Mode Conversion Behavior Research Articles

Prominent phonon transmission across aperiodic superlattice through coherent mode-conversion

In both particle and wave descriptions of phonons, the dense, aperiodically arranged interfaces in aperiodic superlattices are expected to strongly attenuate thermal transport due to phonon-interface scattering or broken long-range coherence. However, non-trivial thermal conductivity is still observed in these structures. In this study, we reveal that incoherent modes propagating in the aperiodic superlattice can be converted, through interference, into coherent modes defined by an approximate dispersion relation. This conversion leads to high transmission across the aperiodic superlattice structure, which contains hundreds of interfaces, ultimately resulting in non-trivial thermal conductivity. Such incoherent-to-coherent mode-conversion behavior is extensively observed in periodic superlattices. This work suggests an effective strategy to manipulate the phonon dispersion relation through layer patterning or material choice, enabling precise control of phonon transmission across aperiodic superlattices.

Influence of spatial coherence on phonon transmission across aperiodically arranged interfaces

In this study, we employ the atomistic wave-packet method to directly simulate coherent phonon transport and scattering dynamics in an aperiodic superlattice structure with aperiodically arranged interfaces. Our investigation reveals that interference dynamics, contingent upon the relative magnitudes of phonon wavelength, spatial coherence length, and average interface spacing, profoundly influence transmission spectra and mode-conversion behavior. Particularly, longer-wavelength phonons possessing larger coherence lengths exhibit more pronounced destructive interference and mode-conversion, leading to generally lower transmission. The insights gained into coherent phonon wave physics from this study can significantly augment the analysis and design of phononic devices.

Scattering of the Fundamental Lamb Modes in Bent Metallic Plates

Abstract Guided elastic waves, propagating through curved waveguides, have attracted significant attention in the recent past, both from the perspectives of assessment of structural integrity and generating novel designs of acoustic waveguides. This article presents investigation of the interaction of the fundamental Lamb modes with a cylindrical bend in thin metallic plates. A hybrid numerical method is exposited, which combines the computational efficacy of the semi-analytical finite element method in modeling the long straight portions of the plate and the versatility of the conventional finite element method in modeling the bent portion. The predictive capabilities of the proposed method are validated using transient finite element simulations. Appropriate modifications to the hybrid method, needed for simulating the multimodal incidence resulting from the point-force actuation, are discussed. Using the hybrid method, the scattering and mode-conversion behavior, imparted by the cylindrical bend, is studied when the two fundamental Lamb modes are used as the incident interrogation signals. The extents of equi-modal and cross-modal contributions in both the reflected as well as transmitted waveforms are quantified in terms of the respective modal energy ratios. Explicable contour charts are presented for comprehending the scattering behavior over a wide range of frequencies and bend angles that span from 0 deg to 180 deg. For two representative cases, the modal displacement patterns inside the bend region are presented and discussed. The present investigation can find its potential use in the analysis of geometrically irregular structures leading to the design of novel acoustic waveguides.

A metamaterial ultrasound mode convertor for complete transformation of Lamb waves into shear horizontal waves

This article reports a new mechanism involving a non-perforated resonant elastic metamaterial to achieve the complete conversion of Lamb waves (A0 and S0) into the fundamental shear horizontal (SH0) wave. The proposed metamaterial ultrasound mode convertor is studied via the observation of the special resonant shear motion of its unit cells, initiating with a conventional additive stub design. Thereafter, such a stubbed structure is further modified to fully couple the Lamb modes with the shear horizontal stub motion. By investigating the band structure of the metamaterial unit cell through modal analysis and tuning the shear resonant motions, a complete SH0 mode generation band within the simultaneous Lamb modes bandgap can be established in a wide frequency range. Such a special bandgap situation enables the complete mode conversion from Lamb waves into shear horizontal waves. The transformation capability of the proposed ultrasound mode convertor is further substantiated via the harmonic analysis of metamaterial chain model, showcasing the frequency spectrum of the transmitted wave modes. The optimal configuration is determined by conducting a parametric study to identify the most effective mode conversion performance. Finally, a coupled-field transient finite element simulation is carried out to acquire the dynamic response of the structure. The frequency-wavenumber analysis of the transmitted wave field illuminates the successful realization of the mode conversion behavior. Experimental demonstrations are presented to validate the numerical predictions. The proposed complete mode conversion capability may possess great potential for wave control and manipulation.

Mode conversion behavior of guided wave in glass fiber reinforced polymer with fatigue damage accumulation

This paper presents a study on the mode conversion effect of guided wave in glass fiber reinforced polymer (GFRP) with fatigue damage accumulation. By using a laser ultrasonic system, a series of time-space diagrams of guided wave has been obtained, which shows the failure process of the specimen under tension-tension fatigue loading. An interesting phenomenon that the mode conversion effect becomes increasing more pronounced with the micro-damage accumulation can be observed in the experiment and further quantitatively evaluated by defining an index MMR (Mode-to-Mode Ratio). An investigation of the relationship between mode conversion effect and matrix crack density is then carried out by means of the finite element method. Influences of various factors such as the density and the randomness of the simulative matrix crack distribution are discussed. Results show that the matrix crack density makes a major contribution to the mode conversion effect in despite of the limited influence caused by the random distributions of the matrix cracks, which offers great potential for composite fatigue evaluation by using the mode conversion effect.

Damage localization method for plates based on the time reversal of the mode-converted Lamb waves

A damage localization method based on the time reversal focusing of the mode-converted scattered Lamb wave is proposed for plate structures with a non-symmetric defect in the thickness direction. Dual transducers are attached symmetrically on the upper and lower surfaces of the plate to selectively emit and receive the lowest-order symmetric (S0) and antisymmetric (A0) modes. The localization of damage is achieved by the numerical time-reversed (TR) simulation of the mode-converted Lamb wave generated at the defect. To investigate the validity of the proposed method, the signals of the Lamb waves in a plate with a partial-thickness notch are numerically simulated by the three-dimensional elastodynamic finite integration technique (EFIT). When the S0 mode is emitted in the damaged plate, not only the S0 mode is scattered but also the A0 mode is generated due to mode conversion at the notch. Similar mode conversion behavior is confirmed when the A0 mode is emitted. The time reversal of the mode-converted scattered Lamb waves creates focused spots at the damage location without using baseline data for the undamaged plate. The proposed method reduces the magnitude of the artifacts compared to the time reversal of the non-mode-converted Lamb wave, and yields the focused spot whose size is associated with the size of the notch and the half wavelength of the time-reversed wave mode. Furthermore, the proposed method is applied to a plate with a notch-type defect adjacent to an a priori known through-thickness hole, demonstrating the damage localization in a relatively complicated structure.

Guided-Wave Properties of Mode-Selective Transmission Line

The so-called mode-selective transmission line or simply “MSTL” is studied theoretically and experimentally. This low-loss and low-dispersion transmission line operates with a frequency-dependent mode-switching behavior. This self-adaptive mode-selective guided-wave structure begins with the propagation of electromagnetic waves over the lower frequency range in the form of a quasi-TEM fundamental mode similar to the microstrip line case, then followed by a fundamental quasi-TE10 mode with reference to rectangular waveguide over the higher frequency region. To gain insight into the physical mechanism and fundamental features of this mode-selective transmission line, a detailed semi-analytical hybrid-mode analysis is developed through the application of a method of lines. This method allows accurate and effective modeling of MSTL guided-wave properties. Propagation characteristics of this proposed mode-agile structure in terms of dispersion, modal, and loss properties are examined, which leads to the establishment of some basic MSTL design guidelines. Numerical results confirm the expected mode conversion and low-loss behavior through the observation of field evolutions along the structure. For experimental verification, a set of MSTL prototypes are fabricated on two different substrates through dissimilar fabrication processes. Measurements are carried out from dc-to-500 GHz using a vector network analyzer. Excellent agreement between theoretical and experimental results is observed. It is confirmed that the low-dispersion and low-loss behavior of MSTL makes it an outstanding integrated waveguide in support of high-performance super-broadband signal transmission and/or ultra-fast pulse propagation in a fully-integrated platform.

Circumventing velocity uncertainty in imaging complex structures

In areas with irregular topography, complex near surface, and complex subsurface, there is much uncertainty in rms velocity estimation for prestack time migration, whereas interval velocity estimation for prestack depth migration is despairingly challenging. We often attribute the velocity uncertainty to various factors, including strong to severe lateral velocity variations, heterogeneity, anisotropy, mode conversion, and three-dimensional behavior of complex structures. Nevertheless, it is not easy to identify the cause of and account for the uncertainty as it often is a combination of the various factors. And the analyst struggles with much difficulty when estimating a velocity field, whether it is for prestack time or depth migration. Velocity uncertainty invariably gives rise to erroneously high or low migration velocities, which then causes two problems with prestack migration: (1) we fail to preserve reflector amplitudes, and (2) we also fail to position the reflectors correctly and focus diffractions to their apexes. We may choose to solve both problems simultaneously as we currently attempt to do with prestack migration workflows, or we may choose to solve them one after the other as we used to do in the 1980s and '90s by workflows that included dip-moveout correction. The quality of image gathers associated with prestack migration may not be adequate for velocity updating and verification and thus may or may not warrant the simultaneous solution. In areas with irregular topography, complex near surface, and complex subsurface, it may not. What then? I propose a workflow, applicable to both 2-D and 3-D seismic data, to solve the two problems with prestack time migration one after the other. The workflow is based on synthesis of a zero-offset wavefield to capture and preserve all reflections and diffractions, followed by zero-offset time migration.

Asymmetric propagation of the first order antisymmetric lamb wave in a tapered plate based on time domain analysis

The asymmetric propagation of the first order antisymmetric (A1) Lamb wave in a tapered plate respectively carved with sharp bottom corner and round bottom corner is theoretically investigated. Through numerical simulation of A1 Lamb wave in time domain, we find that when the thickness of the waveguide abruptly decreases to below the cut-off thickness, about half of the A1 mode is converted into the fundamental symmetrical S0 and antisymmetrical A0 modes to pass through the defected region. Furthermore, the transmitted modes A0 and S0 are completely apart from each other and can be quantitatively evaluated. Conversely, when the thickness change is very smooth, most of the energy of A1 Lamb wave is reflected back. It is the unique mode conversion behavior that leads to great transmission difference value of A1 Lamb wave along the opposite directions. Finally, the influence of geometrical parameters on the transmission coefficient is also studied. The higher efficiency and proper working frequency range can be realized by adjusting the slope angle θ, height h 1 and h 2. The simple asymmetric systems will be potentially significant in applications of ultrasound diagnosis and therapy.

Mode Conversion Behavior of Guided Wave in a Pipe Inspection System Based on a Long Waveguide.

To make clear the mode conversion behavior of S0-mode lamb wave and SH0-plate wave converting to the longitudinal mode guided wave and torsional mode guided wave in a pipe, respectively, the experiments were performed based on a previous built pipe inspection system. The pipe was wound with an L-shaped plate or a T-shaped plate as the waveguide, and the S0-wave and SH0-wave were excited separately in the waveguide. To carry out the objective, a meander-line coil electromagnetic acoustic transducer (EMAT) for S0-wave and a periodic permanent magnet (PPM) EMAT for SH0-wave were developed and optimized. Then, several comparison experiments were conducted to compare the efficiency of mode conversion. Experimental results showed that the T(0,1) mode, L(0,1) mode, and L(0,2) mode guided waves can be successfully detected when converted from the S0-wave or SH0-wave with different shaped waveguides. It can also be inferred that the S0-wave has a better ability to convert to the T(0,1) mode, while the SH0-wave is easier to convert to the L(0,1) mode and L(0,2) mode, and the L-shaped waveguide has a better efficiency than T-shaped waveguide.

616 有限幅を有するCFRP接着構造におけるラム波を用いた剥がれ検知

Debonding in a skin-stringer panel is one of the main problems in actual application of carbon fiber reinforced plastic (CFRP) composite. Although an ultrasonic structural health monitoring technique using mode conversions of Lamb waves can evaluate this defect effectively in infinite-width CFRP laminates, this method cannot be applied directly to the bonded structure with a finite-width stringer. Hence, in this research, the mode conversion behavior in a simple finite-width CFRP laminate with an artificial delamination was investigated as a basic study. The theoretical dispersion curves of Lamb waves in the finite-width laminate were calculated by a semi-analytical finite element method. Then, the experiments were conducted at various frequencies with an acousto-ultrasonic system based on macro fiber composite actuators and fiber Bragg grating sensors for the investigation of Lamb wave propagation behavior in the finite-width laminate. The results indicated that mode conversion occurs in the delaminated area of the finite-width CFRP laminate. Therefore, the mode conversion of Lamb waves has a possibility to detect delamination damage in finite-width composite structures.

Axial Transmission Method for Long Bone Fracture Evaluation by Ultrasonic Guided Waves: Simulation, Phantom and in Vitro Experiments

Mode conversion occurs when the ultrasonic guided waves encounter fractures. The aim of this study was to investigate the feasibility of fracture assessment in long cortical bone using guided-mode conversion. Mode conversion behavior between the fundamental modes S0 and A0 was analyzed. The expressions proposed for modal velocity were used to identify the original and converted modes. Simulations and phantom experiments were performed using 1.0-mm-thick steel plates with a notch width of 0.5 mm and notch depths of 0.2, 0.4, 0.6 and 0.8 mm. Furthermore, in vitro experiments were carried out on nine ovine tibias with 1.0-mm-wide partial transverse gap break and cortical thickness varying from 2.10 to 3.88 mm. The study confirmed that mode conversion gradually becomes observable as fracture depth increases. Energy percentages of the converted modes correlated strongly with fracture depth, as illustrated by the frequency-sweeping experiments on steel phantoms (100–1100 kHz, r2 = 0.97, p < 0.0069) and the fixed-frequency experiments on nine ovine tibias (250 kHz, r2 = 0.97, p < 0.0056). The approaches described, including mode excitation, velocity expressions and energy percentage criteria, may also contribute to ultrasonic monitoring of long bone fracture healing.

Mode conversion behavior of SH guided wave in a tapered plate

The present study explores mode conversion of shear horizontal (SH) guided wave when it impinges on smooth defects in plates. The fundamental (SH 0) and the first higher (SH 1) modes were selectively generated by an electromagnetic acoustic transducer in aluminum plates and the propagating modes at various points were detected by a pinducer. The defects had a flat bottom region and tapered edges. Remaining thickness at the bottom defected region was smaller than the so-called cut-off thickness of SH 1 mode. Both modes exhibited unique mode conversion behaviors in tapered edge, which were interpreted with the dispersion relation. Total reflection of SH 1 mode was also observed at a specific condition. Numerical simulation revealed that the continuous wavenumber change in the tapered region and the consequent zero value at cut-off thickness cause this total reflection.

Damage Identification of Cracked Pipes Based on Reflection Characteristics of Guided Waves

A methodology of damage identification for the circumferentially cracked pipe is presented by using the quantitative relationship between crack size and reflection characteristics of guided waves. Firstly, the reflection characteristics and mode conversion behavior were theoretically studied by the 3-D finite element (FE) analysis. It is found that the reflection coefficients (RCs) of longitudinal L(0,2) and F(1,3) modes quantitatively is related to the circumferential length and radial depth of the crack. Then we present a novel method to quantitatively identify the crack. The feasibility of the proposed method was numerically investigated. The method only requires the longitudinal excitation mode of guided wave in the pipe, which shows the good potential for real application or in-situ damage monitoring. Introduction

A finite element procedure for multiscale wave equations with application to plasma waves

A finite element wave-packet procedure is presented to solve problems of wave propagation in multi-scale behavior. The proposed scheme combines the advantages of the finite element and spectral methods. The basic formulation is presented, and the capabilities of the procedure are demonstrated through the solution of some illustrative problems, including a problem that characterizes the mode-conversion behavior in plasmas.

The transmission of Lamb waves across adhesively bonded lap joints

The transmission of Lamb waves across adhesively bonded lap joints is investigated using finite element analysis. The studies consider three modes for excitation and reception, s0, a0, and a1, applied to lap joints consisting of parallel aluminum sheets bonded with an epoxy adhesive. Transmission coefficient results for a two-dimensional range of bond thicknesses and bond overlap lengths are presented for all three modes. The transmission coefficients are calculated from the spectra of the received and transmitted signals using an approach which is insensitive to the presence of multimode signals and reverberated signals, and which approximates to a power transmission coefficient. Detailed analysis is then performed for one of the modes in order to investigate the nature of the mode conversion in the overlap region of the joint. It is found that the relative amplitudes of the different modes which propagate in the overlap region can be estimated reliably and simply from the properties of the incident wave mode. As well as demonstrating the physics of the mode conversion behavior, the study provides a basis for the selection of modes for nondestructive evaluation (NDE) of the bond region and for measuring the bond dimensions.

Wave leakage from groove NRD structures

It is shown that the offset of grooves in the groove nonradiative dielectric (GNRD) structure causes the leakage of power from the operating mode fields. Numerical results based on a full-wave analysis are provided to show the complex propagation properties of the operating mode as a function of the offset dimension of the grooves and the frequency. Mode conversion behavior between the operating mode and the dominant mode is also illustrated. Some conclusions about the usage of the GNRD guide are drawn.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coupled mode analysis of dielectric planar branching waveguides

The mode conversion behavior of planar dielectric branching waveguides is studied as a function of branching angle, propagation direction of modes, and thickness ratio of branches. The multimode coupled equation and new step approximation method of branch taper are introduced, which give the accurate analysis of mode behavior in the branching waveguide of a large taper angle.

Mode conversion in planar-dielectric separating waveguides

The mode-conversion behavior of branching or separating planar-dielectric waveguides is studied theoretically as a function of branch asymmetry and taper slope. When the taper slope is large in a near-symmetric structure, considerable mode conversion will occur and the structure will act as a power divider. In an asymmetric structure with small taper slope, mode conversion is negligible and the structure will act as a mode splitter. A quantitative formula is presented which can be used to determine the behavior of a given separating structure.