Papers by Keyword: Polycrystal

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Authors: Alexander M. Korsunsky
Abstract: The sin2ψ technique for near-surface and bulk stress evaluation is frequently considered to be the method of reference, largely due to the historical reason of being established early on in the development of experimental study of residual stress, and due to the widespread availability of laboratory X-ray facilities equipped with goniometers allowing ψ-tilting to be carried out. In recent years other diffraction-based techniques of residual strain and residual stress evaluation have been developed, some of them based at large facilities such as synchrotrons, neutron reactors or spallation sources, and others becoming available in the laboratory setting. It is therefore perhaps relevant and timely to review the strengths and shortcomings of the sin2ψ technique in today’s context. In the present study this task is addressed through the use of polycrystal elasto-plastic modelling that allows the determination of equivalent average elastic lattice strains following complex deformation history, and by post-processing of the model results in order to extract the parameters measurable in diffraction experiments. In particular, it is possible to extract the simulated strain values that would be measured at different tilt angles, and to build a family of sin2ψ plots for different reflections. It then becomes possible to assess the degree to which the hypotheses underpinning the principle of this method are enforced or violated; to select the most suitable reflections; and to discuss how the method could be improved or varied to provide more reliable residual stress measurement procedures.
Authors: Sadahiro Tsurekawa, Tadao Watanabe
Authors: Dierk Raabe, Kurt Helming, Franz Roters, Zisu Zhao, Jürgen Hirsch
Authors: Paulo Rangel Rios, Martin E. Glicksman
Abstract: One common point amongst extant theories of abnormal grain growth (AGG) is that they treat this phenomenon in terms of the relative grain size, or grain radius, of the abnormal grains. Topological and metrical quantities of abnormal grains, such as the number of their faces, or their grain boundary curvature, are taken into account only indirectly through the grain size itself. This paper, by contrast, treats AGG in terms of concepts, that include both the boundary curvature and the number of faces of the abnormal grain. Two cases are examined: 1) AGG, in which the matrix grains are fully pinned, so normal grain growth cannot occur; 2) AGG in which the matrix grains are free to evolve, so that normal grain growth ensues simultaneously in the matrix.
Authors: Shu Yan Zhang, Jordan Schlipf, Alexander M. Korsunsky
Abstract: A traditional approach to increasing fatigue resistance of many assemblies involves the creation of regions of compressive residual stress. For example, riveting holes used in modern passenger aircraft are currently subjected to cold expansion using split mandrel tools. The method is relatively expensive and not entirely problem-free. In the present study we consider a method of creating residual stresses around drilled holes referred to as ‘dimpling’, that itself is a variation of a novel technique known as the StressWaveTM process. An experimental procedure is described for the creation of localised regions of significant plastic deformation and residual stress by ‘dimpling’, allowing the production of cold-worked and residually-stressed specimens. The overall aims of this study were to determine thickness-average residual stresses by two different techniques, namely, one destructive technique (Sachs boring) and one non-destructive (high energy X-ray diffraction); and to compare the results. In Sachs boring experiments the variation of strain gauge readings with increasing diameter of the central hole was recorded. A classical elastic-ideally plastic axisymmetric model for plane stress conditions was used in the analysis. Energy dispersive synchrotron X-ray diffraction experiments were performed for non-destructive assessment of residual elastic strains. The two different stress evaluation techniques used in this project led to consistent results. Good correlation was found between the stresses obtained from X-ray diffraction results and those deduced from Sachs boring experiments.
Authors: G.N. Kamaev, S.V. Golod, E.M. Skok, A.K. Fedotov, A.V. Mazanik
Authors: Jesús Galán López, Patricia Verleysen, Soroosh Naghdy, Leo Kestens
Abstract: The use of finite element simulations has become one of the main tools of the mechanical engineer. The method is applied to the analysis and design of engineering structures, the study of manufacturing processes and even to perform virtual experiments. Traditionally, the constitutive laws chosen for finite element analysis have been as simple as possible, mainly due to the limitation imposed by the available computing power. However, the development of more powerful computers and more efficient methods is opening the possibility of using more elaborated (and, most often, more accurate) material models. In particular, polycrystal models capable of predicting not only the mechanical behaviour of the material, but also of the evolution of properties with increasing strain, are particularly well suited for the simulation of forming processes, for which a precise knowledge of the properties of the resulting product is of paramount importance.The present work studies how the Visco Plastic Self-Consistent model (VPSC) can be used in combination with the implicit finite element package Abaqus/Standard to simulate the behaviour of Ti-6Al-4V sheet, and compares it with the more common (and much simpler) Johnson-Cook model. More specifically, the goal of this study is to determine whether or not, with using similar experimental calibration data, the use of the much more complex polycrystal model, justifies the increased complexity and execution time. Using standard tensile experiments at different strain rates, the parameters of the VPSC and Johnson-Cook models are fitted using a minimization method. Then, both models are used in finite element simulations and the results given by both models are compared.
Authors: Sokrates T. Pantelides, D. Maroudas, D.B. Laks
Authors: A.V. Mironov, A.V. Pokoev, D.I. Stepanov, I.S. Trofimov
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