1,721,018 research outputs found
Numerical investigation on hydrogen embrittlement of metallic pipeline structures
Full text linkMotivated by shifting to renewable energy sources, the utilization of hydrogen gas as an energy carrier is discussed to account for expected irregularities in supply. Here, the construction of a new hydrogen pipeline network would be rather expensive such that the usage of the existing natural gas pipeline system is in the focus of investigation. One major problem is that hydrogen embrittles the material and can cause fatal failure especially in case of pipelines which were already damaged during their service time. At the localized stress fields around pre-existing cracks, hydrogen accumulates and converts the material response from ductile to brittle failure. Based on experimental findings, the so-called Hydrogen Enhanced Localized Plasticity (HELP) mechanism is identified to govern hydrogen embrittlement in the prevalent case by reducing the yield stress in a continuum sense. To solve the highly coupled equations of the mechanical model and the transient hydrogen distribution model, an iterative finite element scheme is applied using a discontinuous Galerkin method for time discretisation. A continuum model of a natural gas pipeline with a radial crack is investigated. Therefore, a surrogate model using the boundary layer approach is adopted, whose results are in good agreement with a model of the full pipeline structure. Based on the idea of the local softening effect by the HELP mechanism, three different approaches to account for hydrogen embrittlement are discussed. Firstly, the steady state hydrogen distributions in front of the crack tip are evaluated while in a next step the effect of hydrogen on the mechanical properties is discussed. When investigating the impact of different crack lengths, a notably increasing amount of hydrogen embrittlement is identified. The results highlight the importance of carefully investigating the actual conditions in the specific pipelines and adequately accounting for hydrogen embrittlement in numerical simulations as a basis for ducting hydrogen through the existing pipeline system.Motiviert von der anstehenden Energiewende wird aktuell unter anderem diskutiert, ob Wasserstoffgas ein brauchbarer Energieträger ist, um erwartete Unregelmäßigkeiten in der Energieversorgung zu überbrücken. Da die Konstruktion eines neuen Rohrleitungssystems für Wasserstoff mit hohen Kosten verbunden wäre, wird untersucht, ob das existierende Erdgassystem für diese Zwecke verwendet werden kann. Hierbei erscheint besonders problematisch, dass Wasserstoffgas in das Rohrleitungsmaterial eindringen kann und dieses unter bestimmten Bedingungen versprödet. Dies kann zu fatalem Materialversagen führen, insbesondere bei bereits vorgeschädigten Rohrsegmenten. So sammelt sich an den lokalen Spannungsfeldern der Risse Wasserstoff an, der das Materialversagen von duktil zu spröde transformiert. Basierend auf experimentellen Ergebnissen kann der so genannte Hydrogen Enhanced Localized Plasticity (HELP) Mechanismus im hier untersuchten Fall als vorherrschender Versprödungsmechanismus identifiziert und durch eine gezielte Reduktion der Fließgrenze im Rahmen des verwendeten Kontinuumsmodells berücksichtigt werden. Um das stark gekoppelte Gleichungssystem des mechanischen Modells und des transienten Wasserstoffmodells zu lösen, wird ein iterativer Finite-Element-Ansatz mit der diskontinuierlichen Galerkin Methode für die Zeitdiskretisierung genutzt. Da insbesondere bereits beschädigte Rohre kritisch im Hinblick auf Versagen sind, wird ein Kontinuumsmodell einer Erdgasleitung mit einem radialen Riss untersucht. Hierfür kommt ein Ersatzmodel unter Berücksichtigung des "boundary layer approaches" zum Einsatz, dessen Ergebnisse gut mit denen einer Simulationen eines vollen Rohrausschnittes übereinstimmen. Basierend auf der Idee des HELP Mechanismusses werden drei unterschiedliche Ansätze zur Berücksichtigung von Wasserstoffversprödung vorgestellt. Zunächst wird die stationäre Verteilung der Wasserstoffkonzentrationen bestimmt, woraufhin in einem weiteren Schritt der Einfluss von Wasserstoff auf die Materialeigenschaften mit einbezogen wird. Bei der numerischen Untersuchung des Einflusses verschiedener Risstiefen kann eine ansteigende Tendenz der Wasserstoffversprödung identifiziert werden. Diese Ergebnisse zeigen die Wichtigkeit einer eingehenden Untersuchung der existierenden Rohrstrukturen und die Notwendigkeit der Weiterentwicklung numerischer Methoden, um den Einfluss von Wasserstoffgas auf das Erdgassystem schlussendlich bewerten zu können
Finite Element Analysis of Nonsmooth Frictional Contact
No full textA nonsmooth contact class of algorithms were introduced by Kane et al. [1] and extended to the case of friction by Pandolfi et. al [2]. The formulation specifically addresses geometries for which neither normals nor gap functions can be properly defined, e.g. bodies with corners. The formulation provides the incremental displacements in variational form, following from a minimum principle. Selected numerical examples of application of the algorithm are presented here
Continuum based modelling and simulation of rate-dependent effects in inelastic solids: applications for biological growth and damage mechanics
No full textNowadays, model-based simulation methods play a major role in the research and development of new technologies. This is not least due to the time and cost savings associated with their use. Ideally, however, physically motivated simulation models also provide a deeper insight into the processes depicted, which is otherwise difficult to achieve in many places. In the context of engineering sciences, the prediction of the behaviour of a wide variety of materials under mechanical loading plays a particularly important role. Since almost all materials tend to nonlinear material behaviour under certain circumstances, the modelling can be very challenging depending on the effect under consideration. Especially in the area of rate- and time-dependent modelling of a wide variety of material properties, many open research questions remain yet to be answered. These include, for example, the description of time-dependent growth processes in biological materials or the modelling of rate-dependent damage phenomena. The present cumulative dissertation presents a compilation of the author’s (and his coauthors’) work that has been published on topics of continuum mechanical modelling of rate-dependent material behaviour. After the introduction, the overview of the current state of research and the clarification of the research-relevant questions, four published research papers are presented. The dissertation starts with a paper on the topic of modelling growth processes in artificially grown tissue. Based on experimental observations, a so-called homeostatic state is postulated in this work. Such a state describes a state of tension preferred by the tissue, which the material always tries to adopt through active contraction or expansion. This consideration forms the basis for modelling the corresponding growth-induced change in shape and volume. Following classical models for the description of plastic material behaviour, it is shown that by introducing a homeostatic potential, the development of the growth-related, inelastic strains can be elegantly described. For the description of the temporal component in the evolution of the inelastic strains, a classical Perzyna approach is used. In addition to the theoretical derivation, the numerical realization as well as the implementation in finite element (FE) software will be discussed in the following. Using numerical examples, it is shown that the new formulation is able to predict the growth behaviour more precisely than other well established models. This is especially illustrated by considering the influence of complex boundary conditions. Finally, a first investigation of the prediction quality of the model based on experimental data shows that the developed model is able to reasonably approximate the growth-induced homeostatic stress in the tissue. Besides growth process effects, many other inelastic effects can occur in complex materials. Polymers show a strong dependence in their deformation behaviour with respect to both loading rate and temperature. Furthermore, rate-dependent damage effects play an important role in these materials. In the further course of this dissertation, two articles and one conference proceeding are presented, dealing with the thermodynamically consistent modelling of ratedependent damage behaviour in polymers. In the first two publications on this topic, the purely mechanical continuum model is presented. Based on the multiplicative decomposition of the deformation gradient, the viscoelastic material behaviour is described via a viscous potential. A Perzyna approach is used to model the rate-dependent evolution of the scalar damage variable. The thermodynamically consistent derivation is discussed in the following as well as the numerical treatment of the equations and their implementation in FE software. Finally, it is shown that the developed model is able to adequately represent creep damage and polymers. This is a great advantage compared to classical, rate-independent models, as these are not able to represent this effect in a meaningful way. Building on the previous publications, the last article in this work deals with the thermodynamically consistent extension of the proposed damage model to take thermal effects into account. For this purpose, a fully thermomechanically coupled formulation is presented on the basis of a further decomposition of the deformation gradient. Using appropriate parameter studies and numerical examples, the influence of temperature on the development of damage is investigated. Finally, the results of various structural calculations demonstrate the applicability of this mutliphysical simulation model for various applications. In the last chapter, this dissertation concludes the research questions investigated herein and gives an outlook for further potential research based on the findings of this work
A constitutive contact model for homogenized tread-road interaction in rolling resistance computations
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Micro-feature-motivated numerical analysis of the coupled bio-chemo-hydro-mechanical behaviour in MICP
Full text linkA coupled bio-chemo-hydro-mechanical model (BCHM) is developed to investigate the permeability reduction and stiffness improvement in soil by microbially induced calcite precipitation (MICP). Specifically, in our model based on the geometric method a link between the micro- and macroscopic features is generated. This allows the model to capture the macroscopic material property changes caused by variations in the microstructure during MICP. The developed model was calibrated and validated with the experimental data from different literature sources. Besides, the model was applied in a scenario simulation to predict the hydro-mechanical response of MICP-soil under continuous biochemical, hydraulic and mechanical treatments. Our modelling study indicates that for a reasonable prediction of the permeability reduction and stiffness improvement by MICP in both space and time, the coupled BCHM processes and the influences from the microstructural aspects should be considered. Due to its capability to capture the dynamic BCHM interactions in flexible settings, this model could potentially be adopted as a designing tool for real MICP applications. © 2022, The Author(s)
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
Implementation, Weiterentwicklung und Verifikation eines finiten Strukturelementes für elastisches und inelastisches Materialverhalten bei großen Deformationen.
Full text linkIm Rahmen dieser Bachelorarbeit wird ein dreidimensionales sogenanntes ”Makro-Element“ mit linearen Ansatzfunktionen entwickelt und verifiziert, das für die Lösung statischer Probleme für nichtlinear elastisches und inelastisches Materialverhalten geeignet ist, fokussierend auf Situationen, die unerwünschte Nebeneffekte wie Locking oder Hourglassing hervorrufen können. Das Makro-Element basiert auf dem in (I) entwickelte 3D Cosserat Brick Element, das sich in der Behebung von Locking und Hourglassing bewährt hat. Das Makro-Element wird in kleine Elemente, sogenannte Mikro- oder Subelemente unterteilt, in denen erstmal die Spannungen wie in einem rein homogenen 3D Cosserat Brick Element berechnet und dann auf das ursprüngliche Element abgebildet werden. Aus diesem Grund wurde das Element ”Makro-Element“ getauft. Ein schöner Nebeneffekt bei der Verwendung dieses Elementes ist der Verzicht auf die numerische Integration über das Gebiet aufgrund einer speziell entwickelten Kinematik. In dem ersten Teil dieser Arbeit wird die Theorie des Makro-Elementes für das nichtlineare, elastische Materialverhalten vorgestellt. Das implementierte Element wird einem Patch-Test und anderen Tests unterzogen, um es auf seine Richtigkeit zu überprüfen, und um sein Verhalten in heiklen Situationen im Vergleich zu anderen Elementen zu evaluieren. Im zweiten Teil wird die Theorie um das inelastische, insbesondere um das nichtlineare, plastische Materialverhalten erweitert. Es wird die Von-Mises-Plastizität verwendet mit Beschränkung auf isotrope Verfestigung. Das plastische Modell wird mit Hilfe des Radial-Return-Mapping-Algorithmus numerisch umgesetzt. Diese Erweiterung im Element wird implementiert und für einen speziellen Fall untersucht
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
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