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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]

An increase in seismic stations also having microbarographs has led to increased interest in the field of seismoacoustics. A review of the recent advances in this field can be found in Dannemann Dugick et al. (2023). The goal of this note is to draw the attention of the readers of Dannemann Dugick et al. (2023) to several additional interactions between the solid Earth and atmosphere that have not been classically considered in the field of seismoacoustics. The 15 January 2022 Hunga Tonga-Hunga Ha'api eruption produced acoustic gravity waves that were recorded globally. For example, the Lamb wave from this eruption produced early-arriving and long-lasting tsunami waves. This eruption also provided globally recorded coupling of atmospheric modes with solid Earth modes, providing another example of the complex interactions that can occur at the boundary between the atmosphere and the solid Earth. Even in the absence of large atmospheric signals, collocated pressure sensors at seismic stations can be a useful tool for estimating the local substructure, such at VS30 the average shear velocity of the upper 30 m. Finally, at low frequencies, it is possible to use pressure records to correct out atmospheric disturbances recorded on seismometers. We briefly review the aforementioned, nontraditional seismoacoustic topics that we feel are important to consider as part of the full suite of interactions occurring between the solid Earth and atmosphere. [ABSTRACT FROM AUTHOR]

Kappa (the high-frequency spectral decay slope at near-source distances; often referred to as κ0) is determined at 25 seismograph stations in Eastern Canada using broadband ground-motion modeling approaches. The database comprises Fourier spectra (effective amplitude spectrum for the horizontal component and the vertical component, 0.8-40 Hz) computed from 3318 earthquakes of moment magnitude M 1.5-5 recorded on stations within 150 km. Average kappa values for bedrock sites, having shear-wave velocities from 850 to 2400 m/s, are highly variable, ranging from -29 to +21 ms (horizontal) and -28 to +11 ms (vertical), but appear on average to be near-zero. The values obtained are sensitive to methodology, especially the necessary adjustments to the spectra to account for site amplification effects. Kappa values do not appear to correlate well with site parameters such as rock shear-wave velocity, average shear-wave velocity in the upper 30 m, primary wave velocity, site class, type and age of rock, or instrument housing. This lack of correlation may reflect the noted sensitivities to methodological factors. We conclude that kappa values in rock environments are not reliably estimated from such proxies and should be determined from recorded ground motions at a given location. On average, there is little evidence of significant high-frequency attenuation on rock sites beyond that already accounted for in ground-motion modeling by the empirical parameterization of regional Q-related path effects. [ABSTRACT FROM AUTHOR]

Site-specific seismic hazard studies require the knowledge of the shear-wave velocity VS and the high-frequency site attenuation parameter κ0 at the reference rock level at depth. The latter one (called κ0,ref) is often not available and hard to derive. In this study, we make use of the KiK-net database in Japan that consists of surface and colocated downhole sensors. We select 175 sites where the bottom sensor is deployed at rock or hard-rock conditions resulting in a database with many recordings at VS ≥ 1500 ms. This allows us to tackle two questions: first, is it possible to derive κ0,ref from surface recordings? Second, does reach an asymptotic level at high vs that could be used as a κ0,ref in site-specific seismic hazard studies? Our results show that measures of κ0 derived from S and coda waves are equivalent. Thus, it is not possible to obtain κ0,ref from surface recordings using coda waves. On the other hand, S-wave measurements of κ0 from surface rock sites are close to κ0,ref if VS ≥L760 m/s or if the sedimentary cover is thin. The values of κ0 decrease with increasing VS and reach an asymptotic value. The scatter in the so obtained is high, but it can be reduced by selecting subregions with similar geological conditions. Finally, we observe that borehole and surface are correlated, and that the variability of κ0,ref is only slightly reduced compared to κ0 at the surface. Although we cannot exclude any influence of source effects, our findings indicate that κ0,ref has to be considered as a deep site parameter. [ABSTRACT FROM AUTHOR]

The crustal structure and Quaternary fault zone in the Seoul metropolitan area are investigated using a joint inversion that combines the receiver functions with surface wave dispersion based on seismic data from densely deployed seismic arrays. The uncertainty in inverted models is examined through a set of synthetic experiments. Synthetic tests and comparisons with other methods verify the observed crustal features. The Moho depths range between 28.7 and 32.8 km with relatively shallow depths in the central Seoul metropolitan area. The VP/VSratio varies between 1.67 and 1.85. A Conrad discontinuity with a weak velocity increase is present at a depth of ~20 km. Low-velocity anomalies appear at depths of ~3 km and 6-11 km along the Chugaryeong fault zone and fault junction area. Apparent shear wave velocity contrasts appear across the fault zone where earthquakes are clustered. A high-velocity block in the western Seoul metropolitan area lies along the fault zone that runs across western Seoul. The shear wave velocity models and seismicity suggest that the fault zone may extend to a depth of 11 km at least. [ABSTRACT FROM AUTHOR]

The mainland region of Gujarat, northwest India, is a less investigated region than other parts of India with a low seismicity rate. An MW >4.7 earthquake has not occurred in this region for 15 yr, and no MW >5.5 events since 1971. We analyze the local earthquake catalog and waveforms to examine dynamic triggering in the region by the 2012 MW 8.6 Indian Ocean earthquake, which triggered widespread seismicity globally. Further detection of possibly missing microearthquakes is conducted by applying the matched filter technique to the waveforms. We identify six microearthquakes (~ML 1.0-2.1) triggered during the surface and coda wave of the 2012 mainshock. Also, an earthquake of MW 2.6 was likely triggered five hours after the mainshock near Bhavnagar city, because the record since 2006 would indicate such a magnitude event to have only a 0.8% chance of occurring independently any given day. Indeed, only 35 earthquakes with MW ≤ 2.5 were recorded since 2006 within a 100 km radius of this city. The -statistics indicate an increase in seismicity and further confirm the triggering. The seismicity rate increased immediately after the 2012 mainshock and continued for three days, indicating a possible delayed triggering. The delayed triggering may be due to the crustal fluid, and/or subcritical crack growth model may be responsible for triggering. Our study suggests that dynamic triggering tends to occur near active faults that have ruptured in ancient times. Other recent earthquakes, for example, 2011 Tohoku-Oki, did not trigger seismicity despite significant peak dynamic stresses values. Investigation of dynamic triggering in regions experiencing infrequent earthquakes can be crucial in understanding the origin of such earthquakes, which can be achieved by grasping the ambient stresses and geodynamic mechanisms in a particular region. Thus, we evaluate character and behavior of high-amplitude surface waves to grasp better the undergoing processes and stress transfer in the intraplate mainland region. [ABSTRACT FROM AUTHOR]

• A fractional order elastodynamics model is proposed to investigate guided waves in FGQC nanoplates. • The phonon-phason coupling and nonlocal effects on wave characteristics are studied. • The fractional order considerably influences phason modes, but weakly affects phonon modes. • The phonon-phason coupling and the nonlocal effects promote each other. Due to the coupled phonon-phason coupling and nonlocal effects, dynamic behaviors of quasicrystal nanostructures are highly complex. Mechanics investigations have mainly focused on the statics, but rarely on dynamics, thus limiting their engineering applications. In the present paper, a fractional order elastodynamics model is proposed to investigate guided waves propagating in functionally graded one-dimensional hexagonal quasicrystal nanoplates in the context of the nonlocal theory. In this model, the fractional order can be flexibly adjusted to describe the role of the phason field accurately. Phase velocity and attenuation curves, as well as stress distributions, are illustrated. The phonon-phason coupling and nonlocal effects on wave characteristics are studied. Some new results are revealed: The fractional order considerably influences phason modes, but weakly affects phonon modes; The phonon-phason coupling and the nonlocal effects promote each other. These results establish a theoretical foundation for designing and optimizing quasicrystal nanostructures. [ABSTRACT FROM AUTHOR]

Highlights • A novel Slot-waveguide-based polarization beam splitter is proposed. • The mode properties of TM mode in slot waveguide are significantly modified by the ENZ effect provided by ITO. • The PBS is compatible with most of ITO-based photonic devices and can enrich the application areas of ENZ effect of ITO. Abstract A slot-waveguide-based polarization beam splitter (PBS) assisted by "epsilon-near-zero" (ENZ) material is proposed and investigated. Because of the tunable complex refractive indices of indium tin oxide (ITO), ENZ effect can be achieved in the 1.55 μm regime. By embedding ITO in the slot region of the silicon waveguide, the modal characteristics of the TM mode are significantly modified, whereas that of the TE mode is nearly unchanged. Based on this feature, an asymmetric directional coupler is constructed as a PBS in which the TE mode undergoes a strong coupling to the slot waveguide nearby while the TM mode can pass through the waveguide directly. The results show that with 5.6 μm coupling length and 200 nm gap, the PBS offers 31.1 dB and 31.9 dB extinction ratio (ER) and 0.69 dB and 0.97 dB insertion loss (IL) for TE mode and TM mode, respectively. Furthermore, the impact of carrier concentration and operation wavelength on the PBS performance are investigated in detail. The proposed PBS is compatible with most of the newly developed ITO-based active photonic devices and can be a useful component for on-chip signal processing. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract In vivo mechanical characterization of soft biological tissues has broad applications ranging from disease diagnosis to tissue engineering. Shear wave elastography based on the bulk wave theory has been widely used to measure the mechanical properties of soft tissues. Given that most soft tissues basically have layered structures, the dispersive feature of elastic waves should be considered when the thickness of the interested layer is comparable to or smaller than the wavelength. Bearing this fundamental issue in mind, we propose an ultrasound-based guided wave elastography (GWE) method to characterize the mechanical properties of layered soft tissues. The dispersion relations of guided waves in layered structures were derived first, and its explicit expression was achieved. An inverse approach based on the dispersion relation to characterize the mechanical properties of layered soft tissues was then established. Both finite element analysis (FEA) and phantom experiments were carried out to validate the new method. In vivo experiments on forearm skin demonstrate the usefulness of the present method in characterizing layered soft tissues. Statement of significance Layered soft tissues and artificial soft materials are ubiquitous in both nature and engineering. Imaging their in vivo/in situ mechanical properties finds important applications and remains a great challenge to date. Here, we propose an ultrasound-based guided wave elastography method to in vivo/in situ characterize the elastic properties of layered soft materials. We validate the method via finite element analysis and phantom experiments and further demonstrate its usefulness in practice by performing in vivo measurements on forearm skins. Given that the dispersive feature of elastic waves in layered soft media is considered in our method, it provides the opportunity to assess the intrinsic elastic properties of an individual layer in a non-destructive manner as shown in our experiments. [ABSTRACT FROM AUTHOR]

Many engineering structures must be inspected or monitored throughout their serviceable life to ensure their safe operation. In the case of reinforced concrete structures, the most common cause of premature failure is corrosion of the steel reinforcement bars that must be pre-empted. Modal based techniques are popular, since they can potentially detect damage using sensors placed remotely from the damage site. However, changes in modal parameters due to damage can be masked by their sensitivity to environmental factors and changes to the boundary conditions that are unrelated to damage. In this respect, wave based techniques provide a potential alternative. In this paper, wave propagation is modelled in a damaged steel reinforced concrete beam. The damaged section is modelled in conventional finite elements and this is coupled to wave finite element models (WFE) of the undamaged sections on either side. This hybrid modelling approach facilitates a wave based analysis of a one dimensional structure with potentially geometrically complex damage. A numerical case study is presented for a locally damaged beam represented by a loss of thickness of one reinforcement bar. It is shown that some wave modes, that feature deformation of the cross section, exhibit a strong reflection close to their cut-on frequency. This is due to the difference in cut-on frequency between the damaged and undamaged sections. A damage detection method is outlined in which the amplitudes of incident and reflected waves of low wave number are compared. No a priori knowledge of the dispersion curves is necessary. In numerical simulations, a reduction in the ratio of the reflected to incident wave amplitudes is seen in the vicinity of cut-on frequencies. [ABSTRACT FROM AUTHOR]

Reinforced cement concrete (RCC) structures are prone to damages such as corrosion of reinforcement, surface cracking, debonding and the like. The corrosion process results in two effects, namely, reduction in the diameter of rebar and debonding at the steel/concrete interface. Several researchers have worked to establish a correlation between intrinsic damage features and guide wave signal characteristics. However, the work till date is focused mainly on identifying the individual damage phenomenon, and few researchers have attempted to correlate the combined effect of these damage parameters. Therefore, it is an impending task to study the combined influence of corrosion and debonding on wave characteristics and to establish methods to distinguish and correctly quantify the damage level. A three-dimensional finite element model of a RCC beam is developed in the Abaqus software by using an explicit method in order to study wave propagation in an RCC beam after including the two effects of corrosion. The implicit and explicit models are co-executed using standard-explicit co-simulation. It is observed that the reduction in diameter would result in the diminishing of the guided wave amplitude, whereas debonding results in an increase in the guided wave amplitude. It is found that these two complimentary effects on wave features result in a signal that has the same wave features as undamaged rebar that is surrounded by concrete, which could be termed as the zero effect state. [ABSTRACT FROM AUTHOR]

Concrete filled steel tubes are extensively applied in engineering structures due to their resistance to high tensile and compressive load and convenience in construction. But one major flaw, their vulnerability to environmental attack, can severely reduce the strength and life of these structures. Degradation due to corrosion of steel confining the concrete is one of the major durability problems faced by civil engineers to maintain these structures. The problem accelerates as inner surface of steel tube is in contact with concrete which serves as electrolyte. If it remains unnoticed, it further accelerates and can be catastrophic. This paper discusses a non-destructive degradation monitoring technique for early detection corrosion in steel tubes in CFST members. Due to corrosion, damage in the form of debonding and pitting occurs in steel sections. Guided ultrasonic waves have been used as a feasible and attractive solution for the detection and monitoring of corrosion damages in CFST sections. Guided waves have been utilized to monitor the effect of notch and debond defects in concrete filled steel tubes simulating pitting and delamination of steel tubes from surrounding concrete caused by corrosion. Pulse transmission has been used to monitor the healthy and simulated damaged specimens. A methodology is developed and successfully applied for the monitoring of concrete filled steel tubular sections undergoing accelerated chloride corrosion. The ultrasonic signals efficiently narrate the state of steel tube undergoing corrosion. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop a novel method to determine the severity of a damage in a thin plate. This paper presents a novel fault detection and diagnosis approach employing a new electromagnetic acoustic transducer, called EMAT, together with a complex signal processing method. The method consists in the recognition of a fault that exists within the structure, the fault location, i.e. the identification of the geometric position of damage, and the determining the significance of the damage, which indicates the importance or severity of the defect. The main scientific novelties presented in this paper is: to develop of a new type of electromagnetic acoustic transducer; to incorporate wavelet transforms for signal representation enhancements; to investigate multi-parametric analysis for noise identification and defect classification; to study attenuation curves properties for defect localization improvement; flaw sizing and location algorithm development. [ABSTRACT FROM AUTHOR]

Structural Health Monitoring (SHM) based on ultrasonic guided waves is able to assess the health of complex structures, improving maintenance strategies and overall performances of an aircraft. However, wave propagation analysis in composite structures is quite complex for effective damage identification. To alleviate such difficulties, various parametric information can be simultaneously extracted from propagating waves in view of a more effective detection of damage. A multi-parameter approach is indeed able to detect and localize impact induced damages with an increasing probability of detection. Experimental tests and numerical simulations have been used in combination to improve the methodology. The formers are oriented to optimize the single parameter methodology whose effectiveness and issues have been already investigated. The latter are mostly oriented to evaluate the effectiveness of a multi-parameter approach in view of an increasing probability of detection of the system. Finally, the management of large amount of data is optimized developing a graphic user interface aimed to easily post-process measurements and simulation data and quickly investigate several implemented techniques. [ABSTRACT FROM AUTHOR]

Structural Health Monitoring (SHM) based on guided waves (GWs) is responsive to damage occurrences allowing the assessment of composites integrity. However, online monitoring is quite complex due to a wide range of parameters affecting wave propagation and consequently diagnostic outputs. A useful implementation of a GW-based condition monitoring requires an accurate analysis of reconstruction algorithm and collected data as well. In view of a more effective detection of impact induced damages in composites using a sparse array of sensors, several methods are explored in this paper as a first step towards a comprehensive capability assessment of a system permanently installed on aircraft structures. Various probabilistic reconstruction methods and signal transformation techniques are developed and used to detect hidden flaws with a multiple analysis. From a wide number of simulation carried out it appears that, although a single procedure for the estimation of the structural health is a fast solution for flaw detection, a multiple analysis based on different reconstruction techniques and/or several damage parameters could provide more detailed information about the location and the severity of possible failures if the parameters affecting diagnostics are completely addressed. [ABSTRACT FROM AUTHOR]

This paper discusses a non-contact and in situ damage-monitoring methodology in submerged plates using ultrasonic guided waves. Specific Lamb wave modes sensitive to near-surface and sub-surface damages have been identified. These modes are further utilized for non-contact scanning of the plates to identify the presence, as well as extent, of damage. Effective combination of specific Lamb wave modes leads to comprehensive inspection of the submerged plate structures. The developed methodology is successfully applied for monitoring notches and corrosion in plate. The data generated from scanning the plates have been pictorially represented in the form of defect maps. [ABSTRACT FROM AUTHOR]

Damage detection using Lead-Zirconate-Titanate (PZT) transducers for exciting guided waves was investigated. The analytical model for guided wave excitation using PZT transducers was used in a Finite Element Model for simulating guided waves in a thin aluminum plate with various defect depths. The fundamental symmetric ( S0 ) mode was excited, and four sensors were placed for detecting both the reflected and transmitted waves from defects. The model was used to conduct a parametric study of the effect of the PZT adhesive bonding on the measured signal and hence probability of detection (POD).The model was also used for optimizing signal analysis algorithms, particularly for the case of multiple overlapping modes due to mode conversion from defects. A mode decomposition algorithm is proposed for separating the reflected/transmitted S0 mode from the mode converted antisymmetric mode ( A0 ). The signal processing algorithms were optimized to maximize the POD. [ABSTRACT FROM AUTHOR]

Damage detection using Lead-Zirconate-Titanate (PZT) transducers for exciting guided waves was investigated. The analytical model for guided wave excitation using PZT transducers was used in a Finite Element Model for simulating guided waves in a thin aluminum plate with various defect depths. The fundamental symmetric ( S0 ) mode was excited, and four sensors were placed for detecting both the reflected and transmitted waves from defects. The model was used to conduct a parametric study of the effect of the PZT adhesive bonding on the measured signal and hence probability of detection (POD).The model was also used for optimizing signal analysis algorithms, particularly for the case of multiple overlapping modes due to mode conversion from defects. A mode decomposition algorithm is proposed for separating the reflected/transmitted S0 mode from the mode converted antisymmetric mode ( A0 ). The signal processing algorithms were optimized to maximize the POD. [ABSTRACT FROM AUTHOR]

While rails being inspected by guided waves techniques, dispersion curves and wave structures act as the basis for designing tools and analyzing waveforms. This paper, first, introduces a basic way for calculating dispersion curves and wave structures for guided waves propagation in elastic waveguides of arbitrary cross-section with a semi-analytical finite element method (SAFEM). Then the guided waves dispersion curves and wave structures of vertically vibrating modes in U60 rail are obtained by using semi-analytical finite element method and their applications in guided wave inspection for railhead defect are discussed. Finally, instrumented hammer experimental work and jointed time-frequency analysis are carried out to verify the dispersion curves' numerical results. Finite element method (FEM) modeling and experimental investigations of railhead transverse defect detection shows that the guided waves in rail's vertically vibrating mode are quite fit for inspecting defects in railhead. [ABSTRACT FROM AUTHOR]

The technique for measurement of the group velocity of guided waves was proposed. The technique is based on the spectrum decomposition method and was investigated in the case of A0 and S0 modes of Lamb waves propagating in CFRP composite plate. The obtained measurement results were compared with the theoretical dispersion curves obtained by SAFE method. [ABSTRACT FROM AUTHOR]

A split spectrum processing (SSP) method is proposed to accurately determine the time-of-flight (ToF) of damage-scattered waves by comparing the instantaneous amplitude variation degree (IAVD) of a wave signal captured from a damage case with that from the benchmark. The fundamental symmetrical (S0) mode in aluminum plates without and with a notch is assessed. The efficiency of the proposed SSP method and Hilbert transform in determining the ToF of damage-scattered S0 mode is evaluated for damage identification when the wave signals are severely contaminated by noise. Broadband noise can overwhelm damage-scattered wave signals in the time domain, and the Hilbert transform is only competent for determining the ToF of damage-scattered S0 mode in a noise-free condition. However, the calibrated IAVD of the captured wave signal is minimally affected by noise, and the proposed SSP method is capable of determining the ToF of damage-scattered S0 mode accurately even though the captured wave signal is severely contaminated by broadband noise, leading to the successful identification of damage (within an error on the order of the damage size) using a triangulation algorithm. [ABSTRACT FROM AUTHOR]

This study proposes an optics-based active sensing system for continuous monitoring of underground pipelines in nuclear power plants (NPPs). The proposed system generates and measures guided waves using a single laser source and optical cables. First, a tunable laser is used as a common power source for guided wave generation and sensing. This source laser beam is transmitted through an optical fiber, and the fiber is split into two. One of them is used to actuate macro fiber composite (MFC) transducers for guided wave generation, and the other optical fiber is used with fiber Bragg grating (FBG) sensors to measure guided wave responses. The MFC transducers placed along a circumferential direction of a pipe at one end generate longitudinal and flexural modes, and the corresponding responses are measured using FBG sensors instrumented in the same configuration at the other end. The generated guided waves interact with a defect, and this interaction causes changes in response signals. Then, a damage-sensitive feature is extracted from the response signals using the axi-symmetry nature of the measured pitch-catch signals. The feasibility of the proposed system has been examined through a laboratory experiment. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a model-based probabilistic damage characterisation methodology for beams using guided waves. Damage location, length, depth and Young''s modulus of the material are treats as unknown parameters and are determined using optimisation approach to maximise the probability density function of the damage scenario conditional on the measured guided wave signals. A two-stage optimisation strategy is proposed using simulated annealing to first guarantee the solution is close to the global optimum, a standard simplex search method is then employed to precisely determine the global optimum. In addition to damage characterisation, the proposed methodology is developed based on the Bayesian statistical framework, hence, it allows quantification of the uncertainties associated with damage characterisation results. The accuracy and robustness of the proposed methodology are investigated through a series of numerical case studies in which the spectral element method is employed to model the wave propagation and scattering at the damage. [Copyright &y& Elsevier]

Abstract: Guided waves had been studied for a long time. However, most of all were the kind of Love. And the studying of Rayleigh guided waves are not enough. Rayleigh guided waves which have two components are more complicated than Love guided waves that have only one component. This paper started from elastic theorem, derived the dispersive equation due to the idea model. And through programming computing, the dispersive characters of symmetry and dissymmetry models are obtained, which made some theorem basis for using Rayleigh guided waves in coal field. [Copyright &y& Elsevier]

This study investigated propagation characteristics of piezo-activated guided waves in an aluminium rectangular-section tube for the purpose of damage identification. Changes in propagating velocity and amplitude of the first wave packet in acquired signals were observed in the frequency range from 50 to 250 kHz. The difference in guided wave propagation between rectangular- and circular-section tubes was examined using finite element simulation, demonstrating a great challenge in interpretation of guided wave signals in rectangular-section tubes. An active sensor network, consisting of nine PZT elements bonded on different surfaces of the tube, was configured to collect the wave signals scattered from through-thickness holes of different diameters. It was found that guided waves were capable of propagating across the sharp tube curvatures while retaining sensitivity to damage, even that not located on the surfaces where actuators/sensors were attached. Signal correlation between the intact and damaged structures was evaluated with the assistance of a concept of digital damage fingerprints (DDFs). The probability of the presence of damage on the unfolded tube surface was thus obtained, by which means the position of damage was identified with good accuracy. [ABSTRACT FROM AUTHOR]

The article presents a study which explores the curvature effect on guided wave propagation in curved plates of steel and in hollow steel cylinders. The study involved the use of curved steel plates and hollow steel cylinders having the radius-to-thickness ratios of 10:1, 5:1, 2:1, 1:1 and 0.5:1. Steps on how to achieve a dimensionless frequency and a dimensionless phase or group velocity are cited. One result of the study shows that the group velocity of the F1 mode with a 0.31 dimensionless frequency was higher in the 0.5:1 ratio group.

Abstract: In the past ten years ultrasonic guided wave systems have become widely used for the rapid large-scale examination of pipes and pipelines in pressure containing systems, for the detection of corrosion and other degradation in service. As a result, the characteristics of the guided wave modes existing in pipes have been studied extensively and are well understood. Advantage is taken of the fact that a pipe is essentially a one-dimensional object and that guided waves travelling along it will return to the sensor location for detection. Guided waves may also be applied to other components which are non-cylindrical, either those of constant cross-section such as railway rails or structural I beams, or constructions consisting of plates. However, in both cases the guided wave systems are more complex than in simple cylinders and successful application of this technique for large scale examination of these constructions requires further development of the understanding of the properties of the waves propagating and regularities of their generation. This paper describes work carried out under the EU-funded LRUCM project to develop techniques for detection of corrosion and cracking in railway rails and structural sheet piles used to retain earth embankments, river banks etc. The work described involved identification of wave modes present, derivation of dispersion curves to describe propagation behaviour and evaluation of experimental data to explain mode conversions and interactions with interfaces observed in the results. [Copyright &y& Elsevier]

Abstract: This paper is concerned with guided elastic waves at a periodic array of coplanar disbonds in a solid, and the role they play in the scattering of bulk waves. It is approached as a mixed boundary condition problem pertaining to two elastic half spaces which are joined together within infinitely long regularly spaced strips, and unattached in between. It provides an approach to modeling wave interactions at an array of coplanar cracks, solids with slightly uneven surfaces which are in contact, stoping in mine operations where a narrow seam of a mineral is removed leaving regularly positioned pillars for roof support, and other analogous physical situations. In the analysis, a truncated Fourier series representation of the wave field is invoked, and the boundary conditions are applied at a discrete set of points within the repeat interval at the interface to determine the Fourier coefficients. The dispersion relation for interfacial waves is obtained within a one dimensional folded Brillouin zone scheme, and it is shown that there are branches associated with leaky interfacial waves, which at certain isolated points in wavevector space uncouple from the bulk wave continuum to exist as secluded supersonic interfacial waves. These observations are able to explain striking resonant features in the scattering of bulk waves at the interface. [Copyright &y& Elsevier]

Ultrasonic chaotic excitations combined with sensor prediction algorithms have shown the ability to identify incipient damage (loss of preload) in a bolted joint. In this study we examine a physical experiment on a single-bolt aluminum lap joint as well as a three-dimensional physics-based simulation designed to model the behavior of guided ultrasonic waves through a similarly configured joint. A multiple bolt frame structure is also experimentally examined. In the physical experiment each signal is imparted to the structure through a macro-fiber composite (MFC) patch on one side of the lap joint and sensed using an equivalent MFC patch on the opposite side of the joint. The model applies the waveform via direct nodal displacement and 'senses' the resulting displacement using an average of the nodal strain over an area equivalent to the MFC patch. A novel statistical classification feature is developed from information theory concepts of cross-prediction and interdependence. This damage detection algorithm is used to evaluate multiple damage levels and locations. [ABSTRACT FROM AUTHOR]

Abstract: We design an efficient coupler to transmit light from a strip waveguide into the flatband slow mode of a photonic crystal waveguide with ring-shaped holes. The coupler is a section of a photonic crystal waveguide with a higher group velocity, obtained by different ring dimensions. We demonstrate coupling efficiency in excess of 95% over the 8 nm wavelength range where the photonic crystal waveguide exhibits a quasi-constant group velocity and observe a more than 12-fold intensity enhancement in the slow-light waveguide. An analysis based on the small Fabry–Pérot resonances in the simulated transmission spectra is used for studying the effect of the coupler length and for evaluating the coupling efficiency in different parts of the coupler. The mode conversion efficiency within the coupler is more than 99.7% over the wavelength range of interest. The parasitic reflectance in the coupler, which depends on the propagation constant mismatch between the slow mode and the coupler mode, is lower than 0.6%. [Copyright &y& Elsevier]

Abstract: We show that photonic crystals with ring-shaped holes (RPhCs) exhibit superior properties compared to conventional photonic crystals (PhCs). At low air-fill factors RPhCs can have a larger bandgap than conventional PhCs. Moreover, RPhC waveguides with both high group index and small group velocity dispersion can be designed. RPhC waveguides are also more sensitive to external refractive index changes, which is attractive for sensor applications. Finally we set up a procedure to pattern RPhCs in silicon-on-insulator. [Copyright &y& Elsevier]

Using guided wave dispersion characteristics, an inverse method based on artificial neural network (ANN) is presented to determine the material properties of functionally graded materials (FGM) plates. The group velocities of several lowest modes at several lower frequencies are used as the inputs of the ANN model; the outputs of the ANN are the distribution function of the volume fraction of the FGM plate. The Legendre polynomials method is used to calculate the dispersion curves for the FGM plate. Levenberg-Marquardt algorithm is used as numerical optimization to speed up the training process of the ANN model. [ABSTRACT FROM AUTHOR]

Parametric density concept is proposed for a long-range pipeline health monitoring. This concept is designed to obtain the attenuation of ultrasonic guided waves propagating in underwater pipelines without complicated calculation of attenuation dispersion curves. For the study, three different pipe materials such as aluminum, cast iron, and steel are considered, ten different transporting fluids are assumed, and four different geometric pipe dimensions are adopted. It is shown that the attenuation values based on the parametric density concept reasonably match with the attenuation values obtained from dispersion curves; hence, its efficiency is proved. With this concept, field engineers or inspectors associated with long-range pipeline health monitoring would take the advantage of easier capturing wave attenuation value, which is a critical variable to decide sensor location or sensors interval. [ABSTRACT FROM AUTHOR]

The reflection coefficients of extensional guided modes from notches of different axial, circumferential and through-thickness extent in pipes of different diameters have been studied using finite element analysis. A selection of the predictions has also been validated by experiments. For part-thickness notches of a given circumferential extent and minimal axial extent, the reflection coefficient increases monotonically with depth at all frequencies, and increases with frequency at a given depth. When the wavelength is long compared to the pipe wall thickness, the reflection coefficient from part-thickness notches of a given circumferential extent is a strong function of the defect axial extent, the reflection being a maximum at an axial extent of about 25 per cent of the wavelength and a minimum at 0 and 50 per cent. The reflection coefficient is a linear function of the defect circumferential extent at higher frequencies (with frequency-diameter products greater than about 3000 kHz mm) where a ray theory analysis explains the behaviour, while at low frequencies the reflection coefficient at a given circumferential extent is reduced. In the high-frequency regime, the axial extent of a through-thickness defect has little influence on the reflection coefficient, while it is important at lower frequencies. Three-dimensional finite element predictions in the high-frequency regime have shown that the reflection coefficient from a part-thickness, part-circumferential defect can be predicted by multiplying the reflection coefficient for an axisymmetric defect of the same depth and axial extent by that for a through-thickness defect of the same circumferential extent. [ABSTRACT FROM AUTHOR]

This paper provides an overview of a study on circumferential guided waves in a thick annulus. Both steady state, time-harmonic waves and transient waves are considered. Several solution methods are reviewed and numerical solutions are presented for the propagation of ultrasonic circumferential waves in a thick, curved, two-dimensional annular waveguide. The modal content of the signal and the displacement profiles across the wall thickness are investigated. These studies provide valuable guidance in selecting optimal parameters for use in applications of the guided wave technique to the detection of cracks on the inner surface of annular components. Experimental results show that the technique can be used on parts with complex geometries (e.g. the pitch shaft of a helicopter) to detect cracks that would not be detectable by standard ultrasonic inspection. [ABSTRACT FROM AUTHOR]