Unwanted Waves Research Articles

Electromagnetic Absorption and Mechanical Properties of Natural Rubber Composites Based on Conductive Carbon Black and Fe3O4.

In contemporary civilization, the electromagnetic radiation from electronic devices and communication systems has become a substantial pollutant. High-performance electromagnetic absorbers have become a solution for absorbing unwanted electromagnetic waves. This research proposed a lightweight and flexible electromagnetic absorber produced from natural rubber filled with conductive carbon black (CCB) and Fe3O4. The effect of CCB, Fe3O4, and a combination of CCB and Fe3O4 as a hybrid filler on foam morpholog, electromagnetic reflectivity, tensile strength, and compression set properties were investigated. In addition, the effect of the alternating layered structure of CCB and Fe3O4 on electromagnetic absorption was investigated. The results indicated that the composite foam exhibited an interconnected network structure that enhanced the electromagnetic attenuation in the absorber. CCB increased the electromagnetic absorption of the foam, whereas Fe3O4 had less of an effect. The foam filled with the hybrid filler at the CCB/Fe3O4 ratio of 8/2 exhibited excellent electromagnetic absorption. The composite foam had a higher tensile modulus and higher strength compared to neat foam. The addition of CCB decreased the compression set; however, the compression set was improved by the incorporation of Fe3O4. Composite foams filled with hybrid filler can serve as highly efficient electromagnetic absorbing materials.

Electrospun carbon fibers embedded with core–shell TiC@TiO[formula omitted] nanostructures and their epoxy composites for potential EMI shielding in the Ku band

The demand for developing efficient shielding materials to safeguard electronic instruments from unwanted electromagnetic waves and their associated interference is growing. The present work reports on one such effective material, electrospun TiC@TiO 2 core–shell carbon fiber mats, synthesized by directly electrospinning nano titanium carbide embedded polyacrylonitrile fibers. The morphology of prepared mats and carbonized mats were observed using electron microscopy and the results are presented. The XRD, Raman spectra, XPS and EMI shielding characteristics of these carbon fiber samples were tested and are also presented. The results show the surface oxidation of TiC to TiO 2 during the conversation of the PAN fiber to carbon fiber. The electromagnetic interference shielding effectiveness of these TiC@TiO 2 carbon fiber mats was tested in the Ku band (12–18 GHz) and an excellent shielding effectiveness of ∼-33.31 dB for a single layer (thickness: 0.21 mm) was observed. Further, these mats were impregnated in an epoxy matrix and electrospun carbon fiber reinforced epoxy composites were prepared. The ease of preparation, enhanced mechanical performance and excellent shielding capability makes these TiC@TiO 2 embedded carbon fiber mats and their epoxy composites promising structural EMI shielding materials.

A Metallic Additively Manufactured Metamaterial for Enhanced Monitoring of Acoustic Cavitation‐Based Therapeutic Ultrasound

The combination of ultrasound and microbubbles allows treatment of indications that would be impossible or too risk adverse with conventional surgery. During treatment, subharmonic and ultraharmonic components that can only be generated from microbubbles are of great interest for intraoperative monitoring. However, the microbubble emissions are several orders of magnitude lower in power compared to that of the fundamental frequency component from the ultrasound applicator, resulting in a low signal‐to‐noise ratio (SNR) for monitoring. A 3D acoustic metamaterial (AMM) immersed in water is proposed for suppressing unwanted ultrasound waves, which allows the improved sensitivity for detecting weak microbubble emissions. Numerically, the importance of shear waves on the AMM transfer properties is highlighted, though only longitudinal ultrasound waves are transmitted through water. Experimentally, the design is implemented in titanium using additive manufacturing, with an attenuation level of 40 dB at the fundamental frequency. Consequently, the application of the AMM efficiently improves the SNR for subharmonic and ultraharmonic microbubble emissions by 11.8 and 11.9 dB, respectively. The subharmonic components originally overwhelmed by noise are recovered. This is the first time that AMMs have been applied to passive acoustic monitoring and this work stands to improve treatment outcomes from cavitation‐mediated focused ultrasound therapy.

Enhancing low-frequency water-column acoustic reflections in marine multichannel seismic data for seismic oceanography.

Reflections of low-frequency, broadband airgun signals from ocean water columns have long been collected in modern marine seismic surveys. However, they were barely examined because of their weak amplitude (following from low acoustic impedance contrast in water columns) and the lack of application interests. Nevertheless, in the past decades, a new cross-discipline "seismic oceanography" has developed the interest to use these water-column reflection signals to image the ocean structures. Motivated by seismic oceanography applications, we examined the acoustic multipath structure of marine seismic survey data with a focus on water-column reflections, and developed a two-step matched filtering approach to enhance water-column reflection and suppress the unwanted bubble waves. The approach was applied to process data collected from the Gulf of Mexico and led to an improvement in imaging mesoscale ocean structures when compared with the traditional matched filtering approach. For the specific data we examined, the results reveal a 11.3-dB improvement of signal-to-noise ratio by removing the noise and a 8-dB improvement of signal-to-reverberation ratio by suppressing bubble waves, while not affecting the information of ocean structures embedded in the signals. This study gains insights into features of water-column acoustic reflections and provides better tools for acoustic imaging of mesoscale ocean structures.

Enhanced microwave absorption of biomass carbon/nickel/polypyrrole (C/Ni/PPy) ternary composites through the synergistic effects

With rapid advances in the portable electronics industry, the creation of microwave absorbing materials with high-performance to dissipate the unwanted electromagnetic wave is an important prerequisite. Herein, unique ternary organic-inorganic composites (C/Ni/PPy) were fabricated by hydrothermal method combined with in-situ polymerization approach. The correlation between structure, components, and microwave absorption capability is investigated in detail. The carbon matrix was prepared through a typical “carbonization-activation” procedure. Then, the PPy particles were decorated on the surface of the biomass carbon matrix incorporated with Ni particulates via an in-situ polymerization method. The microwave absorbing results suggest that the C/Ni/PPy ternary composites exhibit stronger dielectric loss and microwave absorption capabilities than C/PPy binary composites. A reflection loss of − 42.09 dB and an outstanding effective absorption bandwidth of 5.24 GHz are achieved simultaneously with a thickness of only 2.4 mm. The excellent microwave absorbing performances can be ascribed to the unique microstructure, enhanced polarization and the synergistic effects between dielectric loss and magnetic loss. The present work achieves a new paradigm to improve microwave absorption characteristics through the combination of dielectric components with magnetic materials. Furthermore, a prototype is proposed, which has broad application prospects for applications in next-generation wearable and portable electronics.

Selective excitation of high-order shear horizontal wave (SH1) by using a piezoelectric interdigital transducer

Shear horizontal (SH) waves have received more and more attention in structural health monitoring (SHM) and nondestructive testing (NDT) since the fundamental mode (SH0) is non-dispersive and only has one displacement component, which is very convenient for signal identification and simulation. In recent years, great progress had been made in exciting single-mode SH0 wave for SHM. However, it is still difficult to excite high-order SH wave modes at present.In this work, a piezoelectric interdigital transducer, named SH-IDT, was proposed for selective excitation of the SH wave. By simplifying the bonded SH-IDT to distributed shearing line forces, an explicit formula for the far-field wave displacement was derived and validated through finite element simulations. Then, two SH1-IDTs working at 1 MHz on a 3 mm-thick plate and 3 MHz on a 1 mm-thick plate were designed and fabricated for exciting the SH1 mode and suppressing other wave modes. Their performances were examined by using a homemade 2D laser Doppler vibrometer. Nearly perfect wave mode selection and high energy concentration were observed. The displacement amplitude ratios between all the unwanted waves and the SH1 wave are smaller than 1/10 and this value turned to be as lower as 1/50 when using another SH1-IDT as the receiver. The half radiation angle of the SH1-IDT was only about 10°. All the experimental results are in good agreement with the simulation results, which validated the design of the SH1-IDT. The high selective capabilities of the designed SH-IDT make it very promising for tomography in SHM and it is expected to be widely used in near future.

Microtremor surveys based on rotational seismology: theoretical analysis with focus on separation of Rayleigh and Love waves in general wavefield of microtremors

SUMMARYThe applicability of rotational seismology to the general wavefield of microtremors is theoretically demonstrated based on a random process model of a 2-D wavefield. We show the effectiveness of taking the rotations (i.e. spatial differentiation) of microtremor waveforms in separating the Rayleigh and Love waves in a wavefield where waves are simultaneously arriving from various directions with different intensities. This means that a method based on rotational seismology (a rotational method) is capable of separating Rayleigh and Love waves without adopting a specific array geometry or imposing a specific assumption on the microtremor wavefield. This is an important feature of a rotational method because the spatial autocorrelation (SPAC) method, a conventional approach for determining phase velocities in microtremor array surveys, requires either the use of a circular array or the assumption of an isotropic wavefield (i.e. azimuthal averaging of correlations is required). Derivatives of the SPAC method additionally require the assumption that Rayleigh and Love waves are uncorrelated. We also show that it is possible to apply a rotational method to determine the characteristics of Love waves based on a simple three-point microtremor array that consists of translational (i.e. ordinary) three-component sensors. In later sections, we assume realistic data processing for microtremor arrays with translational sensors to construct a theoretical model to evaluate the effects of approximating spatial differentiation via finite differencing (i.e. array-derived rotation, ADR) and the effects of incoherent noise on analysis results. Using this model, it is shown that in a short-wavelength range compared to the distance for finite differencing (e.g. $\lambda < 3h$, where $\lambda $ and $h$ are the wavelength and distance for finite differencing, respectively), the leakage of unwanted wave components can determine the analysis limit. It is also shown that in a long-wavelength range (e.g. $\lambda > 3h$), the signal intensity gradually decreases, and thus the effects of incoherent noise increase (i.e. the signal-to-noise ratio, SNR decreases) and determine the analysis limit. We derive the relation between the SNR and wavelength. Although the analysis results quantitatively depend on the array geometry used for finite differencing, the qualitative understanding supported by mathematical expressions with a physically clear meaning can serve as a guideline for the treatment of data obtained from ADR.

Numerical implementation of the Neumann-Michell theory incorporating with the analytical panel integral for ship waves

The numerical implementation approach (named NM_LSA) of the Neumann-Michell (NM) theory by using the Linear-semi-analytical method for panel integral is established to solve ship waves. The NM theory is a boundary-integral equation method, which is discretized by using 4-node isoparametric elements with linear flow potential distribution in this paper. To overcome the difficulty of panel integral of wave component in the Green function, the NM_LSA employs the analytical panel integral related to the elementary wave. The filter function based on parabolic extrapolation within a layer bordering the free surface is used to remove unwanted short waves. The analytical panel integral of elementary wave over a triangular panel is derived and applied to compute the spatial integral of wave potential.In addition, another numerical implementation approach (named NM_LG) of the NM theory by using Linear-gauss method for panel integral is established. Two sets of filter parameters and five hulls are selected to validate the above mentioned numerical methods. The simulation results obtained by different numerical methods show good agreement with experimental measurements. Moreover, the time consumption of wave drag computation per Froude number via the NM_LSA is reduced by about 40 % compared to that via the NM_LG.

A numerically exact nonreflecting boundary condition applied to the acoustic Helmholtz equation

When modeling wave propagation, truncation of the computational domain to a manageable size requires nonreflecting boundaries. To construct such a boundary condition on one side of a rectangular domain for a finite-difference discretization of the acoustic wave equation in the frequency domain, the domain is extended on that one side to infinity. Constant extrapolation in the direction perpendicular to the boundary provides the material properties in the exterior. Domain decomposition can split the enlarged domain into the original one and its exterior. Because the boundary-value problem for the latter is translation-invariant, the boundary Green functions obey a quadratic matrix equation. Selection of the solvent that corresponds to the outgoing waves provides the input for the remaining problem in the interior. The result is a numerically exact nonreflecting boundary condition on one side of the domain. When two nonreflecting sides have a common corner, the translation invariance is lost. Treating each side independently in combination with a classic absorbing condition in the other direction restores the translation invariance and enables application of the method at the expense of numerical exactness. Solving the quadratic matrix equation with Newton’s method turns out to be more costly than solving the Helmholtz equation and may select unwanted incoming waves. A proposed direct method has a much lower cost and selects the correct branch. A test on a 2D acoustic marine seismic problem with a free surface at the top, a classic second-order Higdon condition at the bottom, and numerically exact boundaries at the two lateral sides demonstrates the capability of the method. Numerically exact boundaries on each side, each computed independently with a free-surface or Higdon condition, provide even better results.

Non-Contact Inspection of Railhead via Laser-Generated Rayleigh Waves and an Enhanced Matching Pursuit to Assist Detection of Surface and Subsurface Defects.

Laser ultrasonic technology can provide a non-contact, reliable and efficient inspection of train rails. However, the laser-generated signals measured at the railhead are usually contaminated with a high level of noise and unwanted wave components that complicate the identification of defect echoes in the signal. This study explores the possibility of combining laser ultrasonic technology (LUT) and an enhanced matching pursuit (MP) to achieve a fully non-contact inspection of the rail track. A completely non-contact laser-based inspection system was used to generate and sense Rayleigh waves to detect artificial surface horizontal, surface edge, subsurface horizontal and subsurface vertical defects created at railheads of different dimensions. MP was enhanced by developing two novel dictionaries, which include a finite element method (FEM) simulation dictionary and an experimental dictionary. The enhanced MP was used to analyze the experimentally obtained laser-generated Rayleigh wave signals. The results show that the enhanced MP is highly effective in detecting defects by suppressing noise, and, further, it could also overcome the deficiency in the low repeatability of the laser-generated signals. The comparative analysis of MP with both the FEM simulation and experimental dictionaries shows that the enhanced MP with the FEM simulation dictionary is highly efficient in both noise removal and defect detection from the experimental signals captured by a laser-generated ultrasonic inspection system. The major novelty contributed by this research work is the enhanced MP method with the developments of, first, an FEM simulation dictionary and, second, an experimental dictionary that is especially suited for Rayleigh wave signals. Third, the enhanced MP dictionaries are created to process the Rayleigh wave signals generated by laser excitation and received using a 3D laser scanner. Fourth, we introduce a pioneer application of such laser-generated Rayleigh waves for inspecting surface and subsurface detects occurring in train rails.

Micro-porous MXene/Aramid nanofibers hybrid aerogel with reversible compression and efficient EMI shielding performance

Currently, extensive utilization of electronic devices and wireless equipment require human to take affirmative measures to weaken unwanted electromagnetic wave radiations. Herein, micro-porous structure MXene/aramid nanofibers hybrid aerogel was developed from aramid nanofibers and MXene(Ti3C2Tx) flakes through freeze-drying approach. The robust aramid nanofibers established foundation for oxidation protection and reversible compressibility in skeleton. Due to the unique micro-porous structure, the MXene(Ti3C2Tx)/aramid nanofibers hybrid aerogel remained efficient shielding capacity, whose electromagnetic interference (EMI) shielding effectiveness and specific EMI shielding effectiveness reached ~56.8 dB and 3645.7 dB cm2 g−1 at thickness of 1.9 mm in X-band. Furthermore, the shielding performance could be regulated by MXene(Ti3C2Tx) content and thickness. With increasing MXene(Ti3C2Tx) loading, the porous size of MXene(Ti3C2Tx)/aramid nanofibers hybrid aerogel enlarged, and the MXene(Ti3C2Tx)/aramid nanofibers hybrid aerogel became tough and robust. Under 40% strain, the maximum compressive stress of MXene(Ti3C2Tx)/aramid nanofibers hybrid aerogel with 21 wt% MXene(Ti3C2Tx) flakes content reached ~210 kPa. This work provided feasible avenue for fabricating hybrid aerogel with reversible compressibility and efficient EMI shielding performance simultaneously.

Analysis of Horizontal to Vertical Spectral Ratio (HVSR) Curve Results from Processing using Interpolated Finite Impulse Response (IFIR) Filter

Microtremor is recorded with a device that is very sensitive to subsurface soil vibrations, that unwanted noise frequency waves are often also included. For this reason, processing needs to be done using filters that can attenuate the noise. In the previous research, signal decomposition was performed using the Interpolated Finite Impulse Response (IFIR) filter and resulted attenuation at the desired frequency value. Using the MATLAB program, microtremor data are processed with IFIR filtering, smoothing, calculation of: Horizontal to Vertical Spectral Ratio (H / V) curve, natural frequency (f0) and standard deviation (σ). The same process was carried out using Butterworth, Gaussian, and Cosine filters as a comparison with the Geopsy program. As a result, the amplitude characteristics obtained can provide a value of f 0 < 2.5 Hz. The value of f0 obtained using IFIR filter is also stable, proven by the standard deviation ranging from σ = 0.009 - 0.83 Hz, with an average of σ = 0.132 Hz. In conclusion, the IFIR filter can provide a well-attenuated H/V curve according to the specified cutoff frequency, which was 2 Hz. The IFIR filter is also considered capable of being applied to the HVSR method Microtremor data. Lowpass IFIR filter can attenuate high-frequency noise in microtremor data.

A generalized second-order 3D theory for coupling multidirectional wave propagation from a numerical model to a physical model. Part II: Experimental validation using an I-shaped segmented wavemaker

This paper provides the experimental validation of the generalized second-order three-dimensional (3D) coupling theory outlined by Yang et al. [Z. Yang, S. Liu, X. Ji, and H.B. Bingham, 2020. A generalized second-order 3D theory for coupling multidirectional nonlinear wave propagation from a numerical model to a physical model. Part I: Derivation, implementation and model verification, submitted for publication] using multidirectional nonlinear 3D irregular waves. Based on the second-order theory for two-dimensional (2D) irregular waves described by Yang et al. (2014a), this work provides a second-order dispersive correction for the physical wavemaker signal which improves the nonlinear 3D wave information transfer between the numerical and physical models compared to the first-order method of Zhang et al. (2007). The important coupling equation and its discretization schemes were presented in detail in Part I. For further validating the performance of the second-order 3D coupling model under complex waves and bathymetry conditions, in this Part II, a careful experimental validation has been carried out by establishing a flat coupling basin and a concave-convex coupling basin. In addition, a sequence of progressively more complex numerical target waves has been used and an I-shaped segmented piston-type wavemaker system has been considered for verification of the theory. The effectiveness, adaptability, and precision of the second-order 3D coupling model have been discussed in detail. The effects of the wave parameters and its nonlinearity on the coupling simulation have also been evaluated from the discrepancy error and a wave harmonic analysis. By defining a space synchronization error, the overall error of the spatial wave field has been analyzed. Coupling simulation experiments using oblique regular and irregular waves, unidirectional irregular waves, and multidirectional irregular waves show that the traditional first-order coupling model has limitations when coupling complex waves with strong nonlinearity. When controlling the waves using the second-order 3D coupling signal, the higher harmonics underlying the numerical waves are accurately captured and transferred into the physical model, and the unwanted second-order spurious free waves are substantially reduced. Results obtained from the experimental verification described in this paper demonstrate the strengths of the second-order 3D coupling theory, which are more evident with increased wave nonlinearity.

Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.

Preliminary study on the periodic base isolation effectiveness and experimental validation

Periodic material possesses distinct characteristics that can block some certain frequency waves propagate through the structure, which can be used to reduce the unwanted seismic waves in civil engineering. Base on the Bloch-Floquet theory, the certain frequencies, terms frequency band gaps, are theoretically presented and derived. A numerical model of 1D periodic foundation is established to investigate the filtering effects of elastic waves. The dynamic response and filtering effects of a three-storey steel frame with a 1D periodic foundation and concrete foundation are performed and compared through the numerical method and shaking table tests. Ambient vibration, earthquake ground motions, and sine waves are applied as the input waves. The results show that the seismic waves cannot propagate in the 1D periodic foundation when the frequencies of the waves fall into the frequency band gaps of the foundation. Simultaneously, a single unit cell has proven to possess the same frequency band gaps as those with an infinite number of unit cells and has enough response reduction inside the frequency band gaps of the periodic foundation. Consequently, the proposed periodic foundation can be used to apply in isolating environmental vibration or seismic waves in civil engineering through adjusting the frequency band gaps enveloping the exciting frequencies of the waves.

Simulation and Optimization of Electromagnetic Absorption of Polycarbonate/CNT Composites Using Machine Learning.

Electronic devices that transmit, distribute, or utilize electrical energy create electromagnetic interference (EMI) that can lead to malfunctioning and degradation of electronic devices. EMI shielding materials block the unwanted electromagnetic waves from reaching the target material. EMI issues can be solved by using a new family of building blocks constituted of polymer and nanofillers. The electromagnetic absorption index of this material is calculated by measuring the “S-parameters”. In this article, we investigated the use of artificial intelligence (AI) in the EMI shielding field by developing a new system based on a multilayer perceptron neural network designed to predict the electromagnetic absorption of polycarbonate-carbon nanotubes composites films. The proposed system included 15 different multilayer perception (MLP) networks; each network was specialized to predict the absorption value of a specific category sample. The selection of appropriate networks was done automatically, using an independent block. Optimization of the hyper-parameters using hold-out validation was required to ensure the best results. To evaluate the performance of our system, we calculated the similarity error, precision accuracy, and calculation time. The results obtained over our database showed clearly that the system provided a very good result with an average accuracy of 99.7997%, with an overall average calculation time of 0.01295 s. The composite based on polycarbonate−5 wt.% carbon nanotube was found to be the ultimate absorber over microwave range according to Rozanov formalism.

Terahertz imaging and vibro-thermography for impact response in carbon fiber reinforced plastics

In this paper, terahertz (THz) imaging and vibro-thermography (VT) are implemented for non-destructive evaluation (NDE) of CFRP subjected to impact damage. THz inspection is well-known for its outstanding detection depth in dielectric materials but unreliable in conductive materials like carbon fiber reinforced plastics (CFRP): an unwanted electro-magnetic wave will be excited under the irradiation of THz waves in conductive molecules and interrupted the reflective waves leading to a blur in the detection results. However, our experimental results showed THz imaging is comparable when it’s used to detect surface defects and shallow ones caused by impact loads. We conducted a set of VT tests as a contrast test, and the VT data are processed by Fourier transform and eigenvector decomposition algorithms, i.e. independent component analysis (ICA). Compared with VT, THz imaging show more details on surface defects and shallow-buried ones due to the absence of thermal diffusion effects. THz imaging could perform non-invasive, non-ionizing and non-contact inspection on the impact damage based on the differences in reflective index. In the end, signal-to-noise (SNR) evaluation is used as an effective means to quantitatively compare the results of these two techniques.

Design of an innovative and self-adaptive-smart algorithm to investigate the structural integrity of a rail track using Rayleigh waves emitted and sensed by a fully non-contact laser transduction system

The focus of this study is on locating surface and sub-surface defects that occur in rail tracks using Rayleigh waves that were emitted and sensed by a fully non-contact laser transduction system. As the quality of received signals varies with respect to the rail surface characteristics, spotting the reflection from a defect can be extremely challenging. These signals are in general contaminated with noise and have low repeatability that could hinder the proper identification of the defect signal. In view of this, an innovative signal processing technique called a self-adaptive-smart algorithm (SASA) was designed and developed. In SASA, the incident wave that is the first coming wave-packet is taken as a mother wavelet. A library of possible mother wavelets was designed based on the experimental data. As the incident wave for each sensing point varies because of the physical condition of the rail surface and the laser excitation, the algorithm automatically picks the mother wavelet from the library that generates the largest absolute cross-correlation with the incident wave. SASA is found to be able to suppress the unwanted wave packets from the overall signal leaving behind only the incident wave for a healthy specimen, and the incident wave and its reflection from the defect for a damaged specimen. The functioning of the algorithm was successfully tested by carrying out extensive experiments on a real rail track in the presence of different types of surface and sub-surface defects on its head and web. The obtained results prove the effectiveness of using SASA in localizing defects in rails with a marginal error. Notably, the proposed method has benefits such as being self-adaptive, can help suppress high levels of noise, bring the peak of defect reflected wave in the center, and distinguish between a healthy and damaged sample.

A Two-Step Method for Ionospheric Clutter Mitigation for HFSWR With Two-Dimensional Dual-Polarized Received Array

For high-frequency surface wave radar (HFSWR), the unwanted radio wave originated from the ionosphere is commonly called ionospheric clutter. Its presence seriously affects the performance of the HFSWR and extremely degrades the radar capability to detect target over long distances. To solve this problem, this paper proposes a two-step method for ionospheric clutter mitigation with two-dimensional dual-polarized received array. The proposed method first performs parameter estimation of the clutter, and then gives a polarimetric-adaptive-based oblique projection filter (PAB-OPF) to suppress the clutter. In the first step, due to the fact that vertically and horizontally polarized antennas are at different array elements for reducing mutual coupling and hardware cost, 2-D DOAs are estimated to give phase compensation for polarization phase delay estimation. In the second step, the PAB-OPF is proposed to eliminate phase inconsistency by matching different polarization phase delays for different vertically polarized antennas, and then to suppress the clutter efficiently in space-polarization domain. Error analysis and computational complexity of the proposed method are derived. Experimental results are shown to illustrate the superiority of the proposed method for ionospheric clutter suppression.

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

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.