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Wetting of a solid surface with liquids is an important parameter in the chemical engineering process such as distillation, absorption and desorption. The degree of wetting in packed columns mainly contributes in the generating of the effective interfacial area and then enhancing of the heat and mass transfer process. In this work the wetting of solid surfaces was studied in real experimental work and virtually through three dimensional CFD simulations using the multiphase flow VOF model implemented in the commercial software FLUENT. That can be used to simulate the stratified flows [1]. The liquid rivulet flow which is a special case of the film flow and mostly found in packed columns has been discussed. Wetting of a solid flat and wavy metal plate with rivulet liquid flow was simulated and experimentally validated. The local rivulet thickness was measured using an optically assisted mechanical sensor using a needle which is moved perpendicular to the plate surface with a step motor and in the other two directions using two micrometers. The measured and simulated rivulet profiles were compared to some selected theoretical models founded in the literature such as Duffy & Muffatt [2], Towell & Rothfeld [3] and Al-Khalil et al. [4]. The velocity field in a cross section of a rivulet flow and the non-dimensional maximum and mean velocity values for the vertical flat plate was also compared with models from Al-Khalil et al. [4] and Allen & Biggin [5]. Few CFD simulations for the wavy plate case were compared to the experimental findings, and the Towel model for a flat plate [3]. In the second stage of this work 3-D CFD simulations and experimental study has been performed for wetting of a structured packing element and packing sheet consisting of three elements from the type Rombopak 4M, which is a product of the company Kuhni, Switzerland. The hydrodynamics parameters of a packed column, e. i. the degree of wetting, the interfacial area and liquid hold-up have been depicted from the CFD simulations for different liquid systems and liquid loads. Flow patterns on the degree of wetting have been compared to that of the experiments, where the experimental values for the degree of wetting were estimated from the snap shooting of the flow on the packing sheet in a test rig. A new model to describe the hydrodynamics of packed columns equipped with Rombopak 4M was derived with help of the CFD–simulation results. The model predicts the degree of wetting, the specific or interfacial area and liquid hold-up at different flow conditions. This model was compared to Billet & Schultes [6], the SRP model Rocha et al. [7-9], to Shi & Mersmann [10] and others. Since the pressure drop is one of the most important parameter in packed columns especially for vacuum operating columns, few CFD simulations were performed to estimate the dry pressure drop in a structured and flat packing element and were compared to the experimental results. It was found a good agreement from one side, between the experimental and the CFD simulation results, and from the other side between the simulations and theoretical models for the rivulet flow on an inclined plate. The flow patterns and liquid spreading behaviour on the packing element agrees well with the experimental results. The VOF (Volume of Fluid) was found very sensitive to different liquid properties and can be used in optimization of the packing geometries and revealing critical details of wetting and film flow. An extension of this work to perform CFD simulations for the flow inside a block of the packing to get a detailed picture about the interaction between the liquid and packing surfaces is recommended as further perspective.

This research explores the development of web based reference software for
characterisation of surface roughness for two-dimensional surface data. The reference software used for verification of surface characteristics makes the evaluation methods easier for clients. The algorithms used in this software
are based on International ISO standards. Most software used in industrial measuring
instruments may give variations in the parameters calculated due to numerical changes in
calculation. Such variations can be verified using the proposed reference software.
The evaluation of surface roughness is carried out in four major steps: data capture, data
align, data filtering and parameter calculation. This work walks through each of these steps
explaining how surface profiles are evaluated by pre-processing steps called fitting and
filtering. The analysis process is then followed by parameter evaluation according to DIN EN
ISO 4287 and DIN EN ISO 13565-2 standards to extract important information from the
profile to characterise surface roughness.

Nanotechnology is now recognized as one of the most promising areas for technological
development in the 21st century. In materials research, the development of
polymer nanocomposites is rapidly emerging as a multidisciplinary research activity
whose results could widen the applications of polymers to the benefit of many different
industries. Nanocomposites are a new class of composites that are particle-filled
polymers for which at least one dimension of the dispersed particle is in the nanometer
range. In the related area polymer/clay nanocomposites have attracted considerable
interest because they often exhibit remarkable property improvements when
compared to virgin polymer or conventional micro- and macro- composites.
The present work addresses the toughening and reinforcement of thermoplastics via
a novel method which allows us to achieve micro- and nanocomposites. In this work
two matrices are used: amorphous polystyrene (PS) and semi-crystalline polyoxymethylene
(POM). Polyurethane (PU) was selected as the toughening agent for POM
and used in its latex form. It is noteworthy that the mean size of rubber latices is
closely matched with that of conventional toughening agents, impact modifiers.
Boehmite alumina and sodium fluorohectorite (FH) were used as reinforcements.
One of the criteria for selecting these fillers was that they are water swellable/
dispersible and thus their nanoscale dispersion can be achieved also in aqueous
polymer latex. A systematic study was performed on how to adapt discontinuousand
continuous manufacturing techniques for the related nanocomposites.
The dispersion of nanofillers was characterized by transmission, scanning electron
and atomic force microcopy (TEM, SEM and AFM respectively), X-ray diffraction
(XRD) techniques, and discussed. The crystallization of POM was studied by means
of differential scanning calorimetry and polarized light optical microscopy (DSC and
PLM, respectively). The mechanical and thermomechanical properties of the composites
were determined in uniaxial tensile, dynamic-mechanical thermal analysis
(DMTA), short-time creep tests, and thermogravimetric analysis (TGA).
PS composites were produced first by a discontinuous manufacturing technique,
whereby FH or alumina was incorporated in the PS matrix by melt blending with and
without latex precompounding of PS latex with the nanofiller. It was found that direct melt mixing (DM) of the nanofillers with PS resulted in micro-, whereas the latex mediated
pre-compounding (masterbatch technique, MB) in nanocomposites. FH was
not intercalated by PS when prepared by DM. On the other hand, FH was well dispersed
(mostly intercalated) in PS via the PS latex-mediated predispersion of FH following
the MB route. The nanocomposites produced by MB outperformed the DM
compounded microcomposites in respect to properties like stiffness, strength and
ductility based on dynamic-mechanical and static tensile tests. It was found that the
resistance to creep (summarized in master curves) of the nanocomposites were improved
compared to those of the microcomposites. Master curves (creep compliance
vs. time), constructed based on isothermal creep tests performed at different temperatures,
showed that the nanofiller reinforcement affects mostly the initial creep
compliance.
Next, ternary composites composed of POM, PU and boehmite alumina were produced
by melt blending with and without latex precompounding. Latex precompounding
served for the predispersion of the alumina particles. The related MB was produced
by mixing the PU latex with water dispersible boehmite alumina. The composites
produced by the MB technique outperformed the DM compounded composites in
respect to most of the thermal and mechanical characteristics.
Toughened and/or reinforced PS- and POM-based composites have been successfully
produced by a continuous extrusion technique, too. This technique resulted in
good dispersion of both nanofillers (boehmite) and impact modifier (PU). Compared
to the microcomposites obtained by conventional DM, the nanofiller dispersion became
finer and uniform when using the water-mediated predispersion. The resulting
structure markedly affected the mechanical properties (stiffness and creep resistance)
of the corresponding composites. The impact resistance of POM was highly
enhanced by the addition of PU rubber when manufactured by the continuous extrusion
manufacturing technique. This was traced to the dispersed PU particle size being
in the range required from conventional, impact modifiers.

In this thesis viscoelastic material models are established to investigate the nature of continuous calving processes at Antarctic ice shelves. Physics-based descriptions of calving require appropriate fracture criteria to separate icebergs from the remaining ice shelf. Hence, criteria of the stress, the strain, and the self-similarity criterion are considered within finite-element computations. Crucial parameters in the models to determine the position of calving are the accurate knowledge of the geometry, especially the freeboard height, while the material parameters mainly influence the time span between two successive calving events. The extension to nonlinear material models is necessary to properly analyze the internal forces also for large deformations that occur for longer times of the viscous ice flow.

The main goal of this work was the study of the applicability of a polymer film heat exchanger concept for the applications in the chemical industry, such as the condensation of organic solvents. The polymer film heat exchanger investigated is a plate heat exchanger with very thin (0.025 – 0.1 mm) plates or films, which separate the fluids and enable the heat transfer. After a successful application of this concept to seawater desalination in a previous work, a further step is in chemical engineering, where the good chemical resistance of polymers in aggressive fluids is the challenge.
Two approaches were performed in this work. The first one was experimental and included the study of the chemical and mechanical resistance of preselected films, made of polymer materials, such as polyimide (PI), polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE). To simulate realistic operating conditions in a heat exchanger the films were exposed to a combined thermal (up to 90°C) and mechanical pressure loads (4-6 bar) with permanent contact with the relevant organic solvents, such as toluene, hexane, heptane and tetrahydrofuran (THF). Furthermore, a lab-scale apparatus and a full-scale demonstrator were manufactured in cooperation with two industrial partners. These were used for the investigation of the heat transfer performance for operating modes with and without phase change.
In addition to the experimental work, a coupled finite element –computational fluid dynamics (FEM-CFD)-model was developed, based on the fluid-structure-interaction (FSI). Two major tasks had to be solved here. The first one was the modelling of the condensation process, based on available mathematical models and energy balances. The second one was the consideration of the partially reversible deformation of the used film during operation. Since this deformation changes the geometry of the fluid channels also has an influence on the overall performance of the apparatus, a coupled FEM-CFD model was developed.
During the experimental study of the chemical resistance of the films, the PTFE film showed the best performance, and hence can be used for all four tested solvents. For the polyimide film, failures while exposed to THF were observed, and the PET film can only be used with water and hexane. With the used lab-scale heat exchanger and the full-scale demonstrator competitive overall heat transfer coefficients between 270 W/m²K and 700 W/m²K could be reached for the liquid-liquid (water-water, water-hexane) operation mode without phase change. For the condensation process, overall heat transfer coefficients of up to 1700/m²K could be obtained.
The numerical approach led to a well-functioning coupled model in a very small scale (1 cm²). An upscale, however, failed due to enormous hardware resources necessary required for the simulation of the entire full-scale demonstrator. The main reason for this is the very low thickness of the films, which leads to tiny mesh element sizes (<0.05 mm) necessary to model the deformation of the film. The modelling of the liquid-liquid heat transfer provided an acceptable accuracy (approx. 10%), but at very low rates the deviations were then higher (over 30%). The results of the condensation modelling were ambivalent. One the one hand a physically plausible model was developed, which could map the entire condensation process. On the other hand, the corresponding energy balance revealed major inaccuracy and hence could not be used for the determination of the overall heat transfer and showed the current limits of the FEM-CFD approach.

This work presents a framework for the computation of complex geometries containing intersections of multiple patches with Reissner-Mindlin shell elements. The main objective is to provide an isogeometric finite element implementation which neither requires drilling rotation stabilization, nor user interaction to quantify the number of rotational degrees of freedom for every node. For this purpose, the following set of methods is presented. Control points with corresponding physical location are assigned to one common node for the finite element solution. A nodal basis system in every control point is defined, which ensures an exact interpolation of the director vector throughout the whole domain. A distinction criterion for the automatic quantification of rotational degrees of freedom for every node is presented. An isogeometric Reissner-Mindlin shell formulation is enhanced to handle geometries with kinks and allowing for arbitrary intersections of patches. The parametrization of adjacent patches along the interface has to be conforming. The shell formulation is derived from the continuum theory and uses a rotational update scheme for the current director vector. The nonlinear kinematic allows the computation of large deformations and large rotations. Two concepts for the description of rotations are presented. The first one uses an interpolation which is commonly used in standard Lagrange-based shell element formulations. The second scheme uses a more elaborate concept proposed by the authors in prior work, which increases the accuracy for arbitrary curved geometries. Numerical examples show the high accuracy and robustness of both concepts. The applicability of the proposed framework is demonstrated.

Hydrogels are known to be covalently or ionic cross-linked, hydrophilic three-dimensional
polymer networks, which exist in our bodies in a biological gel form such as the vitreous
humour that fills the interior of the eyes. Poly(N-isopropylacrylamide) (poly(NIPAAm))
hydrogels are attracting more interest in biomedical applications because, besides others, they
exhibit a well-defined lower critical solution temperature (LCST) in water, around 31–34°C,
which is close to the body temperature. This is considered to be of great interest in drug
delivery, cell encapsulation, and tissue engineering applications. In this work, the
poly(NIPAAm) hydrogel is synthesized by free radical polymerization. Hydrogel properties
and the dimensional changes accompanied with the volume phase transition of the
thermosensitive poly(NIPAAm) hydrogel were investigated in terms of Raman spectra,
swelling ratio, and hydration. The thermal swelling/deswelling changes that occur at different
equilibrium temperatures and different solutions (phenol, ethanol, propanol, and sodium
chloride) based on Raman spectrum were investigated. In addition, Raman spectroscopy has
been employed to evaluate the diffusion aspects of bovine serum albumin (BSA) and phenol
through the poly(NIPAAm) network. The determination of the mutual diffusion coefficient,
\(D_{mut}\) for hydrogels/solvent system was achieved successfully using Raman spectroscopy at
different solute concentrations. Moreover, the mechanical properties of the hydrogel, which
were investigated by uniaxial compression tests, were used to characterize the hydrogel and to
determine the collective diffusion coefficient through the hydrogel. The solute release coupled
with shrinking of the hydrogel particles was modelled with a bi-dimensional diffusion model
with moving boundary conditions. The influence of the variable diffusion coefficient is
observed and leads to a better description of the kinetic curve in the case of important
deformation around the LCST. A good accordance between experimental and calculated data
was obtained.

With the burgeoning computing power available, multiscale modelling and simulation has these days become increasingly capable of capturing the details of physical processes on different scales. The mechanical behavior of solids is oftentimes the result of interaction between multiple spatial and temporal scales at different levels and hence it is a typical phenomena of interest exhibiting multiscale characteristic. At the most basic level, properties of solids can be attributed to atomic interactions and crystal structure that can be described on nano scale. Mechanical properties at the macro scale are modeled using continuum mechanics for which we mention stresses and strains. Continuum models, however they offer an efficient way of studying material properties they are not accurate enough and lack microstructural information behind the microscopic mechanics that cause the material to behave in a way it does. Atomistic models are concerned with phenomenon at the level of lattice thereby allowing investigation of detailed crystalline and defect structures, and yet the length scales of interest are inevitably far beyond the reach of full atomistic computation and is rohibitively expensive. This makes it necessary the need for multiscale models. The bottom line and a possible avenue to this end is, coupling different length scales, the continuum and the atomistics in accordance with standard procedures. This is done by recourse to the Cauchy-Born rule and in so doing, we aim at a model that is efficient and reasonably accurate in mimicking physical behaviors observed in nature or laboratory. In this work, we focus on concurrent coupling based on energetic formulations that links the continuum to atomistics. At the atomic scale, we describe deformation of the solid by the displaced positions of atoms that make up the solid and at the continuum level deformation of the solid is described by the displacement field that minimize the total energy. In the coupled model, continuum-atomistic, a continuum formulation is retained as the overall framework of the problem and the atomistic feature is introduced by way of constitutive description, with the Cauchy-Born rule establishing the point of contact. The entire formulation is made in the framework of nonlinear elasticity and all the simulations are carried out within the confines of quasistatic settings. The model gives direct account to measurable features of microstructures developed by crystals through sequential lamination.

In the present work the modelling and numerical treatment of discontinuities in thermo-mechanical solids is investigated and applied to diverse physical problems. From this topic a structure for this work results, which considers the formulation of thermo-mechanical processes in continua in the first part and which forms the mechanical and thermodynamical framework for the description of discontinuities and interfaces, that is performed in the second part. The representation of the modelling of solid materials bases on the detailed derivation of geometrically nonlinear kinematics, that yields different strain and stress measures for the material and spatial configuration. Accordingly, this results in different formulations of the mechanical and thermodynamical balance equations. On these foundations we firstly derive by means of the concepts of the plasticity theory an elasto-plastic prototype-model, that is extended subsequently. In the centre of interest is the formulation of damage models in consideration of rate-dependent material behaviour. In the next step follows the extension of the isothermal material models to thermo-mechanically coupled problems, whereby also the special case of adiabatic processes is discussed. Within the representation of the different constitutive laws, the importance is attached to their modular structure. Moreover, a detailed discussion of the isothermal and the thermo-mechanically coupled problem with respect to their numerical treatment is performed. For this purpose the weak forms with respect to the different configurations and the corresponding linearizations are derived and discretized. The derived material models are highlighted by numerical examples and also proved with respect to plausibility. In order to take discontinuities into account appropriate kinematics are introduced and the mechanical and thermodynamical balance equations have to be modified correspondingly. The numerical description is accomplished by so-called interface-elements, which are based on an adequate discretization. In this context two application fields are distinguished. On the one side the interface elements provide a tool for the description of postcritical processes in the framework of localization problems, which include material separation and therefore they are appropriate for the description of cutting processes. Here in turn one has to make the difference between the domain-dependent and the domain-independent formulation, which mainly differ in the definition of the interfacial strain measure. On the other side material properties are attached to the interfaces whereas the spatial extension is neglectable. A typical application of this type of discontinuities can be found in the scope of the modelling of composites, for instance. In both applications the corresponding thermo-mechanical formulations are derived. Finally, the different interface formulations are highlighted by some numerical examples and they are also proved with respect to plausibility.

Within this thesis we present a novel approach towards the modeling of strong discontinuities in a three dimensional finite element framework for large deformations. This novel finite element framework is modularly constructed containing three essential parts: (i) the bulk problem, ii) the cohesive interface problem and iii) the crack tracking problem. Within this modular design, chapter 2 (Continuous solid mechanics) treats the behavior of the bulk problem (i). It includes the overall description of the continuous kinematics, the required balance equations, the constitutive setting and the finite element formulation with its corresponding discretization and required solution strategy for the emerging highly non-linear equations. Subsequently, we discuss the modeling of strong discontinuities within finite element discretization schemes in chapter 3 (Discontinuous solid mechanics). Starting with an extension of the continuous kinematics to the discontinuous situation, we discuss the phantom-node discretization scheme based on the works of Hansbo & Hansbo. Thereby, in addition to a comparison with the extended finite element method (XFEM), importance is attached to the technical details for the adaptive introduction of the required discontinuous elements: The splitting of finite elements, the numerical integration, the visualization and the formulation of geometrical correct crack tip elements. In chapter 4 (The cohesive crack concept), we consider the treatment of cohesive process zones and the associated treatment of cohesive tractions. By applying this approach we are able to merge all irreversible, crack propagation accompanying, failure mechanisms into an arbitrary traction separation relation. Additionally, this concept ensures bounded crack tip stresses and allows the use of stress-based failure criteria for the determination of crack growth. In summary, the use of the discontinuous elements in conjunction with cohesive traction separation allows the mesh-independent computation of crack propagation along pre-defined crack paths. Therefore, this combination is defined as the interface problem (ii) and represents the next building block in the modular design of this thesis. The description and the computation of the evolving crack surface, based on the actual status of a considered specimen is the key issue of chapter 5 (Crack path tracking strategies). In contrast to the two-dimensional case, where tracking the path in a C0-continuous way is straightforward, three-dimensional crack path tracking requires additional strategies. We discuss the currently available approaches regarding this issue and further compare the approaches by means of usual quality measures. In the modular design of this thesis these algorithms represent the last main part which is classified as the crack tracking problem (iii). Finally chapter 6 (Representative numerical examples) verifies the finite element tool by comparisons of the computational results which experiments and benchmarks of engineering fracture problems in concrete. Afterwards the finite element tool is applied to model folding induced fracture of geological structures.

A general framework for the thermodynamics of open systems is developed in the spatial and the material setting. Special emphasis is placed on the balance of mass which is enhanced by additional source and flux terms. Different solution strategies within the finite element technique are derived and compared. A number of numerical examples illustrates the features of the proposed approach.

Thermoelasticity represents the fusion of the fields of heat conduction and elasticity in solids and is usually characterized by a twofold coupling. Thermally induced stresses can be determined as well as temperature changes caused by deformations. Studying the mutual influence is subject of thermoelasticity. Usually, heat conduction in solids is based on Fourier’s law which describes a diffusive process. It predicts unnatural infinite transmission speed for parts of local heat pulses. At room temperature, for example, these parts are strongly damped. Thus, in these cases most engineering applications are described satisfactorily by the classical theory. However, in some situations the predictions according to Fourier’s law fail miserable. One of these situations occurs at temperatures near absolute zero, where the phenomenon of second sound1 was discovered in the 20th century. Consequently, non-classical theories experienced great research interest during the recent decades. Throughout this thesis, the expression “non-classical” refers to the fact that the constitutive equation of the heat flux is not based on Fourier’s law. Fourier’s classical theory hypothesizes that the heat flux is proportional to the temperature gradient. A new thermoelastic theory, on the one hand, needs to be consistent with classical thermoelastodynamics and, on the other hand, needs to describe second sound accurately. Hence, during the second half of the last century the traditional parabolic heat equation was replaced by a hyperbolic one. Its coupling with elasticity leads to non-classical thermomechanics which allows the modeling of second sound, provides a passage to the classical theory and additionally overcomes the paradox of infinite wave speed. Although much effort is put into non-classical theories, the thermoelastodynamic community has not yet agreed on one approach and a systematic research is going on worldwide.Computational methods play an important role for solving thermoelastic problems in engineering sciences. Usually this is due to the complex structure of the equations at hand. This thesis aims at establishing a basic theory and numerical treatment of non-classical thermoelasticity (rather than dealing with special cases). The finite element method is already widely accepted in the field of structural solid mechanics and enjoys a growing significance in thermal analyses. This approach resorts to a finite element method in space as well as in time.

The goal of this thesis is a physically motivated and thermodynamically consistent formulation of higher gradient inelastic material behavior. Thereby, the influence of the material microstructure is incorporated. Next to theoretical aspects, the thesis is complemented with the algorithmic treatment and numerical implementation of the derived model. Hereby, two major inelastic effects will be addressed: on the one hand elasto-plastic processes and on the other hand damage mechanisms, which will both be modeled within a continuum mechanics framework.

In this contribution a mortar-type method for the coupling of non-conforming NURBS surface patches is proposed. The connection of non-conforming patches with shared degrees of freedom requires mutual refinement, which propagates throughout the whole patch due to the tensor-product structure of NURBS surfaces. Thus, methods to handle non-conforming meshes are essential in NURBS-based isogeometric analysis. The main objective of this work is to provide a simple and efficient way to couple the individual patches of complex geometrical models without altering the variational formulation. The deformations of the interface control points of adjacent patches are interrelated with a master-slave relation. This relation is established numerically using the weak form of the equality of mutual deformations along the interface. With the help of this relation the interface degrees of freedom of the slave patch can be condensated out of the system. A natural connection of the patches is attained without additional terms in the weak form. The proposed method is also applicable for nonlinear computations without further measures. Linear and geometrical nonlinear examples show the high accuracy and robustness of the new method. A comparison to reference results and to computations with the Lagrange multiplier method is given.

The fact that long fibre reinforced thermoplastic composites (LFT) have higher tensile
strength, modulus and even toughness, compared to short fibre reinforced
thermoplastics with the same fibre loading has been well documented in literature.
These are the underlying factors that have made LFT materials one of the most
rapidly growing sectors of plastics industry. New developments in manufacturing of
LFT composites have led to improvements in mechanical properties and price
reduction, which has made these materials an attractive choice as a replacement for
metals in automobile parts and other similar applications. However, there are still
several open scientific questions concerning the material selection leading to the
optimal property combinations. The present work is an attempt to clarify some of
these questions. The target was to develop tools that can be used to modify, or to
“tailor”, the properties of LFT composite materials, according to the requirements of
automobile and other applications.
The present study consisted of three separate case studies, focusing on the current
scientific issues on LFT material systems. The first part of this work was focused on
LGF reinforced thermoplastic styrenic resins. The target was to find suitable maleic
acid anhydride (MAH) based coupling agents in order to improve the fibre-matrix
interfacial strength, and, in this way, to develop an LGF concentrate suitable for
thermoplastic styrenic resins. It was shown that the mechanical properties of LGF
reinforced “styrenics” were considerably improved when a small amount of MAH
functionalised polymer was added to the matrix. This could be explained by the better fibre-matrix adhesion, revealed by scanning electron microscopy of fracture surfaces.
A novel LGF concentrate concept showed that one particular base material can be
used to produce parts with different mechanical and thermal properties by diluting the
fibre content with different types of thermoplastic styrenic resins. Therefore, this
concept allows a flexible production of parts, and it can be used in the manufacturing
of interior parts for automobile components.The second material system dealt with so called hybrid composites, consisting of
long glass fibre reinforced polypropylene (LGF-PP) and mineral fillers like calcium
carbonate and talcum. The aim was to get more information about the fracture
behaviour of such hybrid composites under tensile and impact loading, and to
observe the influence of the fillers on properties. It was found that, in general, the
addition of fillers in LGF-PP, increased stiffness but the strength and fracture
toughness were decreased. However, calcium carbonate and talcum fillers resulted
in different mechanical properties, when added to LGF-PP: better mechanical
properties were achieved by using talcum, compared to calcium carbonate. This
phenomenon could be explained by the different nucleation effect of these fillers,
which resulted in a different crystalline morphology of polypropylene, and by the
particle orientation during the processing when talc was used. Furthermore, the
acoustic emission study revealed that the fracture mode of LGF-PP changed when
calcium carbonate was added. The characteristic acoustic signals revealed that the
addition of filler led to the fibre debonding at an earlier stage of fracture sequence
when compared to unfilled LGF-PP.
In the third material system, the target was to develop a novel long glass fibre
reinforced composite material based on the blend of polyamide with thermoset
resins. In this study a blend of polyamide-66 (PA66) and phenol formaldehyde resin
(PFR) was used. The chemical structure of the PA66-PFR resin was analysed by
using small molecular weight analogues corresponding to PA66 and PFR
components, as well as by carrying out experiments using the macromolecular
system. Theoretical calculations and experiments showed that there exists a strong
hydrogen bonding between the carboxylic groups of PA66 and the hydroxylic groups
of PFR, exceeding even the strength of amide-water hydrogen bonds. This was
shown to lead to the miscible blends, when PFR was not crosslinked. It was also
found that the morphology of such thermoplastic-thermoset blends can be controlled
by altering ratio of blend components (PA66, PFR and crosslinking agent). In the
next phase, PA66-PFR blends were reinforced by long glass fibres. The studies
showed that the water absorption of the blend samples was considerably decreased,
which was also reflected in higher mechanical properties at equilibrium state.
Wie man aus zahlreichen Untersuchungen und Anwendungsbeispielen entnehmen
kann, besitzen langfaserverstärkte Thermoplaste (LFT) eine bessere Zugfestigkeit,
Biege- und Schlagzähigkeit im Vergleich zu kurzfaserverstärkten Thermoplasten. Die
Vorteile in den mechanischen Eigenschaften haben die LFT zu einem
schnellwachsenden Bereich in der Kunststoffindustrie gemacht. Neue Entwicklungen
in Bereich der Herstellung von LFT haben für zusätzliche Verbesserungen der
mechanischen Eigenschaften sowie eine Preisreduzierung der Materialien in den
vergangenen Jahren gesorgt, was die LFT zu einer attraktiven Wahl u.a. als Ersatz
von Metallen in Automobilteilen macht. Es stellen sich allerdings immer noch einige
offene wissenschaftliche Fragen in Bezug auf z.B. die Materialbeschaffenheit, um
optimale Eigenschaftskombinationen zu erreichen. Die vorliegende Arbeit versucht,
einige dieser Fragen zu beantworten. Ziel war es, Vorgehensweisen zu entwickeln,
mit denen man die Eigenschaften von LFT gezielt beeinflussen und so den
Anforderungen von Automobilen oder anderen Anwendungen anpassen oder
„maßschneidern“ kann.
Die vorliegende Arbeit besteht aus drei Teilen, welche sich auf unterschiedliche
Materialsysteme, angepasst an den aktuellen Bedarf und das Interesse der Industrie,
konzentrieren.
Der erste Teil der Arbeit richtet sich auf die Eigenschaftsoptimierung von
langglasfaserverstärkten (LGF) thermoplastischen Styrolcopolymeren und von
Blends aus diesen Materialien. Es wurden passende, auf Maleinsäureanhydride
(MAH) basierende Kopplungsmittel gefunden, um die Faser-Matrix-Haftung zu
optimieren. Weiterhin wurde ein LGF Konzentrat entwickelt, welches mit
verschiedenen thermoplastischen Styrolcopolymeren kompatibel ist und somit als
„Verstärkungsadditiv“ eingesetzt werden kann.Das Konzept für ein neues LGF-Konzentrat auf Basis des kompatiblen
Materialsystems konzentriert sich insbesondere darauf, dass ein Basismaterial für
die Herstellung von Bauteilen bereit gestellt werden kann, mit dessen Hilfe gezielt
verschiedene mechanische und thermomechanischen Eigenschaften durch das
Zumischen von verschiedenen Styrolcopoylmeren und Blends verbessert werden
können. Dieses Konzept ermöglicht eine sehr flexible Produktion von Bauteilen und
wird seine Anwendung bei der Herstellung von Bauteilen u.a. im Interieur von Autos
finden.
Das zweite Materialsystem basiert auf sogenannten hybriden Verbundwerkstoffen,
welche aus Langglasfasern und mineralischen Füllstoffen wie Kalziumkarbonat und
Talkum in einer Polypropylen (PP) - Matrix zusammengesetzt sind. Ziel war es, durch
detaillierte bruchmechanische Analysen genaue Informationen über das
Bruchverhalten dieser hybriden Verbundwerkstoffe bei Zug- und Schlagbelastung zu
bekommen, um dann die Unterschiede zwischen den verschiedenen Füllstoffen in
Bezug auf ihre Eigenschaften zu dokumentieren. Es konnte beobachtet werden, dass
bei Zugabe der Füllstoffe zum LGF-PP normalerweise die Steifigkeit weiter
verbessert wurde, jedoch die Festigkeit und Schlagzähigkeit abnahmen. Weiterhin
zeigten die verschiedenen Füllstoffe wie Kalziumkarbonat und Talkum
unterschiedliche mechanische Eigenschaften auf, wenn sie zusammen mit LGF
Verstärkung eingesetzt wurden: Bei der Zugabe von Talkum wurde u.a. eine deutlich
bessere Schlagzähigkeit als bei der Zugabe von Kalziumkarbonat festgestellt. Dieses
Phänomen konnte durch das unterschiedliche Nukleierungsverhalten des PPs erklärt
werden, welches in einer unterschiedlichen Kristallmorphologie von Polypropylen
resultierte. Weiterhin konnte man durch Messungen der akustischen Emmissionen
während der Zugbelastung eines bruchmechanischen Versuchskörpers aufzeigen,
dass die höhere Bruchzähigkeit von LGF-PP ohne Füllstoffe daraus resultiert, dass
Faser-Pullout schon bei geringeren Kräften vorhanden war.

In this study, the dependence of the cyclic deformation behavior on the surface morphology of metastable austenitic HSD® 600 TWinning Induced Plasticity (TWIP) steel was investigated. This steel—with the alloying concept Mn-Al-Si—shows a fully austenitic microstructure with deformation-induced twinning at ambient temperature. Four different surface morphologies were analyzed: as-received with a so-called rolling skin, after up milling, after down milling, and a reference morphology achieved by polishing. The morphologies were characterized by X-Ray Diffraction (XRD), Focused Ion Beam (FIB), Scanning Electron Microscopy (SEM) as well as confocal microscopy methods and show significant differences in initial residual stresses, phase fractions, topographies and microstructures. For specimens with all variants of the morphologies, fatigue tests were performed in the Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) regime to characterize the cyclic deformation behavior and fatigue life. Moreover, this study focused on the frequency-dependent self-heating of the specimens caused by cyclic plasticity in the HCF regime. The results show that both surface morphology and specimen temperature have a significant influence on the cyclic deformation behavior of HSD® 600 TWIP steel in the HCF regime.

The scientific and industrial interest devoted to polymer/layered silicate
nanocomposites due to their outstanding properties and novel applications resulted
in numerous studies in the last decade. They cover mostly thermoplastic- and
thermoset-based systems. Recently, studies in rubber/layered silicate
nanocomposites were started, as well. It was presented how complex maybe the
nanocomposite formation for the related systems. Therefore the rules governing their
structure-property relationships have to be clarified. In this Thesis, the related
aspects were addressed.
For the investigations several ethylene propylene diene rubbers (EPDM) of polar and
non-polar origin were selected, as well as, the more polar hydrogenated acrylonitrile
butadiene rubber (HNBR). The polarity was found to be beneficial on the
nanocomposite formation as it assisted to the intercalation of the polymer chains
within the clay galleries. This favored the development of exfoliated structures.
Finding an appropriate processing procedure, i.e. compounding in a kneader instead
of on an open mill, the mechanical performance of the nanocomposites was
significantly improved. The complexity of the nanocomposite formation in
rubber/organoclay system was demonstrated. The deintercalation of the organoclay
observed, was traced to the vulcanization system used. It was evidenced by an
indirect way that during sulfur curing, the primary amine clay intercalant leaves the
silicate surface and migrates in the rubber matrix. This was explained by its
participation in the sulfur-rich Zn-complexes created. Thus, by using quaternary
amine clay intercalants (as it was presented for EPDM or HNBR compounds) the
deintercalation was eliminated. The organoclay intercalation/deintercalation detected
for the primary amine clay intercalants, were controlled by means of peroxide curing
(as it was presented for HNBR compounds), where the vulcanization mechanism
differs from that of the sulfur curing.
The current analysis showed that by selecting the appropriate organoclay type the
properties of the nanocomposites can be tailored. This occurs via generating different
nanostructures (i.e. exfoliated, intercalated or deintercalated). In all cases, the
rubber/organoclay nanocomposites exhibited better performance than vulcanizates
with traditional fillers, like silica or unmodified (pristine) layered silicates.The mechanical and gas permeation behavior of the respective nanocomposites
were modelled. It was shown that models (e.g. Guth’s or Nielsen’s equations)
developed for “traditional” vulcanizates can be used when specific aspects are taken
into consideration. These involve characteristics related to the platy structure of the
silicates, i.e. their aspect ratio after compounding (appearance of platelet stacks), or
their orientation in the rubber matrix (order parameter).

Im Zuge der steigenden Anzahl von Einsatzmöglichkeiten der
Faserverbundwerkstoffe in den verschiedensten Industriebereichen spielt die
Entwicklung bzw. Weiterentwicklung neuer und effektiverer Verarbeitungstechniken
eine bedeutende Rolle.
Dabei findet derzeit das Harzinjektionsverfahren (LCM) ausschließlich für kleinere bis
mittlere Stückzahlen seinen Einsatz. Aufgrund der sehr großen Stückzahlen im
Automobilbereich, ist dieses Verfahren hier zurzeit weniger interessant. Daher
werden große Anstrengungen unternommen, das Harzinjektionsverfahren besonders
für solche Bauteile attraktiver zu machen, die gegenwärtig mit Hilfe des Prepreg-
Verfahrens hergestellt werden. Dabei spielt die Reduktion der hier vergleichsweise
hohen Zykluszeit eine tragende Rolle. Die Dauer eines Zyklus wird hierbei
hauptsächlich durch die Vorbereitung und Herstellung der Verstärkungsstruktur
(Preform) sowie durch die Bestückung des Werkzeuges bestimmt. Diese so
genannte Preform-Technik weist daher ein sehr großes Entwicklungspotential auf,
mit dem Ziel, solche Verstärkungsstrukturen herzustellen, die nach der Injektion
keine Nacharbeit erfordern. Solche Strukturen werden auch als „net shape, ready-toimpregnate“-
Preform bezeichnet. Die hierfür notwendigen Techniken stammen
vornehmend aus der Textilindustrie, wie z.B. die direkte Preformtechnik, das Nähen
oder Kleben (Binder-Technik).
Ziel der vorliegenden Dissertation ist es, die Möglichkeiten der Nähtechnik bezogen
auf die Herstellung der Preforms zu untersuchen. Hierfür werden die verschiedenen
Naht- und Verbindungsarten hinsichtlich ihres Einsatzes in der Preformtechnik, wie
die Fixier- und Positionier-, die Füge- oder Verbindungsnaht und die Montagenaht,
untersucht.
Im Rahmen dieser Arbeit wurde zunächst innerhalb einer Studie zur „net shape“-
Preformtechnik eine Versteifungsstruktur entwickelt und hergestellt. Diese Struktur
soll dabei der Veranschaulichung der Möglichkeiten und Einsatzbereiche der
Nähtechnik bei der Preformtechnologie dienen. Zudem kann so ein mehrstufiger
Preformherstellungsprozess demonstriert werden. Ferner zeigt diese Studie, dass
ein hochgradiger, automatisierter Prozess, welcher zudem eine durchgängige
Qualitätskontrolle ermöglicht, realisiert werden konnte. Als ein weiterer Schritt wurde ein Prozess zur Herstellung eine dreidimensionalen
Preform, der die Anwendung verschiedener thermoplastischer,
niedrigtemperaturschmelzender Nähgarne zulässt, ausgearbeitet. Hierbei wurden die
Vorteile der Näh- und der Binder-Technologie miteinander verbunden. Außerdem
konnte durch die bereits formstabile und imprägnierungsfertige Preformstruktur, die
Bestückung des Werkzeuges wesentlich vereinfacht werden. Um die mechanischen
Eigenschaften der Preforms bestimmen zu können, wurden quantitative
Messmethoden erarbeitet. Hierdurch konnten anschließend die Einflüsse der
Orientierung sowie der Stichdichte ermittelt werden. Zudem wurden die folgenden
drei grundlegenden Eigenschaften untersucht: die spezifische Biegesteifigkeit, der so
genannte Rückspringwinkel sowie die Rückstellkraft nach dem Thermoformen
hinsichtlich der verschiedenen Nähtypen.
Um dies zu ergänzen, wurden weiterführende Untersuchungen zu den
Materialeigenschaften der Nähfäden, die bei der dreidimensionalen Preformtechnik
eingesetzt werden können, durchgeführt. Dabei ist neben der niedrigen
Schmelztemperatur die vollständige Auflösbarkeit der Nähgarne in den ungesättigten
Polyester- und Epoxidharzen besonders wichtig. Auf Grund dieser vollständigen
Auflösung der Fäden in der Matrix können die Stichlöcher wieder vollkommen
verschlossen werden. Dadurch kann eine Reduktion des Einflusses solcher
Stichlöcher auf die mechanischen Eigenschaften des Faserverbundwerkstoffes
erreicht werden. Mit Hilfe dieser Untersuchungen wurden schließlich zwei polymere
Nähgarne als vielversprechend beurteilt. Diese weisen eine Schmelztemperatur von
weniger als 100 °C sowie eine gute Lösbarkeit, besonders im Harzsystem RTM 6,
auf.
In der Preformtechnik werden die Nähte nicht nur als Positionier- oder Montagenaht
eingesetzt, sondern können in einer Struktur als auch als Verstärkungselement, eine
so genannte Verstärkungsnaht, verwendet werden. Der Zweck einer solchen Naht ist
die interlaminare Verstärkung von monolitischen oder Sandwichstrukturen. Zudem
besteht die Möglichkeit, diese zur Fixierung von metallischen Funktionselementen
(Inserts) in den Faserverbundwerkstoff zu benutzen. Hinsichtlich diese Möglichkeiten
wurden im Rahmen dieser Arbeit erfolgreich Untersuchung durchgeführt. Dabei
wiesen die eingenähten Krafteinleitungselemente in durchgeführten statischen
Zugversuchen eine annähernd 200 % höhere maximale Zugkraft verglichen mit
entsprechenden Elementen (BigHead®), die nicht durch eine Naht fixiert wurden. Weitere Untersuchungen zeigten auch, dass eine doppelte Naht nicht eine
proportionale Verdoppelung der maximal erreichbaren Zugkraft bewirkt. Der Grund
hierfür liegt an einer partiellen Zerstörung des vorhandenen Nähgarns der ersten
Naht begründet durch den doppelten Einstich in die bereits bestehenden Löcher
beim mehrmaligen Durchlaufen der Nadel. Der größte Verstärkungseffekt konnte
schließlich bei der interlaminaren Einbettung und der Vernähung des Insert erreicht
werden. In diesem Fall kann eine Delamination, wie sie bei lediglich interlaminar
eingebetteten Inserts auftritt, verhindert werden.
Zusätzlich wurden statische Scherversuche durchgeführt, um auch in diesem
Belastungsfall die Versagensart zu untersuchen. Dabei stellte sich heraus, dass nicht
die Nähte sondern der Insert versagte. Auf Grund des Materialbruchs des Inserts,
sowohl in Zug- als auch in Scherversuchen, wurde in einem weiteren Schritt ein
optimiertes Insert entwickelt. Bei diesem wurde der Sockel in soweit modifiziert, dass
die maximale Versagenslast des Nähgarns ermittelt werden konnte. Dabei stellte
sich heraus, dass Glas-, Kohlenstoff- und Aramidfasern sich nur bedingt als
Verstärkungsgarn zur Fixierung von Inserts eignen. Im Gegensatz dazu sind die
Polyestergarne als ausreichende Verstärkung gut geeignet. Weitere Vorteile des
Polyestergarns sind die niedrigeren Kosten sowie die gute Vernähbarkeit.
Anschließend wurde eine solche Verbindung des Inserts mit einem
Faserverbundwerkstoff mit Hilfe der Finite-Elemente-Methode (FEM) simuliert. Dabei
zeigte sich eine gute Übereinstimmung der simulierten Ergebnisse mit denen aus
dem statischen Zugversuch mit dem weiterentwickelten Insert.
Auf Grund der elektrischen Leitfähigkeit von Kohlenstofffasern, können Fäden aus
diesem Material auch als Sensoren zur Überwachung einer Struktur oder Verbindung
eingesetzt werden. Hierfür wurden ebenfalls Untersuchungen durchgeführt. Dabei
konnte mit Hilfe der Änderung des elektrischen Widerstandes auf Schädigungen der
Fasern geschlossen werden. Somit können nicht nur das Bestehen einer
Schädigung, sondern auch der annähernde Ort ermittelt werden. Die
Untersuchungen zeigten somit, dass die Kohlenstofffasern nicht lediglich als
Verstärkung sondern auch als Überwachungssensor bei einem eingebetteten Insert
dienen können.
Im Rahmen aller Untersuchungen konnte das große und vielversprechende Potential
der Nähtechnik bei der Herstellung von Preform-Bauteilen aufgezeigt sowie ein
Einblick in einige von vielen Anwendungsmöglichkeiten gegeben werden.

The noise issue in manufacturing system is widely discussed from legal and health aspects. Regarding the existing laws and guidelines, various investigation methods are implemented in industry. The sound pressure level can be measured and reduced by using established approaches in reality. However, a straightforward and low cost approach to study noise issue using existing digital factory models is not found.
This thesis attempts to develop a novel concept for sound pressure level investigation in a virtual environment. With this, the factory planners are able to investigate the noise issue during factory design and layout planning phase.
Two computer aided tools are used in this approach: acoustic simulation and virtual reality (VR). The former enables the planner to simulate the sound pressure level by given factory layout and facility sound features. And the latter provides a visualization environment to view and explore the simulation results. The combination of these two powerful tools provides the planners a new possibility to analyze the noise in a factory.
To validate the simulations, the acoustic measurements are implemented in a real factory. Sound pressure level and sound intensity are determined respectively. Furthermore, a software tool is implemented using the introduced concept and approach. With this software, the simulation results are represented in a Cave Automatic Virtual Environment (CAVE).
This thesis describes the development of the approach, the measurement of sound features, the design of visualization framework, and the implementation of VR software. Based on this know-how, the industry users are able to design their own method and software for noise investigation and analysis.

The polydispersive nature of the turbulent droplet swarm in agitated liquid-liquid contacting equipment makes its mathematical modelling and the solution methodologies a rather sophisticated process. This polydispersion could be modelled as a population of droplets randomly distributed with respect to some internal properties at a specific location in space using the population balance equation as a mathematical tool. However, the analytical solution of such a mathematical model is hardly to obtain except for particular idealized cases, and hence numerical solutions are resorted to in general. This is due to the inherent nonlinearities in the convective and diffusive terms as well as the appearance of many integrals in the source term. In this work two conservative discretization methodologies for both internal (droplet state) and external (spatial) coordinates are extended and efficiently implemented to solve the population balance equation (PBE) describing the hydrodynamics of liquid-liquid contacting equipment. The internal coordinate conservative discretization techniques of Kumar and Ramkrishna (1996a, b) originally developed for the solution of PBE in simple batch systems are extended to continuous flow systems and validated against analytical solutions as well as published experimental droplet interaction functions and hydrodynamic data. In addition to these methodologies, we presented a conservative discretization approach for droplet breakage in batch and continuous flow systems, where it is found to have identical convergence characteristics when compared to the method of Kumar and Ramkrishna (1996a). Apart from the specific discretization schemes, the numerical solution of droplet population balance equations by discretization is known to suffer from inherent finite domain errors (FDE). Two approaches that minimize the total FDE during the solution of the discrete PBEs using an approximate optimal moving (for batch) and fixed (for continuous systems) grids are introduced (Attarakih, Bart & Faqir, 2003a). As a result, significant improvements are achieved in predicting the number densities, zero and first moments of the population. For spatially distributed populations (such as extraction columns) the resulting system of partial differential equations is spatially discretized in conservative form using a simplified first order upwind scheme as well as first and second order nonoscillatory central differencing schemes (Kurganov & Tadmor, 2000). This spatial discretization avoids the characteristic decomposition of the convective flux based on the approximate Riemann Solvers and the operator splitting technique required by classical upwind schemes (Karlsen et al., 2001). The time variable is discretized using an implicit strongly stable approach that is formulated by careful lagging of the nonlinear parts of the convective and source terms. The present algorithms are tested against analytical solutions of the simplified PBE through many case studies. In all these case studies the discrete models converges successfully to the available analytical solutions and to solutions on relatively fine grids when the analytical solution is not available. This is accomplished by deriving five analytical solutions of the PBE in continuous stirred tank and liquid-liquid extraction column for especial cases of breakage and coalescence functions. As an especial case, these algorithms are implemented via a windows computer code called LLECMOD (Liquid-Liquid Extraction Column Module) to simulate the hydrodynamics of general liquid-liquid extraction columns (LLEC). The user input dialog makes the LLECMOD a user-friendly program that enables the user to select grids, column dimensions, flow rates, velocity models, simulation parameters, dispersed and continuous phases chemical components, and droplet phase space-time solvers. The graphical output within the windows environment adds to the program a distinctive feature and makes it very easy to examine and interpret the results very quickly. Moreover, the dynamic model of the dispersed phase is carefully treated to correctly predict the oscillatory behavior of the LLEC hold up. In this context, a continuous velocity model corresponding to the manipulation of the inlet continuous flow rate through the control of the dispersed phase level is derived to get rid of this behavior.

In the present work, the phase transitions in different Fe/FeC systems were studied by using the molecular dynamics simulation and the Meyer-Entel interaction potential (also the Johnson potential for Fe-C interaction). Fe-bicrystal, thin film, Fe-C bulk and Fe-C nanowire systems were investigated to study the behaviour of the phase transition, where the energetics, dynamics and transformations pathways were analysed.

Solid particle erosion is usually undesirable, as it leads to development of cracks and
holes, material removal and other degradation mechanisms that as final
consequence reduce the durability of the structure imposed to erosion. The main aim
of this study was to characterise the erosion behaviour of polymers and polymer
composites, to understand the nature and the mechanisms of the material removal
and to suggest modifications and protective strategies for the effective reduction of
the material removal due to erosion.
In polymers, the effects of morphology, mechanical-, thermomechanical, and fracture
mechanical- properties were discussed. It was established that there is no general
rule for high resistance to erosive wear. Because of the different erosive wear
mechanisms that can take place, wear resistance can be achieved by more than one
type of materials. Difficulties with materials optimisation for wear reduction arise from
the fact that a material can show different behaviour depending on the impact angle
and the experimental conditions. Effects of polymer modification through mixing or
blending with elastomers and inclusion of nanoparticles were also discussed.
Toughness modification of epoxy resin with hygrothermally decomposed polyesterurethane
can be favourable for the erosion resistance. This type of modification
changes also the crosslinking characteristics of the modified EP and it was
established the crosslink density along with fracture energy are decisive parameters
for the erosion response. Melt blending of thermoplastic polymers with functionalised
rubbers on the other hand, can also have a positive influence whereas inclusion of
nanoparticles deteriorate the erosion resistance at low oblique impact angles (30°).
The effects of fibre length, orientation, fibre/matrix adhesion, stacking sequence,
number, position and existence of interleaves were studied in polymer composites.
Linear and inverse rules of mixture were applied in order to predict the erosion rate of
a composite system as a function of the erosion rate of its constituents and their
relative content. Best results were generally delivered with the inverse rule of mixture
approach.
A semi-empirical model, proposed to describe the property degradation and damage
growth characteristics and to predict residual properties after single impact, was
applied for the case of solid particle erosion. Theoretical predictions and experimental
results were in very good agreement.
Strahlerosionsverschleiß (Erosion) entsteht beim Auftreffen von festen Partikel
auf Oberflächen und zeichnet sich üblicherweise durch einen Materialabtrag aus, der
neben der Partikelgeschwindigkeit und dem Auftreffwinkel stark vom jeweiligen
Werkstoff abhängt. In den letzten Jahren ist die Anwendung von Polymeren und
Verbundwerkstoffen anstelle der traditionellen Materialien stark angestiegen.
Polymere und Polymer-Verbundwerkstoffe weisen eine relativ hohe Erosionsrate
(ER) auf, was die potenzielle Anwendung dieser Werkstoffe unter erosiven
Umgebungsbedingungen erheblich einschränkt.
Untersuchungen des Erosionsverhaltens anhand ausgewählter Polymere und
Polymer-Verbundwerkstoffe haben gezeigt, dass diese Systeme unterschiedlichen
Verschleißmechnismen folgen, die sehr komplex sind und nicht nur von einer
Werkstoffeigenschaft beeinflusst werden. Anhand der ER kann das
Erosionsverhalten grob in zwei Kategorien eingeteilt werden: sprödes und duktiles
Erosionsverhalten. Das spröde Erosionsverhalten zeigt eine maximale ER bei 90°,
während das Maximum bei dem duktilen Verhalten bei 30° liegt. Ob ein Material das
eine oder das andere Erosionsverhalten aufweist, ist nicht nur von seinen
Eigenschaften, sondern auch von den jeweiligen Prüfparametern abhängig.
Das Ziel dieser Forschungsarbeit war, das grundsätzliche Verhalten von
Polymeren und Verbundwerkstoffen unter dem Einfluss von Erosion zu
charakterisieren, die verschiedenen Verschleißmechanismen zu erkennen und die
maßgeblichen Materialeigenschaften und Kennwerte zu erfassen, um Anwendungen
dieser Werkstoffe unter Erosionsbedingungen zu ermöglichen bzw. zu verbessern.
An einer exemplarischen Auswahl von Polymeren, Elastomeren, modifizierten Polymeren und Faserverbundwerkstoffen wurden die wesentlichen Einflussfaktoren
für die Erosion experimentell bestimmt.
Thermoplastische Polymere und thermoplastische- und vernetzte- Elastomere
Die Versuche, den Erosionswiderstand ausgewählter Polymere (Polyethylene
und Polyurethane) mit verschiedenen Materialeigenschaften zu korrelieren, haben
gezeigt, dass es weder eine klare Abhängigkeit von einzelnen Kenngrößen noch von
Eigenschaftskombinationen gibt. Möglicherweise führt die Bestimmung der
Materialeigenschaften unter den gleichen experimentellen Bedingungen wie bei den Erosionsversuchen zu einer besseren Korrelation zwischen ER und
Materialkenngröße.
Modifiziertes Epoxidharz
Am Beispiel eines modifizierten Epoxidharzes (EP) mit verschiedener
Vernetzungsdichte wurde eine Korrelation zwischen Erosionswiderstand und
Bruchenergie bzw. Erosionswiderstand und Vernetzungsdichte gefunden. Die
Modifizierung erfolgte mit verschiedenen Anteilen von einem hygrothermisch
abgebauten Polyurethan (HD-PUR). Der Zusammenhang zwischen ER und
Vernetzungsparametern steht im Einklang mit der Theorie der Kautschukelastizität.
Modifizierungseffizienz in Duromeren, Thermoplasten und Elastomeren
Des weiteren wurde der Einfluss von Modifizierungen von Polymeren und
Elastomeren untersucht. Mit dem obenerwähnten System (d.h. EP/HD-PUR) läßt sich
auch der Einfluss der Zähigkeitsmodifizierung des Epoxidharzes (EP) auf das
Erosionsverhalten untersuchen. Es wurde gezeigt, dass für HD-PUR Anteile von
mehr als 20 Gew.% diese Modifizierung einen positiven Einfluss auf die
Erosionsbeständigkeit hat. Durch Variation der HD-PUR-Anteile können für dieses
EP Materialeigenschaften, die zwischen den Eigenschaften eines üblichen
Duroplasten und eines weniger elastischen Gummis liegen, erzeugt werden.
Deswegen stellt der modifizierte EP-Harz ein sehr gutes Modellmaterial dar, um den
Einfluss der experimentellen Bedingungen zu studieren, und zu untersuchen, ob
verschiedene Erodenten zu gleichen Erosionsmechanismen führen. Der Übergang
vom duroplastischen zum zähen Verhalten wurde anhand von vier Erodenten
untersucht. Aus den Versuchen ergab sich, dass ein solcher Übergang auftritt, wenn
sehr feine, kantige Partikel (Korund) als Erodenten dienen. Die Partikelgröße und -form ist von entscheidender Bedeutung für die jeweiligen Verschleißmechanismen.
Die Effizienz neuartiger thermoplastischer Elastomere mit einer cokontinuierlichen
Phasenstruktur, bestehend aus thermoplastischem Polyester und
Gummi (funktionalisierter NBR und EPDM Kautschuk), wurde in Bezug auf die
Erosionsbeständigkeit untersucht. Große Anteile von funktionalisiertem Gummi (mehr
als 20 Gew.%) sind vorteilhaft für den Erosionswiderstand. Weiterhin wurde
untersucht, ob sich die herausragende Erosionsbeständigkeit von Polyurethan (PUR)
durch Zugabe von Nanosilikaten eventuell noch steigern läßt. Das Ergebnis war,
dass die Nanopartikel sich vor allem bei einem kleinen Verschleißwinkel (30°) negativ
auswirken. Die schwache Adhäsion zwischen Matrix und Partikeln erleichtert den
Beginn und das Wachsen von Rissen. Dies führt zu einem schnelleren
Materialabtrag von der Materialoberfläche.
Faserverbundwerkstoffe
Ferner wurden Faserverbundwerkstoffe (FVW) mit thermoplastischer und
duromerer Matrix auf ihr Verhalten bei Erosivverschleiß untersucht. Es war von
großem Interesse, den Einfluss von Faserlänge und -orientierung zu untersuchen.
Kurzfaserverstärkte Systeme haben einen besseren Erosionswiderstand als die
unidirektionalen (UD) Systeme. Die Rolle der Faserorientierung kann man nur in
Verbindung mit anderen Parametern, wie Matrixzähigkeit, Faseranteil oder Faser-
Matrix Haftung, berücksichtigen. Am Beispiel von GF/PP Verbunden weisen die
parallel zur Verstreckungsrichtung gestrahlten Systeme den geringsten Widerstand
auf. Andererseits findet bei einem GF/EP System die maximale ER in senkrechter
Richtung statt. Eine Verbesserung der Grenzflächenscherfestigkeit beeinflusst die
Erosionsverschleißrate nachhaltig. Wenn die Haftung der Grenzfläche ausreichend
ist, spielt die Erosionsrichtung eine unbedeutende Rolle für die ER. Weiterhin wurde
gezeigt, dass die Präsenz von zähen Zwischenschichten zu einer deutlichen
Verbesserung des Erosionswiderstands von CF/EP- Verbunden führt.
Eine weitere Aufgabenstellung war es, die Rolle des Faservolumenanteils zu
bestimmen. „Lineare, inverse und modifizierte Mischungsregeln“ wurden
angewendet, und es wurde festgestellt, dass die inversen Mischungsregeln besser
die ER in Abhängigkeit des Faservolumenanteils beschreiben können.
Im Anwendungsbereich von Faserverbundwerkstoffen ist nicht nur die Kenntnis
der ER, sondern auch die Kenntnis der Resteigenschaften erforderlich. Ein
halbempirisches Modell für die Vorhersage des Schlagenergieschwellwertes (Uo) für den Beginn der Festigkeitsabnahme und der Restzugfestigkeit nach einer
Schlagbelastung wurde bei der Untersuchung des Erosionsverschleißes
angewendet. Experimentelle Ergebnisse und theoretische Vorhersagen stimmten
nicht nur für duromere CF/EP-Verbundwerkstoffe, sondern auch für
Verbundwerkstoffe mit einer thermoplastischen Matrix (GF/PP) sehr gut überein.

In recent years the consumption of polymer based composites in many engineering
fields where friction and wear are critical issues has increased enormously. Satisfying
the growing industrial needs can be successful only if the costly, labor-intensive and
time-consuming cycle of manufacturing, followed by testing, and additionally followed
by further trial-and-error compounding is reduced or even avoided. Therefore, the
objective is to get in advance as much fundamental understanding as possible of the
interaction between various composite components and that of the composite against
its counterface. Sliding wear of polymers and polymer composites involves very
complex and highly nonlinear processes. Consequently, to develop analytical models
for the simulation of the sliding wear behavior of these materials is extremely difficult
or even impossible. It necessitates simplifying hypotheses and thus compromising
accuracy. An alternative way, discussed in this work, is an artificial neural network
based modeling. The principal benefit of artificial neural networks (ANNs) is their ability
to learn patterns through a training experience from experimentally generated data
using self-organizing capabilities.
Initially, the potential of using ANNs for the prediction of friction and wear properties
of polymers and polymer composites was explored using already published friction
and wear data of 101 independent fretting wear tests of polyamide 46 (PA 46) composites.
For comparison, ANNs were also applied to model the mechanical properties
of polymer composites using a commercial data bank of 93 pairs of independent Izod
impact, tension and bending tests of polyamide 66 (PA 66) composites. Different
stages in the development of ANN models such as selection of optimum network
configuration, multi-dimensional modeling, training and testing of the network were
addressed at length. The results of neural network predictions appeared viable and
very promising for their application in the field of tribology.
A case example was subsequently presented to model the sliding friction and wear
properties of polymer composites by using newly measured datasets of polyphenylene
sulfide (PPS) matrix composites. The composites were prepared by twinscrew
extrusion and injection molding. The dataset investigated was generated from
pin-on-disc testing in dry sliding conditions under various contact pressures and sliding speeds. Initially the focus was placed on exploring the possible synergistic effects
between traditional reinforcements and particulate fillers, with special emphasis on
sub-micro TiO2 particles (300 nm average diameter) and short carbon fibers (SCFs).
Subsequently, the lubricating contributions of graphite (Gr) and polytetrafluoroethylene
(PTFE) in these multiphase materials were also studied. ANNs were trained
using a conjugate gradient with Powell/Beale restarts (CGB) algorithm as well as a
variable learning rate backpropagation (GDX) algorithm in order to learn compositionproperty
relationships between the inputs and outputs of the system. Likewise, the
influence of the operating parameters (contact pressure (p) and sliding speed (v))
was also examined. The incorporation of short carbon fibers and sub-micro TiO2
particles resulted in both a lower friction and a great improvement in the wear resistance
of the PPS composites within the low and medium pv-range. The mechanical
characterization and surface analysis after wear testing revealed that this beneficial
tribological performance could be explained by the following phenomena: (i)
enhanced mechanical properties through the inclusion of short carbon fibers, (ii)
favorable protection of the short carbon fibers by the sub-micro particles diminishing
fiber breakage and removal, (iii) self-repairing effects with the sub-micro particles, (iv)
formation of quasi-spherical transfer particles free to roll at the tribological contact.
Still, in the high pv-range stick-slip sliding motion was observed with these hybrid
materials. The adverse stick-slip behavior could be effectively eliminated through the
additional inclusion of solid lubricant reservoirs (Gr and PTFE), analogous to the
lubricants used in real ball bearings. Likewise, solid lubricants improved the wear resistance
of the multiphase system PPS/SCF/TiO2 in the high pv-range (≥ 9 MPa·m/s).
Yet, their positive effect, especially that of graphite, was limited up to certain volume
fraction and loading conditions. The optimum results were obtained by blending
comparatively low amounts of Gr and PTFE (≈ 5 vol.% from each additive). An introduction
of softer sub-micro particles did not bring the desired ball bearing effect and
fiber protection. The ANN prediction profiles for PPS tribo-compounds exhibited very
good or even perfect agreement with the measured results demonstrating that the
target of achieving a well trained network was reached. The results of employing a
validation test dataset indicated that the trained neural network acquired enough
generalization capability to extend what it has learned about the training patterns to
data that it has not seen before from the same knowledge domain. Optimal brain surgeon (OBS) algorithm was employed to perform pruning of the network
topology by eliminating non-useful weights and bias in order to determine if the
performance of the pruned network was better than the fully-connected network.
Pruning resulted in accuracy gains over the fully-connected network, but induced
higher computational cost in coding the data in the required format. Within an importance
analysis, the sensitivity of the network response variable (frictional coefficient
or specific wear rate) to characteristic mechanical and thermo-mechanical input variables
was examined. The goal was to study the relationships between the diverse
input variables and the characteristic tribological parameters for a better understanding
of the sliding wear process with these materials. Finally, it was demonstrated that
the well-trained networks might be applied for visualization what will happen if a certain
filler is introduced into a composite, or what the impacts of the testing conditions
on the frictional coefficient and specific wear rate are. In this way, they might be a
helpful tool for design engineers and materials experts to explore materials and to
make reasoned selection and substitution decisions early in the design phase, when
they incur least cost.

Epoxy resins have achieved acceptance as adhesives, coatings, and potting compounds,
but their main application is as matrix to produce reinforced composites.
However, their usefulness in this field still limited due to their brittle nature. Some
studies have been done to increase the toughness of epoxy composites, of which the
most successful one is the modification of the polymer matrix with a second toughening
phase.
Resin Transfer Molding (RTM) is one of the most important technologies to manufacture
fiber reinforced composites. In the last decade it has experimented new impulse,
due to its favorable application to produce large surface composites with good technical
properties and at relative low cost.
This research work focuses on the development of novel modified epoxy matrices,
with enhanced mechanical and thermal properties, suitable to be processed by resin
transfer molding technology, to manufacture Glass Fiber Reinforced Composites
(GFRC’s) with improved performance in comparison to the commercially available
ones.
In the first stage of the project, a neat epoxy resin (EP) was modified using two different
nano-sized ceramics: silicium dioxide (SiO2) and zirconium dioxide (ZrO2); and
micro-sized particles of silicone rubber (SR) as second filler. Series of nanocomposites
and hybrid modified epoxy resins were obtained by systematic variation of filler
contents. The rheology and curing process of the modified epoxy resins were determined
in order to define their aptness to be processed by RTM. The resulting matrices
were extensively characterized qualitatively and quantitatively to precise the effect
of each filler on the polymer properties.
It was shown that the nanoparticles confer better mechanical properties to the epoxy
resin, including modulus and toughness. It was possible to improve simultaneously
the tensile modulus and toughness of the epoxy matrix in more than 30 % and 50 %
respectively, only by using 8 vol.-% nano-SiO2 as filler. A similar performance was
obtained by nanocomposites containing zirconia. The epoxy matrix modified with 8 vol.-% ZrO2 recorded tensile modulus and toughness improved up to 36% and 45%
respectively regarding EP.
On the other hand, the addition of silicone rubber to EP and nanocomposites results
in a superior toughness but has a slightly negative effect on modulus and strength.
The addition of 3 vol.-% SR to the neat epoxy and nanocomposites increases their
toughness between 1.5 and 2.5 fold; but implies also a reduction in their tensile modulus
and strength in range 5-10%. Therefore, when the right proportion of nanoceramic
and rubber were added to the epoxy resin, hybrid epoxy matrices with fracture
toughness 3 fold higher than EP but also with up to 20% improved modulus were
obtained.
Widespread investigations were carried out to define the structural mechanisms responsible
for these improvements. It was stated, that each type of filler induces specific
energy dissipating mechanisms during the mechanical loading and fracture
processes, which are closely related to their nature, morphology and of course to
their bonding with the epoxy matrix. When both nanoceramic and silicone rubber are
involved in the epoxy formulation, a superposition of their corresponding energy release
mechanisms is generated, which provides the matrix with an unusual properties
balance.
From the modified matrices glass fiber reinforced RTM-plates were produced. The
structure of the obtained composites was microscopically analyzed to determine their
impregnation quality. In all cases composites with no structural defects (i.e. voids,
delaminations) and good superficial finish were reached. The composites were also
properly characterized. As expected the final performance of the GFRCs is strongly
determined by the matrix properties. Thus, the enhancement reached by epoxy matrices
is translated into better GFRC´s macroscopical properties. Composites with up
to 15% enhanced strength and toughness improved up to 50%, were obtained from
the modified epoxy matrices.

This thesis deals with the development of thermoplastic polyolefin elastomers using recycled polyolefins and ground tyre rubber (GTR). The disposal of worn tyres and their economic recycling mean a great challenge nowadays. Material recycling is a preferred way in Europa owing to legislative actions and ecological arguments. This first step with worn tyres is already done in this direc-tion as GTR is available in different fractions in guaranteed quality. As the traditional applications of GTR are saturated, there is a great demand for new, value-added products containing GTR. So, the objective of this work was to convert GTR by reac-tive blending with polyolefins into thermoplastic elastomers (TPE) of suitable me-chanical and rheological properties. It has been established that bituminous reclamation of GTR prior to extrusion melt compounding with polyolefins is a promising way of TPE production. By this way the sol-content (acetone soluble fraction) of the GTR increases and the GTR particles can be better incorporated in the corresponding polyolefin matrix. The adhesion be-tween GTR and matrix is given by molecular intermingling in the resulting interphase. GTR particles of various production and mean particle size were involved in this study. As polyolefins recycled low-density polyethylene (LDPE), recycled high-density polyethylene (HDPE) and polypropylene (PP) were selected. First, the opti-mum conditions for the GTR reclamation in bitumen were established (160 °C < T < 180 °C; time ca. 4 hours). Polyolefin based TPEs were produced after GTR reclamation in extrusion compounding. Their mechanical (tensile behaviour, set properties), thermal (dynamic-mechanical thermal analysis, differential scanning calorimetry) and rheological properties (both in low- and high-shear rates ) were determined. The PE-based blends contained an ethylene/propylene/diene (EPDM) rubber as compatibilizer and their composition was as follows: PE/EPDM/GTR:bitumen = 50/25/25:25. The selected TPEs met the most important criterion, i.e. elongation at break > 100 %; compression set < 50%. The LDPE-based TPE (TPE(LDPE)) showed better me-chanical performance compared to the TPE(HDPE). This was assigned to the higher crystallinity of the HDPE. The PP-based blends of the compositions PP/(GTR-bitumen) 50/50 and 25/75, whereby the ratio of GTR/bitumen was 60/40, outperformed those containing non-reclaimed GTR. The related blends showed also a better compatibility with a PP-based commercial thermoplastic dynamic vulcanizate (TDV). Surprisingly, the mean particle size of the GTR, varied between < 0.2 and 0.4-0.7 mm, had a small effect on the mechanical properties, however somewhat larger for the rheological behaviour of the TPEs produced.

Nowadays piezoelectric and ferroelectric materials are becoming more and more an interesting part of smart materials in scientific and engineering applications. Precision machining in manufacturing, micropositioning in metrology, common rail systems with piezo fuel injection control in automobile industry, and ferroelectric random access memories (FRAM) in microelectromechanical systems (MEMS) besides commercial piezo actuators and sensors can be very good examples for the application of piezoceramic and ferroelectric materials. In spite of having good characteristics, piezoelectric and ferroelectric materials have significant nonlinearities, which limit the applications in high performance usage. Domain switching (ferroelastic or ferroelectric) is the main reason for the nonlinearity of ferroelectric materials. External excessive electromechanical loads (mechanical stress and electric field) are driving forces for domain switching. In literature, various important experiments related to the non-linear properties of piezoelectric and ferroelectric materials are reported. Simulations of nonlinear properties of piezoelectric and ferroelectric materials based on physical insights of the material have been performed during the last two decades by using micromechanical and phenomenological models. The most significant experiments and models are deeply discussed in the literature survey. In this thesis the nonlinear behaviour of tetragonal perovskite type piezoceramic materials is simulated theoretically using two and three dimensional micromechanical models which are based on physical insights of the material. In the simulations a bulk piezoceramic material which has numerous grains is considered. Each grain has random orientation in properties of polarization and strain. Randomness of orientations is given by Euler angles equally distributed between \(0\) and \(2\pi\). Each element in the micromechanical model has been assumed to have the same properties of the real piezoelectric grain. In the first part of the simulations, quasi-static characteristics of piezoelectric materials are investigated by applying cyclic, rate independent, bipolar, uni-axial and external electrical loading with an amplitude of 2 kV/mm gradually starting from zero value in virgin state. Moreover, the simulations are undertaken for these materials which are subjected to quasi-static, uni-polar, uni-axial mechanical stress, namely compressive stress. The calculations are performed at each element based on linear constitutive equations, nonlinear domain switching and a probability theory for domain switching. In order to fit the simulations to the experimental data, some parameters such as spontaneous polarization, spontaneous strain, piezoelectric and dielectric constants are chosen from literature. The domain switching of each grain is determined by an electromechanical energy criterion. Depending on the actual energy related to a critical energy a certain probability is introduced for domain switching of the polarization direction. Same energy levels are assumed in the electromechanical energy relation for different types of domain switching like 90º and 180º for perovskite type tetragonal or 70.5º and 109.5º for rhombohedral microstructures. It is assumed that intergranular effects between grains can be modelled by such probability functions phenomenologically. The macroscopic response of the material to the applied electromechanical loading is calculated by using Euler transformations and averaging the individual grains. Properties of piezoelectric materials under fixed mechanical stresses are also investigated by applying constant compressive stress in addition to cyclic electrical loading in the simulations. Compressive stress is applied and kept constant before cyclic bipolar electrical loading is implemented. In the following chapters, a three-dimensional micromechanical model is extended for the simulation of the rate dependent properties of certain perovskite type tetragonal piezoelectric materials. The frequency dependent micromechanical model is now not only based on linear constitutive and nonlinear domain switching but also linear kinetics theories. The material is loaded both electrically and mechanically in separate manner with an alternating electrical voltage and mechanical stress values of various moderate frequencies, which are in the order of 0.01 Hz to 1 Hz. Electromechanical energy equation in combination with a probability function is again used to determine the onset of the domain switching inside the grains. The propagation of the domain wall during the domain switching process in grains is modelled by means of linear kinetics relations after a new domain nucleates. Electric displacement versus electric field hysteresis loops, mechanical strain versus mechanical stress and electric displacement versus mechanical stress for different frequencies and amplitudes of the alternating electric fields and compressive stresses are simulated and presented. A simple micromechanical model without using probabilistic approach is compared with the one that takes it into account. Both models give important insights into the rate dependency of piezoelectric materials, which was observed in some experiments reported in the literature. Intergranular effects are other significant factors for nonlinearities of polycrystalline ferroelectric materials. Even piezoelectric actuators and sensors show nonlinearities when they are operated with electrical loading, which is much lower than the coercive electric field level. Intergranular effects are the main cause of such small hysteresis loops. In the corresponding chapter, two basic field effects which are electrical and mechanical are taken into account for the consideration of intergranular effects micromechanically in the simulations of the two dimensional model. Therefore, a new electromechanical energy equation for the threshold of domain switching is introduced to explain nonlinearities stemming from both domain switching and intergranular effects. The material parameters like coercive electric field and critical spontaneous polarization or strain quantities are not implemented in the electromechanical energy relation. But, this relation contains new parameters which consider both mechanical and electrical field characteristics of neighbouring elements. By using this new model, mechanical strain versus electric field butterfly curves under small electrical loading conditions are also simulated. Hence, a rate dependent concept is applied in butterfly curves by means of linear kinetics model. As a result, the simulations have better matching with corresponding experiments in literature. In the next step, the model can be extended in three dimensional case and the parameters of electromechanical energy relation can be improved in order to get better simulations of nonlinear properties of polycrystalline piezoelectric materials.

Process Chain in Automotive Industry - Present Day Demands versus Long Term Open CAD/CAM Strategies
(1997)

The automotive industry was a pioneer in using CAD/CAM technology. Now the car manufacturers development process is almost completely done with this technology. Substantial initiative for the standardisation of CAD/CAM technics comes from the automotive industry, as e.g. for neutral CAD data interfaces. The R&D departments of German car manufacturers have founded a working group ii with the aim to develop a common long term CAD/CAM strategy. One important result is the concept of a future CAx iii architecture based on the standard data structure STEP iv . The commitment of the car manufactures to STEP and open system architectures is in contradiction to their attitude towards suppliers and subcontractors: Recently, more and more contractors are contractually bound to use exactly the same CAD system as the orderer. The German car industry tries to find a way out of this contradiction and to improve the co-operation between the companies in short term. Therefore they proposed a "Dual CAD Strategy", i.e. to put improvements in CAD communication into practice which are possible today - even proprietary solutions - and in parallel to invest in strategic concepts to prepare tomorrow's open system landscape.

Point defects in piezoelectric materials – continuum mechanical modelling and numerical simulation
(2010)

The topic of this work is the continuum mechanic modelling of point defects in piezoelectric materials. Devices containing piezoelectric material and especially ferroelectrics require a high precision and are exposed to a high number of electrical and mechanical load cycles. As a result, the relevant material properties may decrease with increasing load cycles. This phenomenon is called electric fatigue. The transported ionic and electric charge carriers can interact with each other, as well as with structural elements (grain boundaries, inhomogeneities) or with material interfaces (domain walls). A reduced domain wall mobility also reduces the electromechanical coupling effect, which leads to the electric fatigue effect. The materials considered here are barium titanate and lead zirconate titanate (PZT), in which oxygen vacancies is the most mobile and most frequently appearing defect species. Intentionally introduced foreign atoms (dopants) can adjust the material properties according to their field of application by generating electric dipoles with the vacancies. Agglomerations of point defects can strongly influence the domain wall motion. The domain wall can be slowed down or even be stopped by the locally varying fields in the vicinity of the clusters. Accumulations of point defects can be detected at electrodes, pores or in the bulk of fatigued samples. The present thesis concentrates focuses on the self interaction behaviour of point defects in the bulk. A micro mechanical continuum model is used to show the qualitative and the quantitative interaction behaviour of defects in a static setup and during drift processes. The modelling neglects the ferroelectric switching mechanisms, but is applicable to every piezoelectric material. The underlying differential equations are solved by means of analytical (Green's functions) and numerical (Finite Differences with discrete Fourier Transform) methods, depending on the boundary conditions. The defects are introduced as localised Eigenstrains, as electric charges and as electric dipoles. The required defect parameters are obtained by comparisons with atomistic methods (lattice statics). There are no standardised procedures available for the parameter identification. In this thesis, the mechanical parameter is obtained by a comparison of relaxation volumes of the atomic lattice and the continuum solution. Parameters for isotropic and anisotropic defect descriptions are identified. The strength of the electric defect is obtained by a comparison of the electric internal energies of atomistics and continuum. The appearing singularities are eliminated by taking only the energy difference of a infinite crystal and a periodic cell into account. Both identification processes are carried out for the cubic structure of barium titanate, which decouples the mechanical and the electrical problem. The defect interaction is analysed by means of configurational forces. The mechanical defect parameter generates a directional short-range attraction between defects. An electrical defect parameter produces the long-range Coulomb interaction, which predicts a repulsion of two similar charges. Additionally, an interaction with defect dipoles is taken into account. It is shown that a defect agglomeration is possible for any static defect configuration. Finally, defect drift is simulated using a thermodynamically motivated migration law based on configurational forces. In this context, the migration of point defects due to self interaction, and the influence of external fields is investigated.

A fast numerical method for an advanced electro-chemo-mechanical model is developed which is able to capture phase separation processes in porous materials. This method is applied to simulate lithium-ion battery cells, where the complex microstructure of the electrodes is fully resolved. The intercalation of ions into the popular cathode material LFP leads to a separation into lithium-rich and lithium-poor phases. The large concentration gradients result in high mechanical stresses. A phase-field method applying the Cahn-Hilliard equation is used to describe the diffusion. For the sake of simplicity, the linear elastic case is considered. Numerical tests for fully resolved three-dimensional granular microstructures are discussed in detail.

The phase field approach is a powerful tool that can handle even complicated fracture phenomena within an apparently simple framework. Nonetheless, a profound understanding of the model is required in order to be able to interpret the obtained results correctly. Furthermore, in the dynamic case the phase field model needs to be verified in comparison to experimental data and analytical results in order to increase the trust in this new approach. In this thesis, a phase field model for dynamic brittle fracture is investigated with regard to these aspects by analytical and numerical methods

Microfibrillar reinforced composites (MFC) have attracted considerable academic and practical interests after the concept was introduced more than a decade years ago. This new type of composites will be created by blending of two polymers with different melting temperatures and processing the blend under certain thermo-mechanical conditions to generate in-situ formed microfibrils of the higher melting polymer grade of temperature in the blend. The compression molded microfibrillar composites were reported to possess excellent mechanical properties and thus they are promising materials for different applications. In the present work, a typical immiscible polymer blend PET/PP was selected for the preparation of PET/PP, PET/PP/TiO2 microfibrillar reinforced composites. The objective of this study is to analyse the processing-structure-property relationship in the PET/PP based MFCs. The morphology of the PET microfibrils and the dispersion of the TiO2 nanoparticles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and discussed. The crystallization behaviour of PET and PP was studied by means of differential scanning calorimetry (DSC). The thermomechanical and mechanical properties of the composites were determined by dynamic mechanical thermal analysis (DMTA) and uniaxial tensile tests and the related results discussed as a function of the composition of the corresponding system. During stretching of the PET/PP extrudate, the PET dispersed phase was deformed into microfibrils. These microfibrils were still well persevered after compression molding of the drawn strands. Therefore the PET microfibrils acted as the reinforcement for the PP matrix. Compared with neat PP, the tensile properties of the PET/PP MFC were greatly improved. For the PET/PP/TiO2 MFC, the effects of polypropylene grafted maleic anhydride (PP-g-MA, introduced as compatibilizer) and TiO2 particles on the structure and properties of drawn strands and composites were investigated. Upon the addition of PP-g-MA, the preferential location of TiO2 particles changed: they migrated from the PET dispersed phase to the continuous PP matrix phase. This was accompanied with structural changes of the drawn strands. The microfibril formation mechanism was also investigated. After injection molding of the microfibrillar composites, the preferential location of TiO2 particles was still preserved. DMTA analysis of drawn strands, the tensile and impact tests of the composites demonstrated that the mechanical properties of the drawn strands of the microfibrillar composites were strongly dependent on the respective structures of the tested materials. To further investigate the preferential location of TiO2 particles in the PET/PP blend which were discovered during the preparation of PET/PP/TiO2 MFCs, PET/PP/TiO2 ternary nanocomposites were prepared according to four blending procedures. The preferential location of TiO2 nanoparticles was influenced by the blending sequence and the amount of PP-g-MA incorporated. Furthermore, it was discovered that TiO2 nanoparticles exerted a compatibilizing effect on the morphology of the composites. Three different compatibilization mechanisms of nanoparticles were proposed depending on the location of the nanoparticles.

On the Extended Finite Element Method for the Elasto-Plastic Deformation of Heterogeneous Materials
(2015)

This thesis is concerned with the extended finite element method (XFEM) for deformation analysis of three-dimensional heterogeneous materials. Using the "enhanced abs enrichment" the XFEM is able to reproduce kinks in the displacements and therewith jumps in the strains within elements of the underlying tetrahedral finite element mesh. A complex model for the micro structure reconstruction of aluminum matrix composite AMC225xe and the modeling of its macroscopic thermo-mechanical plastic deformation behavior is presented, using the XFEM. Additionally, a novel stabilization algorithm is introduced for the XFEM. This algorithm requires preprocessing only.

On the Effect of Nanofillers on the Environmental Stress Cracking Resistance of Glassy Polymers
(2019)

It is well known that reinforcing polymers with small amounts of nano-sized fillers is one of the most effective methods for simultaneously improving their mechanical and thermal properties. However, only a small number of studies have focused on environ-mental stress cracking (ESC), which is a major issue for premature failures of plastic products in service. Therefore, the contribution of this work focused on the influence of nano-SiO2 particles on the morphological, optical, mechanical, thermal, as well as envi-ronmental stress cracking properties of amorphous-based nanocomposites.
Polycarbonate (PC), polystyrene (PS) and poly(methyl methacrylate) (PMMA) nanocom-posites containing different amounts and sizes of nano-SiO2 particles were prepared using a twin-screw extruder followed by injection molding. Adding a small amount of nano-SiO2 caused a reduction in optical properties but improved the tensile, toughness, and thermal properties of the polymer nanocomposites. The significant enhancement in mechanical and thermal properties was attributed to the adequate level of dispersion and interfacial interaction of the SiO2 nanoparticles in the polymer matrix. This situation possibly increased the efficiency of stress transfer across the nanocomposite compo-nents. Moreover, the data revealed a clear dependency on the filler size. The polymer nanocomposites filled with smaller nanofillers exhibited an outstanding enhancement in both mechanical properties and transparency compared with nanocomposites filled with larger particles. The best compromise of strength, toughness, and thermal proper-ties was achieved in PC-based nanocomposites. Therefore, special attention to the influ-ence of nanofiller on the ESC resistance was given to PC.
The ESC resistance of the materials was investigated under static loading with and without the presence of stress-cracking agents. Interestingly, the incorporation of nano-SiO2 greatly enhanced the ESC resistance of PC in all investigated fluids. This result was particularly evident with the smaller quantities and sizes of nano-SiO2. The enhancement in ESC resistance was more effective in mild agents and air, where the quality of the deformation process was vastly altered with the presence of nano-SiO2. This finding confirmed that the new structural arrangements on the molecular scale in-duced by nanoparticles dominate over the ESC agent absorption effect and result in greatly improving the ESC resistance of the materials. This effect was more pronounced with increasing molecular weight of PC due to an increase in craze stability and fibril density. The most important and new finding is that the ESC behavior of polymer-based nanocomposites/ stress-cracking agent combinations can be scaled using the Hansen solubility parameter. Thus allowed us to predict the risk of ESC as a function of the filler content for different stress-cracking agents without performing extensive tests. For a comparison of different amorphous polymer-based nanocomposites at a given nano-SiO2 particle content, the ESC resistance of materials improved in the following order: PMMA/SiO2 < PS/SiO2 < low molecular weight PC/SiO2 < high molecular weight PC/SiO2. In most cases, nanocomposites with 1 vol.% of nano-SiO2 particles exhibited the largest improvement in ESC resistance.
However, the remarkable improvement in the ESC resistance—particularly in PC-based nanocomposites—created some challenges related to material characterization because testing times (failure time) significantly increased. Accordingly, the superposition ap-proach has been applied to construct a master curve of crack propagation model from the available short-term tests at different temperatures. Good agreement of the master curves with the experimental data revealed that the superposition approach is a suitable comparative method for predicting slow crack growth behavior, particularly for long-duration cracking tests as in mild agents. This methodology made it possible to mini-mize testing time.
Additionally, modeling and simulations using the finite element method revealed that multi-field modeling could provide reasonable predictions for diffusion processes and their impact on fracture behavior in different stress cracking agents. This finding sug-gests that the implemented model may be a useful tool for quick screening and mitigat-ing the risk of ESC failures in plastic products.

The primary objective of this work is the development of robust, accurate and efficient simulation methods for the optimal control of mechanical systems, in particular of constrained mechanical systems as they appear in the context of multibody dynamics. The focus is on the development of new numerical methods that meet the demand of structure preservation, i.e. the approximate numerical solution inherits certain characteristic properties from the real dynamical process.
This task includes three main challenges. First of all, a kinematic description of multibody systems is required that treats rigid bodies and spatially discretised elastic structures in a uniform way and takes their interconnection by joints into account. This kinematic description must not be subject to singularities when the system performs large nonlinear dynamics. Here, a holonomically constrained formulation that completely circumvents the use of rotational parameters has proved to perform very well. The arising constrained equations of motion are suitable for an easy temporal discretisation in a structure preserving way. In the temporal discrete setting, the equations can be reduced to minimal dimension by elimination of the constraint forces. Structure preserving integration is the second important ingredient. Computational methods that are designed to inherit system specific characteristics – like consistency in energy, momentum maps or symplecticity – often show superior numerical performance regarding stability and accuracy compared to standard methods. In addition to that, they provide a more meaningful picture of the behaviour of the systems they approximate. The third step is to take the previ- ously addressed points into the context of optimal control, where differential equation and inequality constrained optimisation problems with boundary values arise. To obtain meaningful results from optimal control simulations, wherein energy expenditure or the control effort of a motion are often part of the optimisation goal, it is crucial to approxi- mate the underlying dynamics in a structure preserving way, i.e. in a way that does not numerically, thus artificially, dissipate energy and in which momentum maps change only and exactly according to the applied loads.
The excellent numerical performance of the newly developed simulation method for optimal control problems is demonstrated by various examples dealing with robotic systems and a biomotion problem. Furthermore, the method is extended to uncertain systems where the goal is to minimise a probability of failure upper bound and to problems with contacts arising for example in bipedal walking.

In the present work, various aspects of the mixed continuum-atomistic modelling of materials are studied, most of which are related to the problems arising due to a development of microstructures during the transition from an elastic to plastic description within the framework of continuum-atomistics. By virtue of the so-called Cauchy-Born hypothesis, which is an essential part of the continuum-atomistics, a localization criterion has been derived in terms of the loss of infinitesimal rank-one convexity of the strain energy density. According to this criterion, a numerical yield condition has been computed for two different interatomic energy functions. Therewith, the range of the Cauchy-Born rule validity has been defined, since the strain energy density remains quasiconvex only within the computed yield surface. To provide a possibility to continue the simulation of material response after the loss of quasiconvexity, a relaxation procedure proposed by Tadmor et al. leading necessarily to the development of microstructures has been used. Thereby, various notions of convexity have been overviewed in details. Alternatively to the above mentioned criterion, a stability criterion has been applied to detect the critical deformation. For the study in the postcritical region, the path-change procedure proposed by Wagner and Wriggers has been adapted for the continuum-atomistic and modified. To capture the deformation inhomogeneity arising due to the relaxation, the Cauchy-Born hypothesis has been extended by assumption that it represents only the 1st term in the Taylor's series expansion of the deformation map. The introduction of the 2nd, quadratic term results in the higher-order materials theory. Based on a simple computational example, the relevance of this theory in the postcritical region has been shown. For all simulations including the finite element examples, the development tool MATLAB 6.5 has been used.

This thesis investigates the electromechanic coupling of dielectric elastomers for the static and dynamic case by numerical simulations. To this end, the fundamental equations of the coupled field problem are introduced and the discretisation procedure for the numerical implementation is described. Furthermore, a three field formulation is proposed and implemented to treat the nearly incompressible behaviour of the elastomer. Because of the reduced electric permittivity of the material, very high electric fields are required for actuation purposes. To improve the electromechanic coupling a heterogeneous microstructure consisting of an elastomer matrix with barium titanate inclusions is proposed and studied.

This thesis is concerned with a phase field model for brittle fracture.
The high potential of phase field modeling in computational fracture mechanics lies in the generality of the approach and the straightforward numerical implementation, combined with a good accuracy of the results in the sense of continuum fracture mechanics.
However, despite the convenient numerical application of phase field fracture models, a detailed understanding of the physical properties is crucial for a correct interpretation of the numerical results. Therefore, the driving mechanisms of crack propagation and nucleation in the proposed phase field fracture model are explored by a thorough numerical and analytical investigation in this work.

In this work the investigation of a (Ti, Al, Si) N system was done. The main point of investigation was to study the possibility of getting the nanocomposite coatings structures by deposition of multilayer films from TiN, AlSiN, . This tries to understand the relation between the mechanical properties (hardness, Young s modulus), and the microstructure (nanocrystalline with individual phases). Particularly special attention was given to the temperature effects on microstructural changes in annealing at 600 °C for the coatings. The surface hardness, elastic modulus, and the multilayers diffusion and compositions were the test tools for the comparison between the different coated samples with and without annealing at 600 °C. To achieve this object a rectangular aluminum vacuum chamber with three unbalanced sputtering magnetrons for the deposition of thin film coatings from different materials was constructed The chamber consists mainly of two chambers, the pre-vacuum chamber to load the workpiece, and the main vacuum chamber where the sputtering deposition of the thin film coatings take place. The workpiece is moving on a car travel on a railway between the two chambers to the position of the magnetrons by step motors. The chambers are divided by a self constructed rectangular gate controlled manually from outside the chamber. The chamber was sealed for vacuum use using glue and screws. Therefore, different types of glue were tested not only for its ability to develop an uniform thin layer in the gap between the aluminum plates to seal the chamber for vacuum use, but also low outgassing rates which made it suitable for vacuum use. A epoxy was able to fulfill this tasks. The evacuation characteristics of the constructed chamber was improved by minimizing the inner surface outgassing rate. Therefore, the throughput outgassing rate test method was used in the comparisons between the selected two aluminum materials (A2017 and A5353) samples short time period (one hour) outgassing rates. Different machining methods and treatments for the inner surface of the vacuum chamber were tested. The machining of the surface of material A (A2017) with ethanol as coolant fluid was able to reduce its outgassing rate a factor of 6 compared with a non-machined sample surface of the same material. The reduction of the surface porous oxide layer on the top of the aluminum surface by the pickling process with HNO3 acid, and the protection of it by producing another passive non-porous oxides layer using anodizing process will protect the surface for longer time and will minimize the outgassing rates even under humid atmosphere The residual gas analyzer (RGA) 6. Summary test shows that more than 85% of the gases inside the test chamber were water vapour (H2O) and the rests are (N2, H2, CO), so liquid nitrogen water vapor trap can enhance the chamber pumping down process. As a result it was possible to construct a chamber that can be pumped down using a turbo molecular pump (450 L/s) to the range of 1x10-6 mbar within one hour of evacuations where the chamber volume is 160 Litters and the inner surface area is 1.6 m2. This is a good base pressure for the process of sputtering deposition of hard thin film coatings. Multilayer thin film coating was deposited to demonstrate that nanostructured thin film within the (Ti, Al, Si) N system could be prepared by reactive magnetron sputtering of multi thin film layers of TiN, AlSiN. The (SNMS) spectrometry of the test samples show that a complete diffusion between the different deposited thin film coating layers in each sample takes place, even at low substrate deposition temperature. The high magnetic flux of the unbalanced magnetrons and the high sputtering power were able to produce a high ion-toatom flux, which give high mobility to the coated atoms. The interactions between the high mobility of the coated atoms and the ion-to-atom flux were sufficient to enhance the diffusion between the different deposited thin layers. It was shown from the XRD patterns for this system that the structure of the formed mixture consists of two phases. One phase is noted as TiN bulk and another detected unknown amorphous phase, which can be SiNx or AlN or a combination of Ti-Al-Si-N. As a result we where able to deposit a nanocomposite coatings by the deposition of multilayers from TiN, AlSiN thin film coatings using the constructed vacuum chamber

Nanoparticle-Filled Thermoplastics and Thermoplastic Elastomer: Structure-Property Relationships
(2012)

The present work focuses on the structure-property relationships of
particulate-filled thermoplastics and thermoplastic elastomer (TPE). In this work
two thermoplastics and one TPE were used as polymer matrices, i.e. amorphous
bisphenol-A polycarbonate (PC), semi-crystalline isotactic polypropylene (iPP),
and a block copolymer poly(butylene terephthalate)-block-poly(tetramethylene
glycol) TPE(PBT-PTMG). For PC, a selected type of various Aerosil® nano-SiO2
types was used as filler to improve the thermal and mechanical properties by
maintaining the transparency of PC matrix. Different types of SiO2 and TiO2
nanoparticles with different surface polarity were used for iPP. The goal was to
examine the influence of surface polarity and chemical nature of nanoparticles on
the thermal, mechanical and morphological properties of iPP composites. For
TPE(PBT-PTMG), three TiO2 particles were used, i.e. one grade with hydroxyl
groups on the particle surface and the other two grades are surface-modified with
metal and metal oxides, respectively. The influence of primary size and dispersion
quality of TiO2 particles on the properties of TPE(PBT-PTMG)/TiO2 composites
were determined and discussed.
All polymer composites were produced by direct melt blending in a twin-screw
extruder via masterbatch technique. The dispersion of particles was examined by
using scanning electron microscopy (SEM) and micro-computerized tomography
(μCT). The thermal and crystalline properties of polymer composites were characterized by using thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The mechanical and thermomechanical properties
were determined by using mechanical tensile testing, compact tension and
Charpy impact as well as dynamic-mechanical thermal analysis (DMTA).
The SEM results show that the unpolar-surface modified nanoparticles are better
dispersed in polymer matrices as iPP than polar-surface nanoparticles, especially
in case of using Aeroxide® TiO2 nanoparticles. The Aeroxide® TiO2 nanoparticles
with a polar surface due to Ti-OH groups result in a very high degree of
agglomeration in both iPP and TPE matrices because of strong van der Waals
interactions among particles (hydrogen bonding). Compared to unmodified
Aeroxide® TiO2 nanoparticles, the other grades of surface modified TiO2 particles
are very homogenously dispersed in used iPP and TPE(PBT-PTMG). The
incorporation of SiO2 nanoparticles into bisphenol-A PC significantly increases
the mechanical properties of PC/SiO2 nanocomposites, particularly the resistance
against environmental stress crazing (ESC). However, the transparency of
PC/SiO2 nanocomposites decreases with increasing nanoparticle content and
size due to a mismatch of infractive indices of PC and SiO2 particles. The different
surface polarity of nanoparticles in iPP shows evident influence on properties of
iPP composites. Among iPP/SiO2 nanocomposites, the nanocomposite
containing SiO2 nanoparticles with a higher degree of hydrophobicity shows
improved fracture and impact toughness compared to the other iPP/SiO2
composites. The TPE(PBT-PTMG)/TiO2 composites show much better thermal and mechanical properties than neat TPE(PBT-PTMG) due to strong chemical
interactions between polymer matrix and TiO2 particles. In addition, better
dispersion quality of TiO2 particles in used TPE(PBT-PTMG) leads to dramatically
improved mechanical properties of TPE(PBT-PTMG)/TiO2 composites.

Due to their superior weight-specific mechanical properties, carbon fibre epoxy composites (CFRP) are commonly used in aviation industry. However, their brittle failure behaviour limits the structural integrity and damage tolerance in case of impact (e.g. tool drop, bird strike, hail strike, ramp collision) or crash events. To ensure sufficient robustness, a minimum skin thickness is therefore prescribed for the fuselage, partially exceeding typical service load requirements from ground or flight manoeuvre load cases. A minimum skin thickness is also required for lightning strike protection purposes and to enable state-of-the-art bolted repair technology. Furthermore, the electrical conductivity of CFRP aircraft structures is insufficient for certain applications; additional metal components are necessary to provide electrical functionality (e.g. metal meshes on the outer skin for lightning strike protection, wires for electrical bonding and grounding, overbraiding of cables to provide electromagnetic shielding). The corresponding penalty weights compromise the lightweight potential that is actually given by the structural performance of CFRP over aluminium alloys.
Former research attempts tried to overcome these deficits by modifying the resin system (e.g. by addition of conductive particles or toughening agents) but could not prove sufficient enhancements. A novel holistic approach is the incorporation of highly conductive and ductile continuous metal fibres into CFRP. The basic idea of this hybrid material concept is to take advantage of both the electrical and mechanical capabilities of the integrated metal fibres in order to simultaneously improve the electrical conductivity and the damage tolerance of the composite. The increased density of the hybrid material is over-compensated by omitting the need for additional electrical system installation items and by the enhanced structural performance, enabling a reduction of the prescribed minimum skin thickness. Advantages over state-of-the-art fibre metal laminates mainly arise from design and processing technology aspects.
In this context, the present work focuses on analysing and optimising the structural and electrical performance of such hybrid composites with shares of metal fibres up to 20 vol.%. Bundles of soft-annealed austenitic steel or copper cladded low carbon steel fibres with filament diameters of 60 or 63 µm are considered. The fibre bundles are distinguished by high elongation at break (32 %) and ultimate tensile strength (900 MPa) or high electrical conductivity (2.4 × 10^7 S/m). Comprehensive researches are carried out on the fibre bundles as well as on unidirectional and multiaxial laminates. Both hybrid composites with homogeneous and accumulated steel fibre arrangement are taken into account. Electrical in-plane conductivity, plain tensile behaviour, suitability for bolted joints as well as impact and perforation performance of the composite are analysed. Additionally, a novel non-destructive testing method based on measurement of deformation-induced phase transformation of the metastable austenitic steel fibres is discussed.
The outcome of the conductivity measurements verifies a correlation of the volume conductivity of the composite with the volume share and the specific electrical resistance of the incorporated metal fibres. Compared to conventional CFRP, the electrical conductivity in parallel to the fibre orientation can be increased by one to two orders of magnitude even for minor percentages of steel fibres. The analysis, however, also discloses the challenge of establishing a sufficient connection to the hybrid composite in order to entirely exploit its electrical conductivity.
In case of plain tensile load, the performance of the hybrid composite is essentially affected by the steel fibre-resin-adhesion as well as the laminate structure. Uniaxial hybrid laminates show brittle, singular failure behaviour. Exhaustive yielding of the embedded steel fibres is confined to the arising fracture gap. The high transverse stiffness of the isotropic metal fibres additionally intensifies strain magnification within the resin under transverse tensile load. This promotes (intralaminar) inter-fibre-failure at minor composite deformation. By contrast, multiaxial hybrid laminates exhibit distinctive damage evolution. After failure initiation, the steel fibres extensively yield and sustain the load-carrying capacity of angularly (e.g. ±45°) aligned CFRP plies. The overall material response is thus not only a simple superimposition but a complex interaction of the mechanical behaviour of the composite’s constituents. As a result of this post-damage performance, an ultimate elongation of over 11 % can be proven for the hybrid laminates analysed in this work. In this context, the influence of the steel fibre-resin adhesion on the failure behaviour of the hybrid composite is explicated by means of an analytical model. Long term exposure to corrosive media has no detrimental effect on the mechanical performance of stainless steel fibre reinforced composites. By trend, water uptake increases the maximum elongation at break of the hybrid laminate.
Moreover, the suitability of CFRP for bolted joints can partially be improved by the integration of steel fibres. While the bearing strength basically remains nearly unaffected, the bypass failure behaviour (ε_{max}: +363 %) as well as the head pull-through resistance (E_{a,BPT}: +81 %) can be enhanced. The improvements primarily concern the load-carrying capacity after failure initiation. Additionally, the integrated ductile steel fibres significantly increase the energy absorption capacity of the laminate in case of progressive bearing failure by up to 63 %.
However, the hybrid composite exhibits a sensitive low velocity/low mass impact behaviour. Compared to conventional CFRP, the damage threshold load of very thin hybrid laminates is lower, making them prone for delamination at minor, non-critical impact energies. At higher energy levels, however, the impact-induced delamination spreads less since most of the impact energy is absorbed by yielding of the ductile metal fibres instead of crack propagation. This structural advantage compared to CFRP gains in importance with increasing impact energy. The plastic deformation of the metastable austenitic steel fibres is accompanied by a phase transformation from paramagnetic γ-austenite to ferromagnetic α’-martensite. This change of the magnetic behaviour can be used to detect and evaluate impacts on the surface of the hybrid composite, which provides a simple non-destructive testing method. In case of low velocity/high mass impact, integration of ductile metal fibres into CFRP enables to address spacious areas of the laminate for energy absorption purposes. As a consequence, the perforation resistance of the hybrid composite is significantly enhanced; by addition of approximately 20 vol.% of stainless steel fibres, the perforation strength can be increased by 61 %, while the maximum energy absorption capacity rises by 194 %.

This thesis treats the extension of the classical computational homogenization scheme towards the multi-scale computation of material quantities like the Eshelby stresses and material forces. To this end, microscopic body forces are considered in the scale-transition, which may emerge due to inhomogeneities in the material. Regarding the determination of material quantities based on the underlying microscopic structure different approaches are compared by means of their virtual work consistency. In analogy to the homogenization of spatial quantities, this consistency is discussed within Hill-Mandel type conditions.

The mechanical properties of semi-crystalline polymers depend extremely on their
morphology, which is dependent on the crystallization during processing. The aim of
this research is to determine the effect of various nanoparticles on morphology
formation and tensile mechanical properties of polypropylene under conditions
relevant in polymer processing and to contribute ultimately to the understanding of
this influence.
Based on the thermal analyses of samples during fast cooling, it is found that the
presence of nanoparticle enhances the overall crystallization process of PP. The results
suggest that an increase of the nucleation density/rate is a dominant process that
controls the crystallization process of PP in this work, which can help to reduce the
cycle time in the injection process. Moreover, the analysis of melting behaviors
obtained after each undercooling reveals that crystal perfection increases significantly
with the incorporation of TiO2 nanoparticles, while it is not influenced by the SiO2
nanoparticles.
This work also comprises an analysis of the influence of nanoparticles on the
microstructure of injection-molded parts. The results clearly show multi-layers along
the wall thickness. The spherulite size and the degree of crystallinity continuously
decrease from the center to the edge. Generally both the spherulite size and the degree
of crystallinity decrease with higher the SiO2 loading. In contrast, an increase in the
degree of crystallinity with an increasing TiO2 nanoparticle loading was detected.
The tensile properties exhibit a tendency to increase in the tensile strength as the core
is reached. The tensile strength decreases with the addition of nanoparticles, while the
elongation at break of nanoparticle-filled PP decreases from the skin to the core. With
increasing TiO2 loading, the elongation at break decreases.

Epoxy belongs to a category of high-performance thermosetting polymers which have been used extensively in industrial and consumer applications. Highly cross-linked epoxy polymers offer excellent mechanical properties, adhesion, and chemical resistance. However, unmodified epoxies are prone to brittle fracture and crack propagation due to their highly crosslinked structure. As a result, epoxies are normally toughened to ensure the usability of these materials in practical applications.
This research work focuses on the development of novel modified epoxy matrices, with enhanced mechanical, fracture mechanical and thermal properties, suitable to be processed by filament winding technology, to manufacture composite based calender roller covers with improved performance in comparison to commercially available products.
In the first stage, a neat epoxy resin (EP) was modified using three different high functionality epoxy resins with two type of hardeners i.e. amine-based (H1) and anhydride-based (H2). Series of hybrid epoxy resins were obtained by systematic variation of high functionality epoxy resin contents with reference epoxy system. The resulting matrices were characterized by their tensile properties and the best system was chosen from each hardener system i.e. amine and anhydride. For tailored amine based system (MEP_H1) 14 % improvement was measured for bulk samples similarly, for tailored anhydride system (MEP_H2) 11 % improvement was measured when tested at 23 °C.
Further, tailored epoxy systems (MEP_H1 and MEP_H2) were modified using specially designed block copolymer (BCP), and core-shell rubber nanoparticles (CSR). Series of nanocomposites were obtained by systematic variation of filler contents. The resulting matrices were extensively characterized qualitatively and quantitatively to reveal the effect of each filler on the polymer properties. It was shown that the BCP confer better fracture properties to the epoxy resin at low filler loading without losing the other mechanical properties. These characteristics were accompanied by ductility and temperature stability. All composites were tested at 23 °C and at 80 °C to understand the effect of temperature on the mechanical and fracture properties.
Examinations on fractured specimen surfaces provided information about the mechanisms responsible for reinforcement. Nanoparticles generate several energy dissipating mechanisms in the epoxy, e.g. plastic deformation of the matrix, cavitation, void growth, debonding and crack pinning. These were closely related to the microstructure of the materials. The characteristic of the microstructure was verified by microscopy methods (SEM and AFM). The microstructure of neat epoxy hardener system was strongly influenced by the nanoparticles and the resulting interfacial interactions. The interaction of nanoparticles with a different hardener system will result in different morphology which will ultimately influence the mechanical and fracture mechanical properties of the nanocomposites. Hybrid toughening using a combination of the block-copolymer / core-shell rubber nanoparticles and block copolymer / TiO2 nanoparticles has been investigated in the epoxy systems. It was found out that addition of rigid phase with a soft phase recovers the loss of strength in the nanocomposites caused by a softer phase.
In order to clarify the relevant relationships, the microstructural and mechanical properties were correlated. The Counto’s, Halpin-Tsai, and Lewis-Nielsen equations were used to calculate the modulus of the composites and predicted modulus fit well with the measured values. Modeling was done to predict the toughening contribution from block copolymers and core-shell rubber nanoparticles. There was good agreement between the predicted values and the experimental values for the fracture energy.

Based on the framework of continuum mechanics two different concepts to formulate phenomenological anisotropic inelasticity are developed in a thermodynamically consistent manner. On the one hand, special emphasis is placed on the incorporation of structural tensors while on the other hand, fictitious configurations are introduced. Substantial parts of this work deal with the numerical treatment of the presented theory within the finite element method.

This thesis is concerned with the modeling of the domain structure evolution in ferroelectric materials. Both a sharp interface model, in which the driving force on a domain wall is used to postulate an evolution law, and a continuum phase field model are treated in a thermodynamically consistent framework. Within the phase field model, a Ginzburg-Landau type evolution law for the spontaneous polarization is derived. Numerical simulations (FEM) show the influence of various kinds of defects on the domain wall mobility in comparison with experimental findings. A macroscopic material law derived from the phase field model is used to calculate polarization yield surfaces for multiaxial loading conditions.

Product development with end-user integration is not an end in itself but a logical necessity due to divergent types of knowledge of the user and the developer of a product. While the user is an expert in regard to the product’s usage the developer is an expert in the product’s construction and functioning. For the development of high-end products both types of expertises were a prerequisite at all times. The efficient and throughout integration of the user’s perspective into existing product development approaches is the core of user-centred product development. Activities that are the basic ingredient of just any user-centred development approach can be roughly categorized into analysis, design and evaluation activities. Research and practice prove the early integration of real end-users within those activities to add significant and sustainable value to product innovation. The instrumental, methodological and procedural impact of globalization tendencies, on modern user-centred product development in particular, is the primary research focus of the field of cross-cultural user-centred product development. This research aims at the further advancement of the methodological foundations of cross-cultural user centred product development approaches based on a stabile and profound theoretical basis. Primary research objects are established user-analysis methodologies, which are mainly based on Western concepts and theories, and their applicability in disparate cultural contexts of the Far East (China and Korea in particular). For facilitating the adaptation of abstract method characteristics to the situational context of method application as foundation of cross-cultural methodological advancement, a model of method localization was developed. In alignment with internationalization and localization activities within product development processes, a framework for localizing user-centred methodologies was developed. Equivalent to internationalization activities of real product development, the abstraction of method traits from specific methodologies is a necessity in a first step. Methodological adaptation with the primary objective of optimizing situational application of a methodology is to be done in a second step – the step of method-localization. This model of method localization and its underlying theories and principles were tested within an extensive empirical study in Germany, China and Korea. Within this study the applicability of six distinct user-centred product development methodologies, each with its very own profile of abstract method traits, was tested with 248 participants in total. Results clearly back the basic hypothesis of method-localization, i.e. that the application of a user-centred methodology rises and falls with the alignment of its characteristic traits with the cross-cultural application context. Beyond, applicability-influencing factors identified within this study could be proven to be valid indicators of adaptation-necessities and –potentials of user-centred product development methodologies.

The primary object of this work is the development of a robust, accurate and efficient time integrator for the dynamics of flexible multibody systems. Particularly a unified framework for the computational dynamics of multibody systems consisting of mass points, rigid bodies and flexible beams forming open kinematic chains or closed loop systems is developed. In addition, it aims at the presentation of (i) a focused survey of the Lagrangian and Hamiltonian formalism for dynamics, (ii) five different methods to enforce constraints with their respective relations, and (iii) three alternative ways for the temporal discretisation of the evolution equations. The relations between the different methods for the constraint enforcement in conjunction with one specific energy-momentum conserving temporal discretisation method are proved and their numerical performances are compared by means of theoretical considerations as well as with the help of numerical examples.

Mechanical and electrical properties of carbon nanofiber–ceramic nanoparticle–polymer composites
(2010)

The present research is focused on the manufacturing and analysis of composites consisting of a thermosetting polymer reinforced with fillers of nanometric dimensions. The materials were chosen to be an epoxy resin matrix and two different kinds of fillers: electrically conductive carbon nanofibers (CNFs) and ceramic titanium dioxide (TiO2) and aluminium dioxide (Al2O3) nanoparticles. In an initial step of the work, in order to understand the effect that each kind of filler had when added separately to the polymer matrix, CNF–EP and ceramic nanoparticle–EP composites were manufactured and tested. Each type of filler was dispersed in the polymer matrix using two different dispersion technologies. CNFs were dispersed in the resin with the aid of a three roll calender (TRC) whereas a torus bead mill (TML) was used in the ceramic nanoparticle case. Calendering proved to be an efficient method to disperse the untreated CNFs in the polymer matrix. The study of the physical properties of undispersed CNF composites showed that the tensile strength and the maximum sustained strain, were more sensitive to the state of dispersion of the nanofibers than the elastic modulus, fracture toughness, impact energy and electrical conductivity (for filler loadings above the percolation threshold of the system). Rheological investigation of the uncured CNF–epoxy mixture at different stages of dispersion indicated the formation of an interconnected nanofiber network within the matrix after the initial steps of calendering. CNF–EP composites showed better mechanical performance than the unmodified polymer matrix. However, the tensile modulus and strength of the CNF composites accused the presence of remaining nanofiber clusters and did not reach theoretically predicted values. Fracture toughness and resistance against impact did not seem to be so sensitive to the state of nanofiber dispersion and improved consistently with the incorporation of the CNFs. The electrical conductivity of the CNF composites saw an eight orders of magnitude percolative enhancement with increasing nanofiber content. The percolation threshold for the achieved level of CNF dispersion was found to be 0.14 vol. %. It was also determined that, for these composites, the main mechanism of electrical transmission was the electron tunnelling mechanism. Ceramic nanoparticle–EP composites were manufactured using TiO2 and Al2O3 particles as fillers in the epoxy matrix. Mechanical dispersion of the nanoparticles in the liquid polymer by means of a torus bead mill dissolver led to homogeneous distributions of particles in the matrix. Remaining particle agglomerates had a mean value of 80 nm. However, micrometer sized agglomerates could clearly be observed in the microscopical analysis of the composites, especially in the TiO2 case. The inclusion of the nanoparticles in the epoxy resin resulted in a general improvement of the modulus, strength, maximum sustained strain, fracture toughness and impact energy of the polymer matrix. Nanoparticles were able to overcome the stiffness/toughness problem. On the other hand, nanoparticle–EP composites showed lower electrical conductivity than the neat epoxy. In general, there were no significant differences between the incorporation of TiO2 or Al2O3 particles. Based on the previous results, CNFs and nanoparticles were combined as fillers to create a nanocomposite that could benefit from the electrical properties provided by the conductive CNFs and, at the same time, have improved mechanical performance thanks to the presence of the well dispersed ceramic nanoparticles. Nanoparticles and CNFs were dispersed separately to create two batches which were blended together in a dissolver mixer. This method proved effective to create well dispersed CNF–nanoparticle–epoxy composites which showed improved electrical and mechanical properties compared with the neat polymer matrix. The well dispersed ceramic nanofillers were able to introduce additional energy dissipating mechanisms in the CNF–EP composites that resulted in an improvement of their mechanical performance. With high volume loadings of nanoparticles most of the reinforcement came from the presence of the nanoparticles in the polymer matrix. Therefore, the observed trends were, in essence, similar to the ones observed in the ceramic nanoparticle–EP composites. The enhancement in the mechanical performance of the CNF composites with the inclusion of ceramic nanoparticles came at the price of an increase in the percolation threshold and a reduction of the electrical conductivity of the CNF–nanoparticle–EP composites compared with the CNF–EP materials. A modified Weber and Kamal’s fiber contact model (FCM) was used to explain the electrical behaviour of the CNF–nanoparticle–EP composites once percolation was achieved. This model was able to fit rather accurately the experimentally measured conductivity of these composites.

The increasing parallelisation of development processes as well as the ongoing trends towards virtual product development and outsourcing of development activities strengthen the need for 3D co-operative design via communication networks. Regarding the field of CAx, none of the existing systems meets all the requirements of very complex process chain. This leads to a tremendous need for the integration of heterogeneous CAx systems. Therefore, MACAO, a platform-independent client for a distributed CAx component system, the so-called ANICA CAx object bus, is presented. The MACAO client is able to access objects and functions provided by different CAx servers distributed over a communication network. Thus, MACAO is a new solution for engineering design and visualisation in shared distributed virtual environments. This paper describes the underlying concepts, the actual prototype implementation, as well as possible application scenarios in the area of co-operative design and visualisation.

An isogeometric Reissner-Mindlin shell derived from the continuum theory is presented. The geometry is described by NURBS surfaces. The kinematic description of the employed shell theory requires the interpolation of the director vector and of a local basis system. Hence, the definition of nodal basis systems at the control points is necessary for the proposed formulation. The control points are in general not located on the shell reference surface and thus, several choices for the nodal values are possible. The proposed new method uses the higher continuity of the geometrical description to calculate nodal basis system and director vectors which lead to geometrical exact interpolated values thereof. Thus, the initial director vector coincides with the normal vector even for the coarsest mesh. In addition to that a more accurate interpolation of the current director and its variation is proposed. Instead of the interpolation of nodal director vectors the new approach interpolates nodal rotations. Account is taken for the discrepancy between interpolated basis systems and the individual nodal basis systems with an additional transformation. The exact evaluation of the initial director vector along with the interpolation of the nodal rotations lead to a shell formulation which yields precise results even for coarse meshes. The convergence behavior is shown to be correct for k-refinement allowing the use of coarse meshes with high orders of NURBS basis functions. This is potentially advantageous for applications with high numerical effort per integration point. The geometrically nonlinear formulation accounts for large rotations. The consistent tangent matrix is derived. Various standard benchmark examples show the superior accuracy of the presented shell formulation. A new benchmark designed to test the convergence behavior for free form surfaces is presented. Despite the higher numerical effort per integration point the improved accuracy yields considerable savings in computation cost for a predefined error bound.