This thesis addresses the experimental and numerical study of vibration-based damage detection (VBDD) techniques in structural health monitoring (SHM) of bridge superstructures. The primary goal of SHM is to ascertain the condition or health of a structure so that decisions can be made with regard to the need for remediation. VBDD techniques are global non-destructive evaluation (NDE) techniques. The principle of VBDD techniques is to detect damage using changes in the dynamic characteristics of a structure caused by the damage. The advantage of VBDD techniques over local NDE techniques is that VBDD techniques can assess the condition of an entire structure at once and are not limited to accessible components. <p>Well controlled laboratory experiments on a half-scale, simply supported steel-free bridge deck and two full-scale, simply supported prestressed concrete girders demonstrated that small scale damage at different locations can be reliably detected and located by VBDD techniques using a relatively small number of sensors (accelerometers or strain gauges) and considering changes to only the fundamental mode of vibration. The resolution of damage localization, defined as the length of the window within which damage could be located when the technique predicts it to be located at a particular point, was 70% of measurement point spacing for the deck and 82% for the girders, provided the damage was not located too close to a simple support.<p>To establish the potential of VBDD techniques in the absence of experimental uncertainty, eigenvalue analyses using finite element models of the deck and the girders were undertaken to investigate ability of five VBDD methods to predict the longitudinal location of damage. It was found that when mode shapes were well-defined with a large number of measurement points, the damage location could be determined with great accuracy using any of the five VBDD techniques investigated. The resolution of longitudinal localization of damage was 40 to 80% of the spacing between measurement points when small numbers of measurement points were used, provided the damage was not located too close to a simple support.<p>The experimental study successfully detected small scale damage under forced resonant harmonic excitation but failed in detecting damage under forced random excitation, although the use of random sources of excitation is more practical in field testing. Transient dynamic analyses on the finite element model of the steel-free bridge deck were performed to investigate the implications of using random forced vibrations to characterize mode shapes to be used to detect damage. It was found that the probability of successful damage localization depends upon the severity of the damage, the number of trials used to obtain the average mode shape, the location of damage relative to the nearest sensor, the distance between the damage and the support, and the magnitude of measurement errors. A method based on the repeatability of measured mode shapes is proposed to calculate the probability of successful damage detection and localization.<p>In summary, results of this research demonstrate that VBDD techniques are a promising tool for structural health monitoring of bridge superstructures. However, although these methods have been shown to be capable of effectively detecting small scale damage under well controlled conditions, a significant amount of challenging work remains to be done before they can be applied to real structures.

<p>The objectives of the thesis work were to study theinteraction between embedded piezoceramic transducers andcomposite structures as well as determine techniques tosimplify the Lamb waves analysis. Firstly, this studyconsidered the design of the embedded piezoceramic transducers.Secondly, the effect of the embedded transducer on thecomposite strength as well as the influence of the mechanicallyloaded composite on the characteristics of the embeddedtransducer were investigated. Finally, to simplify the analysisof such complex Lamb wave responses, two techniques weredeveloped. They were based on the wavelet technique and amodelling technique, respectively.</p><p>The design of the embedded piezoceramic transducers wasimproved by reducing the stress concentrations in the compositeas well as in all components constituting the piezoceramictransducer, that is, the piezoceramic element, interconnectorand conductive adhesive. The numerical analysis showed that thethickness of the interconnector had no significant influence onthe stress state of the piezoceramic transducer. It was alsofound that a compliant conductive adhesive reduced the stressconcentration located at the edge of the piezoceramic element.The structural integrity of composites embedded with theimproved piezoceramic transducer was investigated. Theexperiments, performed in tensile and compressive staticloading, indicated that the strength of the composite was notsignificantly reduced by the embedded piezoceramic transducer.Further investigations were conducted to evaluate theperformance of the improved piezoceramic transducer used as aLamb wave generator embedded in composites subjected tomechanical loading. The tests were conducted in tensile andcompressive static loading as well as fatigue loading. Thestudy showed a large working range of the embedded piezoceramictransducer. A post processing technique based on the waveletswas further assessed in the detection of damage and in thedamage size evaluation. A new wavelet basis was developedspecially for processing the Lamb wave response. This method,focused on the wavelet coefficients from the decomposition Lambwave response, showed promising results in evaluating thedamage size. The wavelets offered a sensitive tool to detectsmall damage, compared to other detection methods, improvingthe damage detection capabilities. The other technique wasdevoted to the simplification of the generated Lamb waves bythe use of multi-element transducers. The transducers weredesigned using both a normal-mode expansion and a FE-method.This technique allowed reducing the effect of a Lamb wave modetowards another. This technique was successfully implemented ina damage detection system in composites.</p><p><b>Keywords:</b>Embedded piezoceramic, transducer, composite,structural integrity, health monitoring, damage detection, Lambwaves, wavelets, normal-mode expansion, FE-method</p>

A novel structural health assessment (SHA) technique is proposed. It is a finite element-based time domain nonlinear system identification technique. The procedure is developed in two stages to incorporate several desirable features and increase its implementation potential. First, a weighted global iteration with an objective function is introduced in the unscented Kalman filter (UKF) procedure in order to obtain stable, convergent, and optimal solution. Furthermore, it also improves the capability of the UKF procedure to identify a large structural system using only a short duration of responses measured at a limited number of dynamic degrees of freedom (DDOFs). The combined procedure is denoted as unscented Kalman filter with weighted global iteration (UKF-WGI). Then, UKF-WGI is integrated with iterative least-squares with unknown input (ILS-UI) in order to increase its implementation potential. The substructure concept is also incorporated in the procedure. The integrated procedure is denoted as unscented Kalman filter with unknown input and weighted global iteration (UKF-UI-WGI). The two most important features of the method are that it does not need information on input excitation and uses only limited number of noise-contaminated response information to identify structural systems. Also, the method is able to identify the defects at the local element level by tracking the changes in the stiffness of the structural elements in the finite element representation. The UKF-UI-WGI procedure is implemented in two stages. In Stage 1, based on the location of input excitation, the substructure is selected. Using only responses at all DDOFs in the substructure, ILS-UI can identify the input excitation time-histories, stiffness parameters of all the elements in the substructure, and two Rayleigh damping coefficients. The outcomes of the first stage are necessary to initiate UKF-WGI. Using the information from Stage 1, the stiffness parameters of all the elements in the structure are identified using UKF-WGI in Stage 2. To demonstrate the effectiveness of the procedure, health assessment of relatively large structural systems is presented. Small and relatively large defects are introduced at different locations in the structure and the capability of the method to detect the health of the structure is examined. The optimum number and location of measured responses are also investigated. It is demonstrated that the method is capable of identifying defect-free and defective states of the structures using minimum information. Furthermore, it can locate defect spot within a defective element accurately. The comparative studies are also conducted between the proposed methods and available methods in the literature. First, it is between the UKF-WGI and extended Kalman filter with weighted global iteration (EKF-WGI) procedure. Then, it is between UKF-UI-WGI and generalized iterative least-squares extended Kalman filter with unknown input (GILS-EKF-UI) procedure, developed earlier by the research team. It is demonstrated that the proposed UKF-based procedures are superior to the EKF-based procedures for SHA.

Σκοπός της παρούσας Πτυχιακής Εργασίας είναι ανάπτυξη μιας διαδικασίας παρασκευής νανοσύνθετων εποξικής ρητίνης/ πολλαπλών νανοσωληνίσκων άνθρακα (MWCNT) σε ένα εύρος περιεκτικοτήτων από 0.1 έως και 1 % κατά βάρος (κ.β) MWCNT. Τα εμποτισμένων δοκίμια έναντι αυτών της καθαρής ρητίνης παρουσίασαν ενισχυμένες μηχανικές ιδιότητες όπως αντοχή σε εφελκυσμό και αυξημένο μέτρο ελαστικότητας. Η αύξηση αυτή μπορεί να αποδοθεί στο υψηλό λόγο μήκους /διαμέτρου καθώς και στην μεγάλη ελεύθερη επιφάνεια των νανοσωληνίσκων (CNTs). Επίσης από τα πειράματα δυναμικής ανάλυσης παρατηρήθηκε αύξηση της θερμοκρασίας υαλώδους μετάβασης με την αύξηση της περιεκτικότητας των CNTs. Στα πλαίσια της ίδιας εργασίας μελετήθηκαν οι ηλεκτρικές ιδιότητες καθώς και οι αισθητήριες ιδιότητες των MWCNT και εξερευνήθηκε η χρήση τους ως νανοαισθητήρες για την παρακολούθηση βλάβης στην εμποτισμένη εποξική ρητίνη. Για τον σκοπό αυτό πραγματοποιήθηκαν πειράματα φόρτισης-αποφόρτισης μονοαξονικού εφελκυσμού με ταυτόχρονη παρακολούθηση της ηλεκτρικής αντίστασης του δοκιμίου. Από την παραπάνω διαδικασία παρατηρήθηκε πως όσο μεγαλύτερη είναι η περιεκτικότητα CNTs στην ρητίνη, τόσο μεγαλύτερη είναι η ευαισθησία της ηλεκτρικής αντίστασης στις αλλαγές του εφαρμοζόμενου φορτίου. Σημειώνεται επίσης πως η εμποτισμένη σε CNTs εποξική ρητίνη παρουσιάζει ηλεκτρική αγωγιμότητα παρουσιάζοντας σε περιεκτικότητα 1% κ.β συμπεριφορά αγωγού, γεγονός που οφείλεται στην αγώγιμη φύση των CNTs. Από τις μετρήσεις ηλεκτρικής αγωγιμότητας παρατηρήθηκε πως το «κατώφλι» αγωγιμότητας επιτυγχάνεται σε περιεκτικότητα 0.3% κ.β MWCNT ενώ, επιβεβαιώνεται η ισχύς της θεωρίας «διήθησης» : σ~ (V-Vc)t δίνοντας τιμή «κρίσιμου» εκθέτη t ίση με 2.05. / The goal of the present study is the development of a manufacturing process of epoxy resin compounds with several multi-wall carbon nanotube (MWCNT) contents per weight. Enhanced mechanical properties of the doped specimens epoxy against the neat epoxy testpieces e.g. tensile strength and modulus of elasticity was achieved and attributed to the high surface area and high aspect ratio of the nanotubes. Moreover the dynamic properties of the nano-doped epoxy polymers were investigated and the relation of glass transition temperature with increasing CNT content was found to be inverse. Another goal of the present work was to use the electrical/sensing properties of MWCNTs as a nano-sensor for the damage detection within the doped matrix material. Therefore loading-unloading tensile tests were performed, along with on-line conductivity monitoring for the nano-doped epoxy polymers. It was noted that all the nano-doped samples were more sensitive to load changes and thus resistance changes. The higher the CNT content per weight was, the higher the sensitivity in load changes. The conductive nature of CNTs has produced conductive epoxy polymers, which exhibit “percolation threshold” at the content of 0.3% wt. MWCNT and enhanced sensing properties. The measurements of electrical conductivity confirm the validity of “percolation” theory: σ~ (V-Vc)t with the critical exponent t equal to 2.05.

This thesis aimed at the development of a fibre optic strain sensor-based damage detection and evaluation system (FODDAS) based on the composite beams. EFPI strain sensors were used with their integrity being assessed. Their performance, either bonded on the surfaces or embedded was examined extensively. They were shown to be adequate and reliable for strain measurements. Through-the-width damages were simulated by artificially-embedded delaminations, which were located at several through-the-thickness locations, each with two different sizes. The overall design considerations were guided by ply stresses and strains which were estimated by using the modified classical lamination theory (CLT). Considerable efforts were devoted to assessing the through-the-thickness mechanical behaviours of the beams containing optical fibres in three-point bending and short beam shear (SBS). They involved various optical fibre orientations with respect to 00 plies / longitudinal axis and at various through-the-thickness locations, each with different number of optical fibres. The understanding of these behaviours paved the way for the evaluation of the beam-based FODDAS. Smart preconditioned beams were subjected to the quasi -static loads whose magnitudes and locations were required to be well controlled. The viability and effectiveness of the beam-based FODDAS was evaluated in terms of strength and strain obtained by the embedded sensor as well as the surface-bonded strain gauges via the cross comparison of ten cases. For the strength, each beam was incrementally loaded up to the ultimate failure either in three-point bending or SBS. After each increment, the beam was unloaded and inspected for damage. For the given locations of EFPI-SS and artificial delamination as well as the sizes of the latter, it was found that the embedded EFPI-SSs were capable of picking up the stiffness degradation when the 10- mm as well as the 20-mm delamination was located at the 29-30 ply interface in the tensile region of a 32-ply quasi-isotropic carbon/epoxy smart composite beam. It was speculated from single tests results that the propagation of the embedded delamination of the sufficient size was able not only to be detected but also to be monitored by the sensors.

abstract: Current economic conditions necessitate the extension of service lives for a variety of aerospace systems. As a result, there is an increased need for structural health management (SHM) systems to increase safety, extend life, reduce maintenance costs, and minimize downtime, lowering life cycle costs for these aging systems. The implementation of such a system requires a collaborative research effort in a variety of areas such as novel sensing techniques, robust algorithms for damage interrogation, high fidelity probabilistic progressive damage models, and hybrid residual life estimation models. This dissertation focuses on the sensing and damage estimation aspects of this multidisciplinary topic for application in metallic and composite material systems. The primary means of interrogating a structure in this work is through the use of Lamb wave propagation which works well for the thin structures used in aerospace applications. Piezoelectric transducers (PZTs) were selected for this application since they can be used as both sensors and actuators of guided waves. Placement of these transducers is an important issue in wave based approaches as Lamb waves are sensitive to changes in material properties, geometry, and boundary conditions which may obscure the presence of damage if they are not taken into account during sensor placement. The placement scheme proposed in this dissertation arranges piezoelectric transducers in a pitch-catch mode so the entire structure can be covered using a minimum number of sensors. The stress distribution of the structure is also considered so PZTs are placed in regions where they do not fail before the host structure. In order to process the data from these transducers, advanced signal processing techniques are employed to detect the presence of damage in complex structures. To provide a better estimate of the damage for accurate life estimation, machine learning techniques are used to classify the type of damage in the structure. A data structure analysis approach is used to reduce the amount of data collected and increase computational efficiency. In the case of low velocity impact damage, fiber Bragg grating (FBG) sensors were used with a nonlinear regression tool to reconstruct the loading at the impact site. / Dissertation/Thesis / Ph.D. Aerospace Engineering 2011

abstract: Composite materials are increasingly being used in aircraft, automobiles, and other applications due to their high strength to weight and stiffness to weight ratios. However, the presence of damage, such as delamination or matrix cracks, can significantly compromise the performance of these materials and result in premature failure. Structural components are often manually inspected to detect the presence of damage. This technique, known as schedule based maintenance, however, is expensive, time-consuming, and often limited to easily accessible structural elements. Therefore, there is an increased demand for robust and efficient Structural Health Monitoring (SHM) techniques that can be used for Condition Based Monitoring, which is the method in which structural components are inspected based upon damage metrics as opposed to flight hours. SHM relies on in situ frameworks for detecting early signs of damage in exposed and unexposed structural elements, offering not only reduced number of schedule based inspections, but also providing better useful life estimates. SHM frameworks require the development of different sensing technologies, algorithms, and procedures to detect, localize, quantify, characterize, as well as assess overall damage in aerospace structures so that strong estimations in the remaining useful life can be determined. The use of piezoelectric transducers along with guided Lamb waves is a method that has received considerable attention due to the weight, cost, and function of the systems based on these elements. The research in this thesis investigates the ability of Lamb waves to detect damage in feature dense anisotropic composite panels. Most current research negates the effects of experimental variability by performing tests on structurally simple isotropic plates that are used as a baseline and damaged specimen. However, in actual applications, variability cannot be negated, and therefore there is a need to research the effects of complex sample geometries, environmental operating conditions, and the effects of variability in material properties. This research is based on experiments conducted on a single blade-stiffened anisotropic composite panel that localizes delamination damage caused by impact. The overall goal was to utilize a correlative approach that used only the damage feature produced by the delamination as the damage index. This approach was adopted because it offered a simplistic way to determine the existence and location of damage without having to conduct a more complex wave propagation analysis or having to take into account the geometric complexities of the test specimen. Results showed that even in a complex structure, if the damage feature can be extracted and measured, then an appropriate damage index can be associated to it and the location of the damage can be inferred using a dense sensor array. The second experiment presented in this research studies the effects of temperature on damage detection when using one test specimen for a benchmark data set and another for damage data collection. This expands the previous experiment into exploring not only the effects of variable temperature, but also the effects of high experimental variability. Results from this work show that the damage feature in the data is not only extractable at higher temperatures, but that the data from one panel at one temperature can be directly compared to another panel at another temperature for baseline comparison due to linearity of the collected data. / Dissertation/Thesis / M.S. Aerospace Engineering 2012

Detecção de dano em estruturas de engenharia de grandes dimensões através da análise de suas características dinâmicas envolve diversos campos de estudo. O primeiro deles trata da identificação dos parâmetros modais da estrutura, uma vez que executar testes de vibração livre em tais estruturas não é uma tarefa simples, necessita-se de um método robusto que seja capaz de identificar os parâmetros modais dessa estrutura a ações ambientais, campo esse chamado de análise modal operacional. Este trabalho trata do problema de detecção de dano em estruturas que possam ser representadas através de modelos em pórticos planos e vigas e que estejam submetidos à ação de vibrações ambientais. A localização do dano é determinada através de um algoritmo de otimização conhecido como Backtracking Search Algorithm (BSA) fazendo uso de uma função objetivo que utiliza as frequências naturais e modos de vibração identificados da estrutura. Simulações e testes são feitos a fim de verificar a concordância da metodologia para ambos os casos. Para as simulações, são utilizados casos mais gerais de carregamentos dinâmicos, e dois níveis de ruído (3% e 5%) são adicionados ao sinal de respostas para que esses ensaios se assemelhem aos ensaios experimentais, onde o ruído é inerente do processo. Já nos ensaios experimentais, apenas testes de vibração livre são executados. Diversos cenários de dano são propostos para as estruturas analisadas a fim de se verificar a robustez da rotina de detecção de dano. Os resultados mostram que a etapa de identificação modal estocástica através do método de identificação estocástica de subespaço (SSI) teve ótimos resultados, possibilitando, assim, a localização da região danificada da estrutura em todos os casos analisados. / Damage detection in large dimensions engineering structures through the analysis of their dynamic characteristics involves several fields. The first one deals with the structure modal identification parameter, since running free vibration tests in such structures is not a simple task, robust methods are needed in order to identify the modal parameters of this structure under ambient vibrations, this field is known as operational modal analysis. This work deals with the problem of damage detection in structures under ambient vibrations that can be represented by FEM using frame and beam elements. The damage location is determined through an optimization algorithm know as Backtracking Search Algorithm (BSA). It uses as objective function the identified natural frequencies and modes of vibration of the structure. Numerical and experimental tests are performed to assess the agreement of the methodology for both cases. For the numerical tests, more general cases of dynamic loads are used, and two noise levels (3% and 5%) are added to the response signal to assessing the robustness of the methodology close to the field conditions, in which noise is inherent of the process. In the experimental tests, only free vibration tests are performed. Several damage scenarios are proposed for the analyzed structures to check the robustness of the damage detection routine. The results show that the stochastic modal identification using the stochastic subspace identification (SSI) method had excellent results, thus allowing the location of the damaged region of the structure in all analyzed cases.

Detecção de dano em estruturas de engenharia de grandes dimensões através da análise de suas características dinâmicas envolve diversos campos de estudo. O primeiro deles trata da identificação dos parâmetros modais da estrutura, uma vez que executar testes de vibração livre em tais estruturas não é uma tarefa simples, necessita-se de um método robusto que seja capaz de identificar os parâmetros modais dessa estrutura a ações ambientais, campo esse chamado de análise modal operacional. Este trabalho trata do problema de detecção de dano em estruturas que possam ser representadas através de modelos em pórticos planos e vigas e que estejam submetidos à ação de vibrações ambientais. A localização do dano é determinada através de um algoritmo de otimização conhecido como Backtracking Search Algorithm (BSA) fazendo uso de uma função objetivo que utiliza as frequências naturais e modos de vibração identificados da estrutura. Simulações e testes são feitos a fim de verificar a concordância da metodologia para ambos os casos. Para as simulações, são utilizados casos mais gerais de carregamentos dinâmicos, e dois níveis de ruído (3% e 5%) são adicionados ao sinal de respostas para que esses ensaios se assemelhem aos ensaios experimentais, onde o ruído é inerente do processo. Já nos ensaios experimentais, apenas testes de vibração livre são executados. Diversos cenários de dano são propostos para as estruturas analisadas a fim de se verificar a robustez da rotina de detecção de dano. Os resultados mostram que a etapa de identificação modal estocástica através do método de identificação estocástica de subespaço (SSI) teve ótimos resultados, possibilitando, assim, a localização da região danificada da estrutura em todos os casos analisados. / Damage detection in large dimensions engineering structures through the analysis of their dynamic characteristics involves several fields. The first one deals with the structure modal identification parameter, since running free vibration tests in such structures is not a simple task, robust methods are needed in order to identify the modal parameters of this structure under ambient vibrations, this field is known as operational modal analysis. This work deals with the problem of damage detection in structures under ambient vibrations that can be represented by FEM using frame and beam elements. The damage location is determined through an optimization algorithm know as Backtracking Search Algorithm (BSA). It uses as objective function the identified natural frequencies and modes of vibration of the structure. Numerical and experimental tests are performed to assess the agreement of the methodology for both cases. For the numerical tests, more general cases of dynamic loads are used, and two noise levels (3% and 5%) are added to the response signal to assessing the robustness of the methodology close to the field conditions, in which noise is inherent of the process. In the experimental tests, only free vibration tests are performed. Several damage scenarios are proposed for the analyzed structures to check the robustness of the damage detection routine. The results show that the stochastic modal identification using the stochastic subspace identification (SSI) method had excellent results, thus allowing the location of the damaged region of the structure in all analyzed cases.

A avaliação do estado das estruturas é um tema de pesquisa muito importante para diversos campos da engenharia e, por isso, estão sendo desenvolvidas metodologias que permitem detectar dano em uma estrutura. O presente trabalho tem como objetivo verificar a aplicabilidade dos algoritmos genéticos (AG) na detecção de dano a partir das mudanças ocorridas, entre as condições com e sem dano, dos parâmetros dinâmicos da estrutura. Três tipos de AGs (binário, real e redundante implícita) são implementados com a finalidade de comparação do desempenho. Os parâmetros dinâmicos da estrutura, sadia e danificada, são determinados a partir do modelo de elementos finitos da estrutura. Medições incompletas e ruidosas foram consideradas visando simular as características da informação obtida por meio de um ensaio dinâmico real. Os AGs implementados são aplicados em estruturas de tipo viga, treliça e pórtico sob diferentes cenários de dano. Resultados mostram o bom desempenho dos AGs para detectar dano em uma estrutura. / The assessment of structural health is an important research topic in many engineering fields and, for that reason, damage detection methodologies are being developed. The goal of this dissertation is to verify the applicability of genetic algorithms (GAs) for detecting damage using dynamic parameters changes between undamaged and damaged condition of the structure. Three different GAs are implemented in order to compare the performance of the algorithms. Undamaged and damaged dynamic parameters are computed using the finite element model of the structure. Incomplete and noisy measurements are considered with the objective of simulating the real condition of the information in a real dynamic test. GAs are applied in some different structures: beam, truss and frame. The results indicate the good performance of the GAs for detecting damage in a structure.