Na indústria automobilística há uma grande preocupação em relação ao custo e ao peso das peças utilizadas em projetos e os adesivos vem sendo utilizados para substituir a solda ponto com esta finalidade. Neste contexto o adesivo epóxi estrutural Betamate 1496 vem sendo empregado pela Volkswagen do Brasil S.A para substituição de solda a ponto na fabricação da porta dianteira do veículo Pólo Hatch. O adesivo Betamate 1496 é utilizado na fabricação da carroçaria do veículo Pólo Hatch da Volkswagen do Brasil SA com essa finalidade. O objetivo do presente trabalho é propor e testar uma metodologia para avaliar o estado de cura do adesivo epóxi Betamate 1496 por meio da determinação do seu módulo de elasticidade e de sua dureza, quando este é empregado na fabricação das portas dianteiras do veículo Pólo Hatch. Estas propriedades foram determinadas pela medição das propriedades mecânicas do adesivo em amostras retiradas em seis posições da porta em duas fases de processamento: estado pré-curado, durante a montagem da porta e estado curado, após a cura final na estufa de pintura. O estudo foi realizado através do ensaio de indentação instrumentada, utilizando os procedimentos experimentais propostos por Franco Jr. e colaboradores e Pintaúde e colaboradores na avaliação das propriedades do polímero. O estado de cura do polímero foi avaliado de modo qualitativo também pela técnica de calorimetria diferencial exploratória (Differential Scanning Calormetry). Os resultados obtidos pela metodologia proposta por Franco Jr. e colaboradores apresentaram grande dispersão de valores, provavelmente pelo fato do método propagar erros de arredondamento na aplicação do algoritmo. Conforme observado nos resultados experimentais obtidos pela metodologia proposta por Pintaúde e colaboradores, no estado pré-curado todas as amostras apresentam valores de módulo de elasticidade muito próximos, em torno de 1,0 + 0,1. GPa, independentemente da posição na porta ou do tempo de descarregamento, já no estado curado observa-se uma diferença significativa dos valores de módulo de elasticidade, E, e dureza H dependendo da posição original da amostra na porta, com as amostras retiradas das posições inferiores apresentando E=0,8 + 0,1 GPa, enquanto que as demais apresentam E=1,0+0,1 GPa, os valores de dureza, H, para todas as amostras curadas foi H=0,03 GPa. A analise qualitativa do ensaio térmico pelas as curvas obtidas no ensaio térmico de DSC mostra que as curvas da amostra no estado pré-curado e curado são idênticas, indicando o ciclo de cura do polímero já se completa na fase de montagem da porta, antes mesmo da cura final na estufa de pintura. / Cost and weight reduction are major concerns of the automobilist industry, this leads to the increasing substitution of spot weld by adhesives in the assembling of automobile parts. In this context Volkswagen do Brazil S.A uses the Betamate 1496 structural epoxy adhesive in the manufacture of the chassis of the Polo v. Hatch. The aim of the present work is to suggest and to test a methodology to evaluate the state of cure of this adhesive by means of the determination of its hardness and elastic modulus when used in the manufacture of the front doors of the Polo v. Hatch vehicle. These properties have been determined by the measurement in six samples removed from different positions at the two phases of processing: after assembling of the chassis and cured state, after final cure at painting. The study was carried through instrumented indentation testing, which used the experimental procedures suggested by Franco Jr. and collaborators and Pintaúde and collaborators. The state of cure of polymer was evaluated also in qualitative way by Differential Scanning Calorimetry. The results obtained by the Franco Jr. methodology show great dispersion of values, probable due to the fact that the method propagates rounding errors in the application of the algorithm. The experimental results obtained by the Pintaúde methodology show that, in the state after assembling of the chassis all samples present elastic modulus, E, around 1,0 + 0.1 GPa, independently of the position in the door. In the cured state is observed depending on the sample’s position at the door: samples taken from the bottom of the door shaved E=0,8 + 0,1 GPa, while the remaining showed E= 1,0 + 0,8 GPa. The hardness, H, of the adhesive is constant (H=0,03 GPa) for all samples. The comparison of DSC curves obtained from the incurred and cured samples, showed that they are very similar, suggesting that the cure cycle of the polymer is already completed at assembling stage of the door.

Les procédés de formage à grande vitesse comme le magnétoformage et le formage électrohydraulique se développent actuellement à l'échelle industrielle. La conception de ces procédés nécessite le recours à la simulation numérique, du fait de leur caractère fortement dynamique et multiphysique. Pour garantir la précision de ces simulations, il est essentiel de disposer de données pertinentes capables de décrire le comportement dynamique des matériaux déformés.Ce travail de thèse se propose de développer des essais mécaniques basés sur le procédé de magnétoformage afin de caractériser le comportement en écrouissage dynamique de matériaux sous forme de tubes ou de tôles. Les conditions d'essai sont ainsi au plus proche des conditions de déformation visées.Le premier essai mis en place chez iCube Research est un essai d'expansion électromagnétique de tube, instrumenté à l'aide de mesures de courant et de vitesse par photonic Doppler velocimeter (PDV). Le choix de l'inducteur, l'optimisation du modèle numérique (LS-Dyna®) et la sensibilité de l'essai aux incertitudes de mesure sont discutés. La procédure d'identification par analyse inverse est mise en place, et sa capacité à identifier les paramètres du modèle de comportement est étudiée. La sensibilité de l'essai aux paramètres du modèle, et leurs corrélations, sont également analysées.Cette procédure a ensuite été adaptée à un essai dédié à des éprouvettes issues de tôles. Le mode de chargement et de déformation produit par cet essai a permis de réduire de façon significative la sensibilité de la caractérisation aux incertitudes expérimentales par rapport à l'essai d'expansion de tube. Une démarche de réduction de modèle a rendu possible l'utilisation de cet essai dans le cadre d'une procédure itérative d'identification de paramètres.Les deux essais ont été appliqués à l'aluminium 1050 à l'état recuit, et ont permis de mettre en évidence une sensibilité marquée à la vitesse de déformation, avec une augmentation de l'écrouissage par rapport au comportement mesuré par des essais de traction uniaxiale quasi-statique. D'autres nuances d'aluminium ou alliages de cuivre ont également été caractérisés par l'un ou l'autre des essais. La procédure mise en place n'est pas restrictive quant au choix du modèle de comportement dynamique. Dans le cadre de cette étude, qui vise essentiellement à caractériser l'écrouissage dans une gamme relativement restreinte de vitesses de déformation (entre 100 et 5000 s-1), le choix s'est porté sur le modèle de Johnson-Cook.Les conditions dynamiques du formage (hautes vitesses de déformation, impacts, mode de déformation en flexion contre une matrice…) peuvent apporter des gains importants en termes d'allongement. La problématique de la caractérisation de limites de formage dynamiques, qui doit faire l'objet de développements ultérieurs, est introduite en dernière partie. / High speed forming processes such as magnetic pulse forming and electrohydraulic forming are currently developing at the industrial scale. Design of these processes requires numerical simulation to take into account the highly dynamic and multiphysics conditions. To ensure the representativeness of the simulations, there is an important need for relevant data describing the material dynamic behavior.In this study, mechanical tests based on magnetic pulse forming have been developed in order to characterize the dynamic strain hardening of materials in the form of tubes or sheets. The testing conditions are then very similar to the real industrial forming conditions.The first test developped at iCube Research is an electromagnetic tube expansion test, instrumented with pulse current measurement devices and photonic Doppler velocimetry (PDV). Design of the inductor, optimization of the numerical model, and test sensitivity to the experimental uncertainties are discussed. The inverse analysis identification procedure is established, and its ability to identify the constitutive model parameters is examined. The sensitivity of the test to the model parameters, and their correlations, are analyzed as well.This procudure is then adapted to a test dedicated to plane samples, cut from sheets. The loading mode and the deformation mode induced by this test enables a significant reduction of the characterization sensitivity relative to experimental uncertainties, as compared to tube expansion testing. A model reduction approach makes it possible to use this test in an iterative parameter identification procedure.Both tests are applied to annealed aluminium 1050, and they reveal a significant strain-rate sensitivity, with a stronger strain-hardening than that measured by quasi-static tensile tests. Ohter aluminium alloys and copper alloys have been dynamically characterized by one of the tests. The developped procedure is not restrictive for the selection of the constitutive model. In this study, which concerns a relatively narrow strain-rate domain (from 100 to 5000 s-1), the Johnson-Cook model has been chosen.The dynamic forming conditions (high strain-rates, impact, deformation mode by flexion against a die…) are likely to offer great improvements in formability. The problem of dynamic forming limits characterization, which will be the subject of further developpements, is introduced as a last chapter.

In this study, we have investigated the relationship between the process-microstructure to predict and modify the material's properties. Understanding these relationships allows the identification and correction of processing deficiencies when the desired properties are not achieved, depending on the microstructure. Hence, the co-relation between process-microstructure-properties helped reduce the number of experiments, materials & tool costs and saved much time. In the case of high entropy alloys, friction stir welding (FSW) causes improved strength due to the formation of fine grain structure and phase transformation from f.c.c to h.c.p. The phase transformation is temperature sensitive and is studied with the help of differential scanning calorimetry (DSC) to calculate the enthalpy experimentally to obtain ΔGγ→ε. The second process discussed is heat treatment causing precipitation evolution. Fundamental investigations aided in understanding the influence of strengthening precipitates on mechanical properties due to the aging kinetics – solid solution and variable artificial aging temperature and time. Finally, in the third case, the effect of FSW parameters causes the thermal profile to be generated, which significantly influences the final microstructure and weld properties. Therefore, a computational model using COMSOL Multiphysics and TC-Prisma is developed to generate the thermal profile for different weld parameters to understand its effect on the microstructure, which would eventually affect and predict the final properties of the weld. The model's validation is done via DSC, TEM, and mechanical testing.

This dissertation addresses two critical issues in the mechanical behaviors and structures of bulk-metallic glasses (BMGs): (1) the effect of composition, fabrication method, and pretreatment of plastic deformation on mechanical properties and structures of BMGs; (2) the mechanical response and structural evolution of BMGs in the elastic and plastic region. (Cu50Zr50)94Al6 and (Cu50Zr50)92Al8 amorphous alloys were used to study the effect of composition on mechanical properties and structures of BMGs. The (Cu50Zr50)94Al6 alloy exhibits lower yield stress and Young’s modulus, higher Poisson’s ratio, worse thermal stability, and better plasticity than (Cu50Zr50)92Al8. Both the topological and chemical effects of Al addition account for the differences of mechanical and physical properties between them. A Zr55Ni5Al10Cu30 glass-forming alloy with injection casting (the melting temperatures are 1,550 K and 1,250 K, respectively) and with suction casting was fabricated. The results indicate that despite their amorphous structures, the suction-casting samples exhibit a lower yield stress, lower Young’s modulus, and larger plastic strain than the injection-casting samples (the melting temperature is 1,550 K) due to more quenched-in free volumes in suction casting, which results from the higher cooling rate. The inhomogeneous plastic deformation in Zr50Cu40Al10 BMG samples was introduced by four-point-bend fatigue. There is almost no difference of the stress-strain behaviors between the deformed and undeformed samples. Elastostatic compression was used to introduce homogeneous deformation in Zr70Cu6Ni16Al8 BMG samples. The preloaded samples are softer with decreases of yield strength and Young’s moduli. Anisotropy was observed in the preloaded samples despite their small magnitudes, which even occurred at a relatively low temperature and applied stress level. The structural evolution of Zr70Cu6Ni16Al8 BMG in the elastic region was analyzed with anisotropic pair density function. The analysis of the first shell of Zr70Cu6Ni16Al8 glass confirms the structural changes in the elastic region. The bond reorientation leads to direction dependent changes in the chemical short-range order. The structural evolution in the plastic region of Zr70Cu6Ni16Al8 BMG is investigated as well. The serrations were observed for both the stress-displacement and full width at half maximum-displacement curves. The excess free volume was measured, which increases with increasing the displacement.

An experimental technique has been presented to probe the mechanical behavior of thin film materials. The method is capable of tensile testing thin films on substrate and free-standing thin film specimens. A mechanical gripper was designed to address the current challenges in gripping thin film specimens. In order to measure the strain field across the gage section, the moire interferometry technique was used and the respective optical setup was designed. A versatile microfabrication process has been developed to fabricate free-standing dog-bone specimens. Aluminum was used as the model material; however, any other metallization material can be integrated in the process. Thin film specimens have been characterized using SEM, AFM, and TEM. A process has been developed to fabrication diffraction gratings on the specimen by FIB milling. Different grating geometries were fabricated and the diffraction efficiency of the gratings was characterized. The structural damage induced by the Ga+ ions during the FIB milling of the specimens was partially characterized using STEM and EDS. In order to extract the strain field information from the moire interferogram data, a numerical postprocessing technique was developed based on continuous wavelet transforms (CWT). The method was applied on simulated uniform and nonuniform strain fields and the wavelet parameters were tuned to achieve the best spatial localization and strain accuracy.

An experimental technique has been presented to probe the mechanical behavior of thin film materials. The method is capable of tensile testing thin films on substrate and free-standing thin film specimens. A mechanical gripper was designed to address the current challenges in gripping thin film specimens. In order to measure the strain field across the gage section, the moire interferometry technique was used and the respective optical setup was designed. A versatile microfabrication process has been developed to fabricate free-standing dog-bone specimens. Aluminum was used as the model material; however, any other metallization material can be integrated in the process. Thin film specimens have been characterized using SEM, AFM, and TEM. A process has been developed to fabrication diffraction gratings on the specimen by FIB milling. Different grating geometries were fabricated and the diffraction efficiency of the gratings was characterized. The structural damage induced by the Ga+ ions during the FIB milling of the specimens was partially characterized using STEM and EDS. In order to extract the strain field information from the moire interferogram data, a numerical postprocessing technique was developed based on continuous wavelet transforms (CWT). The method was applied on simulated uniform and nonuniform strain fields and the wavelet parameters were tuned to achieve the best spatial localization and strain accuracy.

Nanoporous (np) materials with pore size below 100 nano-meters exist naturally in biological and mineral structures, and synthetic np materials have been used industrially for centuries. Np materials have attracted significant research interest in recent decades, as the development of new characterization techniques and nanotechnology allow the observation and design of np materials at a new level. This study focuses on two np materials: nanoporous silicon (np-Si) and nanoporous palladium (np-Pd). Silicon (Si), because of its high capacity to store lithium (Li), is increasingly becoming an attractive candidate as anode material for Li ion batteries (LIB). One significant problem with using Si as an anode is the large strain that accompanies charge-discharge cycling, due to swelling of the Si during Li insertion and deinsertion. Np-Si offers a large amount of free volume for Li absorption, which could allow the anode material to swell without cracking. A new method to fabricate thin films of high-purity (100% Si content) np-Si, which is promising as an anode material for LIB, is demonstrated and discussed in this study. Microstructural characterization, chemical analysis, battery performance testing and mechanical behavior of thin film np-Si are discussed here. Palladium (Pd) is considered an ideal and reliable hydrogen sensor and storage material, due to its fast response and selectivity for hydrogen gas. This research not only demonstrates a method to fabricate np-Pd thin films, but also proposes a method to fabricate bulk np-Pd. The uniformly crack-free and sponge-like np-Pd thin film provides high sensitivity to low concentrations of H2, showing promise as a hydrogen sensor material. Stress changes during hydrogenation/dehydrogenation were measured using wafer curvature. For bulk np-Pd, ultra-fine pore sizes were achieved by electrochemically dealloying bulk PdNi alloy. Mechanical behavior of bulk np-Pd was studied using in-situ transmission electron microscopy (TEM). Scanning electron microscopy (SEM) and x-ray diffraction were also used to characterize the structure and morphology of np-Pd. This doctoral research has involved the optimization of fabrication conditions and investigations of microstructural evolution during processing, yielding an improved understanding of the properties, mechanical behavior and potential applications of np-Si and np-Pd.

Το οπλισμένο σκυρόδεμα αποτελεί στις μέρες μας το πιο διαδεδομένο υλικό για την κατασκευή του φέροντος οργανισμού κτιριακών κατασκευών. Παρά την εξαιρετική επίδοση του οπλισμένου σκυροδέματος ως φέροντος υλικού, διαπιστώθηκε ότι στη διάρκεια ζωής των κατασκευών από οπλισμένο σκυρόδεμα, η οποία συχνά ξεπερνά τα 100 χρόνια, παρατηρείται μια βαθμιαία συσσώρευση βλάβης, ένας από τους κύριους παράγοντες της οποίας εντοπίζεται στη διάβρωση του σιδηροπλισμού. Tα τελευταία χρόνια το πρόβλημα της υποβάθμισης της φέρουσας ικανότητας του οπλισμού σκυροδέματος λόγω βλάβης διάβρωσης έχει προσελκύσει το ενδιαφέρον αρκετών ερευνητών παγκοσμίως. Όμως, μέχρι σήμερα ούτε η τεχνολογική σημασία της υποβάθμισης αυτής μπορεί να εκτιμηθεί ικανοποιητικά, κυρίως λόγω της έλλειψης σχετικών συστηματικών μελετών αλλά και επαρκών πειραματικών δεδομένων, ούτε οι φυσικοί μηχανισμοί που συμβάλλουν στην παρατηρούμενη υποβάθμιση των μηχανικών ιδιοτήτων έχουν γίνει πλήρως κατανοητοί. Κατά την εκπόνηση της παρούσας διδακτορικής διατριβής πραγματοποιήθηκε μια συστηματική μελέτη της μηχανικής συμπεριφοράς χαλύβων οπλισμού σκυροδέματος διαβρωμένων τόσο στο φυσικό τους περιβάλλον εντός του σκυροδέματος όσο και σε συνθήκες εργαστηριακής επιταχυμένης διάβρωσης. Με βάση τη σύγκριση της απώλειας μάζας σε συνάρτηση με το χρόνο έκθεσης στο διαβρωτικό μέσο που καταμετρήθηκε στο εργαστήριο, με την απώλεια μάζας δειγμάτων υλικού τα οποία είχαν διαβρωθεί φυσικά σε κατασκευές, κατέστη δυνατή η εξαγωγή ενός εμπειρικού συντελεστή επιτάχυνσης της βλάβης διάβρωσης όταν το υλικό διαβρώνεται με τη μέθοδο της αλατονέφωσης σε σχέση με τη βλάβη φυσικής διάβρωσης. Κατά την παρούσα εργασία εξετάστηκαν οι χάλυβες S400 και S500s, οι οποίοι χρησιμοποιήθηκαν ευρύτατα για κατασκευές του φέροντος οργανισμού κτιρίων στην Ελλάδα κατά το πρόσφατο παρελθόν, καθώς και ο χάλυβας B500c ο οποίος από τα τέλη του 2006 χρησιμοποιείται στην Ελλάδα σχεδόν αποκλειστικά. Για την εξασφάλιση μια επαρκούς πειραματικής βάσης, πραγματοποιήθηκαν περισσότερες από 500 δοκιμές εφελκυσμού σε διαβρωμένα και μη διαβρωμένα δοκίμια. Οι μηχανικές δοκιμές εφελκυσμού έδειξαν ότι οι ιδιότητες αντοχής του διαβρωμένου υλικού παρουσιάζουν μικρή μόνο υποβάθμιση. Παρά ταύτα, η μείωση της διατομής των ράβδων του σιδηροπλισμού λόγω βλάβης διάβρωσης οδηγεί σε αύξηση των εφαρμοζόμενων τάσεων, αφού το βάρος των δομικών στοιχείων των κατασκευών προφανώς δεν μεταβάλλεται. Αυτό έχει ως συνέπεια τη σημαντική μείωση των συντελεστών ασφαλείας. Αντίθετα με τις ιδιότητες αντοχής, παρατηρήθηκε μια σημαντική μείωση στην παραμόρφωση θραύσης του διαβρωμένου υλικού. Η παραμόρφωση θραύσης και η ειδική ενέργεια παραμόρφωσης είναι κρίσιμες τεχνολογικές ιδιότητες σε συνθήκες σεισμού. Για την κατανόηση των φυσικών μηχανισμών που συμβάλλουν στην παρατηρούμενη υποβάθμιση των μηχανικών ιδιοτήτων, διεξήχθη εκτενής μεταλλογραφική μελέτη καθώς και μελέτη των επιφανειών θραύσης του διαβρωμένου υλικού. Παρατηρήθηκε ότι η διάβρωση είναι ομοιόμορφη, ενώ κατά το μήκος των δοκιμίων εντοπίστηκαν περιοχές με εντονότερη και άλλες με λιγότερο έντονη διάβρωση. Επιπλέον, η ανάλυση των επιφανειών θραύσης έδειξε πλήρως όλκιμη συμπεριφορά θραύσης των δοκιμίων ακόμη και στο έντονα διαβρωμένο υλικό. Επομένως, μπορεί να εξαχθεί το συμπέρασμα ότι η υποβάθμιση των μηχανικών ιδιοτήτων εφελκυσμού του χάλυβα οπλισμού οφείλεται κυρίως στην ανομοιομορφία που παρουσιάζει η διατομή του σιδηροπλισμού κατά μήκος των ράβδων του υλικού λόγω της βλάβης διάβρωσης, με συνέπεια αφενός την τοπική συγκέντρωση τάσεων και αφετέρου την καταπίεση της ομοιόμορφης παραμόρφωσης κατά τον εφελκυσμό του υλικού. Τέλος, στην εργασία αυτή παρουσιάζεται μια μέθοδος με την οποία προσομοιώνεται η βλάβη λόγω διάβρωσης με μια τεχνητή εγκοπή. Η μέθοδος αυτή στηρίζεται στις παραδοχές ότι η ολκιμότητα του υλικού παραμένει αμετάβλητη λόγω της διάβρωσης και η παρατηρούμενη μείωση στην παραμόρφωση θραύσης οφείλεται στην καταπίεση της ομοιόμορφης παραμόρφωσης λόγω των εγκοπών που προκαλεί η διάβρωση στο υλικό οι οποίες διευκολύνουν τη δημιουργία του λαιμού. Επίσης γίνεται η παραδοχή ότι η παρατηρούμενη μείωση των ιδιοτήτων αντοχής οφείλεται στη συγκέντρωση τάσεων που προκαλούν οι εγκοπές αυτές. Δοκιμές εφελκυσμού σε δοκίμια με εγκοπές γνωστής γεωμετρίας έδειξαν ότι η μέθοδος αυτή μπορεί να χρησιμοποιηθεί με ικανοποιητική ακρίβεια για τον υπολογισμό των ιδιοτήτων εφελκυσμού του υλικού χωρίς την ανάγκη πραγματοποίησης μηχανικών δοκιμών. / Reinforced concrete is currently the most common material used for the construction of the load bearing elements of structures. Although this composite material performs exceptionally well, it has been noted that during the life span of reinforced concrete structures, which often exceeds 100 years, a gradual damage accumulation takes place. One of the most influential factors of this damage has been attributed to the corrosion of steel reinforcement. Recently, the degradation of the load bearing ability of steel reinforcement has been an issue under research by several researchers worldwide. Yet to date, the technological importance of this degradation caused by corrosion damage cannot be assessed to a satisfactory degree, mainly due to the lack of relevant experimental studies. Similarly, the physical mechanisms which contribute to the degradation have not been totally resolved. In the framework of the current PhD thesis, a systematic study of the mechanical behaviour of concrete reinforcing steel bars, corroded both in their natural environment (embedded in concrete) and by means of laboratory accelerated corrosion, was performed. By comparing the mass loss as a function of time recorded during the laboratory corrosion tests with the respective mass loss recorded from naturally corroded samples, an empirical acceleration factor was derived for laboratory corrosion damage compared to natural corrosion damage. Reinforcing steel grades S400* and S500s*, which have been used in the recent past for the reinforcement of concrete structures in Greece, as well as steel grade B500c**, which from the end of 2006 is used almost exclusively, were tested. To obtain a sufficient experimental database more than 500 tensile tests on corroded and non-corroded samples were performed. The tensile tests performed have shown only a slight degradation of the strength properties of the corroded steel bars. However, the reduction of the cross sectional area of the corroded bars lead to an increase of the applied stress, as the loads applied to which steel bars in structures are constant over time. This leads to a significant reduction of the safety factors applied during design. On the contrary, a significant reduction of the material’s ductility properties was recorded. Elongation to failure and strain energy density are crucial properties in the case of alternating loading during earthquakes. In order to understand the physical mechanisms which contribute to the recorded degradation of the mechanical properties, an extensive metallographic investigation as well as an investigation of the fracture surfaces of corroded material was performed. From this investigation it was concluded that corrosion damage is uniform without pitting, while along the bars’ length areas more severe corrosion damage was noted. Furthermore, the investigation of the fracture surfaces showed ductile fracture characteristics even of the most severely corroded specimens. It can therefore be concluded that the degradation of the tensile properties of corroded material is caused mainly by the non-uniformity of the corrosion damage and therefore of the cross sectional area along the longitudinal axis of the bars. This leads to a local stress concentration as well as to the depression of the uniform elongation during the tensile testing of the material. Finally, in the current PhD thesis a method by which corrosion damage can be simulated by a fictitious notch is suggested. This method is based on the assumptions that the ductility of the material remains unaffected by corrosion and the recorded reduction of the ductility properties is attributed to the depression of the uniform elongation caused by notches which are formed on the bars due to corrosion exposure and facilitate necking. In addition, the recorded reduction of the strength properties is attributed to the stress concentration caused by these notches. Tensile tests performed on specimens with artificial notches of known geometry have shown that the suggested method can be used to assess to a satisfactory degree the tensile properties of steel reinforcing bars without the need to perform tensile tests. * Names according to the Hellenic regulations. These steel grades are similar to the StIIIs and StIVs steel grades of the DIN regulations respectively. ** Name according to the Hellenic regulations. It is similar to StIV steel grade but with higher ductility requirements compared to S500s.

Ο σκοπός της παρούσης διδακτορικής διατριβής ήταν η θεωρητική και πειραματική μελέτη της επίδρασης της αναπτυσσόμενης βλάβης που προέρχεται από διαφορετικές αιτίες στην μηχανική συμπεριφορά συνθέτων υλικών. Οι πηγές βλάβης που μελετήθηκαν ήταν: 1.Υγροθερμική Κόπωση 2. Θερμική Κόπωση 3. Ύπαρξη κεντρικής κυκλικής οπής διαφορετικών διαμέτρων ή εγκοπής διαφορετικών μηκών 4. Διαφορετικοί ρυθμοί παραμόρφωσης 5. Ερπυσμός και επανάταξη, και 6. Συνδυασμός των παραπάνω, όπως: ερπυσμός μετά από υγροθερμική ή θερμική κόπωση, ερπυσμό σε διαφορετικές θερμοκρασίες και χρονικές διάρκειες. Τα σύνθετα υλικά που μελετήθηκαν ήταν κοκκώδη, ινώδη και τύπου σάντουιτς σύνθετα υλικά. Για την καλύτερη μελέτη της μηχανικής και βισκοελαστικής συμπεριφοράς των συνθέτων υλικών, εφαρμόστηκε ένας σημαντικός αριθμός θεωρητικών μοντέλων, όπως είναι: 1. Το μοντέλο των τεσσάρων παραμέτρων. 2. Το μοντέλο των τριών παραμέτρων. 3. Το μοντέλο εναπομένουσων ιδιοτήτων, RPM. 4. Το θεωρητικό μοντέλο ανάλυσης των διεπιφανειακών ιδιοτήτων σε σάντουιτς υλικά. 5. Το μοντέλο πρόβλεψης βισκοελαστικής συμπεριφοράς στη μη γραμμική περιοχή. Τα συμπεράσματα που προέκυψαν από την παραπάνω μελέτη ήταν ιδιαίτερα ενδιαφέροντα. / The aim of the present investigation is to study not only experimentally but also theoretically the effect of damage due to different damage sources, which analytically are: a) hygrothermal fatigue, b) thermal fatigue, c) the existence of central hole (with different diameters) or notch (with different lengths), d) creep and relaxation, e) different stain rates and f) combination of the previous such as creep after hygrothermal or thermal fatigue and creep on different temperatures and time, on the mechanical and viscoelastic behavior of composites materials (reinforced with fibers and grains ), sandwich structures and different types of resin. For the better observation of the mechanical and viscoelastic behavior of composites materials, it was applied several theoretical models, which are: a) The four parameter model, b) the three parameter model, c) the RPM model (Residual Property Model) for the prediction of the residual mechanical properties of materials after damage from different sources, d) the theoretical model for the analysis of interfacial properties of sandwich structures and e) the viscoelastic model for the determination of the non elastic properties of composites material in function of the applied stress. From the present investigation useful conclusions were conducted for the determination of the mechanical behavior of composites materials (reinforced with fibres and grains or sandwich structures). In addition, the experimental results of this study were in great deal with those results of the application of the already mentioned theoretical models.

La fabrication des composants d'une turbine hydraulique est un procédé complexe, couteux et nécessite beaucoup de métal. Une possibilité d'améliorer ce procédé est d' obtenir des formes complexes, ce qui va réduire la quantité de soudage et de post-traitement. Le nouveau procédé nécessite l'étude de nouveaux matériaux, qui possèdent une déformabilité élevée et répondent aux exigences du matériau pour une application dans les turbines hydrauliques. Les aciers supermartensitiques sont attractifs, car ils possèdent la combinaison exceptionnelle d'une haute résistance à la corrosion, une bonne soudabilité, une résistance à la traction élevée et un comportement ductile.Le but de ce travail est la détermination et la modélisation du comportement d'un acier supermartensique à hautes températures pour contrôler et optimiser le procédé de mise en forme à chaud de composants d'une roue d'une turbine hydraulique.Des paramètres différents affectent la mise en forme à chaud de l'acier. D'un côté, la connaissance de la microstructure et des transformations de phase est nécessaire pour optimiser la température du mise en forme et analyser les résultats, d'un autre côté, des essais mécaniques doivent être réalisés pour déterminer le comportement à haute température de l'acier et ses lois de comportement, ce qui permettra de modéliser le procédé de mise en forme. Ainsi, une étude détaillée de la microstructure et des transformations de l'acier supermartensique est d'abord présentée. Il a été démontré que le traitement thermique peut améliorer la microstructure du matériau et supprimer les hétérogénéités provenant de la fabrication de l'acier. Ensuite, le comportement mécanique à haute température a été étudié. Des essais de traction ont été effectués dans l'intervalle de température de 650 ° C à 1100 ° C à des vitesses de déformation entre 0,0004s-1 et 0,04s-1. Les influences de la température, de la vitesse de déformation, de la taille des grains, de la teneur et de la morphologie des phases sur le comportement mécanique ont été analysés. En outre, la caractérisation qualitative de l'endommagement a été effectuée dans le domaine austénitique. Finalement les lois de comportement ont été déterminées et la modélisation de la mise en forme à chaud d'un acier supermartensitique a pu être entreprise. Des exemples de deformation en flexion, correspondant à des formes réelles complexes, sont présentés.Les résultats de l'étude des aciers inoxydables supermartensitiques et de leurs lois de comportement à chaud permettent d'optimiser le processus de formage. / The current manufacturing of hydraulic turbine components is complex, metal-consuming and expensive process. The one way of improving this process is forming complex shapes that will decrease the amount of welding and post treatment. The new forming process requires the investigation of new materials having good deformability and satisfying the requirements to material for hydraulic turbine application. The supermartensitic steels are attractive as they have exceptional combination of high corrosion resistance, good weldability, tensile strength and ductile behavior.The aim of this work is the determination and modeling of supermartensitic stainless steel behavior at high temperature to control and optimize process of hot forming the parts of hydraulic turbine runner.Different parameters affect the hot forming of steel. On the one hand, the knowledge about steel microstructure and transformation is needed to optimize temperature of forming and analyze results, and on the other hand, the mechanical experiments need to be performed to determine high temperature behavior of steel and constitutive law which allow to model process of forming. Thus firstly detailed study of microstructure and transformation of supermartensitic steel is presented. It has been demonstrated that the heat treatment can improve microstructure of material and exclude disadvantage of steel making. Then the mechanical behavior at high temperature was investigated. Tensile tests were carried out in the interval of temperature from 650°C to 1100°C at strain rate in range from 0,0004s-1 to 0,04s-1. The influences of temperature, strain rate, grain size, content and morphology of phases on mechanical behavior have been analyzed. Additionally brief investigation of cavities at full austenite zone has been done. Finally the constitutive laws were determined and modeling of behavior is presentede Examples of bending process are reported, corresponding to real and complex shapes of turbine parts.The presented investigation of supermartensitic stainless steel and proposed constitutive laws allow optimization the process of hot forming.