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Dataset supporting the University of Southampton Doctoral Thesis "Study of fatigue crack initiation and propagation mechanisms in a directionally solidified superalloy: Effects of microstructure, anisotropy and oxidation"
No full textThis dataset is supporting the University of Southampton Doctoral Thesis "Study of fatigue crack initiation and propagation mechanisms in a directionally solidified superalloy: Effects of microstructure, anisotropy and oxidation"</span
Study of fatigue crack initiation and propagation mechanisms in a directionally solidified superalloy: Effects of microstructure, anisotropy and oxidation
No full textThe Directionally solidified (DS) superalloy CM247LC was first developed by Cannon Muskegon Corporation and is extensively used in turbine blades and vane applications, due to its high strength at elevated temperatures, and excellent resistance to creep comparable with some single crystal (SX) superalloys. Studies on the fatigue behaviour, particularly the interplay between microstructure, oxidation, creep and fatigue at more moderate elevated temperatures (i.e. typical of those seen at stress concentration features like the blade root or at cooling holes) are however relatively few. The thesis provides a deeper understanding of the contribution of these factors to the alloy’s failure mechanisms by systematic test design and careful in-situ and post characterization. Fatigue anisotropy is correlated with the columnar grain elongation direction with the respect to the loading direction and general crack propagation direction. The former relationship induces the anisotropy in the Young’s modulus and yield stress, which affects the entire fatigue life, while the later affects the specific fatigue crack initiation and propagation directions. Much shorter fatigue life is found in the specimen (termed as L or LR) with the columnar grains aligned along the loading direction when the applied maximum stress is close to the yield stress. This is attributed to the much lower Young’s modulus inducing a higher local plastic strain and hence faster crack initiation. The fatigue short crack propagation behaviours are mainly affected by the relationship between the columnar grain elongation direction and crack propagation direction. The effects of microstructural features on the fatigue short crack initiation and propagation have been assessed by data rich imaging approaches. Short cracks preferentially initiate from carbides or pores, instead of at slip bands, even though the intensity of the localized strain is at similar scale to these features in the early stages of crack initiation. The observation of crack evolution in three dimensions (3D) via X-ray CT further illustrates the fatigue cracks propagate along several slip systems simultaneously instead of a single favoured slip system. Oxidation preferentially occurs at the stress/strain localisation features such as slip bands and carbides. The detailed oxidation mechanism of the carbides is revealed by 3D reconstruction of the oxidised carbides at different testing times: (Co, Ni) oxides are formed firstly at the interface between carbides and matrix, then protrude beneath these carbides, finally intruding into the bulk, leading to crack formation inside the carbides. Crack initiation from oxidised carbides is widely observed, but their propagation is blocked by the surrounding oxides in fatigue testing with 1-1-1-1 trapezoidal waveform. Thus, the main crack initiation sites transfer from the surface to the subsurface pores. The phenomenon of oxides prohibiting crack propagation is also shown in the long fatigue crack propagation tests. The degree of crack arrest is linked to testing temperatures and frequencies, as more significant oxidation occurs in the lower frequency and higher temperature tests. Internal (Al, Cr)-rich oxides intrude into the materials, especially ahead of the crack tip, while the external (Co, Ni) oxides fill the crack. Oxidation induced crack closure is thought to be the main factor contributing to observed crack arrests, caused by the thick external oxides. A modified model based on the thickness of the external oxidation layers is proposed to quantitatively evaluate the effective stress intensity, which shows the thicker oxidation layers formed at lower frequencies, causes a dramatic reduction in the crack tip stress intensity and leads to crack arrest. Further detailed analysis on cracks was performed to assess the steps in the internal (Al, Cr)-rich oxide formation. Al-rich oxides form inside the γ՛ in the form of nanoparticles, while Cr-rich oxides are formed at the γ channel in the form of stripes. The potential stress assisted oxidation mechanism is inferred to be: (1) the formation of external oxides depletes γ՛ of Ni, causing the formation of Al-rich oxides inside the γ՛ (2) after the γ՛ is fully oxidized, O diffuses into the γ channel, forming stripe-like Cr-rich oxides; (3) As dense internal oxidation layers are not formed, allowing continuous diffusion of Ni and Co elements, this forms thick external oxidation layers, resulting in oxidation-induced crack closure
Dataset for the publication 'Oxidation induced crack closure in a nickel base superalloy: a novel phenomenon and mechanism assessed via combination of 2D and 3D characterization'
No full textDataset supporting the publication by Y Tan, N Gao, P Reed,
"Oxidation induced crack closure in a nickel base superalloy: A novel phenomenon and mechanism assessed via combination of 2D and 3D characterization",
Materials Science and Engineering: A, Volume 861, 2022, 144311,
ISSN 0921-5093,
https://doi.org/10.1016/j.msea.2022.144311.
The data is derived from three dimensions - X-ray scanning tomography (CT) and two dimensions - scanning electron microscopy (SEM) with attached energy-dispersive Xray spectroscopy (EDX). The data was incorporated into modified models to provide quantitative measurements of oxidation induced crack closure.
The data is presented as:
COD_final.opju
L_and_T_650_1_1_1_and_sinewaveform.opju
Resharp_data.opju
The project was funded by EPSRC (Grant no:EP/M000710/1) and the China Scholarship Council.</span
Oxidation induced crack closure in a nickel base superalloy: a novel phenomenon and mechanism assessed via combination of 2D and 3D characterization
No full textUnderstanding the mechanism of oxidation induced crack closure (OICC) is of great importance in understanding the fatigue resistance of materials operating at intermediate or high temperatures subjected to oxidation. Current work reveals that the occurrence of OICC is most closely related to the test frequencies and temperatures rather than the microstructure in a directionally solidified (DS) superalloy. Characterization techniques in three dimensions - X-ray scanning tomography (CT) and two dimensions - scanning electron microscopy (SEM) with attached energy-dispersive Xray spectroscopy (EDX) are combined to capture the oxides formed within the crack wake. These data are then incorporated into modified models to provide quantitative measurements of oxidation induced crack closure. Both the experimental and modelling results show that the external oxides forming close to the crack tip, result in a high crack tip opening displacement and thereby significant crack closure
The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature
No full textThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress
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
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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