Authors

Jeseung Lee, Jooa Park, Chan Wook Park, Seung Hyun Cho, Yoon Young Kim

Title

Uni-modal retroreflection in multi-modal elastic wave fields

Journal

International Journal of Mechanical Science

Vol/No

Vol. 232

PP

107655

Year/Month

2022-10

Link

https://doi.org/10.1016/j.ijmecsci.2022.107655

Abstract

The phenomenon of retroreflection of light characterized by a single transversely polarized wave has been extensively explored. However, the phenomenon of retroreflection of a longitudinally polarized elastic wave in a solid medium is far behind the current theory because an incident longitudinal wave can be diffusively reflected into multiple wave modes, both longitudinal and transverse. Here, our experiments show that a uniquely engineered meta-corner reflector can perfectly retroreflect an incident longitudinal wave onto the same longi-tudinal wave without generating a transverse wave. The meta-corner reflector consisted of a novel sequential stack of a full longitudinal-to-transverse mode-converting layer and an angle-tuned corner reflector. We present a theoretical foundation for retroreflection and its critical application in measuring the thickness of a thin film coated on a substrate that is usually difficult to measure.

Authors

Jaeyong Kim, Gang-Won Jang, Yoon Young Kim

Title

Joint Modeling Method for Higher-order Beam-based Models of Thin-walled Frame Structures

Journal

International Journal of Mechanical Sciences

Vol/No

Vol. 220

PP

107132

Year/Month

2022-04

Link

https://www.sciencedirect.com/science/article/pii/S0020740322000649

Abstract

Higher-order sectional modes of a thin-walled beam such as distortion and warping significantly affect structural stiffness levels. Because this higher-order effect becomes even greater near the joints of a beam frame structure, finding the correct connection conditions of sectional modes at beam joints is crucial for an accurate analysis. For conventional beam elements based on the Euler-Bernoulli/Timoshenko beam theory, the joint connection conditions are obtained by component-wise matching of the force and moment vectors at the joint node. However, this simple approach is no longer valid for the joints of higher-order beam elements because warping and distortion modes have zero force/moment resultants on the beam cross-section and therefore cannot be considered through the equilibrium condition of their resultants. In this investigation, three-dimensional displacements and rotation angles are set to be continuous at the connection points on a so-called joint section, which is defined as a virtual plane shared by joining beams. We propose to comprise the connection points using the vertices of the joint section and intersection points on the joint axis and impose the continuity conditions at these points using Lagrange multipliers. The proposed joint connection conditions can be applied to a beam frame structure with general section shapes without requiring any geometry-dependent conditions as done in earlier studies. The validity of the proposed method is demonstrated by conducting static and vibration analyses of two-beam joint structures, a T-joint structure, and a vehicle frame structure.

Authors

Jeseung Lee, Minwoo Kweun, Woorim Lee, Chung Il Park, Yoon Young Kim

Title

Perfect transmission of elastic waves obliquely incident at solid-solid interfaces

Journal

Extreme Mechanics Letters

Vol/No

Vol. 51

PP

101606

Year/Month

2022-02

Link

https://www.sciencedirect.com/science/article/pii/S2352431622000025

Abstract

Perfect wave transmission across dissimilar media is critical in wave devices. However, the existing impedance matching theory allows perfect wave transmission only in special cases dealing with electromagnetic waves or acoustic waves in fluids. For the cases where elastic waves are incident at an arbitrary angle from one solid to another, even elaborately designed layers satisfying the conventional theory cannot realize perfect wave transmission. Elastic waves in isotropic solids always carry waves of both longitudinal and transverse modes and this multimodality generally couples the two wave modes at solid–solid interfaces, making the conventional impedance matching theory inapplicable for elastic waves. Here, we establish a novel theory for the perfect transmission through solid–solid interfaces and propose a unique nonresonant anisotropic single-phase metamaterial realizing the theory. The theory dictates intriguing interferences among longitudinal–shear coupled waves existing inside a metamaterial. The theory is shown to be valid for any form of wave transmissions, either mode-preserving or mode-converting. Experiments using highly impedance-mismatched elastic plates were conducted to verify the validity of the proposed theory. A further analysis confirmed transmittance enhancement over a wide range of frequencies and incidence angles. Our findings will be useful in developing new types of powerful wave devices.

Authors

Jun Hyeong Park, Joong Seok Lee, Yoon Young Kim

Title

Directional quantification of power dissipation in sound-absorbing metaporous layers

Journal

Journal of Sound and Vibration

Vol/No

Vol. 512

PP

116375

Year/Month

2021-11

Link

https://doi.org/10.1016/j.jsv.2021.116375

Abstract

Metaporous layers composed of finite-length porous layers with rigid inclusions embedded have recently received much attention given their ability to greatly enhance sound absorption. Compared with homogeneous porous layers, the insertion of rigid inclusions into porous layers can lead to a significant enhancement in the sound-absorbing performance. It is because the resulting inhomogeneity in metaporous layers generates multidirectional effects to dissipate the sound power, which may not be observed in homogeneous porous layers. This implies that directional evaluations of sound power dissipation in metaporous layers can be important for better designs that yield further enhancements of the sound-absorbing performance. In most previous works related to metaporous layers, unfortunately, quantitative studies of the directional characteristics of sound power dissipation inside of the layers are relatively rare. In this work, we investigate the directional effect of rigid inclusions on the dissipation of sound power in metaporous layers quantitatively. By spatially decomposing the dissipated power in orthogonal directions, the directional characteristics of sound power dissipation are evaluated along the thickness and lateral directions of hard-backed metaporous layers. The present directional quantification provides greater insight for those attempting to interpret the performance capabilities of metaporous layers and can be applied as well to the creation of optimal designs for these types of layers.

Authors

Jaemin Jeon, Jaeyong Kim, Jong Jun Lee, Dongil Shin, Yoon Young Kim

Title

Development of deep learning-based joint elements for thin-walled beam structures

Journal

Computers & Structures

Vol/No

Vol. 260

PP

106714-1~17

Year/Month

2022-02

Link

https://www.sciencedirect.com/science/article/pii/S0045794921002364

Abstract

This study presents a new modeling technique to estimate the stiffness matrix of a thin-walled beam-joint structure using deep learning. When thin-walled beams meet at joints, significant sectional deformations occur, such as warping and distortion. These deformations should be considered in the one-dimensional beam analysis, but it is difficult to explicitly express the coupling relationships between the beams’ deformations connected at the joint. This study constructed a deep learning-based joint model to predict the stiffness matrix of a higher-order one-dimensional super element that presents the relationships. Our proposition trains the neural network using the eigenvalues and eigenvectors of the joint's reduced stiffness matrix to satisfy the correct number of zero-strain energy modes overcoming the randomly perturbed error of the deep learning. The deep learning-based joint model produced compliance errors mostly within 2% for a given structural system and the maximum error of 4% in the worst case. The newly proposed methodology is expected to be widely applicable to structural problems requiring the stiffness of a reduction model.

Authors

Chunguang Piao, Sung Hyun Kim, Jun kyu Lee, Won Goo Choi, Yoon Young Kim

Title

Non-invasive ultrasonic inspection of sludge accumulation in a pipe

Journal

Ultrasonics

Vol/No

Vol. 119

PP

106602-1~15

Year/Month

2022-02

Link

https://www.sciencedirect.com/science/article/pii/S0041624X21002225

Abstract

Sludge accumulated inside a fluid-flowing pipe used in a chemical or semiconductor processing factory should be periodically removed to avoid flow blockage that increases undesirable pressure inside the pipe. Accordingly, it is common practice to periodically dismantle a pipe system, clean up the accumulate sludge, and reassemble. Therefore, an accurate estimation of sludge accumulation in the pipe is important to minimize the halting time of a chemical process using the system. Considering the lack of a practically efficient, non-invasive method to estimate the severity of sludge accumulation without interrupting the on-going chemical process, we propose an ultrasonic, non-invasive, real-time inspection method using a pair of ultrasonic wedge transducers installed circumferentially on the outer wall of a pipe at the same axial coordinate. To detect lowly accumulated sludge, an ultrasonic wave path from a transmitting transducer via a test pipe with accumulated sludge to a receiving transducer is carefully designed. The severity of sludge accumulation can then be determined by the amplitude of the longitudinal wave picked up by another transducer installed on the other side of the wall. We performed a series of experiments with steel and PVC pipes that are partially filled with water and sludge of different heights. The experimental results confirmed the effectiveness and practical viability of the proposed inspection method.

Authors

Chung Il Park, Chunguang Piao, Hyuk Lee, Yoon Young Kim

Title

Elastic complementary meta-layer for ultrasound penetration through solid/liquid/gas barriers

Journal

International Journal of Mechanical Sciences

Vol/No

Vol. 206

PP

106619-1~18

Year/Month

2021-09

Link

https://www.sciencedirect.com/science/article/pii/S0020740321003532

Abstract

Efficient ultrasound transmission is crucial in vast areas including noninvasive surgery, structural health monitoring, and underwater detection. However, extreme impedance contrast of interfering media spanning from solid to liquid and even gas seriously harms ultrasound transmission efficiency. Here, we propose a complementary meta-layer (CML) to “virtually cancel out” not only dissimilar solid but also liquid and gas barriers for highly enhanced transmission through the barriers in wave propagation perspective. Our proposition is apparently the first elastic CML and it is uniquely designed in a monolayer. The meta-atom forming a single meta-layer consists of spatially separated dipolar and monopolar resonators with completely independent tunability of effective negative properties. The proposed CML is capable of enhancing the ultrasound transmission by 593%, 1426%, and 3280% respectively for the polymer, water, and air barriers by numerical simulations. The proposed methodology to “cancel out” diverse barriers of highly dissimilar impedances can be groundbreaking in various ultrasound applications.

Authors

Neung Hwan Yim, Jongjun Lee, Jungho Kim, Yoon Young Kim

Title

Big data approach for the simultaneous determination of the topology and end-effector location of a planar linkage mechanism

Journal

Mechanism and Machine Theory

Vol/No

Vol. 163

PP

104375

Year/Month

2021-09

Link

https://www.sciencedirect.com/science/article/pii/S0094114X21001336?via%3Dihub

Abstract

Although significant advances have been made in mechanism synthesis to find a mechanism generating a desired motion at its end-effector, no available synthesis methods can determine the topology of a mechanism and its end-effector location simultaneously. It is generally difficult to pre-determine the location of the end-effector relative to the input drive link because the mechanism synthesis may fail if the end-effector location is incorrectly selected. Therefore, the simultaneous determination of the mechanism topology and its end-effector location can be critically useful to advance automated mechanism synthesis. Motivated by this, we propose a neural network-based big data approach to achieve the simultaneous determination. To implement a big data approach which requires a training dataset consisting of diverse mechanisms of different topologies, we propose the use of a spring-connected rigid block model as a unified means to be able to represent diverse mechanisms. The big data approach is followed by gradient-based shape optimization to determine the detailed dimensions of the mechanism synthesized by the approach. The effectiveness and validity of the proposed method are checked with various mechanism synthesis problems.

Authors

Soomin Choi,Yoon Young Kim

Title

Higher-order beam bending theory for static, free vibration, and buckling analysis of thin-walled rectangular hollow section beam

Journal

Computers & Structures

Vol/No

Vol. 248

PP

106494-1~32

Year/Month

2021-05

Link

https://www.sciencedirect.com/science/article/pii/S004579492100016X

Abstract

In higher-order beam theories, cross-sectional deformations causing complex responses of thin-walled beams are considered as additional degrees of freedom. To fully capture their bending responses, enriched sectional modes departing from Vlasov’s assumptions have been utilized in recent studies. However, due to these bending-related modes, no available higher-order beam bending theory has established explicit stress-generalized force relations that are fully consistent with those by the classical beam theories and earlier studies based on Vlasov’s assumptions. If they are available, physical significance of the bending-related generalized forces can be readily understood. In addition, equilibrium conditions at a joint of multiple thin-walled beams can be explicitly derived. Here, we newly propose a higher-order beam bending theory that not only includes as many bending-related sectional modes as desired, but also provides the desired explicit stress-generalized force relations. To this end, we establish a recursive analysis method that derives hierarchical bending-related sectional modes. We show that this method can yield certain relations among the sectional mode shapes, which are critical in establishing the desired explicit relations. The validity of the present theory is confirmed by calculating the static, free vibration, and buckling responses of several thin-walled rectangular hollow section beams.

Authors

Shin Young Kim, Woorim Lee, Joong Seok Lee, Yoon Young Kim

Title

Longitudinal wave steering using beam-type elastic metagratings

Journal

Mechanical Systems and Signal Processing

Vol/No

Vol. 156

PP

107688-1~18

Year/Month

2021-07

Link

https://www.sciencedirect.com/science/article/pii/S0888327021000832

Abstract

Metagratings have recently received much attention for effective realization of anomalous wave steering. Governed by the diffraction grating theory and the concept of metamaterials, metagratings allow to suppress superfluous propagations of high-order scattering modes in a selective manner such that wavefronts are accurately steered as designated. While numerous metagratings have been reported in electromagnetic wave research field, little has been explored in the elastic wave regime because of difficulties caused by complicatedly coupled motions among various types of elastic waves. In this circumstance, we newly propose a novel elastic metagrating model to realize anomalous steering of longitudinal waves in elastic continuum medium. Modeled by a periodic arrangement of a single or array of so-called beam-type elastic members, the present elastic metagratings resolve the difficulties in steering a specific scattered mode of elastic longitudinal waves by suppressing undesired modes in the scattered wave field. As the metagratings are attached to a boundary of elastic continuum, both longitudinal and flexural motions in the beam-type members are so coupled with elasticity wave solutions in the elastic medium as to thoroughly manipulate the reflected longitudinal waves from the interface. A set of appropriate geometric parameters of the beam-type members are searched for a direct control of the scattered modes quantitatively. As practical applications, the proposed elastic metagratings are applied to realize anomalous reflections and asymmetric splitting of longitudinal elastic waves. As well as experimental verifications, performance of the designed elastic metagratings are physically interpreted in the perspective of phase modulation.

Authors

Soomin Choi and Yoon Young Kim

Title

Higher-order Vlasov torsion theory for thin-walled box beams

Journal

International Journal of Mechanical Sciences

Vol/No

Vol. 195,

PP

106231-1~35

Year/Month

2021-04

Link

https://www.sciencedirect.com/science/article/pii/S0020740320343368

Abstract

Non-negligible sectional deformations, such as warping and distortion, occur in thin-walled beams under a twisting moment. For accurate analysis, these deformations need to be considered as additional kinematic degrees besides the degrees of freedom used in the classical St. Venant torsion theory. Vlasov pioneered to develop a higher-order beam theory for torsion that incorporates warping and distortion, but more sectional deformation modes than those considered in the Vlasov theory are needed to improve solution accuracy. Several theories were developed towards this direction, but no higher-order beam theory for torsion appears to allow explicit F-U and σ-F relations (U: kinematic variables, F: generalized forces, σ: stresses) as established by the Vlasov theory. In that the explicit relations are useful to interpret the physical significance of the generalized forces and can be critical in deriving explicit equilibrium conditions among the generalized forces at a joint of multiple thin-walled beams, a theory allowing the explicit relations needs to be developed. In this study, we newly propose a higher-order Vlasov torsion theory that not only includes as many torsion-related modes as desired but also provides the explicit F-U and σ-F relations that are fully consistent with those by the Vlasov theory. Towards this direction, we show that expressing the σ-U relation only with sectional mode shapes orthogonal to each other is critical in establishing explicit F-U and σ-F relations. We then establish new recursive relations that can be used to express each of derivatives for the sectional mode shapes involved in the σ-U relation as a linear combination of other orthogonal sectional mode shapes. In the developed theory, even stresses at off-centerline positions of the beam cross-section are explicitly related to F. The validity and accuracy of the proposed theory are confirmed by examining displacements, stresses, and eigenfrequencies for several torsion problems. The numerical results by the proposed theory are in good agreement with those by the shell analysis.

Authors

Jaeyong Kim, Soomin Choi*, Yoon Young Kim and Gang-Won Jang

Title

Hierarchical derivation of orthogonal cross-section modes for thin-walled beams with arbitrary sections

Journal

Thin-Walled Structures

Vol/No

Vol. 161

PP

107491-1~21

Year/Month

2021-04

Link

TBD

Abstract

TBD

Authors

Jeong Min Hur, Dong-Sik Seo, Kiyean Kim, Jun Kyu Lee, Kwang Je Lee, Yoon Young Kim and Do-Nyun Kim

Title

Harnessing distinct deformation modes of auxetic patterns for stiffness design of tubular structures

Journal

Materials & Design

Vol/No

Vol.198, 15

PP

109376

Year/Month

2021-01

Link

https://www.sciencedirect.com/science/article/pii/S0264127520309126

Abstract

Auxetic materials with a negative Poisson's ratio show a unique lateral expansion under tension while their deformation under shear is similar to that of normal materials. Here, we present a novel method to design the stiffness of tubular structures by exploiting these load-dependent distinct deformation modes of auxetic materials. Auxetic cutting patterns are engraved on a tube whose bending and torsional stiffness values are predicted computationally for a comprehensive set of design parameters and measured experimentally for representative designs. Results demonstrate that a wide range of bending and torsional stiffness values can be effectively achieved and controlled independently. The proposed design principle is simple and powerful, and hence can be easily extended to design the mechanical properties of non-tubular structures as well.

Authors

Sang Min Han, Yoon Young Kim

Title

Topology Optimization of Linkage Mechanisms Simultaneously Considering Both Kinematic and Compliance Characteristics

Journal

Journal of Mechanical Design

Vol/No

Vol. 143

PP

pp. 061704-1~18

Year/Month

2021-06

Link

https://asmedigitalcollection.asme.org/mechanicaldesign/article/143/6/061704/1086966

Abstract

Studies on the topology optimization of linkage mechanisms have thus far focused mainly on mechanism synthesis considering only kinematic characteristics describing a desired path or motion. Here, we propose a new topology optimization method that synthesizes a linkage mechanism considering both kinematic and compliance (K&C) characteristics simultaneously, as compliance characteristics can also significantly affect the linkage mechanism performance; compliance characteristics dictate how elastic components, such as bushings in a vehicle suspension, are deformed by external forces. To achieve our objective, we use the spring-connected rigid block model (SBM) developed earlier for mechanism synthesis considering only kinematic characteristics, but we make it suitable for the simultaneous consideration of K&C characteristics during mechanism synthesis by making its zero-length springs multifunctional. Variable stiffness springs were used to identify the mechanism kinematic configuration only, but now in the proposed approach, they serve to determine not only the mechanism kinematic configuration but also the compliance element distribution. In particular, the ground-anchoring springs used to anchor a linkage mechanism to the ground are functionalized to simulate actual bushings and to identify the desired linkage kinematic chain. After the proposed formulation and numerical implementation are presented, case studies are considered. In particular, the effectiveness of the proposed method is demonstrated with a simplified two-dimensional vehicle suspension design problem.

Authors

Suh In Kim, Dongil Shin, Sang Min Han, Seok Won Kang, Seungmin Kwon, Yong-Sub Yi & Yoon Young Kim

Title

A novel space-constrained vehicle suspension mechanism synthesized by a systematic design process employing topology optimization

Journal

Structural and Multidisciplinary Optimization

Vol/No

Vol. 62

PP

1497–1517

Year/Month

2020-07

Link

https://doi.org/10.1007/s00158-020-02669-2

Abstract

Demands on larger motors and battery packs in electric vehicles cause a suspension layout issue not appearing in gas-powered vehicles. Motivated by this need, our research aims to design a new-concept suspension applicable to electric vehicles, where given space-related constraints are satisfied without scarifying their kinematic performance. Here, we propose a three-phase design process for the synthesis of rear suspensions of an electric vehicle: concept topology design, kinematic feature identification, and detailed design. For the concept design to determine the mechanism topology, we employ the topology optimization method developed for mechanism synthesis subjected to a reduced suspension design space as well as a tighter condition on the camber rate—known as yielding better vehicle’s dynamic performance. The next phase is to extract the underlying kinematic features of the synthesized suspension obtained by the topology optimization method as it may be difficult to directly figure out how the synthesized mechanism functions kinematically. For the extraction, we propose a connectivity-mapping technique followed by the wrench calculation. This phase is followed by the final detailed design to meet the specific requirements imposed on the target suspensions. The new suspensions designed by the proposed three-phase design approach will be shown to successfully resolve the suspension layout issue typically encountered in electric vehicles.

Authors

Chunguang Piao(정밀연), Xiongwei Yang(Xi’an Jiaotong University), Joshua Minwoo Kweun(기계연), Hyunseok Kim(박형민교수랩), Hyungmin Park, Seung Hyun Cho(표준연) and Yoon Young Kim

Title

Ultrasonic flow measurement using a high-efficiency longitudinal-to-shear wave mode-converting meta-slab wedge

Journal

Sensors and Actuators A: Physical

Vol/No

Vol. 310

PP

pp. 112080-1~16

Year/Month

2020-08

Link

https://doi.org/10.1016/j.sna.2020.112080

Abstract

A non-invasive ultrasonic flowmeter transmits a shear wave into the wall of the pipe hosting the fluid. The shear wave is typically generated through a plastic wedge by the mode conversion of a longitudinal wave excited by a PZT element. Because of a big impedance mismatch between a plastic wedge and a metal pipe, however, conversion efficiency is not high. In this study, we propose a novel metal wedge equipped with an elaborately-designed mode-converting metamaterial slab and show that the designed wedge can fully transmit longitudinal-to-shear mode converted waves into a metal pipe. The design principle and mode-conversion efficiency of the meta-slab wedge and experimental results are presented. The designed meta-slab wedge was used for experiments performed with a water-flowing rectangular pipe and the flow velocity measurement sensitivity with it was increased several times compared with that based on a conventional wedge. Through this study, a metamaterial wedge is found to be a useful alternative to improve mode-converting transmission efficiency in ultrasonic applications.

Authors

Mingye Zheng(BIT, Beijing Institute of Technology), Chung Il Park, Xiaoning Liu(BIT, Beijing Institute of Technology), Rui Zhu(BIT, Beijing Institute of Technology), Gengkai Hu(BIT, Beijing Institute of Technology) and Yoon Young Kim

Title

Non-resonant metasurface for broadband elastic wave mode splitting

Journal

Applied Physics Letters

Vol/No

Vol. 116

PP

pp. 171903-1~5

Year/Month

2020-04

Link

https://aip.scitation.org/doi/10.1063/5.0005408

Abstract

Complex polarizations of elastic waves allow mode conversions between two solids, making wave mode separation extremely difficult even for a narrow frequency range with resonant metamaterials. In this Letter, a non-resonant elastic metasurface design is proposed to achieve a perfect splitting of longitudinal and shear waves in space. The key to this broadband design is a singly polarized solid with engineered static elastic tensor, which provides a tool to tune the polarization through non-resonant microstructure design. Both full-wave simulations and experiments are conducted to validate the mode splitting function of the designed metasurface. Potential applications such as elastic wave shifting and selective wave mode focusing are also demonstrated. This research provides an alternative route to design broadband and compact metasurfaces for elastic wave communications, biomedical diagnosis, and wave-based damage evaluations.

Authors

Dongil Shin, Yoon Young Kim

Title

Data-driven approach for a one-dimensional thin-walled beam analysis

Journal

Computers & Structures

Vol/No

Vol. 231

PP

Year/Month

2020-04

Link

https://www.sciencedirect.com/science/article/pii/S0045794920300109

Abstract

This study presents a data-driven approach for developing a one-dimensional thin-walled beam model. In order to analyze complicated deformations occurring in a thin-walled beam by a beam theory, it is important to identify core cross-sectional deformations that are the bases for the one-dimensional beam analysis. In this study, we derive core cross-sectional deformations through data processing using the shell-based static analysis results of a thin-walled beam. We perform a principal component analysis for the data processing, in which the desired core cross-sectional deformations are obtained without specific assumptions pertaining to the behavior of the beam’s sectional deformations. Then, the core cross-sectional deformations can be obtained in explicit functional forms (shape functions) to facilitate the subsequent one-dimensional higher-order beam analysis. Important issues related to the establishment of a well-defined dataset are addressed. Using the data-driven shape functions, a series of static, vibration and buckling analyses are performed for various thin-walled beams. We demonstrate by numerical examples that the present data-driven results agree well with those obtained by shell analysis results. While the shape functions are obtained only from the static analysis results, the vibration and buckling analyses using them were also found to be sufficiently accurate.

Authors

Soomin Choi(ERC) and Yoon Young Kim

Title

Consistent higher-order beam theory for thin-walled box beams using recursive analysis: Edge-bending deformation under doubly symmetric loads

Journal

Engineering Structures

Vol/No

Vol. 206

PP

pp. 110129

Year/Month

2020-03

Link

https://www.sciencedirect.com/science/article/pii/S0141029619334327

Abstract

Thin-walled box beams generally exhibit complex sectional deformations that are not significant in solid beams. Accordingly, a higher-order beam theory suitable for the analysis of thin-walled box beams should include degrees of freedom representing sectional deformations. In a recent study, a recursive analysis method to systematically derive sectional membrane deformations has been proposed to establish a consistent higher-order beam theory. In this study, another recursive analysis method is proposed that is suitable for the closed-form derivation of new sectional bending deformations representing the bending of edges (or walls) of the cross-section shown in a box beam under doubly symmetric loads. A consistent 1D higher-order beam theory appropriate to include these additional deformation modes as beam degrees of freedom is also established. The proposed theory provides explicit formulas that relate stresses to generalized forces including self-equilibrated forces such as bimoments. Furthermore, sectional modes are hierarchically derived so that the level of solution accuracy can be effectively and systematically controlled. Thus, the accuracy for static displacement/stress calculations and eigenfrequencies can be adjusted to be fully comparable with plate/shell results. When general doubly symmetric loads are applied to a box beam, the edge membrane modes derived in an earlier study can also be used as additional degrees of freedom besides the edge-bending modes derived in this study. The validity of the proposed beam approach is verified through the analyses of static displacements and stresses as well as eigenfrequencies for free vibration problems.

Authors

Soomin Choi and Yoon Young Kim

Title

Consistent higher-order beam theory for thin-walled box beams using recursive analysis : Membrane deformation under doubly symmetric loads

Journal

Engineering Structures

Vol/No

Vol. 197

PP

pp. 109430

Year/Month

2019-10

Link

https://www.sciencedirect.com/science/article/pii/S0141029619313422

Abstract

We propose a consistent higher-order beam theory in which cross-sectional deformations defining degrees of freedom are derived in the framework consistent with the mechanics of the proposed one-dimensional beam theory. This approach contrasts with earlier methods in which the procedure used to derive sectional deformations and the final beam theory are based on models of different levels. An advantage of the proposed consistent approach is that the generalized force-stress relation even for self-equilibrated forces such as bimoments can now be explicitly written. Also, sectional deformations can be systematically derived in closed form by the recursive and hierarchical approach. Accordingly, the accuracy in both displacement and stress can be adjusted so that obtained results are fully comparable with plate/shell results. We mainly conduct analysis of membrane deformations occurring in thin-walled box beams subjected to doubly symmetric loads such as axially-loaded forces. This case is elaborately chosen to better explain the fundamental concepts of our newly proposed approach. A brief description is also provided to show that these concepts are applicable to other types of loads such as bending and torsion. We confirm the accuracy of the theory proposed here by calculating stress and displacement in several examples.

Authors

Won Uk Yoon, Jun Hyeong Park(삼성전자), Joong Seok Lee(충남대) and Yoon Young Kim

Title

Topology optimization design for total sound absorption in porous media

Journal

Computer Methods in Applied Mechanics and Engineering

Vol/No

Vol. 360

PP

pp. 112723-1~18

Year/Month

2020-03

Link

https://doi.org/10.1016/j.cma.2019.112723

Abstract

Acoustic porous layers used for noise reduction have the unique functionality of dissipating sound energy. It is very difficult for finite length porous layers to realize total sound absorption if they are filled only with homogeneous materials. Rigid inclusions inserted in a porous medium will certainly alter the acoustic field, realizing total sound absorption at some frequencies, but finding their optimal distributions by trial and error is intractable, especially for multiple target frequencies. The aim of this study is to develop a finite element-based numerical method to distribute rigid inclusions inside a porous layer having a fixed thickness for the total sound absorption. Without any special treatment for initial layout, multiply divided rigid inclusions are distributed systematically in an optimized porous layer by the present density-based topology optimization formulation. Considering single target frequency problems, we initially assess the effectiveness of the developed formulation with regard to achieving total sound absorption. In these problems, various resonance mechanisms, such as the Helmholtz resonance and the quarter-wavelength resonances, are generated inside the porous layer by the distributed rigid inclusions. Subsequently, we solve problems considering multiple target frequencies. The numerical results show that different types of resonances occur appropriately in the optimized porous–rigid layer to achieve total sound absorption simultaneously for all the different target frequencies considered here.

Authors

Xiongwei Yang, Joshua M. Kweun, Yoon Young Kim

Title

Theory for Perfect Transmodal Fabry-Perot Interferometer

Journal

Scientific Reports

Vol/No

Vol. 8, 69

PP

Year/Month

2018-01

Link

https://www.nature.com/articles/s41598-017-18408-5

Abstract

We establish the theory for perfect transmodal Fabry-Perot interferometers that can convert longitudinal modes solely to transverse modes and vice versa, reaching up to 100% efficiency. Two exact conditions are derived for plane mechanical waves: simultaneous constructive interferences of each of two coupled orthogonal modes, and intermodal interference at the entrance and exit sides of the interferometer with specific skew polarizations. Because the multimodal interferences and specific skew motions require unique anisotropic interferometers, they are realized by metamaterials. The observed peak patterns by the transmodal interferometers are similar to those found in the single-mode Fabry-Perot resonance, but multimodality complicates the involved mechanics. We provide their design principle and experimented with a fabricated interferometer. This theory expands the classical Fabry-Perot resonance to the realm of mode-coupled waves, having profound impact on general wave manipulation. The transmodal interferometer could sever as a device to transfer wave energy freely between dissimilar modes.

Authors

Jeonghan Yu, Sang Min Han and Yoon Young Kim

Title

Simultaneous Shape and Topology Optimization of Planar Linkage Mechanisms Based on the Spring-Connected Rigid Block Model

Journal

Journal of Mechanical Design

Vol/No

Vol. 142

PP

pp. 011401-1

Year/Month

2020-01

Link

https://gasturbinespower.asmedigitalcollection.asme.org/mechanicaldesign/article/955326/Simultaneous-Shape-and-Topology-Optimization-of

Abstract

Using the topology optimization can be an effective means of synthesizing planar rigid-body linkage mechanisms to generate desired motion, as it does not require a baseline mechanism for a specific topology. While most earlier studies were mainly concerned with the formulation and implementation of topology optimization-based synthesis in a fixed grid, this study aims to realize the simultaneous shape and topology optimization of planar linkage mechanisms using a low-resolution spring-connected rigid block model. Here, we demonstrate the effectiveness of simultaneous optimization over a higher-resolution fixed-grid rigid block-based topology optimization process. When shape optimization to change the block shapes is combined with topology optimization to synthesize the mechanism, the use of low-resolution discretized models improves the computation efficiency considerably and helps to yield compact mechanisms with less complexity, making them more amenable to fabrication. After verifying the effectiveness of the simultaneous shape and topology optimization process with several benchmark problems, we apply the method to synthesize a mechanism which guides a planar version of a human's gait trajectory.

Authors

Jae Eun Kim(대구카톨릭대), Sowon Lee(현대차 연구소) and Yoon Young Kim

Title

Mathematical Model Development, Experimental Validation and Design Parameter Study of A Folded Two-Degreeof-Freedom Piezoelectric Vibration Energy Harvester

Journal

International Journal of Precision Engineering and Manufacturing-Green Technology

Vol/No

PP

pp. 1-14

Year/Month

2019-08

Link

https://link.springer.com/article/10.1007%2Fs40684-019-00149-7

Abstract

A folded two-degree-of-freedom piezoelectric vibration energy harvester has been proposed by several researchers including the authors. It is characterized by the reversal of the modal sequence, which enhances the electrical output power near a target operating frequency with very little eigenfrequency separation. However, no appropriate mathematical models have yet been developed, though a well-developed one would clarify the physical behaviors and allow efficient investigations of the effects of design parameters. In this work, a rigorous mathematical model based on the continuous Euler–Bernoulli beam theory is proposed for the first time. It is validated by both a finite element analysis and by experimental measurements. The validated mathematical model is then employed for a design parameter study with four design parameters, in this case the length of each component beam and the magnitude of each proof mass. For certain selected cases from the design parameter study sets, prototypes are fabricated and their performances are measured for verification of the design parameter study. Some useful observations are also drawn from the design parameter study for the design of a high-performance folded two-degree-of-freedom piezoelectric vibration energy harvester. Through all of the results presented in this work, it is shown that the proposed mathematical model will serve as an effective tool for the analysis and design of a folded two-degree-of-freedom piezoelectric vibration energy harvester.

Authors

Byungseong Ahn, Hyuk Lee(기계연), Joong Seok Lee(충남대) and Yoon Young Kim

Title

Topology optimization of metasurfaces for anomalous reflection of longitudinal elastic waves

Journal

Computer Methods in Applied Mechanics and Engineering

Vol/No

Vol. 357

PP

pp. 112582

Year/Month

2019-12

Link

https://www.sciencedirect.com/science/article/pii/S0045782519304530

Abstract

A numerical method based on topology optimization is developed for designing a metasurface that anomalously reflects longitudinal elastic waves. While the analysis and design of metasurfaces and metamaterials have received much attention recently, no numerical method to design elastic metasurfaces for anomalous reflection has been explored. Here, we formulate a density-based topology optimization suitable for finding a set of phase-delaying unit cells forming a metasurface that anomalously reflects longitudinal waves impinged upon it. Each unit cell is designed to yield the specific phase delay between the incident and reflected elastic waves. The transfer matrix approach using the averaged finite element results is employed to efficiently and accurately calculate the phase delay while the wave phenomena are analyzed by the finite element analysis. As case studies, metasurfaces converting normally incident longitudinal waves to anomalously reflected longitudinal waves were designed. Various target anomalous reflection angles were considered including the extreme reflection angle of 90°. To investigate the numerical aspect of the developed method, we investigated the effect of the sizes of the unit cells, both along the perpendicular and normal directions to the wave incidence. The mesh dependence issue was also investigated. Finally, we applied the developed topology optimization method to design metasurfaces for wave focusing and trapping in a waveguide.

Authors

Xiongwei Yang(Xi’an Jiaotong University), Minwoo Kweun, and Yoon Young Kim

Title

Monolayer Metamaterial for Full Mode-Converting Transmission of Elastic Waves

Journal

Applied Physics Letters

Vol/No

Vol. 115, No. 7

PP

pp. 071901-1~5

Year/Month

2019-08

Link

https://aip.scitation.org/doi/10.1063/1.5109758

Abstract

For actual realization of full-power longitudinal-to-transverse (transverse-to-longitudinal) mode-converting transmission, we propose a single-phase “monolayer” anisotropic metamaterial that is easy to fabricate with a dimension of 4–5 times the target wavelength. Full-power mode conversion can be critically useful in ultrasonic applications, but this has not been experimentally validated because a metamaterial consisting of subwavelength unit cells is difficult to fabricate. Here, we achieve full-power mode-converting transmission with a monolayer, and its effective material properties should satisfy the required theoretical conditions. While an S-parameter based method can be employed to retrieve the effective material properties, an elaborate method should be developed to estimate its effective size because the monolayer metamaterial directly interfaces with the background medium. To this end, we propose a T-matrix based method to estimate its size. Finally, ultrasonic experiments performed with the designed monolayer confirmed the realization of nearly full-power mode conversion.

Authors

Chung Il Park, Hong Min Seung(표준연), Yoon Young Kim

Title

Bi-annular shear-horizontal wave MPT tailored to generate SH1 mode in a plate

Journal

Ultrasonics

Vol/No

Vol. 99

PP

pp. 105958-1~7

Year/Month

2019-11

Link

https://www.sciencedirect.com/science/article/pii/S0041624X19302586

Abstract

The use of a specific wave mode is critical in ultrasonic non-destructive evaluations but it is difficult to generate a specific mode, especially a higher mode at a frequency where there exist multiple wave modes. Here, we propose a compact omnidirectional shear-horizontal wave MPT (magnetostrictive patch transducer) having two annular magnetostrictive patches for the generation of a nearly pure SH1 (second shear-horizontal) mode in a plate for frequencies above the first cutoff frequency. While a common wavelength-matching approach would typically require the use of several patches and does not appear completely to eliminate unwanted omnidirectional wave modes, the proposed MPT, with only two annular patches, generates the desired SH1 mode predominantly with the unwanted SH0 (first shear-horizontal) mode nearly eliminated. For the design, the geometries of the annular patches are optimally configured to maximize the ratio of the SH1 mode to the SH0 mode. Numerical simulations and experiments confirm the effectiveness of the proposed bi-annular shear-horizontal wave MPT. Because the SH1 mode near the first cutoff frequency is highly dispersive, the developed transducer is expected to be critically useful in various applications, such as ultrasonic inspections of wall thinning.

Authors

Kiyean Kim, Hyung Jin Lee(표준연), Chung Il Park, Hyuk Lee(기계연) and Yoon Young Kim

Title

Enhanced transduction of MPT for antisymmetric Lamb waves using a detuned resonator

Journal

Smart Materials and Structures

Vol/No

Vol. 28

PP

pp. 075035-1~11

Year/Month

2019-07

Link

https://iopscience.iop.org/article/10.1088/1361-665X/ab20fc

Abstract

We propose a novel method to boost the power of a magnetostrictive patch transducer (MPT) for generating antisymmetric Lamb wave modes in a plate. Because an MPT installed on a plate mainly exerts in-plane deformation onto a test specimen, it cannot efficiently generate antisymmetric wave modes such as the lowest antisymmetric Lamb wave (A 0) mode, unlike the generation of symmetric wave modes. To achieve the high-power generation of antisymmetric modes, we propose an add-on passive boosting element which is referred to here as a detuned resonator. The resulting MPT can serve both as an actuator and a sensor. Here, we present the specific boosting resonator configuration, explain its operating mechanism, and verify its role in increasing the transducer power. A theoretical analysis and experimental validation are carried out.

Authors

Chung Il Park, Hong Min Seung(표준연), Jun Kyu Lee and Yoon Young Kim

Title

Analysis and design of an annular-array MPT for the efficient generation of omnidirectional shear-horizontal waves in plates

Journal

Smart Materials and Structures

Vol/No

Vol. 28

PP

pp. 075005-1~17

Year/Month

2019-07

Link

https://iopscience.iop.org/article/10.1088/1361-665X/ab1adb

Abstract

Despite the fact that omnidirectional shear-horizontal (SH) waves have recently received much attention, there is no systematic design method with which to develop the corresponding transducers with enhanced output and power concentration levels at a target frequency. Here, we propose a method which can be used to design an optimal annular-array magnetostrictive patch transducer (MPT) that fulfills the above-mentioned objectives. The proposed transducer consists of optimally configured multiple magnetostrictive patches, consistently placed permanent magnets, and toroid coils. For the design, the transducer output signals are theoretically predicted using Green's function of guided waves in a plate and the analysis results are then used to develop new high-power annular-array MPTs generating omnidirectional SH waves in a plate. The validity of the analysis and the effectiveness of the design method were confirmed with numerical simulations and pitch-catch wave experiments performed on an aluminum plate. In the experiments, the mode selectivity and omnidirectivity of the designed transducer were also checked. While the proposed method is mainly developed for MPTs, it should be equally useful for the design of similar annular-type piezoelectric and electromagnetic acoustic guided wave transducers.

Authors

Do-Min Kim, Soomin Choi, Gang-Won Jang, and Yoon Young Kim

Title

Buckling analysis of thin-walled box beams under arbitrary loads with general boundary conditions using higher-order beam theory

Journal

Journal of Mechanical Science and Technology (JMST)

Vol/No

Vol. 33 , No.5

PP

pp 2289–2305

Year/Month

2019-05

Link

https://link.springer.com/article/10.1007/s12206-019-0430-y

Abstract

When a higher-order or generalized beam theory is used for the buckling analysis of thin-walled beams, the analysis accuracy critically depends on the number and shapes of the cross-sectional modes associated with warping and distortion. In the study, we propose to use the hierarchically-derived cross-sectional modes consistent with the higher-order beam theory for the analysis of pre-buckling stress and buckling load. The proposed formulation is applicable to any box beams subjected to arbitrary loads and general boundary conditions. We demonstrate the effectiveness of the proposed method by performing buckling analyses for axial, bending, torsional, and general loadings. Length-to-height ratios of the beams are also varied from 1 to 100. If up to fifty cross-sectional and rigid-body modes are employed, the calculated buckling loads are found to match favorably those predicted by the shell finite element analysis. In that a unified buckling analysis under general loads is developed for box beams, the present study is expected to contribute towards new possibilities for the efficient buckling analysis of more general box beam structures involving several joints.

Authors

Neung Hwan Yim, Seok Won Kang and Yoon Young Kim

Title

Topology Optimization of Planar Gear-Linkage Mechanisms

Journal

Journal of Mechanical Design

Vol/No

141

PP

Year/Month

2019-03

Link

http://mechanicaldesign.asmedigitalcollection.asme.org/article.aspx?articleid=2718463

Abstract

Topology optimization for mechanism synthesis has been developed for the simultaneous determination of the number and dimension of mechanisms. However, these methods can be used to synthesize linkage mechanisms that consist only of links and joints because other types of mechanical elements such as gears cannot be simultaneously synthesized. In this study, we aim to develop a gradient-based topology optimization method which can be used to synthesize mechanisms consisting of both linkages and gears. For the synthesis, we propose a new ground model defined by two superposed design spaces: the linkage and gear design spaces. The gear design space is discretized by newly proposed gear blocks, each of which is assumed to rotate as an output gear, while the linkage design space is discretized by zero-length-spring-connected rigid blocks. Another set of zero-length springs is introduced to connect gear blocks to rigid blocks, and their stiffness values are varied to determine the existence of gears when they are necessary to produce the desired path. After the proposed topology-optimization-based synthesis formulation and its numerical implementation are presented, its effectiveness and validity are checked with various synthesis examples involving gear-linkage and linkage-only mechanisms.

Authors

Kiyean Kim, Chung Il Park, Hyuk Lee and Yoon Young Kim

Title

Near-Zero Effective Impedance with Finite Phase Velocity for Sensing and Actuation Enhancement by Resonator Pairing

Journal

Nature Communications

Vol/No

Vol. 9, Article number: 5255

PP

Year/Month

2018-12

Link

https://doi.org/10.1038/s41467-018-07697-7

Abstract

In spite of the extensive studies of zero-index metamaterials, the realization of zero impedance with finite phase velocity has not been explored. Here, we show that this extreme case, realized by elaborately-tuned paired resonators, can effectively enhance sensing and actuation. To explain the formation mechanism of the near-zero effective impedance with finite phase velocity by paired resonators at a target frequency, a theory using an equivalent model based on mechanical longitudinal waves is developed. If the frequency of the extreme property is further tuned at a Fabry–Pérot resonance frequency, highly efficient enhancement is possible. Experiments using a piezoceramic transducer (PZT) installed on the plate region bounded by two resonators confirm that the proposed extreme property mechanism highly enhances the sensing and actuation outputs of the transducer.

Authors

Xiongwei Yang and Yoon Young Kim

Title

Asymptotic theory of bimodal quarter-wave impedance matching for full mode-converting transmission

Journal

Physical Review B

Vol/No

Vol.98

PP

Year/Month

2018-10

Link

https://doi.org/10.1103/PhysRevB.98.144110 ©2018 American Physical Society

Abstract

A quarter-wave impedance-matching element inserted between two dissimilar media allows full-power transmission, but this phenomenon occurs only when the mode of the incident and transmitted waves remains unaltered, either longitudinal (L) or transverse (T). Here, we report our asymptotic bimodal quarter-wave impedance-matching theory for full-power mode-converting transmission. It requires that two coupled quarter waves should be simultaneously formed inside a matching element possessing specific anisotropy for phase matching and L-to-T (T-to-L) mode conversion. Simulations using designed metamaterial matching elements show that nearly full mode-converting transmissions can be realized within a single medium or between two dissimilar media. We expect that our theory can be critically useful for medical and industrial ultrasonic applications wherever highly efficient mode conversion and high-powered transverse waves are required.

Authors

Joo Hwan Oh(UNIST), Seong Jae Choi(KAI), Jun Kyu Lee and Yoon Young Kim

Title

Zero-frequency Bragg gap by spin-harnessed metamaterial

Journal

New Journal of Physics

Vol/No

Vol. 20

PP

pp. 083035-1~20

Year/Month

2018-08

Link

https://doi.org/10.1088/1367-2630/aada38

Abstract

The Bragg gap that stops wave propagation may not be formed from zero or a very low frequency unless the periodicity of a periodic system is unrealistically large. Accordingly, the Bragg gap has been considered to be inappropriate for low frequency applications despite its broad bandwidth. Here, we report a new mechanism that allows formation of the Bragg gap starting from a nearly zero frequency. The mechanism is based on the finding that if additional spin motion is coupled with the longitudinal motion of a mass of a diatomic mechanical periodic system, the Bragg gap starting from a nearly zero frequency can be formed. The theoretical analysis shows that the effective mass and stiffness at the band gap frequencies are all positive, confirming that the formed stop band is a Bragg gap. The periodic system is realized by a spin-harnessed metamaterial which incorporates unique linkage mechanisms. The numerical and experimental validation confirmed the formation of the low-frequency Bragg gap. The zero-frequency Bragg gap is expected to open a new way to control hard-to-shield low-frequency vibration and noise.

Authors

Min Soo Kim(UNIST), Woo Rim Lee, Yoon Young Kim and Joo Hwan Oh(UNIST)

Title

Transmodal elastic metasurface for broad angle total mode conversion

Journal

Applied Physics Letters

Vol/No

Vol. 112

PP

pp. 241905 (1-5)

Year/Month

2018-06

Link

https://doi.org/10.1063/1.5032157

Abstract

It has been long believed that a total mode conversion between longitudinal and shear elastic waves can only be achieved at a certain incidence angle. Here, we show that a total mode conversion can be achieved for a broad range of incidence angles by a specially designed elastic metasurface, namely, transmodal metasurface. From the generalized reflection law, we found that the incident longitudinal wave can be totally converted to a reflected shear wave over a broad range of incidence angles if a sufficiently large phase gradient is introduced at the boundary. Numerical and experimental investigations with a specially engineered transmodal metasurface showed that the total mode conversion can be achieved for wide incidence angles from 19° to 90°, which was impossible to be achieved previously. The proposed idea of the transmodal metasurface can open up an advanced avenue for tailoring elastic wave modes as an outstanding alternative to generating shear waves.

Authors

Dongil Shin, Soomin Choi, Gang-Won Jang, Yoon Young Kim

Title

Higher-order beam theory for static and vibration analysis of composite thin-walled box beam

Journal

Composite Structures

Vol/No

206

PP

140-154

Year/Month

2018-12

Link

https://www.sciencedirect.com/science/article/pii/S0263822318305312

Abstract

A higher-order beam theory suitable for accurate analysis of composite thin-walled box beams is developed. Because anisotropic and laminate effects in composite beams produce deformation patterns that do not appear in isotropic beams, accurate analyses for composite beams require elaborately defined sectional shape functions that describe local cross-sectional deformations. Here, we present a new systematic method to define these functions and establish a one-dimensional finite element based on a higher-order beam theory for composite thin-walled box beams. The validity of the developed approach is checked by solving static and eigenvalue problems with composite thin-walled box beams, and by comparing the obtained numerical results with those obtained by ABAQUS shell elements.

Authors

Xiongwei Yang and Yoon Young Kim

Title

Topology optimization for the design of perfect mode-converting anisotropic elastic metamaterials

Journal

Composite Structures

Vol/No

Vol. 201, No.

PP

pp. 161-177

Year/Month

2018-10

Link

https://doi.org/10.1016/j.compstruct.2018.06.022

Abstract

This work is concerned with the topology optimization of anisotropic elastic metamaterials exhibiting perfect mode conversion, a newly discovered phenomenon that an incident longitudinal (transverse) mode is solely and maximally converted to a transmitted transverse (longitudinal) mode. The wave phenomenon occurs at a series of interference frequencies due to elaborate multimodal interferences, known as the perfect transmodal Fabry-Perot interferences. Because the metamaterial must satisfy unique anisotropic relations among its effective stiffness, design of its unit cell is difficult without a systematic strategy. Here, we propose a topology optimization method based on the effective material properties to design such artificial composites. The homogenization method is employed to evaluate the effective material properties and the anisotropy requirements are treated as a special form of constraints. Because there is no natural mass constraint, we propose to maximize the effective longitudinal-transverse coupling stiffness for stable convergence. The sensitivity analysis is performed analytically within the finite element framework to update the design variables. The validity and effectiveness of the developed method are verified by considering different lattice types and interference cases. Considering the wide potential applications of anisotropic metamaterials in industrial applications, the developed numerical method can be an important and critically useful design tool.

Authors

Dong Kyun Kim, Jun Kyu Lee, Hong Min Seung, Chung Il Park and Yoon Young Kim

Title

Omnidirectional shear horizontal wave based tomography for damage detection in a metallic plate with the compensation for the transfer functions of transducer

Journal

Ultrasonics

Vol/No

Vol. 88, No.

PP

pp. 72-83

Year/Month

2018-08

Link

https://doi.org/10.1016/j.ultras.2018.03.009

Abstract

Guided-wave based damage detection for plates has been widely studied for structural health monitoring. Because most of the earlier studies used dispersive Lamb waves, substantial efforts had to be made to alleviate the dispersive and multi-modal nature of Lamb waves and the effect of surface conditions. In contrast, shear-horizontal (SH) waves have better propagation characteristics suitable for the detection of damages in plates, but SH waves have not been widely used due to the lack of efficient methods to generate and sense omnidirectional SH waves. The objective of this study is to construct diagnostic images of damaged plates by using omnidirectional SH waves with a special emphasis on the compensation of the frequency-dependence of the SH wave transducers. The compensation is necessary to have reliable diagnostic images because its frequency-dependent characteristics considerably can affect imaging quality if they are not considered. Consequently, simplified, yet effective, models representing the transfer functions of the omnidirectional SH wave magnetostrictive patch transducer (OSH-MPT) are developed in this paper. To visualize the position and shape of the structural damages in a metallic plate, the virtual time-reversal imaging method is used and two alternative techniques are considered to compensate for the effects of the transfer functions of transducers in the imaging processes. The imaging results after the compensation appear quite promising, suggesting that the omnidirectional SH wave based enhanced time-reversal imaging can be an efficient inspection method for plate structures.

Authors

Seok Won Kang, Yoon Young Kim

Title

Unified topology and joint types optimization of general planar linkage mechanisms

Journal

Structural and Multidisciplinary Optimization

Vol/No

57/5

PP

1955-1983

Year/Month

2018-03

Link

https://link.springer.com/article/10.1007/s00158-017-1887-x

Abstract

We propose a gradient-based topology optimization method to synthesize a planar linkage mechanism consisting of links and revolute/prismatic joints, which converts an input motion to a desired output motion at its end effector. Earlier gradient-based topology optimization methods were mainly applicable to the synthesis of linkage mechanisms connected by revolute joints only. The proposed method simultaneously determines not only the topology of planar linkage mechanisms but also the required revolute and/or prismatic joint types. For the synthesis, the design domain is discretized into rectangular rigid blocks that are connected to each other by the newly proposed revolute and prismatic joint elements, the joint states of which vary depending on the corresponding design variables. The new concept of joint elements is materialized thorough an elaborately configured set of zero-length springs whose stiffnesses vary as the functions of the design variables. Therefore, any connectivity state among unconnected, rigidly connected, revolute joint, and prismatic joint states can be represented by properly adjusting the stiffnesses. After presenting our modeling, formulation, and sensitivity analysis, the developed method is tested with verification examples. Then the developed method is extended to be able include additional shape design variables and applied to solve a realistic problem of synthesizing a finger rehabilitation robotic device. We expect the developed method to play a critical role in synthesizing a wide class of general linkage mechanisms.

Authors

Jieun Yang, Joong Seok Lee, Hyeong Rae Lee, Yeon June Kang, and Yoon Young Kim

Title

Slow-wave metamaterial open panels for efficient reduction of low-frequency sound transmission

Journal

Applied Physics Letters

Vol/No

112/9

PP

091901

Year/Month

2018-02

Link

http://aip.scitation.org/doi/10.1063/1.5003455

Abstract

Sound transmission reduction is typically governed by the mass law, requiring thicker panels to handle lower frequencies. When open holes must be inserted in panels for heat transfer, ventilation, or other purposes, the efficient reduction of sound transmission through holey panels becomes difficult, especially in the low-frequency ranges. Here, we propose slow-wave metamaterial open panels that can dramatically lower the working frequencies of sound transmission loss. Global resonances originating from slow waves realized by multiply inserted, elaborately designed subwavelength rigid partitions between two thin holey plates contribute to sound transmission reductions at lower frequencies. Owing to the dispersive characteristics of the present metamaterial panels, local resonances that trap sound in the partitions also occur at higher frequencies, exhibiting negative effective bulk moduli and zero effective velocities. As a result, low-frequency broadened sound transmission reduction is realized efficiently in the present metamaterial panels. The theoretical model of the proposed metamaterial open panels is derived using an effective medium approach and verified by numerical and experimental investigations.

Authors

Byungseong Ahn, Hyung Jin Lee, Yoon Young Kim

Title

Topology Optimization of Anisotropic Metamaterials Tracing the Target EFC and Field Polarization

Journal

Computer Methods in Applied Mechanics and Engineering

Vol/No

333

PP

176-196

Year/Month

2018-05

Link

https://www.sciencedirect.com/science/article/pii/S0045782518300197

Abstract

Equi-frequency contours (EFCs) can be used to predict certain intrinsic wave behaviors of a metamaterial, and are therefore frequently used for analysis and design. However, information about field polarization (FP) dictating whether particle motion along an EFC is longitudinal, shear, or longitudinal–shear coupled should also be considered to accurately predict the actual wave phenomena of a metamaterial. For instance, anomalous polarization can be used to create mode converters characterized only when both EFCs and FPs are simultaneously considered. In this work, we aim to develop a topology optimization method to design anisotropic metamaterials that trace target EFCs and FPs. Earlier studies on the topology optimization method have considered EFCs or dispersion curves; however, there is no topology optimization that considers the EFC and FP simultaneously. As a means to trace the FP, we introduce a scalar measure equivalent to the modal assurance criterion and derive its sensitivity with respect to density design variables. We present various case studies to design metamaterial unit cells to trace target EFCs and FPs corresponding to anomalous polarization. Furthermore, mode converters made of the metamaterials designed by the developed topology optimization method are created and their performance in converting a longitudinal wave mode to a transverse wave mode is evaluated.

Authors

Hyuk Lee, Jun Kyu Lee, Hong Min Seung, Yoon Young Kim

Title

Mass-stiffness substructuring of an elastic metasurface for full transmission beam steering

Journal

Journal of the Mechanics and Physics of Solids

Vol/No

112

PP

577-593

Year/Month

2018-03

Link

https://www.sciencedirect.com/science/article/pii/S0022509617308475

Abstract

The metasurface concept has a significant potential due to its novel wavefront-shaping functionalities that can be critically useful for ultrasonic and solid wave-based applications. To achieve the desired functionalities, elastic metasurfaces should cover full 2π phase shift and also acquire full transmission within subwavelength scale. However, they have not been explored much with respect to the elastic regime, because the intrinsic proportionality of mass-stiffness within the continuum elastic media causes an inevitable trade-off between abrupt phase shift and sufficient transmission. Our goal is to engineer an elastic metasurface that can realize an inverse relation between (amplified) effective mass and (weakened) stiffness in order to satisfy full 2π phase shift as well as full transmission. To achieve this goal, we propose a continuum elastic metasurface unit cell that is decomposed into two substructures, namely a mass-tuning substructure with a local dipolar resonator and a stiffness-tuning substructure composed of non-resonant multiply-perforated slits. We demonstrate analytically, numerically, and experimentally that this unique substructured unit cell can satisfy the required phase shift with high transmission. The substructuring enables independent tuning of the elastic properties over a wide range of values. We use a mass-spring model of the proposed continuum unit cell to investigate the working mechanism of the proposed metasurface. With the designed metasurface consisting of substructured unit cells embedded in an aluminum plate, we demonstrate that our metasurface can successfully realize anomalous steering and focusing of in-plane longitudinal ultrasonic beams. The proposed substructuring concept is expected to provide a new principle for the design of general elastic metasurfaces that can be used to efficiently engineer arbitrary wave profiles.

Authors

Jun Kyu Lee, Hong Min Seung, Chung Il Park, Joo Kyung Lee, Do Hyeong Lim, Yoon Young Kim

Title

Magnetostrictive patch sensor system for battery-less real-time measurement of torsional vibrations of rotating shafts

Journal

Journal of Sound and Vibration

Vol/No

414

PP

245-258

Year/Month

2018-02

Link

https://www.sciencedirect.com/science/article/pii/S0022460X17307939

Abstract

Real-time uninterrupted measurement for torsional vibrations of rotating shafts is crucial for permanent health monitoring. So far, strain gauge systems with telemetry units have been used for real-time monitoring. However, they have a critical disadvantage in that shaft operations must be stopped intermittently to replace telemetry unit batteries. To find an alternative method to carry out battery-less real-time measurement for torsional vibrations of rotating shafts, a magnetostrictive patch sensor system was proposed in the present study. Since the proposed sensor does not use any powered telemetry system, no battery is needed and thus there is no need to stop rotating shafts for battery replacement. The proposed sensor consists of magnetostrictive patches and small magnets tightly bonded onto a shaft. A solenoid coil is placed around the shaft to convert magnetostrictive patch deformation by shaft torsional vibration into electric voltage output. For sensor design and characterization, investigations were performed in a laboratory on relatively small-sized stationary solid shaft. A magnetostrictive patch sensor system was then designed and installed on a large rotating propulsion shaft of an LPG carrier ship in operation. Vibration signals were measured using the proposed sensor system and compared to those measured with a telemetry unit-equipped strain gauge system.

Authors

Joo Hwan Oh, Shuibao Qi, Yoon Young Kim, Badreddine Assouar

Title

Elastic Metamaterial Insulator for Broadband Low-Frequency Flexural Vibration Shielding

Journal

Physical Review Applied

Vol/No

8

PP

054034-1~11

Year/Month

2017-11

Link

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.054034

Abstract

Achieving stop band over broadband at low-frequency range has remained a great scientific challenge in spite of various efforts made using metamaterials or other technologies. In this work, we propose an idea that creates a stop band for broadband at low-frequency range. The dual mechanism of shear stiffening and rotation softening is initiated here to achieve a broad stop band at low-frequency range. Through analytical, numerical, and experimental studies, we reveal the underlying physical mechanism and confirm the effectiveness of this metamaterial on vibration shielding for flexural elastic wave covering 235 to 4520 Hz. This work opens an avenue for the development of elastic metamaterials with performance and functionalities that are highly desirable in many fields such as vibration shielding.

Authors

Hyung Jin Lee, Je-Ryung Lee, Seung Hwan Moon, Tae-Jin Je, Eun-chae Jeon, Kiyean Kim, Yoon Young Kim

Title

Off-centered Double-slit Metamaterial for Elastic Wave Polarization Anomaly

Journal

Scientific Reports

Vol/No

7

PP

15378

Year/Month

2017-11

Link

https://www.nature.com/articles/s41598-017-15746-2

Abstract

The polarization anomaly refers to the polarization transition from longitudinal to shear modes along an equi-frequency contour of the same branch, which occurs only in some anisotropic elastic media, but the lack of natural materials exhibiting desired anisotropy makes its utilization impossible for potential novel applications. In this paper, we present a unique, non-resonant type elastic metamaterial made of off-centered, double-slit unit cells. We show that its wave polarization characteristics that determine the desired anomalous polarization for a certain application are tailorable. As an application, a mode converting wedge that transforms pure longitudinal into pure shear modes is designed by the proposed metamaterial. The physics involved in the mode conversion is investigated by simulations and experiments.

Authors

Ngoc-Linh Nguyen, Gang-Won Jang, Soomin Choi, Jaeyong Kim, Yoon Young Kim

Title

Analysis of thin-walled beam-shell structures for concept modeling based on higher-order beam theory

Journal

Computers and Structures

Vol/No

195

PP

16-33

Year/Month

2018-01

Link

https://www.sciencedirect.com/science/article/pii/S0045794917307575

Abstract

Many engineering structures consist of thin-walled beams and shells. Especially for fast design in an early design stage, a simplified analysis using beams for load-carrying members and shells for panels is very useful, but there appears no accurate beam-shell combined finite element model. The main reason is that the Timoshenko or Euler beam elements are incapable of representing significant sectional deformations near beam joints or near beam-panel interfaces. Although some progress has been made in developing higher-order beam elements that can accurately capture the sectional deformations, there is no investigation to develop higher-order beam and shell combined models useful to analyze various engineering structures. The main contribution of this work is to present the first attempt to model structures made of thin-walled closed beams and shells in terms of higher-order beam elements and shell elements and to establish the matching conditions between the dissimilar field variables of higher-order beam and shell elements along their interfaces. For the finite element analysis of a whole structure, the interface matching conditions are imposed through Lagrange multipliers. High accuracy of the proposed higher-order beam-shell method is demonstrated through static and modal analyses of various structures including a simplified model of a vehicle body-in-white (BIW).

Authors

Joo Hwan Oh, Hong Min Seung, Yoon Young Kim

Title

Doubly negative isotropic elastic metamaterial for subwavelength focusing: Design and realization”, Journal of Sound and Vibration

Journal

Journal of Sound and Vibration

Vol/No

410

PP

169-186

Year/Month

2017-12

Link

http://www.sciencedirect.com/science/article/pii/S0022460X17306314

Abstract

In spite of much progress in elastic metamaterials, tuning the effective density and stiffness to desired values ranging from negatives to large positives is still difficult. In particular, simultaneous realization of double negativity and isotropy, critical in sub-wavelength focusing, is very challenging since anisotropy is usually unavoidable in resonance-based metamaterials. The main difficulty is that there is no established systematic design method for simultaneous achieving of double negativity and isotropy. Thus, we propose a unique elastic metamaterial unit cell with which simultaneous realization can be achieved by an explicit step-by-step approach. The unit cell of the proposed metamaterial can be accurately modeled as an equivalent mass-spring system so that the effective properties can be easily controlled with the design parameters. The actual realization was carried out by acquiring the desired properties in sequential steps which is in detail. The specific application for this study is on sub-wavelength focusing, which will be demonstrated by waves from a single point source focused on a region smaller than half the wavelength. Actual experiments were performed on an aluminum plate where the designed metamaterial flat lens was imbedded. The results acquired through simulations and experiments suggest potential applications of the proposed metamaterial and the systematic design approach in advanced acoustic surgery or non-destructive testing.

Authors

Young Kwan Ahn, Hyung Jin Lee, Yoon Young Kim

Title

Conical Refraction of Elastic Waves by Anisotropic Metamaterials and Application for Parallel Translation of Elastic Waves

Journal

Scientific Reports

Vol/No

7

PP

10072

Year/Month

2017-08

Link

https://www.nature.com/articles/s41598-017-10691-6

Abstract

Conical refraction, which is quite well-known in electromagnetic waves, has not been explored well in elastic waves due to the lack of proper natural elastic media. Here, we propose and design a unique anisotropic elastic metamaterial slab that realizes conical refraction for horizontally incident longitudinal or transverse waves; the single-mode wave is split into two oblique coupled longitudinal-shear waves. As an interesting application, we carried out an experiment of parallel translation of an incident elastic wave system through the anisotropic metamaterial slab. The parallel translation can be useful for ultrasonic non-destructive testing of a system hidden by obstacles. While the parallel translation resembles light refraction through a parallel plate without angle deviation between entry and exit beams, this wave behavior cannot be achieved without the engineered metamaterial because an elastic wave incident upon a dissimilar medium is always split at different refraction angles into two different modes, longitudinal and shear.

Authors

Suh In Kim, Seok Won Kang, Yong-Sub Yi, Joonhong Park, Yoon Young Kim

Title

Topology optimization of vehicle rear suspension mechanisms

Journal

International Journal for Numerical Methods in Engineering

Vol/No

PP

1-22

Year/Month

2017-06

Link

http://onlinelibrary.wiley.com/doi/10.1002/nme.5573/full

Abstract

As suspensions critically affect the ride and handling performance of vehicles, considerable efforts have been made to improve their design by an optimization method. In this paper, we propose a topology optimization–based method for suspension synthesis by using a 3-dimensional model constructed with nonlinear bars and zero-length springs that discretize the 3-dimensional space between the chassis frame and a vehicle wheel. For optimization, bar cross-sections and spring stiffness values are used as design variables alongside the nodal positions of bar elements as shape optimization variables to simultaneously optimize the topology and shape. To verify the proposed approach, 2 types of design problems were solved: recovering known suspension mechanisms for a given set of wheel trajectories and synthesizing unknown suspension mechanisms that satisfy several design constraints typically used in the automobile industry. Through these examples, possibilities to design new and advanced suspensions by the proposed optimization method are clearly demonstrated.

Authors

Joon Hee Jung, Gang-Won Jang, Dongil Shin, Yoon Young Kim

Title

One-dimensional analysis of thin-walled beams with diaphragms and its application to optimization for stiffness reinforcement

Journal

Computational Mechanics

Vol/No

PP

1-19

Year/Month

2017-08

Link

https://link.springer.com/article/10.1007/s00466-017-1452-x

Abstract

This paper presents a method to analyze thin-walled beams with quadrilateral cross sections reinforced with diaphragms using a one-dimensional higher-order beam theory. The effect of a diaphragm is reflected focusing on the increase of static stiffness. The deformations on the beam-interfacing boundary of a thin diaphragm are described by using deformation modes of the beam cross section while the deformations inside the diaphragm are approximated in the form of complete cubic polynomials. By using the principle of minimum potential energy, its stiffness that significantly affects distortional deformation of a thin-walled beam can be considered in the one-dimensional beam analysis. It is shown that the accuracy of the resulting one-dimensional analysis is comparable with that by a shell element based analysis. As a means to demonstrate the usefulness of the present approach for design, position optimization problems of diaphragms for stiffness reinforcement of an automotive side frame are solved.

Authors

Sang Min Han, Suh In Kim, Yoon Young Kim

Title

Topology optimization of planar linkage mechanisms for path generation without prescribed timing

Journal

Structural and Multidisciplinary Optimization

Vol/No

56

PP

501-517

Year/Month

2017-09

Link

https://link.springer.com/article/10.1007/s00158-017-1712-6

Abstract

The synthesis of planar linkage mechanisms by a topology optimization method has recently received much attention because both their numbers and dimensions can be simultaneously determined without baseline layouts. To synthesize a mechanism to produce a desired path at its end-effector, a desired path can be defined with or without prescribed timing. While earlier topology optimizations of mechanisms were all concerned with paths with prescribed timing, no study to deal with the topology optimization of mechanisms for path generation without prescribed timing is carried out in spite of its importance. The aim of this study is to propose and set up a gradientbased topology optimization formulation to synthesize planar linkage mechanisms that generate desired paths without prescribed timing. To this end, the desired path of the end-effector is expressed by the centroid distance function which is then represented by its Fourier descriptors. Then a topology optimization formulation using the Fourier descriptors is developed and the sensitivity analysis based on the Fourier descriptors is derived. Several numerical case studies are considered to verify the effectiveness of the proposed formulation. Some numerical issues appearing with the use of the Fourier descriptors are also investigated.

Authors

Joshua Minwoo Kweun, Hyung Jin Lee, Joo Hwan Oh, Hong Min Seung, and Yoon Young Kim

Title

Transmodal Fabry-Pérot Resonance: Theory and Realization with Elastic Metamaterials

Journal

Physical Review Letters

Vol/No

118

PP

205901

Year/Month

2017-05

Link

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.205901

Abstract

We discovered a new transmodal Fabry-Pérot resonance where one elastic-wave mode is maximally transmitted to another. It occurs when the phase difference of two dissimilar modes through an anisotropic layer becomes odd multiples of π under the reflection-free and weak mode-coupling assumptions. Unlike the well-established Fabry-Pérot resonance, the transmodal resonance must involve two coupled elastic waves between longitudinal and shear modes. The investigation into the origin of wiggly transmodal transmission spectra suggests that efficient broadband mode conversion can be achieved if the media satisfy the structural stability condition to some degree. The new resonance mechanism, also experimentally characterized, opens up new possibilities for manipulating elastic wave modes as an effective alternative to generating shear-mode ultrasound.

Authors

Joong Seok Lee, Il Kyu Lee, Hong Min Seung, Jun Kyu Lee and Yoon Young Kim

Title

Subwavelength elastic joints connecting torsional waveguides to maximize the power transmission coefficient

Journal

Journal of Sound and Vibration

Vol/No

392

PP

154-169

Year/Month

2017-03

Link

http://www.sciencedirect.com/science/article/pii/S0022460X16307611

Abstract

Joints with slowly varying tapered shapes, such as linear or exponential profiles, are known to transmit incident wave power efficiently between two waveguides with dissimilar impedances. This statement is valid only when the considered joint length is longer than the wavelengths of the incident waves. When the joint length is shorter than the wavelengths, however, appropriate shapes of such subwavelength joints for efficient power transmission have not been explored much. In this work, considering one-dimensional torsional wave motion in a cylindrical elastic waveguide system, optimal shapes or radial profiles of a subwavelength joint maximizing the power transmission coefficient are designed by a gradient-based optimization formulation. The joint is divided into a number of thin disk elements using the transfer matrix approach and optimal radii of the disks are determined by iterative shape optimization processes for several single or bands of wavenumbers. Due to the subwavelength constraint, the optimized joint profiles were found to be considerably different from the slowly varying tapered shapes. Specifically, for bands of wavenumbers, peculiar gourd-like shapes were obtained as optimal shapes to maximize the power transmission coefficient. Numerical results from the proposed optimization formulation were also experimentally realized to verify the validity of the present designs.

Authors

Jieun Yang, Joong Seok Lee and Yoon Young Kim

Title

Multiple slow waves in metaporous layers for broadband sound absorption

Journal

Journal of Physics D: Applied Physics

Vol/No

50

PP

015301

Year/Month

2017-01

Link

http://iopscience.iop.org/article/10.1088/1361-6463/50/1/015301/meta

Abstract

Sound absorption for a broad frequency range requires sound dissipation. The mechanics of acoustic metamaterials for non-dissipative applications has been extensively studied, but sound absorption using dissipative porous metamaterials has been less explored because of the complexity resulting from the coupling of its dissipative mechanism and metamaterial behavior. We investigated broadband sound absorption by engineering dissipative metaporous layers, which absorb sound by the mechanism of multiple slow waves, and combined local and global resonance phenomena. A set of rigid partitions of varying lengths was elaborately inserted in a hard-backed porous layer of a finite thickness. An effective medium theory was used to explain the physics involved; high performance at a low-frequency range was found to be mainly due to the formation of global resonances caused by multiple slow waves over the thickness of the metaporous layer, while enhancement at a high-frequency range was attributed to the combined effects of the global resonances and the local resonances directly related to the sizes of the inserted partitions.

Authors

Kyung Ho Sun, Jae Eun Kim, Byeong Ock Kim, Yoon Young Kim

Title

Optimal configuration of magnetoelectric composites under various mechanical boundary conditions

Journal

Composites Science and Technology

Vol/No

142

PP

221-226

Year/Month

2017-04

Link

http://www.sciencedirect.com/science/article/pii/S0266353816311848

Abstract

The magnetoelectric (ME) coupling effect of magneto-electro-elastic composites is generated by the product property of magnetostriction and piezoelectricity via elastic deformation. Since mechanical deformation and interlayer stress interaction directly influence the extrinsic ME effect, composite configurations between magnetostrictive and piezoelectric materials critically influence the energy conversion efficiency of ME composites, as also do the imposed mechanical boundary conditions (BCs). Here, we aim to identify the material configurations in the composites under different BCs, in order to maximize their ME effects. The problem is set up as optimization problems solved by a genetic algorithm while the required multiphysics simulation is performed by finite element analysis. For five major mechanical BCs, we determined optimal ME laminate layer configurations and compared their magneto-electric conversion efficiency with that by typical sandwiched laminate composites. We also investigated how much the efficiency can be increased if arbitrary-arranged material distributions are allowed. The present study is expected to provide useful guidelines for the design of ME composites, and offers the possibility of finding new ME composite structures.

Authors

Yongping Zheng, Dae-Soo Yang, Joshua M. Kweun, Chenzhe Li, Kui Tan, Fantai Kong, Chaoping Liang, Yves J. Chabal, Yoon Young Kim, Maenghyo Cho, Jong-Sung Yu, and Kyeongjae Cho

Title

Rational design of common transition metal-nitrogen-carbon catalysts for oxygen reduction reaction in fuel cells

Journal

Nano Energy

Vol/No

30

PP

443-449

Year/Month

2016-12

Link

http://www.sciencedirect.com/science/article/pii/S2211285516304566

Abstract

Bio-inspired non-precious-metal catalysts based on iron and cobalt porphyrins are promising alternatives to replace costly platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. However, the exact nature of the active sites is still not clearly understood, and further optimization design is needed for practical applications. Here, we report a rational catalyst design process by combining density functional theory (DFT) calculations and experimental validations. Two sets of square-planar (MNxC4−x) and square-pyramid (MNxC5−x) active centers (M=Mn, Fe, Co, Ni) incorporated in graphene were examined using DFT. Fe-N5 and Co-N4 sites were identified theoretically to have the best performance in fuel cells, while Ni-NxC4−x sites catalyze the most H2O2 byproduct. Graphene samples with well-dispersed incorporations of metals were synthesized, and the following electrochemical measurements show an excellent agreement with the theoretical predictions, indicating that a successful design framework and systematic understanding toward the catalytic nature of these materials are established.

Authors

In Seop Choi, Gang-Won Jang, Soomin Choi, Dongil Shin and Yoon Young Kim

Title

Higher order analysis of thin-walled beams with axially varying quadrilateral cross sections

Journal

Computers & Structures

Vol/No

179

PP

127-139

Year/Month

2017-01

Link

http://www.sciencedirect.com/science/article/pii/S0045794916302565

Abstract

A thin-walled beam finite element with a varying quadrilateral cross section is formulated based on a higher order beam theory. For the calculation of distortions, the beam frame approach, which models the cross section by using two-dimensional Euler beams, is used. Distortions induced by the Poisson’s effect and warpings are analytically derived. Three-dimensional displacements at an arbitrary point of a present beam element can be described by interpolating three-dimensional displacements at the end sections. Straight and curved thin-walled beams with varying cross sections are solved to show the validity of the proposed approach.

Authors

Xiong Wei Yang, Joong Seok Lee and Yoon Young Kim

Title

Effective mass density based topology optimization of locally resonant acoustic metamaterials for bandgap maximization

Journal

Journal of Sound and Vibration

Vol/No

383

PP

89=107

Year/Month

2016-11

Link

http://www.sciencedirect.com/science/article/pii/S0022460X16303522

Abstract

Because effective material properties are essential concepts in the analyses of wave phenomena in metamaterials, they may also be utilized in the optimal design of metamaterials. In this work, we propose a topology optimization method directly using the Effective Mass Density (EMD) concept to maximize the first bandgaps of two-dimensional solid Locally Resonant Acoustic Metamaterials (LRAMs). When the first bandgap is characterized by the negative EMD, the bandgap maximization can be formulated efficiently as a topology optimization problem to broaden the frequency zone of the negative EMD values. In this work, EMD is calculated by considering the macroscopic isotropy of LRAMs in the long wavelength limit. To facilitate the analytical sensitivity analysis, we propose an elaborate calculation scheme of EMD. A sensitivity averaging technique is also suggested to guarantee the macroscopically isotropic behavior of the LRAMs. In the present study, the coating layer interfacing the core and the matrix of a ternary LRAM is chosen as the design region because it significantly influences the bandgap. By considering several numerical examples, the validity of this method is verified, and the effects of the mass constraint ratios on the optimized results are also investigated.

Authors

Woorim Lee, Hyuk Lee, Yoon Young Kim

Title

Experiments of wave cancellation with elastic phononic crystal

Journal

Ultrasonics

Vol/No

72

PP

128-133

Year/Month

2016-12

Link

http://www.sciencedirect.com/science/article/pii/S0041624X16301342

Abstract

The objective of this work is to experimentally demonstrate that two incident beams of ultrasonic waves can be cancelled by using an elastic phononic crystal (PC) prism. Although PCs are known to be used for wave cancellation, there appears no experimental demonstration especially for elastic waves. Here, we use an elastic PC prism embedded in an aluminum plate, which can split an input incident beam into multiple output beams. Two signals of different incident angles are reversely sent to the prism for the wave cancellation experiment. For successful wave cancellation experiments, the magnitudes and phase difference of the input sources were carefully tuned. The experimental results were found to agree well with the predictions from numerical simulations.

Authors

Soomin Choi, Yoon Young Kim

Title

Exact matching at a joint of multiply-connected box beams under out-of-plane bending and torsion

Journal

Engineering Structures

Vol/No

124

PP

96-112

Year/Month

2016-10

Link

http://www.sciencedirect.com/science/article/pii/S0141029616302693

Abstract

Accurate analysis of thin-walled box beams meeting at a joint requires not only consideration of higher-order deformation degrees (such as warping and distortion) but also exact matching conditions at the joint. Especially when more than two box beams are connected at a joint, the deformation of the beam-joint system is so complicated that no one-dimensional beam analysis has yet predicted its structural behavior correctly. Since a beam theory incorporating higher-order deformations is available, the main difficulty is determining the exact matching conditions at the joint. In this paper, we derive the exact matching conditions for five field variables—bending deflection, bending/torsional rotations, warping, and distortion—of multiply connected box beams under out-of-plane bending and torsional loads. The derived relations are valid irrespective of the number of beams and angles. We introduced a new concept called “edge resultants” besides conventional (sectional) resultants, and demonstrated its effectiveness for exact derivation and physical interpretations of the derived equations. The accuracy and validity of the proposed theory are checked by comparing the predicted results with those of shell finite element analysis.

Authors

Seok Won Kang, Suh In Kim and Yoon Young Kim

Title

Topology optimization of planar linkage systems involving general joint types

Journal

Mechanism and Machine Theory

Vol/No

104

PP

130–160

Year/Month

2016-10

Link

http://www.sciencedirect.com/science/article/pii/S0094114X16300817

Abstract

The simultaneous synthesis of the number and dimension of planar linkage mechanisms has opened a new possibility in mechanism synthesis. Mechanisms with revolute joints were a main concern so far but planar mechanisms with general joints including prismatic joints cannot be synthesized. This study aims to overcome the current difficult by proposing a new synthesis method. The key idea in the proposed approach is to parameterize joint types directly, unlike earlier methods parameterizing ground link skeletons connected by revolute joints. So, we propose a new concept of “double-springs” that can be used to connect a finite number of ground rectangular rigid blocks that discretize a given synthesis domain. Rigid blocks are elaborately connected by two sets of one-dimensional springs, i.e., double-springs and their stiffness values are varied by design variables. The connectivity of the double-springs dictated by the stiffness values determines the synthesized link mechanisms. We explain why it is crucial to use the double-spring concept for the synthesis of general joints such as prismatic joints. Various benchmark problems were considered to demonstrate the validity of the proposed method.

Authors

Joo Kyung Lee and Yoon Young Kim

Title

Tuned double-coil EMATs for omnidirectional symmetric mode lamb wave generation

Journal

NDT&E International

Vol/No

83

PP

38–47

Year/Month

2016-10

Link

http://www.sciencedirect.com/science/article/pii/S0963869516300482

Abstract

This paper investigates a method to generate only symmetric omnidirectional Lamb waves and proposes specially-tuned double-coil EMATs. While methods to generate omnidirectional Lamb waves have been reported, the symmetric mode is typically accompanied by unwanted antisymmetric modes. The double spiral coil works as a wavelength filter but the relation between the coil geometries and the wavelength of the symmetric mode that minimizes the unwanted antisymmetric mode has not been established. Axisymmetric analysis was performed to find the relation. Excellent rejection of antisymmetric modes with successful symmetric mode generation was confirmed by experimental results that favorably agree with the theoretical results.

Authors

Soomin Choi and Yoon Young Kim

Title

Analysis of two box beams-joint systems under in-plane bending and axial loads by one-dimensional higher-order beam theory

Journal

International Journal of Solids and Structures

Vol/No

90

PP

69-94

Year/Month

2016-07

Link

http://www.sciencedirect.com/science/article/pii/S0020768316300270

Abstract

If two thin-walled box beams meet at a joint, significant flexibilities that cannot be dealt with by the classical Euler or Timoshenko beam theory are observed. Especially under in-plane bending and axial loads, the deformation of the two box beams-joint system near the joint region is so complicated that no theoretical one-dimensional approach that interprets its mechanical behavior correctly has yet been proposed. To establish an effective higher-order beam theory, we introduce a new additional bending distortion degree representing anticlastic curvature effects and also redefine the section-shape functions of the bending warping and bending distortion degrees. In box beams-joint systems, it is crucial to find the matching conditions among field variables at the joint, but no exact conditions applicable for the systems under in-plane bending and axial loads are available. In this paper, we newly derive the explicit form of the transformation matrix relating six field variables of two box beams at a joint–axial displacement, transverse displacement, in-plane bending/shear rotation, bending warping, and two bending distortions. The accuracy and validity of the developed higher-order beam theory and the exact matching conditions are checked by comparing the present beam based results and ABAQUS shell analysis results for various box beams with different joint angles.

Authors

Hyuk Lee, Joo Hwan Oh, Hong Min Seung, Seung Hyun Cho and Yoon Young Kim

Title

Extreme Stiffness Hyperbolic Elastic Metamaterial for Total Transmission Subwavelength Imaging

Journal

Scientific Reports

Vol/No

6

PP

24026

Year/Month

2016-04

Link

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4819191/

Abstract

Subwavelength imaging by metamaterials and extended work to pursue total transmission has been successfully demonstrated with electromagnetic and acoustic waves very recently. However, no elastic counterpart has been reported because earlier attempts suffer from considerable loss. Here, for the first time, we realize an elastic hyperbolic metamaterial lens and experimentally show total transmission subwavelength imaging with measured wave field inside the metamaterial lens. The main idea is to compensate for the decreased impedance in the perforated elastic metamaterial by utilizing extreme stiffness, which has not been independently actualized in a continuum elastic medium so far. The fabricated elastic lens is capable of directly transferring subwavelength information from the input to the output boundary. In the experiment, this intriguing phenomenon is confirmed by scanning the elastic structures inside the lens with laser scanning vibrometer. The proposed elastic metamaterial lens will bring forth significant guidelines for ultrasonic imaging techniques.

Authors

Hong Min Seung, Chung Il Park and Yoon Young Kim

Title

An omnidirectional shear-horizontal guided wave EMAT for a metallic plate

Journal

Ultrasonics

Vol/No

69

PP

58-66

Year/Month

2016-07

Link

http://www.sciencedirect.com/science/article/pii/S0041624X16300014

Abstract

We propose a new electromagnetic acoustic transducer (EMAT) for generation and measurement of omnidirectional shear-horizontal (SH) guided waves in metallic plates. The proposed EMAT requires a magnetic circuit configuration that allows omnidirectional SH wave transduction. It consists of a pair of ring-type permanent magnets that supply static magnetic fluxes and a specially wound coil that induces eddy currents. The Lorentz force acting along the circumferential direction is induced by the vertical static magnetic flux and the radial eddy current in a plate, resulting in omnidirectional SH wave generation. To maximize the transducer output at given excitation frequencies, optimal EMAT configurations are determined by numerical simulations and validated by experiments. The omnidirectivity of the proposed EMAT is also confirmed by the simulations and experiments.

Authors

Joo Hwan Oh, Young Eui Kwon, Hyung Jin Lee and Yoon Young Kim

Title

Elastic metamaterials for independent realization of negativity in density and stiffness

Journal

Scientific Reports

Vol/No

6

PP

23630

Year/Month

2016-03

Link

http://www.readcube.com/articles/10.1038/srep23630

Abstract

In this paper, we present the realization of an elastic metamaterial allowing independent tuning of negative density and stiffness for elastic waves propagating along a designated direction. In electromagnetic (or acoustic) metamaterials, it is now possible to tune permittivity (bulk modulus) and permeability (density) independently. Apparently, the tuning methods seem to be directly applicable for elastic case, but no realization has yet been made due to the unique tensorial physics of elasticity that makes wave motions coupled in a peculiar way. To realize independent tunability, we developed a single-phased elastic metamaterial supported by theoretical analysis and numerical/experimental validations.

Authors

Joo Hwan Oh, Hong Min Seung, and Yoon Young Kim

Title

Adjoining of Negative Stiffness and Negative Density Bands in an Elastic Metamaterial

Journal

APPLIED PHYSICS LETTERS

Vol/No

108(9)

PP

093501-1~5

Year/Month

2016-03

Link

http://scitation.aip.org/content/aip/journal/apl/108/9/10.1063/1.4943095

Abstract

In this paper, we explore an elasticmetamaterial adjoining the frequency band of negative density and that of negative stiffness. If the band-adjoining occurs, a fully continuous widened stop band covering the entire ranges of the negative density and stiffness can form. The resulting stop band can be useful for various vibration applications such as vibration shielding. We show that the band-adjoining frequency is characterized by a standing wave with peculiar “unbalanced” motions. Experiments with the symmetric Lamb waves were conducted to validate the theoretical prediction and the effects of perturbed adjoining band on wave physics are also investigated.

Authors

Chenzhe Li, Sampreetha Thampy, Yongping Zheng, Joshua M Kweun, Yixin Ren, Julia Y Chan, Hanchul Kim, Maenghyo Cho, Yoon Young Kim, Julia W P Hsu and Kyeongjae Cho

Title

Thermal stability of mullite RMn2O5 (R  =  Bi, Y, Pr, Sm or Gd): combined density functional theory and experimental study

Journal

Journal of Physics: Condensed Matter

Vol/No

28/12

PP

125602

Year/Month

2016-02

Link

http://iopscience.iop.org/article/10.1088/0953-8984/28/12/125602

Abstract

Understanding and effectively predicting the thermal stability of ternary transition metal oxides with heavy elements using first principle simulations are vital for understanding performance of advanced materials. In this work, we have investigated the thermal stability of mullite RMn2O5 (R  =  Bi, Pr, Sm, or Gd) structures by constructing temperature phase diagrams using an efficient mixed generalized gradient approximation (GGA) and the GGA  +  U method. Simulation predicted stability regions without corrections on heavy elements show a 4–200 K underestimation compared to our experimental results. We have found the number of d/f electrons in the heavy elements shows a linear relationship with the prediction deviation. Further correction on the strongly correlated electrons in heavy elements could significantly reduce the prediction deviations. Our corrected simulation results demonstrate that further correction of R-site elements in RMn2O5 could effectively reduce the underestimation of the density functional theory-predicted decomposition temperature to within 30 K. Therefore, it could produce an accurate thermal stability prediction for complex ternary transition metal oxide compounds with heavy elements.

Authors

Yoon Young Kim, Jae Chun Ryu, Eun Il Kim, Hyeongkee Kim, Byungseong Ahn

Title

Variational Art Algorithm for Image Generation

Journal

Leonardo

Vol/No

49(3)

PP

226-231

Year/Month

2016-06

Link

http://www.mitpressjournals.org/doi/abs/10.1162/LEON_a_00914#.Vtz3RvmLSUk

Abstract

We propose the variational art algorithm, a virtual system-based optimization algorithm developed for generating images. Observing that the topology optimization method used for multi-physics system design can produce two- or three-dimensional layouts without baselines, we propose to expand it beyond engineering applications for generating images. In doing so, we devise a virtual physical system for image generation and in this work, we have chosen a heat-path system as a virtual system for which one could possibly “interpret” the optimization-based process of image generation as simultaneous drawing of multiple strokes in painting.

Authors

Dongil Shin, Soomin Choi, Gang-Won Jang, Yoon Young Kim

Title

Finite element beam analysis of tapered thin-walled box beams

Journal

Thin-Walled Structures

Vol/No

102

PP

205-214

Year/Month

2016-05

Link

http://www.sciencedirect.com/science/article/pii/S0263823116300283

Abstract

This paper presents a one-dimensional finite element analysis of tapered thin-walled box beams under out-of-plane loads and twisting moments by developing new C0-continuous tapered higher-order beam elements. While higher-order sectional deformations such as distortion were considered earlier, there was no theoretical or finite element analysis on the tapering effect of variable cross-sectioned box beams. Case studies considering tapered box beams with various height-to-width ratios and a system of joint structure with a tapered box beam show that the results by the developed method are comparable with the shell finite element results unlike the stepped Timoshenko beam element results.

Authors

Joshua Minwoo Kweun, Chenzhe Li, Yongping Zheng, Maenghyo Cho, Yoon Young Kim, Kyeongjae Cho

Title

Bulk-Surface relationship of an electronic structure for high-throughput screening of metal oxide catalysts

Journal

Applied Surface Science

Vol/No

370(1)

PP

279-290

Year/Month

2016-05

Link

http://www.sciencedirect.com/science/article/pii/S0169433216302628

Abstract

Designing metal-oxides consisting of earth-abundant elements has been a crucial issue to replace precious metal catalysts. To achieve efficient screening of metal-oxide catalysts via bulk descriptors rather than surface descriptors, we investigated the relationship between the electronic structure of bulk and that of the surface for lanthanum-based perovskite oxides, LaMO3 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu). Through density functional theory calculations, we examined the d-band occupancy of the bulk and surface transition-metal atoms (nBulk and nSurf) and the adsorption energy of an oxygen atom (Eads) on (001), (110), and (111) surfaces. For the (001) surface, we observed strong correlation between the nBulk and nSurf with an R-squared value over 94%, and the result was interpreted in terms of ligand field splitting and antibonding/bonding level splitting. Moreover, the Eads on the surfaces was highly correlated with the nBulk with an R-squared value of more than 94%, and different surface relaxations could be explained by the bulk electronic structure (e.g., LaMnO3 vs. LaTiO3). These results suggest that a bulk-derived descriptor such as nBulk can be used to screen metal-oxide catalysts.

Authors

Joong Seok Lee, Elke Deckers, Stijn Jonckheere, Wim Desmet, Yoon Young Kim

Title

A direct hybrid finite element-wave based modelling technique for efficient analysis of poroelastic materials in steady-state acoustic problems

Journal

Computer Methods in Applied Mechanics and Engineering

Vol/No

304(1)

PP

55-80

Year/Month

2016-06

Link

http://www.sciencedirect.com/science/article/pii/S0045782516300287

Abstract

This work proposes a hybrid modelling technique for efficient analysis of poroelastic materials, which are widely used for noise reduction in acoustic problems. By combining the finite element method and the wave based method in a direct manner, the proposed hybrid technique maximises the advantages and compensates the drawbacks of both numerical methods. The considered poroelastic domain described by Biot’s theory is divided into two groups of domains according to their geometrical characteristics and boundary conditions. The group with complex geometries and/or boundary conditions leading to singularities is discretised into a large number of small finite elements. The other group consisting of large, geometrically moderate poroelastic domains is partitioned into wave based subdomains where the field variables are expanded with analytical poroelastic wave functions. Both groups modelled by the finite element method and the wave based method, respectively, are combined in a hybrid framework in this work to ensure their interacting dynamic behaviours. The properties of the hybrid model are investigated and are compared to existing modelling methods for some numerical examples. The proposed direct hybrid modelling technique provides stable predictions and exhibits fast convergence performances for the analysis of poroelastic materials, especially when singularities arise in the poroelastic domain.

Authors

Hong Min Seung, Yoon Young Kim

Title

Generation of omni-directional shear-horizontal waves in a ferromagnetic plate by a magnetostrictive patch transducer

Journal

NDT&E International

Vol/No

80

PP

6-14

Year/Month

2016-06

Link

http://www.sciencedirect.com/science/article/pii/S0963869516000116

Abstract

It this study, omni-directional shear-horizontal wave transduction is performed using a specially configured magnetostrictive patch transducer in a ferromagnetic plate. For a ferromagnetic plate, unavoidable magnetic flux leakage into the plate not only results in poor transduction efficiency but also generates unwanted waves within the plate. These problems must be overcome to inspect ferromagnetic plates using the transducer. Therefore, we investigate the reasons for the poor performance and propose a method to improve its performance. The effectiveness of the proposed method was validated through simulations and experiments.

Authors

Min Kyung Lee and Yoon Young Kim

Title

Add-on unidirectional elastic metamaterial plate cloak

Journal

Scientific Reports

Vol/No

6

PP

20731

Year/Month

2016-02

Link

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4748288/pdf/srep20731.pdf

Abstract

Metamaterial cloaks control the propagation of waves to make an object invisible or insensible. To manipulate elastic waves in space, a metamaterial cloak is typically embedded in a base system that includes or surrounds a target object. The embedding is undesirable because it structurally weakens or permanently alters the base system. In this study, we propose a new add-on metamaterial elastic cloak that can be placed over and mechanically coupled with a base structure without embedding. We designed an add-on type annular metamaterial plate cloak through conformal mapping, fabricated it and performed cloaking experiments in a thin-plate with a hole. Experiments were performed in a thin plate by using the lowest symmetric Lamb wave centered at 100 kHz. As a means to check the cloaking performance of the add-on elastic plate cloak, possibly as a temporary stress reliever or a so-called “stress bandage”, the degree of stress concentration mitigation and the recovery from the perturbed wave field due to a hole were investigated.

Authors

Kiyean Kim, Hyung Jin Lee, Joo Kyung Lee and Yoon Young Kim

Title

Effects of slits in a patch of omnidirectional Lamb-wave MPT on the transducer output

Journal

Smart Materials and Structures

Vol/No

25

PP

035019-1~15

Year/Month

2016-02

Link

http://iopscience.iop.org/article/10.1088/0964-1726/25/3/035019/meta

Abstract

An omnidirectional Lamb-wave magnetostrictive patch transducer (MPT) uses a thin circular magnetostrictive patch excited by the axisymmetric turns of a plane coil placed over it. Since the dynamic magnetic field applied by the coil induces an eddy current only on the top surface due to the skin effect, the mechanical deformation in the patch is confined only near the surface, considerably limiting the transducer output power. This study shows that if a radial slit is introduced in the patch, the circular flow of the eddy current on the top surface only becomes broken, and thus the eddy current flows on both the top and bottom surfaces. As a consequence, there is a substantial increase in the dynamic magnetic field in the patch and, in turn, an increased transducer output power. Interestingly, the material type, either metallic or nonmetallic, of the test waveguide plate affects the magnitude of the eddy current near the bottom surface and the overall magnetic field. If the number of slits is over a certain number, say, 8, and the slits are symmetrically made in the patch, the omni-directivity of the resulting MPT is virtually ensured. Finally, the present findings are verified both numerically and experimentally.

Authors

Do-Min Kim, Suh In Kim, Soomin Choi, Gang-Won Jang, Yoon Young Kim

Title

Topology optimization of thin‐walled box beam structures based on the higher‐order beam theory

Journal

International Journal for Numerical Methods in Engineering

Vol/No

106(7)

PP

576-590

Year/Month

2016-05

Link

http://onlinelibrary.wiley.com/doi/10.1002/nme.5143/full

Abstract

The investigation aims to formulate ground-structure based topology optimization approach by using a higher-order beam theory suitable for thin-walled box beam structures. While earlier studies use the Timoshenko or Euler beams to form a ground-structure, they are not suitable for a structure consisting of thin-walled closed beams. The higher-order beam theory takes into an additional account sectional deformations of a thin-walled box beam such as warping and distortion. Therefore, a method to connect ground beams at a joint and a technique to represent different joint connectivity states should be investigated for streamlined topology optimization. Several numerical case studies involving different loading and boundary conditions are considered to show the effectiveness of employing a higher-order beam theory for the ground-structure based topology optimization of thin-walled box beam structures. Through the numerical results, this work shows significant difference between optimized beam layouts based on the Timoshenko beam theory and those based on a more accurate higher-order beam theory for a structure consisting of thin-walled box beams.

Authors

Jun Hyeong Park, Hyung Jin Lee, and Yoon Young Kim

Title

[Published Online] Characterization of anisotropic acoustic metamaterial slabs

Journal

JOURNAL OF APPLIED PHYSICS

Vol/No

119

PP

034901-1~9

Year/Month

2016

Link

http://scitation.aip.org/content/aip/journal/jap/119/3/10.1063/1.4939868

Abstract

In an anisotropic acoustic metamaterial, the off-diagonal components of its effective mass density tensor should be considered in order to describe the anisotropic behavior produced by arbitrarily shaped inclusions. However, few studies have been carried out to characterize anisotropic acoustic metamaterials. In this paper, we propose a method that uses the non-diagonal effective mass density tensor to determine the behavior of anisotropic acoustic metamaterials. Our method accurately evaluates the effective properties of anisotropic acoustic metamaterials by separately dealing with slabs made of single and multiple unit cells along the thickness direction. To determine the effective properties, the reflection and transmission coefficients of an acoustic metamaterial slab are calculated, and then the wave vectors inside of the slab are determined using these coefficients. The effective material properties are finally determined by utilizing the spatial dispersion relation of the anisotropic acoustic metamaterial. Since the dispersion relation of an anisotropic acoustic metamaterial is explicitly used, its effective properties can be easily determined by only using a limited number of normal and oblique plane wave incidences into a metamaterial slab, unlike existing approaches requiring a large number of wave incidences. The validity of the proposed method is verified by conducting wave simulations for anisotropic acoustic metamaterial slabs with Z-shaped elastic inclusions of tilted principal material axes.

Authors

Hocheol Lee, Yoon Young Kim

Title

Semi-analytic formulation and its experimental verification to determine the radiation patterns of ultrasonic guided waves generated on a plate by a magnetostrictive patch transducer

Journal

Journal of Mechanical Science and Technology

Vol/No

29(12)

PP

5309-5316

Year/Month

2015-12

Link

http://link.springer.com/article/10.1007%2Fs12206-015-1133-7

Abstract

In the non-destructive evaluation of plate structures using ultrasonic guided waves, the radiation patterns of ultrasonic transducers that generate and measure elastic guided waves must be analyzed. This work develops a semi-analytic formulation to predict these patterns and verifies this method experimentally. Magnetostriction is employed as the transduction mechanism between electrical and mechanical energies to control the direction of ultrasonic guided wave emission. This study also shows that the proposed semi-analytic method, which consists of a numerical method and an analytic formulation, effectively estimates radiation patterns even when the static and dynamic magnetic fields are arbitrarily distributed.

Authors

Heung Son Lee,Yoon Young Kim

Title

Guided wave scattering analysis for a plate with arbitrarily shaped elastic inclusions using the T-matrix method

Journal

Journal of Sound and Vibration

Vol/No

360

PP

97-111

Year/Month

2016-01

Link

http://www.sciencedirect.com/science/article/pii/S0022460X15007063

Abstract

Guided wave scattering by an arbitrarily shaped elastic inclusion in a flat plate is analyzed by using the three-dimensional elasticity. An inclusion is decomposed into a number of small subscatterers and the field scattered by the subscatterers is solved by using a multiple scattering theory. The transition matrix relating the scattering field by a single subscatterer and an incident field is utilized for the multiple scattering analysis, but near-field interactions among subscatterers must be considered because of the closely packed arrangement of subscatterers. This calls for an analysis of nonpropagating wave modes and requires consideration of their influences on near-field interactions. Most earlier guided wave scattering analyses were based on an approximate plate theory and the wavefunction expansion techniques based on the three-dimensional elasticity were developed only for a cavity with a small range of shapes and sizes. Therefore, this investigation presents new results applicable for wave scattering by an elastic inclusion with a wide range of shapes and sizes in a plate. Several numerical tests were conducted to ensure stable solution convergence; the scattered wave fields generated by using the developed method for some case studies were compared with those of finite element analysis.

Authors

Young Kwan Ahn, Joo Hwan Oh, Pyung Sik Ma, Yoon Young Kim

Title

Dispersion analysis with 45°-rotated augmented supercells and applications in phononic crystal design

Journal

Wave Motion

Vol/No

61

PP

63-72

Year/Month

2016-03

Link

http://www.sciencedirect.com/science/article/pii/S0165212515001468

Abstract

The supercell approach can be useful for tailoring dispersion curves of phononic crystals, but the interpretation of the dispersion curves in the supercells can be often intriguing. Supercells formed by integer multiples of an original unit cell along its lattice axes are common, but there are also important situations requiring supercells formed by non-integer multiples of an original unit cell and its rotation with respect to its lattice axes. In these cases, not only dispersion branch folding, but also branch overlapping not found in common supercells, take place, which complicates the correct interpretation of band structures. In this study, we consider 45°-rotated augmented supercells and analyze why and how branch folding and overlapping take place. For the analysis, the relation between the first Brillouin zone of an original cell and that of the corresponding 45°-rotated augmented supercell is investigated. The analysis of the folding and overlapping mechanism found in the dispersion curve of the supercell is also useful for interpreting which branches can be excited over a target wavevector direction. The usefulness of the findings of this supercell-based dispersion analysis is demonstrated in unit cell design problems. Specifically, we show how to interpret correctly the dispersion curves of phononic crystals made of unit cells optimized by bandgap maximizing topology optimization when the optimized unit cells turn out to be 45°-rotated augmented supercells. Conversely, throughout design optimization iterations, the original period of a unit cell that is initially set at the beginning of design optimization can be maintained if branch overlapping is forced not to occur.

Authors

Joong Seok Lee, Sungmin Yoo, Young Kwan Ahn and Yoon Young Kim

Title

Broadband sound blocking in phononic crystals with rotationally symmetric inclusions

Journal

The Journal of the Acoustical Society of America

Vol/No

138(3)

PP

EL217-EL222

Year/Month

2015-09

Link

http://scitation.aip.org/content/asa/journal/jasa/138/3/10.1121/1.4929625

Abstract

This paper investigates the feasibility of broadband sound blocking with rotationally symmetric extensible inclusions introduced in phononic crystals. By varying the size of four equally shaped inclusions gradually, the phononic crystal experiences remarkable changes in its band-stop properties, such as shifting/widening of multiple Bragg bandgaps and evolution to resonance gaps. Necessary extensions of the inclusions to block sound effectively can be determined for given incident frequencies by evaluating power transmission characteristics. By arraying finite dissimilar unit cells, the resulting phononic crystal exhibits broadband sound blocking from combinational effects of multiple Bragg scattering and local resonances even with small-numbered cells.

Authors

Il Kyu Lee, Hong Min Seung and Yoon Young Kim

Title

Realization of High-performance Bandpass Filter by Impedance-mirroring

Journal

Journal of Sound and Vibration

Vol/No

355

PP

86-92

Year/Month

2015-10

Link

http://www.sciencedirect.com/science/article/pii/S0022460X15004885

Abstract

Ideal bandpass filters must have sharp roll-off and high transmission over a target pass band simultaneoulsy, but it is still difficult to make such filters of a reasnoable size. To achieve such performance, we propose a novel finite phononic crystal filter made of impedance-mirrored elements each of which constists of two layers having different impedances. For broad bandpass filtering, one of the two layers is selected to be one eighth wavelength long at the center frequency of the passband and the other layer, one quarter wavelength long with a matched impedance. For narrow bandpass filtering, the broad bandpass filter is slightly modified in such a way that the concept of the impedance-mirroring is applied globally over the finite periodic structure filter, not within each unit cell. To check the performance of the proposed filters, experiments using torsional elastic waves in a rod were conducted.

Authors

Hyung Jin Lee, Joo Kyung Lee and Yoon Young Kim

Title

Elastic Metamaterial-based Impedance-varying Phononic Bandgap Structures for Bandpass Filters

Journal

Journal of Sound and Vibration

Vol/No

353

PP

58-74

Year/Month

2015-09

Link

http://www.sciencedirect.com/science/article/pii/S0022460X15004204

Abstract

In this study, we propose a novel impedance-variation scheme in a half-quarter-wave stack (HQWS) structure to achieve desired bandpass filtering performances such as quasi-flat tops, steep bandedges and wide surrounding bandgaps. Specifically, only the characteristic impedances of constituent half-wave layers are varied without altering the phase shift of π at the center frequency of the target passband that is the Fabry–Perot resonance frequency of the unperturbed original HQWS structure. Because the simultaneous control of characteristic impedance and phase shift in each of the constituent half-wave layers is not achievable only by layer-sizing, the varied layers must be realized by metamaterials. So, specially-configured aluminum-based metamaterials having a double-slit void inhomogeneity are engineered and the actual wave transmission performance of the metamaterial-realized HQWS structure is examined numerically and experimentally.

Authors

Young Eui Kwon, Hoe Woong Kim and Yoon Young Kim

Title

High-frequency lowest torsional wave mode ultrasonic inspection using a necked pipe waveguide unit

Journal

Ultrasonics

Vol/No

62

PP

237-243

Year/Month

2015-09

Link

http://www.sciencedirect.com/science/article/pii/S0041624X15001420

Abstract

We propose an effective method to transmit only the non-dispersive lowest torsional wave mode at a high frequency range even above the cutoff frequency of the third torsional mode. Unlike existing methods that tune the wavelength or phase of the target wave mode, the proposed method is based on the thickness change and the cutoff phenomenon. A specially configured necked waveguide, consisting of three regions of which the middle region is thinner than the so-called cutoff thickness, is put in end-to-end contact with a test pipe to transmit only the first torsional wave mode to a test pipe. After explaining the underlying role of the proposed necked waveguide, we propose a technique to mainly transmit the lowest torsional wave mode at a frequency where higher modes can also propagate. Numerical simulations and damage detection experiments were carried out to show the effectiveness of the proposed method.

Authors

Yoon Young Kim and Young Eui Kwon

Title

Review of magnetostrictive patch transducers and applications in ultrasonic nondestructive testing of waveguides

Journal

Ultrasonics

Vol/No

62

PP

3-19

Year/Month

2015-09

Link

http://www.sciencedirect.com/science/article/pii/S0041624X15001341

Abstract

A magnetostrictive patch transducer (MPT) is a transducer that exploits the magnetostrictive phenomena representing interactions between mechanical and magnetic fields in ferromagnetic materials. Since MPT technology was mainly developed and applied for nondestructive ultrasonic testing in waveguides such as pipes and plates, this paper will accordingly review advances of this technology in such a context. An MPT consists of a magnetic circuit composed of permanent magnets and coils, and a thin magnetostrictive patch that works as a sensing and actuating element which is bonded onto or coupled with a test waveguide. The configurations of the circuit and magnetostrictive patch therefore critically affect the performance of an MPT as well as the excited and measured wave modes in a waveguide. In this paper, a variety of state-of-the-art MPT configurations and their applications will be reviewed along with the working principle of this transducer type. The use of MPTs in wave experiments involving phononic crystals and elastic metamaterials is also briefly introduced.

Authors

Jun Kyu Lee, Joo Kyung Lee and Yoon Young Kim

Title

Analysis of magnetic interferences in ultrasonic magnetostrictive patch transducers Division

Journal

Smart Materials and Structures

Vol/No

24(8)

PP

085026-1~9

Year/Month

2015-07

Link

http://iopscience.iop.org/0964-1726/24/8/085026/pdf/0964-1726_24_8_085026.pdf

Abstract

Magnetostrictive patch transducers are operated by the magnetostrictive principle, the coupling phenomenon between magnetic and elastic fields. When multiple magnetostrictive patch transducers are used in close proximity as for ultrasonic imaging of a damaged plate, the magnetic fields used to drive transducers can interfere with each other, resulting in distortion in the ultrasonic waves transmitted and measured by the transducers. While a number of recent papers have investigated the development and applications of the transducers, no study has been reported on the analysis of the effects of the magnetic interference. In this paper, we analyze the effects of both the dynamic actuating and the static biasing magnetic fields of a magnetostrictive patch transducer on the occurrence of undesired cross-talk signals in a receiving transducer and also on the radiation pattern of an adjacent transmitting transducer. The analysis is performed both experimentally and semi-analytically and a guideline is suggested to minimize the undesirable interference. While the specific transducer used in this study is an omni-directional Lamb wave magnetostrictive patch transducer, the findings from this study can be utilized for any type of magnetostrictive patch

transducers.

Authors

Joo Hwan Oh, Young Kwan Ahn and Yoon Young Kim

Title

Maximization of operating frequency ranges of hyperbolic elastic metamaterials by topology optimization

Journal

Structural and Multidisciplinary Optimization

Vol/No

PP

Year/Month

2015-12

Link

http://link.springer.com/article/10.1007%2Fs00158-015-1288-y

Abstract

Hyperbolic elastic metamaterials developed for sub-wavelength resolution allow wave propagation in the radial direction but prohibit wave propagation in the circumferential direction. Recently, a two-dimensional elastic metamaterial truly exhibiting the hyperbolic behavior has been realized and also experimented but there is a practically important design issue that its operating frequency range should be widened. Motivated by this need, the present investigation aims to set up a topology optimization formulation to maximize the operating frequency range. Because different wave physics are involved along the circumferential and radial directions, the topology optimization requires the extraction of the key physical phenomena along the two different directions. In doing so, the wave physics occurring in the hyperbolic elastic metamaterial is analyzed by using equivalent discrete models and the findings from the analysis are used to set up a topology optimization problem. The topology optimization that maximizes the operating frequency range of the hyperbolic elastic metamaterial is newly formulated by using the finite element method. After the metamaterial configuration maximizing the frequency range is found, the mechanics hidden in the optimized configuration is explained in some details by using analytic mass-spring model.

Authors

Pyung Sik Ma, Hyung Jin Lee and Yoon Young Kim

Title

Dispersion Suppression of Guided Elastic Waves by Anisotropic Metamaterial

Journal

The Journal of the Acoustical Society of America

Vol/No

138(1)

PP

EL77~EL82

Year/Month

2015-07

Link

http://scitation.aip.org/content/asa/journal/jasa/138/1/10.1121/1.4922766

Abstract

This investigation presents a method to engineer a metamaterial exhibiting the desired anisotropic wave behavior with the specific applications toward the dispersion suppression of elastic guided waves. In the proposed approach, effective anisotropic properties required for dispersion suppression were first determined. Then the slowness curves for the metamaterial were used to find the specific unit cell configuration through inverse design. When the metamateral layers were attached to the homogeneous waveguide, the target guided mode was shown to exhibit little dispersion. Detailed engineering procedures were given, and the direct numerical simulations were performed to confirm the effectiveness of the proposed approach.

Authors

Jun Hyeong Park, Pyung Sik Ma, Yoon Young Kim

Title

Design of Phononic Crystals for Self-collimation of Elastic Waves Using Topology Optimization Method

Journal

Structural and Multidisciplinary Optimization

Vol/No

51(6)

PP

1199-1209

Year/Month

2015-06

Link

http://link.springer.com/article/10.1007%2Fs00158-014-1206-8

Abstract

Self-collimating phononic crystals (PCs) are periodic structures that enable self-collimation of waves. While various design parameters such as material property, period, lattice symmetry, and material distribution in a unit cell affect wave scattering inside a PC, this work aims to find an optimal material distribution in a unit cell that exhibits the desired self-collimation properties. While earlier studies were mainly focused on the arrangement of self-collimating PCs or shape changes of inclusions in a unit cell having a specific topological layout, we present a topology optimization formulation to find a desired material distribution. Specifically, a finite element based formulation is set up to find the matrix and inclusion material distribution that can make elastic shear-horizontal bulk waves propagate along a desired target direction. The proposed topology optimization formulation newly employs the geometric properties of equi-frequency contours (EFCs) in the wave vector space as essential elements in forming objective and constraint functions. The sensitivities of these functions with respect to design variables are explicitly derived to utilize a gradient-based optimizer. To show the effectiveness of the formulation, several case studies are considered.

Authors

Hongjin Kim, Sowon Lee, Chulmin Cho, Jae Eun Kim, Byeng Dong Youn and Yoon Young Kim

Title

An experimental method to design piezoelectric energy harvesting skin using operating deflection shapes and its application for self-powered operation of a wireless sensor network

Journal

Journal of Intelligent Material Systems and Structures

Vol/No

26(9)

PP

1128–1137

Year/Month

2015-06

Link

http://jim.sagepub.com/content/26/9/1128

Abstract

This work proposes a more practical way of designing the piezoelectric energy harvesting skin, which facilitates the application of the proposed idea to any practical engineering facilities with vibration. Unlike earlier studies to make use of analytical model for piezoelectric energy harvesting design, this work proposes an experimental method to design a piezoelectric energy harvesting skin using operating deflection shapes of a target mechanical system. This method is more practical in the sense that actual vibration of a target mechanical system can be characterized far better for the piezoelectric energy harvesting design. The key idea is to extract continuous in-plane normal strain values from discrete out-of-plane deformations of a target mechanical system. From multiple strain inflection lines, piezoelectric energy harvesting skin segments are designed to maximize a total power output. It is demonstrated that four piezoelectric energy harvesting skin segments designed by the proposed idea can generate sufficient power from the outdoor condensing unit of an air conditioning for self-powered operation of a wireless sensor network.

Authors

Heung Son Lee, Yoon Young Kim

Title

Multipole expansion of Green’s function for guided waves in a transversely isotropic plate

Journal

Journal of Mechanical Science and Technology

Vol/No

29(5)

PP

1899-1906

Year/Month

2015-05

Link

http://link.springer.com/article/10.1007%2Fs12206-015-0411-8

Abstract

The multipole expansion of Green’s function in a transversely isotropic plate is derived based on the eigenfunction expansion method. For the derivation, Green’s function is expressed in a bilinear form composed of the regular and singular Lamb-type (or shear-horizontal) wave eigenfunctions. The specific form of the derived Green’s function facilitates the handling of general scattering problems in an elastic plate when numerical methods such as the methods of the null-field integral equations are employed. In the derivation, the integral transform of an arbitrary guided wave field is first constructed by the Lamb-type and shear horizontal wave eigenfunctions that work as the kernel functions. After showing that the thickness-dependent parts of the eigenfunctions are orthogonal to each other in the transformed space, Green’s function is explicitly derived by using the orthogonality. As an application of the derived Green’s function, a scattering problem is solved by the transition matrix method.

Authors

Jieun Yang, Joong Seok Lee and Yoon Young Kim

Title

Metaporous Layer to Overcome the Thickness Constraint for Broadband Sound Absorption

Journal

Journal of Applied Physics

Vol/No

117(17)

PP

174903

Year/Month

2015-05

Link

http://scitation.aip.org/content/aip/journal/jap/117/17/10.1063/1.4919844

Abstract

The sound absorption of a porous layer is affected by its thickness, especially in a low-frequency range. If a hard-backed porous layer contains periodical arrangements of rigid partitions that are coordinated parallel and perpendicular to the direction of incoming sound waves, the lower bound of the effective sound absorption can be lowered much more and the overall absorption performance enhanced. The consequence of rigid partitioning in a porous layer is to make the first thickness resonance mode in the layer appear at much lower frequencies compared to that in the original homogeneous porous layer with the same thickness. Moreover, appropriate partitioning yields multiple thickness resonances with higher absorption peaks through impedance matching. The physics of the partitioned porous layer, or the metaporous layer, is theoretically investigated in this study.

Authors

Joong Seok Leea, Peter Göransson, Yoon Young Kim

Title

Topology Optimization for Three-phase Materials Distribution in a Dissipative Expansion Chamber by Unified Multiphase Modeling Approach

Journal

Computer Methods in Applied Mechanics and Engineering

Vol/No

287

PP

191-211

Year/Month

2015-04

Link

http://www.sciencedirect.com/science/article/pii/S0045782515000249

Abstract

The sound attenuation performance of a dissipative expansion chamber is a combinational result of reflective and dissipative effects. Although it is well known that the performance can be substantially improved by altering the distribution of constituent materials, the process of finding an optimal distribution of the materials still remains a challenge. This work proposes a new design method for interior space of a dissipative expansion chamber by using topology optimization method. Different from the existing topology optimizations for expansion chamber designs based on simplified material modeling, the present design deals with fully-modeled multiphase constituent materials, such as acoustic, poroelastic and elastic one. Difficulties in the optimization formulation for the multiphase material distribution arise from extremely-different acoustic behavior of the materials and the use of various governing equations for different phase materials. To systematically vary the attributes of the chamber interior space, a unified multiphase modeling approach that allows continuous variations between the three-phase materials within the same implementation is employed with an elaborately-derived penalty parameters of material interpolation functions. Various design examples are successfully solved for wide frequency bands and the optimal configurations clearly demonstrate the importance of using specific configurations tuned to different target frequencies.

Authors

Joong Seok Lee

Title

A new unified modeling of acoustically-coupled multilayers by (US,uw) displacement formulation based poroelastic transfer matrix

Journal

Journal of Mechanical Science and Technology

Vol/No

29(2)

PP

663-676

Year/Month

2015-02

Link

http://www.materic.or.kr/info/scholar/content.asp?p_id=173891&s_code=JP&s_kinds=s_title&s_word=&mode=&field_code=1

Abstract

This paper is devoted to propose a new unified modeling technique to efficiently present acoustically-coupled multilayers consisting of air, elastic and poroelastic layers. The present work is motivated by a need for more efficient modeling in multilayer design optimization, such as topology optimization. Aporoelastic transfer matrix derived from Biot's theory is employed for a unified modeling ofmultilayers. The existing unified modeling approaches were derived by the field variables of the solid- and fluid-phase displacements. In this work, the solid-phase displacement (US) and a compound form (uw) of the solid- and fluid-phase displacements are used as the generalized coordinates to develop a new unified modeling technique. When the alternative formulation is used, the physics of wave propagation in multilayers can be better explained. Moreover, it also allows the macroscopic material coefficients to be directly controlled for the design optimization applications compared with the existing unified modeling techniques.

Authors

Hyuk Lee, Joo Hwan Oh, Yoon Young Kim

Title

Multiple beam splitting in elastic phononic crystal plates

Journal

Ultrasonics

Vol/No

56

PP

178-182

Year/Month

2015-02

Link

http://www.sciencedirect.com/science/article/pii/S0041624X14002972

Abstract

This work presents an experimental evidence for triple beam splitting in an elastic plate with an embedded elastic phononic crystal (PC) prism and elaborates on its working mechanism. While there were reports on negative refraction and double beam splitting with PCs, no experimental evidence on the splitting of triple or more ultrasonic elastic beams through PCs has been shown yet. After the experimental results are presented in case of triple beam splitting, further analysis is carried out to explain how triple or more beams can be split depending on elastic PC prism angles.

Authors

Hwa Jung Kim, Ju Seung Lee, Hoe Woong Kim, Heung Sun Lee and Yoon Young Kim

Title

Numerical simulation of guided waves using equivalent source model of magnetostrictive patch transducers

Journal

Smart Materials and Structures

Vol/No

24(1)

PP

015006-1~18

Year/Month

2015-01

Link

http://iopscience.iop.org/0964-1726/24/1/015006

Abstract

While magnetostrictive patch transducers have become more widely used for damage inspection of waveguides such as plates or pipes, the numerical simulation of guided waves excited by the transducers is very limited. Recently, one-way, coupled time-harmonic, finite element analysis based on the linearized magnetostrictive coupling equation has been conducted, but transient analysis must be carried out to extract more information including the reflected waves from cracks. Naturally, fully coupled or one-way coupled multiphysics transient finite element analysis would require substantial computational resources and cost. In order to utilize an efficient structural finite element method or code, we newly propose equivalent distributed force models that can describe the actuation mechanism of the magnetostrictive patch transducer without explicitly solving coupled multiphysics equations. Once equivalent force models are established, transient wave simulations in inspected waveguides can be accomplished efficiently through any structural finite element code. The essence of this approach is to establish the equivalent actuating force models useful for various types of magnetostrictive patch transducers currently available. The validity of the developed models is checked by investigating the wave radiation patterns of the Rayleigh–Lamb and shear–horizontal waves in test waveguides, and the simulated results obtained with the proposed models are compared against the available experimental results.

Authors

Jae Eun Kim, Hongjin Kim, Hansol Yoon, Yoon Young Kim, Byeng D. Youn

Title

An Energy conversion model for cantilevered piezoelectric vibration energy harvesters using only measurable parameters

Journal

International Journal of Precision Engineering and Manufacturing-Green Technology

Vol/No

2(1)

PP

51-57

Year/Month

2015-01

Link

http://link.springer.com/article/10.1007%2Fs40684-015-0007-x

Abstract

Accurate predictions of the amount of harvestable energy available from ambient vibrations are important for design of energy harvesters and for their integration in specific applications. This need has motivated the development of many mathematical models for piezoelectric energy harvesters (PEHs). Existing models, however, require material and geometric PEH data that are often incorrect and/or unavailable. As a more accurate and practical means to meet this need, we propose an energy conversion model of a cantilevered PEH that requires only geometric data and modal parameters that can be directly measured using a standard vibration test. The newly proposed model facilitates calculation of the maximum output power and thus enables visualization of the harvestable energy from the target vibrating structure. Prediction accuracy of the proposed model was confirmed in our study through finite element analysis and experimental results. Practical use of the proposed energy conversion model was demonstrated by applying it to a cooling fan unit of a boiler facility in a power plant.

Authors

Dong Kyun Kim, Seok-Whan Chung, Byung-Gil Jeong, Seog-Woo Hong, and Hyungjae Shin

Title

An Indirectly Clamped Capacitive Micromachined Ultrasonic transducer with a High Electromechanical Coupling Factor

Journal

Sensors and Actuators A: Physical

Vol/No

203

PP

82-91

Year/Month

2013.12

Link

http://www.sciencedirect.com/science/article/pii/S0924424713004160

Abstract

An indirectly clamped capacitive micromachined ultrasonic transducer (CMUT) with a high electromechanical coupling factor is reported. An indirectly clamped diaphragm includes a circular bottom plate hanging from a ring-type middle plate below the inner end of an annular top plate. This design allows the bottom plate to vibrate in a piston-like manner. In this way, the average gap variation between electrodes can be made larger than the average diaphragm displacement. The deflection shape of the proposed diaphragm mainly depends on two geometric parameters: the relative radial position of the middle plate and the ratio of the top and bottom plate thickness. Parameter simulation results show that the optimal inner-to-outer radius ratio of the annular top plate ranges from 0.4 to 0.6 for a top and bottom plate thickness ratio of 0.5–1.5. Simulations of 8 × 8 CMUT arrays show that the ratio of the gap variation to the diaphragm displacement and the coupling factor were 1.62 and 2.49 times, respectively, than that for a conventional CMUT array. Measurements of static displacements of fabricated indirectly clamped diaphragms showed that the circular bottom plate moved uniformly as expected. In addition, immersion test results showed that the output pressure of a fabricated indirectly clamped CMUT array is 1.72 times higher than that of a flat clamped CMUT array under the same conditions. In conclusion, high transmission and reception efficiencies for a CMUT can be achievable using the proposed indirectly clamped structure.

Authors

Suh In Kim and Yoon Young Kim

Title

Topology optimization of planar linkage mechanisms

Journal

International Journal for Numerical Methods in Engineering

Vol/No

98(4)

PP

265-286

Year/Month

2014.04

Link

http://onlinelibrary.wiley.com/doi/10.1002/nme.4635/abstract

Abstract

In spite of increasing interest in gradient-based topology optimization of linkage mechanisms, it is still difficult to solve practical, realistic problems. Besides the apparent difficulty resulting from high nonlinearity, the optimization problem faces other major difficulties: difficulty to satisfy the discrete DOF condition with continuous design variables and lack of intrinsic mechanisms to generate distinct black-and-white layouts. To deal with the DOF issue, we propose a new formulation, which maximizes a single objective function, the energy transmittance efficiency. It is shown that the efficiency function maximization handles DOF redundancy and deficiency simultaneously. To obtain distinct linkage layouts, a common practice is to introduce an artificial mass constraint and/or to remove unnecessary links during optimization. However, we do not use any artificial mass constraint but post-process the optimized result to obtain the final layout by a special post-processing algorithm. In this study, the linkage design model consists of nonlinear ground bars and zero-length springs. The springs are used to fix bar-connecting nodes to the ground, generating pinned joints. After verifying the effectiveness of the proposed approach for four-bar linkage synthesis, we synthesize an automobile steering mechanism satisfying the Ackermann condition. The steering mechanism problem is solved here for the first time.

Authors

Young Eui Kwon, Hyun Joong Jeon, Hoe Woong Kim, and Yoon Young Kim

Title

Waveguide Tapering for Beam-width Control in a Waveguide Transducer

Journal

Ultrasonics

Vol/No

54(3)

PP

953-960

Year/Month

2014.03

Link

http://www.sciencedirect.com/science/article/pii/S0041624X13003302

Abstract

In a waveguide transducer that transmits an ultrasonic wave through a waveguide unit to a test structure, it is most preferred to send a non-dispersive ultrasonic wave of a narrow beam width. However, there is an unresolved conflict between the generation of the non- or less-dispersive wave and the transmission of a narrow-beam wave into a test structure. Among others, the thickness of the waveguide unit in a waveguide transducer is the key variable determining these two conflicting criteria, but the use of a uniformly-thick waveguide of any thickness cannot fulfill the two conflicting criteria simultaneously. In this study, we propose a specially-engineered tapered waveguide unit for the simultaneous satisfaction. An excitation unit is installed at the end of the thin region of the tapered waveguide and generates only the lowest non-dispersive shear-horizontal wave. Then the generated wave propagates through the tapered region of the waveguide unit and reaches the thick region of the waveguide with insignificant mode conversion to higher modes. If the tapered waveguide is used, the surviving lowest mode in the thick region of the waveguide is shown to carry most of the transmitted power and is finally propagated into a test structure. Because the beam size of the propagated wave and the thickness of the contacting waveguide region are inversely related, the thick contacting region of the tapered waveguide ensures narrow beam width. Numerical and experimental investigations were performed to check the effectiveness of the proposed waveguide-tapering approach.

Authors

Joo Hwan Oh, Hong Min Seung and Yoon Young Kim

Title

A truly hyperbolic elastic metamaterial lens

Journal

Applied Physics Letters

Vol/No

104

PP

073503

Year/Month

2014.02

Link

http://scitation.aip.org/content/aip/journal/apl/104/7/10.1063/1.4865907

Abstract

Sub-wavelength imaging is possible if metamaterial lenses realizing hyperbolic or elliptic Equi-Frequency Contours (EFCs) are used. Theoretically, lenses exhibiting hyperbolic EFCs allow imaging with unlimited resolution, but only metamaterials of elliptic EFCs producing limited resolution have been so far realized in elastic field. Thus, an elastic metamaterial lens realizing truly hyperbolic EFCs can lead to superior-resolution ultrasonic imaging. This Letter presents the realization of an elastic lens exhibiting truly hyperbolic EFCs and its experimental verification.

Authors

Pyung Sik Ma, Young Eui Kwon, and Yoon Young Kim

Title

Wave Dispersion Tailoring in an Elastic Waveguide by Phononic Crystals

Journal

Applied Physics Letters

Vol/No

103(15)

PP

151901-1~4

Year/Month

2013.10

Link

http://scitation.aip.org/content/aip/journal/apl/103/15/10.1063/1.4824476

Abstract

Waveguides may inevitably excite undesired modes and induce dispersion-related distortion. This is true when an elastic homogeneous waveguide is used to carry a pulse of which the center frequency is above its cutoff frequency. We show that these problems can be avoided if a properly-tailored phononic crystal (PC) structure is inserted inside a waveguide. In engineering the PC, we open the band gap of the undesired wave mode and make the dispersion curve of the desired wave branch straight in a target frequency band. Numerical simulations and ultrasonic experiments using the engineered PC confirmed the validity of the proposed approach

Authors

Gang-Won Jang, Soo Min Choi, Yoon Young Kim

Title

Analysis of Three Thin-Walled Box Beams Connected at a Joint under Out-of-Plane Bending Loads

Journal

Journal of Engineering Mechanics

Vol/No

139(10)

PP

1350–1361

Year/Month

2013.10

Link

Abstract

When thin-walled box beams meet at an angled joint, the joint region exhibits significant flexibilities that cannot be modeled by the Timoshenko or Euler beam theory especially at a joint of multiple box beams. Conventionally, the flexibilities are represented by artificial spring elements when the Timoshenko beam theory is used. On the other hand, this investigation presents a higher-order beam analysis applicable

to three thin-walled box beams connected at a joint under out-of-plane bending loads; no artificial spring element will be used. Because the higher-order theory includes the sectional distortion and warping degrees of freedom, it accurately predicts the structural behavior of thinwalled straight box beams. The main issue in joint analysis is how to match all field variables of bending displacement, bending rotation, warping, and distortion at the joint—no investigation applicable to the analysis of a joint of three box beams has been reported so far. The difficulties result from the fact that the standard vector transformation useful for the Timoshenko beam is not applicable, because the higher-order beam theory involves variables producing zero resultant. In this work, a three-beam joint-matching condition using the Lagrange multipliers is proposed where the three-dimensional displacements calculated by the higher-order beam theory are imposed to be continuous along shared edges of interfacing beams. The validity of the developed matching approach is tested with several numerical examples.Aspecial consideration is also discussed to deal with the case when joint angles between two of the three beams are unequal.

Authors

Joo Kyung Lee, Hoe Woong Kim and Yoon Young Kim

Title

Omnidirectional Lamb Waves by Axisymmetrically - Configured Magnetostrictive Patch Transducer

Journal

IEEE-UFFC

Vol/No

60(9)

PP

1928-1934

Year/Month

2013.09

Link

http://dx.doi.org/10.1109/TUFFc.2013.2777

Abstract

This work presents the generation of omnidirectional

Lamb waves by a new magnetostrictive patch transducer

(MPT) and investigates its generation mechanism. Although

MPTs have been widely used for wave transduction in plates

and pipes, no investigation reports the generation of omnidirectional

Lamb waves in a plate by an MPT. For the generation,

we propose an axisymmetrically-configured MPT that

installs multiple axisymmetric turns of coil outside of a permanent

cylindrical magnet located above the center of a circular

magnetostrictive patch. After confirming the omnidirectivity

of the proposed MPT experimentally, the mechanism of the

Lamb wave generation and its frequency characteristics are

investigated. It is also shown that the Lamb wave is most efficiently

generated in a test plate when its wavelength is equal

to two-thirds of the magnetostrictive patch diameter. If this

wavelength–patch diameter relation holds, the second radial

extensional vibration mode of the patch of the proposed MPT

is shown to be the mode responsible for generating the Lamb

wave in a plate.

Authors

Jae Eun Kim and Yoon Young Kim

Title

Power Enhancing by Reversing Mode Sequence in Tuned Mass-Spring Unit Attached Vibration Energy Harvester

Journal

AIP Advances

Vol/No

3(7)

PP

072103-1~7

Year/Month

2013.07

Link

http://aipadvances.aip.org/resource/1/aaidbi/v3/i7/p072103_s1

Abstract

We propose a vibration energy harvester consisting of an auxiliary frequency-tuned mass unit and a piezoelectric vibration energy harvesting unit for enhancing output power. The proposed integrated system is so configured that its out-of-phase mode can appear at the lowest eigenfrequency unlike in the conventional system using a tuned unit. Such an arrangement makes the resulting system distinctive: enhanced output power at or near the target operating frequency and very little eigenfrequency separation, not observed in conventional eigenfrequency-tuned vibration energy harvesters. The power enhancement of the proposed system is theoretically examined with and without tip mass normalization or footprint area normalization

Authors

Joo Hwan Oh, Kyung Ho Sun and Yoon Young Kim

Title

Time-harmonic Finite Element Analysis of Guided Waves Generated by Magnetostrictive Patch Transducers

Journal

Smart Materials and Structures

Vol/No

22

PP

085007-1~13

Year/Month

2013.07

Link

Abstract

Authors

Hong Min Seung, Hoe Woong Kim and Yoon Young Kim

Title

Development of an Omni-directional Shear-Horizontal Wave Magnetostrictive Patch Transducer for Plates

Journal

Ultrasonics

Vol/No

53(7)

PP

1304-1308

Year/Month

2013.09

Link

http://dx.doi.org/10.1016/j.ultras.2013.03.015

Abstract

As an effective tool to inspect large plates, omni-directional guided wave transducers have become more widely used to form phased-array inspection systems. While omni-directional Lamb wave transducers have been successfully utilized in the systems, omni-directional Shear-Horizontal (SH) wave transducers have not been investigated. In this paper, we propose an omni-directional SH magnetostrictive patch transducer that consists of an annular magnetostrictive patch, a toroidal coil and a permanent magnet. After presenting the unique configuration of the proposed transducer and its working principle, the omni-directivity of the developed transducer is verified through simulations and experiments conducted in an aluminum plate. The frequency characteristics of the proposed transducer depending on the patch size are also investigated as the underlying reference data for future construction of an SH phased-array system.

Authors

Joo Hwan Oh, Yoon Jae Kim and Yoon Young Kim

Title

Wave attenuation and dissipation mechanisms in viscoelastic phononic crystals

Journal

Journal of Applied Physics

Vol/No

113(10)

PP

106101-1~3

Year/Month

2013.03

Link

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=6479649&reason=concurrency

Abstract

This work investigates wave attenuation and dissipation mechanisms in viscoelastic phononic crystals (VPCs) having different inclusion types in a long-wavelength regime. After investigating the intrinsic damping properties of VPCs for different inclusion sizes and materials, we carried out wave simulations revealing the energy dissipation by a finite VPC structure inserted inside an elastic medium. The simulations, supported by physical reasoning, showed that air- and metal-embedded VPCs can indeed dissipate more wave energy than pure viscoelastic media in low and high frequency ranges, respectively.