Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Vassilios John Papazoglou. / Ph.D.

Neste trabalho desenvolve-se uma formulação teórica consistente para a análise não-linear, física e geométrica, de pórticos planos de aço, através do Método dos Elementos Finitos (MEF), considerando barras com imperfeições iniciais e tensões residuais nas suas seções transversais, visando analisar os critérios de resistência adotados para projeto das colunas de aço, baseados no conceito de resistência-última, e estudar o comportamento não-linear dos pórticos de aço considerando estes dois fatores. São feitas considerações qualitativas sobre a influência das imperfeições iniciais e das tensões residuais no cálculo da resistência das colunas. e é dada uma visão geral sobre a evolução dos critérios usados na prática para dimensionamento das colunas de aço. Apresenta-se o desenvolvimento da teoria geral, que leva em conta a presença das tensões residuais auto-equilibradas nas equações de equilíbrio do elemento. Este desenvolvimento é feito dentro de uma formulação Lagrangiana utilizando a técnica corrotacional para a dedução consistente da matriz de rigidez tangente do elemento de pórtico plano com imperfeição inicial. Para implementação desta formulação é desenvolvido um programa em linguagem FORTRAN para micro-computador capaz de fazer a análise não-linear elasto-plástica de pórticos planos, baseado num processo incremental-iterativo. Utiliza-se o modelo de fatias para avaliar os coeficientes de rigidez do elemento, modelo este que se mostrou bastante adequado, permitindo o acompanhamento da plastificação ao longo da altura da seção e a consideração de qualquer modelo de distribuição das tensões residuais. Finalmente, os exemplos apresentados mostram a grande potencialidade da formulação desenvolvida. São analisados vários casos cujos resultados são comparados com os obtidos por outros autores, demonstrando a precisão e correção da teoria proposta. A aplicação na análise de pórticos de andares múltiplos confirma a validade da formulação e demonstra a sua aplicabilidade nos casos práticos. / A consistent finite element formulation to consider material and geometric nonlinearities of steel plane frames is presented. Residual stresses and initial imperfections are considered to evaluate steel columns failure criteria based on the ultimate strength and also to investigate the nonlinear behavior of steel plane frames. Some qualitative considerations are made on the initial imperfections and residual stresses influence on the practical steel columns design. A general theory considering self equilibrated residual stresses is developed based on the Lagrangian formulation. Corotacional technique is used to obtain the tangent stiffness matrix of plane frame taking the initial imperfection into account. A stand alone code has been written in FORTRAN and implemented on a microcomputer platform to perform incremental-iterative analysis of nonlinear elastoplastic plane frame problems. The frame element is made up of layers such that the plastic region can be readily identified and any kind of residual stresses through the cross section area can be taken into account in the analysis. Some examples are presented and their results compared to others in the literature. The multistory building analyses using the plane frame element presented in this research has shown to be very effective and useful to practical applications.

A trefilação de barras é um dos mais antigos processos de conformação mecânica e pode ser definido como um processo de manufatura por deformação plástica, onde o fio-máquina (matéria-prima) é tracionado passando através de uma ferramenta cônica, causando uma redução da área da seção transversal e, aumentando o comprimento total. Este processo introduz tensões residuais no produto final. Parâmetros como ângulo de fieira, coeficiente de atrito e redução, bem como a anisotropia do material tem grande influência nestas tensões. Neste trabalho avaliou-se a influência dos parâmetros citados nas tensões residuais resultantes do processo de trefilação de barras redondas de aço AISI 1045 com diâmetro inicial de 20,46 mm e final, após trefilação, de 20,25 mm, através de simulação numérica, comparando os resultados obtidos numericamente com dados experimentais. Além disso, inseriram-se heterogeneidades nos modelos computacionais de trefilação para que fossem verificados seus efeitos sobre a distorção (empenamento) das barras. Foram criados seis modelos para simular as tensões residuais e para estes, realizadas diferentes validações numéricas, as quais sugerem uma grande proximidade entre o problema físico e o modelo computacional. O atrito teve uma maior influência na força de trefilação do que nas tensões residuais após trefilação. A utilização de duas fieiras para realizar a trefilação mostrou uma redução das tensões residuais trativas na superfície, sendo que reduzindo a distância entre as mesmas, observou-se uma redução ainda maior destas. A consideração da anisotropia do material também mostrou influência considerável no valor das tensões e deve ser levada em conta nas simulações. Observou-se também, pelas comparações entre as simulações, que pelas variações realizadas, os valores das tensões residuais simulados aproximaram-se dos valores experimentais medidos por difração de Nêutrons, porém os resultados ainda não foram satisfatórios. Neste trabalho também são discutidas as possíveis razões das diferenças observadas. / The wire drawing is one of the oldest mechanical forming processes and it may be defined as a manufacturing process for plastic deformation, where the wire rod (raw material) is pulled passing through a conical tool, causing a reduction in cross section area and increasing the total length. This process introduces residual stresses in the final product. Parameters such as die angle, friction coefficient, area reduction and the anisotropy of the material has great influence on these stresses. In this study was evaluated the influence of the parameters mentioned in the residual stresses resulting from the drawing process of round bars of AISI 1045 steel with initial diameter of 20.46 mm and final, after drawing, of 20.25 mm. Through numerical simulation, a comparison of numerical results and experimental data was carried out. Furthermore, heterogeneities were inserted in the drawing computational models to check their effects on distortion (warpage) of the bars. Six models were created to simulate the residual stresses with following numerical validations, showing good approximation between the physical problem and the computational model. Friction had a greater influence on the drawing force than in residual stresses after drawing. The use of two dies to perform the drawing showed a reduction of residual stresses, however if the distance between two dies is reduced, lower levels of resulting tensile residual stresses in the surface are seen. The consideration of the material anisotropy also showed considerable influence on the value of the residual stresses and it should be taken into account in the simulations. Concerning the comparison of simulated and measured residual stresses by the neutron diffraction method, the results approach similar profiles, however the results were still not satisfactory. This work discusses the possible reasons for the observed difference in terms of residual stresses.

Para muitos componentes e estruturas de engenharia a determinação precisa do estado de tensões residuais presente é de fundamental importância para a avaliação de sua integridade estrutural. O método do furo cego (MFC) é um dos métodos mais difundidos para a medição de tensões residuais, no entanto, por se tratar de um ensaio relativamente complexo, a determinação da sua incerteza de medição apresenta uma série de dificuldades, as quais se refletem na inexistência de estudos na literatura que abordem esse assunto de forma completa. Este trabalho tem como objetivo a determinação da incerteza de medição do MFC englobando todas as fontes de incerteza em potencial do método. Para tanto, foi elaborado um procedimento envolvendo a determinação e caracterização das fontes de incerteza tanto na parte experimental como no tratamento matemático dos dados. A partir da caracterização das fontes de incerteza detectou-se como fator principal na determinação da incerteza o erro do operador, que foi possível através do desvio de repetitividade. A fonte de incerteza devido ao erro do operador foi a de determinação mais complexa e envolveu o projeto e a construção de uma máquina especialmente desenvolvida para a aplicação de um estado de tensões homogêneo e que permitisse medir com precisão as tensões residuais com o método do furo cego. Técnicas de medição de tensões residuais por difração de raios-X e monitoramento de tensão com extensometria de resistência elétrica foram utilizadas para a verificação da homogeneidade das tensões nas amostras. A metodologia se mostrou adequada, conduzindo a resultados que permitiram determinar com sucesso a incerteza de medição através de planilhas eletrônicas. / The characterization of the residual stress state of engineering components and structures is of fundamental importance for assessments of their structural integrity. The hole drilling method is one of the most commonly used methods; however, due to its relative complexity, quantifying its measurement uncertainties is not straightforward. It is a semi-destructive method, which means that multiple measurements cannot be repeated in a same location, and stress distributions mean that it is difficult to guarantee that a number of points in the same state will be available in a given volume. This study suggests a global uncertainty measurement which includes a study of repeatability related to the operator. This is thought to be novel in the existing literature. For this, a standard sample and a machine which guarantees a homogeneous stress distribution in the sample were developed. To ensure the homogeneity of the stresses, x-ray diffraction measurements were performed. The results obtained show good repeatability for different operators and good results for the uncertainty of measurements for the method.

Les pièces de structure aéronautique utilisées dans l’aéronautique sont fabriquées en plusieurs étapes. Des étapes, comme le traitement thermique, génèrent des contraintes résiduelles. Les enlèvements de matière réalisés par les opérations d’usinage peuvent alors conduire à la réorganisation des contraintes résiduelles dans la matière et ainsi à la déformation de la pièce. La mesure in-situ de ces déformations devient nécessaire lorsque ce phénomène est étudié. Ces travaux de thèse abordent cette problématique dans le cadre du projet SIMP-Aero. L’objectif de ces travaux de thèse est d’adapter la méthode de corrélation d’images numériques à la mesure de la déformation de pièce pendant l’usinage, c’est-à-dire dans un centre d’usinage. Pour cela, plusieurs améliorations sont apportées à la méthode. Premièrement, les mouvements du système optique sont pris en compte afin que ceux-ci n’altère pas la qualité des mesures. Ensuite, les copeaux présents sur les images sont détectés et filtrés par un algorithme. Au final, la méthode développée permet de mesurer des champs de déplacement durant toute la séquence d’usinage, sans devoir l’interrompre, avec une incertitude de mesure de l’ordre du centième de millimètre. / Structural aluminum alloy parts used in aeronautics are manufactured in several steps, from forming processes and heat treatments to final machining. Some of the process steps induce residual stresses. The material removal during machining release these residual stresses and thus, leads to the part deformation. The in-situ measurement of these deformations becomes necessary when this phenomenon is studied. The present work address this problematic in the context of the ANR SIMP-Aero Project. It aims to define a reliable experimental technique dedicated to the measurement of part deformations during machining of large aeronautical parts. The backbone of the technique relies on Digital Image Correlation (DIC). Mainly as a consequence of the harsh constraints environment of machining, the customization of DIC is required. First, movements of the optical system are quantified and compensated for the proper measurement of the workpiece displacement. Then, the metal chips that fly between the observed surface and the acquisition system are detected and filtered by the algorithm. Finally, the developed method enables the measurement of displacement fields throughout the whole machining sequence, without interrupting it, and a measurement uncertainty of around one hundredth of a millimeter is ensured.

This thesis provides an in depth look at the application of Shot peening to three alloys that cover a wide range of stabilizer compositions (Ti-64, Ti-5553 and Ti21S). Here the effects of Shot peening on the material were studied by characterizing the changes to subsurface microstructres in a variety of starting microstructures across all alloys studied. the study also compared the evolution of residual stress across all conditions and its effect on precipitation during post shot peening heat treatments. <div>Additionally, the study developed a novel shot peening media providing proof of concept for the idea of self-peening a material. This being the practice of peening the target material with the same material as media. Here the production of hardened Ti media was achieved to then self peen titanium alloys. the results from conventional and self-peened samples are compared in terms of corrosion resistance, surface hardening, surface roughness, and residual stress evolution.</div>

[pt] Esta pesquisa teve como objetivo avaliar o efeito do jateamento de partículas (shot-peening) sobre a vida-fadiga do aço estrutural R4, largamente adotado na fabricação de componentes estruturais para sistemas de ancoragem de unidades flutuantes. Inicialmente, amostras cilíndricas do material foram retiradas das seções retas de elos tipo Kenter. Em seqüência, algumas amostras foram temperadas e revenidas em diferentes tempos de tratamento, com o objetivo de se promover uma homogeneidade microestrutural, bem como propriedades mecânicas similares àquelas dos elos tratados industrialmente. As amostras foram submetidas a ensaios de dureza e tração e, com base nos resultados, providenciou-se o tratamento térmico de todas as amostras. Dando continuidade a etapa experimental, corpos de prova para ensaios de fadiga foram usinados das amostras tratadas e submetidos a jateamento com microesferas de aço temperado sob pressões de 20 psi, 30 psi e 40 psi. Finalmente, corpos de prova nas condições sem jateamento e após jateamento foram submetidos a ensaios de fadiga por flexão rotativa, com o objetivo de se levantar a curva tensão versus número de ciclos para a falha (curva S-N) do aço estrutural R4 nas diferentes condições de superfície. A extensão da vida em fadiga do aço estrutural R4 foi relacionada com o jateamento de microesferas. De acordo com os resultados, houve uma influência benéfica do tratamento de superfície sobre a vida-fadiga do material. Tal influência foi caracterizada por um maior número de ciclos para a falha e aumento do limite de resistência à fadiga. Os resultados mostraram que a extensão da vida útil esteve diretamente relacionada com a intensidade do jateamento, significando que uma maior intensidade de jateamento provocou uma maior extensão da vida-fadiga do material. / [en] The objective of the present work was to evaluate the effect of shot peening on the fatigue life of a grade R4 structural steel, largely adopted for fabricating offshore mooring chains. Initially, cylindrical samples of the material were cut of from the straight sections of Kenter links. In the sequence, a number of specimens were austenitized ed and tempered making use of different treatment times, in order to promote a microstructural homogeneity as well as mechanical properties similar to those associated with industrial heat treatments. The specimens were subjected to hardness and tensile tests and on the basis of the results, all samples were heat treated. After that, fatigue specimens were machined from the treated samples and shot peened with quenched steel shots under pressures of 20 psi, 30 psi and 40 psi. Finally, rotating bend fatigue tests were performed with unpeened and shot peened specimens aiming to establish the stress-life curves of the grade R4 structural steel. The fatigue life extension of the grade R4 structural steel was related to the shot peening. Regarding the results, the surface treatments affected the fatigue life of the material beneficially. This influence was characterized by a longer fatigue cycles and a fatigue life improvement. The results showed that the fatigue life extension was directly related to the shot peening intensity, i.e., the fatigue life extension has increased when increasing the shot peening intensity.

In this thesis structural optimization of castings and thermomechanical analysis of castings are studied. In paper I an optimization algorithm is created by using Matlab. The algorithm is linked to the commercial FE solver Abaqus by using Python script. The optimization algorithm uses the successive response surfaces methodology (SRSM) to create global response surfaces. It is shown that including residual stresses in structural optimization of castings yields an optimal shape that differs significantly from the one obtained when residual stresses are excluded. In paper II the optimization algorithm is expanded to using neural networks. It is tested on some typical bench mark problems and shows very promising results. Combining paper I and II the response surfaces can be either analytical functions, both linear and non-linear, or neural networks. The optimization is then performed by using sequential linear programming or by using a zero-order method called Complex. This is all gathered in a package called StuG-OPT. In paper III and IV focus is on the thermomechanical problem when residual stresses are calculated. In paper III a literature review is performed and some numerical simulations are performed to see where numerical simulations can be used in the industry today. In paper IV simulations are compared to real tests. Several stress lattices are casted and the residual stresses are measured. Simulations are performed by using Magmasoft and Abaqus. In Magmasoft a J2-plasticity model is used and in Abaqus two simulations are performed using either J2-plasticity or the ”Cast Iron Plasticity” available in Abaqus that takes into account the different behavior in tension and compression for grey cast iron. / <p>Report code: LIU-TEK-LIC-2007:34.</p>

Solid oxide fuel cell platforms consisting of alternating cellular layers of yttria-stabilized zirconia electrolyte and Fe-Ni metallic interconnects (Fe45Ni, Fe47.5Ni, Fe50Ni) were produced through the co-extrusion of two particulate pastes. Subsequent thermal treatment in a hydrogen atmosphere was used to reduce iron and nickel oxides and co-sinter the entire structure. Issues surrounding this process include the constrained sintering of the layers and the evolution of residual stress between the dense, fired layers. Sintering curves for individual components of the layers were measured by dilatometry to ascertain each materials impact on overall sintering mismatch. X-ray diffraction, scanning electron microscopy and weight loss were utilized to examine phase evolution within the Fe-Ni alloys during reduction. YSZ powders densified above ~1050C and shrinkage was rapid above the sintering temperature. Shrinkage of the interconnect occurred in two stages: reduction and the initial stages of sintering concluded around ~600C, plateauing shortly and continuing at ~900C as pore removal and grain growth ensued simultaneously. Constrained sintering resulted in the formation of remnant porosity within the interconnect layers. Interconnect compositions were chosen in efforts to minimize disparities in thermal expansion with the electrolyte. Residual strains on the surfaces of the layers were measured by x-ray diffraction. Corresponding stresses were calculated using the sin2y method. Grain growth within the interconnect prohibited random planes to be measured so stress measurements were confined to the ceramic layers. Various material properties such as thermal expansion were collected and employed in a modified finite element model to estimate residual stresses in the platform. A method for determining a crucial parameter, the zero stress temperature was outlined and incorporated. Modeled values were found to agree well with XRD values, providing indirect confirmation of the zero stress temperature calculations. Discrepancies were attributed to microcracks found within the layer that arose due to residual stress values surpassing the tensile strength of the zirconia.

Residual stresses play an important role in the performance of machined components. Component characteristics that are influenced by residual stress include fatigue life, corrosion resistance, and part distortion. The functional behavior of machined components can be enhanced or impaired by residual stresses. Because of this, understanding the residual stress imparted by machining is an important aspect of understanding machining and overall part quality. Machining-induced residual stress prediction has been a topic of research since the 1950s. Research efforts have been primarily composed of experimental findings, analytical modeling, finite element modeling, and various combinations of those efforts. Although there has been significant research in the area, there are still opportunities for advancing predictive residual stress methods. The objectives of the current research are as follows: (1) develop a method of predicting residual stress based on an analytical description of the machining process and (2) validate the model with experimental data. The research focuses on predicting residual stresses in machining based on first principles. Machining process output parameters such as cutting forces and cutting temperatures are predicted as part of the overall modeling effort. These output parameters serve as the basis for determining the loads which generate residual stresses due to machining. The modeling techniques are applied to a range of machining operations including orthogonal cutting, broaching, milling, and turning. The strengths and weaknesses of the model are discussed as well as opportunities for future work.