No description available.

Copper (Cu) has high thermal conductivity and is thus ideal for high heat flux, thermal heat sink applications in fusion power applications. Divertor designs for future fusion power plants will expose Cu alloys to extreme thermal (>10 MW m^-2) and neutron fluxes (200 dpa) that destabilise the microstructure of Cu. To improve stability, strength and creep resistance, alloying additions are used commercially, but these compromise thermal conductivity. Dispersed oxide particles may offer the opportunity for improved mechanical stability and creep resistance even at very low volume fractions (<1%) while avoiding large reductions in thermal conductivity. However there are few studies on the processing-performance of oxide dispersion strengthened Cu alloys. In this thesis, novel oxide dispersion strengthened Cu alloys were prepared by room temperature mechanical alloying of Y₂O₃ and the mechanism of dispersion investigated. A small fraction of Y₂O₃, up to 1%, was shown to disperse effectively during mechanical alloying at room temperature in Cu. Both severe fragmentation and some local disassociation of the Y and O occurred, allowing for re-precipitation of fine nanoparticles <10 nm during consolidation and exposure to elevated temperatures. A model alloy of Cu-2 wt.% Y₂O₃ alloy had a mean oxide particle diameter of 7.1 ± 6.0 nm and a mean particle number density of 8.24 x 10²² m^-3 following consolidation. Ternary Ti additions were investigated for nanoparticle refinement and best design alloy with a composition of Cu-0.25Y₂O₃-0.25Ti was produced that had a mean nanoparticle diameter of 3.2 ± 1.5 nm and a mean particle number density of 2.36 x 10²³ m^-3 , which after thermal ageing for 545 h at 350 °C, was largely unchanged at 3.8 ± 1.7 nm, and 1.74 x 10²³ m^-3 respectively. Comparing favourably with commercial Al₂O₃ dispersion strengthened Cu, the alloy had a narrower particle size distribution and a higher particle number density. The fine dispersed oxide nanoparticles gave good grain boundary pinning, retaining an ultrafine mean grain size of 220 nm after thermal ageing. Thermal conductivity of the Ti-containing alloy was 332 ± 16W m_-1 K_-1 , and the addition of Ti increased the thermal conductivity with increasing temperature. The creep resistance was evaluated by small punch testing and in-house produced alloys outperformed commercial alloys at 350 °C. The work in this thesis indicates that mechanically alloyed Cu-Y₂O₃ or Cu-Y₂O₃-Ti alloys, with further development and evaluation, have potential as thermal heat-sink materials for fusion divertor application.

Fatigue and dispersion strengthening are reviewed generally. Experiments are described in which thin film transmission electron microscopy, as well as optical and electron fractography were employed to elucidate the mechanism of fatigue failure in dispersion strengthened nickel. It was found that whereas fatigue crack initiation occurs as in conventional materials, the propagation of both fatigue and tensile cracks is abnormal. The material fails in shear wherever possible although this is prevented where triaxial tensile stresses exist. Particle-matrix detachment does not occur during either tensile or fatigue stressing. / Thesis / Master of Science (MS)

Approved for public release; distribution is unlimited / The US Navy is currently developing a new family of high-strength , low-alloy steels which derive a significant portion of their strength from copper precipitation. These highly weldable steels require little or no preheat. resulting in substantial cost savings. The first of these steels. HSLA-80, has been certified for ship construction, but recent studies have indicated some susceptibility to stress relief cracking in weldments. HSLA-100, a modification of HSLA-80, is now being considered for several higher-strength naval structures. Stress-relief cracking has not been studied previously in this steel and is the subject of investigation in this work. The steel weldments were loaded below their yield strength, heated to temperatures of 550°-650° C, and permitted to stress relieve for one hour. At all temperatures, the steel exhibited susceptibility to stress relief cracking in certain stress ranges. Optical and scanning electron microscopy exhibited intergranular cracking which always traversed the coarse-grained region of the heat-affected zone. Auger and transmission electron microscopy indicated high concentrations of alloying elements at the grain boundaries. Stress-relief cracking was associated with the diffusion of alloying elements to the prior austenite grain boundaries. / http://archive.org/details/stressreliefcrac00mcnu / Captain, Canadian Forces

Ο σκοπός αυτής της διπλωματικής εργασίας είναι η σύγκριση της αποτελεσματικότητας της ενίσχυσης, στοιχείων από φέρουσα τοιχοποιία, με μανδύες συνθέτων υλικών, οι οποίοι αποτελούνταν από στρώσεις ανθρακοϋφάσματος, με οργανική ή ανόργανη μήτρα και με τοποθέτηση ράβδων οπλισμού, από ανθρακονήματα, μέσα στους συνεχείς αρμούς της τοιχοποιίας. Αρχικά δίνονται μορφολογικά και γενικά τεχνικά χαρακτηριστικά. Γίνεται μια σύντομη ιστορική ανασκόπηση για την τοιχοποιία και ακολουθεί ο διαχωρισμός της σε βασικές κατηγορίες. Αναφέρονται οι πρώτες ύλες των τεχνητών λιθοσωμάτων και η διαδικασία παραγωγής τους και στη συνέχεια σημειώνονται τα βασικά είδη τους, με τις συνήθεις διαστάσεις και οι τεχνικές προδιαγραφές τους. Γίνεται ακόμα αναφορά στις κατηγορίες και στις ιδιότητες των κονιαμάτων και των επιχρισμάτων που χρησιμοποιούνται. Ακολουθεί μια περίληψη της μηχανικής της τοιχοποιίας. Περιγράφεται η λειτουργία της άοπλης τοιχοποιίας και ο προσδιορισμός της αντοχής της, υπό θλιπτικά, καμπτικά και διατμητικά φορτία, κατά τον Ευρωκώδικα 6 (EC6) και Τάσιο. Περιγράφονται επίσης τα ελαστικά χαρακτηριστικά της τοιχοποιίας και ο τρόπος, με τον οποίο αυτή συμπεριφέρεται υπό οριζόντια πλευρική φόρτιση, ενώ φέρει θλιπτικά φορτία. Στη συνέχεια γίνεται περιγραφή των ενισχύσεων κατασκευών με σύνθετα υλικά. Κατηγοριοποιούνται τα σύνθετα υλικά και δίνονται οι ιδιότητες των συνθέτων ινο-πλισμένων υλικών. Αναφέρονται εν συντομία οι ιδιότητες των υλικών, ινών και ρητινών, που χρησιμοποιούνται συνήθως για την κατασκευή ινοπλισμένων πολυμερών. Ακόμα αναφέρονται τα βασικά συστήματα ενίσχυσης και οι ιδιότητες των συνθέτων υλικών. Ακολουθεί η μικρομηχανική των υλικών αυτών και οι επιπτώσεις, των περιβαλλοντικών συνθηκών, στην ανθεκτικότητά τους. Επίσης, γίνεται μια περιληπτική αναφορά στις τεχνικές ενισχύσεων δομικών στοιχείων με σύνθετα υλικά. Έπεται μια σύντομη αναφορά στις βάσεις σχεδιασμού ενισχύσεων με σύνθετα υλικά. Δίνονται οι καταστατικοί νόμοι των υλικών για οριακή κατάσταση αντοχής, τόσο για πλήρη συνεργασία όσο και για αποκόλληση του συνθέτου υλικού από την κατασκευή, και για οριακή κατάσταση λειτουργικότητας. Επίσης σχολιάζεται και το θέμα της συνάφειας των συνθέτων υλικών με το υπόστρωμα (τοιχοποιία). Ακολούθως περιγράφονται τα υλικά κατασκευής και ενίσχυσης, τα πειραματικά δοκίμια και δίνονται οι μηχανικές τους ιδιότητες. Ακόμα γίνεται περιγραφή των ειδών των δοκιμίων, που δημιουργήθηκαν, της διαδικασίας ενισχύσεώς τους, σύμφωνα με τους τρόπους που είχαν αποφασιστεί, δηλαδή με κατασκευή μανδυών και με τοποθέτηση ράβδων οπλισμού από σύνθετα υλικά. Επίσης περιγράφονται εν συντομία ο μηχανικός εργαστηριακός εξοπλισμός, που χρησιμοποιήθηκε για την εκτέλεση των πειραματικών διαδικασιών. Στην συνέχεια γίνεται αναλυτική προσομοίωση για κάθε είδος δοκιμίου και ενίσχυσης, από όπου εξάγεται και η θεωρητική τιμή της αντοχής τους, και περιγραφή της πειραματική διαδικασίας, που ακολούθησε για τον έλεγχό τους. Ακολούθως, γίνεται σύγκριση μεταξύ των τρόπων ενίσχυσης και αναφορά των σχετικών πλεονεκτημάτων και τα μειονεκτημάτων τους, ανά κατηγορία δοκιμίων. Τέλος, δίνονται τα συμπεράσματα της εργασίας, που συνοπτικά είναι ότι η χρήση α-νόργανης μήτρας για την κατασκευή μανδυών προσφέρει, με σχετική μείωση της αντοχής, μεγαλύτερη παραμορφωσιμότητα και ικανότητα απορρόφησης ενέργειας και ότι η ενίσχυση, με τοποθέτηση ράβδων ανθρακονήματος, μέσα στους συνεχείς αρμούς της τοιχοποιίας, είναι επισφαλής, λόγω της μη ικανοποιητικής συνάφειάς τους με το συνδετικό κονίαμα, αποτελέσματα, τα οποία μπορούν να χρησιμοποιηθούν για την καλύτερη διαστασιολόγηση των ενισχύσεων με σύνθετα υλικά. Επίσης αναφέρονται κάποια θέματα που χρήζουν περαιτέρω μελέτης, όπως η τοποθέτηση των ράβδων ανθρακονημάτων σε άλλες θέσεις και διευθύνσεις, το απαιτούμενο ποσοστό ινών και η πιθανή τροποποίηση των πειραματικών διατάξεων, ώστε να πραγματοποιηθεί ακριβέστερη προσομοίωση των πραγματικών στοιχείων και φορτίσεων, μιας κατασκευής. / The purpose of this study is the comparison between the use of fibre-reinforced polymers (FRP) as strengthening materials for masonry walls with jackets of polymeric matrix (organic and inorganic) using carbon fabrics and strips (into the constant joints of masonry). First of all, is discribed the history of masorny and its categories. Then, are mentioned the first materials for bricks, the procedure for their produce and their sizes. Also, are mentioned the categories of mortar and plaster. Moreover, is discribed the mechanics of masonry according the Eurocode 6(EC6) and the professor Tasios. Furthermore, are reported the categories of composite materials and their qualities. Also, is annotated the connection between the composite materials and the masonry (substratum). Apart from this, are described the materials that were used to form and reinforce elements of masonry wall. Also, are described the types of elements of masonry wall, that were built, the procedure for their reinforcement and the mechanical equipment of the laboratory, where expirements took place. Moreover, are annotated the analytical model for every type of masonry element, from where theoritical values were taken and the experimental procedure. Furthermore, is presenced the comparison between every type of reinforcment and their advantages or disadvantages. Finally, are decribed subjects for future usage and results of this study.

War, terrorist attacks, explosions, progressive collapse and other unforeseen circumstances have damaged many structures, including buildings and bridges in war- torn countries such as Iraq. Most of the damaged structural members, for example, beams, columns and slabs, have not totally collapsed and can be repaired. Nowadays, carbon fibre reinforced polymer (CFRP) is widely used in strengthening and retrofitting structural members. CFRP can restore the load- carrying capacity of damaged structural members to make them serviceable. The effect of using CFRP to repair the damaged beams has not been not properly addressed in the literature. This research has the aim of providing a better understanding of the behaviour of reinforced concrete beams strengthened or repaired with CFRP strip under impact loading. Experimental and analytical work were conducted in this research to investigate the performance of RC beams strengthened or repaired using CFRP. To study the impact behaviour of the CFRP reinforced concrete beams, a new heavy drop weight impact test machine has been designed and manufactured to conduct the experimental work. Twelve RC beams were tested experimentally under impact load. The experimental work was divided into two stages; stage 1 (strengthened) and stage 2 (repair). At stage 1, three pairs of beams were tested under impact loading. External bonded reinforcement (EBR) and near surface mounted (NSM) techniques were used to strengthen the RC beams to find the most effective technique. Three pairs of beams were tested in stage 2 (repair). Different degrees of damages were induced using different impact energies. NSM technique was used to repair the damaged beams using CFRP strip. Stiffness degradation method was used to assess the degree of damage in beams due to impact. The study investigated the stiffness, bending load, impact energy, deflection and mode of failure of CFRP strengthened or repaired beams under impact loading. The distribution of the stresses, strains, accelerations, inertia forces, and cracks in the beam under impact loading was also investigated in this study. Empirical equations were proposed in this research to predict the bending load and maximum deflection of the damaged and repaired beams under impact loading. For validation purposes, finite element analysis was used with the LUSAS package. The FEA results were compared with the experimental load-deflection curves and ultimate failure load results. In this research, to simulate a real situation, different models were used to simulate the bonding between the CFRP and concrete and also between steel bars and concrete. In these FEA models, the bonding between the concrete and the CFRP was modelled using the Drucker-Prager model. To simulate the bonding between steel and concrete, a joint element was used with spring constants to model the bond between steel bars and surrounding concrete. The analytical results were compared with the experimental results. In most previous research, FEA has been used to simulate the RC beams under impact loading without any damage. In this thesis, a new 3D FEA model was proposed to simulate and analyse the damaged RC beams under impact loading with different degrees of damage. The effect of the damage on concrete stiffness and the bonding between the steel bars and the concrete were investigated in FEA model. The damage was modelled by reducing the mechanical properties of the concrete and the bonding between steel bars and concrete. This thesis has contributed to improving knowledge of the behaviour of damaged beams repaired with CFRP, and the experimental work conducted, together with the numerical analysis, have provided essential data in the process of preparing a universal standard of CFRP design and construction. In the FEA model, the damage to the beams due to impact loading was successfully modelled by reducing the beam stiffness.

No description available.

Les alliages ODS (Oxide Dispersion Strengthened) sont des matériaux candidats pour la réalisation du gainage combustible des réacteurs nucléaires de Génération IV de type RNR-Na. Leurs propriétés mécaniques à haute température sont assurées par une dispersion d’oxydes nanométriques qui peut être modifiée sous l’effet de la température et notamment lors du passage par l’état liquide. Les procédés d'assemblage en phase solide sont donc préconisés. La méthode de soudage étudiée dans cette thèse est le soudage par résistance en bout. Une double approche simulation numérique-expérimentation a été mise en place pour étudier l’influence des paramètres opératoires sur le soudage et la soudure. L’ensemble des essais (expérimentaux et numériques) est réalisé selon la méthode des plans d’expériences factoriels complets. Un alliage ODS base fer à 20 % de chrome a été utilisé comme alliage d’essai.L’analyse de l’influence des paramètres opératoires sur le soudage est réalisée dans un premier temps. Les résultats montrent que l’étape de soudage peut se découper en trois phases. Dans une première phase, la température augmente au niveau des interfaces. Dans une seconde phase, le procédé de soudage est gouverné par le changement de section macroscopique lorsque le courant passe du bouchon massif à la gaine plus étroite. La température augmente alors dans la partie de gaine dépassant de l’électrode. Lorsque la température atteint une valeur suffisamment élevée, la partie de gaine dépassant de l’électrode se déforme, entrainant un affaissement général des pièces et constituant la troisième phase de l’étape de soudage. L’influence des paramètres opératoires sur les phénomènes physiques lors de l’opération d’assemblage est évaluée. Il est alors possible d’influer sur les sollicitations thermiques et mécaniques à l’aide des paramètres opératoires. L’analyse de l’influence des sollicitations thermomécaniques sur la soudure est réalisée dans un second temps. Sur les plages de paramètres opératoires étudiées, les résultats révèlent sur certaines soudures la présence de défauts de compacité ainsi qu’une modification de la microstructure et de la dispersion des oxydes. Les défauts de compacité peuvent être liés à des phénomènes thermiques et mécaniques au contact entre pièces. Les modifications de la microstructure sont liées à des phénomènes de recristallisation dynamique ou de fusion locale et donc à des phénomènes thermiques mais aussi mécaniques avec la présence de déformations importantes. Les modifications de la microstructure sont alors reliées à une modification de la dispersion d’oxydes. A l’aide des paramètres opératoires il est possible d’influer sur les températures et les déformations afin de limiter les modifications de la structure de l’alliage ODS ainsi que l’apparition de défaut de compacité. A l’aide de l’ensemble de ces résultats, la procédure de soudage est adaptée sur un alliage ODS à 9 % de chrome, nuance envisagée pour la réalisation des futurs gainages combustibles. L’influence des propriétés matériaux sur le soudage et la soudure est alors discutée en comparant les deux nuances ODS de compositions différentes mais aussi en comparant les résultats obtenus sur l’alliage ODS à 20 % de chrome avec un alliage non renforcé de composition semblable. / ODS steels (Oxide Dispersion Strengthened) are candidate materials for fuel cladding in Sodium Fast Reactors (SFR), one of the concepts at study for the forth generation of nuclear power plant. These materials have good mechanical properties at high temperature due to a dispersion of nanometer-sized oxides into the matrix. Previous studies have shown that melting can induce a decrease of the mechanical properties at high temperatures due to modifications of the nanometer-sized oxide dispersion. Therefore the fusion welding techniques are not recommended and the solid state bonding has to be evaluated. This study is focused on resistance upset welding.Welding experiments and numerical simulations of the process are coupled in this thesis. All the trials (experimental and numerical) are built using the experimental design method in order to evaluate the effects of the process parameters on the welding and on the weld. A 20Cr ODS steel is used in order to conduct the study.The first part is dedicated to the study of the influence of the process parameters on the welding. The simulation shows that the welding steps can be divided in three stages. First, the temperature of the contact between pieces increases. Second, the process is driven by the pieces geometry and especially the current constriction due to the thinness of the clad compare to the massive plug. Therefore, the heat generation is mainly located in the clad part out of the electrode leading to its collapse which is the third stage of the welding step. The evaluation of the process parameters influences on the physical phenomena (thermal, mechanical …) occurring during the welding step allow to adjust them in order to influence the thermal and mechanical solicitation undergone by the pieces during the welding process.The second part is dedicated to the study of the influence of the physical phenomena on the welds. In the process parameter range, some welds exhibit compactness defects or a modification of the microstructure and of the oxide dispersion. Compactness defects are related to thermal and mechanical phenomena occurring at the contact between pieces. The modification of the microstructure is related to dynamical recrystallization or to a local fusion. The dynamical recrystallization occurring in the clad due to high deformation and high temperature is linked to modification of the oxide dispersion.Using the effects of the process parameters on the welding and on the weld, it is possible to adjust the temperature and the deformation in order to avoid the compactness defects and the modification of the oxide dispersion. All these results are then apply to the welding of a 9Cr ODS steel which is a candidate alloy for the SFR fuel cladding. The effects of the material properties on the welding and the weld are then discussed by comparing the two alloy with a different chromium content but also by comparing results on the 20Cr-ODS with a material of similar chemical composition but without the oxide dispersion.

Alloys used in fission and in future fusion reactors are subjected to extreme conditions including high temperatures, corrosive and intense radiation environments. Understanding the processes occurring at the microscopic level during radiation events is essential for the further development of them. As a prospective candidate material for new reactors, oxide dispersion strengthened (ODS) steels have shown good radiation resistance and the ability to trap He into fine scale bubbles, thus preventing swelling and preserving high-temperature strength. This thesis represents the findings obtained by performing computational studies of radiation effects in pure iron, Y-Ti-O systems and a simplified model of ODS using Molecular Dynamics (MD) and on-the-fly Kinetic Monte Carlo (otf-KMC) techniques. MD studies of radiation damage were carried out in a perfect body-centred cubic (bcc) iron matrix (alpha-Fe) in which yttria nanoparticles are embedded as a simplified model of an ODS steel. The results have shown how the nanoparticles interact with nearby initiated collision cascades, through cascade blocking and energy absorption. Fe defects accumulate at the interface both directly from the ballistic collisions and also by attraction of defects generated close by. The nanoparticles generally remain intact during a radiation event and release absorbed energy over times longer than the ballistic phase of the collision cascade. Also the nanoparticles have shown ability to attract He atoms as a product of fission and fusion reactions. Moreover, studies showed that He clusters containing up to 4 He atoms are very mobile and clusters containing 5 He or more become stable by pushing an Fe atom out of its lattice position. The radiation damage study in the Y-Ti-O materials showed two types of residual damage behaviour: when the damage is localized in a region, usually close to the initial primary knock-on atom (PKA) position and when PKA is directed in the channelling direction and creates less defects compared to the localised damage case, but with a wider spread. The Y2TiO5 and Y2Ti2O7 systems showed increased recombination of defects with increased temperature, suggesting that the Y-Ti-O systems could have a higher radiation resistance at higher temperatures. The otf-KMC technique was used to estimate the influence of the prefactor in the Arrhenius equation for the long time scale motion of defects in alpha-Fe. It is shown that calculated prefactors vary widely between different defect types and it is thus important to determine these accurately when implementing KMC simulations. The technique was also used to study the recombination and clustering processes of post-cascade defects that occur on the longer time scales.

Lamellar graphite iron (LGI) is commonly used in diesel engine applications such as piston rings–cylinder liner where an excellent combination of physical and tribological properties is essential to avoid scuffing and bore polishing issues. The excellent tribological behaviour of LGI alloys is related to the graphite lamellas, which act as solid lubricant agents by feeding onto the tribosurfaces under sliding conditions. However, increasingly tighter emissions and fuel economy legislations and the higher demands on enhanced power and durability have encouraged both engine designers and manufacturers to introduce pearlitic compacted graphite irons (CGI) as an alternative material replacing LGI, although the poor machinability of pearlitic CGI alloys compared to the LGI remains a challenge. The focus of this study is placed on investigating how the microstructure of LGI and CGI alloys affects their mechanical and tribological properties. This was initially undertaken by investigating representative, worn lamellar cast iron piston rings taken from a two-stroke large-bore heavy-duty diesel engine. As known that it is tribologically essential to keep the graphite open under sliding conditions, in particular under starved lubrication regimes or unlubricated conditions to avoid scuffing issues; however, this study revealed the closure of a majority of graphite lamellas; profoundly for those lamellas that were parallel to sliding direction; due to the severe matrix deformation caused by abrasion. Both microindentation and microscratch testing, which were used to crudely simulate the abrasion under starved lubricated condition in combustion chamber, suggested a novel mechanism of activating the graphite lamellas to serve as lubricating agents in which the matrix deformation adjacent to the graphite initially resulted in fracturing and then extrusion of the graphite lamellas. Additionally, in order to investigate the relation between matrix constituents, mechanical properties and machinability of cast iron materials, solution-strengthened CGI alloys were produced with different levels of silicon and section thicknesses. The results showed significant improvements in mechanical properties and machinability while deteriorating the ductility. Moreover, multiple regression analysis, based on chemical composition and microstructural characteristics was used to model the local mechanical properties of high Si ferritic CGI alloys, followed by implementing the derived models into a casting process simulation which enables the local mechanical properties of castings with complex geometries. Very good agreement was observed between the measured and predicted microstructure and mechanical properties.