<p>This doctoral thesis presents a modeling method fordemonstrating the behavior of inclusions and their surroundingmatrix during plastic deformation of steels.</p><p>Inclusions are inescapable components of all steels. Moreknowledge about their behavior in processes such as rolling andforging is necessary for carrying out the forming processes ina more proper way so that the properties of the final productare improved. This work is focussed on deformation ofinclusions together with void formation at the inclusion-matrixinterface. The topic of the work is analyzed by differentFE-codes.</p><p>The relative plasticity index is considered as an importantmeasure for describing the deformability of inclusions. Theindex could be analyzed quantitatively, enabling a deeperunderstanding of the deformation mechanisms. The workingtemperature is found to be an important process parameter. Thisis very clear when the deformation of silicate inclusions in alow-carbon steel is studied during hot rolling. Here a narrowtransition temperature region exists, meaning that theinclusion behaves as non-plastic at lower temperatures and asplastic at higher. The results are in agreement withexperiments published by other authors.</p><p>Regarding void formation, the simulations have been carriedout by utilizing an interfacial debonding criterion. Thedifference in yield stress between the matrix and the inclusionis one common reason for void initiation and propagation.During large compressive deformation the evolution of voidsgoes through a sequence of shapes, from convex with two cuspsto concave with three cusps together with self-welding lines.It is concluded that the formation of voids is alwaysassociated with a large relative sliding between the inclusionand the matrix.</p><p>In order to study the local behavior of the material closeto inclusions during hot rolling a mesomechanical approach isused. Uncoupled macro- and micro- models have been developed.By means of the macro-model, the stress-strain historythroughout each sub-volume of the steel is evaluated. Thestress components or velocity fields are recorded with respectto time as history data. No consideration is taken to theexistence of inclusions. The micro-model, which includes bothinclusion and steel matrix, utilizes the stress components orthe velocity fields from the macro-model as boundaryconditions.</p><p><b>Keywords</b>: Inclusion; Steel; Plastic deformation; Void;Rolling; Forging; Finite Element; Mesomechanical approach.</p>

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension-compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with i) the precipitation structure and ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the precipitate average radius and is thus linked to the shearable/non shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually lead to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al, a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the precipitation structure main characteristics –such as the average precipitate radius, length and volume fraction– allows one to estimate accurately the internal stress contribution to the flow stress with this model.

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In this thesis ultrasonic Lamb wave measurements are performed to detect material nonlinearity in aluminum sheets. When a Lamb wave propagates, higher harmonic wave fields are generated and under certain conditions the second harmonic is cumulative. When these conditions hold the Lamb waves are serviceable for material nonlinearity measurements. For generation, a wedge transducer combination is used. The detection of the Lamb wave are performed with either a laser interferometer or a second wedge transducer combination and the results are benchmarked. A short time Fourier transformation (STFT) is applied to the detected signal to extract the amplitudes of the first and second harmonics. A relative ratio of the first and second harmonics is deduced from nonlinear wave theory to assign the nonlinearity of the material. To verify the capability of the measurement setup and to show that cumulative second harmonics are generated, measurements for different propagation distances are performed. Further measurements on plasticly deformed specimens are carried out to examine the change of the material nonlinearity as a function of plasticity.

Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter. Increasing the knowledge about the heat treatment process is one task in the direction towards this goal. The aim of this work is to investigate and model the heat treatment process for Al-Si casting alloys. Three alloys containing Mg and/or Cu were cast using the gradient solidification technique to achieve three different coarsenesses of the microstructure and a low amount of defects. Solution treatment was studied by measuring the concentration of Mg, Cu and Si in the α-Al matrix using wavelength dispersive spectroscopy (WDS) after various times at a solution treatment temperature. A diffusion based model was developed which estimates the time needed to obtain a high and homogenous concentration of alloying elements for different alloys, temperatures and coarsenesses of the microstructure. It was shown that the yield strength after artificial ageing is weakly dependent on the coarseness of the microstructure when the solution treatment time is adjusted to achieve complete dissolution and homogenisation. The shape and position of ageing curves (yield strength versus ageing time) was investigated empirically in this work and by studying the literature in order to differentiate the mechanisms involved. A diffusion based model for prediction of the yield strength after different ageing times was developed for Al-Si-Mg alloys. The model was validated using data available in the literature. For Al-Si-Cu-Mg alloys further studies regarding the mechanisms involved need to be performed. Changes in the microstructure during a heat treatment process influence the plastic deformation behaviour. The Hollomon equation describes the plastic deformation of alloys containing shearable precipitates well, while the Ludwigson equation is needed when a supersaturated solid solution is present. When non-coherent precipitates are present, none of the equations describe the plastic deformation well. The evolution of the storage rate and recovery rate of dislocations was studied and coupled to the evolution of the microstructure using the Kocks-Mecking strain hardening theory.

Le magnésium est le plus léger des métaux, ce qui lui confère un fort potentiel pour être utilisé dans des applications où l’allégement des structures est requis. Pour autant, sa résistance mécanique est très faible, et doit donc être augmentée afin de rivaliser avec d’autres métaux légers tels que l’aluminium ou le titane. Une solution consiste à renforcer le magnésium et ses alliages en introduisant des nanoparticules d’oxydes. De par sa structure cristalline hexagonale compacte, le magnésium présente des propriétés plastiques complexes telles qu’une très forte anisotropie plastique et une prédisposition au maclage. La compréhension de ces mécanismes de déformation est essentielle pour le développement de nanocomposites plus performants en vue d’une utilisation industrielle plus répandue. Dans ce travail, nous nous sommes intéressés à l'élaboration et à la caractérisation de nanocomposites de magnésium pur renforcés par des particules d’oxydes. Différentes techniques ont été testées pour l’élaboration des nanocomposites : la solidification assistée aux ultrasons et le procédé de friction malaxage. L’homogénéité de la dispersion des particules a été vérifiée en 2D par observations en microscopie électronique et également en 3D par tomographie aux rayons X. On montre ainsi que le procédé de friction malaxage permet d'obtenir une distribution homogène des particules, tout en réduisant leur taille. Des essais de traction ont permis de mettre en évidence une augmentation de la limité d’élasticité pour une fraction volumique aussi faible que 0.3 %. Afin d’isoler le rôle des particules de celui des joints de grains sur le comportement plastique du nanocomposite, nous avons réalisé des essais de micro-compression sur des micro-piliers monocristallins usinés par canon à ions focalisés (FIB) dans des échantillons ayant préalablement subis un traitement thermique favorisant la croissance anormale des grains. Différentes orientations cristallines et tailles de micro-piliers ont été testées en vue d'étudier l’influence des particules d’une part sur la plasticité dans le plan basal par mouvement de dislocations et d’autre part sur la déformation par maclage. Contre toute attente, les essais sur monocristaux favorablement orientés pour un glissement basal ne montrent pas l’effet durcissant observé macroscopiquement. Nous attribuons cet effet à la densité initiale de dislocations mobiles, plus importante dans les nanocomposites que dans le magnésium pur, du fait des concentrations de contraintes autour des particules. Ces densités initiales de dislocations mobiles tendent également à supprimer l'effet de taille classiquement observé dans le magnésium pur. Les particules modifient également le mécanisme de déformation par maclage en favorisant l’apparition simultanée de plusieurs macles dans le micro-pilier qui interagissent entre elles au cours de la déformation alors que les micro-piliers de magnésium pur présentent généralement une macle unique (dans certains cas deux) qui envahi tout le monocristal. Ces résultats constituent une contribution originale à la compréhension du rôle des nanoparticules dans la déformation plastique des monocristaux de nanocomposites à base de magnésium. / Magnesium is the lightest of all structural metals, which gives it a huge potential to be used in applications that require lightweighting. However, its strength needs to be increased in order to compete with other light metals such as aluminum and titanium. A solution is the reinforcement of magnesium and its alloys with the addition of oxide nanoparticles. The hexagonal close packed crystalline structure is responsible for the complex plasticity of magnesium, which is characterized by a very strong plastic anisotropy as well as a complex twinning activity. Understanding these deformation mechanisms is crucial for the development of more performant nanocomposites, allowing widespread industrial application. The present work focuses on the processing and characterization of magnesium based nanocomposites reinforced with oxide particles. Two different processing techniques have been compared: friction stir processing and ultrasound assisted casting. The homogeneity of the dispersion of the reinforcement particles has been verified in 2 and 3 dimensions using electron microscopy and X-ray tomography, respectively. Friction stir processing produces nanocomposites with a more homogeneous dispersion of particles, while reducing their size. Tensile tests have shown strengthening of magnesium with the addition of a volume fraction of only 0.3 % of reinforcement. An annealing heat treatment has then been performed in order to promote abnormal grain growth and single crystalline microcolumns for microcompression testing have been machined by focused ion beam (FIB). The purpose is to isolate the role of particles. The orientation dependent mechanism of deformation and the size effects have been studied in order to understand the influence of the reinforcement particles on the plasticity for orientations favorable for basal slip or tensile twinning. Differently from the strengthening observed macroscopically, no clear strengthening effect is observed on microcolumns when dislocation glide operates. The reason is the higher density of potentially mobile dislocations that is generated due to stress concentrations around the reinforcement particles. In addition, the size effects usually observed on pure magnesium have also been suppressed with the addition of particles. The reinforcement particles seem to affect the twin nucleation stress and twin morphology: particles induce the nucleation of multiple twins inside a microcolumn, whereas in pure magnesium, only one or two twins have been observed. These results provide relevant insights on the role of nanoparticles on the onset of plastic deformation, as well as size effect, in single crystalline magnesium nanocomposites.

abstract: Composite materials are now beginning to provide uses hitherto reserved for metals in structural systems such as airframes and engine containment systems, wraps for repair and rehabilitation, and ballistic/blast mitigation systems. These structural systems are often subjected to impact loads and there is a pressing need for accurate prediction of deformation, damage and failure. There are numerous material models that have been developed to analyze the dynamic impact response of polymer matrix composites. However, there are key features that are missing in those models that prevent them from providing accurate predictive capabilities. In this dissertation, a general purpose orthotropic elasto-plastic computational constitutive material model has been developed to predict the response of composites subjected to high velocity impacts. The constitutive model is divided into three components – deformation model, damage model and failure model, with failure to be added at a later date. The deformation model generalizes the Tsai-Wu failure criteria and extends it using a strain-hardening-based orthotropic yield function with a non-associative flow rule. A strain equivalent formulation is utilized in the damage model that permits plastic and damage calculations to be uncoupled and capture the nonlinear unloading and local softening of the stress-strain response. A diagonal damage tensor is defined to account for the directionally dependent variation of damage. However, in composites it has been found that loading in one direction can lead to damage in multiple coordinate directions. To account for this phenomena, the terms in the damage matrix are semi-coupled such that the damage in a particular coordinate direction is a function of the stresses and plastic strains in all of the coordinate directions. The overall framework is driven by experimental tabulated temperature and rate-dependent stress-strain data as well as data that characterizes the damage matrix and failure. The developed theory has been implemented in a commercial explicit finite element analysis code, LS-DYNA®, as MAT213. Several verification and validation tests using a commonly available carbon-fiber composite, Toyobo’s T800/F3900, have been carried and the results show that the theory and implementation are efficient, robust and accurate. / Dissertation/Thesis / Doctoral Dissertation Civil and Environmental Engineering 2016

Orientadores: Rubens Caram Junior, Alexandra de Oliveira França Hayama / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T11:40:06Z (GMT). No. of bitstreams: 1 Lopes_JulianaFelettoSilveiraCosta_M.pdf: 9637733 bytes, checksum: 7bc2d0a22ca6a34be43ea82403ecff91 (MD5) Previous issue date: 2010 / Resumo: O controle da composição e a aplicação de tratamentos termo-mecânicos de ligas de titânio tipo ß permite otimizar microestruturas em relação à aplicações como biomaterial ortopédico. Amostras das ligas Ti-30Nb e Ti-35Nb (% em peso) foram preparadas em forno de fusão a arco voltaico, homogeneizadas a 1000 oC por 8 h e resfriadas em água, obtendo-se microestrutura formada pela fase ß combinada com a fase martensita ortorrômbica. Na seqüência, essas amostras seguiram duas rotas de processamento: uma envolvendo deformação plástica, seguida por tratamento térmico de envelhecimento e outra envolvendo deformação plástica seguida por recristalização e envelhecimento. Os resultados mostram que a microestrutura das amostras homogeneizadas é composta por grãos grosseiros, com aproximadamente 3 mm de diâmetro. A caracterização microestrutural das amostras deformadas mostra a presença de heterogeneidades de deformação, tais como bandas de cisalhamento. O tratamento térmico de envelhecimento foi realizado com o objetivo de estudar a precipitação das fases ? e a. A aplicação de tais tratamentos térmicos permitiu a otimização das microestruturas e do comportamento mecânico das ligas estudadas. / Abstract: This work deals with studies of titanium alloys used in orthopedic implants. The control of composition and application of thermo-mechanical treatments allows one to optimize microstructures regarding applications as orthopedic biomaterial. Samples of Ti-30Nb and Ti-35Nb (wt %) alloys were obtained by arc melting, solution heat treated at 1000 ºC for 8 hours and water quenched, resulting in microstructures composed by ß phase and orthorhombic martensite phase. Following, samples followed two processing routes: one involving plastic deformation followed by aging heat treatment and another involving plastic deformation followed by recrystallization and aging heat treatment. In the solution condition, the sample microstructures were formed by coarse grains, with approximate diameter of 3 mm. Microstructural characterization of deformed samples shows the presence of deformation heterogeneity, as shear bands. Aging heat treatment was accomplished with the aim of studying precipitation of ? and a phases. Application of the mentioned heat treatments allowed optimization of microstructure and mechanical behavior of the studied alloys. / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The crack-tip displacement concept has been applied to austenitic steels at 25°C and in the temperature range 400-800°C. The measurement of a maximum load crack-tip displacement in a notch bend test, provided a good indication of fracture-toughness since it included a portion of slot: crack-growth. The ability of these alloys to tolerate slow crack-growth, and even to arrest cracks without becoming unstable, is of considerable engineering importance since many service failures originate at stress concentrations produced by welding of poor design. Slow crack-growth was intermittent and highly dependent upon microstructural irregularities in the immediate vicinity of the crack-tip. The stress concentration at the crack-tip could be relieved (and the fracture-toughness improved) by localized plastic deformation. The degree of stress relief depended upon the locality and proximity of the various microstructural features as determined by the alloy manufacturing process. The temperature sensitivity of the bond strengths of the various interfaces particularly in cast alloys, had a marked-effect on fracture-toughness. This may have been the result of segregation of trace elements e.g. lead to the interface. Above the equicohesive temperature, the greatest contribution to fracture-toughness was associated with the presence of large amounts of second phase particles in the boundaries. In many cases, and particularly cast alloys, more than one crack formed at the notch root. Only one of these cracks propagated to failure. Heterogeneous cracking of this kind (itself an indication of fracture-toughness), lead to difficulties in correlating an initiation C. O. D. with the macroscopic plastic properties of the material. Similar correlations were however possible with the maximum load crack opening displacement.