1,720,964 research outputs found
High-throughput screening the micro-mechanical properties of polyimide matrix composites at elevated temperatures
No full textIn this work, a novel high-throughput methodology based on combinations of nanoindentation, indentation creep and push-in methods, is proposed to measure in-situ the micro-mechanical properties of typical polymer matrix composites at a wide temperature range. The Young's modulus and strain rate sensitivity of a polyimide matrix and the interfacial shear strength in a quartz fiber reinforced polyimide matrix composite are measured at 25-350 degrees C for the first time. The results highlight a linear softening of the polyimide matrix at high temperatures, which is evidenced by the approximate linear decrease of Young's modulus from approximate to 5.0 GPa at 25 degrees C to approximate to 1.1 GPa at 350 degrees C. In comparison, the strain rate sensitivity of the polyimide matrix is increased, from approximate to 0.032 at 25 degrees C to approximate to 0.062 at 350 degrees C. This evidences a stronger visco-plasticity of polyimide at higher temperatures. The shear strength of the fiber/matrix interface is also temperature dependent. As the testing temperature increases from 25 to 300 degrees C, the shear strength is decreased from approximate to 147 MPa to approximate to 40 MPa. Specially, the interfacial strength is extremely low at 350 degrees C (approximate to 4 MPa), evidencing a failure of the composite at this temperature
High-throughput investigation of temperature-dependent creep mechanisms in Mg-Zn alloys using nanoindentation
No full textA novel high-throughput methodology is proposed to investigate the influence of Zn alloying and temperature on the creep mechanisms of Mg through a combination of a Mg/Mg-Zn diffusion couple and advanced nanomechanical testing. Systematic nanoindentation creep tests were conducted at room temperature (RT) and elevated temperatures (100 degrees C, 200 degrees C and 300 degrees C) within individual grains in a Mg/Mg-Zn diffusion couple with Zn content ranging from 0 to 0.8 at%. Electron backscattered diffraction-assisted trace analysis was employed to characterize the activated deformation mechanisms, including twinning and dislocation slip. The activation volume and creep activation energy were analyzed to elucidate the fundamental creep mechanisms as functions of temperature and Zn content. At the low temperature regime (RT-100 degrees C), the creep activation energy was approximate to 80 kJ mol(-1), indicating that twinning dominated deformation mechanisms. At the high temperature regime (>200 degrees C), all alloys converge to a relatively high activation energy, of the order of 150 kJ mol(-1). This is in good agreement with the activation energy reported for cross-slip deformation mechanisms. The results indicate that the derived activation energy aligns with the transition in creep deformation mechanisms, shifting from twin boundary migration to cross-slip. Moreover, Zn alloying significantly enhances creep resistance, particularly within the temperature range of RT to 200 degrees C
Deformation mechanisms of basal slip, twinning and non-basal slips in Mg-Y alloy by micropillar compression
No full textMicropillar compression technique was employed to study the microscale deformation mechanisms of basal slip, twinning and non-basal slips at selected grains in a Mg-2 wt.% Y alloy. The results suggest a critical resolved shear stress (tCRSS) for basal slip 12.5 +/- 1.7 MPa, and for twin nucleation and twin growth 38.5 +/- 1.2 MPa and 33.8 +/- 0.7 MPa, respectively. The higher values compared to those in pure Mg suggests a more balanced deformation in Mg alloy with Y addition. The activation of dislocations in the twinned orientation is highlighted, which leads to strong work hardening in twinned favorable orientation [1010]. In addition, at prismatic-slip favorable orientation [1120], a twinning-to-prismatic slip transition was observed when elevating temperature from 25 degrees C to 100 degrees C and 250 degrees C. Specially at 250 degrees C, twinning was completely prohibited, and pure prismatic slip was triggered. The measured tCRSS for prismatic slip at 250 degrees C was 39.7 +/- 0.3 MPa, much higher than that for pure Mg at the same temperature. Finally, at pyramidal-slip favorable orientation [0001], an abnormal strengthening was observed at 100 degrees C and 250 degrees C due to activation of pyramidal slips. Decompositions of dislocations and Y segregation at stacking faults are the main mechanisms leading to the hightemperature strengthening in Mg-Y alloy
Effect of dilute atom Gd on critical resolved shear stress and anisotropic deformation mechanism of Mg-Gd alloy
No full textFormability and ductility of Mg alloy are highly dependent on its mechanical anisotropy and are mainly dominated by the anisotropic deformations of basal slip, tension twining and non-basal slips. This work is engaged to measuring critical resolved shear stress (CRSS) for basal slip, twin nucleation/growth and pyramidal slip of a Mg-1 wt% Gd alloy by micropillar compressions. The effects of rare earth (RE) element, Gd, on the CRSS and the deformation mechanisms are discussed, and the mechanical anisotropy is determined. The results suggest that Gd atom is capable of balancing the anisotropy ratio of hard mode to soft mode, by triggering significant strengthening in basal slip of Mg alloy. The strengthening effect is related to the misfit-volume induced fluctuations in solute concentrations and the wavy solutes/dislocation interaction energies when Gd is added. As a result, Gd solute is advantageous to reduce the mechanical anisotropy, thus benefits to formability and ductility of Mg alloys. The presented results can provide a framework to understand basically the physical origin of mechanical anisotropy and formability of Mg-RE alloys and can be guidelines to design high ductility Mg alloys for next-generation lunar exploration
Temperature-dependent activation of prismatic slip in dilute Mg-Zn alloys: A micromechanical study
No full textUnderstanding the temperature-dependent deformation mechanisms in magnesium alloys is crucial for their application in lunar exploration payloads, where materials experience extreme temperature variations. Through in-situ micropillar compression along the [11 (2) over bar0] orientation, the effects of Zn alloying on prismatic slip in Mg-Zn alloys were systematically investigated at 25 degrees C and 100 degrees C. The results revealed a remarkable transition in deformation mechanisms: from twinning-dominated behavior at room temperature to prismatic slip-controlled deformation at elevated temperatures. Notably, the critical resolved shear stress (CRSS) for prismatic slip at 100 degrees C in Mg-Zn alloys was measured for the first time, demonstrating an unexpected solute softening effect where CRSS decreased from similar to 37 MPa to similar to 32 MPa as Zn concentration increased from 0.5 to 1.0 at%. Through detailed microstructural analysis, this softening behavior was attributed to the reduction in Peierls-Nabarro lattice resistance, which was facilitated by elevated temperature promotion of cross-slip of dislocations from basal to prismatic planes. This mechanism was further evidenced by characteristic wavy slip traces and distinct strain hardening behavior. These findings provide fundamental insights into temperature-controlled activation of prismatic slip, offering new pathways for enhancing the formability of Mg alloys at elevated temperatures
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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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