Kaiserslautern - Fachbereich Maschinenbau und Verfahrenstechnik
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The broad engineering applications of polymers and composites have become the
state of the art due to their numerous advantages over metals and alloys, such as
lightweight, easy processing and manufacturing, as well as acceptable mechanical
properties. However, a general deficiency of thermoplastics is their relatively poor
creep resistance, impairing service durability and safety, which is a significant barrier
to further their potential applications. In recent years, polymer nanocomposites have
been increasingly focused as a novel field in materials science. There are still many
scientific questions concerning these materials leading to the optimal property
combinations. The major task of the current work is to study the improved creep
resistance of thermoplastics filled with various nanoparticles and multi-walled carbon
nanotubes.
A systematic study of three different nanocomposite systems by means of
experimental observation and modeling and prediction was carried out. In the first
part, a nanoparticle/PA system was prepared to undergo creep tests under different
stress levels (20, 30, 40 MPa) at various temperatures (23, 50, 80 °C). The aim was
to understand the effect of different nanoparticles on creep performance. 1 vol. % of
300 nm and 21 nm TiO2 nanoparticles and nanoclay was considered. Surface
modified 21 nm TiO2 particles were also investigated. Static tensile tests were
conducted at those temperatures accordingly. It was found that creep resistance was
significantly enhanced to different degrees by the nanoparticles, without sacrificing
static tensile properties. Creep was characterized by isochronous stress-strain curves,
creep rate, and creep compliance under different temperatures and stress levels.
Orientational hardening, as well as thermally and stress activated processes were
briefly introduced to further understanding of the creep mechanisms of these
nanocomposites. The second material system was PP filled with 1 vol. % 300 nm and 21 nm TiO2
nanoparticles, which was used to obtain more information about the effect of particle
size on creep behavior based on another matrix material with much lower Tg. It was
found especially that small nanoparticles could significantly improve creep resistance.
Additionally, creep lifetime under high stress levels was noticeably extended by
smaller nanoparticles. The improvement in creep resistance was attributed to a very
dense network formed by the small particles that effectively restricted the mobility of
polymer chains. Changes in the spherulite morphology and crystallinity in specimens
before and after creep tests confirmed this explanation.
In the third material system, the objective was to explore the creep behavior of PP
reinforced with multi-walled carbon nanotubes. Short and long aspect ratio nanotubes
with 1 vol. % were used. It was found that nanotubes markedly improved the creep
resistance of the matrix, with reduced creep deformation and rate. In addition, the
creep lifetime of the composites was dramatically extended by 1,000 % at elevated
temperatures. This enhancement contributed to efficient load transfer between
carbon nanotubes and surrounding polymer chains.
Finally, a modeling analysis and prediction of long-term creep behaviors presented a
comprehensive understanding of creep in the materials studied here. Both the
Burgers model and Findley power law were applied to satisfactorily simulate the
experimental data. The parameter analysis based on Burgers model provided an
explanation of structure-to-property relationships. Due to their intrinsic difference, the
power law was more capable of predicting long-term behaviors than Burgers model.
The time-temperature-stress superposition principle was adopted to predict long-term
creep performance based on the short-term experimental data, to make it possible to
forecast the future performance of materials.
The main concern of this contribution is the computational modeling of biomechanically relevant phenomena. To minimize resource requirements, living biomaterials commonly adapt to changing demands. One way to do so is the optimization of mass. For the modeling of biomaterials with changing mass, we distinguish between two different approaches: the coupling of mass changes and deformations at the constitutive level and at the kinematic level. Mass change at the constitutive level is typically realized by weighting the free energy function with respect to the density field, as experimentally motivated by Carter and Hayes [1977] and computationally realized by Harrigan and Hamilton [1992]. Such an ansatz enables the simulation of changes in density while the overall volume remains unaffected. In this contribution we call this effect remodeling. Although in principle applicable for small and large strains, this approach is typically adopted for hard tissues, e.g. bone, which usually undergo small strain deformations. Remodeling in anisotropic materials is realized by choosing an appropriate anisotropic free energy function. <br> Within the kinematic coupling, a changing mass is characterized through a multiplicative decomposition of the deformation gradient into a growth part and an elastic part, as first introduced in the context of plasticity by Lee [1969]. In this formulation, which we will refer to as growth in the following, mass changes are attributed to changes in volume while the material density remains constant. This approach has classically been applied to model soft tissues undergoing large strains, e.g. the arterial wall. The first contribution including this ansatz is the work by Rodriguez, Hoger and McCulloch [1994]. To model anisotropic growth, an appropriate anisotropic growth deformation tensor has to be formulated. In this contribution we restrict ourselves to transversely isotropic growth, i.e., growth characterized by one preferred direction. On that account, we define a transversely isotropic growth deformation tensor determined by two variables, namely the stretch ratios parallel and perpendicular to the characteristic direction. <br> Another method of material optimization is the adaption of the inner structure f a material to its loading conditions. In anisotropic materials this can be realized by a suitable orientation of the material directions. For example, the trabeculae in the human femur head are oriented such that they can carry the daily loads with an optimum mass. Such a behavior can also be observed in soft tissues. For instance, the fibers of muscles and the collagen fibers in the arterial wall are oriented along the loading directions to carry a maximum of mechanical load. If the overall loading conditions change, for instance during a balloon angioplasty or a stent implantation, the material orientation readapts, which we call reorientation. The anisotropy type in biomaterials is often characterized by fiber reinforcement. A particular subclass of tissues, which includes muscles, tendons and ligaments, is featured by one family of fibers. More complex microstructures, such as arterial walls, show two fiber families, which do not necessarily have to be perpendicular. Within this contribution we confine ourselves to the first case, i.e., transversely isotropic materials indicated by one characteristic direction. The reorientation of the fiber direction in biomaterials is commonly smooth and continuous. For transverse isotropy it can be described by a rotation of the characteristic direction. Analogous to the theory of shells, we additionally exclude drilling rotations, see also Menzel [2006]. However, the driving force for these reorientation processes is still under discussion. Mathematical considerations promote strain driven reorientations. As discussed, for instance, in Vianello [1996], the free energy reaches a critical state for coaxial stresses and strains. For transverse isotropy, it can be shown that this can be achieved if the characteristic direction is aligned with a principal strain direction. From a biological point of view, depending on the kind of material (i.e. bone, muscle tissue, cartilage tissue, etc.), both strains and stresses can be suggested as stimuli for reorientation. Thus, whithin this contribution both approaches are investigated. <br> In contrast to previous works, in which remodeling, growth and reorientation are discussed separately, the present work provides a framework comprising all of the three mentioned effects at once. This admits a direct comparison how and on which level the individual phenomenon is introduced into the material model, and which influence it has on the material behavior. For a uniform description of the phenomenological quantities an internal variable approach is chosen. Moreover, we particularly focus on the algorithmic implementation of the three effects, each on its own, into a finite element framework. The nonlinear equations on the local and the global level are solved by means of the Newton-Raphson scheme. Accordingly, the local update of the internal variables and the global update of the deformation field are consistently linearized yielding the corresponding tangent moduli. For an efficient implementation into a finite element code, unitized update algorithms are given. The fundamental characteristics of the effects are illustrated by means of some representative numerical simulations. Due to the unified framework, combinations of the individual effects are straightforward.
Die enorme Nachfrage nach neuartigen Naturstoffen sowie Leitstrukturen für die (Teil-) Synthese pharmazeutischer Produkte verlangt ständig nach neuen Innovationen. Ein bedeutender Impuls ist dabei von der Marinen Biotechnologie zu erwarten, die seit einigen Jahren versucht, die sich bietenden medizinischen Potentiale auszuschöpfen. So können neuartige Wirkstoffe bzw. die für deren Produktion verantwortlichen Organismen und Enzyme isoliert und bereitgestellt werden. Häufig ist das Wachstum der Organismen jedoch stark limitiert sowie eine Produktion der Ziel-Metabolite nur unter bestimmten Bedingungen möglich, weshalb ein enormes Interesse an neuen Optimiermethoden zur Lösung des Problems besteht. Neuartige Lösungsansätze sollten daher im Rahmen dieser Arbeit verfolgt werden. Untersucht wurde eine „biochemische“ Wachstums- und Produktionsoptimierung mariner Bakterien durch einen Zusatz von Homoserinlactonen zum Kultivierungsmedium. Dabei konnte neben einem verbesserten Wachstum diverser Prokaryonten eine leichte Variation im Metabolitspektrum ermittelt sowie eine geringfügig erhöhte Produktion biologisch aktiver Substanzen detektiert werden. Soll das Wachstum von (marinen) Mikroorganismen optimiert werden, wird vom Anwender häufig eine unstrukturierte one-factor-at-a-time-Methode angewendet. Da diese in der Regel keine Beeinflussung der zu optimierenden variablen Parameter beachtet, wurden in dieser Arbeit alternativ rechnergestützte Optimierungen zur Umgehung dieses Problems durchgeführt. Zum Einsatz kam dabei eine Kombination aus einem Genetischen Algorithmus, welcher die Identifikation des globalen Optimums erlaubt und einem Simplex-Algorithmus, der zur Verfeinerung des bereits aufgefundenen Optimums dient. Hiermit konnte die Produktion einer marinen L-Serindehydratase im Rahmen einer Nährmediumsoptimierung um mehr als 50 % im Vergleich zum Referenzmedium gesteigert werden. Des Weiteren wurde ein im Polyketidscreening positiv aufgefallenes Bakterium (Halomonas marina) als Modellorganismus für ein neuartiges rechnergestütztes Verfahren eingesetzt, welches eine schnelle Identifizierung der Güte von Wachstumsparametern erlaubt und dabei eine repetitiv geführte Batch-Betriebsweise von Bioreaktoren nutzt. Ein weiterer Schwerpunkt dieser Arbeit ist die Charakterisierung einer rekombinant exprimierten Tryptophan-5-Halogenase. Hierbei konnte nach der Identifizierung des Aktivitätsoptimums hinsichtlich des einzustellenden pH-Werts sowie der Temperatur die Halbwertszeit des Enzyms bei verschiedenen Temperaturen bestimmt und mittels 2 D-Gelelektrophorese der pI-Wert des Enzyms erstmalig determiniert werden. Ein Fokus dieser Untersuchungen lag auf der Optimierung der Enzymkinetik. Dabei konnte insgesamt eine Verbesserung der Ausbeute um den Faktor 1,6 erzielt werden.
Wässrige Lösungen sowohl neutraler als auch ionischer Polymerer gewinnen ein zunehmendes Interesse in vielen Bereichen. Für den Einsatz solcher Systeme muss deren Phasenverhalten bekannt sein. Die vorliegende Arbeit liefert einen Beitrag zum Phasengleichgewicht solcher Systeme. Als Bausteine der Polymere werden dabei sowohl die neutrale organischen Komponente Vinylpyrrolidon (VP) als auch ein ionisches Monomer auf Basis von Imidazolium (3-Methyl-1-vinyl-1H-Imidazoliummethylsulfat - QVI), als niedrigmolekularer Elektrolyt ausschließlich Natriumsulfat verwendet. Die vorliegende Arbeit hat zum Ziel, das Phasenverhalten einer Reihe technisch interessierender Systeme in experimentellen Untersuchungen bei 25 und 65°C zu bestimmen und damit eine Datenbasis für theoretische Arbeiten zu liefern, die im Anschluss an diese Untersuchungen im Zusammenhang mit der Entwicklung von Modellen zur Korrelation bzw. Vorhersage solcher Phasengleichgewichte vorgesehen sind. Die Grenze zwischen einem einphasigen, flüssigen Bereich und Zwei- bzw. Drei-Phasen-Gebieten wurde durch visuelle Bestimmung der Trübung bei der Titration einer wässrigen Lösung (entweder des Polymeren oder des Salzes) bestimmt. Die Zusammensetzung der koexistierenden Phasen wurde in Phasengleichgewichtsexperimenten bestimmt, bei denen Proben der koexistierenden Phasen analysiert wurden. Dazu wurden mehrere Analysenmethoden entwickelt/erprobt (z. B. die Gefriertrocknung, die thermische (gravimetrische) Analyse, die Gaschromatographie und die Ionenchromatographie). Insgesamt wurden für 42 Systeme der Verlauf der Trübungskurve und für 34 Systeme das Phasengleichgewicht bestimmt. Dabei handelte es sich überwiegend um ternäre Systeme aus einem Polymeren (auf Basis von VP und/bzw. QVI), Natriumsulfat und Wasser, teilweise auch um quaternäre Systeme aus den zuvor erwähnten Komponenten und einem der Monomeren (VP bzw. QVI). Dabei zeigte die Mehrzahl der untersuchten Systeme eine flüssig-flüssig Entmischung, teilweise jedoch auch nur die häufiger anzutreffenden Fest-Flüssig-Phasengleichgewichte (z.B. Ausfall eines Salzes als Feststoff). Die experimentellen Untersuchungen wurden insbesondere bei hohen Polymerkonzentrationen durch die Zähigkeit der wässrigen Lösungen erschwert. Neben den Untersuchungen zum Phasengleichgewicht in ternären und quaternären Systemen wurden im Hinblick auf die in weiterführenden Arbeiten geplanten Modellierungsarbeiten auch experimentelle Untersuchungen an binären Subsystemen durchgeführt. Dabei handelte es sich ausschließlich um sogenannte isopiestische Messungen an wässrigen Lösungen der Polymere bzw. der Monomere. In solchen Untersuchungen wird der Einfluss der Wechselwirkungen zwischen den Molekülen eines in Wasser gelösten Stoffes auf den Dampfdruck der Lösung bestimmt. Der dabei quantitativ bestimmte Einfluss von Art und Menge des Polymeren auf den Dampfdrucks des Lösungsmittels soll in weiterführenden Arbeiten zur Bestimmung von Parametern thermodynamischer Modelle zur Beschreibung der Gibbsschen Exzessenergie wässriger Polymerlösungen verwendet werden. Die Ergebnisse der experimentellen Untersuchungen zum Flüssig-Flüssig bzw. Fest-Flüssig-Phasengleichgewicht lassen sich folgendermaßen charakterisieren: In fast allen Systemen mit Polymeren auf Basis von Vinylpyrrolidon wurden Flüssig-Flüssig-Gleichgewichte mit einer salzreichen, nahezu polymerfreien wässrigen Phase und einer polymerreichen wässrigen auch salzhaltigen flüssigen Phase gefunden. D. h. in einem Gibbsschen Dreiecksdiagramm, in dem die Zusammensetzung einer ternären Mischung (mit Hilfe des Konzentrationsmaßes „Massenanteil“ ausgedrückt) durch einen Punkt dargestellt wird zeigt die Phasengrenze zwischen dem einphasigen und dem mehrphasigen Gebiet eine starke Asymmetrie. Der Wassergehalt der polymerreichen Phase ist dabei häufig deutlich geringer als der Wassergehalt der salzreichen Phase. In Systemen mit (Natriumsulfat und) Polymeren auf Basis des Imidazoliumsalzes QVI wurden dagegen überwiegend Fest-Flüssig-Gleichgewichte beobachtet. Es zeigte sich, dass die Molmasse der verwendeten Polymere nur einen vergleichsweise geringen Einfluss auf die Ausdehnung des einphasigen flüssigen Gebietes hat. I. d. R. nimmt mit steigender Molmasse die Ausdehnung der Mischungslücke zu. Auch der Temperatureinfluss auf die beobachteten Phasengleichgewichte ist relativ gering. Dies war im Fall der Polymere auf Basis von Vinylpyrrolidon aus früheren Untersuchungen an wässrigen, salzfreien Lösungen dieser Polymere zu erwarten. Wie schon erwähnt, sind die Ergebnisse der vorliegenden Arbeit im Zusammenhang mit dem Einsatz solcher polymer- und salzhaltigen Systeme in verschiedenen Bereichen von Interesse. Sie bilden aber auch die Datenbasis für anstehende theoretische Arbeiten, die sich mit der Entwicklung thermodynamischer Modelle zur Beschreibung von Phasengleich- gewichten salz- und polymerhaltiger, wässriger Systeme beschäftigen werden.
In recent years, nanofiller-reinforced polymer composites have attracted considerable
interest from numerous researchers, since they can offer unique mechanical,
electrical, optical and thermal properties compared to the conventional polymer
composites filled with micron-sized particles or short fibers. With this background, the
main objective of the present work was to investigate the various mechanical
properties of polymer matrices filled with different inorganic rigid nanofillers, including
SiOB2B, TiOB2B, AlB2BOB3B and multi-walled carbon nanotubes (MWNT). Further, special
attention was paid to the fracture behaviours of the polymer nanocomposites. The
polymer matrices used in this work contained two types of epoxy resin (cycloaliphatic
and bisphenol-F) and two types of thermoplastic polymer (polyamide 66 and isotactic
polypropylene).
The epoxy-based nanocomposites (filled with nano-SiOB2B) were formed in situ by a
special sol-gel technique supplied by nanoresins AG. Excellent nanoparticle
dispersion was achieved even at rather high particle loading. The almost
homogeneously distributed nanoparticles can improve the elastic modulus and
fracture toughness (characterized by KBICB and GBICB) simultaneously. According to
dynamic mechanical and thermal analysis (DMTA), the nanosilica particles in epoxy
resins possessed considerable "effective volume fraction" in comparison with their
actual volume fraction, due to the presence of the interphase. Moreover, AFM and
high-resolution SEM observations also suggested that the nanosilica particles were
coated with a polymer layer and therefore a core-shell structure of particle-matrix was
expected. Furthermore, based on SEM fractography, several toughening
mechanisms were considered to be responsible for the improvement in toughness,
which included crack deflection, crack pinning/bowing and plastic deformation of
matrix induced by nanoparticles.
The PA66 or iPP-based nanocomposites were fabricated by a conventional meltextrusion
technique. Here, the nanofiller content was set constant as 1 vol.%. Relatively good particle dispersion was found, though some small aggregates still
existed. The elastic modulus of both PA66 and iPP was moderately improved after
incorporation of the nanofillers. The fracture behaviours of these materials were
characterized by an essential work fracture (EWF) approach. In the case of PA66
system, the EWF experiments were carried out over a broad temperature range
(23~120 °C). It was found that the EWF parameters exhibited high temperature
dependence. At most testing temperatures, a small amount of nanoparticles could
produce obvious toughening effects at the cost of reduction in plastic deformation of
the matrix. In light of SEM fractographs and crack opening tip (COD) analysis, the
crack blunting induced by nanoparticles might be the major source of this toughening.
The fracture behaviours of PP filled with MWNTs were investigated over a broad
temperature range (-196~80 °C) in terms of notched impact resistance. It was found
that MWNTs could enhance the notched impact resistance of PP matrix significantly
once the testing temperature was higher than the glass transition temperature (TBgB) of
neat PP. At the relevant temperature range, the longer the MWNTs, the better was
the impact resistance. SEM observation revealed three failure modes of nanotubes:
nanotube bridging, debonding/pullout and fracture. All of them would contribute to
impact toughness to a degree. Moreover, the nanotube fracture was considered as
the major failure mode. In addition, the smaller spherulites induced by the nanotubes
would also benefit toughness.
The main goal of this work is to examine various aspects of `inelastic continuum mechanics': first, fundamental aspects of a general finite deformation theory based on a multiplicative decomposition of the deformation gradient with special emphasis on the incompatibility of the so-called intermediate configuration are discussed in detail. Moreover, various balance of linear momentum representations together with the corresponding volume forces are derived in a configurational mechanics context. Subsequent chapters are consequently based on these elaborations so that the applied multiplicative decomposition generally serves as a fundamental modelling concept in this work; after generalised strain measures are introduced, a kinematic hardening model coupled with anisotropic damage, a substructure evolution framework as well as two different growth and remodelling formulations for biological tissues are presented.
Elastomeric and other rubber-like materials are often simultaneously exposed to short- and long-time loads within engineering applications. When aiming at establishing a general simulation tool for viscoelastic media over these different time scales, a suitable material model and its corresponding material parameters can only be determined if an appropriate number of experimental data is taken into account. In this work an algorithm for the identification of material parameters for large strain viscoelasticity is presented. Thereby, data of multiple experiments are considered. Based on this method the experimental loading intervals for long-time experiments can be shortened in time and the parameter identification procedure is now referred to experimental data of tests under short- and long-time loads without separating the parameters due to these different time scales. The employed viscoelastic material law is based on a nonlinear evolution law and valid far from thermodynamic equilibrium. The identification is carried out by minimizing a least squares functional comparing inhomogeneous displacement fields from experiments and FEM simulations at given (measured) force loads. Within this optimization procedure all material parameters are identified simultaneously by means of a gradient based method for which a semi-analytical sensitivity analysis is calculated. Representative numerical examples are referred to measured data for different polyurethanes. In order to show the general applicability of the identification method for multiple tests, in the last part of this work the parameter identification for small strain plasticity is presented. Thereby three similar test programs on three specimen of the aluminum alloy AlSi9Cu3 are analyzed, and the parameter sets for the respective individual identifications, and for the combination of all tests in one identification, is compared.
Induktionsschweißen kann sowohl für das Schweißen von thermoplastischen Faser-
Kunststoff-Verbunden als auch für das Verbinden von Metall/Faser-Kunststoff-
Verbunden eingesetzt werden. Nach Betrachtung der Möglichkeiten einer solchen
Verbindung wurde festgestellt, dass die Verbindungsqualität durch die
Oberflächenvorbehandlung des metallischen und des polymeren Fügepartners und
durch die Prozessbedingungen bestimmt wird.
Verschiedene neue Werkzeuge (z.B. spezielle Probenhalterungen, temperierbarer
Anpressstempel, Erwärmungs- und Konsolidierungsrolle) wurden entwickelt und in
die Induktionsschweißanlage zur Herstellung von Metall/Faser-Kunststoff-Verbunden
integriert. Topografische Analysen mittels Rasterelektronenmikroskopie und
Laserprofilometrie zeigen einen großen Einfluss der Vorbehandlungsmethoden auf
die Oberflächenrauhigkeit. Zusätzlich ändert die Vorbehandlung die physikalischen
(Oberflächenenergie) und die chemischen Eigenschaften (Atomkonzentration). Die
Eigenschaften der Verbindungen wurden zuerst anhand von Zugscherprüfungen und
parallel durch Oberflächenanalysen untersucht. Die Ergebnisse dieser
Untersuchungen zeigen:
• Die Vorbehandlungsmethoden Korundstrahlen und Sauerbeizen führen bei
dem metallischen Fügepartner zu den höchsten Verbundfestigkeiten. Die
Atmosphären-Plasmareinigung des polymeren Fügepartners ergibt eine
Zunahme der Zugscherfestigkeit von ca. 10 % sowie auch eine Verkleinerung
des Vertrauensbereiches.
• Die Zugscherfestigkeit hängt vom Prozessdruck und damit vom Fließverhalten
des Polymers in der Fügezone ab.
• Die Orientierung der Prüfkraft relativ zur Faserorientierung hat keinen Einfluss
auf die Zugscherfestigkeit der eingesetzten faserverstärkten Materialien.
• Die Leinwand-Bindung, mit mehr polymerreichen Zonen, führt zu einem
geringen Anstieg der Zugscherfestigkeit im Vergleich zu einer Atlas 1/4-
Bindung. Die Gelege-Struktur ergibt durch Faserverschiebungen ähnliche
Festigkeiten wie die Leinwand-Bindung. Es zeigt sich, dass die
Verbundfestigkeit durch das Polymer bestimmt wird. • Die Zugscherfestigkeit gewinnt einen großen Anstieg durch eine zusätzliche
Polymerfolie in der Fügezone. Die Schliffbilder zeigen eine polymere
Zwischenschichtdicke von 5 bis 20 μm für AlMg3-CF/PA66.
• Durch den gezielten Einsatz verschiedener Vorbehandlungsmethoden
(Korundstrahlen mit zusätzlichem Polymer) kann die Zugscherfestigkeit auf bis
zu 14 MPa für AlMg3-CF/PA66-Verbunde und 18 MPa für DC01-CF/PEEKVerbunde
gegenüber dem unbehandelten Zustand verdoppelt werden. Weitere Untersuchungen an den Prozessparametern ergaben für DC01-CF/PEEKVerbunde,
dass folgende Einstellungen zu einer weiteren Steigerung der
Zugscherfestigkeit auf 19 MPa führen:
• Eine Starttemperatur des Anpresstempels von 370 °C.
• Eine Haltezeit von 7 Minuten.
• Eine Abkühlrate von 6 °C/min.
Für AlMg3-CF/PA66 zeigte sich, dass eine Anpresstemperatur von 10 °C zu einer
Zugscherfestigkeit von 14,5 MPa führt. Diese beiden Zugscherfestigkeiten sind
lediglich 10 – 15 % geringer als die unter optimalen Bedingungen hergestellten
Klebeverbindungen.
Erste Untersuchungen zeigen, dass bei galvanischer Korrosion von Metall/FKVVerbunden
eine schnelle Abnahme der Zugscherfestigkeit erfolgt. Hierfür wurden die
Proben drei Wochen in Wasser gelagert. Beim direkten Kontakt zwischen
Kohlenstofffaser und Aluminium erklärt sich dies durch Korrosion in der Fügezone.
Dabei sinken die Zugscherfestigkeiten der Proben bis auf 5 MPa. Bei Proben mit
einer Glasfaserlage als Isolationsschicht zeigen sich keine Korrosionsprodukte und
die Zugscherfestigkeit nimmt um 30 % bis auf 8 – 9 MPa ab.
Bei in Salzwasser gelagerten Proben ist die galvanische Korrosion deutlich stärker
ausgeprägt. Bereits nach einer Woche besitzen die acetongereinigten Proben mit
zusätzlichem Polymer lediglich eine Restzugscherfestigkeit von 3 bis 4 MPa. Die
korundgestrahlten Proben zeigen Korrosionsprodukte am Rande der Fügezone und
in der Fügezone, weisen aber dennoch eine Zugscherfestigkeit von ca. 10 MPa auf.
Die glasfaserverstärkten Proben zeigen weder Korrosionsprodukte noch eine
Abnahme der Zugscherfestigkeit. Dynamisch thermografische Analysen wurden in verschiedenen Umgebungsgasen
durchgeführt, um die Zersetzungstemperatur des faserverstärkten Polymers zu
bestimmen. Im Falle von CF/PA66 führte dies nicht zu einer Vergrößerung des
Prozessfensters, da die Zersetzung hauptsächlich thermisch und nicht thermooxidativ
ist. Die festgestellte Zersetzungstemperatur von CF/PEEK in Luft betrug
550 °C. Die Vergrößerung des Prozessfensters ist für CF/PA66 gering und zeigte
auch keinen Anstieg in der Zugscherfestigkeit nach dem Schweißen in Stickstoff.
Trotzdem hat das Induktionsschweißen unter Schutzgas ein großes Potential für
gesättigte Kohlenwasserstoffe wie z.B. glasfaserverstärktes Polypropylen. Hier wurde
die Zersetzungstemperatur von 230 °C in Luft auf 390 °C in Stickstoff erhöht.
Es wurde ein Demonstrator bestehend aus einem Aluminium-Profil und einer
CF/PA66-Platte hergestellt, womit gezeigt werden konnte, dass die erworbenen
Kenntnisse auch für die industrielle Anwendung umsetzbar sind. Mittels analytischer
Modelle und FE-Berechnungen wurde die induktive Erwärmung erfolgreich
nachgebildet.
Sewn net-shape preform based composite manufacturing technology is widely
accepted in combination with liquid composite molding technologies for the
manufacturing of fiber reinforced polymer composites. The development of threedimensional
dry fibrous reinforcement structures containing desired fiber orientation
and volume fraction before the resin infusion is based on the predefined preforming
processes. Various preform manufacturing aspects influence the overall composite
manufacturing processes. Sewing technology used for the preform manufacturing
has number of challenges to overcome which includes consistency in preform quality,
composite quality, and composite mechanical properties.
Experimental studies are undertaken to investigate the influence of various sewing
parameters on the preform manufacturing processes, preform quality, and the fiber
reinforced polymer composite quality and properties. Sewing thread, sewing machine
parameters, shortcomings of sewing process, and remedies are explained according
to their importance during preforming and liquid composite molding. The stitches and
fiber free zone in the form of ellipse that are generated in the thickness direction were
investigated by evaluating the laminate micrographs. Correlation between ellipse
formation phenomenon, sewing thread, and sewing machine parameters is
established. A statistical tool, analysis of variance, was used to emphasize the major
preform processing factors influencing the preform imperfections.
For assessing the preform quality, the observations of sewing thread requirements
for preform and structural sewing were well documented during the experimental
studies and explained according to their significance in the composite processing.
Furthermore, selection criteria for sewing thread according to end application are
discussed in detail. Investigations on polyester sewing thread as a high speed
preform manufacturing element are also performed. Applicability of polyester sewing
thread for the preform sewing and challenges to be overcome for its extensive
utilization in the composite components are explained. Apart from this, influence of
physical structure of sewing thread on the laminate quality and properties are
explained and relationship between them is discussed in brief. Furthermore,
challenges caused due to applied spin-finishes and sizing and remedies for the same
are discussed. Sewing threads made of high performance fibers that are available in the market,
e.g., carbon, glass, and Zylon are studied for effect of thread material on through-thethickness
laminate properties. Threads made up of carbon or glass fibers are very
rigid and produces number of defects, which is a major cause of concern. Optimized
sewing procedure has been implemented to minimize the in-plane and through-thethickness
imperfections and to improve mechanical properties and surface
characteristics of composite laminate.
Preform sewing process and final ready to impregnate preforms were analyzed for
quality appearance. The sewing defects and their influence on composite structure
are monitored. Preform compressibility before and after the sewing operations are
intensively studied and correlation with sewing parameters is developed. Influence of
sewing process parameters on the warpage and change in preform area weight are
also explained in detail. Results of analytical experiments can help to improve further
exploitation of sewn preforms for composite manufacturing and overall preform and
laminate quality.
For almost thirty years bast fibers such as flax, hemp, kenaf, sisal and jute have been
used as reinforcing material in the molding process of both thermoplastic and thermoset
matrices. The main application areas of these natural fiber reinforced composites
in Europe are limited almost exclusively to the passenger car range. Typical
components are door panels, rear parcel shelves, instrument boards and trunk linings.
Natural fiber reinforced composites have become prevalent due to their good
mechanical properties and their low production costs. The main advantage in applying
natural fibers as reinforcement in composite materials is the price. Nowadays this
argument becomes more and more important due to the scarcity of synthetic raw materials
and consequently, their rising prices. Additionally the low density (approx.
1.5 g/cm³) of natural fibers confers them a very good lightweight potential. Other advantageous
features of natural fiber composites include very good processing and
acoustic properties. Further benefits such as good life cycle assessment and easier
processability compared to glass fiber material should also be taken into account.
Disadvantages of natural fibers are unevenness of the fiber quality and varying fiber
characteristics due to differences in soil, climate and fiber separation and their low
heat resistance (at temperatures exceeding 220 °C, some fiber components start
thermal degradation). Another disadvantage of natural fiber reinforced materials is
that with some matrices (mostly thermoplastic polymers) a sufficient impregnation of
the fibers can only be achieved if the fiber content in the composite is kept low, usually
below 50 wt.-%. Under these conditions the best performance of the natural fiber
reinforcement can not be realized. Another disadvantage of non-impregnated thermoplastic
prepregs is the long processing cycle times, which result from heating up the enclosed air in the prepreg during the process. Alternatively pure natural fiber
based non-woven fabrics are impregnated with thermoset systems. Due to the relatively
simple handling compared to alternative procedures, the thermoforming of thermoset
bonded prepregs is a very promising method for manufacturing natural fiber
reinforced components.
In this work, a novel general concept for natural fiber reinforced composites with a
natural fiber content of approx. 80 wt.-% and a thermoset matrix is developed. A suitable
material combination as well as an optimal process execution that help to meet the technical requirements for the natural fiber reinforced composites will be demonstrated.
Hemp and kenaf have been chosen as reinforcement fibers. In this work it is shown
that hemp and kenaf can be used as successful reinforcement alternatives to the
more established flax fibers in composite materials. Short flax fibers, which are commonly
used as reinforcement in composites (approx. 67 % for the German automotive
applications), are the “waste” of the long fiber production and their availability
and price strongly depend on the demand of the long fibers from the textile industry
and therefore their cost can strongly fluctuate, as it has been demonstrated in the
past few years. In contrast fiber plants such as hemp and kenaf are especifically cultivated
for technical applications and their availability and price is much more stable.
For their application a profound knowledge of the structural and mechanical properties
of the fibers is indispensable. In this work single filament tensile tests on these
two types of natural fibers are carried out. The cross-section area of both fibers, necessary
for the calculation of the tensile properties, was intensively studied using light
microscopy and Scanning Electron Microscopy (SEM) analysis. Because the occurrence
of flaws within the fiber is random in nature, tensile strength data of these fibers
was statistically analyzed using the Weibull distribution. The strengths were estimated
by means of Weibull statistics and then were compared to experimentally
measured strengths.
For a better handling of the material, both kenaf and hemp fibers were manufactured
to needle punched fiber mats. For the impregnation of the natural fiber mats, a Foulard-
process with the thermoset matrix as an aqueous solution was employed. The
reproducibility of this impregnation process was examined. Different matrix Systems with different chemical compositions were applied on the needle punched fiber mats.
The impregnated prepregs were heated and consolidated to components in a onestep-
process. A big advantage of this procedure is the short cycle times, since no
additional pre-heating process is required, in contrast to thermoplastic bonded prepregs.
Additionally, a parameter study of the mechanical properties of the composites
was performed. The best matrix system satisfying the work conditions and properties
of the composites was chosen to carry out the next working step, namely optimization of the compression molding process for the thermoset bonded natural fiber prepregs.
Apart from the material composition of prepregs, general processing parameters
such as temperature, time and pressure play a decisive role for the quality of structures
made of natural fiber reinforced polymers. The impregnated prepregs were
consolidated in a one-step-process to components. A systematic parameter study of
the influence of the relevant process parameters on the characteristics of manufactured
components was performed. Mould temperatures over 200 °C lead to thermal
degradation of the fibers. This temperature should not be exceeded when working
with natural fibers. Furthermore, the composites clearly display a dependence on the
processing pressure. The flexural properties increase with increasing manufacturing
pressures between 15 and 60 bar, reaching a maximum at 60 bar. At higher pressures
(80 to 200 bar) a decrease of the flexural properties is demonstrated. SEM images
of the fracture surface of the composites show that the decrease of the mechanical
properties is related to structural damage of the fiber.
A new technology allowing pressing under vacuum conditions was developed and tested. The press is equipped with a vacuum chamber and achieves very short cycle
processing times (up to less than one minute during the compression molding). The
aspiration connections of the vacuum chamber ensure that the residual moisture and
the condensation products of the matrix chemical reaction could be directly evacuated.
This type of press ensures also very safe processing and working conditions.
The properties of the components fulfill the technical specifications for natural fiber
reinforced polymers for use in the car interior. This versatile composite material is not
only limited to automotive applications, but may also be used for product manufacturing
in other industries. This work shows that the process parameters can be optimized
to fit a particular application.