153 research outputs found
Divi Norberti archiepiscopi Magdebvrgensis, Præmonstratensium fundatoris, vita /
No full textDedicatie: Joannes Brazius. Drempeldichten: Jean Polit, Lambertus Moraeus, Claudius BourguignonDe Theux, XavierMachiels, J. Catalogus van de boeken gedrukt vóór 1600 ; M 104Europeana-GoogleBook
A novel, total-iterative approach to model quasi-brittle materials
No full textIn quasi-brittle materials, such as reinforced concrete, localisation of initially diffuse cracking evolving in localised cracking patterns consists of a numerically challenging task. With conventional iterative methods, convergence of the numerical solution scheme to model crack localisation is often difficult to obtain. On the other hand, conventional total approaches, such as the Sequentially Linear Approach, although robust, fail to approximate properly the underlying material law. In the present work, a new model is introduced, designated the Total Iterative Approach, in which the internal damage variables are updated iteratively. It is found that this approach is robust, allows for the correct approximation of the material law and is a powerful tool for the analysis of softening behaviour. Some examples are presented to illustrate the performance of the model.Green Open Access added to TU Delft Institutional Repository ''You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials- Mechanics- Management & Desig
Multiscale Computational Modeling of Brittle and Ductile Materials under Dynamic Loading
No full textThe computational homogenization method enables to derive the overall behavior of heterogeneous materials from their local-scale response. In this method, a representative volume element (RVE) is assigned to a macroscopic material point and the constitutive law for the macroscopic model at that point is obtained by solving a boundary value problem for the RVE. However, the standard computational homogenization scheme cannot be used when strain localization occurs and does not account for dynamic effects at the local-scale. Furthermore, in the computational homogenization scheme, at each iteration, a boundary value problem should be solved for RVEs associated to the integration points of macroscopic elements which leads to high computational cost. When the problem is nonlinear (material and/or geometrical nonlinearities), the computational cost may become more than used for direct numerical simulation (DNS).This study aims at developing computational and numerical homogenization schemes which account for strain localization, dynamic effects at the local-scale and large deformations and strains. Furthermore, strategies are presented to decrease the computational cost while preserving accuracy. Different heterogeneous structures consisting of quasi-brittle materials, hyperelastic materials and polymer materials are studied and proper homogenization schemes are presented
Discrete fiber models beyond classical applications: Rigid line inclusions, fiber-based batteries, challenges
No full textReinforced composites are used in many industrial and multi-functional applications. The efficiency of the reinforcements depends mainly on the aspect ratio, material properties, and the adhesion between matrix and reinforcement. Particularly, high aspect ratio fillers and inclusions have gained popularity due to their unique material and geometrical features, where a fundamental understanding of composites hierarchical structure and behavior is crucial for the optimal design and performance. There is however a lack of robust numerical modeling frameworks that are able to accurately represent composites with high aspect ratio reinforcements. Ideally the expensive mesh generation of the standard finite element method or the simplifying assumptions adopted by smeared type or mean-field approaches should be avoided.A group of numerical techniques here referred to as "embedded methods" eliminate mesh conformity restrictions and significantly reduce the computational cost of the standard finite element method, while still benefiting from the advantages of a direct numerical analysis. In formulating the embedded models, enrichment techniques and different element technologies are considered, and physical assumptions are investigated. Limitations of the classical embedded models are highlighted through numerical examples, on the basis of which possible enhancements are discussed. We specifically highlight the important roles of field gradients continuity/discontinuity and the element size, order, and regularity extensions on the smoothness of the solutions.A computationally efficient embedded model is then applied to the study of failure and inclusion orientation effects in planar composites. A detailed study is also performed for dense fiber-reinforced composites, where homogenized mechanical properties are extracted and various forms of neutrality of thin fibers are demonstrated. In this context, a part of this thesis is dedicated to one-to-one comparisons between results obtained using the standard finite element method and embedded techniques. This led to a range of model and geometry parameters under which predictions of embedded technique are reliable. Comparisons are reported in terms of homogenized properties and local field variables, namely relative displacement between inclusions and matrix (slips).Finally as a preliminary step towards multi-functional fiber-based structural batteries, an electro-chemical system characterized by composite cathode in a half cell configuration is considered. The main point of difference with common composite batteries is that active material particles are cast in form of high aspect ratio fibers, which are efficiently discretized by use of the embedded technique. A discrete definition of fibers, unlike the case of mean-field approaches, allows to define local fields and interfacial conditions between fibers and electrolyte and is crucial for the accurate modelling of a battery cell with fiber-based electrodes.Applied Mechanic
Computational modeling of failure in composites under fatigue loading conditions
No full textApplied Mechanic
Numerical Determination of Permeability in Unsaturated Cementitious Materials
No full textTo assess durability of cement-based materials, permeability is commonly considered as an important indicator. It is defined as the rate of movement of an agent (liquid or gas) through the porous medium under an applied pressure. Although permeability can be directly measured in laboratories, these experimental tests generally require specialized equipment and long periods of time to be completed, so they are laborious and expensive. For economic and ecological interests, numerical models are considered as an attractive alternative. Up until now, however, permeability of virtual cement seems to exceed experimental data by several orders of magnitude. Full saturation however, as generally assumed in numerical evaluations, does not realistically represent the experiments. Modelling fluid flow through unsaturated cement-based materials constitutes the focal point of this thesis. It is shown that the saturation degree has a significant effect on the permeability
Multiscale modeling of strain rate effects in FRP laminated composites
No full textFiber reinforced polymer composites are increasingly used in impactresistant devices, automotives, and aircraft structures due to their high strengthtoweight ratios and their potential for impact energy absorption. Dynamic impact loading causes complex deformation and failure phenomena in composite laminates. Moreover, the high loading rates in impact scenarios give rise to a significant change in mechanical properties (e.g. elastic modulus, strength, fracture energy) and failure characteristics (e.g. failure mechanisms, energy dissipation) of polymer composites. In other words, both mechanical deformation and failure are strainrate dependent. The contributing mechanisms can be roughly classified as viscous material behavior, changes in failure mechanism, inertia effects and thermome chanical effects. These effects involve multiple length and time scales. In experiments it is difficult to isolate single mechanisms contributing to the overall ratedependency. Therefore, it is difficult to quantify the contribution of each mechanism at different scales. The aim of this thesis is to establish a multiscale numerical framework in which three of the contributing mechanisms, i.e. the viscous material behavior, changes in fracture mechanisms and inertia effects, can be investigated at different scales. The research in this thesis is divided into four parts, one related to the macroscale, where the composite material is treated as homogeneous, and three on exploring possibilities to include microscale information, taking into account the microstructure of fibers and matrix.Applied Mechanic
The coastal system of the Volta delta, Ghana: Strategies and opportunities for development
No full textThe Volta delta is a very dynamic environment, forming the interface between the Volta river and the Atlantic ocean. The delta is a home for many communities, settled both at the shorelines and more inland. Furthermore the delta provides great natural values and a habitat for many species.The coastline of the Volta delta suffers from erosion, threatening these communities and natural habitats. Furthermore the delta largely consists of low-lying lands which are prone to flooding. The large scale geology and morphology of the delta are explored and the natural forcing of the coastal system is determined in terms of waves, currents and human impact. Lastly flood protection and mitigation strategies are explored for the area. It must be noted that in ultimo hard protection measures will not provide a definitive solution. Maintenance is key in any kind of coastal protection strategy. Hard structures are very costly to construct and maintain, although nourishments may not be suitable in every location. More dynamic solutions and retreating from the most hazardous locations would be a better and more sustainable way forward.This report is written for the Delft Deltas Infrastructure and Mobility Initiative (DIMI) Volta delta special case. In extension on the workshop in the Volta delta, early October 2017, a literature review is produced to bundle the existing knowledge of the coastal system of the Volta delta and to process this knowledge in practical handles for design.Rivers, Ports, Waterways and Dredging EngineeringCoastal Engineerin
Accelerating finite element analysis using machine learning
No full textWe study the acceleration of the finite element method (FEM) simulations using machine learning (ML) models. Specifically, we replace computationally expensive (parts of) FEM models with efficient ML surrogates. We develop three methods to speed up FEM simulations. The primary difference between these models is their degree of intrusion into the FEM source code. Here, we enumerate them from the most to the least intrusive. In the first contribution, we tackle two bottlenecks of a FEM model equipped with a viscoplastic constitutive equation namely, solving the linear system of equations and evaluating the force vector. To tackle the former, we use a proper orthogonal decomposition (POD) method. And we tackle the latter with a discrete empirical interpolation method (DEIM).We observe that DEIM does not effectively speed up such a highly nonlinear FEM model. As a remedy, we divide the time domain into subdomains using a clustering algorithm. Then we construct a set of DEIM points for each cluster. By doing so, we manage to increase the efficiency of the POD-DEIM scheme. We, however, observe that the POD-DEIM scheme is sometimes unstable. The source of this instability is, to the extent of our knowledge, an open question. In the second contribution, we consider a FE2 scheme. The micro model is a FEM model equipped with a viscoplastic constitutive equation. The evaluation of the micro model is the computational bottleneck in this framework. Therefore, we develop a recurrent neural network as a surrogate for the micro model. In this contribution, we also propose a simple but effective sampling technique to collect stress-strain data points. The RNN model is trained based on this data. We also discuss how the RNN model becomes inaccurate when extrapolating. For these scenarios, we discuss how to improve the RNN by collecting more data and retraining. In the third contribution, we develop a surrogate for the entire FEM simulation of a multi-physics problem. Specifically, we consider the FEM simulation of the electrochemical-mechanical interactions in a Li-ion battery. We propose a variant of the convolutional neural network (CNN), namely the HydraNet. The HydraNet takes the geometry of the battery and predicts all solution fields of the FEM model. Solution fields are either output of the solver or that of the post-processing. The HydraNet accepts inputs in the form of image-like fields. We discuss how to encode the geometry of the battery into a set of image-like fields. We argue that the degree of intrusion of these methods to the FEM source code is inversely related to their industrial applicability. As a result, we believe that the first method (POD-DEIM) will mostly remain an academic contribution, while the other two could have potential industrial applications. The central use case of these methods is in a multi-query application such as uncertainty quantification, design, and real-time simulations
A multi-scale approach towards reusable steel-concrete composite floor systems
No full textTraditionally welded headed studs have been used to generate composite interaction between a steel beam and a (cast in-situ) concrete floor. This permanent connection impairs the demountability of the structural components and therefore demolition of the composite floor system is inevitable at the end of the functional service life. The demolition of functionally obsolete but technically sound building components is in contradiction with the globally prevailing ambition of more sustainable development of the built environment through reduced demand for primary resources and reduced emissions of harmful substances. This dissertation aims to overcome the need for demolition of composite floor systems by developing methods, tools and recommendations to enable easy demountability of the structural components. The recommendations are both based on practical experience obtained by full-scale laboratory experiments on a demountable composite floor system consisting of large prefabricated concrete floor elements (2.6 × 7.2 m), and on the (analytical) methods and tools developed to predict the response of the floor system during execution (e.g. instability) and service life (e.g. deflection and stresses).Steel & Composite Structure
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