Energy harvesting is a promising and evolving field of research capable of supplying power to systems in a broad range of applications. In particular, the ability to gather energy directly from the environment without human intervention makes energy harvesting an excellent option for powering autonomous sensors in remote or hazardous locations. This dissertation examines the possibility of using energy harvesting in new and innovative ways to power wireless sensor nodes placed in the substructures of highway bridges for structural health monitoring. Estimates for power requirements are established, using a wireless sensor node from National Instruments as an example system. Available power in a bridge environment is calculated for different energy sources, including solar radiation, wind, and vibration from traffic. Feasibility of using energy harvesting in such an application is addressed for both power availability and cost as compared with grid power or primary batteries. An in-depth functional analysis of existing energy-harvesting systems is also presented, with insights into where innovation would be most beneficial in future systems. Finally, the development of a suite of complementary energy-harvesting devices is described. Because conditions on bridges may vary, multiple solutions involving different energy domains are desired, with the end user able to select the harvester most appropriate for the specific installation. Concept generation techniques such as mind-mapping and 6-3-5 (C-Sketch) are used to produce a wide variety of concepts, from which several promising concept variants are selected. The continued development for one concept, which harvests vibration using piezoelectric materials, is described. Analytical modeling is presented for static and dynamic loading, as well as predicted power generation. Two proof-of-concept prototypes are built and tested in laboratory conditions. Through the development of this prototype, it is shown that the example wireless sensor node can successfully be powered through energy harvesting, and insights are shared concerning the situations where this and other energy harvesters would be most appropriate. / text

Η συνεχώς αυξανόμενη χρήση σύνθετων υλικών στην αεροδιαστημική βιομηχανία στις κρίσιμες δομικές εφαρμογές έχει οδηγήσει στην ανάγκη ανάπτυξης μη καταστροφικών μεθόδων βλάβης. Οι έως τώρα χρησιμοποιούμενες μέθοδοι για την παρακολούθησης βλάβης όπως είναι η ενσωμάτωση πιεζο-κεραμικών αισθητήρων και οπτικών ινών σε σύνθετες κατασκευές έχουν ως αποτέλεσμα την συγκέντρωση υψηλών τάσεων και ροών στην κατασκευή. Η παρούσα διατριβή αποτελεί μια πρωτότυπη προσπάθεια στη ευρύτερη επιστημονική και τεχνολογική κατεύθυνση της ενσωμάτωσης της νάνο-τεχνολογίας στο τομέα σχεδιασμού και κατασκευής νέων συνθέτων υλικών. Πρωταρχικός στόχος είναι η επίτευξη αυξημένων δυνατοτήτων για αποτελεσματική παρακολούθηση της αναπτυσσόμενης βλάβης στο υλικό και κατά προέκταση ελέγχου της δομικής ακεραιότητας των κατασκευών τους. Στο πλαίσιο αυτής της προσπάθειας προτείνεται η καινοτόμα ιδέα της αξιοποίησης των νάνο-σωληνίσκων άνθρακα ως εμποτισμένους αισθητήρες βλάβης μέσα σε συμβατικά ινώδη σύνθετα υλικά άνθρακα με απώτερο στόχο τον καλύτερο μη καταστροφικό έλεγχο της βλάβης με τη μέτρηση της ηλεκτρικής αντίστασης. Οι εξαιρετικά αγώγιμοι ΝΣΑ όταν εμποτιστούν σε πολυμερή (κύρια υλικά μήτρας συνθέτων υλικών) δημιουργούν ένα ηλεκτρικό δίκτυο (percolation network) το οποίο παρέχει την δυνατότητα αγωγής ηλεκτρικού φορτίου αυξάνοντας πολλές τάξεις μεγέθους την αγωγιμότητα του πολυμερούς. Η παρουσία του προαναφερθέντος ηλεκτρικού δικτύου ταυτόχρονα με αυτό των αγώγιμων ινών άνθρακα σε σύνθετα υλικά όπου η μήτρα έχει εμποτιστεί με ΝΣΑ αναμένεται να αυξήσει άμεσα ή έμμεσα τη διακριτική ικανότητα ανίχνευσης της βλάβης μέσω της μέτρησης της ηλεκτρικής αντίστασης. Αρχικά έγινε η επιλογή των υλικών και της διαδικασίας παραγωγής με βάση την υπάρχουσα βιβλιογραφία. Μια τυποποιημένη ρητίνη αεροδιαστημικών εφαρμογών επιλέχθηκε ως υλικό εποξικής μήτρας και χρησιμοποιήθηκε σε όλη τη διάρκεια της εργασίας, όπως αντίστοιχα ο τύπος και οι προδιαγραφές των ανθρακονημάτων που αποτέλεσαν την ινώδη ενίσχυση άνθρακα. Επίσης από την υπάρχουσα βιβλιογραφία διερευνήθηκαν οι διαθέσιμοι τύποι και είδη νάνο-σωληνίσκων άνθρακα (ΝΣΑ) και έγινε η επιλογή τους βάση των ηλεκτρικών ιδιοτήτων τους. Η ερευνητική μεθοδολογία που ακολουθήθηκε στην συνέχεια για την επίτευξη του παραπάνω στόχου μπορεί να χωριστεί σε δύο φάσεις. Στην πρώτη φάση έλαβε χώρα η διερεύνηση και ο προσδιορισμός των καταλληλότερων διαδικασιών/ μεθοδολογιών για ενσωμάτωση των ΝΣΑ στην εποξική μήτρα, που αποτελεί σημαντικό παράγοντα της ποιότητας της διασποράς και ως εκ τούτου της επίτευξης βέλτιστης ηλεκτρικής αγωγιμότητας. Στο στάδιο αυτό, αναζητήθηκαν στη βιβλιογραφία όλες οι υπάρχουσες τεχνικές ανάμιξης και αναγνωρίστηκαν οι βασικές παράμετροι που επηρεάζουν τη διαδικασία. Πιο συγκεκριμένα το ιξώδες του συστήματος (ρητίνη+ΝΣΑ), ο χρόνος και η θερμοκρασία της ανάδευσης και η χημική τροποποίηση των ΝΣΑ αποτέλεσαν ορισμένες από τις παραμέτρους που εξετάστηκαν. Η αξιολόγηση της ποιότητας των μίξεων έγινε με κριτήριο την επίτευξη βέλτιστης ηλεκτρικής αγωγιμότητας μέσω μετρήσεων AC και DC σε συνδυασμό με την χρήση SEM. Μία από τις βέλτιστες τεχνικές ανάδευσης επιλέχθηκε στη συνέχεια για τη μελέτη της επίδρασης της περιεκτικότητας σε ΝΣΑ στην ηλεκτρική αγωγιμότητα της εποξικής ρητίνης σε ένα εύρος περιεκτικοτήτων από 0-1% κ.β. Μέσω αυτής της μεθόδου προσδιορίστηκαν το «κατώφλι διήθησης» ή αλλιώς η κρίσιμη περιεκτικότητα καθώς και η βέλτιστη περιεκτικότητα των ΝΣΑ σε εποξική ρητίνη. Η πρώτη φάση ολοκληρώθηκε με τον ηλεκτρομηχανικό χαρακτηρισμό των νάνο-ενισχυμένων με ΝΣΑ πολυμερών μέσω της μέτρησης της μεταβολής της ηλεκτρικής αντίστασης κατά την παραμόρφωσή τους υπό (α) την επίδραση μόνο-αξονικού ψευδό-στατικού εφελκυστικού φορτίου και (β) την υποβολή εναλλασσόμενου εφελκυστικού φορτίου ως Κυκλική Φόρτιση Αποφόρτιση-Επαναφόρτιση. Η πειραματική μακροσκοπική συμπεριφορά κατέδειξε ενδιαφέρουσα συμπεριφορά που δεν απαντάται σε προγενέστερα αγώγιμα πολυμερή (πχ. Carbon Black, Carbon -Nano-Fibers κτλ.). Στην δεύτερη φάση η έρευνα επικεντρώθηκε στη παρασκευή και στον ηλεκτρομηχανικό χαρακτηρισμό των ινωδών συνθέτων υλικών, όπου η μήτρα είναι η προαναφερθείσα τροποποιημένη με ΝΣΑ. Πρώτο βήμα αποτέλεσε η μελέτη της επίδρασης της παρουσίας και ειδικότερα της περιεκτικότητας των ΝΣΑ στην εποξική μήτρα στην ανισότροπη ηλεκτρική αγωγιμότητα τέτοιων υλικών. Αρχικά επιλέχθηκε ένας ισχυρά ανισότροπος τύπος, αυτός του πολύστρωτου συνθέτου μονής διεύθυνσης (unidirectional CFRP) [0]16. Στο πλαίσιο αναπτύχθηκαν με χρήση της με CFRPs [0]16 με μήτρα τριών διαφορετικών περιεκτικοτήτων σε ΝΣΑ 0% (υλικό αναφοράς), 0.5% και 1% κ.β. Τα δοκίμια που προέκυψαν υποβλήθηκαν σε ηλεκτρομηχανικό χαρακτηρισμό σε (α) μονο-αξονικό ψευδό-στατικό εφελκυσμό και σε (β) Κυκλική Εφελκυστική Φόρτιση-Αποφόρτιση-Επαναφόρτιση. Τα αποτελέσματα επιβεβαίωσαν τις αρχικές εκτιμήσεις και η παρουσία των ΝΣΑ αυξάνει την ευαισθησία της ηλεκτρικής απόκρισης με την εφαρμοζόμενη φόρτιση. Στη συνέχεια, έχοντας αποδείξει ότι η παρουσία των ΝΣΑ έχει θετική επίδραση στην ηλεκτρική παρακολούθηση της βλάβης ακολούθησε η μελέτη της ηλεκτρομηχανικής απόκρισης σε συνθήκες κόπωσης, που αντιπροσωπεύει και τον πιο διαδεδομένο τύπο φόρτισης στις κατασκευές από σύνθετα υλικά. Επίσης για τα ίδια δοκίμια μετρήσεις ηλεκτρικής αγωγιμότητας AC και DC έδειξαν πως η παρουσία των ΝΣΑ στην μήτρα μειώνει την ανισοτροπία του υλικού ως προς αυτή την ιδιότητα. Συνέχεια της παραπάνω έρευνας αποτέλεσε η ανάπτυξη ψευδό- ισότροπων ινωδών σύνθετων υλικών CFRPs [0,+45,90,45]nS (τυπική σύνθετη δομή αεροπορικών κατασκευών) μήτρας εποξικής ρητίνης και εμποτισμένης εποξικής ρητίνης με ΝΣΑ περιεκτικότητας 0.5% κ.β. στην παρούσα φάση εκτός από τα προαναφερθέντα πειράματα κατά κύριο λόγω διερευνήθηκε η ηλεκτρομηχανική απόκριση του υλικού σε κόπωση. Η συχνότητα της κόπωσης παρέμεινε σταθερή, όπως και ο λόγος της ελάχιστης εφαρμοζόμενης τάσης προς την μέγιστη (λόγος R), ενώ εξετάστηκαν τρία (3) διαφορετικά επίπεδα φόρτισης (Stress levels) με σκοπό την μελέτη πιθανή εξάρτησης της ηλεκτρομηχανικής απόκρισης. Παράλληλα καταγράφηκε και η θερμοκρασία του υλικού για την αφαίρεση τυχόν επιδράσεων της στη απόκριση της ηλεκτρικής αντίστασης. Ταυτόχρονα, χρησιμοποιήθηκε σύστημα ακουστικής εκπομπής με απώτερο στόχο την συσχέτιση των μετρήσεων των 2 τεχνικών ΜΚΕ. Η ανάλυση των πειραματικών αποτελεσμάτων έδειξε ότι η συμπεριφορά των ψευδους ισότροπων συνθέτων υλικών σε σχέση αυτής των μονής διεύθυνσης και αυτής των ορθότροπων υλικών εμπεριέχει πιο σύνθετη απόκριση λόγω της ύπαρξης των [45ο] στρώσεων. Παράλληλα, η ηλεκτρική απόκριση τους συσχετίστηκε με τις μετρήσεις της υποβάθμισης του μέτρου ελαστικότητας αλλά και τις μετρήσεις της ακουστικής εκπομπής επιβεβαιώνοντας «ηλεκτρικά» χαρακτηριστικές περιοχές βλάβης. Βάση αυτών των παρατηρήσεων έγινε μια απόπειρα μελέτης της δυνατότητας πρόγνωσης της τελικής αστοχίας μέσω της μετρούμενης ηλεκτρομηχανικής απόκρισης με πολύ ενθαρρυντικά αποτελέσματα. Προς σε αυτή τη κατεύθυνση, η παρουσία των ΝΣΑ έδειξε μια πιο ομαλή ηλεκτρομηχανική συμπεριφορά που βελτιστοποιεί την αξιοπιστία της πρόβλεψης, παρατήρηση που αποτελεί και την ολοκλήρωση του κύριου όγκου έρευνας της παρούσας διδακτορικής εργασίας. / The increasing usage of composite materials in the aerospace industry in critical structural applications has proved the need for health monitoring of those structures. In the past piezo-ceramic particles and optic fibres have been integrated into composite structures so as to serve as health monitoring sensors but they introduced flaws and high stress concentration to the structures. In the current PhD thesis a different approach was used; to use the material itself as an inherent structural monitoring sensor. In order to achieve that the matrix material of the epoxy reinforced composite has to be doped with Carbon Nanotubes (CNTs). Carbon nanotubes are reported to have excellent mechanical, thermal and electrical properties. The CNTs have to be dispersed into the epoxy matrix and above certain per weight percentage into the matrix where they form a conductive percolating network. By monitoring the changes of the electrical resistivity of this network is possible to track any load, strain or even damage stages of the material. In other words a step towards a multifunctional material is made since an enhancement also in the mechanical properties is expected from the presence of the CNTs. At the first chapters of the PhD an extensive evaluation of the dispersion method takes place. Different times and speeds of the high-shear mechanical device that was used for the dispersion of the Multi-Wall CNTs (MWCNTs) into the epoxy polymer are evaluated via electrical measurements (both DC and AC) along with SEM investigations. The next step involved the investigation on the % p.wt of the CNTs into the polymer matrix ranging from 0-1%. With this method both the optimum CNT concentration and the percolation network were established. Beside the aforementioned tests, tensile tests with on line Electrical Resistivity Changes Measurement (ERCM) along the loading axis, took place using a digital multimeter. The tensile tests were both quasi static and loading-unloading. It was proved that the load-strain variation can be tracked for the nano-doped polymers. The percolation threshold for the given system was calculated for both AC and DC to be above 0.3% p.wt and that the optimum CNT content was established at 0.5% while at 0.1% the CNTs tended to behave as flaws due to the large number of agglomerations. The next chapter describes the work of using the conclusion of the previous chapter to manufacture unidirectional CFRPs with aerospace epoxy matrix with 0,0.1,0.5 and 1% p.wt CNT. The CFRP were manufactured by wet lay-up and autoclave methods. The specimens were subjected to both quasi static and loading-unloading tensile test with on-line ERCM. The nano-enhanced composites proved to have inherent sensing abilities and as the CNT content increased the more sensitive the load monitoring was. Moreover, for the loading-unloading tests (5 cycles) it was noted that the nano-composites were able to track the initial damage of the coupons by irreversible ERCM changes. The AC and DC conductivity was also measured and it was proved that the presence of the CNTs in the matrix reduce the electrical anisotropy of the composite. The 0.5 % p.wt CNTs into the epoxy matrix proved to be the optimised content for FRP manufacturing since not only proved to give very good all overall results but also it is easier to manufacture when compared with 1% (high viscosity). Further in the research two quasi-isotropic CFRPs were manufactured as above with 0 and 0.5% CNTs. The quasi lay-up was chosen so as to be representative of a typical aerospace structural component. Besides the aforementioned tensile tests with ERCM, the main investigation took place on tension-tension fatigue tests with ERCM. In this case due to the nature of the samples (many fiber contacts) the differences in the ERC measurements were not as obvious as before, but by further mathematically-statistically analyzing the results a model of estimation of fatigue life life is presented. The principal of health monitoring of composite structures enhanced with CNTs, was also proved for cross-ply Kevlar FRPs (KFRPs) with 1% p.wt in the matrix material and with the same amount of CNTs in gramms in as received form spread by hand at the mid-plane. Finally one can highlight the results of the current PhD thesis as follows: the principle of manufacturing and testing a multi-functional composite material with the use of CNTs has been proved. The dispersion of the nano-fillers took place using a high shear mixing device and the parameters were optimised. The percolation threshold was calculated above 0.3% p.wt and wt and that the optimum CNT content was established at 0.5% while at 0.1% the CNTs tended to behave as flaws due to the large number of agglomerations. The nano-reinforced polymers became semi-conductors and showed the ability to track load-starin variation via ERCM. The nano-reinforced polymer was used as matrix materials for CFRPs. The optimum CNT contents was established at 0.5% p.wt. The presence of CNTs not olny improved the mechanical properties but also provided to the materials sensing abilities via ERCM on several load cases i.e.tensile and fatigue. The proof of concept was also demonstrated using KFRPs.

I present RAACD, a software suite that detects misbehaving computers in large computing systems and presents information about those machines to the system administrator. I build this system using preexisting anomaly detection techniques. I evaluate my methods using simple synthesized data, real data containing coerced abnormal behavior, and real data containing naturally occurring abnormal behavior. I find that the system adequately detects abnormal behavior and significantly reduces the amount of uninteresting computer health data presented to a system administrator.

Unscheduled aircraft maintenance causes a lot problems and costs for aircraft operators. This is due to the fact that aircraft cause significant costs if flights have to be delayed or canceled and because spares are not always available at any place and sometimes have to be shipped across the world. Reducing the number of unscheduled maintenance is thus a great costs factor for aircraft operators. This thesis describes three methods for aircraft health monitoring and prediction; one method for system monitoring, one method for forecasting of time series and one method that combines the two other methods for one complete monitoring and prediction process. Together the three methods allow the forecasting of possible failures. The two base methods use decision trees for decision making in the processes and genetic optimization to improve the performance of the decision trees and to reduce the need for human interaction. Decision trees have the advantage that the generated code can be fast and easily processed, they can be altered by human experts without much work and they are readable by humans. The human readability and modification of the results is especially important to include special knowledge and to remove errors, which the automated code generation produced.

In this thesis, miniaturized wavelength interrogators based on planar lightwave circuits (PLCs) are investigated and developed for the optical fiber sensing applications in the aircraft structural health monitoring (SHM) and optofluidic analysis. Two interrogation systems based on an arrayed waveguide grating (AWG) and an Echelle diffractive grating (EDG) are developed and used to convert the optical sensing signals into strain, temperature, vibration, damage, and humidity information for the aircraft SHM. A fiber Bragg grating (FBG) sensing system using developed interrogators is then demonstrated in a field test for aircraft SHM applications. For optofluidic analysis, a PLCs based optofluidic device consisting of two on-chip lens sets is built to enhance the optical manipulation capability of particles. Then, a solution to a multi-functional Lab-on-a-Chip platform for optofluidic analysis is proposed, which integrates the developed particle maneuvering device, grating-structured sensors, and miniaturized interrogators.

[pt] O presente trabalho apresenta uma modelagem da geração de ondas guiadas em estruturas laminadas compósitas através de atuadores piezoelétricos interdigitais.O estudo foi desenvolvido a partir de uma técnica que se baseia na teoria discreta de Reddy e modela os campos de deslocamento e esforços e velocidades generalizadas nas interfaces de uma estrutura laminada. O modelo desenvolvido representa trechos da estrutura em questão através de suas matrizes de impedância, relacionando os esforços e velocidades generalizadas nas interfaces de cada trecho. Após a validação do modelo através da comparação com o método dos elementos finitos, via um pacote comercial, onde foi mostrado que os campos calculados pelos dois métodos obtiveram resultados bastante aproximados nas diferentes faixas de freqüência, foi demonstrada a capacidade do modelo proposto de representar atuadores piezoelétricos interdigitais gerando modos guiados desacoplados em vigas laminadas compósitas. Foram modeladas vigas de alumínio e de ARALL e, após o cálculo das suas curvas de dispersão e dos pontos em que os seis primeiros modos são aproximadamentenão dispersivos, projetaram-se atuadores capazes de gerar de forma desacoplada alguns dos modos guiados nas vigas. A técnica proposta, que pode ser facilmente adaptada pa4ra a simulação da excitação de ondas de Lamb em placas, é capaz de modelar com precisão a rseposta em altas freqüências e pode servir como uma ferramenta valiosa para o desenvolvimento de atuadores interdigitais na monitoração em tempo real da integridade estrutural de compósitos laminados. / [en] The present work is concerned with modeling interdigital piezoeletric actuators for the excitation of guide waves in composite laminated beams. The proposed model is based on Reddys layerwise laminate theory, which relies on piece- wise constant interpolations of the displacement and eletric potential distributions along thethickness of a laminated structure. The laminated beam is also divided in pieces along its span direction, and each piece is represented by an impedance matrix that associates generalized forces with particle velocities on the interfaces of each piece, and also with the applied voltage along the electrodes of the piezoeletric element. Validation of the model was performed with the aid of a commercial Finite Element code. The model was explored to simulate the excitation of uncoupled guided wave modes incomposite laminated beams fabricated from aluminum or ARALL, which is a laminate made ofalternating layers of aluminum and arami-epoxy fiber reinforced composite. From the frequency spectrum of guided waves in the laminated beam, frequencies amd wavelengths at which the first six modes are approximately non-dispersive where determined. With this information, interdigital actuators, capable of generating uncoupled, or weakly coupled, guided modes in the beam were simulated. The proposed model, which is able to accurately reproduce responses in the high frequency/short wavelength range, could be easily adapted to simulate the extation of Lamb waves in laminated plates, and may be a valuable tool in the development of interdigital actuators for health monitoring of laminated composites.

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.

Damage identification is an important topic in structural assessment and structural health monitoring (SHM). Vibration-based identification techniques use modal data to identify the existence, location and severity of possible damages in structures, often via a numerical model updating procedure. Among other factors influencing the practicality and reliability of a damage identification approach, two are of primary interest to this study. The first one concerns the amount and quality of modal data that can be used as ‘response’ data for the model updating. It is generally recognised that natural frequencies can be measured with relatively high accuracy; however, their number is limited. Mode shapes, on the other hand, are susceptible to larger measurement errors. Seeking additional modal frequency data is therefore of significant value. The second one concerns the errors at the numerical (finite element) model level, particularly in the representation of the effect of damage on the dynamic properties of the structure. An inadequate damage model can lead to inaccurate and even false damage identification. The first part of the thesis is devoted to enhancing the modal dataset by extracting the so called ‘artificial boundary condition’ (ABC) frequencies in a real measurement environment. The ABC frequencies correspond to the natural frequencies of the structure with a perturbed boundary condition, but can be generated without the need of actually altering the physical support condition. A comprehensive experimental study on the extraction of such frequencies has been conducted. The test specimens included steel beams of relatively flexible nature, as well as thick and stiffer beams made from metal material and reinforced concrete, to cover the typical variation of the dynamic characteristics of real-life structures in a laboratory condition. The extracted ABC frequencies are subsequently applied in the damage identification in beams. Results demonstrate that it is possible to extract the first few ABC frequencies from the modal testing in different beam settings for a variety of ABC incorporating one or two virtual pin supports. The inclusion of ABC frequencies enables the identification of structural damages satisfactorily without the necessity to involve the mode shape information. The second part of the thesis is devoted to developing a robust model updating and damage identification approach for beam cracks, with a special focus on thick beams which present a more challenging problem in terms of the effect of a crack than slender beams. The priority task has been to establish a crack model which comprehensively describes the effect of a crack to reduce the modelling errors. A cracked Timoshenko beam element model is introduced for explicit beam crack identification. The cracked beam element model is formulated by incorporating an additional flexibility due to a crack using the fracture mechanics principles. Complex effects in cracked thick beams, including shear deformation and coupling between transverse and longitudinal vibrations, are represented in the model. The accuracy of the cracked beam element model for predicting modal data of cracked thick beams is first verified against numerically simulated examples. The consistency of predictions across different modes is examined in comparison with the conventional stiffness reduction approach. Upon satisfactory verification, a tailored model updating procedure incorporating an adaptive discretisation approach is developed for the implementation of the cracked beam element model for crack identification. The updating procedure is robust in that it has no restriction on the location, severity and number of cracks to be identified. Example updating results demonstrate that satisfactory identification can be achieved for practically any configurations of cracks in a beam. Experimental study with five solid beam specimens is then carried out to further verify the developed cracked beam element model. Both forward verification and crack damage identification with the tested beams show similar level of accuracy to that with the numerically simulated examples. The cracked beam element model can be extended to crack identification of beams with complex cross sections. To do so the additional flexibility matrix for a specific cross-section type needs to be re-formulated. In the present study this is done for box sections. The stress intensity factors (SIF) for a box section as required for the establishment of the additional flexibility matrix are formulated with an empirical approach combining FE simulation, parametric analysis and regression analysis. The extended cracked beam element model is verified against both FE simulated and experimentally measured modal data. The model is subsequently incorporated in the crack identification for box beams. The successful extension of the cracked beam element model to the box beams paves the way for similar extension to the crack identification of other types of sections in real-life engineering applications.

Made available in DSpace on 2014-06-11T19:27:14Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-11-24Bitstream added on 2014-06-13T18:55:43Z : No. of bitstreams: 1 franco_vr_me_ilha.pdf: 5148348 bytes, checksum: 722b347f89e5e9a0aa5c379afe0dadba (MD5) / Financiadora de Estudos e Projetos (FINEP) / Este trabalho apresenta o estudo e desenvolvimento de uma técnica de monitoramento da integridade estrutural, para identificação e caracterização de falhas estruturais através da metodologia das ondas de Lamb utilizando materiais piezelétricos como sensores e atuadores. Ondas de Lamb são uma forma de perturbação elástica que se propaga guiada entre duas superfícies paralelas livres. Ondas de Lamb são formadas quando o atuador excita a superfície da estrutura com um pulso depois de receber um sinal. Quando uma onda propaga na superfície de uma placa, ela chega em um PZT sensor por diferentes caminhos. Um caminho é quando a onda atinge o sensor diretamente, ou seja, sem obstáculos no caminho em que ela se propaga. Outro caminho possível é quando a onda chega ao sensor após se propagar sobre descontinuidades existentes na superfície da estrutura. Com as várias características dos sinais recebidos, e com o uso de certas técnicas de processamento de sinais, essas falhas podem ser identificadas, realizando-se a ação correta tentando evitar a total falha da estrutura. Nesse contexto, diferentes testes experimentais foram realizados em diferentes tipos de estruturas. Redes de sensores e atuadores piezelétricos foram acopladas na superfície dessas estruturas, a fim de se fazer a configuração das ondas de Lamb. Os PZTs atuadores excitaram a estrutura em altas faixas de frequência. Diferentes tipos de falhas estruturais foram simuladas, através do aumento de massa, alteração de rigidez e através de cortes na borda das estruturas. Quatro índices de falha foram utilizados para detectar a presença da falha na estrutura, são eles: Root- Means-Square Deviation (RMSD), Índice de Falha Métrica (IFM), Norma H2 e Correlation Coefficient Deviation Mean (CCDM). Estes índices foram computados através dos sinais de entrada e de saída no domínio da frequência... / This work presents the study and development of a Structural Health Monitoring technique for identification and characterization of structural damages based on Lamb waves methodology using piezoelectric materials as actuators and sensors. Lamb waves are a form of elastic perturbation that remains guided between two parallel free surfaces. Lamb waves are formed when the actuator excites the structure’s surface with a pulse after receiving a signal. When the wave propagates on the structure, it comes in a PZT sensor from different paths. One path is when the wave reaches the sensor directly, i.e. without obstacles in the path in which it propagated. Another possible path is when the wave reaches the sensor after spreads on discontinuities in the structure’s surface. Damages can be detected and located through several features of the received signals and with the use of certain techniques of signal processing. In this context, several experimental tests were performed on different kinds of structures. Piezoelectric actuators and sensors networks were attached on the surface of these structures in order to make the Lamb waves configuration. The PZTs actuators excited the structure in high frequency ranges. Different kinds of structural damages were simulated by increasing mass, reduction of stiffness and cuts through the edge of the structures. Four damage-sensitive indexes were used to detect the presence of the damage in the structure: Root-Means-Square Deviation (RMSD), Metric Damage Index (MDI), H2 Norm and Correlation Coefficient Deviation (CCDM). These indices were computed in the frequency domain. The results showed the viability of the Lamb waves methodology for Structural Health Monitoring system using smart materials as actuators and sensors