work, we present a new methodology to design a phononic crystal flat lens for super-resolution imaging. We exploit the anisotropy of equal frequency contours (EFCs) of a square lattice for broadband subwavelength imaging of flexural elastic waves in an aluminum plate. We further demonstrate that the image resolution can be significantly improved by breaking the diffraction limit, which is 0.5 times the wavelength. The plate-EFCs are designed to be smaller than the PC-EFCs so that the evanescent waves in the plate are amplified by the propagating modes in the PC. We obtained a super-resolution of 0.4 times the wavelength, which can be improved further with design optimization.

Ultrasound waves can propagate through an intact human skull and alter nerve activity through targeted delivery. Low-intensity focused ultrasound (LIFU) has shown great promise for the modulation of brain function and reversal of neurological and psychiatric dysfunction. In this work, acoustic holographic lenses are designed using time-reversal and phase conjugation techniques to compensate for skull aberrations as well as pattern the ultrasonic field to target precise locations in the brain. We verify our work using numerical simulations and submerged experiments using a 3D printed skull phantom. Multiphysics simulations were also implemented to study the effects of elastic wave propagation, i.e. shear effects and attenuation of the skull.

This conference presentation was prepared for the Active and Passive Smart Structures and Integrated Systems XVII conference at SPIE Smart Structures + Nondestructive Evaluation, 2023

This conference presentation was prepared for the Active and Passive Smart Structures and Integrated Systems XVII conference at SPIE Smart Structures + Nondestructive Evaluation, 2023

We investigate theoretically and experimentally a nonlocal micropolar metabeam shunted with piezoelectric elements modulated by electronic feed-forward control. Since the nonlocal feed-forward control breaks inversion symmetry, the proposed metabeam supports nonreciprocal flexural wave propagation, featuring unidirectional amplification/attenuation and non-Hermitian skin effect. The unidirectional wave propagation is attributed to the energy conversion between electronic and mechanical parts. Also, the nonlocal metabeam is capable of realizing mechanical roton-like dispersion whose reciprocity can be broken by the programmable feed-forward control. The nonlocal micropolar metabeam could pave the ways for designing non-Hermitian topological mechanical insulators and metamaterials.

This conference presentation was prepared for the Active and Passive Smart Structures and Integrated Systems XVII conference at SPIE Smart Structures + Nondestructive Evaluation, 2023

Topological pumping provides a robust way to transport waves against disorders and defects. Surface wave is a key object studied in seismology and is widely used in electronic devices such as filters, oscillators, and transformers. Here, we extend the topological pumping to the surface wave with synthetic dimension by attaching topologically modulated elastic pillars. To guarantee the eigenmodes are a surface mode, we design the system whose eigenmodes locate below the sound cone. Then, we use the tight-binding model to simplify the problem to discrete matrix equations. After that, we solve the equations by using WKB approximation and obtain the adiabatic theorem. By setting the instantaneous eigenmode from right edge mode to bulk mode to left edge mode, we demonstrate the topological pumping numerically and experimentally. Finally, we discuss the immunization of the topological pumping from disorders and defects.

This talk will investigate the dispersion properties of piezoelectric metamaterials with non-local circuit interactions. A general lumped-element dispersion modeling framework is developed and applied to piezoelectric media with non-local circuit coupling. While the contribution from the underlying mechanical lattice is always Hermitian, it is straightforward to make the overall system non-Hermitian using unbalanced or directional electrical interactions. In this way, piezoelectric metamaterials are a natural venue to explore non-Hermitian acoustic phenomena. Case studies are presented for various non-local circuit interactions and electrical lattices, highlighting non-reciprocal acoustic propagation, topological properties, and system stability.

Inspired by the locomotion exhibited by snakes, elephant’s trunks or octopus tentacles, continuum robots can follow complex paths even in confined spaces. They stand out due to their high miniaturization potential, especially using SMA wires as actuator elements which allow for reducing the size of the driving unit drastically. To be able to not only bend in a constant curvature, but to recreate more complex shapes, like an S-shape, it is necessary to stack multiple independently controllable modules onto each other. In this paper, we present the electrical, mechanical as well as the actuator design process of an SMA driven continuum robot composed of two independently controllable modules. First, characterization measurements as well as an outlook on the future work conclude the paper.

Focused ultrasound (FU) is an emerging non-invasive and non-ionizing therapeutic technology that has the potential to treat a variety of medical conditions. FU can heat up, destroy, or change target tissue while minimizing damage to tissue outside the target area by precisely focusing on ultrasonic beams. Understanding and characterizing high-intensity FU is essential for planning and administering therapeutic procedures safely. Modeling nonlinear ultrasound is a computationally demanding problem. The complex diffraction structure requires the use of accurate diffraction models and a fine spatial numerical grid. In this work, we will propose an accelerated and more efficient strategy in which the nonlinearity in the simulation is monitored, and a corresponding increase in the resolution required to resolve higher harmonics is actively implemented.

This conference presentation was prepared for the Active and Passive Smart Structures and Integrated Systems XVII conference at SPIE Smart Structures + Nondestructive Evaluation, 2023

A piezoelectric energy harvester (PEH) consists of an electromechanical structure and an electric circuit that influence each other. An implicit coupling between a finite element simulation of the electromechanical structure and an electronic circuit simulator, recently introduced by the authors, is applied to the analysis of a PEH. The PEH is equipped with a self-powered electric circuit and the nonlinear and non-ideal behavior of its transistors and diodes and the nonlinear material behavior of the electromechanical structure are considered in the simulations. Harmonic and shock-like base excitations are applied and the behavior of the PEH is carefully analyzed.

Publisher's Note: This conference presentation, originally published on 28 April 2023, was withdrawn on 10 April 2024 per author request.

Non-Hermitian systems have brought great attention to researchers on wave manipulation by introducing losses. Here, we introduce an LR-shunted resonator for the observation of an exceptional point (EP) via tuning the shunting resistance. Through theoretical, numerical, and experimental studies, we demonstrate that by changing the circuit parameters, such as inductance and resistance, such a simple design supports the non-Hermitian degeneracy, namely the exceptional point. Moreover, near the EP, the mechanical resonance splitting has a square-root dependence on the resistance variation. We further numerically and experimentally demonstrate programmable perfect flexural wave absorption at the low-frequency range using this LR-shunted resonator. The absorption spectrum could be further enhanced with an additional shunted negative capacitor. Our approach provides alternative solutions for nondestructive structural health monitoring with enhanced sensitivity and perfect wave absorption.

Controlling and steering elastic waves through isotropic material slabs is of particular interest in engineering applications. This can be achieved by using acoustic holograms. Elastic wave manipulation through solid structures remains a challenge for locally charging sensors or devices. Unlike fluid mediums, wave propagation through solid structures involves the propagation of longitudinal and shear waves that need to be accounted for when designing the hologram. Numerical simulations and experiments are conducted to verify our approach and to show the capability of holograms to pattern elastic waves. This work was supported by the U.S. National Science Foundation (NSF) under CAREER grant No. CMMI 2143788, which is gratefully acknowledged.

Through-wall ultrasound power transfer (TWUPT) systems have gained interest in many engineering applications thanks to their superior efficiency and wireless connection. In this paper, we propose the use of non-uniform electrode configurations attached to the face of the piezoelectric transducer and receiver for selective mode excitations. A finite element model is created to simulate the TWUPT system and predict the best electrode patterns. For a wide range of vibration modes, the simulation results are compared to their experimental counterparts. This work was supported by the U.S. National Science Foundation (NSF) under grant No. ECCS 1711139 and CAREER No. CMMI 2143788, which are gratefully acknowledged.

Wheelchair headrests generally provide support with a fixed position or a single lateral rotation with manually adjustable paddings and forehead straps. The lack of functionality in headrests limits the user from performing daily activities and causes discomfort and fatigue. This paper presents the design of a tendon constrained inflatable for wheelchair headrest interface that demonstrates tailorable rigid load-bearing thresholds for accommodating different users, compact stow for easy transfer from a wheelchair and for daily activities, and scalable thresholds with respect to pressure for temporary high stabilization of the user’s head when fatigued or against vibrations/motions of a vehicle.