This thesis concerns a study of the three-body abrasive wear behaviour of two groups of materials with different abrasive particles using the Dry Sand Rubber Wheel (DSRW) test method. This investigation can be divided into three sections: In the first section, the abrasion of a range of steels with an ash from a biomass power station was compared with that observed for abrasion with a conventional silica abrasive. It was seen that the wear rate of the steels when abraded with silica increased in proportion to the applied load and decreased with the hardness of the steel. However, the bottom-ash was more friable than the silica abrasive, and as such, significantly more abrasive crushing was observed during the tests with the bottom-ash abrasive. It is proposed that the wear is dominated by abrasion by the larger particles in the distribution, and that damage is limited by the maximum load which the particles can sustain before failing. In the second section, the motion of particles in the DSRW test with silica abrasive against a range of steels, as a function of applied load and the hardness of the steels was studied. The results showed that particle rolling through the contact is favoured by low applied loads and low testpiece hardness whereas particle sliding through the contact is favoured by high applied loads and high testpiece hardness. A model was proposed to provide an analysis of the motion of particles in the DSRW test. The effect of hardness on particle rotation is well predicted by the model, but the effect of the applied load on particle motion observed experimentally is opposite to that which is predicted by the model. The shortcomings of the model are discussed, and the model has been qualitatively modified to account for this discrepancy. In the third section, five different WC-metal cermet powders were deposited as coatings by HVOF thermal spraying. These were a WC-nickel alloy, a WC-iron alloy and three types of WC-Co powders with different carbide grain sizes. Characterisation of the coatings showed decomposition of WC during spray process for all the coatings. The results show different solubilities of W and C in the binders and different precipitation characteristics. DSRW tests were performed to assess the wear resistance of the coatings with silica and alumina abrasives. It was found that the coatings had different wear rates and mechanisms when abraded with silica compared with alumina. The differences in the wear behaviour of the coatings are due to the differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates.

This thesis describes the investigation and development of damage modelling for composites materials at their micro-scale (e.g. fibre, matrix). A damage model for elastic materials, based on a "local" damage approach, has been introduced to predict failure onset and simulate the post-failure behaviour of unidirectional threedimensional representative volume elements (RVE) or unit cells with hexagonal distribution of the fibres over the cross section. The damage model consists of three parts: an elastic model, a failure criterion and the post-failure behaviour. Modifications of von Mises criteria and Maximum Principal Stress criterion have been considered to evaluate failure in the matrix whilst for the fibre in general the Maximum Principal Stress criterion has been used. The damage model has been implemented into the commercial code ABAQUS with subroutines in FORTRAN (UMAT and USDFLD). The material properties in the residual stress analyses are considered temperature dependant to simulate the volumetric contraction during the manufacturing process. Hence, the overall residual stress introduced from curing was determined by considering two ontributions: volume shrinkage of matrix resin from the crosslink polymerization during isothermal curing and thermal contraction of both resin and fibre as a result of cooling from the curing temperature to room temperature. Finally, three different typologies of 3D unit cells have been investigated. The first kind of micro-model is based on a symmetric distribution of the fibres and the unit cells have two phases, i.e.: matrix and fibre. The second typology of unit cells is still based on a uniform architecture but include a three-dimensional interphase between fibre and matrix. As in real composites at their constituent level fibres are randomly distributed. The mutual distance between fibres represents a critical factor for the ultimate mechanical properties of the micro-composites. Hence the last kind of micro-models account for this non-uniform position of fibres within the RVE although they consists of only two phases. FEM analyses have indicated that predicted damage initiation and evolution are clearly influenced by the presence of residual stresses in all the three different typologies of unit cells analysed. The numerical analyses on the numerical models have proved that, in general, the overall mechanical properties are strongly influenced by the presence of residual stress, fibre volume fraction, fibre distribution and interphasial properties. In particular on transverse tensile loading, residual stresses produces beneficial results in terms of ultimate strength while in the case of longitudinal loading (parallel to the fibres) the matrix, due to the high compressive stress, undergoes a premature failure although.

Les propriétés thermiques des nanomatériaux offrent une palette variée de comportements. Parfois, le transfert thermique est dégradé comparé à l'échelle macroscopique, produisant un échauffement dans les nanodispositifs électroniques et une augmentation de l'efficacité des modules thermoélectriques. D'autres fois, la conductivité thermique atteint des niveaux supérieurs à ceux des matériaux massifs. Cette thèse étudie le transport de chaleur dans des nanostructures de silicium par simulation à l'échelle atomique. La méthode de la dynamique moléculaire est utilisée dans un cadre original basé sur l'exploitation de transitoires de température, la dynamique moléculaire d'approche à l'équilibre (AEMD i.e. Approach-to-Equilibrium Molecular Dynamics). Après avoir présenté et illustré l'AEMD dans le cas du silicium massif, nous étudions la conductivité thermique de nanofils lisses en fonction de leur diamètre et de la longueur. Nous montrons que le profil de température est conforme à l'équation de la chaleur et que la conductivité thermique des nanofils sature à grande longueur. Nous étudions ensuite l'effet d'une nanostructuration par évidement et de la rugosité de surface. Nos résultats sont comparés à une approche de simulation alternative pour explorer le poids relatif des collisions phononiques internes et surfaciques. Enfin, nous nous intéressons à la conductivité thermique de membranes de silicium nanostructurées fabriquées au sein du laboratoire. Nous montrons que, pour réduire davantage la conductivité thermique et ainsi augmenter l'efficacité de conversion d'un dispositif thermoélectrique, il faudra davantage de nanostructuration que des évidements cylindriques. / The thermal conductivity of nanostructures varies on a large scale compared to bulk materials. Sometimes the thermal transfer is worse due to the reduction of thermal conductivity, at the origin of self-heating in electronic devices, but valuable for thermoelectric devices. In other cases, the thermal conductivity reaches impressive levels compared to bulk materials. The origin of these behaviors is at the atomic level, and the in the propagation of the heat carriers, the phonons. In this thesis, we study heat transport in silicon nanostructures by atomic scale simulations. Molecular dynamics is used here in the framework of a methodology we recently developed, the approach-to-equilibrium molecular dynamics (AEMD). This methodology relies on the creation and exploitation of temperature transients. First we present the principles of AEMD in the case of bulk silicon. Then we determine the thermal conductivity of smooth nanowires versus diameter and length. We show that the temperature profiles do comply with the heat equation, and that the thermal conductivity saturates at long length. Afterwards we study nanostructuring effects by hollowing, and nanowires with rough surfaces. In this last case, we compare our results with an alternative simulation approach and investigate the relative importance of intrinsic and surface phonon scattering. Finally, we calculate the thermal conductivity of nanopatterned silicon membranes for thermoelectric devices. We show that to further decrease the thermal conductivity, it will be necessary to introduce other sources reduction than cylidrincal hole etching, and AEMD will be the appropriate tool for an optimisation.

Οι προσπάθειες παρέμβασης στη φθορά των ιστορικών μνημείων μέχρι σήμερα έχουν δύο κατευθύνσεις: την «επούλωση των τραυμάτων» και την επιβράδυνση ή πλήρη παρεμπόδιση περεταίρω φθοράς. Η παρούσα εργασία έχει ως στόχο τη μελέτη του μηχανισμού διάλυσης του μαρμάρου Carraras σε συνθετικό νερό βροχής (artificial rain water) και σε σταθερά θερμοκρασία και pH. Η επιλογή του μαρμάρου Carraras έγινε λόγω της σύστασής του (>98% ασβεστίτης) . Από τα αποτελέσματα των πειραματικών μετρήσεων θα ληφθούν συμπεράσματα για τον τρόπο διάλυσης του μαρμάρου και θα βοηθήσουν στην κατανόηση του μηχανισμού της διάλυσης. Οι εργασίες που πραγματοποιούνται για τον σκοπό αυτό είναι : Η μέτρηση του ρυθμού διάλυσης του μαρμάρου σε διαφορετικές συνθήκες ακορεστότητας. Ο προσδιορισμός του μηχανισμού διάλυσης από το διάγραμμα του ρυθμού διάλυσης R συναρτήσει της σχετικής ακορεστότητας σ. Η διερεύνηση της δυνατότητας επιβράδυνσης ή και πλήρους αναστολής της διάλυσης του μαρμάρου παρουσία υδατοδιαλυτών ενώσεων. / Corrotion of marbles from water

Η ηλεκτρική απόκριση ελαστομερικών συστημάτων HNBR, FKM και μειγμάτων αυτών, καθώς και νανοσύνθετων μειγμάτων HNBR/FKM ενισχυμένων με νανοσωλήνες άνθρακα πολλαπλού τοιχίου είναι το αντικείμενο της παρούσης εργασίας. Η ηλεκτρική απόκριση προσδιορίζεται με την μέθοδο της διηλεκτρικής φασματοσκοπίας ευρέως φάσματος. / In the present work the electric response of nanocomposites material consisted of elastomer blends as matrix and modified multiwall carbon nanotubes (MWCNT) as reinforcing phase, is studied. The elastomer blends we studied was a mixture of HNBR and FKM elastotmers in 50/50 and 75/25 ratio respectively. Carbon nanotubes 10phr were added in both blends. Specifically, the dielectric response and the conductivity of elastomer nanocomposites is examined with parameters the content in MWCNT, FKM, the temperature and the frequency of the applied electric field. The dielectric response of those composite elastomer systems was studied in a wide frequency and temperature range.

This thesis aims to model the uniaxial deformation of a class of materials consisting of microscopic spherical shells embedded in a rubber matrix. These shells are assumed to buckle as the stress on the material increases. To motivate the analysis we consider the paradigm problem of the debonding of a distribution of cylindrical inclusions in an elastic material undergoing antiplane shear, with bonded and debonded inclusions playing the role of unbuckled and buckled shells respectively. We begin the modelling of the microsphere-containing material by considering the buckling of an isolated embedded shell inclusion with a uniaxial stress field at infinity, using Koiter's theory of shallow shells. The resulting energy functional is solved as an eigenvalue problem by the Rayleigh-Ritz method. Subsequently, we analyse the buckling criterion asymptotically in the limit as the thickness ratio tends to zero by analogy with the WKB analysis of a beam on a variable-stiffness substrate. To model the shell after buckling we consider the simplified case of an embedded shell with a crack around its equator. The system is solved by expressing the displacements in the shell and matrix as series of Love stress functions, with the resulting infinite system of equations solved numerically with the aid of a convergence acceleration method. Finally we consider a composite material consisting of a homogenised dilute distribution of buckled and unbuckled shells, with the proportion of each type of shell dependent on the stress applied to the material, according to an asymptotic formula relating the size of the inclusions and the critical buckling stress that was obtained previously.

Honeycomb sandwich structures (commonly referred to as honeycomb sandwich panels) have found wide spread application in the aerospace industry thanks to their excellent properties, in particular their high strength-to-weight and high stiffness-to-weight ratios. Surrey Satellite Technology Ltd. (SSTL), like many other space companies, often use honeycomb sandwich panels as part of the primary and secondary structures of the small satellites they develop. Although honeycomb panels have been used for the past 50 years gaining a better understanding of these sandwich structures, and the methods and solutions used to produce structural assemblies from them is still a major concern in the aerospace industry. Whether directly or indirectly, there are still significant research efforts ongoing that affect these areas. This work focuses on some of these issues and covers several research fields including material science, tribology and adhesive bonding technology. The first area of focus of this work deals with the structural performance of honeycomb panels alone and mainly concentrates on hexagonal honeycomb cores. An experimental investigation using the rail shear test was conducted to study the shear behaviour of hexagonal honeycomb cores. This involved both static and fatigue tests using numerous honeycomb panel test samples with the loading direction at various angles to the core ribbon. From these tests it was found that core shear strength did not have a linear relationship with loading orientation and that contrary to what is commonly assumed the transverse direction (to the ribbon) is not always necessarily the weakest orientation. The optimal design and performance of the load introduction points was the second area of focus for this work which covers equipment inserts and bolted joints. Two types of inserts where investigated in this work: hot bonded inserts and cold bonded inserts. A study on hot bonded and cold bonded inserts was conducted to assess their performance and effectively compare the two insert systems. A large portion of the study was experimental and involved carrying out numerous insert pull-out tests to measure static pull strength capability. From the study it was found that contrary to what was expected cold bonded potted inserts outperformed the hot bonded inserts in terms of static strength capability. Using finite element it was found that this was due to the different filler materials used for the two insert systems. The last area covered in this work concerns friction grip bolted joint between honeycomb panels. Here a simple method to analyze the efficiency of shear joint units is proposed. An extensive test campaign was also carried out to determine the influence of various parameters on the friction coefficient. Surface abrasion was found to be a reliable way of achieving high values of friction coefficient.

In this work multi-scale CT techniques have been developed to characterise the material structure of PMCs, from the whole engineering structure geometry down to individual fibre level. The techniques have first been applied to a 'model' aerospace grade carbon/epoxy notched laminate loaded in-situ in tension to failure. The material structure and damage mechanisms of internally pressurised experimental cylinders have then been investigated and compared to the notched laminate. It was found the damage accumulation of both samples to be comparable, where fibre breaks were the dominant strength controlling mechanism. The data provides, to the authors knowledge, the first direct internal 3D measurement of the accumulation of fibre damage for commercial CFRP materials under structurally relevant load conditions. A high level of confidence is placed in the measurements, as the failure processes are viewed internally at the relevant micromechanical length-scales, as opposed to previous indirect and/or surface-based methods. Whilst fibre breaks are the dominant composite damage mechanism considered in the work, matrix damage was also seen to occur in advance of extensive fibre breaks. The formation of clusters of broken fibres were observed at high loads in both sample types. The largest clusters were observed in the notched laminate sample, consisting of a group of eleven breaks and a group of fourteen breaks. In comparison, clusters of only four neighbouring breaks were observed in the pressure vessel samples. A correlation between fibre volume fraction and fibre breaks was found, in which higher fibre volume fractions result in higher fibre break densities. No strong correlation was found between the location of matrix damage and fibre breaks in both sample types. Initial analysis showed some correlation between fibre breaks and voids, however further work has been recommended to confirm this. A simple 3D Finite Element Analysis was carried out for the pressure vessel to give an understanding of the stress partitioning through the composite layers and confirm the damage found experimentally. The data sets of the accumulation of fibre breaks with load provide evidence to validate or inform existing micromechanical models, for two different carbon fibre systems, where previous experimental findings are limited. A detailed comparison of the results of this work and the multi-scale micromechanical model of Blassiau and co-workers has been made, in which the underlying assumptions have been discussed.

Les réfractaires alumine-carbone sont largement utilisés dans la coulée continue de l'acier. Ils sont responsables du contrôle du flux d'acier et de sa protection contre l'oxydation. Cependant, pour améliorer leur propre résistance à l'oxydation, plusieurs additifs tels que des carbures, des métaux, des composés à bas point de fusion et des frittes de verre sont ajoutés en tant qu'antioxydants. Dans cette étude, l'influence de ces additifs, ainsi que des conditions de cuisson, sur les propriétés liées à la résistance aux chocs thermiques des matériaux réfractaires à matrice carbonée a été étudiée. Des matériaux modèles, ayant une composition simplifiée par rapport aux matériaux industriels, ont été étudiés pour faciliter la compréhension des interactions entre les différents constituants du composite. Le comportement des matériaux réticulés (avant cuisson) et cuits a été étudié. La première partie de l'étude a montré que les antioxydants peuvent modifier les propriétés du réfractaire par différents mécanismes : cristallisation de la liaison carbone (B4C), guérison des microfissures (borax et fritte de verre) et formation de nouvelles phases (Al-Si). Cependant, puisque les additifs métalliques sont les antioxydants les plus largement utilisés dans les produits de Vesuvius, la deuxième partie de l'étude a porté sur leur impact (Al-Si,Al-Mg et Al) sur les propriétés clés influençant la résistance aux chocs thermiques. En plus, l'impact des conditions de cuisson (température et atmosphère) a également été étudié. Il a été prouvé que tous les antioxydants métalliques réagissent avec d'autres constituants pour former de nouvelles phases qui à la fois rigidifient le réfractaire et augmentent sa valeur du coefficient de dilatation thermique. Malheureusement, un tel comportement entraîne une détérioration de la résistance aux chocs thermiques. De plus, il s'est avéré que l'atmosphère de cuisson a une influence négligeable sur la réactivité des additifs et donc sur les propriétés finales du réfractaire. Ainsi, la température de cuisson est le paramètre qui a l'impact le plus important sur l'évolution réfractaire. Tous les résultats obtenus facilitent le choix des antioxydants et des conditions de cuisson pour obtenir les propriétés souhaitées du réfractaire. / Alumina-carbon refractories are widely used in the continuous casting of steel. They are responsible for the steel flow control and its protection against oxidation. However, to improve their own oxidation resistance, several additives such as carbides, metals, low melting point compounds and glass frits are added as antioxidants. In this study, the influence of these additives, as well as firing conditions, on the properties related to the thermal shock resistance of carbon-bonded refractories was studied. Model materials, having simplified composition compared to the real industrial ones, were investigated tofacilitate the comprehension of interactions between different constituents of the composite. Behavior of both cured (before firing) and fired materials was studied. The first part of the study proved that antioxidants may modify the properties of the refractory through different mechanisms: crystallization of the carbon bond (B4C), microcracks healing (anhydrous borax and glass frit) and formation of new phases (Al-Si). However, since the metallic additives are the most widely used antioxidants in Vesuvius' products, the second part of the study was focused on their (Al-Si, Al-Mg and Al) impact on the key properties influencing the thermal shock resistance. What is more, the impact of firing conditions (temperature and atmosphere) was also investigated. It was proved that all metallic antioxidants react with other constituents to form new phases which both rigidify the refractory and increase its value of the coefficient of thermal expansion. Unfortunately, such behavior results in worsening of the thermal shock resistance. Moreover, it turned out that the firing atmosphere has a negligible influence on the additives reactivity and thus the final properties of the refractory. Thus, firing temperature is the parameter that has the most important impact on the refractory evolution. All the obtained results facilitate the choice of antioxidants and firing condition to obtain desired properties of the refractory.

Le travail de la thèse porte sur une étude globale des interactions qui surgissent sur le système projectile/protection/cible lors d’un impact balistique non perforant. L’objectif principal consiste à l’analyse et à l’évaluation des modes d’endommagement des projectiles et des protections d’une part, et les déformations occasionnées sur la cible, d’autre part. Une étude expérimentale a permis la caractérisation mécanique des différents matériaux utilisés à savoir : la gélatine, la plastiline, ainsi que l’analyse des réponses balistiques utilisant des protections en composite, en aluminium et textiles. L’accent a été mis sur la recherche des lois de comportement appropriées pour les différents composants du système balistique utilisant des multi-matériaux. Une simulation numérique du thorax humain a été développée, exploitée et validée sur la base d’un impact non pénétrant. La simulation numérique a été réalisée en prenant en compte à la fois le thorax et les différents organes du corps humain en intégrant aussi la présence de protections. / This work deals with interactions occurring on the projectile/protection/target system during a non-perforating ballistic impact. The first main goal concerns the analysis and evaluation of the processes’ damage projectiles and protections. The second one concerns the deformation of the target. An experimental study has allowed the mechanical characterisation of different used materials: the gelatine, the plastilina, and the analysis of the ballistic response of the composite, aluminium and textile based-protections. The research carried out here is focused on the appropriate material laws of different components of the ballistic system implicating the multi-materials. A numerical simulation of the human thorax has been developed, implemented and validated thanks to a non-penetrating ballistic impact. The numerical simulation has been realised by taking into account of both the thorax and the different organs of the human body protected by a ballistic protection.