Cette recherche porte sur les fils multifilamentaires de verre utilisés pour renforcer les matériaux à base de ciment. Elle est focalisée sur les interactions mécaniques de ce type de fils, constitués d'un assemblage de milliers de filaments micrométriques, avec un micro-béton et sur le rôle spécifique de l'imprégnation du fil par cette matrice cimentaire. Trois pré-conditionnements des fils ont été employés lors de la fabrication des éprouvettes afin de moduler les conditions d'imprégnation par la matrice cimentaire. L'imprégnation de 5 fils multi-filamentaires par la matrice cimentaire a été caractérisée et les paramètres d'imprégnation ont été définis en s'appuyant sur des observations MEB, ainsi que des essais de porosimétrie au mercure et des essais spécifiques d'écoulement le long du fil enrobé. Des essais classiques d'arrachement de type pull-out ont été utilisés pour la caractérisation mécanique. L'analyse des liens entre les propriétés mécaniques et les paramètres d'imprégnation ont permis de mieux comprendre les micro-mécanismes d'interaction filaments / matrice cimentaire et d'expliquer le comportement macroscopique à l'arrachement. / This research deals with multi-filaments glass yarns used as reinforcement of cement based materials. It focuses on the mechanical interactions of these yarns, made of thousands of micrometric filaments, with a micro-concrete and on the specific part of the impregnation of the yarn by the cementitious matrix. Modulated impregnation conditions of the yarns were obtained by using three different manufacturing processes for the samples preparation. The impregnation of 5 multi-filament yarns by the cementitious matrix has been characterized and physical parameters of impregnation were determined using SEM investigations, mercury intrusion porosity measurements and specific tests of flow all along the embedded yarn. Classical pull-out tests have been used for the mechanical characterisation. The study of the links between the mechanical properties and the physical parameters of impregnation allowed accessing a better understanding of the filaments / cementitious matrix interaction micro-mechanisms, and explaining the macroscopic pull-out behaviour.

Devido à fragilidade do concreto, o controle e combate da fissuração são de importância fundamental em estruturas de concreto armado. Uma maneira de melhorar as propriedades do concreto à tração é pelo emprego de fibras. A presente pesquisa é uma tentativa de fornecer diretrizes para o dimensionamento de estruturas de concreto armado com fibras, e armadura convencional sob condições de serviço. Apresenta-se inicialmente, um estudo do comportamento do material à tração. Um modelo probabilístico/micro-mecânico fundamentado na mecânica de fratura, e capaz de prever o comportamento pós-fissuração do compósito é apresentado. O modelo prevê a relação tensão-abertura de fissura do compósito levando em conta os seguintes micro-mecanismos: travejamento de agregado e fibras, a ruptura das fibras, os efeitos de: atrito local (snubbing effect), esmagamento da matriz, Cook-Gordon, e da pré-tração das fibras. Em nível estrutural, dois modelos macro-mecânicos são apresentados. O primeiro modelo tem premissa na teoria clássica de fissura, e o segundo na mecânica de dado. O primeiro modelo é ajustado para aplicação na previsão de espaçamento e aberturas de fissura em estruturas de concreto armado com fibras discretas e aleatoriamente dispostas. É demostrado que o modelo micro-mecânico pode alimentar perfeitamente o modelo macro-mecânico. Ensaios de tração com elementos de placas de argamassa com fibras armada com tela ou fios foram realizados. Os resultados teóricos previstos pelo modelo foram comparados com os obtidos do programa experimental, e mostram uma boa concordância, comprovando a validade do modelo apresentado. / Due to the brittleness of concrete, the control and prevention of cracking in reinforced concrete structures are of prime importance. One way of improving the tensile properties of concrete is by the addition of fibres. The present research is a trial to provide guidelines for the design of fibre reinforced concrete structures under service loads. First of all, a study of the tensile behaviour of the composite material is presented. A probabilistic/fracture mechanics based micromechanical model, capable of predicting the poscracking behaviour of the material is presented. The model predicts the tensile stress-crack width relationship, accounting for the following micromechanisms: fibre and aggregate bridging, fibre rupture, local snubbing, matrix spalling, the Cook-Gordon interface effect, and fibre prestressing. At the structural level, two macromechanical models are presented. One is founded on the classical theory of cracking, while the other, a shear lag model, is founded on the continuum damage mechanics. The first model is adjusted for application to the prevision of crack width and crack spacing in fibre reinforced concrete structures with short discrete and randomly dispersed fibres. It is shown that the micromechanical model fits very well in the macrostructural model. Tensile tests with mortar specimens reinforced with continuous steel wires or meshes and PVA or polypropylene fibres were carried out. The theoretical results predicted by the model were compared with results obtained from the experimental program, and show very good agreement, confirming the validity of the theoretical model.

A new nonlinear fracture analysis framework is developed for the mode-I and II fracture response of thick-section fiber reinforced polymeric (FRP) composites. This framework employs 3D micromechanical constitutive models for the nonlinear material behavior along with cohesive elements for crack growth. Fracture tests on various cracked geometries are used to verify the prediction of the failure loads and the crack growth behavior. A commercially available pultruded E-glass/polyester and vinylester thick-section FRP composite material was used to demonstrate the proposed fracture approach along with the nonlinear constitutive modeling. A new Infra-red thermography technique is derived to measure the surface strain field near the crack tip in the linear response range. Mode I and II fracture toughness tests for pultruded composites are also examined using the eccentrically loaded, single-edge-notch tension, ESE(T), single-edge-notch tension, SEN(T), and a butterfly specimen with an Arcan-type fixture. Material nonlinearity and crack growth effects were observed during the tests and investigated using the proposed analysis framework. The effect of material orthotropy on the stress intensity factor solutions was addressed using the virtual crack closure technique. The analytic and experimental results support the use of the ESE(T) specimen for the measuring the mode-I fracture toughness and the butterfly shaped specimen for measuring the mode-II toughness. The calibrated cohesive models were able to predict the measured crack growth in both modes I and II for various crack geometries. A mixed mode failure criterion is proposed and verified with test results. Examples are presented for using this criterion and crack growth analyses. The experimental and analytical results of this study can form a foundation for using fracture-based methods for the design of structures using these materials.

Frequency reference oscillator is a critical component of modern radio transceivers. Currently, most reference oscillators are based on low-frequency quartz crystals that are inherently bulky and incompatible with standard micro-fabrication processes. Moreover, their frequency limitation (<200MHz) requires large up-conversion ratio in multigigahertz frequency synthesizers, which in turn, degrades the phase-noise. Recent advances in MEMS technology have made realization of high-frequency on-chip low phase-noise MEMS oscillators possible. Although significant research has been directed toward replacing quartz crystal oscillators with integrated micromechanical oscillators, their phase-noise performance is not well modeled. In addition, little attention has been paid to developing electronic frequency tuning techniques to compensate for temperature/process variation and improve the absolute frequency accuracy. The objective of this dissertation was to realize high-frequency temperature-compensated high-frequency (>100MHz) micromechanical oscillators and study their phase-noise performance. To this end, low-power low-noise CMOS transimpedance amplifiers (TIA) that employ novel gain and bandwidth enhancement techniques are interfaced with high frequency (>100MHz) micromechanical resonators. The oscillation frequency is varied by a tuning network that uses frequency tuning enhancement techniques to increase the tuning range with minimal effect on the phase-noise performance. Taking advantage of extended frequency tuning range, and on-chip temperature-compensation circuitry is embedded with the sustaining circuitry to electronically temperature-compensate the oscillator. Finally, detailed study of the phase-noise in micromechanical oscillators is performed and analytical phase-noise models are derived.

Σκοπός της παρούσας εργασίας ήταν η κατασκευή, η πειραματική και θεωρητική μελέτη πολυμερικών μίκρο- και νάνο- συνθέτων υλικών. Μελετήθηκε η επίδραση στη μηχανική συμπεριφορά τους φθοροποιών παραγόντων όπως η απορρόφηση υγρασίας και κατασκευαστικών παραμέτρων όπως η γεωμετρία και η μέθοδος διασποράς της ενίσχυσης. Επίσης, έγινε σύγκριση των πειραματικών αποτελεσμάτων με την πρόβλεψη των αναλυτικών μοντέλων MPM όσον αφορά το μέτρο Ελαστικότητας συναρτήσει της περιεκτικότητας σε ενίσχυση και RPM όσον αφορά την εναπομένουσα θραυστομηχανικής συμπεριφοράς μετά από υγροθερμική γήρανση. Τέλος, έγινε μικρομηχανική μοντελοποίηση της ελαστικής και βισκοελαστικής διεπιφανειακής συμπεριφοράς με το μοντέλο της υβριδικής ενδιάμεσης φάσης νανοσυνθέτων σε συστήματα νανοσωλήνων άνθρακα-εποξειδικής ρητίνης και νανοσωλήνων TiO2-οστεοκυττάρων. Στο πρώτο μέρος έγινε πειραματική μελέτη πολυμερικών μίκρο-συνθέτων υλικών. Για τη μελέτη της επίδρασης της γεωμετρίας της ενίσχυσης στη μηχανική και θραυστομηχανική συμπεριφορά κατασκευάστηκαν σύνθετα εποξικής μήτρας ενισχυμένα με μίκρο-σωματίδια γυαλιού ινώδους, σφαιρικής και γεωμετρίας φυσαλίδας και πραγματοποιήθηκαν πειράματα κάμψης τριών σημείων και compact tension αντίστοιχα. Για τους τρεις παραπάνω τύπους ενίσχυσης προκύπτει ότι τα σύνθετα ενισχυμένα με μικροσφαιρίδια γυαλιού υπερτερούν από πλευράς καμπτικής δυσκαμψίας και μηχανικής αντοχής. Ο λόγος είναι η δυνατότητα να παραλαμβάνουν τόσο εφελκυστικά όσο και θλιπτικά φορτία σε αντίθεση με τις ίνες και τις φυσαλίδες. Από άποψη στερρότητας, ΚIC υπερτερούν τα σύνθετα με μικροϊνίδια λόγω της αντίστασης στη διάδοση ρωγμών μέσω της εξόλκυσης τους και της γεφύρωσης ρωγμών, ενώ τα σύνθετα με φυσαλίδες γυαλιού παρουσιάζουν αυξημένη ενέργεια θραύσης λόγω της αυξημένης ένδοσης και άρα της δυνατότητας αποθήκευσης ενέργειας. Επίσης, για μικρές περιεκτικότητες σε ενίσχυση η συμπεριφορά των υλικών είναι όλκιμη. Η έναρξη της ρωγμής ακολουθείται από ευσταθή διάδοσή της πριν την καταστροφική αστοχία υπό την επίδραση μηχανισμών διάχυσης ενέργειας και ανάσχεσης της ρωγμής. Αντίθετα, μετά από μία κρίσιμη περιεκτικότητα η θραύση του υλικού γίνεται ψαθυρή λόγω του βαθμού συσσωμάτωσης των εγκλεισμάτων. Η ασταθής διάδοση της αρχικής ρωγμής συμβαίνει αμέσως μετά την εκκίνηση αυτής. Τα πειραματικά αποτελέσματα επιβεβαιώθηκαν από μικροφωτογραφίες SEM και οπτικού στερεομικροσκοπίου. Σε επόμενο στάδιο μελετήθηκε πειραματικά η επίδραση περιβαλλοντικών παραγόντων στη θραυστομηχανική συμπεριφορά των συνθέτων ενισχυμένων με σφαιρίδια γυαλιού. Πιο συγκεκριμένα, διεξήχθηκαν πειράματα compact tension σε δοκίμια μετά από υγροθερμική γήρανση για χρονικά διαστήματα που υπερβαίνουν το χρόνο κορεσμού απορρόφησης υγρασίας. Προέκυψε ότι για μικρές περιεκτικότητες σε ενίσχυση η δυσθραυστότητα των υλικών ακολουθεί μάλλον εκθετική μείωση. Αντίθετα για υλικά με μεγάλη περιεκτικότητα σε ενίσχυση τα οποία παρουσιάζουν ψαθυρή θραύση, αρχικά παρατηρείται αύξηση ή έναρξη της υποβάθμισης του της θραυστομηχανικής συμπεριφοράς από κάποιο κρίσιμο χρόνο εμβάπτισης. Τέλος, από την εφαρμογή σε αυτά των θεωρητικών του μοντέλων MPM και RPM προέκυψε πολύ ικανοποιητική σύγκλιση με τα πειραματικά αποτελέσματα. Στο δεύτερο μέρος έγινε πειραματική μελέτη της επίδρασης του τρόπου διασποράς των νανοσωλήνων στη μηχανική και θερμομηχανική συμπεριφορά καθώς και στη δομή νανοσυνθέτων. Για την ανάμειξη των νανοσωλήνων χρησιμοποιήθηκαν οι μέθοδοι της υπερήχησης και της μηχανικής ανάμειξης σε αναμείκτη υψηλών στροφών. Έγινε μηχανικός χαρακτηρισμός της στατικής και της δυναμικής θερμομηχανικής συμπεριφοράς τους, από όπου επιβεβαιώθηκε ο κρίσιμος ρόλος που διαδραματίζει η ποιότητα της διασποράς των νανοεγκλεισμάτων στις μακροσκοπικές ιδιότητες του νανοσυνθέτου. Παρατηρήθηκε ότι τα νανοσύνθετα υλικά που κατασκευάστηκαν με τη μέθοδο της υπερήχησης πλεονεκτούν από πλευράς στατικής και δυναμικής μηχανικής συμπεριφοράς έναντι αυτών που κατασκευάστηκαν με τη μέθοδο της μηχανικής ανάμειξης με μόνη εξαίρεση τη μείωση της Tg. Παρ’ όλα εγείρεται το ζήτημα της καταστροφής της δομής της μήτρας λόγω της μεγάλης ισχύος των υπερήχων που εστιάζεται σε μικρή περιοχή, καθώς και της μειωμένη ικανότητα αποδοτικής διασποράς νανοσωλήνων σε μεγαλύτερη ποσότητα ρητίνης. Επιπλέον, ελήφθησαν φάσματα υπερύθρου της καθαρής ρητίνης και των νανοσυνθέτων που κατασκευάστηκαν με τη μέθοδο της υπερήχησης από όπου παρατηρήθηκε αύξηση της πυκνότητας των σταυροδεσμών στην περίπτωση των νανοσυνθέτων, γεγονός που αποδίδεται στη θερμική συμπεριφορά των νανοσωλήνων άνθρακα. Τα πειραματικά αποτελέσματα βρέθηκαν σε πλήρη συμφωνία με τα συμπεράσματα από τις μικροφωτογραφίες SEM των επιφανειών θραύσης των νανοσυνθέτων. Ήταν δυνατό να φανεί ξεκάθαρα η ποιότητα της διασποράς των νανοσωλήνων καθώς και οι μηχανισμοί αστοχίας και ενίσχυσης του υλικού σε νάνο-κλίμακα. Στην περίπτωση της μηχανικής ανάμειξης σε αναμείκτη υψηλής ταχύτητας υπήρχε συσσωμάτωση των νανοσωλήνων, ενώ στην περίπτωση της ανάμειξης με υπερήχηση επιτεύχθηκε καλή ποιότητα διασποράς των νανοεγκλεισμάτων στη μήτρα χωρίς την ύπαρξη συσσωματωμάτων μεγέθους άνω των 5-10 νανοσωλήνων. Από πειράματα compact tension που πραγματοποιήθηκαν παρατηρήθηκε σημαντική αύξηση κατά 58,51% στον KIC και 55,25% στον GIC σε σχέση με την καθαρή ρητίνη για περιεκτικότητα μόλις 0,1wt% σε νανοσωλήνες άνθρακα. Η βελτιωμένη θραυστομηχανική συμπεριφορά σχετίζεται με τη μικρή απόσταση ανάμεσα στα νανοσωματίδια και το εξαιρετικά μεγάλο αριθμό νανοσωλήνων σε δεδομένο όγκο συνθέτου. Παρ, όλα αυτά από πειράματα που πραγματοποιήθηκαν μετά από υγροθερμική γήρανση έδειξαν ότι η απορρόφηση υγρασίας επιφέρει απώλεια της ενισχυτικής ικανότητας των νανοσωλήνων για χρόνους εμβάπτισης μεγαλύτερους των 24 ωρών. Δεδομένου ότι η περιοχή της ενδιάμεσης φάσης επηρεάζει σημαντικά τη μακροσκοπική συμπεριφορά ενός συνθέτου υλικού έγινε εφαρμογή των αναλυτικών μοντέλων της υβριδικής ενδιάμεσης φάσης και της βισκοελαστικής υβριδικής ενδιάμεσης φάσης σε συστήματα εποξειδικής ρητίνης – νανοσωλήνων άνθρακα και οστεοκυττάρων - νανοσωλήνων TiO2. Για τα νανοσύνθετα που κατασκευάστηκαν με τη μέθοδο της υπερήχησης υπολογίστηκε η τιμή του συντελεστή πρόσφυσης k=0,90 , ενώ για αυτά που κατασκευάστηκαν με μηχανική ανάμειξη σε αναμείκτη υψηλών στροφών η τιμή του συντελεστή πρόσφυσης υπολογίστηκε k=0,20. Η κακή ποιότητα πρόσφυσης έχει ως συνέπεια τη μη αποτελεσματική μεταφορά των φορτίων από τη μήτρα στα εγκλείσματα. Επίσης, έγινε πρόβλεψη της μεταβολής του πάχους της βισκοελαστικής ενδιάμεσης φάσης συναρτήσει του χρόνου υπό την επίδραση σταθερής φόρτισης. Από τη μοντελοποίηση του συστήματος νανοσωλήνων TiO2-οστεοβλαστών με το μοντέλο της υβριδικής ενδιάμεσης φάσης προκύπτει ότι υπάρχει αλληλοκάλυψη των ενδιάμεσων φάσεων που αναπτύσσονται μεταξύ γειτονικών νανοσωλήνων, ακόμα και για καλή ποιότητα πρόσφυσης. Το γεγονός εξηγεί την ελλιπή ανάπτυξη των κυττάρων, καθώς λόγω της αγκύρωσης στην επιφάνεια των νανοσωλήνων και του μικρού διακένου, η ένδοση στην περιοχή μεταξύ των νανοσωλήνων είναι στην πραγματικότητα σημαντικά μικρότερη από αυτή που έχει αρχικά υποτεθεί. / The current master thesis was realized during the years 2010-2012 under the supervision of Prof G.Papanicolaou at the Composite Materials Group, in the Department of Mechanical and Aeronautical Engineering at University of Patras, Greece. The aim of the thesis was the experimental and analytical study of polymer micro- and nano- composites. There was investigated the effect of damage through hygrothermal ageing as well as the effect of manufacturing parameters such as the fillers’ dispersion method and geometry upon the composites’ mechanical and fracture behavior. Experimental findings were compared with the predictions of the RPM and MPM models, concerning the materials response after hygrothermal ageing and the composites’ elastic modulus as a function of filler’s weight fraction, respectively. Also, the interphasial elastic and viscous behavior was investigated by application of the elastic and viscoelastic hybrid interphase models which were both developed by Prof. G.Papanicolaou et al. The two different composite systems considered were Carbon nanotubes- Epoxy and TiO2 nanotubes osteoblast cells. The first part of the study involved the manufacturing, the experimental characterization and analytical modeling of polymer microcomposites. The effect of fillers geometry upon the mechanical and fracture behavior of composites reinforced with spherical, fibrous and hollow bubble E-glass microinclusions was investigated by the means of three point bending and compact tension tests. It was observed that the composites reinforced with spherical inclusions demonstrated superior stiffness and flexural strength, which was attributed to the ability to take up both tensile and compressive loadings, unlike the other two types of fillers. However, glass fibril reinforced composites showed increased fracture toughness in effect of the mechanisms of fiber pull out and crack closure were considered to be fracture toughness. Finally there was observed increased fracture energy in the case of and glass bubble filled composites due to the inclusions increased compliance and thus ability to store more elastic energy prior to crack propagation. SEM microphotographs obtained confirmed experimental findings of ductile fracture and energy dissipation mechanisms for low vf and brittle fracture after a critical volume fraction. Next, the effect of environmental parameters upon the materials fracture behavior was investigated. For low volume fractions in glass microspheres the materials residual fracture toughness was found to decrease exponentially as a function of immersion time in water. On the contrary, for higher volume fractions the there can be observed an increase in toughness or initiation of damage after a critical immersion time. In any case, experimental findings were in close convergence with the predictions analytical models applied: MPM concerning the elastic modulus as a function of vf, and RPM concerning the residual KIC and GIC as a function of ageing time. The thesis’ second part involved the experimental and analytical investigation of the effect of the fillers dispersion method in epoxy nanocomposites. There were applied two dispersion methods of carbon nanotubes in the resin, namely ultrasonication and high speed shearing. From the three point bending characterization and DMTA tests it was concluded that except for the Tg, all static and dynamic properties of nanocomposites manufactured by the ultrasonication method, are superior to the ones of nanocomposites manufactured by the shearing method. This was confirmed by SEM micrographs observations from which there was a clear indication that nanocomposites produced by the shearing method, were characterized by a limited aggregation while those manufactured by the ultrasonication method showed that individual nanotubes are scattered in the matrix and no aggregation was observed. Moreover, results of FTIR analysis indicated increased crosslink density in the nanocomposites compared to the neat resin, however, the decrease in Tg and increase in damping are an indication of damage of the epoxy and nanotubes structure during the ultrasonication procedure. Compact tension tests indicated an increase by 58.51% in the fracture toughness and by 55.25% in the fracture energy of the nanocomposites reinforced with 0.1wt% in MWCNT’s with respect to the neat resin. However, experimental findings indicated that the toughening of nanotubes is lost after hygrothermal ageing of the nanocomposites. The RPM prediction concerning the nanocomposites residual fracture behavior was in close agreement with experimental results. Given that the interphasial phenomena affect the global behavior of a composite, the hybrid interphase model was applied to the carbon nanotubes-epoxy system. For the nanocomposites that were manufactured with the ultrasonication method the adhesion efficiency coefficient value was calculated k=0.70 while for the ones manufactured by high speed shearing the respective value was k=0.20. For the two given adhesion conditions the variation of the viscoelastic interphase thickness was also predicted as a function of loading time. The hybrid interphase model was also applied in order to investigate the interphasial elastic and viscoelastic behavior in a human osteoblast cells-TiO2 nanotubes system. The interphase overlapping was considered to be leading to TiO2 nanotube arrays mediocre biocompatibility due to the anchoring of cells to nanotubes and increased stiffness in the area between nanotubes.

A pragmatic method for selecting components and devices from a database or parameterized models is developed in this thesis. The quantitative framework presented here is sufficiently general to accommodate an entire device assembly, a component, or a sub-assembly. The details pertaining to a device or a component are classified into three sets of variables: (i) user-specifications, s (ii) device parameters, p , and (iii) device characteristics, c . Functional, practical, and performance-related attributes that a user can provide comprise user-specifications. Since, most often, a specification cannot be specified as a single number, we allow the user to enter a range with lower and upper bounds. Device parameters comprise the geometry and material properties, and device characteristics include functional requirements and performance criteria. Thus, for a device, all its functional and utility attributes are contained in the union of sets s and c , whereas the geometry and the material properties are in set p . The equations governing the physical behavior of the device are written in terms of s , p , and c . These equations may sometimes be readily available; when they are not, it may be necessary to formulate them as required. By solving the governing equations along with the inequalities that arise from the lower and upper bounds on s , we obtain feasible ranges on p and c . Then, for any pair of device characteristics, a 2D feasible map is drawn, to visually portray the consequences of user-specifications. If the feasible map is null, small, or large, it indicates that the user-specifications are infeasible, stringent, or there is much scope for design, respectively. This can be inferred even before the designs are considered. Juxtaposed on the feasible map are points or lozenges corresponding to the quantitative attributes of the entries in the database. The ones that lie inside the feasible map can be reckoned as meeting the user-specifications and thus, enabling selection. On the other hand, if there is no database or none of the devices in the database lie inside the feasible map, we can identify the feasible ranges of all the design parameters for every point inside the feasible map. This information is useful to the designer to redesign and arrive at feasible designs by using parameterized models of the device. A Graphical User Interface (GUI) is developed to facilitate the user-interaction. The utility of the selection framework is demonstrated with a variety of case-studies including miniature pumps, heat pulse-based soil-moisture sensors, springs, flywheels, compliant mechanisms, micromechanical suspensions, etc. The latter two use kineto-elastic characteristics of deformable components. The framework, when used for materials selection, can be seen as an extension of Ashby’s materials selection method. This is also illustrated with two examples.

L’objectif de ce travail est d’étudier les couplages entre les propriétés hygroélastiques des papierset leur microstructure. L’exploitation d’images de papiers modèles acquises par microtomographieà rayons X a permis de caractériser de manière inédite l’évolution des propriétésmicrostructurales de ces matériaux en fonction de leurs conditions d’élaboration ainsique lors d’essais où ils ont été placés sous atmosphère à humidité relative contrôlée.Ces résultats constituent un apport nouveau pour la connaissance de la nature statistique desdescripteurs des propriétés des fibres (dimensions et orientation) et de leurs contacts (surface,degré de liaison), de l’architecture des réseaux fibreux que forment les papiers (nombrede contacts entre fibre) et pour la taille des volumes élémentaires représentatifs des propriétésmicrostructurales et élastiques de ces milieux. Sur la base de ces informations, différentsmodèles, plus oumoins raffinés, ont été élaborés dans le cadre de la théorie de l’homogénéisationdes structures périodiques discrètes, pour décrire les propriétésmécaniques des papiers.Cette approche apporte un éclairage nouveau sur le rôle des liaisons entre fibres sur leur comportement mécanique. / The objective of this work is to study the coupling between the hygroelastic properties ofpapers and their microstructure. The use of images of models acquired by X-ray microtomographypapers allowed the characterization in an unprecedentedmanner of the evolutionofmicrostructural properties of thesematerials according to their production conditions andduring tests where they were placed in atmosphere at controlled relative humidity. These resultsprovide a new contribution to the knowledge of the statistical nature of the descriptorsof fibre properties (size and orientation) and their contacts (surface, bonding degree ratio), ofthe architecture of fibrous networks that papers constitute (number of fibre-to-fibre bonds)as well as of the size of the representative elementary volumes of microstructural and elasticproperties. Based on this information, various models, more or less sophisticated, were developedin the framework of the theory of homogenisation of discrete periodic structures todescribe the mechanical properties of paper. This approach sheds new light on the role offibre-to-fibre bonds on themechanical behaviour of thesematerials.

Devido à fragilidade do concreto, o controle e combate da fissuração são de importância fundamental em estruturas de concreto armado. Uma maneira de melhorar as propriedades do concreto à tração é pelo emprego de fibras. A presente pesquisa é uma tentativa de fornecer diretrizes para o dimensionamento de estruturas de concreto armado com fibras, e armadura convencional sob condições de serviço. Apresenta-se inicialmente, um estudo do comportamento do material à tração. Um modelo probabilístico/micro-mecânico fundamentado na mecânica de fratura, e capaz de prever o comportamento pós-fissuração do compósito é apresentado. O modelo prevê a relação tensão-abertura de fissura do compósito levando em conta os seguintes micro-mecanismos: travejamento de agregado e fibras, a ruptura das fibras, os efeitos de: atrito local (snubbing effect), esmagamento da matriz, Cook-Gordon, e da pré-tração das fibras. Em nível estrutural, dois modelos macro-mecânicos são apresentados. O primeiro modelo tem premissa na teoria clássica de fissura, e o segundo na mecânica de dado. O primeiro modelo é ajustado para aplicação na previsão de espaçamento e aberturas de fissura em estruturas de concreto armado com fibras discretas e aleatoriamente dispostas. É demostrado que o modelo micro-mecânico pode alimentar perfeitamente o modelo macro-mecânico. Ensaios de tração com elementos de placas de argamassa com fibras armada com tela ou fios foram realizados. Os resultados teóricos previstos pelo modelo foram comparados com os obtidos do programa experimental, e mostram uma boa concordância, comprovando a validade do modelo apresentado. / Due to the brittleness of concrete, the control and prevention of cracking in reinforced concrete structures are of prime importance. One way of improving the tensile properties of concrete is by the addition of fibres. The present research is a trial to provide guidelines for the design of fibre reinforced concrete structures under service loads. First of all, a study of the tensile behaviour of the composite material is presented. A probabilistic/fracture mechanics based micromechanical model, capable of predicting the poscracking behaviour of the material is presented. The model predicts the tensile stress-crack width relationship, accounting for the following micromechanisms: fibre and aggregate bridging, fibre rupture, local snubbing, matrix spalling, the Cook-Gordon interface effect, and fibre prestressing. At the structural level, two macromechanical models are presented. One is founded on the classical theory of cracking, while the other, a shear lag model, is founded on the continuum damage mechanics. The first model is adjusted for application to the prevision of crack width and crack spacing in fibre reinforced concrete structures with short discrete and randomly dispersed fibres. It is shown that the micromechanical model fits very well in the macrostructural model. Tensile tests with mortar specimens reinforced with continuous steel wires or meshes and PVA or polypropylene fibres were carried out. The theoretical results predicted by the model were compared with results obtained from the experimental program, and show very good agreement, confirming the validity of the theoretical model.