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The collection and analysis of acoustic emission (AE) from weld circumferential cracks are crucial for ensuring pipeline safety. However, the unclear propagation characteristics and mode features of AE present challenges in array design and mode identification. In this manuscript, a novel mode feature extraction method for circumferential crack AE signals is proposed. This method involves three primary steps: firstly, a L-shaped flexible sensor array is designed to capture raw AE signals from both axial and circumferential directions. Next, the collected signals are filtered and evaluated to extract effective multi-mode components through the Kalman filtering and recursive plot (RP). Finally, the time–frequency features and mode types are extracted through combining the Hilbert-Huang transform (HHT) and dispersion curves. Results indicate that during the crack growths, both axial and circumferential direction AE signals contain multi-mode components, specifically L(0,1), F(1,1) and F(1,2) modes, with durations spanning 30–500 μs. Additionally, the axial modes predominantly occur within 200–300 kHz range, whereas the circumferential modes span both low and high frequency bands, specifically 40–50 kHz and 200–300 kHz, respectively. Modes across different frequencies indicate distinct structural behaviours, including crack growths and plastic deformations. The proposed method provides attempts for the pipe weld AE monitoring and multi-mode analysis. [ABSTRACT FROM AUTHOR]

A high-order layerwise multiresolution method that utilizes Daubechies wavelet and scaling functions for the approximation of state variables is presented for the enhanced simulation of guided waves in composite strips. The multiresolution approximation yields a hierarchical set of equations of motion involving the coarse component of generalized displacements, and finer components can be added on the coarse one, forming improved predictions until the desired precision is accomplished. The multiresolution approach is combined with a high-order layerwise laminate theory, enabling the accurate prediction of both symmetric and antisymmetric wave modes, the modeling of surface traction, and localized intra-ply and delamination damage types. Numerical results for the simulation of guided waves in laminated composite strips are presented, exhibiting significant reduction in computing times and remarkable convergence rates compared to single-resolution approaches and traditional finite element methods. Moreover, it is shown that each resolution can model specific bandwidths of wavenumbers, thus providing unique inherent capabilities to localize and isolate coexisting wave modes and detect converted and reflected waves, induced by degraded material regions and delaminations. [ABSTRACT FROM AUTHOR]

AbstractThe purpose of this article is to establish mathematical model for an early detection method of polyethylene pipe damage using linear guided waves and then perform a digital analysis to determine the efficiency of the technique in identifying different defects in pipeline materials. This method enhances the reliability of engineering pipeline infrastructures for both industrial and life-support purposes while also enabling the prevention of critical incidents such as ruptures, punctures, fissures, fatigue, and other events that may result in the release of transported agents into the surrounding environment. [ABSTRACT FROM AUTHOR]

Flexural guided waves that preserve the circumferential focusing characteristics provide insights into the structural integrity and thus have great application potential. An improved transducer called cross-exciting circumferential phased array is proposed for generating pure flexural mode in pipes. It is divided into two cross-exciting sub-arrays, with opposing excitation angles with respect to the pipe axis, forming a standing wave in the circumference and thereby a clear circumferential focus. The excitation response of the proposed transducer is theoretically analyzed with the normal mode expansion method. Finite element simulations of flexural mode excitation with the proposed transducer and a conventional circumferential array are further compared. The simulation results agree well with the theoretical predictions. Pure flexural modes of circumferential order of one are successfully generated. The excited signals are comprehensively analyzed. Furthermore, purity improvements are investigated with respect to the excitation frequency, element number, element width, and axial length. The excitation frequency should be appropriately chosen to suppress the unwanted modes. The element number should be sufficiently large to prevent the excitation of high circumferential order modes. The element width has negligible effect on the target flexural mode purity, whereas the element axial length significantly should be carefully chosen. [ABSTRACT FROM AUTHOR]

The dispersion analysis with array-based methods enables us to detect and analyse the multimode dispersive waves with weak amplitudes. Over the past years, there has been widespread extraction of higher normal modes from earthquake records and ambient noise, which have been extensively employed in near-surface and crustal investigations. Notably, the frequent reporting of leaky modes in recent observations has gained significant attention. This is because leaky modes can provide more constraints on subsurface structures. Particularly, a subset of leaky modes is found to be sensitive to P -wave velocities, which has the potential to compensate for the limitations of traditional surface wave inversion. Nevertheless, due to the lack of knowledge of the excitation and practical application of leaky modes, it is urgent to have an effective guide in selecting certain leaky modes for practical inversion combined with other imaging methods. To this end, the theoretical investigation into the excitation of leaky modes for multilayered models is undertaken. By computing theoretical seismograms for various source mechanisms and source depths with the discrete wavenumber and the normal-mode summation methods, respectively, the qualitative evaluation of the leaky-mode contributions to the synthesized seismograms is conducted. Additionally, the impact of the source depth for the same source mechanism is meticulously examined by the dispersion spectra. Furthermore, with the accurate computation of leaky modes, we categorize the leaky modes into PL and OP (organ-pipe) modes according to their distinctive properties, that is, their attenuation and eigen-displacements, which may be used to explain very well the occurrence of certain leaky modes on the dispersion spectrum and be indicative of the reasonable choice of PL modes to put them into practical inversion for P -velocity structures. [ABSTRACT FROM AUTHOR]

This study investigates guided-wave reflection and transmission at a water pipe joint. The system comprises a linearly elastic pipe filled with water with a joint that is modeled as a discontinuity of the solid region. Wave reflection and transmission are solved using the finite element method (FEM) with radiation conditions for reflected and transmitted guided waves into infinite waveguides. For the radiation conditions, the reflected and transmitted waves are expressed by modal expansion using the semi-analytical finite-element (SAFE) dispersion analysis method. This study extends the hybrid SAFE-FEM to the coupled fluid–solid axisymmetric problem. Numerical results demonstrate that the hybrid SAFE-FEM provides sufficiently accurate solutions. The propagation modes, similar to the modes in a solid pipe, are strongly or perfectly reflected by the joint. However, the modes are transmitted through the joint with little scattering after they converge to the modes in a water bar. The crossing of dispersion curves with those for modes in a solid pipe causes mode conversion and induces scattering attenuation. [ABSTRACT FROM AUTHOR]

Guided waves are the perfect carriers of electromagnetic signals in planar miniaturized devices due to their high localization and controlled propagation direction. However, it is still a challenge to control the polarization of propagating guided waves. In this work, the broadband polarization TE‐TM degeneracy of highly localized guided waves propagating along an all‐dielectric metasurface and a subwavelength chain of dielectric high‐index cylinders is discovered, both theoretically and experimentally. Using the discovered near‐field polarization degree of freedom, the polarization transformation for guided waves propagating along a subwavelength chain of dielectric cylinders at any frequency within the finite spectral range is demonstrated experimentally. Namely, the simplest planar near‐field polarization device – the quarter‐wave‐retardation (linear‐to‐circular) polarization transformer of guided waves is implemented. The results obtained discover the polarization degree of freedom for guided waves paving the way toward numerous applications in flat optics and planar photonics. [ABSTRACT FROM AUTHOR]

AbstractIn the present paper, the acoustoelastic theory of guided waves is developed for several plates symmetries. The goal is to illustrate how the stress effect changes as the anisotropic symmetry of the plate changes. Accordingly, phase velocity change of SH0 mode can reach about 0.13%, 0.42%, 0.065% and 0.8% for the monoclinic, trigonal, tetragonal and orthorhombic plates, respectively. Evanescent Lamb waves for the different plates are also calculated. In addition, we show that the different plates symmetries provide a unique observation of Lamb dispersion curves with multiple ZGV points. [ABSTRACT FROM AUTHOR]

In this work, probabilistic damage quantification under varying loading conditions in a non-stationary, guided-wave environment is being tackled via the synergistic integration between Time-varying Autoregressive (TAR) models and Gaussian Process regression models (GPRMs). Applying these TAR-GPRMs onto an aluminum coupon with simulated damage under different loading conditions fitted with piezoelectric sensors/actuators, the TAR models show the capability of capturing the non-stationarity in the ultrasonic guided-wave signals generated by the actuators under varying plate-loading conditions. They also provide insights to the maintainer by showing when in time the guided-wave time series deviate the most. In order to take advantage of that, a time instant selection algorithm was developed to allow flexibility in choosing the time instant(s) at which probabilistic damage quantification should be done. Finally, this quantification task is tackled by GPRMs, in which multiple GPRMs are trained using the TAR model parameters under varying conditions, and then used to predict damage size and/or loading state. While this framework is much more powerful in terms of tapping into the dynamics of how guided-waves change with varying conditions compared to simpler forms of GPRMs (such as damage index-trained GPRMs), training of TAR-GPRMs is far more complex. The advantages and challenges associated with the proposed TAR-GPRM approach is presented herein along with potential open areas for research in this regard. [ABSTRACT FROM AUTHOR]

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Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to its limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use. Unfortunately, the FBG sensors and the equipment needed for interrogation is quite expensive, and hence their number is restricted. In the previous work by the authors, the number and location of the actuators was optimized for developing a SHM system with a single sensor and multiple actuators. But through the use of the phenomenon of acoustic coupling, multiple locations on the structure may be interrogated with a single FBG sensor. As a result, a sensor network with multiple sensing locations and a few actuators is feasible and cost effective. This paper develops a two-step methodology for the optimization of an actuator–sensor network harnessing the acoustic coupling ability of FBG sensors. In the first stage, the actuator–sensor network is optimized based on the application demands (coverage with at least three actuator–sensor pairs) and the cost of the instrumentation. In the second stage, an acoustic coupler network is designed to ensure high-fidelity measurements with minimal interference from other bond locations (overlap of measurements) as well as interference from features in the acoustically coupled circuit (fiber end, coupler, etc.). The non-sorting genetic algorithm (NSGA-II) is implemented for finding the optimal solution for both problems. The analytical implementation of the cost function is validated experimentally. The results show that the optimization does indeed have the potential to improve the quality of SHM while reducing the instrumentation costs significantly. [ABSTRACT FROM AUTHOR]

In the present study, we have estimated the coda Q (QC) and the geometrical spreading factor (gamma) for the Saurashtra region that is located in the western part of the Deccan Volcanic Province, Gujarat, India. The waveform data from the seismic network operated by the Institute of Seismological Research, Gandhinagar, are used for the analysis. We have used records of a total of 221 local earthquakes with epicentral distances less than 70 km from the seismic stations. The moment magnitude range of the earthquakes is 2.5-5.3, and the depth range is 1-22 km. The QC has been estimated at five central frequencies (1.5, 3, 6, 12, and 20 Hz) using 20, 30, and 40 s window lengths for vertical, north-south, and east-west components, respectively. The obtained QC values show that the attenuation in the region has a strong dependence on frequency and points toward a heterogeneous medium. The geometrical spreading factors are also estimated for two cases, that is, with and without Q at the same central frequencies and window lengths. The average geometrical spreading factors with Q are 1.37, 1.48, 1.40, 1.08, and 1.04, at central frequencies 1.5, 3, 6, 12, and 20 Hz, respectively, whereas it is 1.59, 1.53, 1.47, 1.19, and 1.14, respectively, in the absence of Q. The estimated values also indicate that the medium is heterogeneous in the Saurashtra region. The estimated region-specific coda Q and the geometrical spreading factor may help in the modeling of strong ground motions and assessment of the seismic hazard for the Saurashtra region. [ABSTRACT FROM AUTHOR]

In geophysical inference problems, quantification of data uncertainties is required to balance the data-fitting ability of the model and its complexity. The transdimensional hierarchical Bayesian approach is a powerful tool to evaluate the level of uncertainty and determine the complexity of the model by treating data errors and model dimensions as unknown. In this article, we take account of the uncertainty through the whole procedure, thus developing a two-step fully Bayesian approach with coupled uncertainty propagation to estimate the crustal isotropic and radial anisotropy (RA) model based on Rayleigh and Love dispersion as well as receiver functions (RFs). First, 2D surface-wave tomography is applied to determine period-wise ambient noise phase velocity maps and their uncertainty for Rayleigh and Love waves. Probabilistic profiles of the isotropic average VS and RA as a function of depth are then derived at station sites by inverting the local surface-wave dispersion and model errors and RFs jointly. The workflow is applied to a temporary seismic broadband array covering all of Sri Lanka. The probabilistic results enable us to effectively quantify the uncertainty of the final RA model and provide robust inferences. The shear-wave velocity results show that the range of Moho depths is between 30 and 40 km, with the thickest crust (38-40 km) beneath the central Highland Complex. Positive RA (VSH > VSV) observed in the upper crust is attributed to subhorizontal alignment of metamorphic foliation and stretched layers resulting from deformation. Negative RA (VSH > VSV) in the midcrust of central Sri Lanka may indicate the existence of melt inclusions and could result from the uplift and folding process. The positive RA in the lower crust could be caused by crustal channel flow in a collision orogeny. [ABSTRACT FROM AUTHOR]

Determining the depths of small crustal earthquakes is challenging in many regions of the world, because most seismic networks are too sparse to resolve trade-offs between depth and origin time with conventional arrival-time methods. Precise and accurate depth estimation is important, because it can help seismologists discriminate between earthquakes and explosions, which is relevant to monitoring nuclear test ban treaties and producing earthquake catalogs that are uncontaminated by mining blasts. Here, we examine the depth sensitivity of several physics-based waveform features for ~8000 earthquakes in southern California that have well-resolved depths from arrival-time inversion. We focus on small earthquakes (2 < ML < 4) recorded at local distances (<150 km), for which depth estimation is especially challenging. We find that differential magnitudes (MW/ML-MC) are positively correlated with focal depth, implying that coda wave excitation decreases with focal depth. We analyze a simple proxy for relative frequency content, ɸ ≊ log10(M0) + 3log10(fc)ɸ, and find that source spectra are preferentially enriched in high frequencies, or "blue-shifted," as focal depth increases. We also find that two spectral amplitude ratios Rg 0.5-2 Hz/Sg 0.5-8 Hz and Pg/Sg at 3-8 Hz decrease as focal depth increases. Using multilinear regression with these features as predictor variables, we develop models that can explain 11%-59% of the variance in depths within 10 subregions and 25% of the depth variance across southern California as a whole. We suggest that incorporating these features into a machine learning workflow could help resolve focal depths in regions that are poorly instrumented and lack large databases of well-located events. Some of the waveform features we evaluate in this study have previously been used as source discriminants, and our results imply that their effectiveness in discrimination is partially because explosions generally occur at shallower depths than earthquakes. [ABSTRACT FROM AUTHOR]

Guided waves propagating in mechanical structures have proved to be an essential technique for applications, such as structural health monitoring. However, it is a well-known problem that when using non-stationary guided wave signals, dispersion, and high-order vibrational modes are excited, it becomes cumbersome to detect and identify relevant information. A typical method for the characterization of these non-stationary signals is based on time–frequency (TF) mapping techniques. This method produces 2D images, allowing the study of specific vibration modes and their evolution over time. However, this approach has low resolution, increases the size of the data, and introduces redundant information, making it difficult to extract relevant features for their accurate identification and classification. This paper presents a method for identifying discontinuities by analyzing the data in the TF maps of Lamb wave signals. Singular Value Decomposition (SVD) for low-rank optimization and then perform foreground feature extraction on the maps were proposed. These foreground features are then analyzed using Principal Component Analysis (PCA). Unlike traditional PCA, which operates on vectorized images, our approach focuses on the correlation between coordinates within the maps. This modification enhances feature detection and enables the classification of discontinuities within the maps. To evaluate unsupervised clustering of the dimensionally reduced data obtained from PCA, we experimentally tested our method using broadband Lamb waves with various vibrational modes interacting with different types of discontinuity patterns in a thin aluminum plate. A Support Vector Machine (SVM) classifier was then implemented for classification. The results of the experimental data yielded good classification effectiveness within reasonably low computational time despite the large matrixes of the TF maps used. [ABSTRACT FROM AUTHOR]

In this article, a set of equations is derived to find incident and load power explicitly in terms of load and source reflection coefficients in a lossless transmission line mismatched to both source and load impedances. A transmission line can be mismatched as the frequency varies if the source and load impedances are frequency dependent. Unlike in a scenario, where the transmission line is either matched to the source or load, the incident and load power depends on the length of the transmission line when both the source and load impedances are not matched to the line. The equations derived show that the power varies with the line length with a period of half wavelength. The maximum and minimum incident and load power with the corresponding line lengths are derived. The use of the Smith chart to find these lengths and the ratio of maximum to minimum is also described. Finally, three applications of the results including an additional version of the Friis transmission equation and the bandwidth improvement of power transfer for frequency dependent source and load impedances are presented. [ABSTRACT FROM AUTHOR]

In unbounded media, the acoustic attenuation as function of frequency is related to the frequency-dependent sound velocity (dispersion) via Kramers–Kronig dispersion relations. These relations are fundamentally important for better understanding of the nature of attenuation and dispersion and as a tool in physical acoustics measurements, where they can be used for control purposes. However, physical acoustic measurements are frequently carried out not in unbounded media but in acoustic waveguides, e.g., inside liquid-filled pipes. Surface acoustic waves are also often used for physical acoustics measurements. In the present work, the applicability of Kramers–Kronig relations to guided and surface waves is investigated using the approach based on the theory of functions of complex variables. It is demonstrated that Kramers–Kronig relations have limited applicability to guided and surface waves. In particular, they are not applicable to waves propagating in waveguides characterised by the possibility of wave energy leakage from the waveguides into the surrounding medium. For waveguides without leakages, e.g., those formed by rigid walls, Kramers–Kronig relations remain valid for both ideal and viscous liquids. Examples of numerical calculations of wave dispersion and attenuation using Kramers–Kronig relations, where applicable, are presented for unbounded media and for waveguides formed by two rigid walls. [ABSTRACT FROM AUTHOR]

The present study deals with the investigation of the oscillatory morphology of guided slow body MHD modes in inhomogeneous magnetic waveguides that appear in the solar photospheric plasmas in the forms of pores or sunspots. The eigenvalues and eigenfunctions related to these waves in an isothermal plasma are obtained numerically by solving a Sturm-Liouville problem with Dirichlet boundary conditions set at the boundary of the waveguide. Our results show that the inhomogeneities in density (pressure) and magnetic field have a strong influence on the morphology of waves, and higher-order more are sensitive to the presence of inhomogeneity. Our results suggest that he identification of modes just by a simple visual inspection can lead to a misinterpretation of the nature of modes. [ABSTRACT FROM AUTHOR]

Guided wave array-based structural health monitoring (SHM) is a promising solution for diagnosing damage in metal-connected structures. In this field, the reconstruction algorithm for probabilistic inspection (RAPID) is one of the most widely used algorithms for performing damage localization. In this paper, a density clustering RAPID based on an array-compensated damage index is proposed. A new probability distribution function was constructed based on a new damage index, which is adaptive to different elements in the sensor array to compensate for performance variation. Then, the imaging matrix of the RAPID algorithm was density-clustered to obtain the location and degree of damage. Finally, the method was verified by experiments on a stiffened aluminum plate. The experimental results demonstrate that the method achieves damage localization and enables quantitative damage diagnosis. [ABSTRACT FROM AUTHOR]

Composite pressure vessels are seeing increasing demand in the oil and gas sector due to their excellent corrosion resistance. However, the assessment of the fatigue state of those structures still an open question. The goal of this work is use elastic wave data to access the fatigue damage (exudation). The Dynamic Time Warping method is proposed as a means of extracting features from guided wave ultrasound data that can describe the on-going fatigue induced damage of glass-fibre reinforced plastic pipes under fatigue-cycle loading. To test its efficiency, three pipe samples were fatigue tested to failure under internal pressure cycles with maximum values of 45 bar, 55 bar and 65 bar, and minimum pressures equal to 10% of the maximum, at a frequency of 0.8 Hz. A Guided Wave monitoring system consisting of a set of permanently attached piezoelectric sensors produced signals which were processed to obtain the Dynamic Time Warping distance, that was then used to obtain a Damage Index that expresses the cumulative fatigue damage suffered by the samples for each loading level. These results were comparable to data obtained from surface-mounted strain-gauges, even though temperature variations of up to 20 °C occurred during the tests and no direct temperature compensation was applied to the GW signals. The Dynamic Time Warping distance presents smaller influence of temperature and was able to better access the exudation of the samples. [ABSTRACT FROM AUTHOR]

In this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a 3-D laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low-frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves. [ABSTRACT FROM AUTHOR]