In der Arbeit werden spezielle Kupferlote im MIG- und Laserlötverfahren an Stählen mit unterschiedlichem Festigkeitsverhalten untersucht. Die Ergebnisse sollen dazu beitragen, den Lötprozess durch den Einsatz spezieller Kupferbasislote zu optimieren und durch reduzierten Energieeintrag ein homogeneres Eigenschaftsfeld im Bereich der Fügestelle zu erzeugen. Den Verarbeitern dieser Werkstoffe soll die Möglichkeit gegeben werden, diese Werkstoffe rationeller und mit höherer Effektivität zu verarbeiten. Im Ergebnis der Arbeit sollen Verbesserungen der Eigenschaften der Lötnähte erzielt werden, die besonders in der Dünnblechverarbeitung mit Schwerpunkt Karosseriebau Anwendung finden. Wesentliche Ziele sind die Erhöhung der Festigkeitseigenschaften, eine Erhöhung der Fügegeschwindigkeit, die Verbesserung des Phosphatierungsverhaltens sowie eine Reduzierung der eingebrachten Wärmeenergie. Die Vielfältigkeit dieser Anforderungen macht es notwendig, die Versuche sowohl im Laser- als auch im MIG-Lötverfahren durchzuführen. Die Lötverfahren werden in der Praxis für unterschiedliche Anforderungen innerhalb der Karosserie eingesetzt. Das Fügen von hochfesten Strukturelementen oder Außenhautbauteilen erfordert in Abhängigkeit von den Anforderungen die Verwendung ausgewählter Zusatzwerkstoffe. Die Vielfältigkeit der Werkstoffe und der Anforderungen spiegelt sich in den Untersuchungen der vorliegenden Arbeit wieder. Für weitergehende Untersuchungen, speziell im hochfesten Blechbereich, soll die Arbeit entsprechende Grundlagen bieten. Als Vorlage für die Erarbeitung von experimentellen und theoretischen Methodiken der Prozessbetrachtung werden neben typischen Kupferloten neu entwickelte Lotlegierungen verwendet. Bei der Betrachtung der Kupferlegierungen werden die unterschiedlichen Einflüsse auf den Fügeprozess definiert und beschrieben. Es wird festgestellt, dass niedrig schmelzende Lote mit ausgewählter Legierungszusammensetzung im Gegensatz zu Eisenbasis-Schweißdrähten einen geringeren negativen Einfluss auf das Gefüge der Bleche im Nahtbereich ausüben. Um die thermische Beanspruchung, besonders in der Wärmeeinflusszone, während des Fügeprozesses gering zu halten, kann zusätzlich eine geeignete Stromquellentechnik zum Einsatz kommen. Mit Hilfe des „kalten“ Lichtbogens ist es möglich, die eingebrachte Streckenenergie weiter zu reduzieren. Faktoren, die den Energieeintrag beeinflussen, werden in der vorliegenden Arbeit in experimentellen und theoretischen Untersuchungen hinsichtlich ihrer Wirkung auf das Festigkeitsverhalten betrachtet. Es werden durch geeignete Legierungskombinationen die Einflüsse auf die Steigerung der Lötgeschwindigkeit und auf eine Verbesserung des Phosphatierungsverhaltens untersucht. Die Ergebnisse dieser Untersuchungen liefern die Informations- und Beweisbasis für die erarbeiteten Legierungssysteme und ermöglichen es, den optimierten Lötprozess an hochfesten Stahlblechen wissenschaftlich zu betrachten. Die Auswertung der wissenschaftlichen Experimente, dargestellt in den angefügten ausführlichen Tabellen, stellen die Zusammenhänge zwischen der Legierungsauswahl und der eingebrachten Streckenenergie dar. Die Erkenntnisse aus der vorliegenden Arbeit sollen für das Fügen von hochfesten Blechen die Entscheidung über die Auswahl geeigneter Zusatzwerkstoffe erleichtern. Die Ergebnisse der theoretischen Untersuchungen anhand mathematischer Modelle zur Beschreibung der physikalischen Prozesse der Wärmezufuhr durch Verwendung eines ausgewählten Lotes in Kombination mit geeigneter Stromquellentechnologie sind die Grundlage für die Optimierung des Lötprozesses. Die vorgeschlagenen Modelle zur Entwicklung und Optimierung von Lichtbogenlötprozessen mit neu entwickelten Lotlegierungen wurde im Rahmen der vorgelegten Arbeit an realen Blechqualitäten angewendet und überprüft. In den Ergebnissen hat sich bestätigt, dass die Verwendung spezieller Kupferlote zu verbesserten Verarbeitungseigenschaften führen, und damit Konzepte zum wirtschaftlich verbesserten Fügen angeboten werden.

In der Arbeit werden spezielle Kupferlote im MIG- und Laserlötverfahren an Stählen mit unterschiedlichem Festigkeitsverhalten untersucht. Die Ergebnisse sollen dazu beitragen, den Lötprozess durch den Einsatz spezieller Kupferbasislote zu optimieren und durch reduzierten Energieeintrag ein homogeneres Eigenschaftsfeld im Bereich der Fügestelle zu erzeugen. Den Verarbeitern dieser Werkstoffe soll die Möglichkeit gegeben werden, diese Werkstoffe rationeller und mit höherer Effektivität zu verarbeiten. Im Ergebnis der Arbeit sollen Verbesserungen der Eigenschaften der Lötnähte erzielt werden, die besonders in der Dünnblechverarbeitung mit Schwerpunkt Karosseriebau Anwendung finden. Wesentliche Ziele sind die Erhöhung der Festigkeitseigenschaften, eine Erhöhung der Fügegeschwindigkeit, die Verbesserung des Phosphatierungsverhaltens sowie eine Reduzierung der eingebrachten Wärmeenergie. Die Vielfältigkeit dieser Anforderungen macht es notwendig, die Versuche sowohl im Laser- als auch im MIG-Lötverfahren durchzuführen. Die Lötverfahren werden in der Praxis für unterschiedliche Anforderungen innerhalb der Karosserie eingesetzt. Das Fügen von hochfesten Strukturelementen oder Außenhautbauteilen erfordert in Abhängigkeit von den Anforderungen die Verwendung ausgewählter Zusatzwerkstoffe. Die Vielfältigkeit der Werkstoffe und der Anforderungen spiegelt sich in den Untersuchungen der vorliegenden Arbeit wieder. Für weitergehende Untersuchungen, speziell im hochfesten Blechbereich, soll die Arbeit entsprechende Grundlagen bieten. Als Vorlage für die Erarbeitung von experimentellen und theoretischen Methodiken der Prozessbetrachtung werden neben typischen Kupferloten neu entwickelte Lotlegierungen verwendet. Bei der Betrachtung der Kupferlegierungen werden die unterschiedlichen Einflüsse auf den Fügeprozess definiert und beschrieben. Es wird festgestellt, dass niedrig schmelzende Lote mit ausgewählter Legierungszusammensetzung im Gegensatz zu Eisenbasis-Schweißdrähten einen geringeren negativen Einfluss auf das Gefüge der Bleche im Nahtbereich ausüben. Um die thermische Beanspruchung, besonders in der Wärmeeinflusszone, während des Fügeprozesses gering zu halten, kann zusätzlich eine geeignete Stromquellentechnik zum Einsatz kommen. Mit Hilfe des „kalten“ Lichtbogens ist es möglich, die eingebrachte Streckenenergie weiter zu reduzieren. Faktoren, die den Energieeintrag beeinflussen, werden in der vorliegenden Arbeit in experimentellen und theoretischen Untersuchungen hinsichtlich ihrer Wirkung auf das Festigkeitsverhalten betrachtet. Es werden durch geeignete Legierungskombinationen die Einflüsse auf die Steigerung der Lötgeschwindigkeit und auf eine Verbesserung des Phosphatierungsverhaltens untersucht. Die Ergebnisse dieser Untersuchungen liefern die Informations- und Beweisbasis für die erarbeiteten Legierungssysteme und ermöglichen es, den optimierten Lötprozess an hochfesten Stahlblechen wissenschaftlich zu betrachten. Die Auswertung der wissenschaftlichen Experimente, dargestellt in den angefügten ausführlichen Tabellen, stellen die Zusammenhänge zwischen der Legierungsauswahl und der eingebrachten Streckenenergie dar. Die Erkenntnisse aus der vorliegenden Arbeit sollen für das Fügen von hochfesten Blechen die Entscheidung über die Auswahl geeigneter Zusatzwerkstoffe erleichtern. Die Ergebnisse der theoretischen Untersuchungen anhand mathematischer Modelle zur Beschreibung der physikalischen Prozesse der Wärmezufuhr durch Verwendung eines ausgewählten Lotes in Kombination mit geeigneter Stromquellentechnologie sind die Grundlage für die Optimierung des Lötprozesses. Die vorgeschlagenen Modelle zur Entwicklung und Optimierung von Lichtbogenlötprozessen mit neu entwickelten Lotlegierungen wurde im Rahmen der vorgelegten Arbeit an realen Blechqualitäten angewendet und überprüft. In den Ergebnissen hat sich bestätigt, dass die Verwendung spezieller Kupferlote zu verbesserten Verarbeitungseigenschaften führen, und damit Konzepte zum wirtschaftlich verbesserten Fügen angeboten werden.

Performance of buildings and bridges during past earthquakes has indicated that many of these structures are vulnerable to seismic damage and structural collapse. The deficiencies in pre-1970s design codes have resulted in poor performance of reinforced concrete structures during seismic excitations. The Richter Magnitude 6.6 - 1971 San Fernando Earthquake raised awareness for seismic retrofit needs of existing buildings for the first time. The majority of deficiencies of vulnerable concrete columns can be overcome through seismic retrofits that involve additional transverse reinforcement. This can be done either by providing reinforced concrete, steel, or fibre-reinforced polymer (FRP) jackets around existing columns; or by applying transverse prestressing to columns (RetroBelt System). The research project presented in this thesis involves a seismic retrofit methodology for seismically deficient building and bridge columns, utilizing the use of high-strength packaging straps as external reinforcement for transverse prestressing. The emphasis in the project is placed on experimental research. Three seismically deficient full-size reinforced concrete columns, with a circular, a square and a rectangular cross- section, either critical in shear or flexure, were designed, built and tested under simulated seismic loading. The results indicate that external prestressing of columns in transverse direction with high-strength steel straps improves ductility and energy dissipation capacity of seismically deficient columns. They further indicate that current analytical techniques can be used to predict the force-displacement relationships of columns. A design approach is presented for the retrofit methodology investigated.

Ultra-high strength steels is known to be susceptible for hydrogen embrittlement at very low concentrations of hydrogen. In this thesis three methods to prevent or reduce the hydrogen embrittlement in martensitic steel, with tensile strength of 1500 MPa, were studied. First, a barrier layer of aluminium designed to prevent hydrogen to enter the steel, which were deposited by vacuum evaporation. Second, a decarburization process of the steels surface designed to mitigate the induced stresses from cutting. Last, a hydrogen relief treatment at 150°C for 11 days and 200°C for 4 days, to reduce the hydrogen concentration in the steel. The effect of the hydrogen embrittlement was analyzed by manual measurements of the elongations after a slow strain rate testing at 5*10-6 mm/s, and the time to fracture in an in-situ constant load test with a current density of 1.92 mA/cm2 in a 0.5 M Na2SO4 solution. The barrier layer showed an increase in time to fracture, but also a decrease in elongations. The decarburized steel had a small increase in the time to fracture, but not enough to make it a feasible process. The hydrogen relief treatment showed a general decrease in hydrogen concentrations, but the elongation measurements was irregular although with a tendency for improvement. The simplicity of the hydrogen relief treatment makes it an interesting process to reduce the influence of hydrogen embrittlement. However, more investigations are necessary.

This thesis illustrates parameter optimization of compressive strength, flexural strength and impact resistance of steel fiber reinforced high strength concrete (SFRHSC) by statistical design and analysis of experiments. Among several factors affecting the compressive strength, flexural strength and impact resistance of SFRHSC, five parameters that maximize all of the responses have been chosen as the most important ones as age of testing, binder type, binder amount, curing type and steel fiber volume fraction. Taguchi and regression analysis techniques have been used to evaluate L27(313) Taguchi&amp / #65533 / s orthogonal array and 3421 full factorial experimental design results. Signal to noise ratio transformation and ANOVA have been applied to the results of experiments in Taguchi analysis. Response surface methodology has been employed to optimize the best regression model selected for all the three responses. In this study Charpy Impact Test, which is a different kind of impact test, have been applied to SFRHSC for the first time. The mean of compressive strength, flexural strength and impact resistance have been observed as around 125 MPa, 14.5 MPa and 9.5 kgf.m respectively which are very close to the desired values. Moreover, this study is unique in the sense that the derived models enable the identification of underlying primary factors and their interactions that influence the modeled responses of steel fiber reinforced high strength concrete.

In this thesis, novel bulk Al-based alloys with high content of Al have been produced by powder metallurgy methods from amorphous and partially amorphous materials. Different processing routes, i.e. mechanical alloying of elemental powder mixtures, controlled pulverization of melt-spun glassy ribbons and gas atomization, have been employed for the production of the Al-based powders. Among the different processing routes, gas atomization is the best choice for the production of Al-based amorphous and partially amorphous powders as precursors for the subsequent consolidation step because it allows the production of large quantities of powders with homogeneous properties (e.g. structure and thermal stability) along with a uniform size distribution of particles. Amorphous and nanocrystalline powders have to be consolidated to achieve dense bulk specimens. However, consolidation of these phases is not an easy task and special care has to be taken with respect to accurate control of the consolidation parameters in order to achieve dense bulk specimens without inducing undesirable microstructural transformations (e.g. crystallization and grain coarsening) or insufficient particle bonding. Consequently, the effect of temperature on viscosity as well as on phase formation has been studied in detail in order to select the proper consolidation parameters. Following their characterization, the Al-based powders have been consolidated into bulk specimens by hot pressing (HP), hot extrusion and spark plasma sintering (SPS) and their microstructure and mechanical properties have been extensively investigated. Consolidation into highly-dense bulk samples cannot be achieved without extended crystallization of the glassy precursors. Nevertheless, partial or full crystallization during consolidation leads to remarkable mechanical properties. For example, HP Al84Gd6Ni7Co3 samples display a remarkably high strength of about 1500 MPa, which is three times larger than the conventional high-strength Al-based alloys, along with a limited but distinct plastic deformability (3.5 – 4%). Lower strength (930 MPa) but remarkably larger plastic strain exceeding 25 % has been achieved for the Al87Ni8La5 gas-atomized powders consolidated by SPS above their crystallization temperature. Similarly, HP Al90.4Y4.4Ni4.3Co0.9 bulk samples display high compression strength ranging between 820 and 925 MPa combined with plastic strain in the range 14 – 30%. Finally, preliminary tensile tests for the Al90.4Y4.3Ni4.4Co0.9 alloy reveal promising tensile properties comparable to commercial high-strength Al-based alloys. The mechanical behavior of the consolidated specimens is strictly linked with their microstructure. High strength and reduced plasticity are observed when a residual amorphous phase is present. On the other hand, reduced strength but enhanced plastic deformation is a result of the complete crystallization of the glass and of the formation of a partially or fully interconnected network of deformable fcc Al. These results indicate that the combined devitrification and consolidation of glassy precursors is a particularly suitable method for the production of Al-based materials characterized by high strength combined with considerable plastic strain. Through this method, the mechanical properties of the consolidated samples can be varied within a wide range of strength and ductility depending on the microstructure and the consolidation techniques used. This might open a new route for the development of innovative high-performance Al-based materials for transport applications.

Made available in DSpace on 2014-12-17T14:48:14Z (GMT). No. of bitstreams: 1 JanainaSS_DISSERT.pdf: 4645317 bytes, checksum: 1f500f6eaf05e8ac34d38ba500240ab2 (MD5) Previous issue date: 2013-04-22 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The reinforced concrete structures are largely used in buildings worldwide. Upon the occurrence of fire in buildings, there is a consensus among researchers that the concrete has a high resistance to fire, due mainly to its low thermal conductivity. However, this does not mean that this material is not affected by exposure to high temperatures. Reduction of the compressive strength, modulus of elasticity, discoloration and cracking, are some of the effects caused by thermal exposure. In the case of concretes with higher resistance occurs even desplacamentos explosives, exposing the reinforcement to fire and contributing to reducing the support capacity of the structural element. Considering the above, this study aims to examine how the compressive strength and porosity of concrete are affected when subjected to high temperatures. Were evaluated concrete of different resistances, and even was the verified if addition fibers of polyethylene terephthalate (PET) in concrete can be used as an alternative to preventing spalling. The results indicated that explosive spalling affect not only high strength concrete whose values of this study ranged from 70 to 88 MPa, as well as conventional concrete of medium strength (52 MPa) and the temperature range to which the concrete begins to suffer significant changes in their resistance is between 400 ? C and 600 ? C, showing to 600 ? C a porosity up to 188% greater than the room temperature / As estruturas de concreto armado s?o largamente utilizadas nas edifica??es em todo o mundo. Quando da ocorr?ncia de inc?ndio em edifica??es, ? consenso entre os estudiosos, que o concreto apresenta uma elevada resist?ncia ao fogo, devido principalmente a sua baixa condutividade t?rmica. No entanto, isto n?o significa que esse material n?o seja afetado pela exposi??o a elevadas temperaturas. Redu??o de resist?ncia ? compress?o e no m?dulo de elasticidade, altera??o na colora??o e aparecimento de fissuras, s?o alguns dos efeitos causados pela exposi??o t?rmica. No caso de concretos com resist?ncia mais elevada, ocorre ainda desplacamentos explosivos, expondo as armaduras ao fogo, e contribuindo assim para a redu??o da capacidade suporte do elemento estrutural. Diante do exposto, o presente trabalho tem por objetivo analisar como a resist?ncia ? compress?o e a porosidade do concreto s?o afetadas quando submetido a elevadas temperaturas. Foram avaliados concretos de diferentes resist?ncias, e ainda foi verificado se a incorpora??o de fibras de politereftalato de etileno (PET), em matriz de concreto, pode ser usada como alternativa a preven??o do lascamento. Os resultados indicaram que lascamentos explosivos acometem n?o somente os concretos de alta resist?ncia, cujos valores desta pesquisa variaram de 70 a 88 MPa, como tamb?m o concreto convencional de m?dia resist?ncia (52 MPa), e que a faixa de temperatura para o qual o concreto come?a a sofrer altera??es expressivas em sua resist?ncia fica entre 400 ?C e 600 ?C, apresentando aos 600 ?C uma porosidade at? 188% maior que a apresentada a temperatura ambiente

O desenvolvimento do concreto de alta resistência - CAR foi um importante avanço na tecnologia de concreto, entretanto, a despeito de suas inúmeras vantagens como material estrutural, o seu emprego tem sido limitado, por ter se revelado mais susceptível à fissuração nas primeiras idades. Isto se deve à ocorrência do fenômeno da retração autógena, particularmente mais intenso nestes concretos que nos de resistência normal, uma vez que no CAR, há significativamente maior quantidade de material cimentício e menor quantidade de água, o que dá origem a uma estrutura porosa muito refinada logo nas primeiras idades, gerando altas magnitudes de tensões capilares. Além do estudo sobre o entendimento do fenômeno, as pesquisas atualmente têm buscado formas de mitigá-lo a fim de contribuir para estruturas mais duráveis. Diante do exposto, esta pesquisa investigou o comportamento do CAR, no tocante às propriedades mecânicas, elásticas e viscoelásticas e à durabilidade, quando empregado um aditivo redutor de retração - ARR que pode se configurar como estratégia mitigadora para a redução da retração autógena, bem como verificar a sua influência sobre a microestrutura e hidratação da pasta de cimento. Os resultados indicam que o ARR é eficaz na redução da retração autógena e retração por secagem, sem alterar de forma relevante as propriedades mecânicas e elásticas: a resistência à compressão sofre uma pequena queda de 5% com o uso de 2% de ARR em relação ao concreto referência, contudo, as demais propriedades não são alteradas com o uso do ARR. Quanto ao efeito sobre a fluência, não se obtiveram resultados conclusivos. A durabilidade, medida pelos ensaios de penetrabilidade a íons cloretos, permeabilidade à água, carbonatação natural e absorção capilar e por imersão, não é comprometida com a incorporação do aditivo redutor de retração. Do ponto de vista microestrutural, observou-se que o ARR altera o volume total de poros, embora de uma forma não muito expressiva; e ainda constatou-se que este aditivo afeta a velocidade de hidratação das pastas de cimento, e que possivelmente interage com compostos de hidratados da pasta, sem, no entanto, alterar as características macroestruturais do material. / The development of high strength concrete - HSC represented an important advance in concrete technology. However, even knowing that this kind of concrete has several advantages as a structural material, its application is limited by the early ages cracking. This occurrence is due to the autogenous shrinkage phenomenon, once HSC has a greater amount of cementitious material and a lower amount of water in relation to a normal-strength concrete. This condition implies in a greatly refinement of pore structure at early ages which lead to a higher magnitudes of capillary tension than the one observed in a normal-strength concrete. Beyond to study the phenomenon, much research has been conducted in many countries in order to reduce autogenous shrinkage and contribute to more durable structures. So, this research aims to investigate the effectiveness of shrinkage-reducing admixtures – SRA in decreasing the autogenous shrinkage of HSC, and mainly, verify its influence on viscoelastic, elastic and mechanicals properties and durability. The effect of SRA on microstructure and on the cement paste hydration was also investigated. The results show that SRA is effective in reducing the autogenous shrinkage and drying shrinkage without remarkable changes in elastic and mechanical properties. There were not conclusive results related to the creep property. The concrete durability under the action of aggressive agents (such as water, CO2 and chloride) was not influenced by the SRA, information provided by the results of chloride penetration, natural carbonation, water permeability, capillary absorption and absorption of water tests. In a microstructural point of view, it was observed that the addition of SRA results in a small rise in total pore volume. Besides, the results suggest that the SRA affects the rate of cement hydration and it can interact to the hydrated products of paste without implying in great influences on the macrostructural characteristics of the material.

Submitted by Marcelo Soares Amorim null (marcelo.amorim@anac.gov.br) on 2016-04-18T20:56:24Z No. of bitstreams: 1 MSA.pdf: 55664014 bytes, checksum: 32a3e8c76677251f96eaeb9c6694cd92 (MD5) / Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2016-04-20T19:48:01Z (GMT) No. of bitstreams: 1 amorim_ms_me_guara.pdf: 55664014 bytes, checksum: 32a3e8c76677251f96eaeb9c6694cd92 (MD5) / Made available in DSpace on 2016-04-20T19:48:01Z (GMT). No. of bitstreams: 1 amorim_ms_me_guara.pdf: 55664014 bytes, checksum: 32a3e8c76677251f96eaeb9c6694cd92 (MD5) Previous issue date: 2016-03-22 / Os aços avançados de alta resistência são de importância na indústria moderna da mobilidade. Na indústria automobilística, por exemplo, é previsto que o uso destes materiais cresça de maneira acentuada nos próximos anos. Algumas características destes materiais podem ser melhoradas por meio de tratamento térmico, seja ele em toda a extensão do produto ou em partes específicas para as quais se deseja um dado comportamento mecânico. Neste trabalho se propôs a investigação inicial do efeito de um tratamento térmico local por radiação laser, uma forma controlada de adição de energia, nas propriedades mecânicas e na microestrutura de dois destes materiais. Os materiais estudados foram os aços dual phase (DP 600) e o transformed induced plasticity (TRIP 750), e a contribuição pretendida aqui foi prover informações sobre a interação entre a radiação laser e os materiais de estudo, dentro de determinados parâmetros de processamento térmico. Para a caracterização dos materiais, conforme recebidos e após os tratamentos térmicos a laser propostos, foram realizados ensaios metalográficos, de difratometria de raios-X, de dureza e de tração. Verificou-se que, para a faixa de parâmetros de processo experimentada, houve mudanças significativas nas características dos materiais estudados, notadamente um aumento na dureza superficial, alcançando valores correspondentes ao dobro dos relativos aos materiais sem tratamento, nos limites de escoamento, com aumentos de até 15% com relação ao material-base, e de resistência, com aumentos até cerca de 30% com relação ao material-base. Por outro lado, verificou-se uma diminuição no alongamento específico dos materiais, quando submetidos a ensaios de tração, de até 80% com relação aos materiais sem tratamento térmico. / The advanced high strength steels are of importance in modern mobility industry. In the automotive industry, for example, it is anticipated that the use of such materials will be increased in the near future. Some characteristics of these materials can be improved through heat treatments, considering the entire mass of the product or in specific regions of a part, for which it is necessary a particular mechanical behavior. This work focused on the initial investigation of a local heat treatment by laser radiation, a controlled form of energy addition, in the mechanical properties and microstructure of two of these materials. The studied materials are the dual phase steel (DP 600) and the transformed induced plasticity steel (TRIP 750), and the desired contribution here was to provide information about the interaction between the laser radiation and the studied materials for a given range of thermal processing parameters. To characterize the materials as received and after proposed laser heat treatments, metallographic evaluation, X-ray diffraction, and hardness and tensile tests were performed. It was found that, for the range of process parameters investigated, there were significant changes in the characteristics of the materials, notably an increase in surface hardness, reaching values corresponding to twice of those for materials without treatment, in the yield limits, with increases of up to 15% with respect to the base material, and strength limits, with increases up to about 30% with respect to the base material. On the other hand, there was a decrease in the elongation of materials, when subjected to tensile tests, up to 80% in comparison to materials without heat treatment.

Ce travail de recherche donc concerne l'étude du système ultrasonique de fatigue en torsion de type indirect pouvoir étudier le comportement des matériaux métalliques à haute résistance, essentiellement au domaine de fatigue gigacyclique. Il existe actuellement deux types de système ultrasonique de torsion : un système qualifié de "direct" dans lequel le convertisseur délivre une rotation permettant, de manière simple et directe, la mise en vibration de torsion du système ; un système qualifié de "indirect" qui utilise un convertisseur longitudinal et nécessite une transformation en rotation pour créer la vibration en résonance de torsion. Premièrement, la présente d'étude sur le système indirect permet de comprendre son comportement de fatigue et ses propriétés. Cette étude est présentée à partir des calculs analytiques et numériques afin de dimensionner et modéliser ce système. Ensuite, l'optimisation du système à l'aide de la méthode des éléments finis est réalisée et un système plus effectif et plus robuste est obtenu. Des comparaisons entre les résultats numériques et les résultats de calibration sont proposées et une interprétation des écarts constatés est proposée avec les nouvelles simulations numériques. Deuxièmement, une campagne expérimentale à 20 kHz au-delà de 10⁸ cycles est menée en utilisant le système optimisé sur les aciers 16MnCr5 et 50CrV4 du projet FreqTigue. La méthode de l'escalier est utilisé dans les essais afin de décrire la courbe S-N, d'estimer la résistance à la fatigue et d'obtenir la durée de vie en fatigue. Des comparaisons des courbes S-N à différentes fréquences et sollicitations sont présentées. Les surfaces de rupture des éprouvettes sont analysées pour tenter d'identifier les causes de la rupture. / This research work concerns the study of the torsional ultrasonic fatigue system of indirect type which is able to study the behavior of high-strength metallic materials, mainly in the domain of gigacyclic fatigue. With the developments of technique, there are currently two types of ultrasonic torsional system: the first one is described as "direct" and its converter delivers a rotation which allows the system vibrate in torsion; the second one is described as "indirect", which uses a longitudinal converter and requires the transformation of a translation into a rotation to create a desired torsional resonant vibration in the specimen. Firstly, the study of the indirect system could understand the fatigue behavior and the properties of this system. This study is presented with the analytical and numerical calculations in order to determine and model the indirect system. Then, the optimization of this system by using the finite element method is performed and a more efficient and robust system is developed. Some comparisons between the numerical results and calibration’s results are proposed and an interpretation of differences is proposed by using new numerical simulations. Secondly, the tests of the 16MnCr5 and 50CrV4 steels of the FreqTigue project are carried out at 20 kHz and beyond 10⁸ cycles by using this optimized system. The staircase method is used in the tests to describe the S-N curve, to estimate the fatigue strength and to obtain the fatigue life. Then, the comparisons of the S-N curves at different vibration frequencies and stresses are presented, and the ruptured surfaces of the specimen are also analyzed to identify the causes of the rupture.