Les aciers bainitiques refroidis à l'air libre sont de plus en plus utilisés à la place des aciers martensitiques trempés et revenus pour la réalisation de pièces forgées dans le secteur automobile. Ces aciers permettent la réalisation d'économies significatives de temps et d'argent lors de leur fabrication. Leur résistance mécanique élevée est obtenue grâce à une composition chimique complexe, générant une microstructure multi-échelle et multiphasée lors du refroidissement à l'air libre. Dans un souci d'amélioration continue de cet acier, il devient nécessaire de comprendre plus en profondeur les mécanismes physiques mis en jeu lors de la décomposition de l'austénite. L'objectif principal de cette thèse est de contribuer à cette compréhension en établissant des liens quantitatifs entre la composition chimique, la microstructure et les propriétés mécaniques pour cette gamme d'aciers dans des conditions représentatives du forgeage. L'influence de plusieurs éléments d'alliage sur la microstructure brute de forge, ainsi que quelques synergies entre ces éléments, a été établie grâce à l'étude de diagrammes TRC spécifiques à l'application visée. Les mécanismes de rupture dominants et les paramètres microstructuraux contrôlant la résilience ont été identifiés par l'étude des faciès de rupture, de l'endommagement dans le volume et par des caractérisations microstructurales ciblées. Les contributions respectives de chaque mécanisme classique de durcissement structural ont été déterminées de manière quantitative, sur la base de plusieurs hypothèses et de paramètres microstructuraux. Au passage, des informations précieuses sur le comportement global en traction de ces aciers ont été récoltées grâces à l'étude des mécanismes d'endommagement et de rupture en traction.Tous ces résultats ont permis l'identification des paramètres microstructuraux, comme la taille de paquet bainitique et la fraction de constituants microstructuraux secondaires, qui doivent être pris en considération lors de l'optimisation de la composition chimique. / Air-cooled bainitic steel grades are increasingly being considered as substitutes to quenched & tempered martensitic steels in the realization of automotive forged parts. They allow significant manufacturing cost and time reductions. To compete with martensitic steels, high mechanical strength is provided by a complex chemical composition, leading to a multi-scale and multi-constituent microstructure after air-cooling. In order to optimize such chemical composition, need in rationalization of the steel grade development has emerged.The main aim of this Ph.D project was to build a physically based knowledge of the steel grade by making quantitative links between chemical composition, microstructure and mechanical properties in as-forged condition.The influence of several alloying elements on the as-forged microstructure, as well as some synergies, has been established using relevant CCT diagrams.Dominant fracture mechanisms and controlling microstructural parameters in concern of impact toughness have been identified with the help of comprehensive fracture surface, cross-section observations and targeted quantitative microstructural characterization.The respective contributions to yield strength of classical strengthening mechanisms have been quantitatively determined, based on several hypothesis and the use of several microstructural parameters. At the same, precious information on tensile behaviour has also been deduced from the observation of the fractured tensile test specimens.All these results allow identifying the key microstructural parameters, such as bainitic packet size or secondary microstructural constituents content, that have to be targeted in the alloy design.

This document describes work performed related to the determination of how elemental additions affect the interfacial fracture toughness of thermal barrier coatings at the bond coat/thermally grown oxide interface in gas turbines. These turbines are exposed to cyclical thermal loading, therefore a simulation was designed to model this interface in a temperature cycle between 200 K and 1000 K that included oxide growth between 2 μm and 27 μm. The fracture toughness of this interface was then determined to elucidate the function of elemental additions. It was shown that minimal concentrations of atomic species, such as hafnium and yttrium cause notable increases in the toughness of the bond coat/thermally grown oxide interface, while other species, such as sulphur, can dramatically reduce the toughness. Furthermore, it was shown that, contrary to some empirical results, the addition of platinum has a negligible effect on the fracture toughness of this interface.

Structural integrity assessment of nuclear components assessed using the R6 Failure Assessment Diagram approach requires an understanding of the limiting condition in terms of both fracture and plastic collapse. For ductile materials, such as stainless steels used for nuclear components, including the primary pipe-work of a Pressurised Water Reactor (PWR), the limiting condition defined by plastic collapse is likely to occur prior to the initiation of fracture. This is due to the relatively low yield stress of the material and the high fracture toughness. If this is the case, structural integrity may be solely assessed on plastic collapse criteria, with little or no reference to fracture toughness; thus considerably simplifying the assessment procedure, whilst maintaining the integrity of the plant. Nevertheless, an in-depth understanding of fracture under plastic collapse conditions is required to make a robust case for single parameter assessments based on a plastic collapse criterion alone. The challenge in this project lay in understanding and predicting ductile fracture initiation under large-scale yielding conditions, i.e. outside the normal validity limits of conventional elastic-plastic fracture mechanics as plastic collapse conditions are achieved. The approach developed in this research has explored three fracture assessment methods: (a) two parameter fracture mechanics based on the J-integral and a refined Q-parameter calculated closer to the crack-tip under widespread plasticity than is conventionally the case, (b) two local approach methods based on critical void growth ratio defined by Rice and Tracey, and (c) a local approach method based on the critical work of fracture. All three methodologies were found to adequately describe failure across a range of constraint conditions. The fracture toughness constraint dependence of 304(L) stainless steel was studied experimentally and analytically. Significant constraint loss was shown to occur in nominally high constraint fracture toughness specimens due to extensive plastic deformation at fracture initiation. Furthermore, significant fracture toughness constraint dependence was observed experimentally. An analytical method using local approach criteria was developed to predict high constraint fracture toughness, required for structural integrity assessments, and to quantify the constraint dependence fracture toughness as a function of two parameter fracture mechanics based on the J-integral and the refined Q-parameter. The influence of constraint on the prediction of failure in a stainless steel pipe containing a fully circumferential crack of various depths was investigated analytically for a range of loading conditions. A refined constraint independent failure assessment methodology was developed using local approach analyses. Using this methodology, the pipe component was shown to consistently fail by plastic collapse irrespective of the crack depth or loading condition. The conservatism of the conventional structural integrity assessment was quantified and shown to vary with crack depth and with loading conditions. This research has suggested that failure in a 304(L) stainless steel pipe will be by plastic collapse prior to ductile initiation for a limited range of defects and loading conditions. Further analytical studies and experimental work will be required to demonstrate whether this observation is general for a wider range of defects and loading conditions.

The operating efficiency and reliability of modern jet engines have undergone significant improvement largely owing to the advances of the materials science over the past 60 years. The use of both superalloys and TBCs in engine components such as turbine blades has made it possible for jet engines to operate at higher temperatures, allowing an optimal balance of fuel economy and thrust power. Despite the vast improvement in high temperature capability of superalloys, the utilization of TBCs has brought the concern of coating adhesion during their usage. TBCs are prone to spallation failure due to interfacial rumpling, which is driven primarily by thermal coefficient mismatch of the multi-layered structure. Although interfacial degradation of TBCs has been widely studied by detailed numerical and analytical models, the predicted results (i.e. stress state and rumpling amplitude) often deviate from that obtained by experiments. This is largely due to the lack of consideration of the influence of bond coat and substrate chemistry on the interfacial evolution of TBC systems. It is only in recent year that more and more study has been focused on studying the role of chemistry on the interfacial degradation of TBCs. The purpose of this PhD project is to clarify how the bond coat and substrate chemical compositions dictate the mechanisms of interfacial degradation, leading to the final spallation. A cross-sectional indentation technique was utilized to quantitatively characterize the adhesion of oxide-bond coat interface among 5 systematically prepared TBC systems. The adhesion of isothermally exposed oxide-bond coat interface was then correlated with different microstructure parameters, in an attempt to identify the key parameters controlling the TBC spallation lifetime. EBSD and EPMA analyses were conducted on the bond coat near the oxide-bond coat interface, in order to understand the relationship between the key parameters and specific alloying elements. The results clearly demonstrated that the phase transformation of bond coat near the oxide-bond coat interface plays the dominant role in the degradation of interfacial adhesion. Particularly, the co-existence of gamma prime and martensitic phases, each having very different thermomechanical response under thermal exposure, can generate a misfit stress in the TGO layer, and ultimately causes early TBC spallation. In addition, the phase transformation behavior has been closely associated with the inherent chemistry of the bond coat and substrate.

La ténacité et l’énergie de surface de fracture de verres, de vitrocéramiques et de composites à matrice en verre ont été étudiées. Tout d'abord, un test de flexion bi-axiale (configuration anneau/anneau) a été réalisé sur des verres de silice et des verre-à-vitres avec différentes conditions de surface afin d’identifier la relation entre le défaut de surface, la résistance et la ténacité du verre. Ensuite, trois méthodes d’expérimentation ont été mises en œuvre, principalement la méthode de poutre de flexion à pré-entaille droit (SEPB), la méthode de la poutre entaillée en chevron (CNB) et la méthode de la poutre avec indentations Vickers (VIF), afin de déterminer la ténacité de quatre verres produits industriellement et de déterminer les avantages et les inconvénients des différents méthodes sélectionnées. La méthode qui est apparue la plus fiable et auto-cohérente, la méthode SEPB (Single Edge Precrack Beam), a été appliquée à la détermination de la ténacité de nombreux verres et vitrocéramiques, afin d’étudier l’influence de la composition et de la microstructure sur les caractéristiques de fissuration (KIC et énergie de fissuration, γ). Enfin, l’influence de la température et de l'environnement sur la ténacité a été étudiée à l'aide de la méthode SEPB. Deux verres d'oxyde ont été testés à des températures élevées et avec une vitesse de charge de 10 MPa∙√m/s, une température de transition de 1,11Tg a été trouvée. Quatre autres verres d'oxyde ont été testés en environnement inerte et les mêmes valeurs de ténacité ont été obtenues à partir de deux vitesses de charge (100 fois) différentes. / Fracture toughness and fracture surface energy of commercial and laboratory glasses, glass-ceramics and glass matrix composites have been studied. First, bi-axial bending test (RoR configuration) was performed on fused silica and window float glasses with different surface conditions to identify the relationship between the surface flaw, the strength and fracture toughness. After, three experiment methods, mainly single-edge precracked beam (SEPB), chevron-notched beam (CNB) and Vickers indentation fracture (VIF) were performed to determine the fracture toughness of four commercial known glasses and to determine the advantages and inconveniences of the different selected methods. The method that is appeared as the most reliable and self-consistent, the SEPB (Single Edge Precrack Beam) method, was applied to determine the toughness of the large amount of glasses and glass-ceramics, to study the influence of the composition and the microstructure on the characteristics of cracking (KIC and fracture energy, γ). Last but not least, the influence of the temperature and environment on the fracture toughness was studied by means of the SEPB method. Two oxide glasses were tested in elevated temperatures and with the loading rate of 10 MPa∙√m/s, a transition temperature of 1.11Tg was found. Four other oxide glasses were tested in the inert environment and the same fracture toughness values were obtained from (100 times) two different cross-head speeds.

The influence of two lubricating and cleaning agents and diesel fuel on environmental stress cracking of polyamide reinforced by glass fibers and polycarbonate was studied in this master thesis. Testing of environmental stress cracking was performed by the method of critical bending deformation. Bergen elliptical strain jig was used for testing. The test liquids caused varying levels of environmental stress cracking of amorphous polycarbonate, but they did not cause environmental stress cracking of glass fiber reinforced polyamide. The test method used allows testing the resistance of both materials against environmental stress cracking of any liquid. The results can be used to design products that are expected to be exposed to corrosive liquids. Quantification of the influence of stress free corrosion on the static and impact properties of polycarbonate and glass fiber reinforced polyamide was performed by accelerated test. Specimens were exposed to test liquid and an elevated temperature of 70 °C. The mechanical properties of the tested materials were significantly affected by elevated temperature exposure. The yield strength of the polycarbonate has increased and the toughness has decreased due to physical aging. The tensile strength of glass fiber reinforced polyamide has increased due to a decrease of the moisture content of the material.

The diploma thesis is focused on determining mechanical properties of alloys produced by powder metallurgy methods. Two alloys were studied, namely a highly entropic CrMnFeCoNi alloy and an oxide dispersion-strenthened FeAlO alloy. Both alloys were made in laboratory amount of test material and miniature test specimens were used to measure their mechanical properties. The main goal of the diploma thesis was to develop a crack preparation methodology, to examine and to evaluate the properties of miniature test specimens for materials with low levels of resilience. In the analytical part of the thesis, tensile and deformation properties and fracture toughness of both alloys were measured. For the highly-entropic CrMnFeCoNi alloy, the thesis focused on the effect of the strain rate on the properties of the alloy. For the oxide dispersion-strenthened FeAlO, the effect of temperature in the 23-600 °C temperature range on its mechanical properties was studied.

In this work, the effect of adding short deformable polyvinylalcohol fibers into dimethacrylate resins on mechanical properties of composite materials was investigated. Furthermore, micro- or nano-fillers were added into the matrix. The effect of filler/short fibers combination on mechanical properties of so-called hybrid composites was studied. Composites were characterized by differential compensation photocalorimetry (DPC), termogravimetric (TGA) and dynamic mechanical (DMA) analysis. The fracture surfaces were examined using scanning electron (SEM) and confocal laser (CLSM) microscopy. Elastic modulus and strength, the critical value of stress intensity factor and strain energy release rate and the viscoelastic properties of composite materials were measured. The mechanical properties of composite materials are affected not only by the type of filler/reinforcement, but also by properties of the matrix. These properties also depend on the way of network formation during photo-initiated polymerization. For this reason, heat of polymerization, degree of conversion and the maximum polymerization rate of mixture of dimethacrylate monomers were determined.

The aim of this thessis is heat treatment and choice of material for Woodworking optimalization. Industrial Knives in conjunction with company Pilana Tools Knives. Knives of this company are used in different parts of the world, among others in northern states too, where are working in extreme conditions about temperature low below of freezing point. That’s why was tested many samples in another working conditions.

This diploma work deals with relationship between composition and mechanical properties of polymeric composite with polypropylene matrix and magnesia hydroxide as a filler. The influence of content of filler (20,40,60 w.%) and temperature (-30°C +60°C) on dynamic fracture toughness is observed. The regresion function describing the dependence of studied mechanical properti on temperature for all experimental materials where found. Experimental data were confront with data gained on the same materials then 10 years ago and influence of time on mechanical properties of follow - up composites is evaluate.