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An exhaust manifold of a truck engine is subjected to tough conditions. As the truck is started, operated and shut down, it becomes subjected to thermal cycling up to around 800°C. At such high temperatures, corrosion, fatigue and creep are active degradation mechanisms. As can be imagined, the interplay between the three complicates materials selection. It is desired to have a versatile grade of high durability which is not too expensive. At the moment, a ferritic, ductile cast iron designated SiMo51 is used for the application. However, due to the rough conditions, it is considered to be on the verge of its operational limit. As a consequence, there is an ongoing search for candidate materials. In this study, the ductile cast irons SiMo51, SiMo1000, D5S and the cast steel HK30 have been included. In the past, there have been several studies describing corrosion and fatigue of the cast materials used for exhaust manifolds. However, on the subject of creep of cast materials, little is known. The present study aims to reveal creep tendencies of cast materials and to do it in several ways. More precisely, three creep-testing methods were employed: the conventional constant-load creep-test, the “Sequential tensile test (STT)” and “Stress relaxations with thermal cycling (SRTC)”. The first one is the traditional one. It is tedious, usually lasting months or years. The second one is a tensile test in which the strain rate is changed in sequences as specimen deformation proceeds. Here, the idea is that a slow tensile test is not different from a conventional creep test. In the third one, stress relaxations are provoked as a specimen is thermally cycled in a locked state. Since stress relaxations are a consequence of creep deformation, the relaxation data gathered from isothermal holds can be directly compared to results from the isothermal constant-load creep-test and STT. When thermally cycled in a locked state, the materials display a loop character in σ, ε and T which provides extensive information about the mechanical properties over the selected temperature interval. In a logarithmic Norton plot, the creep strain rate is plotted as a function of stress. By plotting STT-data in such Norton plots, it was shown that the creep behaviour of the included materials is well represented by Norton’s law. Furthermore, it was found that the creep strain rates and stress relaxations, measured during isothermal holds in SRTC, in several cases show perfect coincidence with tensile test data obtained through STT. At 700°C, data from all three tests were inserted in the same Norton plot. At higher stress levels, the SRTC-curve follows the STT-curve and at lower stresses, when the creep regime is entered, it bends down and unites with data obtained by the constant-load creep tests. Additionally, it was seen that a relatively high degree of pre-deformation can give a critical stress below which creep deformation stops completely. / <p>The Financier is Scania CV. </p>

The current thesis focuses on the deformation behaviour and strain distribution in the microstructure of ductile iron during tensile loading. Utilizing Digital Image Correlation (DIC) and in-situ tensile test under optical microscope, a method was developed to measure high resolution strain in microstructural constitutes. In this method, a pit etching procedure was applied to generate a random speckle pattern for DIC measurement. The method was validated by benchmarking the measured properties with the material’s standard properties. Using DIC, strain maps in the microstructure of the ductile iron were measured, which showed a high level of heterogeneity even during elastic deformation. The early micro-cracks were initiated around graphite particles, where the highest amount of local strain was detected. Local strain at the onset of the micro-cracks were measured. It was observed that the micro-cracks were initiated above a threshold strain level, but with a large variation in the overall strain. A continuum Finite Element (FE) model containing a physical length scale was developed to predict strain on the microstructure of ductile iron. The materials parameters for this model were calculated by optimization, utilizing Ramberg-Osgood equation. For benchmarking, the predicted strain maps were compared to the strain maps measured by DIC, both qualitatively and quantitatively. The DIC and simulation strain maps conformed to a large extent resulting in the validation of the model in micro-scale level. Furthermore, the results obtained from the in-situ tensile test were compared to a FE-model which compromised cohesive elements to enable cracking. The stress-strain curve prediction of the FE simulation showed a good agreement with the stress-strain curve that was measured from the experiment. The cohesive model was able to accurately capture the main trends of microscale deformation such as localized elastic and plastic deformation and micro-crack initiation and propagation.

[pt] Efeitos da taxa de deformação nas propriedades mecânicas podem ser significativos na simulação do comportamento dinâmico de estruturas deformadas plasticamente, e o objetivo deste trabalho é avaliá-los através de duas tarefas complementares: (i) desenvolver e qualificar uma metodologia apropriada para a medição de propriedades mecânicas dinâmicas, incluindo os requisitos necessários para medir e ajustar da melhor forma possível os dados experimentais, quantificando a incerteza a eles associada; e (ii) verificar a influência da estratégia de medição das propriedades mecânicas na previsibilidade do comportamento de estruturas sujeitas a impactos.Os modelos que descrevem o comportamento mecânico dinâmico são baseados na taxa de deformação de/dt, mas freqüentemente seus parâmetros são medidos em testes controlados pela velocidade do carregamento. Todavia, a taxa de/dt não permanece constante nos testes feitos sob velocidade fixa, o que aumenta a incerteza das previsões baseadas nos parâmetros assim medidos. Para avaliar quantitativamente a influência das estratégias de controle nas propriedades mecânicas, primeiro foi estabelecido um procedimento estatístico apropriado para determinar o número de corpos de prova necessários para medir propriedades dinâmicas dentro de um nível de confiança desejado. Todos os dados foram medidos em duplicata, controlando as taxas de deformação ou a velocidade de carregamento imposta nos corpos de prova, numa larga faixa, 10(-4) menor ou igual a de/dt menor ou igual a 10/s, em testes particularmente feitos com cuidado. A aquisição e tratamento dos dados foram feitos através de um programa desenvolvido em Labview. Um programa desenvolvido em Mathcad foi usado para otimizar os parâmetros dos diversos modelos estudados ao conjunto de dados medidos via um algoritmo Levenberg–Marquardt. A partir deste estudo é proposto um novo modelo que descreve adequadamente o patamar de escoamento, e minimiza os erros de ajuste na região de transição elastoplástica, de grande importância na análise estrutural. Um pórtico simples foi escolhido para verificar o efeito das propriedades dinâmicas nas simulações numéricas feitas no programa LS-DYNA, tradicionalmente usado para modelar impactos elastoplásticos. Vários destes pórticos foram submetidos a impactos numa máquina de queda livre (dropweight), para induzir deformações plásticas variáveis em função das velocidades de impacto. As deformações medidas foram comparadas com as previstas usando as propriedades obtidas sob as duas estratégias de controle, e ajustadas pelos diversos modelos estudados. Desta forma pode-se finalmente quantificar a influência da metodologia de medição das propriedades mecânicas dinâmicas na previsibilidade do comportamento de estruturas sujeitas a impactos elastoplásticos. / [en] The strain rate effects in the mechanical properties can be significant in the simulation of the dynamic behavior in plastic deformed structures, this work has the objective to evaluate these effects through two complementary tasks: (i) to develop and qualify an appropriate methodology for the measurement of dynamic mechanical properties, including the requirements necessary to measure and to adjust of the best possible way the experimental data, quantifying the uncertainty they associated; and (ii) to verify the influence of the strategy of measurement of the mechanical properties in the prediction of the behavior of structures submitted to impacts.The models that describe the dynamic mechanical behavior are based on the strain rate de/dt, but usually its parameters are measured in controlled tests by the speed of loading. However, the rate de/dt does not remain constant in the tests realized with fixed speed, which it increases the uncertainty of the predictions based on the measured parameters. For a quantitative evaluation of the influence of the control strategies in the mechanical properties, was first established a appropriated statistical procedure to determine the number of specimens necessary to measure dynamic properties with a acceptable level of reliability. All the data were measured twice, controlling the strain rate or the loading speed imposed in the specimens, in a wide range, 10(-4) less than or equal de/dt less than or equal 10/s, in tests particularly realized with care. The acquisition and data processing were made through a program developed in Labview. A program developed in Mathcad was used to optimize the parameters of the diverse models studied to the data set measured by a Levenberg-Marquardt algorithm. From this study is proposed a new model that describes the yieldind adequately, and minimizes the errors of fitting in the region of transition elasto-plastic, that plays a great role in the structural analysis.A simple porch was chosen to verify the effect of the dynamic properties in the numerical simulations made in program LS-DYNA, used traditionally to simulate elasto-plastics impacts. Several of these porches had been submitted to impacts in a machine of free fall (drop-weight), to induce variable plastic strains in function of the impact speeds. The measured strains were compared with the predicted ones using the properties obtained with the two strategies of control, and adjusted by the diverse studied models. In such way the influence of measurement methodology of the dynamic mechanical properties in the prediction of the behavior of structures submitted to elasto-plastics impacts can be quantified.

The aim of this research was to analyze the mechanical and thermal properties of composites andhybrid composites prepared with four types of jute fibers and two different resins; biobased thermosetresins acrylated epoxidized soybean oil (AESO) and mathacrylated anhydride modified soybean oil(MMSO). The processing technique used was vacuum injection molding (VIM). Tensile and, flexuraltestings and dynamic mechanical and thermal analysis (DMTA) were used to characterize thecomposites’ properties. The results showed that the AESO composites have better tensile and flexuralproperties. This may be due to the fact that the curing conditions were quite the same for both AESOand MMSO composites but MMSO composites showed different behavior during curing step. Theywere completely cured in a shorter time compared to AESO composites. Having equal curing time forboth resins’ composites can damage the structure of MMSO composites and hybrids. Tan delta peak forthe MMSO reinforced composites occurs at higher temperatures, compared to AESO reinforcedcomposites, which means better thermal properties for MMSO reinforced composites.

Foram ensaiadas quatro cerâmicas VMK 95, Omega 900, IPS d. Sign e Cergogold. Os ensaios realizados foram: 1) resistência de união, por ensaio de tração, à dentina humana, de cones (6 x 3 x 5mm) de cerâmica, unidos por meio de cimentos fosfato de zinco, resinosos Rely X e Panávia F (n=10); 2) rugosidade pelos parâmetros Ra, Rt, Ry, Rz e Rp, de superfícies tratadas com ácido fluorídrico a 10% e polidas (n=10); 3) durezas Knoop e Vickers (n=4); 4) análise por microscopia de força atômica de superfícies polidas apenas (n=5). Os dados foram analisados pela análise de variância e testes de Tukey foram feitos quando necessário. A resistência de união com o fosfato de zinco e o parâmetro de rugosidade Ra foram analisados em separado, por apresentarem variâncias bem menores que as correspondentes aos demais agentes cimentantes e parâmetros. Os resultados mostraram que a resistência de união com o fosfato de zinco foi muito baixa (1,61 a 2,91MPa), nas condições dos ensaios. Com cimentos resinosos a resistência média variou de 16,61 a 18,47MPa. O cimento Rely X apresentou menor resistência em média, mas isto também depende da cerâmica.As rugosidades das superfícies tratadas com ácido fluorídrico a 10% foram bem maiores do que nas polidas. Nessas o parâmetro Ra variou de 2,15 a 4,26µm e os demais parâmetros de 10,99 a 21,93µm. Nas superfícies polidas o Ra variou de 0,53 a 0,84µm. Os demais parâmetros apresentaram médias de 2,01 a 5,23µm. A dureza Knoop variou de 418,9 a 493,8 kgf/mm2 e a Vickers de 492,5 a 572,2 kgf/mm2. Esta última apresentou valores maiores. A microscopia de força atômica permitiu visualizar irregularidades de superfícies. As principais conclusões foram: o cimento de fosfato de zinco, na condição do ensaio (por tração) apresentou resistência de união muito baixa; com os cimentos resinosos a resistência de união foi bem alta; as rugosidades foram muito altas após o tratamento superficial com ácido fluorídrico a 10%, mas, as polidas apresentaram valores bem baixos; as durezas Vickers fornecem valores maiores que a Knoop. / Four ceramics (VMK 95, Omega 900, IPS d.Sign and Cergogold) were tested regarding: 1) tensile bond strength to human dentin of ceramic cones (6 x 3 x 5mm), cemented with zinc phosphate, and resin cements, Rely X and Panávia F (n=10); 2) roughness by means of parameters Ra, Rt, Ry, Rz and Rp, of HF (10%) surface treated and polished surfaces (n=10); 3) Knoop and Vickers hardness (n=4); 4) atomic force microscopy; only of polished surfaces (n=5). Data were analyzed by ANOVA and when necessary were used Tukey’s tests. Tensile bond strength with zinc phosphate cement and Ra roughness parameter were analyzed separately because of their low variances when compared with those of resin cements and the other roughness parameter. Results showed that tensile bond strength of zinc phosphate was very low (1.61 to 2.91MPa) in this test condition (tensile test). Resin cements presented tensile bond strength at range 16.61 to 18.47MPa. Cement Rely X presented lower strength, but it depends also of ceramic. HF (10%) treated surfaces presented much higher roughness than polished ones. In HF (10%) treated surfaces parameter Ra ranged from 2.15 to 4.26µm and the other parameter ranged from 10.99 to 21.93µm. Roughness of polished surfaces Ra ranged from 0.53 to 0.84µm. Range of the other parameter was from 2.01 to 5.23µm. Knoop hardness ranged from 418.9 to 493.8 kgf/mm2 and Vicker’s from 492.5 to 572.2 kgf/mm2. This type presented higher values. Atomic force microscopy permited good vision of surface irregularities. Main conclusions were: zinc phosphate cement at test conditions (tensile) presented low bond strength; resin cements presented high tensile bond strength; surface treatment with HF (10%) led to high roughness but in polished one it was low; Vicker’s hardness presented higher values than Knoop one.

High manganese TWIP (TWinning Induced Plasticity) steels are particularly attractive for automotive applications because of their exceptional properties of strength combined with an excellent ductility. However, as austenitic steels, they appear to be sensitive to liquid zinc embrittlement during welding, the liquid zinc arising from the melted coating due to the high temperatures reached during the welding process. In this framework, the cracking behaviour of a high manganese austenitic steel has been investigated in relation to the liquid metal embrittlement (LME) phenomenon by hot tensile tests carried out on electro-galvanized specimens using a Gleeble 3500 thermomechanical simulator. The influence of different parameters such as temperature and strain rate on cracking behaviour has been studied. Embrittlement appears within a limited range of temperature depending on experimental conditions. Conditions for which cracking occurs could be experienced during welding processes. The existence of a critical stress above which cracking appears has been evidenced and this critical stress can be used as a cracking criterion. Finally, the study of the influence of different parameters such as time of contact between steel and liquid zinc before stress application, coating and steel on LME occurrence provides understanding elements of LME mechanism and permits to suggest solutions for preventing cracking during spot welding of such steels.

The aim of this study is to investigate the mechanical properties of continuous fiber reinforced composite tubes, produced by filament winding technique. For this purpose, tensile and split-disk tests were performed with specimens produced with five different fiber materials and two different resin systems. Longitudinal tensile and hoop tensile properties of the selected specimens were determined and the effect of reinforcement direction on the mechanical performance of these composites was investigated. In addition, the effect of a filament-winding processing variable (fiber tension) on longitudinal and hoop tensile properties of the selected composites was obtained. A slight increase in hoop/longitudinal tensile properties of specimens was observed for specimens wounded with tension and with winding angles greater than 60o. The tests were performed according to American Society for Testing and Materials (ASTM) standards. The split-disk tests of selected composite specimens were simulated by the finite element method. For this purpose, a commercial finite element package program was used. Experimental results were used both as input in terms of material data for the finite element study and for comparison purposes. A good agreement was obtained between the simulation results and the experimental data.

Electrospinning, un procédé original de mise en forme de polymère par application d'un champ électrique élevé, est largement utilisé pour la synthèse de membranes non tissées nanofibreuses. Les membranes électrospinnées ont une forte porosité et un rapport surface / volume élevé. En effet, ces matériaux ont suscité beaucoup d'intérêt et d'études au cours des dernières décennies, ce qui ouvre la voie à de nombreuses applications telles que la détection, l'ingénierie tissulaire ou la livraison de médicaments. La recherche actuelle vise à avoir des membranes fibreuses avec une architecture contrôlée utilisant différents types de collecteurs.Le développement de nanofibres à base de biopolymères et une stratégie thérapeutique pour la régénération des tissus mous.Le premier objectif de cette thèse était de développer de nouveaux matériaux biocompatibles et bio résorbables à l'aide de fibres à l'échelle nanométrique obtenues par électrospinning. En outre, cette étude a examiné l'influence de la viscosité, de la concentration et de la tension superficielle des solutions de polymère sur les fibres obtenues. En outre, le débit, la tension appliquée et les paramètres environnementaux (température et humidité) ont également été optimisés au cours de la production de nanofibres.Les fibres ont été obtenues à partir de PEO, polymère biocompatible. C'est un polymère linéaire qui se compose de segments éthylène et éther [-CH2CH2O-]n. L'oxyde d'éther peut être utilisé pour interagir avec des espèces hydrophiles. En raison de sa solubilité dans l'eau, sa non-toxicité et sa capacité à être électrospinné, le PEO a été utilisé comme additif dans des solutions de biopolymères pour permettre la formation d'électrodes fibreuses. La résistance mécanique du PEO dépend de la masse moléculaire, de la conformation des chaînes polymères et de la taille des fibres ainsi que la structure du réseau.Le deuxième effort majeur de cette thèse s'est concentré sur le contrôle des mailles fibreuses. Une telle activité de recherche est justifiée par l'influence attendue de la morphologie du réseau de fibres sur les propriétés mécaniques des membranes et leur caractère biomimétique qui favorise la colonisation et la croissance des cellules du tissu hôte. Le contrôle de cette structure a été réalisé grâce au développement de collecteurs.L'objectif de la thèse est de fabriquer des structures fibreuses non tissées aléatoires et structurées par electrospinning. Ces structures fibreuses sont obtenues à partir de Poly (oxyde d'éthylène), PEO, en solutions à différentes concentrations et masses moléculaires.Le dépôt de fibres est réalisé sur deux types de collecteurs: a) Feuille d'aluminium, b) Collecteur micro-structuré (dimension 3 × 3 cm). Les analyses morphologiques des membranes ont été menées à l'aide d'une microscopie électronique à balayage (MEB) et leurs propriétés mécaniques sous traction ont été réalisées à l'aide du rhéomètre ARESG2.La morphologie des matériaux electrospinnées passe graduellement d'une structure de type perles à des fibres uniformes lorsque la concentration et la masse moléculaire augmentent. Une étude comparative des propriétés morphologiques et mécaniques (essai de traction) des deux structures fibreuses a été réalisée. Cette étude a montré qu'il est possible d'avoir une distribution de fibres formant une cellule primitive très uniforme dans un réseau de dimension 3 × 3 cm. Ce réseau structuré a une contrainte à la rupture plus importante que celle du réseau fibreux aléatoire obtenu conventionnellement avec une feuille d'aluminium. / Electrospinning, an original polymer process under high electric fields to produce a network of thin fiber having a micrometer diameter, is widely used for the synthesis of nanofibrous non-woven membranes. The fabricated electrospun membranes have a high porosity and a high surface to volume ratio. Indeed, they reveal much interest and have been much developed in the last decades, which paves the way for numerous applications such as sensing, tissue engineering or drug delivery. Current research aims to have fibrous membranes with a controlled architecture using various types of collectors.This thesis is part of a global and emerging project that focuses on the production of structured scaffolds nanofibers based on biopolymers and dedicated to the therapeutic strategy for the regeneration of soft tissues.In the present work, the first focus was to develop new biocompatible and bioresorbable materials composed of nanoscale fibers obtained by electrospinning. In addition, this study examined the influence of viscosity, concentration, and surface tension of PEO solutions on the obtained fibers. Further, the flow rate, applied voltage and environmental parameters (temperature and humidity) were also optimized in the course of nanofibers production.Biocompatible fibers have been obtained by using PEO. It is a linear polymer that consists of ethylene and ether segments [-CH2CH2O-]n. The ether oxygen allows this polymer to interact with other hydrophilic species, while the ethylene part participates in hydrophobic interactions. Due to its water solubility, non-toxicity and electrospinn ability, PEO has been used as an additive in biopolymer solutions to enable the formation of electrospun fibers. The mechanical strength of the PEO depended on the molecular weight, the conformation of the polymer chains and the fiber scale, the structure of the network.The second major effort of this thesis focused on the control of the mesh fibers. Such research activity is justified by the expected influence of the morphology of the fiber network on the mechanical properties of scaffolds and their biomimetic character that could favor the colonization and growth of the cells of the host tissue. The control of this structure has been achieved through the development of collectors.The objective of this project is making non-woven fibrous structures in uncontrolled architecture as well as non-woven with controlled architecture by using the electrospinning process. These fibrous structures are obtained from Poly(ethylene oxide), PEO, solutions with different concentration and molecular weight. The deposit of fibers is made on two types of collectors: a) Aluminum foil, b) micro-structured collector (dimension 3×3 cm). The morphological analyses of the membranes were investigated using scanning electron microscopy (SEM) and their mechanical properties were characterized by tensile test using the ARESG2 rheometer. The morphology of the electrospun polymer gradually changes from beads to uniform fibers with increasing polymer concentration and molecular weight. A comparative study of the morphological and mechanical (tensile test) properties, of both fibrous structures is performed. This study showed that it is possible to have a distribution of fiber forming a very uniform primitive cell in a network of dimension 3×3 cm. This structured network has a strain at the break more important than that for the network fibers, which are collected on Aluminum foil.