1,720,966 research outputs found
Fretting wear of alloy steels at the blade tip of steam turbines
Full text linkIn order to reduce blade resonant vibration amplitude in turbomachinery, blades are assembled with a mutual interlocking at the tip. The aim of this study is to investigate the wear mechanism at the contact interface of the blade shroud in steam turbines. Experimental data are available concerning the wear mechanism at interfaces of aircraft engines blades, while the literature regarding the same effect on steam turbines is less rich. Moreover, the transposition of the results from the aero-engine to the steam turbine is difficult, because materials and working conditions are different. To overcome this lack of knowledge an experimental campaign was set up to investigate this wear mechanism under the specific conditions and with the distinctive materials used in steam turbines.
Two base materials (alloy steels) were tested under different conditions: surface treatment (with and without laser quenching), temperature and normal load. Dissipated energies were determined from the hysteresis loops measured during the tests and were correlated to the test conditions. Profiles of worn surfaces were measured, and volume losses were accurately computed with a procedure that takes into account the roughness of the surfaces.
Experiments were conducted both at room and low temperature (150 °C). At room temperature the surface temperature increased to 60-70 °C, due to the heat generated in the wear process. Comparison of volume losses at room and low temperature showed that at 150°C the volume losses decreased dramatically. This behavior was explained with a brittle-ductile transition. In other words, the same wear mechanism, adhesion and abrasion respectively in stick and gross slip condition, give very different results for a small softening effect of the material. Moreover, experimental results showed much more sensitive wear rates to the heat treatment than to the steel type
Partial-gross slip fretting transition of martensitic stainless steels
Full text linkBased on critical reviews of the well-known dependence of fretting process upon the sliding amplitude, experiments were performed to verify this dependence. One of the main critical points is that the experiments which led to this result were performed controlling the imposed displacement amplitude instead of the real sliding amplitude. Therefore, the difference between the real displacement amplitude and the imposed amplitude due to compliances of the test rigs components was not considered. Fretting tests were performed using a high precision test rig. One of the main peculiarities of this test rig is that there is no difference between the imposed sliding amplitude and the real amplitude. The fretting process parameters of experiments were room temperature, two normal load (contact pressure 15, 25 MPa), four strokes (10, 15, 20, 50 μm), two martensitic stainless steels (X20Cr13, M152) and different durations from 15 to 160 M-cycles. Friction coefficient was computed using the hysteresis loops measured during the wear test. The worn surfaces were measured using an optical instrument based on focus variation. Wear volumes were accurately computed with a procedure that takes in to account the roughness of the surfaces. Results show that the friction coefficient is independent of slip amplitude, normal load and steel type if the hysteresis loops shape is parallelogrammatic and the contact surfaces are effectively conformal. When these conditions are not observed, the friction coefficient is dependent on normal load, even in contrast with its increase. Wear volume shows linear evolution in gross slip regime while it is non-linear in partial slip. Wear rate is independent of slip amplitude and normal load in partial slip regime at low ratio amplitude contact length. In contrast, wear rate depends on slip amplitude and exhibits a sharp increase near the transition partial-gross slip
Fretting wear damage mechanism of CoMoCrSi coatings
No full textSuperalloy coatings of the CoMoCrSi family (e.g. Tribaloy® T800) are applied to mitigate wear effects at high temperature. These coatings are extensively used on the contact surfaces of the shroud of turbine blades. If severe wear occurs on these contact surfaces the blade interlocking decreases, reducing the stiffness of the assembly, altering its dynamic behaviour, and increasing the risk of fatigue failure. Fretting is the expected damage mechanics on these mating surfaces. The study presented in this paper investigates the fretting damage mechanism of interfaces coated with CoMoCrSi alloys. The experimental plan includes fifteen combinations of the test parameters: two contact geometries, three deposition processes, four temperatures, three normal loads and three strokes. Wear at different number of cycles was also explored. Moreover, two types of contact geometries were investigated, namely point contact (sphere-on-flat) and flat-on-flat. The friction coefficient was computed using the hysteresis loops measured during the fretting tests. The topography of the contact surfaces was measured at different fretting wear cycles to estimate the volume loss. Wear grooves were observed by scanning electron microscopy. Results of point contact experiments at room temperature exhibited a steady friction coefficient independent of the normal load. Wear volumes showed a sharp increasing in wear rate at high dissipated energy while the trend was linear at lower dissipated energy. Oxidation was found more dependent on substrate than on temperature, stroke and wear cycles. Wear volumes and wear rate on flat-on-flat specimens coated with welded T800 were higher at 400 C than at room temperature and at high temperature (800 C). At room temperature, wear volumes of welded T800 applied by single layer were much higher than in dual layer. At room temperature and at low dissipated energy the wear rate of the point contact geometry was lower than flat-on-flat. At high energy, the wear rate of point contact tends to the flat-on-flat wear rate
Fretting wear of CMSX4 at blade tip interface with and without coating
No full textFretting wear is a complex phenomenon that occurs at component interfaces that are subjected to low amplitude oscillation under high contact pressure. In turbomachinery fretting occurs also at the blade tip interfaces where shrouds, that have the aim to reduce the blade resonant vibration amplitude, are machined. To diminish the fretting damage coatings are applied to the blade tips. The aim of this study is to compare the fretting wear behaviour of single crystal CMSX-4 superalloy interfaces with and without plasma sprayed T-800 coating. Experiments have been conducted with hemispherical surface in contact with a flat surface of the same materials at temperature of 800 °C. The hysteresis cycles have been measured through the experiment. The comparison of the hysteresis cycles shown that the tangential contact stiffness of the coated surfaces is greater then that of the surfaces without coating. At the end of wear process, the mating surfaces have been characterized by three-dimensional optical interferometry and SEM analysis. After 10×106 wear cycles, the uncoated surfaces show a large change in the contact parameters and fretting cracks on the flat surface. On the other hand, the coated surfaces do not shows a measurable change in the contact parameters while the coating damage on the flat surface leads to predict an incipient catastrophic wear
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
Injectable supramolecular hydrogels based on custom-made poly(ether urethane)s and α-cyclodextrins as efficient delivery vehicles of curcumin
No full textA strategy to enhance drug effectiveness while minimizing controversial effects consists in exploiting host-guest interactions. Moreover, these phenomena can induce the self-assembly of physical hydrogels as effective tools to treat various pathologies (e.g., chronic wounds or cancer). Here, two Poloxamers®/Pluronics® (P407/F127 and P188/F68) were utilized to synthesize various LEGO-like poly(ether urethane)s (PEUs) to develop a library of tunable and injectable supramolecular hydrogels for drug delivery. Three PEUs were synthesized by chain extending Poloxamer/Pluronic with 1,6-cyclohexanedimethanol or N-Boc serinol. Other two amino-functionalized and highly responsive polymers were obtained thorough Boc-group cleavage. For hydrogel design, the spontaneous self-assembly of the poly(ethylene oxide) domains of PEUs with α-cyclodextrins was exploited to form poly(pseudo)rotaxanes (PPRs). PPR-derived channel-like crystals were characterized by X-Ray powder diffraction, Infra-Red and Proton Nuclear Magnetic Resonance spectroscopies. Cytocompatible hydrogel formulations were designed at PEU concentrations between 1% and 5% w/v and α-cyclodextrin at 10% w/v. Supramolecular gels showed good mechanical performances (storage modulus up to 20 kPa) coupled with marked thixotropic and self-healing properties (mechanical recovery over 80% within 30 s after cyclic rupture) as assessed through rheology. Hydrogels exhibited stability and high responsiveness in watery environment up to 5 days: the release of less stable components as suitable drug carriers was coupled with high swelling (doubling the content of fluids with respect to their dry mass) and shape retention. Curcumin was encapsulated into the hydrogels at high concentration (80 μg ml−1) through its complexation with α-cyclodextrins and delivery tests showed controllable and progressive release profiles up to four days
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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