1,720,996 research outputs found
Study of solidification cracking in advanced high strength automotive steels
No full textAdvanced high-strength steels (AHSS), which are increasingly used in the automotive industry, meet many functional requirements such as high strength and crash resistance. Some of these steels contain high amounts of alloying elements, which are required to achieve the necessary mechanical properties, but render these steels susceptible to weld solidification cracking. Weld solidification cracking results from the complex interplay between mechanical and metallurgical factors. Our recent work is focused on studying solidification cracking in dual phase (DP) and transformation induced plasticity (TRIP) steelsusing the following modeling and experimental strategies:1. A finite element (FE) based model was constructed to simulate the dynamic thermal and mechanical conditions that prevail during bead-on-plate laser welding. To vary the restraint, laser welding was carried out on single sided clamped specimens at increasing distances from the free edge. In TRIP steel sheets, solidification cracking was observed when welding was carried out close to the free edge and at a certain minimum distance, no cracking was observed. For the no cracking condition, in situ strain evolution during laser welding was measured by means of digital image correlation to validate the strain from the Fe-model. Subsequently, a phase field model was constructed using the validated thermal cycles from the FE-model to simulate the microstructural evolution at the tail of a weld pool, where primary dendrites coalesce atthe weld centerline. From the phase field model, elemental segregation and stress concentration are used to explain the cracking susceptibility in TRIP and DP steels. For DP steel, both the experimental and modeling results indicate a higher resistance to solidification cracking.2. A phase field model was constructed to simulate the directional solidification in TRIP and DP steels. The thermal cycle and temperature gradient were derived from the in-situ solidification experiments conducted using high temperature laser scanning confocal microscopy (HTLSCM). The model showed that longer and narrower interdendritic liquid channels exist in the case of TRIP steel. For the TRIP steel, both the phase field model and atom probe tomography revealed notable enrichment of phosphorus, which leads to a severe undercooling in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability, resulting in solidification cracking.The overall study shows that a combination of factors is responsible for the susceptibility of a material to solidification cracking. These include particularly mechanical restraint, solidification temperature range, solidification morphology, solute segregation and liquid feeding capability.(OLD) MSE-5(OLD) MSE-
Residual stress measurements and model validation of single and double pulse resistance spot welded advanced high strength steel
No full textAdvanced high strength steels (AHSS) are increasingly used in automotive industry; thousands of resistance spot welds are applied to car body-in-white. High alloying levels of AHSS result in lower weldability. Residual stresses play an essential role on the formation of defects and the mechanical performance of the weld. An electrical-thermal-metallurgical-mechanical finite element model was constructed to simulate the temperature and stress distribution during single and double pulse resistance spot welding. The models are validated by ex-situ synchrotron X-ray diffraction stress measurements. In this paper, single pulse and double pulse resistance spot welds were made on 1.3 mm thin sheets of a 3rd generation AHSS. Depth resolved stress measurements in two orthogonal directions were carried out using high-resolution powder diffraction at beamline ID22 of the European Synchrotron Research Facility. A monochromic 70 keV X-ray was used to record the d-spacing of (200) bcc planes in transmission mode. The strains were calculated from the shift in the d-spacing of the planes. The stresses were calculated by the biaxial Hook’s law. The numerical and experimental results show that the residual stresses in the weld nugget zone and the heat affected zone of the welds are tensile in nature, whereas the base material experiences compressive stresses. Lower residual stresses at the weld nugget and HAZ were obtained by applying a second current pulse. The simulated results show a good agreement with the residual stresses measured. This study provides a better understanding of the stress distribution in resistance spot welds and allows prediction of stresses as a result of welding conditions applied.(OLD) MSE-1(OLD) MSE-
Fügen mit Laserstrahlung
No full textErgebnisse des Laserstrahlfügens von Hartmetallen und Stahl werden vorgestellt. Durch gezielte Temperaturführung kann die Bildung von Poren und Rissen vermeiden werden. Im zweiten Teil werden Simulationsergebnisse der Bearbeitung mit Laserstrahlung vorgestellt
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
Modellierung des Verformungs- und Versagensverhaltens punktgeschweißter Mischverbindungen zwischen mikrolegierten und pressgehärteten Stählen
No full textIn der vorliegenden Arbeit wurden Widerstandspunktschweißverbindungen zwischen einem mikrolegierten HC340LAD-Stahl und pressgehärtetem 22MnB5 bei quasistatischen Belastungen charakterisiert und mit der Finite Elemente Methode (FEM) modelliert. Die Tragfähigkeiten, das Verformungs- und das Bruchverhalten der Mischverbindungen wurden hierbei mithilfe von Detailmodellen (FEM), sowie mit einem erweiterten analytischen Versagensmodell berechnet. In beiden Fällen zeigte der Vergleich mit experimentellen Daten sehr gute Übereinstimmungen. Für punktgeschweißte 22MnB5-Bleche wurde erstmals isoliert untersucht, inwieweit die Erweichungszone, die beim Schweißen durch das Anlassen des martensitischen Ausgangsgefüges auftritt, die mechanischen Eigenschaften der Bleche beeinflusst. Sowohl experimentell als auch rechnerisch wurde gezeigt, dass die Erweichungszone bei Zugbelastungen der Bleche den versagenskritischen Gefügebereich darstellt und deren Festigkeit und Verformungsvermögen beschränkt. Möglichst hohe Festigkeiten, sowie eine hohe Ausnutzung des Verformungsvermögens der Bleche können nur dann erreicht werden, wenn die geometrischen Abmessungen der Erweichungszone möglichst klein sind
Electron Beam Welding of TZM plates
Full text linkRecently, many efforts have been made to join molybdenum and its alloys, such as TZM, due to their interest for industrial applications. However, it has a poor weldability due to high thermal conductivity and high melting temperature, sensitivity to gas impurities and lack of solid state transformations leading to coarse grain formation. Electron beam welding (EBW) is a high energy density process, and welding TZM can result in a smaller fusion and heat affected zones. The aim of this study was to perform an exploratory investigation on the ability of EBW to weld TZM, focusing on the major process parameters, the clamping system and evaluate the effect on the weld tensile strength. The selection of the parameter values and their refinement was performed using design of experiments.
Sound welds were obtained, free of pores, cracks and misalignments. A dedicated clamping system was designed, manufactured, and proved to be of major importance. Tensile strength was close to 80% of the base material, which is remarkably good for this alloy. Pre- and post-weld treatment had no significant effect on the welding strength. However, further work is needed to minimize grain coarsening, eventually by stirring the molten pool using beam deflection
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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