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Nichtlineare Versagensanalyse von dünnwandigen Faser-Kunststoff-Verbund-Bauteilen unter besonderer Berücksichtigung von out-of-plane Rovingwelligkeiten (2014)

Maik Ziermaier

Bei der Herstellung hochbelasteter Strukturbauteile aus Faser-Kunststoff-Verbund (FKV) wird verbreitet auf textile Halbzeuge wie Gewebe oder vernähte Biaxial-Gelege zurückgegrif-fen. Diese Halbzeuge zeigen im Verbund mit Kunststoffen periodische out-of-plane Roving-welligkeiten. Die Größe der Welligkeiten hängt unter anderem von den Fertigungsparametern der trockenen Halbzeuge ab. Durch das Verständnis der effektiven Kausalitäten zwischen Rovingwelligkeiten und mechanischem Verhalten soll eine bessere rechnerische Abschätzung der Materialkennwerte erzielt werden. In dieser Arbeit wurde unter anderem der Einfluss der Welligkeitsparameter Amplitude und Wellenlänge auf die faserparallelen Kennwerte an unidirektional verstärkten Proben unter-sucht. Dafür wurden gezielt unterschiedliche Amplituden und Wellenlängen mit Hilfe von unidirektionalen Geweben in die Probekörper eingebracht. Der Einfluss der Rovingwelligkei-ten auf die Steifigkeiten war kleiner als auf die Festigkeiten. Bei Letzten war zu beobachten, dass die Druckfestigkeiten mehr von den Ondulationen beeinflusst wurden als die Zugfestig-keiten. Außerdem wurden die Welligkeiten und die mechanischen Kennwerte von textilen FKV-Geweben und -Gelegen bestimmt. Bei der Auswahl der untersuchten Halbzeuge war ein wichtiges Kriterium, dass diese auch in der Praxis Anwendung finden. Im nächsten Schritt wurde ein vereinfachtes Finite-Elemente-Welligkeitsmodell entwickelt, welches es ermöglicht, die faserparallelen Kennwerte ohne zeit- und kostenintensive Materi-alversuche zu bestimmen. Speziell für Gewebe-Materialien mit großen Rovingwelligkeiten ist dieses Modell in der Lage, deutlich bessere Abschätzungen als vorhandene Methoden zu ge-ben. Weiterhin wurde auf dieser Basis ein Regressionsmodell für unidirektional verstärkte Materialien abgeleitet, welches auch dem Konstrukteur ohne Erfahrungen im Umgang mit Finiten-Elemente-Programmen die Anwendung des Welligkeitsmodells ermöglicht. Der Vorteil des entwickelten Welligkeitsmodells wurde in einem dreistufigen Validierungs-programm nachgewiesen. Dieses beinhaltet den Übergang auf multidirektionale Laminate sowie komplexere Bauteilgeometrien. Die verbesserte Prognose mit Hilfe des Welligkeitsmo-dells zeigte sich vor allem bei Materialien mit großen Rovingwelligkeiten und bei Bauteilen die aufgrund eines Faserbruchs durch eine Druckbelastung versagten.

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

Ruijuan Zhou

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.

Evaluation of whole-body vibrations and improvement of the driver’s seat performance on a compact wheel loader (2016)

Xiaojing Zhao

Whole-body vibrations (WBV) have adverse effects on ride comfort and human health. Suspension seats have an important influence on the WBV severity. In this study, WBV were measured on a medium-sized compact wheel loader (CWL) in its typical operations. The effect of short-term exposure to the WBV on the ride comfort was evaluated according to ISO 2631-1:1985 and ISO 2631-1:1997. ISO 2631-1:1997 and ISO 2631-5:2004 were adopted to evaluate the effect of long-term exposure to the WBV on the human health. Reasons for the different evaluation results obtained according to ISO 2631-1:1997 and ISO 2631-5:2004 were explained in this study. The WBV measurements were carried out in cases where the driver wore a lap belt or a four-point seat harness and in the case where the driver did not wear any safety belt. The seat effective amplitude transmissibility (SEAT) and the seat transmissibility in the frequency domain in these three cases were analyzed to investigate the effect of a safety belt on the seat transmissibility. Seat tests were performed on a multi-axis shaking table in laboratory to study the dynamic behavior of a suspension seat under the vibration excitations measured on the CWL. The WBV intensity was reduced by optimizing the vertical and the longitudinal seat suspension systems with the help of computational simulations. For the optimization multi-body models of the seat-dummy system in the laboratory seat tests and the seat-driver system in the field vibration measurements were built and validated.

Fracture of Nanoparticle Filled Polymer Composites (2007)

Hui Zhang

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.

Continuum Mechanical Modeling of Dry Granular Systems: From Dilute Flow to Solid-Like Behavior (2014)

Clément Zemerli

In this thesis, we develop a granular hydrodynamic model which covers the three principal regimes observed in granular systems, i.e. the dilute flow, the dense flow and the solid-like regime. We start from a kinetic model valid at low density and extend its validity to the granular solid-like behavior. Analytical and numerical results show that this model reproduces a lot of complex phenomena like for instance slow viscoplastic motion, critical states and the pressure dip in sand piles. Finally we formulate a 1D version of the full model and develop a numerical method to solve it. We present two numerical examples, a filling simulation and the flow on an inclined plane where the three regimes are included.

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

Tobias Zangmeister

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.

Modellierung des Phasengleichgewichts bei der Quellung von ionischen und nichtionischen Hydrogelen in wässrigen Lösungen (2007)

Orlov Yury

Die vorliegende Arbeit hat zum Ziel, Methoden zur Beschreibung der Quellung von (nichtionischen und ionischen) Hydrogelen in wässrigen Lösungen zu entwickeln und zu erproben. Die Modelle setzen sich aus einem Beitrag, der die Gibbssche Energie von Flüssigkeiten beschreibt und aus einem Beitrag für die Helmholtz-Energie des Netzwerks zusammen. Die Beschreibung der elastischen Eigenschaften des Netzwerks beruht auf der Phantom-Netzwerk-Theorie. Die Gibbssche Exzessenergie der Flüssigkeiten wurde mit verschiedenen Modellen, abhängig vom Typ der untersuchten Systeme (wässrig/salzhaltige Lösung oder wässrig/organische Lösung), beschrieben. Bei der Modellierung wurden der Einfluss der Zusammensetzung des Netzwerks (z.B. Konzentration von Vernetzer und/bzw. von ionischem Komonomeren) und der Einfluss der Zugabe von weiteren Komponenten in der das Gel umgebenden wässrigen Lösung auf den Quellungsgrad behandelt. Als weitere Komponenten wurden einerseits anorganische Salze (Natriumchlorid,Dinatriumhydrogenphosphat) und andererseits organische Lösungsmittel (Ethanol, Aceton, Essigsäure, 1-Butanol,Methylisobutylketon) behandelt. Zur Modellierung des Quellverhaltens von nichtionischen IPAAm-Gelen in NaCl bzw. Na2HPO4-haltigen Lösungen wurde das VERS-Modell in Kombination mit der Phanom-Netzwerk-Theorie verwendet. Bei der Erweiterung dieser Modelle auf ionische Gele wurde eine gute Beschreibung nur dann erzielt, wenn sowohl das Dissoziationsgleichgewicht des ionischen Komonomeren Natriummethacrylat als auch eine Korrektur im Phantom-Netzwerk-Modell berücksichtigt wurden. Wenn beide Korrekturen allein aus wenigen experimentellen Daten für die Quellung ionischer Gele in wässrigen Lösungen von NaCl bestimmt wurden, gelingen nicht nur eine gute Korrelation für den Quellungsgrad, sondern auch zuverlässige Vorhersagen (sowohl Erweiterung auf andere (IPAAm/NaMA)Gele als auch bei Verwendung von Na2HPO4 anstelle von NaCl). Die Modellierung der Einflüsse der untersuchten Salze (NaCl bzw. Na2HPO4) auf das Quellverhalten sowohl nichtionischer VP-Gele als auch ionischer (VP/NaMA)Gele erfolgt auch in Kombination des VERS-Modells mit der Phanom-Netzwerk-Theorie. Die vorgeschlagene Methode liefert sowohl für nichtionische VP-Gele als auch für ionische (VP/NaMA)Gele in wässrigen salzhaltigen Lösungen eine gute Übereinstimmung zwischen Experiment und Rechnung. Die Quellungsgleichgewichte nichtionischer IPAAm-Gele in Mischungen aus Wasser und einem organischen Lösungsmittel (Ethanol, Aceton, Essigsäure, Butanol, Methylisobutylketon) lassen sich mit Hilfe der Kombination aus dem UNIQUAC-Modell und einem Free-Volume-Beitrag mit der Phantom-Netwerk-Theorie beschreiben. Bei der Erweiterung des Modells auf ionische (IPAAm/NaMA)Gele wurde der Einfluss der Konzentration des organischen Lösungsmittels auf die Dissoziationskonstante von Natriummethacrylat berücksichtigt. Ähnlich zur Modellierung der Quellung im System (IPAAm/NaMA)Gel-Wasser-Salz wurden die Abweichungen von der Phantom-Netzwerk-Theorie mit Hilfe eines empirischen Faktors betrachtet. Mit diesen Modellvorstellungen gelingt sowohl eine gute Korrelation als auch zuverlässige Vorhersage des Quellungsgrades von ionischen Gelen in wässrig/organischen Lösungen. Bei den theoretischen Studien zur Quellung von Gelen auf Basis von Vinylpyrrolidon in wässrig/organischen Lösungsmittelgemischen erwies sich, dass eine Kombination aus dem UNIQUAC-Modell (jedoch ohne kombinatorischen Beitrag) und der Phantom-Netzwerk-Theorie die besten Ergebnisse bei der Beschreibung des Quellverhaltens der nichtionischen VP-Gele liefert. Bei der Erweiterung des Modells auf ionische (VP/NaMA)Gele wurde eine Abhängigkeit des Dissoziationsgrades von Natriummethacrylat von der Ethanol-, bzw. der Acetonkonzentration berücksichtigt.

Polymerwerkstoff-Direktverschraubung: Einsatz von Experiment und Simulation zur Analyse des Vorspannkraftverlaufs (2012)

Yuan Yao

Für die lösbare Verbindung von Bauteilen aus Polymerwerkstoffen sind Direktver-schraubungen das kostengünstigen Fügeverfahren. Nachteilig sind allerdings die durch das viskoelastische Verhalten der Polymerwerkstoffe bedingte zeit- und tem-peraturabhängige Abnahme der Vorspannkraft und deren weitere Beeinflussung durch den Wärmeausdehnungsunterschied von Polymerwerkstoff und Metall. Ziel der Arbeit ist es, durch den gekoppelten Einsatz von Experiment und FE-Simulation den Einfluss von Zeit und Temperatur auf die Vorspannkraft bei Direktverschraubungen in Polymerwerkstoffen zu beschreiben. Der Tubus aus Polymerwerkstoff, der auf-grund der Faserorientierung inhomogen und anisotrop ist, wird ortsaufgelöst unter-sucht, und hierauf aufbauend werden die lokalen Werkstoffparameter für die FE-Analyse ermittelt. Die FE-Analyse der Direktverschraubung selbst wird experimentell anhand des Vorspannkraftverlaufs sowie der Oberflächendeformation überprüft. Beim betrachteten Verbindungssystem ist deutlich zu beobachten, dass der Vor-spannkraftabbau hauptsächlich im Bereich der Entlastungsbohrung bis zu einer be-stimmten Einschraubtiefe stattfindet.

Sound Simulation and Visualization in virtual Manufacturing Systems (2013)

Xiang Yang

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.

Characterization, Modeling and Prediction of the Creep Resistance of Polymer Nanocomposites (2007)

Jinglei Yang

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.

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