Efficient computational mesoscale modeling of concrete under cyclic loading

Tesi amb diferents seccions retallades per drets de l'editor.Concrete is a complex material and can be modeled on various spatial and temporal scales. While simulations on coarse scales are practical for engineering applications, a deeper understanding of the material is gained on finer scales. This is at the cost of an increased numerical effort that can be reduced by the three methods developed and used in this work, each corresponding to one publication. The coarse spatial scale is related to fully homogenized models. The material is described in a phenomenological approach and the numerous parameters sometimes lack a physical meaning. Resolving the three-phase mesoscopic structure consisting of aggregates, the mortar matrix and the interfaces between them allow to describe similar effects with simpler models. This work addresses two computational challenges related to mesoscale modeling. First, aggregate particles take up a high volume fraction and an efficient particle-packing algorithm is required to generate non-overlapping, random esostructures. Enforcing an additional distance between the aggregates is essential to obtain undistorted meshes for finite element simulations, but further complicates the packing problem. An event-driven molecular-dynamics algorithm is applied to this problem that, in contrast to traditional methods, allows movement and a dense arrangement of the aggregates. This allows creating concrete mesostructures with realistic aggregate volume fractions. The second challenge concerns stability problems in mesoscale simulations of concrete fracture. The geometric complexity and the combination of three material laws for each of the phases leads to numerical instabilities, even for regularized material models. This requires tiny time steps and numerous iterations per time step when integrated with a classic backward Euler scheme. The implicit–explicit (IMPL-EX) integration extrapolates internal variables that account for the nonlinear behavior. This linearizes the equations, provides additional robustness and a computational speedup. In combination with a novel time step control method, a three-dimensional mesoscale compression test is accelerated by a factor of 40, compared to an adaptive backward Euler algorithm. The life time of concrete under cyclic loads is commonly predicted with empirical Wöhler lines. They relate the number of endured cycles with the applied load amplitude and can be included in constitutive formulations. They can, however, hardly be generalized to geometries and load configurations other than the ones tested. On a finer temporal scale, fatigue failure is modeled by the accumulation of damage within each loading cycle. This resolves the whole process of failure, includes stress redistributions and size effects and can easily be extended to multiphysics phenomena. The third computational challenge solved here is the efficient temporal integration that would not be feasible in a naive cycle-by-cycle integration of thousands or millions of cycles. The cost of evaluating a single cycle is reduced by reformulating the problem in the frequency space. It is sufficient to equilibrate the structure once for each Fourier coefficient which significantly speeds up this evaluation. The accumulated damage of one cycle is integrated in time using an adaptive cycle jump concept. For a two dimensional void test structure, the combination of both techniques leads to a 25 times faster simulation compared to the full integration. These three main contributions decrease the numerical cost of mesoscale simulations, allow larger and more detailed models and are a basis to deepen the understanding of the complex failure patterns in concrete.El hormigón es un material complejo y puede ser modelado en varias escalas espaciales y temporales. Mientras que las simulaciones en escalas gruesas son prácticas para aplicaciones de ingeniería, se obtiene una comprensión más profunda del material en escalas más finas. Esto es a costa de un mayor esfuerzo numérico que puede ser reducido por los tres métodos desarrollados y utilizados en este trabajo, cada uno de los cuales corresponde a una publicación. La escala espacial gruesa está relacionada con modelos totalmente homogeneizados. El material se describe con un enfoque fenomenológico y los numerosos parámetros a veces carecen de significado físico. La resolución de la estructura mesoscópica trifásica formada por los áridos, la matriz de mortero y las interfaces entre ellos permite describir efectos similares con modelos más sencillos. Este trabajo aborda dos retos computacionales relacionados con el modelado a mesoescala. En primer lugar, las partículas agregadas absorben una fracción de gran volumen y se requiere un algoritmo eficiente de empaquetamiento de partículas para generar mesoestructuras aleatorias que no se solapen. Hacer cumplir una distancia adicional entre los agregados es esencial para obtener mallas no distorsionadas para simulaciones de elementos finitos, pero complica aún más el problema de empaquetado. A este problema se le aplica un algoritmo de dinámica molecular impulsado por eventos que, a diferencia de los métodos tradicionales, permite el movimiento y una disposición densa de los agregados. Esto permite crear mesoestructuras de hormigón con fracciones de volumen de agregado realistas. El segundo reto se refiere a los problemas de estabilidad en las simulaciones mesoescalares de fracturas de hormigón. La complejidad geométrica y la combinación de tres leyes materiales para cada una de las fases conduce a inestabilidades numéricas, incluso para modelos materiales regularizados. Esto requiere pequeños pasos de tiempo y numerosas iteraciones por paso de tiempo cuando se integra con un esquema clásico de Euler hacia atrás. La integración implícita- explícita (IMPL-EX) extrapola variables internas que dan cuenta del comportamiento no lineal. Esto linealiza las ecuaciones, proporciona robustez adicional y una aceleración computacional. En combinación con un nuevo método de control de paso en el tiempo, una prueba de compresión tridimensional de mesoescala es acelerada por un factor de 40, en comparación con un algoritmo adaptativo de Euler hacia atrás. La vida útil del hormigón bajo cargas cíclicas se predice comúnmente con las líneas empíricas de Wöhler. Relacionan el número de ciclos soportados con la amplitud de carga aplicada y pueden ser incluidos en formulaciones constitutivas. Sin embargo, difícilmente pueden generalizarse a geometrías y configuraciones de carga distintas a las probadas. En una escala temporal más fina, la falla por fatiga es modelada por la acumulación de daño dentro de cada ciclo de carga. Esto resuelve todo el proceso de fracaso, incluye redistribuciones de estrés y efectos de tamaño, y puede extenderse fácilmente a fenómenos multifísicos. El tercer reto computacional resuelto aquí es la integración temporal eficiente que no sería factible en una integración costosa de miles o millones de ciclos ciclo a ciclo. El costo de evaluar un solo ciclo se reduce reformulando el problema en el espacio de frecuencias. Es suficiente equilibrar la estructura una vez para cada coeficiente de Fourier, lo que acelera significativamente esta evaluación. El daño acumulado de un ciclo se integra en el tiempo utilizando un concepto de salto de ciclo adaptativo. Para una estructura de prueba de vacío bidimensional, la combinación de ambas técnicas conduce a una simulación 25 veces más rápida en comparación con la integración completa.Postprint (published version

Implicit–explicit integration of gradient-enhanced damage models

Quasi-brittle materials exhibit strain softening. Their modeling requires regularized constitutive formulations to avoid instabilities on the material level. A commonly used model is the implicit gradient-enhanced damage model. For complex geometries, it still shows structural instabilities when integrated with classical backward Euler schemes. An alternative is the implicit–explicit (IMPL-EX) integration scheme. It consists of the extrapolation of internal variables followed by an implicit calculation of the solution fields. The solution procedure for the nonlinear gradient-enhanced damage model is thus transformed into a sequence of problems that are algorithmically linear in every time step. Therefore, they require one single Newton–Raphson iteration per time step to converge. This provides both additional robustness and computational acceleration. The introduced extrapolation error is controlled by adaptive time-stepping schemes. This paper introduced and assessed two novel classes of error control schemes that provide further performance improvements. In a three-dimensional compression test for a mesoscale model of concrete, the presented scheme was about 40 times faster than an adaptive backward Euler time integration

Implicit–explicit integration of gradient-enhanced damage models

This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. This material may be found at https://ascelibrary.org/doi/10.1061/%28ASCE%29EM.1943-7889.0001608.Quasi-brittle materials exhibit strain softening. Their modeling requires regularized constitutive formulations to avoid instabilities on the material level. A commonly used model is the implicit gradient-enhanced damage model. For complex geometries, it still shows structural instabilities when integrated with classical backward Euler schemes. An alternative is the implicit–explicit (IMPL-EX) integration scheme. It consists of the extrapolation of internal variables followed by an implicit calculation of the solution fields. The solution procedure for the nonlinear gradient-enhanced damage model is thus transformed into a sequence of problems that are algorithmically linear in every time step. Therefore, they require one single Newton–Raphson iteration per time step to converge. This provides both additional robustness and computational acceleration. The introduced extrapolation error is controlled by adaptive time-stepping schemes. This paper introduced and assessed two novel classes of error control schemes that provide further performance improvements. In a three-dimensional compression test for a mesoscale model of concrete, the presented scheme was about 40 times faster than an adaptive backward Euler time integration.The research was supported by the Federal Institute for Materials Research and Testing, Berlin, Germany and by the German Research Foundation (DFG) under project Un224/7-1. Additionally, the research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 320815 (ERC Advanced Grant Project "Advanced tools for computational design of engineering materials" COMP-DES-MAT).Peer ReviewedPostprint (author's final draft

Pharmacological and non-pharmacological treatments for major depressive disorder: review of systematic reviews

This study aims to summarise the evidence on more than 140 pharmacological and non-pharmacological treatment options for major depressive disorder (MDD) and to evaluate the confidence that patients and clinicians can have in the underlying science about their effects

Simulation games as tools for integrative dynamic learning: The case of the management course at the University of Algarve

Today, in order to people or organizations survive in a changing environment it is essential to adapt. Learning provided to people is a key feature for an active response since it implies acquiring knowledge, skills and competencies to cope successfully with different circumstances. Literature has focused on how digital games support education because simulators represent dynamic models of real situations; so, their goal is to ensure that the player denotes his decisions consequences. When teaching certain skills through these games, a reflection stage is crucial to evaluate the experiences gathered during the simulation and promote knowledge appliance by participants into the real world. Due to its multiple scientific contributions, gaming can overlap a valid solution to prepare learners understanding regarding complex contexts. This research denotes an ongoing PhD research about the characteristics of a management course unit (at the University of Algarve, Portugal) that explores a business simulator- Cesim Global Challenge- for learning purposes, as well as the effectiveness of an integrative approach (new learning environments) on students’ engagement and dynamic learning outcomes. From the earlier empirical data is understandable game-based learning advantages and disadvantages within Management and Entrepreneurship courses

User testing of an adaptation of fishbone diagrams to depict results of systematic reviews

Abstract Background Summary of findings tables in systematic reviews are highly informative but require epidemiological training to be interpreted correctly. The usage of fishbone diagrams as graphical displays could offer researchers an effective approach to simplify content for readers with limited epidemiological training. In this paper we demonstrate how fishbone diagrams can be applied to systematic reviews and present the results of an initial user testing. Methods Findings from two systematic reviews were graphically depicted in the form of the fishbone diagram. To test the utility of fishbone diagrams compared with summary of findings tables, we developed and pilot-tested an online survey using Qualtrics. Respondents were randomized to the fishbone diagram or a summary of findings table presenting the same body of evidence. They answered questions in both open-ended and closed-answer formats; all responses were anonymous. Measures of interest focused on first and second impressions, the ability to find and interpret critical information, as well as user experience with both displays. We asked respondents about the perceived utility of fishbone diagrams compared to summary of findings tables. We analyzed quantitative data by conducting t-tests and comparing descriptive statistics. Results Based on real world systematic reviews, we provide two different fishbone diagrams to show how they might be used to display complex information in a clear and succinct manner. User testing on 77 students with basic epidemiological training revealed that participants preferred summary of findings tables over fishbone diagrams. Significantly more participants liked the summary of findings table than the fishbone diagram (71.8% vs. 44.8%; p < .01); significantly more participants found the fishbone diagram confusing (63.2% vs. 35.9%, p < .05) or indicated that it was difficult to find information (65.8% vs. 45%; p < .01). However, more than half of the participants in both groups were unable to find critical information and answer three respective questions correctly (52.6% in the fishbone group; 51.3% in the summary of findings group). Conclusions Fishbone diagrams are compact visualizations that, theoretically, may prove useful for summarizing the findings of systematic reviews. Initial user testing, however, did not support the utility of such graphical displays

Political Discourse in the Hospital Heterotopia

To what extent do we pay attention to the text and images that cover our hospital walls and do we offer any critique either as professionals or service users? In the past we might have expected to see functional or helpful instructions about where to go (or not to go) and in more well‐endowed buildings, perhaps we would see some works of art, sculpture, stained glass even, with the intention to encourage, distract or even forewarn us. However, it is now common in UK hospitals, for wall space to be used as a portal for a range of institutional political messages, that convey information about everything from its own values, behaviours to advertisements for products and services to requirements for rule following. Michel Foucault's ideas about Heterotopic space can help us to see that hospitals tend to fall (awkwardly) between being a public and personal health care space, and this is a possible explanation for the confused material culture and messages that are shared there. This paper draws on ethnographic methods to reflect on personal experience in order to offer a critique of the contemporary political discourse which has become 'literally' written onto our hospital walls

Literary ethnography of evidence-based healthcare : accessing the emotions of rational-technical discourse

In this article I revisit the idea of literary ethnography (proposed by Van de Poel-Knottnerus and Knottnerus, 1994) as a method for investigating social phenomena constituted principally through literature. I report the use of this method to investigate the topic of evidence-based healthcare, EBHC. EBHC is a field of discourse much built upon a dichotomy between rationality and emotionality. In this context literary ethnography, a particular type of discourse analysis, is valuable for allowing researchers to bring the emotional currents of technical-rational discourse into conscious awareness. In such discourses, emotions are not written out by name. The researcher must discern emotional phenomena by experiencing the discourse, and (try to) bring them into intelligible expression. As I clarify this process I develop Van de Poel-Knottnerus and Knottnerus’ method theoretically, look to destabilise the rationalityemotionality dichotomy foundational to discourse around EBHC, and so transgress its conventional lines of thought.PostprintPeer reviewe

'Getting people on board': Discursive leadership for consensus building in team meetings

Meetings are increasingly seen as sites where organizing and strategic change take place, but the role of specific discursive strategies and related linguistic-pragmatic and argumentative devices, employed by meeting chairs, is little understood. The purpose of this article is to address the range of behaviours of chairs in business organizations by comparing strategies employed by the same chief executive officer (CEO) in two key meeting genres: regular management team meetings and ‘away-days’. While drawing on research from organization studies on the role of leadership in meetings and studies of language in the workplace from (socio)linguistics and discourse studies, we abductively identified five salient discursive strategies which meeting chairs employ in driving decision making: (1) Bonding; (2) Encouraging; (3) Directing; (4) Modulating; and (5) Re/Committing. We investigate the leadership styles of the CEO in both meeting genres via a multi-level approach using empirical data drawn from meetings of a single management team in a multinational defence corporation. Our key findings are, first, that the chair of the meetings (and leading manager) influences the outcome of the meetings in both negative and positive ways, through the choice of discursive strategies. Second, it becomes apparent that the specific context and related meeting genre mediate participation and the ability of the chair to control interactions within the team. Third, a more hierarchical authoritarian or a more interpersonal egalitarian leadership style can be identified via specific combinations of these five discursive strategies. The article concludes that the egalitarian leadership style increases the likelihood of achieving a durable consensus. Several related avenues for research are outlined