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    A remeshing approach for the finite cell method applied to problems with large deformations
    The primary goal of the proposed research project is to further develop the finite cell method (FCM) - a high-order fictitious domain approach - for large deformation analysis including finite strain problems of elastoplasticity. Foamed materials will be used as a demonstrator application, as they feature complex geometries and a complex deformation behavior. Recently, we introduced a remeshing strategy to improve the robustness of the FCM for large deformations. Here, the deformed structure is remeshed when finite cells are distorted too severely. Since the FCM applies Cartesian grids, the remeshing is simple and can be carried out fully automatically. The method performs well for hyperelastic finite strain problems, and the structure under consideration can be deformed much further when applying the remeshing strategy. Nevertheless, there are still many open questions which we would like to address in this project. First of all, different remeshing criteria need to be developed and investigated in order to reliably determine when remeshing has to be initiated during the analysis. Another considerable challenge in elastoplastic analysis – because of the non-smooth data – is the interpolation of history data from the old to the new mesh. Therefore, error-controlled interpolation algorithms need to be developed. Since the computation of the element/cell matrices accounts for a significant part of the overall effort in high-order methods, special attention will be placed on this point. This is of great importance since the finite cell method requires a dense set of integration points to resolve the geometry of the broken cells. In addition, the nonlinear material models with an increased numerical effort at each integration point, combined with the repeated computation of the tangent stiffness matrix will increase the numerical effort. Thus, we aim at reducing the number of integration points. This can be achieved by applying the moment fitting where a quadrature rule is derived for every broken cell. To this end, we will extend and study the moment fitting for the case of finite strain elastoplasticity. In order to judge the robustness, accuracy, and efficiency of the overall remeshing approach, we will consider carefully selected benchmark problems and compare the proposed method with other finite element formulations.
      84
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    Extension of fictitious domain methods for vibroacoustic problems – Analysis of heterogeneous, foamed damping materials
    Even today, the prediction of the acoustical behaviour of components made of materials exhibiting a highly heterogeneous microstructure isa very demanding and challenging task. There are several reasons that make this problem so difficult to solve. On the one hand, the microstructure can only be accurately resolved if a large number of inite elements are deployed. On the other hand, all physically relevant interactions between the structural and fluid domain need to be taken into account. As mentioned before the body-fitted discretization of highly complex structures demands a large number of finite elements (degrees of freedom) and thus results in inacceptable computational times. Consequently, alternative methods, such as fictitious domain approaches, are required. Over the last years fictitious domain methods, such as the finite cell method (FCM), have proven their capabilities. To account for the vibroacoustic properties of microstructured materials these methods need to be extended. Therefore, the acoustic wave equation in the time domain or the Helmholtz equation in the frequency domain need to discretized by means of the FCM. Moreover, suitable coupling strategies that result in a weak or strong coupling need to be devised. Here, the main advantage of fictitious domain methods is the ability to take complex geometrical features into account while being able to straightforwardly superimpose cells with mechanical and fluidic properties. Thus, an efficient and robust strategy for vibroacoustic problems can be set up. Despite the application of nonconforming discretizations the numerical effort is still considerably large. Therefore, a second idea uses simplified numerical models based on homogenization techniques. To this end, the structure is assumed to consist of a homogeneous medium where the microstructure is neglected. In spite of this model simplification it is still expected to achieve reasonable results for specific applications. Besides the development of numerical methods a second focus is put on a comprehensive validation procedure. In this context, different experimental set-ups are deployed. To check the vibrational behavior of the structure under investigation a 3D laserscanning-vibrometer is used. In addition, the frequency-dependent acoustic parameters are measured by means of simple experimental set-ups such as an impedance tube and the results are compared to the numerically obtained values. As a last step the sound pressure radiation is tested in an anechoic room using microphone arrays and far field microphones. The acquired data are the foundation for illustrating the performance of the proposed methods.
      53
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    Numerical modelling of partially cemented soils in the stagnation zone
    The primary goal of the planned research project is to gain a better understanding of the micromechanical deformation behaviour of multiphase porous composite materials. For this purpose, a multi-phase composite material commonly used in civil engineering and especially in geotechnics in many problems will be investigated in detail: cement-bound sand. For example, self-hardening suspension can be used as a stabilizing fluid in the construction of diaphragm walls. Due to the manufacturing process, there are transition zones at the edges of the components between almost completely cement-filled grain structure and pure grain structure. In this area, known as stagnation zone, the load transfer depends on the position and contact points of the individual grains. In this case, the loads applied are carried by the cement matrix on the one hand and by the sand grains embedded in it on the other. The load transfer is strongly influenced by the heterogeneous arrangement and the sphericity of the individual grains, the degree of mixing and the cement content. In order to improve macroscopic FE-models relevant for practice, a bridge between the microscopic and macroscopic material behaviour of porous composite materials is to be established. Based on imaging techniques such as X-ray computed tomography three-dimensional FE models will be generated directly from the image data obtained by scanning cement-bound sand samples. Using numerical methods, deformation analyses can be carried out on the models obtained in this way. At the Institute of Geotechnical Engineering and Construction Management (Prof. Grabe) the Finite Element Method (FEM) is mainly used. In addition, the Finite Cell Method (FCM) is further developed at the Institute of Ship Structural Design and Analysis (Prof. Düster). These two numerical approaches are to be applied and optimized to microstructural problems in the course of the planned research project. The aim is to derive macroscopic material characteristics of the complex multi-phase microstructures by numerical homogenization. This allows the characterisation of the macroscopic material behaviour of such composite materials. The macroscopic material behaviour will be investigated by means of laboratory experiments. Uniaxial compression tests and triaxial tests on sand samples with different cement contents are planned. Due to the extensive planned investigations in the soil mechanics laboratory, at the CT scanner as well as the numerical investigations and developments, elastic material behaviour will be assumed in this first project phase. In a possible second project phase, non-linear material models, among others, are to be used for investigations of larger deformations or for modelling damages in the composite material.
      219
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    A two-scale simulation approach of the collision behavior of double hull vessels filled with particles
    To reduce the environmental, economical, and safety consequences of ship collisions, it is compulsory that hull structures of all oil tankers arrange for double sides and bottoms. It is clear that double hulls lead to an increase of crash-worthiness and mitigate the possibility of severe damage in ships body and oil spill. Recently, various studies have been conducted to improve the efficiency of double hulls with adding some granular materials into the space between the two walls. In the current research, the collision of double hull vessels filled with energy absorbing granular materials will be studied on both micro and macro scales. For this purpose, the compaction behavior of granulates will be studied at first using DEM. Then considering a representative volume element (RVE) and homogenization technique, the mechanical properties of the particles will be identified for the continuum model. At the next step, since the granular material must be modeled by different methods in different sub-domains (DEM and FEM), a bridging or coupling must be established in the domains interface using, for example, the Arlequin method. At the end, the collision problem in double hull vessels will be simulated and the effects of influencing parameters can be investigated. Project partners: Prof. M. Dosta (TUHH)
      113
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    GRK 2462: Teilprojekt C1 - Herstellung funktioneller Granulate und Agglomerate mit großen Energiedissipationsdichten
    In diesem Projekt sollen granulare Füllstoffe mittels Wirbelschichtsprühgranulation hergestellt werden. Die Füllstoffe sollen zur Schadensreduktion bei Kollisionen in Schiffsdoppelwände eingebracht werden, wodurch die auf die äußere Wand einwirkende Kraft nach innen übertragen wird und die Energiedissipation durch Bruch des Füllmaterials stattfindet. Das Füllmaterial muss einer Vielzahl von Anforderungen genügen und beispielsweise eine möglichst geringe Masse aufweisen. Zudem soll das Material nicht toxisch sein, sich sehr gut fördern und pumpen lassen und hydrophb sein. Die Herstellung der Granulate erfolgt mit der Wirbelschichtsprühgranulation. Das Grundprinzip besteht darin, dass das fluidisierte Partikelbett mit einem sehr gute Wärme-, Stoff- und Impulstransport einhergeht. Durch das Eindüsen einer Flüssigkeit (Supsension, Lösung, Schmelze) werden die Tropfen fein auf den Partikeln verteilt und ummanteln dadurch den innenliegenden Kern. Durch die Verwendung und Kombination geeigneter Materialien und die Optimierung der Betriebsparameter lassen sich so Granulate mit hoher Energiedissipationsdichte generieren.
      101
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    Modelling, simulation and experimental investigation of cables
    This project aims at a detailed experimental characterization and simulation of the multiaxial deformation behavior of cables undergoing finite deformations and contact. Cables show anisotropic and inelastic behavior under load as their structure consists of at least two layers of different kinds of materials. Even though their main purpose is to conduct electric current or other media, their mechanical properties gain importance since they determine the durability and lifetime of a cable. Moreover, situations where the cable has contact to or slides over another part occur often during assembly. Therefore, this proposal deals on one hand with basic experiments that allow for the investigation of inelastic and anisotropic behavior of cables under multiaxial deformations, e.g. combining bending and torsion. On the other hand, a new type of experiment will be realized where application-related contact problems, such as bending of a cable with contact and friction, can be investigated. During this project, numerical experiments using an anisotropic finite element formulation will be executed. Therefore, the cables will be discretized with finite elements of higher order which are suitable to represent the long and slender structure of a cable very well. The anisotropic deformation behavior of cables under load will be described with an anisotropic constitutive law valid for finite deformations and elastoplastic behavior. A contact formulation will be developed and implemented to simulate the application-related load cases of cables in contact with a contact partner. The main objective of this project is the ideal combination of experiments and numerical formulations in order to simulate the anisotropic and inelastic behavior of cables under load realistically.
      162