1,721,168 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Lebensdauer und Schädigungsentwicklung martensitischer Stähle für Niederdruck-Dampfturbinenschaufeln bei Ermüdungsbeanspruchung im VHCF-Bereich
Full text linkLow-pressure steam turbine blades are usually made of martensitic steels with Cr contents between 9 and 12%, which combine good corrosion resistance, high mechanical strength and sufficient ductility. The inhomogeneous flow field behind the vanes generates high-frequency oscillations above 1 kHz. In addition, the blades with lengths up to 1.5 m are operated at rotational speeds up to 3000 rpm, resulting in large centrifugal forces leading to the superposition of extremely high mean stresses. Also resonance oscillations during start-up and shutdown cannot be completely excluded. Currently, the components are designed using high safety factors against S-N curves with an assumed asymptotic fatigue limit above 107 load cycles. Nevertheless, fatigue cracks are observed even at high number of cycles, starting fromthe blade root without pre-damage by erosion or steam droplet impingement. While fatigue failure usually occurs at the surface, fatigue cracks at very high number of cycles (> 108) initiate at oxides or intermetallic inclusions below the surface. This transition between both failure mechanisms in the Very High-Cycle Fatigue (VHCF) regime is in the focus of numerous current research activities, because numbers of cycles above 108 can be attained in a viable period of time using the recently developed high-frequency testing techniques operated at 20 kHz. Also for wind turbines, gas turbines, bearings, springs, etc. VHCF issues become increasingly important.Within this work, the fatigue life and damage behavior of a martensitic Cr-steel during fatigue loading with and without high mean stresses at number of cycles to failure above 108 was analyzed. On the one hand, the studies gave insights into the relation between fatigue life and fatigue damage evolution of the investigated group of high-strength steels in the very high cycle fatigue regime (up to 2∙109). In particular, the influence of high mean stresses on the VHCF behavior (fracture origin, crack growth, fatigue life) which was not investigated in detail before is studied and the crack initiation and propagation mechanisms are analyzed by electron microscopy (SEM, TEM / FIB). With this, the work contributes to the reliable design of future low-pressure steam turbines. The results show that in particular non-metallic inclusionsin the steel cause fracture by fatigue cracks initiated in the volume under very high cycle fatigue conditions. This fatigue behavior can be described very well by means of fracture mechanics approaches over a wide range of load ratios
Variations on the Author
Full text link“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
Thermomechanical Properties of Glass-Ceramic Solid Oxide Fuel Cell Sealant Materials
No full textThe reliable operation of solid oxide fuel cell stacks depends strongly on the structural integrity of the sealing materials. In this respect, failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. The current study concentrates on the glass ceramics abbreviated H-P (YSZ particle reinforcement) and B. Indentation tests have been carried out to determine elastic modulus, hardness and fracture toughness. Bending tests and compressive tests were carried out at room temperature and typical stack operation temperature for bar and head-to-head jointed glass ceramic sealant specimens, similar as in a stack application with different degree of crystallization and annealed temperature, from which fracture stresses and viscosity values are derived. The results reveal a decrease of the strength for the partially crystallized sealant H-P at operation relevant temperatures that can be associated with the viscous deformation of the material, whereas a stable strength was obtained for the annealed sealant H-P and non-annealed sealant B at operation relevant temperatures that can be associated with the enhanced crystallization of the materials. The experimentally obtained strength data of sealant H-P and B as a function of loading rate are used to assess the effect of subcritical crack growth yielding a failure time under static loading via a strength-probability- time plot that can be used for life time prediction. Fractographic analyses based on a combination of optical, confocal and scanning electron microscopy gives insight into the failure origins
Mechanical Behaviour and Microstructure of Reactive-Air-Brazed Metal/Ceramic Joints
No full textThe microstructure evolution dependent deformation and damage mechanisms of reactive air brazed metal/ceramic joints were studied under long term isothermal/thermal cycling conditions. The mechanical behavior and microstructural properties of joints produced using three Ag-base brazes, i.e. a Cu-containing braze (Ag4Cu, as a state of the art SOFC metal braze), an Al-containing (Ag0.5Al, as a newly developed advanced braze) and a pure Ag braze were compared. Ambient temperature delamination, high temperature shear and thermal cycling tests were applied to characterize the thermomechanical properties of the joints. The interfaces prove to be the weakest link of the joints in ambient temperature delamination testing. The long-term operability of the joints was found to be limited by aging induced degradation of the braze/steel and/or the braze/ceramic interfaces. In high temperature shear testing, competing failure of the interfaces and the braze matrix was discovered. Aging in air strengthened both the interface and the braze matrix, thus enhancing the high temperature shear and creep resistance of the joints. Thermal cycling induced mechanical twinning of the initially single crystalline matrices of the Ag and the Ag4Cu braze, which in turn promotes recrystallization of the matrix. Both processes compensate thermal mismatch stresses arising from temperature change and thermal expansion coefficient mismatch. With the introduction of an initially polycrystalline matrix – in case of an advanced Ag0.5Al braze – both mechanisms were exploited to reach greatly increased resistance against rapid thermal cycling. Considering the progress achieved by the advanced Ag0.5Al braze in thermal stress compensation, it could be stated that reactive air brazes have great potential especially for rapid thermal cycling application. Nevertheless brazing properties and thermomechanical stability still need further improvement. Future R&D activities in reactive air brazing should focus on joint stability during combined long term thermal cycling operation under dual atmosphere conditions
Thermomechanische Charakterisierung neu entwickelter Feuerfestwerkstoffe
Full text linkIn high temperature processes in the basic industry, e.g. steel mining, refractories are applied. To achieve a sufficient thermal shock resistance carbon containing refractories are used in steel mining, despite the fact that the utilization of carbon leads to higher heat losses and carbon pickup. To reduce the heat loss, to govern the carbon pickup and to enable innovative process routines for the clean steel technology novel carbon-reduced and carbon-free refractories are being developed.To assess the application potential and to support the ongoing improvement of selected carbon-reduced and carbon-free refractories the thermal shock performance is investigated focusing on the understanding of the microstructural mechanisms of crack propagation. The thermomechanical investigations concentrate on controlled crack propagation experiments and thermal shock tests. Controlled isothermal crack propagation experiments with macroscopic and microscopic observation are based on wedge splitting and compact tension test. To determine the resistance against crack propagation as an estimation of the potential thermal shock damage analytical relations have been applied. The thermal shock experiments focused on upward mode thermal shocks via electron beam tests facilities. Due to the very fast (ms) upward mode thermal shock in vacuum in contrast to conventional thermal shocks in air or water effects related to chemical interactions can be excluded.Due to a dependency on the sample size only carbon-free, alumina-based refractories were investigated. The influence due to the addition of zirconia and titania is shown. Thermal shock experiments with electron beam were mainly performed on carboncontaining refractories, since carbon-free refractories tended to electrical charging. For the investigated carbon-containing refractories the thermal shock damage is shown in dependence of different parameters (duration, number)
Mechanical Properties and Microstructure of Dense Ceramic Membranes for Oxygen Separation in Zero-Emission Power Plants
No full textIn the present work, the mechanical properties as well as microstructure of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF 5582) dense ceramic membranes for oxygen separation were investigated. The main supplier of the material was the Fraunhofer-Institut für Keramische Technologien und Systeme (IKTS), Hermsdorf, Germany. High temperature uniaxial compressive tests on tubular specimens allow the precise determination of material's creep resistance under varied load in the wide range of application relevant temperatures and various oxygen partial pressures. Additionally, it allows to establish the influence of grain size on creep. The results of the creep test were described by the steady state creep equation, which allows to predict the creep resistance of the material under varied conditions (like: stress, oxygen pressure, grain size, temperature). Creep tests performed in vacuum, revealed the existence of two creep regimes, characterised by different activation energy. Change of creep mechanism occurs at around 800°C. In air, creep behavior is much more complicated: during heating sequence, also two regimes are visible, however transition temperature is shifted to 850°C. This difference is related to materials instability and hexagonal phase formation. Moreover, hysteresis was found - creep rates obtained at 850°C during cooling sequence are significantly higher than one obtained during heating. In order to verify creep results obtained for air, additional specimens with cylindrical geometry were tested. Furthermore, additional creep tests were conducted on a newly developed BaCo0.4Fe0.4Zr0.2O3-delta (BCFZ) material. Complementary to creep characterization, also bending test on O-ring specimens, machined from tubes, were carried out. Temperature dependence of Young's modulus and strength were established. Subsequent fractographic analyses revealed elongated pores and agglomerates as fracture origins. Additionally, micromechanical measurements provided information concerning the micro-hardness and Young's modulus at the room temperature. In order to gain information about material stability, a long-term (2 weeks) annealing of BSCF 5582 in the air in the range of application-relevant temperatures (750-950°C) was performed. The study was complemented by qualitative microstructural investigation performed by means of light microscopy and revealed significant material instability. Precipitates of hexagonal phase are present after annealing in air below 850°C, and their volume fraction is temperature-dependent (maximum occurs at 750°C). Post operation analysis of tubes operated in a demonstration unit under conditions expected in a real power-plant operation allows to investigate microstructural changes in the material and find the axial temperature profile in the membrane tube. Finally, all collected results were summarized and conclusions were drawn
Mechanische Eigenschaften von Höchstleistungswerkstoffen für Hochtemperaturwärmeübertrager und PkW-Abgasanlagen
No full textThis work was performed at Forschungszentrum Jülich, IEK-2, within the joint research project “Development of high performance materials for high temperature heat exchangers and automobile exhaust systems – Ferrit 950“, funded by the German federal ministry of education and research. The production of compact, powerful and efficient internal combustion engines leads to an increase of temperatures and mechanical loads in the exhaust system. Currently used chromia forming materials don’t show sufficient strength in the temperature range of 900-1000°C. In Solid Oxide Fuel Cell (SOFC) systems high temperature heat exchangers are used to pre-heat the process gases. Chromia forming materials show high chromium evaporation, which could lead to accelerated ageing of the SOFC cathode. On the other hand, current alumina forming materials show relatively low high temperature strength. The aim of this work was the development of both, a new chromia forming ferritic material with increased strength at 900-1000°C for the use in automotive exhaust systems and an alumina forming ferrite for the use in high temperature heat exchangers on the basis of the material Crofer 22 H. The increase of strength ought to be achieved by solid solution and precipitation strengthening by Laves phases with enhanced dissolution temperature. Main focus of the subproject of Forschungszentrum Jülich - IEK2 was the experimental investigation of the high temperature material properties as well as accompanying microstructural investigation. High temperature strength, short time creep strength up to 1000 hours, the evolution of precipitates at temperatures of 900-1000°C as well as the thermomechanical fatigue behavior were investigated. By increasing the dissolution temperature of the Laves phase an increase in short time stability of precipitation structure and long term stability of the grain structure could be achieved at temperatures up to 950°C. Due to the addition of aluminum the materials for the use in high temperature heat exchangers show a reduction of the number of precipitates of around 50% in comparison to Crofer 22 H. In materials with high carbon and nitrogen as well as low titanium and aluminum contents, small amounts of the Fe3Nb3X phase have been detected. In comparison to the Laves phase this phase exhibited better high temperature stability. Because of dynamic strain ageing, caused by Al atoms, the Al-added materials show superior high temperature strength and creep strength as well as a very strong increase of the thermomechanical fatigue strength compared to the standard material X6CrAl18-4 at maximum temperature - in spite of the lower amount of Laves phase. Because of a higher number of precipitates the new chromia forming materials for the use in automotive exhaust systems show an increase in high temperature strength, short time creep strength and thermomechanical fatigue strength in comparison to the standard material 1.4509
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