No description available.

This thesis deals with problems with manufacturing of high frequency elliptic and hyperbolic optic mirrors, which makes one of the most important functional parts of parabolic satellites made for biggest and highest placed international astronomical radio-telescope in the world - ALMA, which is built on Chajnantor plateau in Chille in South America. Individual chapters are designed in the flow of technological process of manufacturing, containing theoretical and practical analysis of the problems. Content of the thesis leads especially to description and clarification of individual causes of problems during manufacturing.

This thesis deals with computational modeling of the influence of residual stresses in idealized geometry of blood vessels and subsequent application of acquired knowledge to abdominal aortic aneurysm. In the terms of quality of the computational model, we reduced the uncertainties that are included in the computational model without considering the influence of residual stresses. The basic assumption of homogenization significant peaks of the stress between inner and outer vessel wall was met for each level of the computational model Methods that have been used are: deformation method (opening angle method), inverse mechanics of large deformations, fictitious temperature – for linear elastic material and hyperelastic material defined by the constitutive model. Numerical verification was carried out using program ANSYS.

This thesis is dealing with the issue of residual stress present in the wall of a polymer pipe and the influence on its lifetime. Experimental data obtained by the ring slitting method are evaluated and the tangential residual stress in the wall of polypropylene pipe is determined. The evaluation is carried out using a new methodology based on the curved beam theory. The method is verified using numerical simulation. 3D numerical model is used to verify the behavior of the pipe, when there are both tangential and axial residual stresses present in the pipe wall, because the presence of axial stress causes a rise in the magnitude of tangential residual stress. A correction of the tangential stress values corresponding to the pipe length is then proposed. It is shown, that the distribution of the tangential residual stress does not depend significantly on the dimensions or ma-terial of the extruded pipe and a general equation is proposed to describe the distribution. This general distribution is then involved in the calculations of the pipe lifetime that are carried out using a method based on the linear elastic fracture mechanics. A significantly lower lifetimes are obtained when taking the residual stress into account.

Des influences de l’humidité sur le comportement à l’oxydation sous air de l’alliage Inconel 600 ont été étudiées à haute température (de 600°C à 900°C) pour différentes durées d’oxydation (de 24 heures à 150 heures). La cinétique d’oxydation a été étudiée par l’analyse thermogravimétrie : à une température relativement basse (de 600°C à 700°C), la cinétique suit une seule loi parabolique ; à plus haute température (de 800°C à 900°C), une évolution en deux étapes a été observée, chaque étape suit une loi parabolique. En présence de la vapeur d’eau (de 0% à 19% d’humidité absolue), à toutes les températures étudiées, la teneur de Cr en surface de l’oxyde est plus élevée que celle oxydée sans vapeur d’eau. L’oxydation à une même température, la prise de masse diminue en présence de la vapeur d’eau. La morphologie de la surface oxydée est étudie par MEB-FEG. Avec la présence d’humidité, la couche d’oxyde est plus compacte et homogène. En couplant avec la diffraction des rayons X en faible incidence (GIXRD), deux couches d’oxyde ont été observées dans la section transversale quand la température d’oxydation est plus de 700°C avec ou sans l’humidité: la couche externe contient principalement de la phase NiO et la couche interne contient de spinelle NiCr2O4 et de la phase Cr2O3. Les fractions de chaque phase dépendent des conditions d'oxydation (température, taux humidité, durée d’oxydation etc…). Les études de contraintes dans les couches d’oxydes ont été effectuées par la méthode classique des sin2ψ en diffraction des rayons pour les contraintes résiduelles, ainsi que par la méthode de déflexion in situ pour les contraintes de croissance. Les contraintes résiduelles dans les 2 couches oxydes sont en compression, tandis que le niveau de contraintes résiduelles de compression dans la couche NiO (<600 MPa) est moins important que celui dans la couche de NiCr2O4 (> 800 MPa). En présence de la vapeur d’eau, les contraintes résiduelles en compression augmentent de façon très importante. Les analyses et les évaluations des contraintes (résiduelles, croissance et thermique) ont été effectuées. Ces résultats indiquent que le mécanisme d'oxydation n’est pas seulement un processus chimique, mais un processus mécano-chimique complexe qui fait intervenir le comportement mécanique. Les analyses par SIMS et par XPS ont été réalisées sur des échantillons oxydés pour étudier le mécanisme de diffusion atomique et le rôle de l’humidité sur l’oxydation. / Humidity influences on the oxidation behavior of the alloy in air Inconel 600 were investigated at high temperature (600°C to 900°C) for various oxidation periods (24 hours to 150 hours) .The oxidation kinetics were studied by thermo gravimetric analysis: at a relatively low temperature (600°C to 700°C), the kinetics following a single parabolic law; at a higher temperature (800°C to 900°C), a two-stage evolution was observed, each step follows a parabolic law. In the presence of water vapor (from 0% to 19% absolute humidity) at all temperatures studied, the content of Cr in the oxide surface is higher than that oxidized without steam. Oxidation at the same temperature, the weight gain decreases in the presence of water vapor.The morphology of the oxidized surface is studied by FEG-SEM. With the presence of water vapor, the oxide layer is more compact and homogeneous. By coupling with the X-ray diffraction in low incidence (GIXRD), two oxide layers were observed in the cross section when the oxidation temperature is over 700°C with or without humidity: the outer layer contains mainly of the NiO phase and the inner layer contains spinel NiCr2O4 and Cr2O3 phase. The fractions of each phase depend on the oxidation conditions (temperature, humidity, oxidation period etc ...).Constraints studies in the oxide layers have been made by the classical method of ray diffraction sin2ψ to residual stresses as well as the in situ method for deflection of the growth stresses. Residual stresses in the two oxide layers are in compression, while the level of compressive residual stress in the NiO layer (<600 MPa) is less than that in the layer of NiCr2O4 (> 800 MPa). In the presence of water vapor, the compressive residual stress increases dramatically. The analyzes and assessments of constraints (residual, growth and thermal) were performed. These results indicate that the oxidation mechanism is not only a chemical process, but a complex chemical mechanical process involving the mechanical behavior. Analysis by SIMS and XPS were performed on samples oxidized to study the atomic scattering mechanism and the role of humidity on the oxidation.

No description available.

No description available.

Additive manufacturing (AM) processes may introduce large residual stresses in the as-built part, in particular the laser powder bed fusion process (L-PBF). The residual stress state is an inherent consequence of the heterogeneous heating and subsequent cooling during the process. L-PBF has become renowned for its “free complexity” and rapid prototyping capabilities. However, it is vital to ensure shape stability after the component is removed from the build plate, which can be problematic due to the residual stress inducing nature of this manufacturing process. Residual stresses can be analyzed via many different characterization routes (e.g. X-ray and neutron diffraction, hole drilling, etc.), both quantitatively and qualitatively. From an industrial perspective, most of these techniques are either prohibitively expensive, complex or too slow to be implementable during the early prototyping stages of AM manufacturing. In this work a deformation based method employing a specific geometry, a so called “keyhole”-geometry, has been investigated to qualitatively evaluate the effect of different stress relief annealing routes with respect to macroscopic part deformation, mechanical properties and microstructure. Previous published work has focused on structures with open geometry, commonly referred to as bridge-like structures where the deformation required for analysis occurs during removal from the build plate. The proposed keyhole-geometry can be removed from the build plate without releasing the residual stresses required for subsequent measurement, which enables bulk manufacturing on single build plates, prior to removal and stress relief annealing.  Two L-PBF manufactured austenitic stainless steel alloys were studied, 316L and 21-6-9. Tensile specimen blanks were manufactured and the subsequent heat treatments were carried out in pairs of keyhole and tensile blank. Both a contact (micrometer measurement), and a non-contact (optical profilometry) method were employed to measure the residual stress induced deformation in the keyholes. The annealing heat treatment matrix was iteratively expanded with input from the deformation analysis to find the lowest temperature at which approximately zero deformation remained after opening the structure via wire electrical discharge machining. The lowest allowable annealing temperature was sought after to minimize strength loss.  After stress relief annealing at 900 ℃ for 1 hour, the 316L keyhole-geometry was considered shape stable. The lateral micrometer measurement yielded a length change of 1 µm, and a radius of 140 m (over the 22 mm top surface) was assigned from curve fitting the top surface height profiles. The complementary microstructural characterization revealed that this temperature corresponded to where the last remains of the cellular sub-grain structures disappears. Tensile testing showed that the specimen subjected to the 900 ℃ heat treatment had a marked reduction in yield stress (YS) compared to that of the as-built: 540 MPa → 402 MPa, whereas ultimate tensile strength (UTS) only reduced slightly: 595 MPa → 570 MPa. The ductility (4D elongation) was found to be ~13 % higher for the specimen heat treated at 900 ℃ than that of the as-built specimen, 76% and 67% respectively.  For alloy 21-6-9 the residual stress induced deformation minimum (zero measurable deformation) was found after stress relief heat treatment at 850 ℃ for 1 hour. Slight changes in the microstructure were observable through light optical microscopy when comparing the different heat treatment temperatures. The characteristic sub-grain features associated with alloy 316L were not verified for alloy 21-6-9. Similar to the results for 316L, UTS was slightly lower for the tensile specimen subjected to the heat treatment temperature required for shape stability (850 ℃) compared to the as-built specimen: 810 MPa → 775 MPa. The measured ductility (4D elongation) was found to be approximately equal for the as-built (47%), and heat treated (48%) specimen. As-built material exhibited a YS of 640 MPa while the heat treated specimen had a YS of 540 MPa. For alloy 21-6-9, the lateral micrometer deformation measurements were compared with stress relaxation testing performed at 600 ℃, 700℃ and 800 ℃. Stress relaxation results were in good agreement with the results from the lateral deformation measurements.  The study showed that for both steel alloys, the keyhole method could be successfully employed to rapidly find a suitable stress relief heat treatment route when shape stability is vital.

Irradiation of a reactor pressure vessel (RPV) causes a shift of the ductile to brittle transition region towards higher temperature regions. In the event of a pressurized thermal shock (PTS), where the temperature drops drastically, the ductile to brittle transition region might be entered for irradiated ferritic steel. Hence, there is a risk of brittle cleavage fracture. Cleavage fracture is a transgranular unstable fracture initiated by cracked second phase particles and rapidly propagated over grain boundaries. The warm pre-stressing (WPS) effect can be helpful as it increases the apparent fracture toughness of ferritic steel pre-loaded in the ductile temperature region, which is the case for a PTS. This effect has been proven effective for virgin material, but the impact of residual stress fields on the WPS effect have not been investigated thoroughly. Utilizing a finite element model of notched three-point bending specimens and a non-local probabilistic model for fracture prediction the effect of residual stresses on the WPS effect was investigated in this thesis. Regarding the crack tip state, expressed as J, the probability of fracture was alike for both material with and without residual stresses, however a significant loss of load bearing capacity was found comparing them two. The magnitude of this loss depends on pre-load level as well as specimen size. This loss however, was also found when not considering the WPS effect. / Bestrålning av ferritiska reaktortankar orakar en förskjutning av den duktil-spröda omslagstemperaturen till högre temperaturer. Vid en trycksatt termisk shock (TTS) sjunker temperaturen drastiskt i reaktorn och omslagstemperaturen kan nås. Därför uppstår en risk för klyvbrott. Klyvbrott är en transgranulär ostabil spricktillväxt initierad av sprickor i sekundärfaspartiklar som propagerar över korngränserna om spänningstillståndet är gynnsamt. Varm förbelastning, eller warm pre-stressing (WPS) kan vara fördelaktig eftersom den höjer den effektiva brottsegheten hos ferritiskt stål som förbelstats i den duktila temperaturregionen, som är fallet för TTS. Den här effekten har visats effektiv för material utan restspänningar, men för fallet med restspäninningar saknas utförliga undersökningar av WPS-effekten. Med hjälp av en finita element model av tre-punkt böjprovstavar och en icke-lokal sannolikhetsmodell för prediktion av brott så undersöktes vilken effekt restspänningar har på WPS-effekten. Brottsannolikheten visade sig vara lika för både material med och utan restspännigar om man syftar på sprickspetstillståndet, uttryckt som J. Däremot syns en tydlig förlust i lastbärande förmåga mellan de två fallen. Storleksordningen på förlustern beror både på förbelastningsnivå och provstavsstorlek. Den förlusten kunde dock finnas även för fall utan någon WPS effekt.

No description available.