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High Temperature Oxidation Behavior of Cast and Additive Manufactured Co-Base Alloy MAR-M-509
No full textEffect of Alloying Elements in the Substrate Material on Interdiffusion Processes in MCrAlY Coating Systems
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Modelling Coating Microstructure During Aluminising and Following Diffusion Heat Treatment
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Load distribution in the ballasted track superstructure under static and dynamic loading conditions
Get PDFEisenbahnbrücken mit Schotteroberbau werden für dynamische Berechnungen oft vereinfacht als einzelne Euler-Bernoulli-Balken modelliert, weil diese Wahl der Modellbildung aufgrund ihrer Einfachheit am wenigsten Eingangsparameter benötigt, welche bei komplexeren Modellbildungen nicht bekannt sein können oder für welche in der Fachliteratur stark streuende Angaben zu finden sind. Allerdings werden bei solchen vereinfachten Modellierungsverfahren große Abweichungen zwischen Berechnung und Messung in Bezug auf Brückenschwingungen beobachtet, welche neben den unsicheren Eingangsdaten auch durch die fehlende Berücksichtigung der Lastverteilung im Schotteroberbau erklärt werden können. Im Gegensatz zu einem einzelnen Euler-Bernoulli-Balken können die getrennt betrachteten Tragwerks- und Oberbaubalken miteinander durch Feder-Dämpfer-Elemente verbunden werden, um die mechanische Wirkung des Schotteroberbaues im Rahmen eines Koppelbalkenmodells explizit zu erfassen. Standardmäßig wird für die Kopplung eine lineare Federkennlinie angenommen, wobei die Kopplungssteifigkeit einen großen Einfluss auf die Lastverteilung und auf die rechnerische Brückenschwingung hat. Eine Versuchsreihe zeigt jedoch, dass das Schotterbett (neben einem nichtlinearen Dämpfungsverhalten) eine nichtlineare Federkennlinie hat, für welche noch keine Untersuchungen der Lastverteilung durchgeführt worden sind. Es bleibt zudem unbekannt, ob der Übergang von einer statischen auf eine dynamische Belastung sich ebenfalls auf die Lastverteilung auswirken würde. Aus diesen Gründen wird in der vorliegenden Diplomarbeit die Lastverteilung eines sowohl statisch als auch dynamisch belasteten elastisch gebetteten Gleisbalkens für verschiedene Oberbau-Steifigkeitsverhalten und für unterschiedliche Variationen der Eingangsparameter untersucht. Bei der Annahme sowohl einer linearen Federkennlinie als auch einer versuchstechnisch ermittelten nichtlinearen Federkennlinie für das Schotterbett wurde eine vom Lastniveau bzw. von der Lastamplitude unabhängige Lastverteilung festgestellt. Einen wesentlichen Einfluss auf die Lastverteilung zeigen hingegen die Schotterbettsteifigkeit, die Dämpfung des Oberbaues sowie die Anregungsfrequenz. Auf Basis der gewonnenen Ergebnisse werden einwirkungsseitige Lastverteilungsansätze für statische und dynamische Achslasten zum Einsatz im vereinfachten Euler-Bernoulli-Einzelbalkenmodell aufgestellt.Railway bridges with ballasted tracks are often modelled as simplified single Euler-Bernoulli beams for dynamic calculations as this modelling approach, due to its simplicity, requires the fewest input parameters that, in case of more complex models, may be unknown or be subject to considerable variation in the technical literature. However, such simplified modelling methods often lead to significant discrepancies between calculated and measured bridge vibrations. These deviations can be attributed not only to the uncertain input data but also to the disregard of load distribution in a ballasted track superstructure. In contrast to a single Euler-Bernoulli beam model, the separately considered structural and track beams can be coupled via spring-damper elements to explicitly account for the mechanical behaviour of the ballast bed within a more sophisticated coupled beam model. A linear spring characteristic is typically assumed for this purpose, whereby the coupling stiffness has a significant influence on the load distribution and on the computed bridge vibration. However, a series of experiments demonstrates that the ballast (in addition to exhibiting a nonlinear damping behaviour) has a nonlinear spring characteristic, for which no investigations into load distribution have been conducted to date. Moreover, it remains unclear whether the transition from static to dynamic loading also affects the load distribution. For these reasons, this diploma thesis investigates the load distribution of an elastically supported track beam under both static and dynamic loading, considering various stiffness behaviours of the track superstructure and different variations of input parameters. Assuming a linear spring characteristic as well as an experimentally derived nonlinear spring characteristic for the ballast bed, the load distribution was found to be independent of the load level or load amplitude. In contrast, the ballast bed stiffness, the damping intensity of the track superstructure and the excitation frequency were observed to have a significant effect on the load distribution. Based on the obtained results, action-side load distribution models for static and dynamic axle loads are developed for application in the simplified single Euler-Bernoulli beam model
Oxidation behavior of wrought, cast and additive manufactured Co-base alloys for high temperature applications
No full textVarious high temperature industrial processes require construction materials that possess an optimal balance of mechanical strength, corrosion resistance and fabricability. Among the main candidate materials considered for the long-term service at temperatures exceeding 600°C are metallic Fe-, Co- and Ni-base alloys. The present study followed the oxidation mechanisms of wrought, cast and additive manufactured (AM) Co-base alloys during air exposures at 950-1050°C to address the effects of alloy microstructure, minor alloying elements and specimen thickness on oxidation kinetics and subscale depletion processes. Substantial differences in oxidation behavior between cast and AM (produced by Selective Laser Melting) versions of alloy MAR-M-509 were correlated with the alloy grain morphology and size as well as distribution of the strengthening precipitates. Faster oxidation of the cast alloy was shown to be mainly induced by: (1) oxidation of coarse primary Ta-rich carbides, that oxidized rapidly forming thick oxide nodules and promoting development of cracks/pores in the adjacent Cr2O3. Cracking of the scale and its spallation provided direct access of nitrogen to the Cr-depleted base metal causing internal nitridation. (2) Incorporation of other alloy constituents (Co, Ni, W) into the scale was observed from Cr-depleted alloy thus decreasing the scale grain size and increasing its growth kinetics. In contrast, the microstructure of the AM MAR-M-509 featured fine carbide precipitates that were found to dissolve in the subscale region forming internal oxides and resulting in more homogeneous oxide scale compared to the cast alloy. EDX analyses in the subscale zone of AM specimens further revealed virtually flat Cr concentration profiles, contrary to the cast alloy which exhibited strong Cr depletion. Such behavior was proposed to be the result of enhanced Cr diffusion due to contribution of grain-boundary transport in the fine-grained AM alloy. High temperature exposures of the wrought Co-Cr alloys Haynes 25 and Haynes 188 showed significantly faster oxidation of the former material. Relatively high Mn content in Haynes 25 facilitated formation of Cr/Mn-spinel and increased the scaling rate of Cr2O3, which was found to exhibit a significant part of inward growth. TEM and APT studies of the chromia scale revealed a presence of intragranular Cr/Mn-particles possibly nucleating simultaneously with Cr2O3. Substantially better performance of Haynes 188 was interpreted in terms of differences in composition of Cr, Ni, Mn and especially La. The chromia scale formed on the alloy surface featured a distinct duplex morphology with coarse columnar oxide grains in the outer part and fine equiaxed grains in the inner layer. Observed sub-parabolic oxidation of Haynes 188 was attributed to the slow inward growth of the inner chromia layer as a result of reactive element incorporation into the scale, which could suppress outward cation transport. Dependence of oxidation kinetics on specimen thickness was analysed by oxidizing 0.15-1mm sections of wrought alloy Haynes 188. Exposures at 950 and 1000°C showed higher oxidation kinetics of thinner specimens with more pronounced Cr depletion in agreement with the common observations reported for Fe-Cr and Ni-Cr alloys. Higher oxidation rate of thin Haynes 188 specimens could be clearly observed already after initial hours of isothermal exposures in a thermobalance. The possible explanation of this effect was referred to low intrinsic creep strength of thin sections resulting in more pronounced response to oxide growth stresses. In contrast, during the testing at 1050°C the increase of oxidation-induced Cr loss with decreasing section thickness was not monotonous and showed a maximum in the 0.5 mm thick specimen. This result was correlated with rapid exhaustion of Mn reservoir in thinner specimens that appeared to reduce Cr depletion kinetics due to less pronounced Mn incorporation into the scale
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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