Ce travail de thèse, s'inscrit dans un accord co-tutelle entre l’École Polytechnique de Thiès du Sénégal, et de l'Université de Franche-Comté. Il a été réalisé au Département de Mécanique Appliquée de l'institut FEMO-ST à Besançon (France). [...]L'objectif de cette thèse est de contribuer à la caractérisation du comportement thermo-hygro-mécaniques des fibres libériennes élémentaires issues du chanvre. Les fibres élémentaires testées dans ce travail ont des diamètres extérieurs moyens d'environ 20µm et des longueurs utiles de l'ordre de 10mm. Des essais couplant des chargements mécaniques, hygroscopiques et thermiques sont réalisés afin d'apporter des éléments de compréhension sur le comportement des fibres et de collecter une base de données sur leurs performances mécaniques.Trois types de comportement sont identifiés sur les fibres en traction quasi statique en fluage pour les différents niveaux hygro-thermique testés. Une rigidification de la fibre est révélée sous chargements cycliques progressifs dans des conditions environnementales constantes. Une chute du module d' Young initial de la fibre est remarquée quand l'humidité relative augmente. Une accélération de la vitesse de déformation différée de la fibre est obtenue en fluage lorsque des cycles d'humidité relative sont appliqués. Une baisse des propriétés mécaniques des fibres sous l'effet de la température est également observée. Elle dépend directement du niveau de la durée de l'exposition à la température. / The thesis work joins in the framework in joint supervision between the Polytechnic school in Thies (Sénégal) and the University of Franche-Comté (France)[...]The aim of this thesis is to contribute to the characterization of thermo-hygro-mechanical behaviour of elementary hemp fibres. These fibres used in this work have an average diameter approximately 20µm and useful length of only 10 mm. The méchanical, hygroscopic and thermal coupling testing are realized to provide some understanding of the behaviour of the hemp fibres and to collect details from a database on their mechanical performance.From the uniaxial quasi static tensile tests and creep tests, three categories of behaviours are measured and observed. How ever subject to constant environmental conditions, a fibre hardening is observed under cyclic tensile loading. It was also found that a drop of the initial Young 's modulus of the hemp fibres when the relative humidity gradually increases. Thus, the strain rates measured on the fibres during creep exhibited a speeding up relative humidity cycles are imposed . We found also that the mechanical properties of the fibres decrease under temperature effects. It depends directly on the level and exposure time to temperature

La course à l'allègement des aéronefs constitue aujourd'hui l'un des enjeux principaux de l'industrie aéronautique. Le remplacement des rivets par des soudures FSW sur les voilures permettrait de gagner jusqu'à 20% de la masse totale de la voilure. Ce travail réalisé en collaboration avec Dassault-Aviation s'inscrit dans le cadre du projet ADSAM (Assemblage de Structures Aéronautiques Métalliques) visant à fabriquer un démonstrateur de caisson de voilure soudé par FSW. Cette étude traite du comportement mécanique en traction monotone quasi-statique et en fatigue oligocyclique (10⁴ à 10⁵ cycles) d'alliages d'aluminium (séries 2000 et 7000) soudés par FSW. L'objectif est d'étudier l'effet des hétérogénéités microstructurales induites par le soudage sur les hétérogénéités de comportement mécanique dans les joints soudés. La corrélation d'images numériques a été utilisée pour définir la réponse mécanique (σnominale – εvraie) dans chacune des zones constituant ces soudures. En complément, des essais sur des mini-éprouvettes prélevées dans l'épaisseur des joints ont permis d'établir le comportement local (σvraie – εvraie) dans chacune des zones du joint. Sous chargement de traction monotone, la rupture des soudures FSW a été observée dans la zone de localisation des déformations anélastiques située aux minima des profils de microdureté. En revanche, les fissures de fatigue ne démarrent pas nécessairement dans ces zones de localisation. Des analyses microscopiques des faciès de rupture ont permis d'identifier deux mécanismes principaux d'amorçage de fissure : (i) rupture ou (ii) décohésion de particules intermétalliques. D'autre part, les essais cycliques montrent une adaptation élastique des matériaux dans les différentes zones du joint (noyau, ZAT et ZATM) après stabilisation autour d'une déformation moyenne non nulle. En terme de durée de vie, les résultats d'essais sur éprouvettes entaillées traitées OAC (représentative de zones critiques sur structure) montrent que le soudage par FSW réduit jusqu'à 50% la tenue en fatigue de ces alliages. L'amorçage des fissures de fatigue est dû aux piqûres créées par le traitement d'OAC. Les résultats obtenus dans le cadre de ce travail ouvrent des perspectives intéressantes pour modéliser le comportement mécanique des joints soudés par FSW afin d'estimer leur durée de vie en fatigue oligocyclique. / Mass reduction of aeronautic structures is an important challenge for aeronautic industries. FSW is an efficient joining process to replace rivets : the mass reduction using this welding process could reach 20% for aircraft wings. This work carried out in collaboration with Dassault-Aviation was done in the framework of ADSAM project. The FSW process significantly modifies the microstructure and the local material properties, the overall mechanical response of the joint is largely governed by the response of the different zones of FSW joint. In the present study, the global and local mechanical responses under monotonic quasi-static tension and cyclic tension-tension (R=0.1) in low cycle fatigue regime (10⁴ to 10⁵ cycles) were carried out on FSW joints made of aluminum alloys (2000 and 7000 series). Full field strain measurements were carried out on FSW welded specimens using digital image correlation techniques assuming an iso-stress configuration (σlocal = σnominal ). Additionally tension tests parallel to the welding direction have been performed on micro specimens machined in each representative zone of FSW joints. These tests allow us to determine the local mechanical response (σlocal − εlocal) of the joint. Local mechanical properties were determined for each representative weld zone (nugget, TMAZ and HAZ). The full field anelastic strain localization scenario prior to fracture shows a good correlation with the microhardness profile along the FSW joints. Under monotonic loading, fracture was observed at the strain localization zones unlike under cyclic loading. In fatigue, the fracture zones are very dispersed for each weld configuration. SEM observations led to the identification of two crack initiation mechanisms:(i) fracture of intermetallic particles or (ii) decohesion between intermetallic particles and the matrix. Digital image correlation under cyclic loading shows an elastic shakedown of the studied materials in each zone of the joint around a non null mean strain. In terms of number of cycles to failure, fatigue test results on ACO treated notched specimens (representative of structure critical locations) show that the FSW process reduce up to 50% the faigue life of the base materials. The crack initiation was associated to ACO pits. The results of this study open interesting prospects for modelling the mechanical behavior of FSW joints and to assess their fatigue life.

Resíduos de mineração, dispostos em barragens de rejeito, são materiais de baixas densidades e resistências, em geral – muitas vezes, suscetíveis à liquefação. Desta forma, técnicas de melhoramento de solos, tais como o Deep Soil Mixing, surgem como possibilidade à estabilização destes materiais. Neste sentido, baseado nesta técnica, este trabalho visa avaliar o comportamento mecânico de resíduos de mineração de ouro estabilizados com cimento, comparativamente aos rejeitos no estado natural, analisando fatores de influência (quantidade de agente cimentante, umidade inicial das amostras e líquido empregado na moldagem). Para este propósito, ensaios de resistência à compressão simples, de compressão diametral, de ondas ultrassônicas e triaxiais não-drenados (CIU) foram empregados. Os resultados mostram que os resíduos de mineração, nas condições representativas de campo, apresentam baixas resistências e suscetibilidade à liquefação a baixas tensões confinantes. Por sua vez, os ensaios com resíduo estabilizado com cimento revelaram que: o aumento nas taxas de cimentação, a diminuição dos teores de umidade iniciais das amostras e uso de licor proveniente das barragens de rejeito nas moldagens, repercutiram, em geral, em aumentos nas resistências (qu e qt) e na rigidez inicial (G0) das misturas As análises de variância comprovaram que todos os fatores controláveis estudados foram significativos para as respostas avaliadas e demonstraram que a influência da dosagem de cimento foi muito mais pronunciada, em relação às outras variáveis. O comportamento tensão-deformação das misturas, sob condições não-drenadas, foi típico de materiais cimentados submetidos à baixas tensões confinantes – com geração de poro-pressões negativas devidas à tendência à dilatação. Ainda, ficou demonstrado que o índice porosidade/teor volumétrico de cimento (η/Civ) é adequado para a previsão do comportamento mecânico das misturas resíduo-cimento, considerando os parâmetros estudados – ampliando a possibilidade de aplicação deste índice. Foi observado, que existe uma proporcionalidade direta entre as resistências à tração e à compressão das misturas, que independe da relação η/Civ, sendo esta traduzida através de um valor escalar único (ξ = 0,17). / Mine tailings are materials of low in situ densities and strengths, in general – often susceptible to liquefaction. In view of this, soil improvement methods such as Deep Soil Mixing can be alternatives for the stabilization of these materials. In this sense, based on DSM techniques, this research aims to analyze the mechanical behaviour of cemented gold mine tailings, comparing to its natural condition, evaluating factors that influence the stabilization (cement content, initial water content and type of water for preparing specimens). For this purpose, unconfined compressive tests, splitting tensile tests, ultrasonic pulse velocity tests and undrained triaxial (CIU) tests were used. The results showed that remoulded mine tailings samples presented low strength and liquefaction susceptibility under low confining pressures. In its turn, the tests with cemented gold tailings revealed that: increasing the cement content, decreasing initial water content and using water from the tailings pond (liquor) in moulding provided strength (qu and qt) and initial stiffness (G0) gains on the mixtures of gold mine tailings-Portland cement. In this regard, the variance analysis statistically demonstrated that all the factors chosen on the experiment were significant to the parameters assessed It is also showed that the cement content influence was much more pronounced than other factors. Furthermore, the stress-strain behaviour of the blends is typical of cemented materials under low confining pressures and undrained conditions – with negative pore-pressures build-up. In addition, it is demonstrated that the porosity/cement index controls the mechanical performance of gold tailings-Portland cement blends, considering the whole range of densities and cement contents studied – what broadens the applicability of such index. Finally, it was found a single relationship between tensile (qt) and compressive strength (qu) equal to 0.17, being independent of the porosity/cement ratio.

Dans le but de vérifier l'efficacité des dispositifs de scellement ou des barrières ouvragées dans le stockage géologique des déchets radioactifs, l'Institut de Radioprotection et de Sûreté Nucléaire (IRSN) a mis en œuvre le projet expérimental SEALEX (SEALing performance EXperiments) auquel ce travail est étroitement lié. Dans le cadre de ce projet, des essais in-situ sont effectués à l'échelle représentative et dans des conditions naturelles sur un mélange compacté de bentonite et de sable. Ce matériau de mélange a été choisi pour sa faible perméabilité et surtout pour sa capacité de gonflement qui permet de colmater les vides existant dans le système, notamment le vide technologique correspondant au vide radial entre le noyau de scellement et la roche hôte et qui est inévitable au cours de l'installation du noyau dans le forage. Une fois les vides scellés, le gonflement à volume constant engendre une pression de gonflement aussi bien sur la roche hôte (radiale) que sur les structures de confinement en béton (axiale). Le comportement de ce matériau dans ces conditions de couplages hydromécaniques est alors étudié dans ce travail. La microstructure du matériau à son état initial a été premièrement examinée par micro-tomographie rayons-X. Ceci a permis de voir la distribution des grains de bentonite et de sable ainsi que le réseau de pores dans l'échantillon. Des macro-pores se sont retrouvés concentrés à la périphérie de l'échantillon ainsi qu'entre les grains de sable, ce qui pourra affecter à court terme la perméabilité. L'hydratation du même matériau en condition de gonflement limité a été ensuite observée par une photographie 2D et par la micro-tomographie aux rayons-X. Le mécanisme de gonflement par production de gel de bentonite, la cinétique de gonflement, la diminution de densité et l'homogénéisation du matériau final on été analysés. L'hydratation en conditions de gonflement empêché a été aussi étudiée par des essais où la pression de gonflement a été mesurée dans deux directions : radialement et axialement. La différence retrouvée entre les pressions de gonflement axiales et radiales a évoqué la présence d'une anisotropie de microstructure qui a été analysée en fonction de la masse volumique sèche de bentonite dans le mélange. Des essais en modèle réduit reproduisant à une échelle 1/10ème les essais in situ (SEALEX) ont été également effectués afin d'étudier le comportement du noyau compacté après la reprise des vides au cas d'un accident détruisant les éléments de confinement. Des mesures locales de pression de gonflement le long des échantillons ont permis de mettre en évidence l'évolution du gradient de densité durant le gonflement axial. Finalement une comparaison entre les résultats obtenus dans ce travail et ceux d'un essai in situ (SEALEX) a été faite. Une bonne correspondance entre les valeurs d'humidités relatives a été retrouvée pour les mêmes longueurs d'hydratation tout en prenant en compte la saturation par le vide technologique radial. Par contre, la comparaison des évolutions et des valeurs de pressions de gonflement était plus compliquée vu les différences de configurations des essais / In order to verify the effectiveness of the geological high-level radioactive waste disposal, the French Institution of Radiation protection and Nuclear Safety (IRSN) has implemented the SEALEX project to control the long-term performance of swelling clay-based sealing systems, and to which this work is closely related. Within this project, In-situ tests are carried out on compacted bentonite-sand mixture in natural conditions and in a representative scale. This material is one of the most appropriate sealing materials because of its low permeability and good swelling capacity. Once installed, this material will be hydrated by water from the host-rock and start swelling to close all gaps in the system, in particular the internal pores, rock fractures and technological voids. Afterwards, swelling pressure will develop. In the present work, laboratory experiments were performed to investigate the sealing properties under this complex hydro-mechanical conditions taking into consideration the effect of technological voids. The microstructure of the material in its initial state was first examined by microfocus X-ray computed tomography (µCT). This allowed identification of the distribution of grains of sand and bentonite as well as the pores in the sample. Macro-pores are found concentrated at the periphery of the sample and between the grains of sand, which could affect in the short term the permeability. The hydration of the same material in limited swelling conditions was then observed by 2D photography and 3D µCT. The swelling mechanism with bentonite gel production, the swelling kinetics, the density decrease and the homogenisation of the material were analyzed. The hydration in the conditions of prevented swelling was also studied by swelling pressure tests with radial and axial measurements of swelling pressure. The difference found between the axial and radial swelling pressures suggested the presence of an anisotropic microstructure. Mock-up tests at a 1/10 scale of the in situ SEALEX tests were carried out for the study of the recovery capacity of the mixture in case of an accident causing the failure of the confining structures. Local measurements of swelling pressures along the sample allowed analysis of the density gradient evolution during axial swelling. Finally, a comparison between the laboratory results and those from an in-situ test was done, showing a good fitting in the relative humidity curves for the same infiltration length while considering the saturation effect from the technological void. The swelling pressure comparison was more complex because of the different configurations of the tests (existence of technological void in-situ that could affect the kinetics)

Dans un contexte politique où les enjeux énergétiques et environnementaux sont de plus en plus importants, il faut favoriser les techniques économes en énergie et plus respectueuse de l'environnement. Parmi ces pratiques, l'utilisation d’enrobés à l’émulsion de bitume s'est révélée être prometteuse. La fabrication et la conception d’une chaussée comprenant une couche d’enrobé à l’émulsion sont très empiriques et reposent sur des compétences locales qui tendent à en limiter le développement. La première étape pour concevoir une chaussée consiste à connaître le comportement mécanique réversible engendrées par de lourd trafic de chaque matériau qui la compose. Lors d'une deuxième étape, ces valeurs doivent être comparées aux critères de durabilité mécanique des matériaux. Dans le cas de l'enrobé à l’émulsion, aucune loi de comportement mécanique n'a été établie pour tenir compte à la fois de son états frais et durcis. Ce travail de thèse vise donc à pallier à ce manque et à améliorer les connaissances acquises sur ce matériau. Pour cela, un modèle évolutif pour l’enrobé à l’émulsion intégrant l'évolution du matériau de son état frais à l'état durci a été développé. De plus, un nouvel essai de caractérisation a été développé et mis en place pour permettre de suivre l’évolution des performances mécaniques réversibles du matériau. Les résultats obtenus à partir des différentes campagnes d’essais ont permis de caler le modèle et de montrer la pertinence du modèle. / Given a political context in which energy and environmental stakes have become increasingly dominant, road engineering practices have favoured saving energy and protecting the environment. Among these practices, the use of cold mixes treated with bitumen emulsion has proven to be a suitable technique. Cold mix design however, as well as the design of pavements including cold mix asphalt layers, is highly empirical and based on local skills. From prior experience, the transposition of established local rules from one site to another and their application to medium or heavy traffic pavements are not simple steps and tend to limit the development of this environmentally-friendly pavement technique. The first step in designing a pavement consists of knowing the stress-strain relationship of its constitutive materials in order to determine the stresses and strains generated by heavy lorry traffic. During a second step, these values must be compared with the mechanical durability of materials by reliance on damage tests comprising large numbers of load cycles. In the case of CMA, no mechanical behaviour law has been established to take into account both the fresh and cured states. This work aims to improve the knowledge acquired on this material. For this, an evolutiv model for the cold mix asphalt with bitumen emulsion incorporating the evolution of the material from its fresh state to its cured state has been developed. In addition, a new characterization test was developed and put in place to monitor the evolution of the reversible mechanical performance of the material. The results obtained from the test campaigns helped to calibrate the model and show the relevance of the model.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

The increasing use of polyethylene in diverse applications motivates the need for understanding how its molecular properties relate to the overall behaviour of the material. Although microstructure and mechanical properties of polymers have been the subject of several studies, the irreversible microstructural rearrangements occurring at large deformations are not completely understood. The purpose of this thesis is to describe how the concepts of Continuum Damage Mechanics can be applied to modelling of polyethylene materials under different loading conditions. The first part of the thesis consists of the theoretical formulation and numerical implementation of a three-dimensional micromechanical model for crystalline polyethylene. Based on the theory of shear slip on crystallographic planes, the proposed model is expressed in the framework of viscoplasticity coupled with degradation at large deformations. Earlier models aid in the interpretation of the mechanical behaviour of crystalline polyethylene under different loading conditions; however, they cannot predict the microstructural damage caused by deformation. The model, originally due to Parks and Ahzi (1990), was further developed in the light of the concept of Continuum Damage Mechanics to consider the original microstructure, the particular irreversible rearrangements, and the deformation mechanisms. Damage mechanics has been a matter of intensive research by many authors, yet it has not been introduced to the micromodelling of semicrystalline polymeric materials such as polyethylene. Regarding the material representation, the microstructure is simplified as an aggregate of randomly oriented and perfectly bonded crystals. To simulate large deformations, the new constitutive model attempts to take into account existence of intracrystalline microcracks. The second part of the work presents the theoretical formulation and numerical implementation of a three-dimensional constitutive model for the mechanical behaviour of semicrystalline polyethylene. The model proposed herein attempts to describe the deformation and degradation process in semicrystalline polyethylene following the approach of damage mechanics. Structural degradation, an important phenomenon at large deformations, has not received sufficient attention in the literature. The modifications to the constitutive equations consist essentially of introducing the concept of Continuum Damage Mechanics to describe the rupture of the intermolecular (van der Waals) bonds that hold crystals as coherent structures. In order to model the mechanical behaviour, the material morphology is simplified as a collection of inclusions comprising the crystalline and amorphous phases with their characteristic average volume fractions. In the spatial arrangement, each inclusion consists of crystalline material lying in a thin lamella attached to an amorphous layer. To consider microstructural damage, two different approaches are analyzed. The first approach assumes damage occurs only in the crystalline phase, i.e., degradation of the amorphous phase is ignored. The second approach considers the effect of damage on the mechanical behaviour of both the amorphous and crystalline phases. To illustrate the proposed constitutive formulations, the models were used to predict the responses of crystalline and semicrystalline polyethylene under uniaxial tension and simple shear. The numerical simulations were compared with experimental data previously obtained by Bartczak et al. (1994), G‘Sell and Jonas (1981), G‘Sell et al. (1983), Hillmansen et al. (2000), and Li et al. (2001). Our model’s predictions show a consistently good agreement with the experimental results and a significant improvement with respect to the ones obtained by Parks and Ahzi (1990), Schoenfeld et al. (1995), Yang and Chen (2001), Lee et al. (1993b), Lee et al. (1993a), and Nikolov et al. (2006). The newly proposed formulations demonstrate that these types of constitutive models based on Continuum Damage Mechanics are appropriate for predicting large deformations and failure in polyethylene materials.

The increasing use of polyethylene in diverse applications motivates the need for understanding how its molecular properties relate to the overall behaviour of the material. Although microstructure and mechanical properties of polymers have been the subject of several studies, the irreversible microstructural rearrangements occurring at large deformations are not completely understood. The purpose of this thesis is to describe how the concepts of Continuum Damage Mechanics can be applied to modelling of polyethylene materials under different loading conditions. The first part of the thesis consists of the theoretical formulation and numerical implementation of a three-dimensional micromechanical model for crystalline polyethylene. Based on the theory of shear slip on crystallographic planes, the proposed model is expressed in the framework of viscoplasticity coupled with degradation at large deformations. Earlier models aid in the interpretation of the mechanical behaviour of crystalline polyethylene under different loading conditions; however, they cannot predict the microstructural damage caused by deformation. The model, originally due to Parks and Ahzi (1990), was further developed in the light of the concept of Continuum Damage Mechanics to consider the original microstructure, the particular irreversible rearrangements, and the deformation mechanisms. Damage mechanics has been a matter of intensive research by many authors, yet it has not been introduced to the micromodelling of semicrystalline polymeric materials such as polyethylene. Regarding the material representation, the microstructure is simplified as an aggregate of randomly oriented and perfectly bonded crystals. To simulate large deformations, the new constitutive model attempts to take into account existence of intracrystalline microcracks. The second part of the work presents the theoretical formulation and numerical implementation of a three-dimensional constitutive model for the mechanical behaviour of semicrystalline polyethylene. The model proposed herein attempts to describe the deformation and degradation process in semicrystalline polyethylene following the approach of damage mechanics. Structural degradation, an important phenomenon at large deformations, has not received sufficient attention in the literature. The modifications to the constitutive equations consist essentially of introducing the concept of Continuum Damage Mechanics to describe the rupture of the intermolecular (van der Waals) bonds that hold crystals as coherent structures. In order to model the mechanical behaviour, the material morphology is simplified as a collection of inclusions comprising the crystalline and amorphous phases with their characteristic average volume fractions. In the spatial arrangement, each inclusion consists of crystalline material lying in a thin lamella attached to an amorphous layer. To consider microstructural damage, two different approaches are analyzed. The first approach assumes damage occurs only in the crystalline phase, i.e., degradation of the amorphous phase is ignored. The second approach considers the effect of damage on the mechanical behaviour of both the amorphous and crystalline phases. To illustrate the proposed constitutive formulations, the models were used to predict the responses of crystalline and semicrystalline polyethylene under uniaxial tension and simple shear. The numerical simulations were compared with experimental data previously obtained by Bartczak et al. (1994), G‘Sell and Jonas (1981), G‘Sell et al. (1983), Hillmansen et al. (2000), and Li et al. (2001). Our model’s predictions show a consistently good agreement with the experimental results and a significant improvement with respect to the ones obtained by Parks and Ahzi (1990), Schoenfeld et al. (1995), Yang and Chen (2001), Lee et al. (1993b), Lee et al. (1993a), and Nikolov et al. (2006). The newly proposed formulations demonstrate that these types of constitutive models based on Continuum Damage Mechanics are appropriate for predicting large deformations and failure in polyethylene materials.

Διαχρονικά, ένας από τους βασικότερους στόχους της αεροπορικής βιομηχανίας είναι η μείωση του βάρους των αεροχημάτων προκειμένου να επιτευχθεί αύξηση του οφέλιμου φορτίου και παράλληλα μείωση των περιβαλλοντικών επιπτώσεων μέσω της μείωσης εκπομπής ρύπων. Στο πλαίσιο αυτό, η αξιοποίηση ελαφρύτερων μεταλλικών υλικών, όπως είναι για παράδειγμα τα κράματα μαγνησίου, μπορεί να αποτελέσει σημαντική τεχνολογική καινοτομία. Παρολ’ αυτά, μέχρι σήμερα η χρήση των κραμάτων μαγνησίου, και ειδικότερα των ελατών προιόντων, είναι εξαιρετικά περιορισμένη κυρίως λόγω της υψηλής διαβρωτικότητάς τους και δευτερευόντως λόγω της υποδεέστερης συμπεριφοράς ανοχής σε βλάβη σε σύγκριση με τα ευρέως χρησιμοποιούμενα κράματα αλουμινίου και τιτανίου. Στην παρούσα εργασία πραγματοποιείται μια συστηματική μελέτη της μηχανικής συμπεριφοράς δύο προηγμένων ελατών κραμάτων μαγνησίου της οικογένειας ΑΖ, συγκεκριμένα του ΑΖ31 και του ΑΖ61, λαμβάνοντας υπόψη τους μηχανισμούς παραμόρφωσης, συσσώρευσης βλάβης και αστοχίας που λαμβάνουν χώρα στη μικροδομή των υλικών. Ιδιαίτερη βαρύτητα δίνεται στη συμπεριφορά κόπωσης του κράματος ΑΖ31. Επιπλέον, μελετάται η επίδραση της προηγηθείσας βλάβης διάβρωσης στη μηχανική συμπεριφορά των υλικών. Για την αξιολόγηση της μηχανικής επίδοσης των εν λόγω κραμάτων πραγματοποιήθηκε εκτενής πειραματική μελέτη η οποία περιελάμβανε το χαρακτηρισμό της μικροδομής των υλικών, μηχανικές δοκιμές εφελκυσμού, κόπωσης και διάδοσης ρωγμής κόπωσης τόσο σε αδιάβρωτα όσο και σε προ-διαβρωμένα δοκίμια και ακολούθησε μεταλλογραφική ανάλυση και μελέτη των επιφανειών θραύσης των αντίστοιχων δοκιμίων των πειραματικών δοκιμών. Τα αποτελέσματα των δοκιμών εφελκυσμού έδειξαν ανισοτροπία των υλικών στις διευθύνσεις έλασης και κάθετα σε αυτήν. Από τη μεταλλογραφική ανάλυση που επακολούθησε προέκυψε ότι η παρατηρούμενη ανισοτροπία έχει αφετηρία τη γωνιακή διασπορά των πόλων των επιπέδων βάσης του κρυσταλλικού πλέγματος του υλικού. Επιπλέον, από το μεταλλογραφικό έλεγχο παρατηρήθηκε εμφανής διακύμανση της πυκνότητας των διδυμιών κατά μήκος των δοκιμίων εφελκυσμού και διαπιστώθηκε ο καθοριστικός ρόλος των διδυμιών στην πλαστική διαρροή στη διεύθυνση της έλασης. Ως προς τη συμπεριφορά κόπωσης, παρατηρήθηκε ότι οι καμπύλες S-N παρουσιάζουν μια πολύ ήπια μετάβαση από την περιοχή της ολιγοκυκλικής στην πολυκυκλική κόπωση, δηλαδή ότι η διάρκεια ζωής σε κόπωση εξαρτάται ισχυρά από μικρές μεταβολές της τάσης. Οι ρωγμές κόπωσης στο κράμα ΑΖ31 εκκινούν πρόωρα σε σημεία ασυμβατότητας πλαστικής παραμόρφωσης (π.χ. όρια των κόκκων) λόγω της αδυναμίας ενεργοποίησης των απαραίτητων 5 συστημάτων ολίσθησης που απαιτεί το κριτήριο του von Mises. Ως εκ τούτου το υλικό οδηγείται σε ψαθυρούς μηχανισμούς εκκίνησης και διάδοσης των ρωγμών κόπωσης. Για την καλύτερη κατανόηση του μηχανισμού κόπωσης του κράματος ΑΖ31 πραγματοποιήθηκαν δοκιμές νανο-διεισδύσεων σε διαφορετικά ποσοστά της διάρκειας ζωής προκειμένου να γίνει αντιληπτός ο μηχανισμός συσσώρευσης βλάβης στο αρχικό στάδιο της συνολικής διαδικασίας. Τα αποτελέσματα των εν λόγω δοκιμών έδειξαν ότι η επιφανειακή σκληρότητα του υλικού δεν παρουσιάζει ουσιαστική μεταβολή με τους κύκλους καταπόνησης μέχρι την εμφάνιση της ρωγμής. Τα στοιχεία αυτά αποτελούν μια ισχυρή ένδειξη ότι το υλικό αδυνατεί να συσσωρεύσει βλάβη υπό τη μορφή κυκλικής πλαστικότητας, με αποτέλεσμα την πρόωρη εκκίνηση των ρωγμών κόπωσης. Οι μηχανικές δοκιμές σε προ-διαβρωμένο υλικό έδειξαν, όπως ήταν αναμενόμενο, μια σημαντική υποβάθμιση της συνολικής μηχανικής συμπεριφοράς των υλικών. Η υποβάθμιση αυτή αποδίδεται στην προοδευτική ανάπτυξη των τριμμάτων διάβρωσης κάτω από εφελκυστικά φορτία. Τα τρίμματα διάβρωσης δρούν ως εγκοπές, αυξάνοντας τοπικά την τάση και παράλληλα μειώνοντας τη φέρουσα διατομή των δοκιμίων, με αποτέλεσμα το υλικό να αστοχεί χωρίς να προλάβει να δεχθεί σημαντικές πλαστικές παραμορφώσεις. Στην περίπτωση της κυκλικής φόρτισης η παρουσία των εγκοπών διάβρωσης έχει ως αποτέλεσμα τη δημιουργία συγκέντρωσης τάσεων στα άκρα τους, διευκολύνοντας έτσι την εκκίνηση και διάδοση των ρωγμών κόπωσης. Συμπερασματικά, η παρούσα εργασία παρέχει σαφείς ενδείξεις ότι το κύριο μειονέκτημα των κραμάτων μαγνησίου για χρήση σε αεροπορικές δομές είναι η συμπεριφορά κόπωσης, η οποία αποδίδεται στην κρυσταλλική δομή του μαγνησίου, και δευτερεύον μειονέκτημα είναι η υψηλή διαβρωτικότητα αυτών των υλικών η οποία οδηγεί σε σημαντική υποβάθμιση της μηχανικής συμπεριφοράς. / Permanent objective of the aeronautical industry is the weight reduction of airframe, systems and interior components in order to increase operational capacity and reduce environmental impact via reduction of fuel consumption. In this frame, the utilization of low weight materials, like magnesium alloys, could represent a break through solution. Yet, the aeronautical application of magnesium alloys remains very limited due to the high corrosion susceptibility and the poor damage tolerance behaviour as compared to other structural alloys like aluminum and titanium. In the present work, a systematic investigation of the mechanical behaviour of two advanced rolled AZ magnesium alloys, namely AZ31 and AZ61, was conducted by taking into account the deformation mechanisms, damage accumulation mechanisms and failure mechanisms taking place in the microstructure of the materials. The present work mainly focuses on the fatigue behaviour of AZ31 alloy. Furthermore, the effect of prior corrosion damage on the mechanical behaviour has also been assessed. To accomplish the above objective a thorough experimental investigation was performed including microstructural characterization, tensile tests, constant amplitude fatigue tests and constant amplitude fatigue crack growth tests on both parent and pre-corroded specimens. The experimental results were supported by extensive metallographic and fractographic investigation. The tensile tests performed revealed anisotropy of the yield strength of the materials between rolling and transverse direction. The metallographic analysis has shown that the observed anisotropy is attributed to the near basal texture of the alloys and the angular spread of basal poles towards the rolling direction. Furthermore, the metallographic investigation indicates a clear variation in twinning density across the specimen length and the decisive role of twins in plastic deformation has been pointed out. Concerning the fatigue behaviour, it was observed that the S-N curves exhibit a very smooth transition from low to high cycle fatigue regime, indicating very high stress sensitivity on the fatigue life of the materials. Fatigue cracks in AZ31 alloy initiate in an early stage between strain incompatibility points (e.g. grain boundaries) due to difficulties in satisfying the von Mises criterion. As a result, the initiation and propagation mechanisms of the fatigue cracks are characterized as cleavage. In order to understand the fatigue mechanism of magnesium alloy AZ31 in the early stages of fatigue damage accumulation process, nano-indentation measurements at different percentages of the fatigue life of the AZ31 alloy have been performed and hardness alteration was obtained. The obtained results have shown that nano-hardness remains unchangeable with fatigue cycles until crack initiation. This has been interpreted as a lack of the material’s ability to accumulate damage in terms of cyclic plasticity at the early stages resulting in very early crack initiation. This is a major disadvantage for application where fatigue life is of primary importance. The mechanical tests on pre-corroded specimens have shown a significant degradation of the overall mechanical behaviour of the materials. Tensile properties degradation due to prior corrosion damage is attributed to the progressive notch effect of the developed pits, which increase locally the applied stress and in parallel reduce the ability of the material to accumulate large amounts of plastic deformation. In the case of cyclic loading the presence of corrosion pits results in the development of stress concentration, facilitating essentially the initiation and propagation of fatigue cracks. Concluding, the present work provides evidence that the major disadvantage of magnesium alloys for use in aeronautical structures is their fatigue behaviour, which is attributed to the hexagonal structure of magnesium, and secondarily the high corrosion susceptibility of magnesium which leads to significant degradation of the mechanical performance of the alloys.