Search

Introduction: Guided waves being highly sensitive to temperature variations, temperature compensation algorithms, such as Optimal Baseline Selection, Baseline Signal Stretch and Scale Transform demonstrate effective performance under limited conditions. Dynamic Time Warping (DTW) has shown excellent compensation performance, however it comes with a substantial computational burden of O(N*N), where N represents the number of samples in each signal. Methodology: DTW works by construction of cost matrix that maps every point in one time series to all the points in the other time series, that results in complexity O(N*N).This problem can be solved by narrowing the search window using global constraints. The two most common constraints in the literature are the Sakoe-Chiba band and the Itakura Parallelogram. This paper uses Sakoe-Chiba band as a global constraint, the Sakoe-Chiba band is defined through a window size parameter which determines the largest temporal shift allowed from the diagonal. Temperature compensation performance of DTW Sakoe-Chiba is tested using the available OGW dataset #2 provided by Jochen Moll et al. The OGW dataset has been generated using 12 number of piezoelectric transducers bounded to CFRP (Carbon Fiber Reinforced Plastic) plate and varying the temperature from 20-60 degree with an increment of 0.5 degree. The signal is recorded for 1300 microseconds, that results in 13000 samples/time stamps(N). Initial results show that the Sakoe-Chiba constraint based DTW performs well and at a significantly lower cost, as indicated below. Result: The largest temporal shift (r) is estimated using Local Peak Coherence (LPC),for signal at 20 and 60 degree r is 135 samples. This results in complexity O(N*2r) which is very less than conventional DTW compensation technique O(N*N). In the final paper, the analysis will be replicated across a range of temperatures, and the performance of damage detection will be thoroughly discussed., Full text license: CC-BY-4.0;Funder: Centre of Hydrogen Energy Systems Sweden (CH2ESS)

Acousto-ultrasonic composite transducers (AUCTs), comprising piezoceramic materials in a reinforced polymeric matrix, show promise for structural health monitoring in composite structures. Challenges arise when integrating AUCTs onto highly loaded thermoplastic composites, especially low-surface-energy materials like polyaryletherketone composites. To address this, the study explores the viability of attaching AUCTs to low-melting polyaryletherketone carbon fiber-reinforced thermoplastic composite structures using ultrasonic welding. This welding technique forms a joint where the interface material fuses with the AUCT embedment and the structure matrix, providing a reliable and automatable process. The investigation includes a comparative analysis of an ultrasonic welded joint with an external energy director and a reference AUCT system integrated using a vacuum bagging oven procedure. Results highlight the potential of AUCT configurations integrated by ultrasonic welding as an alternative solution, acknowledging challenges that persist for further development and increased reliability in structural health monitoring applications., Group Zarouchas

With the improvements in computational power and advances in chip and sensor technology, the applications of machine learning (ML) technologies in structural health monitoring (SHM) are increasing rapidly. Compared with traditional methods, deep learning based SHM (Deep SHM) methods are more efficient and have a higher accuracy. However, due to the black box nature of deep learning, the trained models are usually difficult to interpret, which blocks their practical application. Therefore, it is of great importance to develop explainable artificial intelligence (XAI) methods to understand the internal decision-making mechanisms of damage classification in Deep SHM. In this paper, a novel XAI algorithm named Deep Gradient-weighted Class Activation Mapping (Deep Grad CAM) is proposed by combining the existing method Grad CAM with the convolutional neural network (CNN) deconvolution mechanism. In this paper, Deep Grad CAM is used to interpret a one-dimensional convolutional neural network trained to detect bolt loosening based on guided wave propagation. The interpretation performance of Deep Grad CAM is compared with Grad CAM, and their performances are quantified using Infidelity. The results show that the Infidelity of Deep Grad CAM is much smaller than that of Grad CAM, indicating significant improvements in explanation accuracy and reliability., Structural Integrity & Composites, Group Yue, Group Groves

Honeycomb sandwich structures (HSS), due to their high strength/stiffness to density ratio, are widely used in aerospace industry. Efficient and reliable techniques are required to detect damages in HSS. While conventional damage detection methods cannot meet the requirements, ultrasonic guided waves, due to their long propagating range and sensitivity to different types of defects, have the potential to significantly improve the state-of-the art in the NDE of HSS. However, the literature on ultrasound propagation in HSS is rather sparse, especially on the detailed characteristics of the waves and their interaction with realistic defects in HSS components. When the defect is smaller, excitation signal with higher frequency is required. However, there is no published research on this subject.The major objective of this research is to study high frequency wave propagation in HSS with or without the presence of defects. A numerical searcher is developed to solve the dispersion equations in HSS under very high frequency excitations. With this tool, solutions for high frequency Lamb waves in isotropic media are obtained and compared with numerical simulations. To obtain the solution in layered media, the global matrix method (GMM) is used and solutions for force responses are obtained by the residue theorem. However, the conventional method has singularity issues causing numerical instability in certain frequencies. A new formulation of the method is developed to solve the problem. The HSS is later homogenized to a three-layered medium and the dispersion curves for this model is studied with changes in the core material properties. At the end, the conventional damping model for Lamb waves is validated through the method and the corresponding damping coefficients are obtained experimentally. An experiment is performed to obtain the signal features when disbond damages are introduced in the HSS. The characteristics of the waves incident from different angles are studied to

© 2000 Elsevier Science Ltd. We wish to thank Prof. Patterson for his kind help and interesting suggestions and J. Pelayo, C. Cosculluela and F. Villuendas, of the Departamento de Física Aplicada of the Universidad of Zaragoza (Spain) for providing us with the sensors. This work has been supported by European Union project SOFIE (Spectroscopy using Optical Fibers in the Marine Environment), Contract Number MAS3-CT97-0157 and by Project HID98-0719 `Teledetección in-situ del grado de salinidad del agua por una red de sensores de fibra óptica, of the Plan Nacional de I+D of Spain., In this work we present an application of a simple quasi-geometrical model to analyze the behavior of compound waveguide structures used as fiber-optic sensors. This theoretical model is based on the adjustment of the parameters of the structure from the experimental measures to predict the observed behavior of the device. It also takes into account the non-monocromaticity of the used source. In this way, it can be used as a design criterion for this kind of structures. It is applied to a refractive index fiber-optic sensor based on the excitation of surface plasmon in a metal layer by the light guided by a monomode fiber., European Union, Plan Nacional de I+D of Spain, Depto. de Óptica, Fac. de Ciencias Físicas, TRUE, pub

Acousto-ultrasonic composite transducers (AUCT), which are made of piezoceramic materials embedded in a reinforced polymeric matrix, are promising for the health monitoring of composite structures. However, when they are integrated into highly loaded thermoplastic composite structures, ensuring proper joining properties is a challenge. The conventional approach of attaching the AUCT using adhesive may not be sufficiently reliable in aeronautic applications for low surface energy materials such as polyaryletherketone composites, where surface treatments are needed for adhesion. Welding techniques can be used to create a joint in which the interface material interfuses with the AUCT embedment and the structure matrix, resulting in a homogeneous interface with properties comparable to the host structure matrix throughout its service life. With this in mind, the main objective of the present work is to investigate the viability of attaching AUCT to low-melting polyaryletherketone carbon fiber reinforced thermoplastic composite structures using the ultrasonic welding (UW) procedure and characterize the joint performance. The ultrasonic welded joint using an external energy director in the interface is investigated by comparing the findings to those of a reference AUCT system integrated into the structure with autoclave co-consolidation. Infrared thermography is employed to monitor the process, and a parameter study of the UW process is carried out. The AUCT survivability during the UW process is determined by measuring the capacitance, and C-scan is used to assess joint quality. The results show the challenges of attaching AUCT to thermoplastic composite structures using UW and surviving the procedure., Structural Integrity & Composites, Aerospace Structures & Materials

A multiresolution finite wavelet domain method, that utilizes Daubechies wavelet and scaling functions for the hierarchical approximation of state variables, is presented. The multiresolution approximation yields a hierarchical set of equations of motion involving the coarse component of generalized displacements, while additional equations of finer components are subsequently added. A coarse solution is first calculated, and finer solutions can be sequentially superimposed on the coarse solution until convergence to the final solution is achieved. Moreover, it is shown that each resolution can model specific bandwidths of wavenumbers, thus providing a unique capability to separate coexisting wave modes and detect converted and reflected waves in the presence of damage. Two wavelet-based beam elements are explored, the first encompasses the Timoshenko shear beam theory and the second a high-order layerwise laminate theory for the accurate prediction of both symmetric and antisymmetric guided waves. Numerical results illustrate the inherent property of the method to a priori localize and isolate coexisting guided wave modes and their conversions, induced by different material regions and weak or debonded layer interfaces, thus demonstrating the method’s intrinsic capabilities towards the design of wave-based SHM systems., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Structural Integrity & Composites

This paper describes the design and implementation of an ultrasonic non-contact air-coupled technique (UNCACT) using antisymmetric Lamb waves (ALW) for NDT assessments in novel composite sandwich plates of a car body shell. This technique is complemented with a C-Scan image implementation using guided waves. The finite element method (FEM) was developed using Comsol 6.1 for the interpretation of the several wave modes presented in the experiments, including the ALW mode. This FEM model is indispensable for the correct interpretation of the received signals and contributes to a better implementation of this technology. This is a novel contribution building upon previously reported work. Additionally, the phase velocity method (PVM) was applied for the verification of the ALW mode in the portion of the RF signal necessary for the C-Scan image.

In the aerospace industry, the non-destructive evaluation (NDE) of reusable composite structural components in launch vehicles requires the development and implementation of efficient and reliable techniques. Most conventional damage detection methods do not meet these requirements. Recently develop methods using guided ultrasonic waves have the potential to improve the efficiency and reliability of damage detection in aerospace structures. However, they rely on previously collected baseline data that can be different in similar parts in composite structures due to their manufacturing tolerances. To overcome this difficulty, databases from analytical and numerical models may be employed to be correlated with data collected during the NDE of the structures. Nonetheless, the lack of knowledge of the exact material properties -- together with manufacturing tolerances -- could lead to erroneous conclusions. Evaluation of these uncertainties can be difficult and occasionally infeasible. Rapid and reliable guided ultrasonic wave-based techniques, which include uncertainty in models and material parameters and can localize and characterize flaws in large composite structures, could gain acceptance for the NDE of reusable structures.In this work, the first objective is to study guided waves formed by the initiation of microcracks in composites, aiming to improve the current passive NDE techniques. Based on an approach developed by seismologists to study earthquake sources, an efficient model has been developed based on a new form of the Green's function to identify the ultrasonic waves produced during the crack formation. The second objective in this work is to develop a methodology to implement the ultrasonic guided wave based technique as one of the qualifications criteria in composite manufacturing. A baseline-free technique to localize delamination in fiber-reinforced composites is proposed. The technique compares data from waves propagating in the healthy regions of th

The application of structural health monitoring (SHM) in composite airframe structural elements under long-term realistic fatigue loading needs to consider the structural behavior on the global level, which is an intricate task. The overall structural stiffness is a key design parameter for composite structures and the stiffness degradation under fatigue loading is closely related to the damage accumulation and failure mechanism which can be used as an indicator for the structural degradation. Therefore, this paper investigates the use of guided waves in axial stiffness degradation estimation for stiffened carbon fiber reinforced polymer (CFRP) composite panels under post-buckling compression-compression (C-C) fatigue loads. Impacted or artificially debonded stiffened composite panels are tested under fatigue until failure and guided waves are acquired using a network of piezoelectric (PZT) sensors at fixed cycle intervals. The guided wave phase velocity along the loading direction is extracted to estimate the axial stiffness degradation with the consideration of mode conversion and failure of PZT sensors. The estimated stiffness of five stiffened composite panels matches well with the stiffness calculated from the load–displacement curves. The estimated stiffness is also assessed using prognostic performance metrics and shows good potential for being used as a health indicator for prognostic purposes., Structural Integrity & Composites

In the aerospace industry, the non-destructive evaluation (NDE) of reusable composite structural components in launch vehicles requires the development and implementation of efficient and reliable techniques. Most conventional damage detection methods do not meet these requirements. Recently develop methods using guided ultrasonic waves have the potential to improve the efficiency and reliability of damage detection in aerospace structures. However, they rely on previously collected baseline data that can be different in similar parts in composite structures due to their manufacturing tolerances. To overcome this difficulty, databases from analytical and numerical models may be employed to be correlated with data collected during the NDE of the structures. Nonetheless, the lack of knowledge of the exact material properties -- together with manufacturing tolerances -- could lead to erroneous conclusions. Evaluation of these uncertainties can be difficult and occasionally infeasible. Rapid and reliable guided ultrasonic wave-based techniques, which include uncertainty in models and material parameters and can localize and characterize flaws in large composite structures, could gain acceptance for the NDE of reusable structures.In this work, the first objective is to study guided waves formed by the initiation of microcracks in composites, aiming to improve the current passive NDE techniques. Based on an approach developed by seismologists to study earthquake sources, an efficient model has been developed based on a new form of the Green's function to identify the ultrasonic waves produced during the crack formation. The second objective in this work is to develop a methodology to implement the ultrasonic guided wave based technique as one of the qualifications criteria in composite manufacturing. A baseline-free technique to localize delamination in fiber-reinforced composites is proposed. The technique compares data from waves propagating in the healthy regions of th

Time-warping is a signal processing technique that, when applied to an isolated measurement of a transient signal that propagates in a waveguide, allows contributions to that signal from individual mode numbers to be isolated and extracted. Dispersion curves for individual mode numbers can, in turn, be recovered. Isolation of contributions associated with individual mode numbers is possible because after time-warping-a special environmentally dependent non-uniform sampling-is applied, the signal energy corresponding to each mode number is isolated in the frequency spectrum of the time-warped signal. Here we derive the time-warping transform for teleseismic Love waves, assuming the Earth structure is approximately known, and we illustrate the utility of time-warping using both synthetic and measured seismograms.

To perform active structural health monitoring (SHM), guided waves (GW) have received great interest as they can inspect large areas with a few sensors and are sensitive to barely-visible structural damages. Fiber Bragg grating (FBG) sensors offer several advantages such as small size, low weight and ability to be embedded but their use has been limited for GW sensing due to their limited sensitivity while using spectrometers. FBG sensors in the edge-filtering configuration have overcome this issue with reasonable sensitivity and there is a renewed interest in their use. It is well known that when subjected to a transverse strain, the circular cross-section of the fiber deforms into an elliptical shape generating the birefringence phenomenon. This deformation, influences the coupling mode of the light inside the FBG and hence, modifies the resulting reflectivity spectrum. This paper investigates how controlled changes in the reflectivity spectrum can be introduced using different transverse loads. The effect of the modified spectrum on the sensitivity of the FBG for GW measurements is then studied. The study also investigates the effect of the transverse strain on the coupling of the GW from the structure into the fiber., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Structural Integrity & Composites

Time-warping is a signal processing technique that, when applied to an isolated measurement of a transient signal that propagates in a waveguide, allows contributions to that signal from individual mode numbers to be isolated and extracted. Dispersion curves for individual mode numbers can, in turn, be recovered. Isolation of contributions associated with individual mode numbers is possible because after time-warping-a special environmentally dependent non-uniform sampling-is applied, the signal energy corresponding to each mode number is isolated in the frequency spectrum of the time-warped signal. Here we derive the time-warping transform for teleseismic Love waves, assuming the Earth structure is approximately known, and we illustrate the utility of time-warping using both synthetic and measured seismograms.

Ultrasonic flow meters (UFMs) based on transducer arrays offer several advantages. With electronic beam steering, it is possible to tune the steering angle of the beam for optimal signal-tonoise ratio (SNR) upon reception. Moreover, multiple beams can be generated to propagate through different travel paths, covering a wider section of the flow profile. Furthermore, in a clamp-on configuration, UFMs based on transducer arrays can perform self-calibration. In this manner, userinput is minimized and measurement repeatability is increased. In practice, transducer array elements may break down. This could happen due to aging, exposure to rough environments, and/or rough mechanical contact. As a consequence of inactive array elements, the measured transit time difference contains two offsets. One offset originates from non-uniform spatial sampling of the generated wavefield. Another offset originates from the ill-defined beam propagating through a travel path different from the intended one. In this paper, an algorithm is proposed that corrects for both of these offsets. The algorithm also performs a filtering operation in the frequency-wavenumber domain of all spurious (i.e., flow-insensitive) wave modes. The advantage of implementing the proposed algorithm is demonstrated on simulations and measurements, showing improved accuracy and precision of the transit time differences compared to the values obtained when the algorithm is not applied. The proposed algorithm can be implemented in both in-line and clamp-on configuration of UFMs based on transducer arrays., ImPhys/Medical Imaging, Electronic Instrumentation

The recording and processing of acoustic emissions can be used to identify and localise damage mechanisms occurring in engineering structures. In plate-like structures, acoustic emissions propagate through the structure as guided waves. With a measurement location away from the source location, dispersion effects in the guided wave distort the acoustic emission signal. The distortion of the original signal hampers identification of damage mechanisms. This research describes and assesses a method to reconstruct the original acoustic emission signal using dispersion compensation. Simulations and experiments are performed involving thick glass-fibre reinforced plastic laminates. The signal reconstruction on the simulated data gives a reasonable representation of the simulated signal at the location of interest. In the experimental case, similarity slightly degrades. Deviation in arrival time between original measurement and reconstruction is attributed to a possible discrepancy in material properties in reality versus the properties used in the reconstruction., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Aerospace Structures & Computational Mechanics, Ship Hydromechanics and Structures

The application of structural health monitoring (SHM) in composite airframe structural elements under long-term realistic fatigue loading needs to consider the structural behavior on the global level, which is an intricate task. The overall structural stiffness is a key design parameter for composite structures and the stiffness degradation under fatigue loading is closely related to the damage accumulation and failure mechanism which can be used as an indicator for the structural degradation. Therefore, this paper investigates the use of guided waves in axial stiffness degradation estimation for stiffened carbon fiber reinforced polymer (CFRP) composite panels under post-buckling compression-compression (C-C) fatigue loads. Impacted or artificially debonded stiffened composite panels are tested under fatigue until failure and guided waves are acquired using a network of piezoelectric (PZT) sensors at fixed cycle intervals. The guided wave phase velocity along the loading direction is extracted to estimate the axial stiffness degradation with the consideration of mode conversion and failure of PZT sensors. The estimated stiffness of five stiffened composite panels matches well with the stiffness calculated from the load–displacement curves. The estimated stiffness is also assessed using prognostic performance metrics and shows good potential for being used as a health indicator for prognostic purposes., Structural Integrity & Composites

This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

A method based on ultrasonic wave propagation is applied for the determination of the viscosity of low viscous liquids. A waveguide is used to remotely transmit the ultrasonic waves from a shear piezoelectric transducer into the liquid. At the solid-liquid interface, a guided wave mode, the shear mode, is used to extract the liquid viscosity. The energy of the reflected ultrasonic wave depends upon its operating frequency, the physical properties of the liquid (viscosity and density), and the waveguide (density and shear modulus). The results show that the attenuation of the waves, and thus the viscosity of the liquid, can be retrieved using this method. Measurements on water, ethanol, and mixtures of water/glycerol illustrate that the method can monitor changes in attenuation due to the viscosity of the liquid. The range of viscosities measured was between 0.8 and 60 mPa s. Compared to literature values, the relative error for these measurements was lower than 12% while the uncertainty in the measurements was lower than 5%. Besides its ability to measure low viscosities, this method offers advantages such as the capability to perform in-situ measurements of liquids in harsh environments, the omission of mechanical parts, and the possibility to handle small volumes of liquid. These features make this method suitable for low viscous liquids that are radioactive, corrosive and at high temperature., RST/Reactor Physics and Nuclear Materials, RST/Technici Pool, ImPhys/Computational Imaging, RST/Radiation, Science and Technology

This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

We image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from ∼42 000 local and ∼180 teleseismic events occurring between 2012 and 2017. The RA linear array has short aperture (∼470 m long with 12 strong motion sensors) and recorded for the entire analysed time window, whereas the RR is a large three-component nodal array (97 geophones across a ∼2.4 km × 1.4 km area) that operated for about a month in September-October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2 and F3 from SW to NE that have accommodated several Mw > 6 earthquakes in the past 15 000 yr. Waveform changes in the fault normal direction indicate structural discontinuities that are consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes ∼0.05 s delay at stations RR26-31 in the teleseismic P arrivals compared with the SW-most station, and generates both P-and S-type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is ∼100 m wide, ∼4 km deep with a Q value of ∼20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multiscale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

We analyze seismograms recorded by four arrays (B1–B4) with 100 m station spacing and apertures of 4–8 km that cross the surface rupture of the 2019 Mw 7.1 Ridgecrest earthquake. The arrays extend from B1 in the northwest to B4 in the southeast of the surface rupture. Delay times between P wave arrivals associated with ∼1,200 local earthquakes and four teleseismic events are used to estimate local velocity variations beneath the arrays. Both teleseismic and local P waves travel faster on the northeast than the southwest side of the fault beneath arrays B1 and B4, but the velocity contrast is less reliably resolved at arrays B2 and B3. We identify several 1–2 km wide low-velocity zones with much slower inner cores that amplify S waveforms, inferred as damage zones, beneath each array. The damage zones at arrays B2 and B4 also generate fault-zone head and trapped waves. An automated detector, based on peak ground velocities and durations of high-amplitude waves, identifies candidate fault-zone trapped waves (FZTWs) in a localized zone for ∼600 earthquakes at array B4. Synthetic waveform modeling of averaged FZTWs, generated by ∼30 events with high-quality signals, indicates that the trapping structure at array B4 has a width of ∼300 m, depth of 3–5 km, S wave velocity reduction of ∼20% with respect to the surrounding rock, Q-value of ∼30, and S wave velocity contrast of ∼4% across the fault (faster on the northeast side). The results show complex fault-zone internal structures (velocity contrasts and low-velocity zones) that vary along fault strike.

A method based on ultrasonic wave propagation is applied for the determination of the viscosity of low viscous liquids. A waveguide is used to remotely transmit the ultrasonic waves from a shear piezoelectric transducer into the liquid. At the solid-liquid interface, a guided wave mode, the shear mode, is used to extract the liquid viscosity. The energy of the reflected ultrasonic wave depends upon its operating frequency, the physical properties of the liquid (viscosity and density), and the waveguide (density and shear modulus). The results show that the attenuation of the waves, and thus the viscosity of the liquid, can be retrieved using this method. Measurements on water, ethanol, and mixtures of water/glycerol illustrate that the method can monitor changes in attenuation due to the viscosity of the liquid. The range of viscosities measured was between 0.8 and 60 mPa s. Compared to literature values, the relative error for these measurements was lower than 12% while the uncertainty in the measurements was lower than 5%. Besides its ability to measure low viscosities, this method offers advantages such as the capability to perform in-situ measurements of liquids in harsh environments, the omission of mechanical parts, and the possibility to handle small volumes of liquid. These features make this method suitable for low viscous liquids that are radioactive, corrosive and at high temperature., RST/Reactor Physics and Nuclear Materials, RST/Technici Pool, ImPhys/Computational Imaging, RST/Radiation, Science and Technology

This work is devoted to experimental studies of the influence of dielectric inhomogeneities on electromagnetic waves in a transmission line. A series of experiments is described, united by one aim: the development of new methods and approaches in the study, parameters reconstruction and the creation of new functional materials. During the work, the related tasks were successfully solved: refinement of the waveguide method for measuring the electrophysical parameters of materials, verification of an experimental study of the half-waveguide measuring method, designing of an experiment for measuring the parameters of materials in free space, discernibility and resonance of inhomogeneities in transmission lines. All experiments were carried out in the microwave range., Данная работа посвящена экспериментальным исследованиям влияния диэлектрических неоднородностей на электромагнитные волны в линии передачи. Описывается целая серия экспериментов, объединенных одной задачей: развитие новых методов и подходов при изучении, восстановлении параметров и создании новых функциональных материалов. В ходе работы были успешно решены сопутствующие задачи: уточнение волноводного метода измерения электрофизических параметров материалов, верификация экспериментального исследования полуволноводного метода измерения параметров материалов, планирование эксперимента по измерению параметров материалов в свободном пространстве, различимость и резонанс неоднородностей в линиях передачи. Все эксперименты были проведены в диапазоне СВЧ.

Blunt impact damage (BID) is known to potentially cause significant strength reduction on composite structures even when the damage is barely noticeable. Therefore, non-destructive evaluation (NDE) to assess blunt-impacted composite aircraft structures is a crucial procedure to determine existence and severity of the damage to examine airworthiness. Ultrasonic guided waves (UGWs) test method has been studied herein as an NDE technique to evaluate large composite structures, because the typically thin-walled aircraft structures effectively act as has guided waves propagation paths. UGW scanning methods were performed only from the external skin-side using pitch-catch method: an actuator and a receiver located at a distance apart to scan the structure within. The actuator and the receiver transducers were placed on the adjacent skin-to-shear tie bolted joint regions for scanning of internal shear tie and C-frame components, and placed at opposite sides of the hat stringer on the skin bay for investigation of co-cured stringer components. UGWs transmission through bolted joint structure was examined to determine the test frequency that allowed wave transmission through bolted joints at low attenuation. And UGW tests from the skin-to-shear tie joint regions identified significant wave energy drop at the test frequency from the comparison of the pristine and the damaged (HEWABI) shear ties. Furthermore, damage characteristics were analyzed from the UGW measurements of the stringer scan followed by successful blunt impacted stringer delamination damage detection. Features extracted from the UGW test results showed exceptional match to the damage location, severity, and modes determined from the conventional NDE methods. Finally, UGW measurements-based residual strength estimation was studied from simple notched coupon tests and is an on-going study for more complex structures with realistic damage modes.

林 高弘, 薄板構造中のガイド波とその拡散場を利用した損傷画像化技術, 材料, 2020, 69(8), pp. 593-598, https://doi.org/10.2472/jsms.69.593.

Determination of soil material damping is known to be difficult and uncertain, especially in the offshore environment. Using an advanced inversion methodology based on multichannel spectral analysis, Scholte and Love wave measurements are used to characterize subsea soil from a North Sea site. After normalization, a determinant-based objective function is used in a genetic algorithm optimization to estimate the soil shear modulus. The inverted shear-modulus profile is comparable to previously published results for the same data, although a higher degree of certainty is achieved in the near-surface layers. The half-power bandwidth method is used for extracting the attenuation curve from the measurements and efficient reference data points are chosen based on wavelet compression. The material-damping ratio inversion is performed using a modified stochastic optimization algorithm. Accounting for measurement errors, the material-damping ratio profile is retrieved from the fundamental-mode Scholte wave with a high degree of certainty. Furthermore, a method is proposed for identifying the frequency dependence of the material-damping ratio from in situ measurements. No evidence for frequency dependence is found and the small-strain soil material-damping ratio at this site can be said to be frequency independent for the measured conditions., ImPhys/Computational Imaging, Offshore Engineering, Engineering Structures, Dynamics of Structures

Blunt impact damage (BID) is known to potentially cause significant strength reduction on composite structures even when the damage is barely noticeable. Therefore, non-destructive evaluation (NDE) to assess blunt-impacted composite aircraft structures is a crucial procedure to determine existence and severity of the damage to examine airworthiness. Ultrasonic guided waves (UGWs) test method has been studied herein as an NDE technique to evaluate large composite structures, because the typically thin-walled aircraft structures effectively act as has guided waves propagation paths. UGW scanning methods were performed only from the external skin-side using pitch-catch method: an actuator and a receiver located at a distance apart to scan the structure within. The actuator and the receiver transducers were placed on the adjacent skin-to-shear tie bolted joint regions for scanning of internal shear tie and C-frame components, and placed at opposite sides of the hat stringer on the skin bay for investigation of co-cured stringer components. UGWs transmission through bolted joint structure was examined to determine the test frequency that allowed wave transmission through bolted joints at low attenuation. And UGW tests from the skin-to-shear tie joint regions identified significant wave energy drop at the test frequency from the comparison of the pristine and the damaged (HEWABI) shear ties. Furthermore, damage characteristics were analyzed from the UGW measurements of the stringer scan followed by successful blunt impacted stringer delamination damage detection. Features extracted from the UGW test results showed exceptional match to the damage location, severity, and modes determined from the conventional NDE methods. Finally, UGW measurements-based residual strength estimation was studied from simple notched coupon tests and is an on-going study for more complex structures with realistic damage modes.

Determination of soil material damping is known to be difficult and uncertain, especially in the offshore environment. Using an advanced inversion methodology based on multichannel spectral analysis, Scholte and Love wave measurements are used to characterize subsea soil from a North Sea site. After normalization, a determinant-based objective function is used in a genetic algorithm optimization to estimate the soil shear modulus. The inverted shear-modulus profile is comparable to previously published results for the same data, although a higher degree of certainty is achieved in the near-surface layers. The half-power bandwidth method is used for extracting the attenuation curve from the measurements and efficient reference data points are chosen based on wavelet compression. The material-damping ratio inversion is performed using a modified stochastic optimization algorithm. Accounting for measurement errors, the material-damping ratio profile is retrieved from the fundamental-mode Scholte wave with a high degree of certainty. Furthermore, a method is proposed for identifying the frequency dependence of the material-damping ratio from in situ measurements. No evidence for frequency dependence is found and the small-strain soil material-damping ratio at this site can be said to be frequency independent for the measured conditions., ImPhys/Computational Imaging, Offshore Engineering, Engineering Structures, Dynamics of Structures

Acoustic emission (AE) sensors and ultrasonic transducers were characterized for the detection of Rayleigh waves (RW). Small aperture reference sensors were characterized first using the fracture of glass capillary tubes in combination with a theoretical displacement calculation, which utilized finite element method (FEM) and was verified by laser interferometer. For the calibration of 18 commercial sensors and two piezoceramic disks, a 90° angle beam transducer was used to generate RW pulses on an aluminum transfer block. By a substitution method, RW receiving sensitivity of a sensor under test was determined over the range of frequency from 22 kHz to 2 MHz. Results were compared to the sensitivities to normally incident waves (NW) and to other guided waves (GW). It was found that (1) NW sensitivities are always higher than RW sensitivities, (2) differences between NW and RW receiving sensitivities are dependent on frequency and sensor size, (3) most sensors show comparable RW and GW receiving sensitivities, especially those of commonly used AE sensors, and (4) the receiving sensitivities of small aperture (1 mm diameter) sensors behave differently from larger sensors.

Acoustic emission (AE) sensors and ultrasonic transducers were characterized for the detection of Rayleigh waves (RW). Small aperture reference sensors were characterized first using the fracture of glass capillary tubes in combination with a theoretical displacement calculation, which utilized finite element method (FEM) and was verified by laser interferometer. For the calibration of 18 commercial sensors and two piezoceramic disks, a 90° angle beam transducer was used to generate RW pulses on an aluminum transfer block. By a substitution method, RW receiving sensitivity of a sensor under test was determined over the range of frequency from 22 kHz to 2 MHz. Results were compared to the sensitivities to normally incident waves (NW) and to other guided waves (GW). It was found that (1) NW sensitivities are always higher than RW sensitivities, (2) differences between NW and RW receiving sensitivities are dependent on frequency and sensor size, (3) most sensors show comparable RW and GW receiving sensitivities, especially those of commonly used AE sensors, and (4) the receiving sensitivities of small aperture (1 mm diameter) sensors behave differently from larger sensors.

Polarization degeneracy of electromagnetic plane waves in vacuum and in any bulk isotropic media is the keynote operational principle of many optical devices such as polarizers and interferometers. However, surface and guided waves spectra are typically either not degenerated at all or meet degeneracy only at very specific dispersion points. In this work, we offer a design of a periodic photonic structure based on zinc oxide (ZnO) nanocylinders providing the broadband polarization degeneracy of the guided waves in the near-IR frequency range. We analyze the impact of the spatial dispersion and substrate on the degeneracy breaking. We offer the design based on ZnO nanocylinders with aluminum-doped zinc oxide (AZO) substrate, achieving the degeneracy in the vicinity of a telecommunication wavelength, for the practical implementation. Finally, we propose and verify numerically a potentially important device - waveguide polarizer, which is the analogue to the $\pi$/4 plate for the guided waves.

La tolérance aux dommages appliquée aux structures aéronautiques est un concept qui requiert une bonne compréhension des modes d’endommagement et une bonne maîtrise des outils de surveillance et d’inspection. Dans ce contexte, la présente thèse de doctorat a pour objectif principal de développer des stratégies expérimentales et des modèles numériques afin d’étudier différents types d’endommagement dans des structures aéronautiques et d’évaluer leurs effets sur les propriétés mécaniques et la durabilité. Pour atteindre cet objectif, quatre structures ont été sélectionnées de façon à couvrir l’ensemble des principaux modes d’endommagement observés en service. Les géométries et les scénarios d’endommagement de ces structures ont été définis en collaboration avec les partenaires industriels dans le cadre des projets CRIAQ DPHM501 et CRIAQ COMP407. Dans un premier temps, on a abordé la fissuration par fatigue dans des structures en aluminium à rivets où une étude préliminaire a été menée pour comparer la réponse ultrasonore obtenue par vibromètre laser entre un défaut artificiel usiné par électro-érosion et une fissure de fatigue. Cette étude a permis de juger la représentativité de ce type de défaut et sa validité pour la mise en place d’une technique de surveillance embarquée. Par la suite, une approche de surveillance s’appuyant sur la technique d’émission acoustique, la corrélation d’image et l’inspection par ondes guidées a été appliquée à une structure avec ligne de rivets. Cette démarche a permis de corréler la perte de rigidité, l’activité acoustique, et la chute d’amplitude des ondes guidées à l’évolution de la taille de la fissure. Dans une deuxième phase, on s’est intéressé à l’évaluation de l’effet d’un défaut de fabrication (délaminage) sur la formation d’endommagement et sur la dégradation de la rigidité dans un matériau composite. Pour cela, des plaques en CFRP tissé avec et sans défaut inséré ont été testées en fatigue. L’application de la stratégie de su, Damage tolerance in aerospace structures is a concept that requires a good understanding of damage modes and monitoring techniques. In this context, the present research defines as a main objective the development of experimental strategies and numerical models to investigate different damage scenarios in typical structure configurations and evaluate their impact on mechanical properties and fatigue life. To achieve this, four different structures with increased-complexity level are selected such that the principal in service damage scenarios are covered. The geometry and damage mode for these structures are defined in collaboration with industrial partners in CRIAQ DPHM501 and COMP407 projects. Firstly, fatigue crack propagation in aluminium riveted structure was investigated, a comparison study was conducted to compare the acoustic response obtained by laser vibrometer between artificial defect made by EDM and real fatigue crack. Based on the obtained results, it was possible to judge the capacity of artificial notch to mimic real fatigue crack for the implementation of SHM approach. Thereafter, in-situ monitoring approach was developed using acoustic emission, digital image correlation and ultrasonic guided waves for tracking fatigue cracks on riveted structure. Thus, it was possible to correlate the stiffness decrease, the acoustic emission activities and the wave amplitude decay to crack size propagation. Secondly, the effect of manufacturing defect (delamination) on the damage development and stiffness degradation in composite material was evaluated. Hence, plain weave CFRP plates with and without embedded defect were tested under fatigue loading. By applying the monitoring strategy, it was possible to measure the average stiffness degradation and its values around the defect, the acoustic activity from the inspection area was also registered and localized by a planar location algorithm, which provided insight on damage development sequences. Additio

We demonstrate that Bound states In the Continuum (BICs) are supported in planar anisotropic structures where the optic axes are arbitrarily oriented. Moreover, we reveal fundamental new topological properties of these BICs that depend on the relative orientation of the optic axes in the core and substrate., Peer Reviewed, Postprint (published version)

In this paper, a support stiffness monitoring scheme based on torsional guided waves for detecting loss of rigidity in a support of cylindrical structures is presented. Poor support performance in cylindrical specimens such as a pipeline setup located in a sloping terrain may produce a risky operation condition in terms of the installation integrity and the possibility of human casualties. The effects of changing the contact forces between support and the waveguide have been investigated by considering variations in the load between them. Fundamental torsional T(0,1) mode is produced and launched by a magnetostrictive collar in a pitch-catch configuration to study the support effect in the wavepacket propagation. Several scenarios are studied by emulating an abnormal condition in the support of a dedicated test bench. Numerical results revealed T(0,1) ultrasonic energy leakage in the form of SH0 bulk waves when a mechanical coupling between the cylindrical waveguide and support is yielded. Experimental results showed that the rate of ultrasonic energy leakage depends on the magnitude of the reaction forces between pipe and support; so different levels of attenuation of T(0,1) mode will be produced with different mechanical contact conditions. Thus, it is possible to relate a measured attenuation to variations in the supports condition. Results of each scenarios are presented and discussed demonstrating the feasibility and potential of tracking of the amplitude of the T(0,1) as an indicator of abnormal conditions in simple supports., Peer Reviewed, Postprint (published version)

Les systèmes SHM (Structural Health Monitoring) basés sur la propagation d'ondes ultrasonores guidées sont utilisés pour des structures de grandes dimensions, par exemple dans les secteurs de l'aéronautique ou du génie civil. Les ondes de Lamb ou SH sont généralement employées car elles se propagent sur de longues distances dans des structures planes tout en sondant l'épaisseur des pièces. Cependant, des modes moins conventionnels se propagent dans les guides d'ondes de section droite finie, tels que les barreaux, les rails ou les tuyaux. Le nombre de modes peut être très élevé dans ce type de guide, et il est important de bien sélectionner un mode particulier. Les méthodes actuelles de sélectivité modale, basées sur l'emploi de plusieurs émetteurs, considèrent habituellement des éléments PZT identiques (même sensibilité, même réponse en fréquence...) et ne prennent pas en compte les conditions réelles de montage et leurs éventuelles imperfections (couplage variable entre traducteurs, mauvais alignement, différence de réponse de l'électronique...). Ce travail présente une méthodologie générale pour la sélectivité modale dans des guides à section droite finie, à l'aide de plusieurs éléments piézoélectriques disposés à leur surface. Cette sélectivité est basée sur la mesure expérimentale préalable, à l'aide d'un vibromètre laser 3D, des amplitudes des modes générés par chaque élément excité individuellement. Une procédure d'optimisation permet d'inverser le problème afin de maximiser l'amplitude du mode désiré, alors obtenue en excitant simultanément tous les émetteurs. Le problème à inverser requiert la connaissance des courbes de dispersion ainsi que des déformées modales des modes, obtenues en utilisant la méthode SAFE 2D. La méthodologie est testée à travers des simulations numériques et des mesures expérimentales sur un barreau d'aluminium de section rectangulaire instrumenté avec huit éléments PZT à sa surface. L'efficacité de la méthode pour générer différents, SHM systems (Structural Health Monitoring) based on ultrasonic guided waves propagation are used for large structures, e.g. in Aerospace or Civil Engineering. Lamb or SH waves are usually employed as they propagate over long distances in plate-like structures while probing the entire thickness. However less conventional modes propagate in waveguides with finite crosssection, such as bars, rails or pipes. The number of modes can be very high even at low frequency in this type of guide, and it is important to carefully select a specific mode. Current methods for modal selectivity, based on the use of several emitters, usually consider identical PZT elements (same sensitivity, same frequency response, etc.) and do not account for real experimental conditions and possible differences (variable coupling between transducers, flawed alignment, variable electronic response, etc.). This work presents a global methodology for modal selectivity in waveguides with finite cross-section, using several piezoelectric elements attached to their surface. This selectivity is based on experimental measurements, with a 3D laser vibrometer, of the amplitudes of the modes generated by each emitter. An optimization process allows to inverse the problem in order to maximize the amplitude of the desired mode, then generated by exciting all the emitters at once. This process requires knowing dispersion curves as well as the displacements of the various modes, calculated with SAFE 2D method. The methodology is tested through numerical simulations and experiments on an aluminium rectangular bar instrumented with 8 PZT elements on top. The method efficiency to generate different pure modes, and to detect and locate calibrated defects, is demonstrated for the aluminium bar. Its potential for SHM application of more complex structures is studied, like a rail or an adhesively bonded composite structure.

The implementation of damage-detection methods for continuously assessing structural integrity entails systems with attractive features such as storage capabilities, memory capacity, computational complexity and time-consuming processing. In this sense, embedded hardware platforms are a promising technology for developing integrated solutions in Structural Health Monitoring. In this paper, design, test, and specifications for a standalone inspection prototype are presented, which take advantage of piezo-diagnostics principle, statistical processing via Principal Component Analysis (PCA) and embedded systems. The equipment corresponds to a piezoelectric active system with the capability to detect defects in structures, by using a PCA-based algorithm embedded in the Odroid-U3 ARM Linux platform. The operation of the equipment consists of applying, at one side of the structure, wide guided waves by means of piezoelectric devices operated in actuation mode and to record the wave response in another side of the structure by using the same kind of piezoelectric devices operated in sensor mode. Based on the nominal response of the guide wave (no damages), represented by means of a PCA statistical model, the system can detect damages between the actuated/sensed points through squared prediction error (Q-statistical index). The system performance was evaluated in a pipe test bench where two kinds of damages were studied: first, a mass is added to the pipe surface, and then leaks are provoked to the pipe structure by means of a drill tool. The experiments were conducted on two lab structures: (i) a meter carbon-steel pipe section and (ii) a pipe loop structure. The wave response was recorded between the instrumented points for two conditions: (i) The pipe in nominal conditions, where several repetitions will be applied to build the nominal statistical model and (ii) when damage is caused to the pipe (mass adding or leak). Damage conditions were graphically recognized through, Peer Reviewed, Postprint (published version)

Pulsed laser equipment can be used to generate elastic waves through the instantaneous reaction of thermal expansion or ablation of the material; however, we cannot control the waveform generated by the laser in the same manner that we can when piezoelectric transducers are used as exciters. This study investigates the generation of narrowband tone-burst waves using a fiber laser of the type that is widely used in laser beam machining. Fiber lasers can emit laser pulses with a high repetition rate on the order of MHz, and the laser pulses can be modulated to a burst train by external signals. As a consequence of the burst laser emission, a narrowband tone-burst elastic wave is generated. We experimentally confirmed that the elastic waves agreed well with the modulation signals in time domain waveforms and their frequency spectra, and that waveforms can be controlled by the generation technique. We also apply the generation technique to defect imaging with a scanning laser source. In the experiments, with small laser emission energy, we were not able to obtain defect images from the signal amplitude due to low signal-to-noise ratio, whereas using frequency spectrum peaks of the tone-burst signals gave clear defect images, which indicates that the signal-to-noise ratio is improved in the frequency domain by using this technique for the generation of narrowband elastic waves. Moreover, even for defect imaging at a single receiving point, defect images were enhanced by taking an average of distributions of frequency spectrum peaks at different frequencies.

There are many advantages for NDE (Non-Destructive Evaluation) by using guided ultrasonic waves for structural diagnostic. Guided waves are composed of longitudinal waves and shear waves. From the data acquisition respect, guided waves provide higher ranges compared to bulk wave testing. However, many challenges appear when guided waves are used for detecting, such as many modes corresponding to one frequency, and dispersion properties on the group velocity and phase velocity. This thesis simulates guided-wave propagation by using the SAFE (Semi-Analytical Finite Element) method. From this method, only the cross section needs to be discretized. The wave propagation direction is approximated by analytical harmonic solutions. By building many large complex stiffness matrices, material viscoelastic property is considered. The solutions, such as group velocity dispersion curves (without material attenuation), phase velocity dispersion curves, energy velocity dispersion curves (with material attenuation), are generated from the SAFE codes. Moreover, SAFE codes can simulate forced solutions more efficiently than full discretization methods of traditional finite element analysis.Two applications of guided wave propagation are shown on this thesis, multilayered anisotropic composites and railroad tracks.

There are many advantages for NDE (Non-Destructive Evaluation) by using guided ultrasonic waves for structural diagnostic. Guided waves are composed of longitudinal waves and shear waves. From the data acquisition respect, guided waves provide higher ranges compared to bulk wave testing. However, many challenges appear when guided waves are used for detecting, such as many modes corresponding to one frequency, and dispersion properties on the group velocity and phase velocity. This thesis simulates guided-wave propagation by using the SAFE (Semi-Analytical Finite Element) method. From this method, only the cross section needs to be discretized. The wave propagation direction is approximated by analytical harmonic solutions. By building many large complex stiffness matrices, material viscoelastic property is considered. The solutions, such as group velocity dispersion curves (without material attenuation), phase velocity dispersion curves, energy velocity dispersion curves (with material attenuation), are generated from the SAFE codes. Moreover, SAFE codes can simulate forced solutions more efficiently than full discretization methods of traditional finite element analysis.Two applications of guided wave propagation are shown on this thesis, multilayered anisotropic composites and railroad tracks.

We present a method to simulate two-dimensional scattering by dielectric objects embedded in a dielectric layered medium with transverse magnetic polarization through a domain integral equation formulation. A mixed spatial-spectral discretization is employed with both a spatial and a spectral representation along the direction of the layer interfaces. In the spectral domain, a discretization on a path through the complex plane is used on which the Green function is well behaved. To calculate the field-material interaction in the spatial domain, an auxiliary field is employed similar to the Li factorization rules. Numerical results show that this auxiliary-field formulation significantly improves accuracy, compared to a formulation that directly employs the electric field.

There are many advantages for NDE (Non-Destructive Evaluation) by using guided ultrasonic waves for structural diagnostic. Guided waves are composed of longitudinal waves and shear waves. From the data acquisition respect, guided waves provide higher ranges compared to bulk wave testing. However, many challenges appear when guided waves are used for detecting, such as many modes corresponding to one frequency, and dispersion properties on the group velocity and phase velocity. This thesis simulates guided-wave propagation by using the SAFE (Semi-Analytical Finite Element) method. From this method, only the cross section needs to be discretized. The wave propagation direction is approximated by analytical harmonic solutions. By building many large complex stiffness matrices, material viscoelastic property is considered. The solutions, such as group velocity dispersion curves (without material attenuation), phase velocity dispersion curves, energy velocity dispersion curves (with material attenuation), are generated from the SAFE codes. Moreover, SAFE codes can simulate forced solutions more efficiently than full discretization methods of traditional finite element analysis.Two applications of guided wave propagation are shown on this thesis, multilayered anisotropic composites and railroad tracks.

The internal structure of the Clark fault in the trifurcation area of the San Jacinto fault zone is imaged using seismograms recorded by a dense linear array (Jackass Flat, JF) crossing the surface trace of the fault and an adjacent array (TR) to the SW. Delay times between phase arrivals associated with ∼3500 local earthquakes and nine teleseismic events are used to estimate velocity variations within the arrays. The teleseismic P waves travel faster beneath the TR than the JF array, in contrast to larger scale tomographic results. Statistical analysis of local P-wave delay times indicates that the entire JF array, with an aperture of ∼400 m, is inside a low-velocity damage zone. This low-velocity zone is bounded on the NE side by a shallow bimaterial interface generating fault zone head waves, and it contains an inner zone of more intense damage generating fault zone trapped waves. The P-wave velocity contrast across the local bounding bimaterial interface is 10-15 per cent. The trapping structure is associated with a width of ∼200 m, S-wave velocity reduction of ∼35 per cent with respect to the surrounding rock, Q-value of ∼20 and depth of ∼3.5 km. The imaging results suggest that the main seismogenic fault is near the SW end of the JF array, in agreement with a prominent geomorphologic feature. The existence of intense local damage on the crustal block with faster larger scale velocity at depth is consistent with common propagation of earthquake ruptures in the area to the NW.

Pulsed laser equipment can be used to generate elastic waves through the instantaneous reaction of thermal expansion or ablation of the material; however, we cannot control the waveform generated by the laser in the same manner that we can when piezoelectric transducers are used as exciters. This study investigates the generation of narrowband tone-burst waves using a fiber laser of the type that is widely used in laser beam machining. Fiber lasers can emit laser pulses with a high repetition rate on the order of MHz, and the laser pulses can be modulated to a burst train by external signals. As a consequence of the burst laser emission, a narrowband tone-burst elastic wave is generated. We experimentally confirmed that the elastic waves agreed well with the modulation signals in time domain waveforms and their frequency spectra, and that waveforms can be controlled by the generation technique. We also apply the generation technique to defect imaging with a scanning laser source. In the experiments, with small laser emission energy, we were not able to obtain defect images from the signal amplitude due to low signal-to-noise ratio, whereas using frequency spectrum peaks of the tone-burst signals gave clear defect images, which indicates that the signal-to-noise ratio is improved in the frequency domain by using this technique for the generation of narrowband elastic waves. Moreover, even for defect imaging at a single receiving point, defect images were enhanced by taking an average of distributions of frequency spectrum peaks at different frequencies.

Since mechanical stress in structures affects issues such as strength, expected operational life and dimensional stability, a continuous stress monitoring scheme is necessary for a complete integrity assessment. Consequently, this paper proposes a stress monitoring scheme for cylindrical specimens, which are widely used in structures such as pipelines, wind turbines or bridges. The approach consists of tracking guided wave variations due to load changes, by comparing wave statistical patterns via Principal Component Analysis (PCA). Each load scenario is projected to the PCA space by means of a baseline model and represented using the Q-statistical indices. Experimental validation of the proposed methodology is conducted on two specimens: (i) a 12.7 mm (1/2”) diameter, 0.4 m length, AISI 1020 steel rod, and (ii) a 25.4 mm (1”) diameter, 6m length, schedule 40, A-106, hollow cylinder. Specimen 1 was subjected to axial loads, meanwhile specimen 2 to flexion. In both cases, simultaneous longitudinal and flexural guided waves were generated via piezoelectric devices (PZTs) in a pitch-catch configuration. Experimental results show the feasibility of the approach and its potential use as in-situ continuous stress monitoring application., Peer Reviewed, Postprint (published version)