Additive manufacturing of carbon fiber reinforced thermoplastic polymer composites

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Simulation of the thermoforming process of UD fiber-reinforced thermoplastic tape laminates

Einer der entscheidendsten Prozessschritte bei der Herstellung von kontinuierlich faserverstärkten Kunstoffen ist die Umformung von zweidimensionalen Halbzeugen in komplexe Geometrien. Hierbei spielt das nicht-isotherme Stempelumformverfahren von unidirektional (UD) faserverstärkten thermoplastischen Tape-Laminaten aufgrund geringer Zykluszeiten, Materialeffizienz und Recyclingfähigkeit insbesondere in der Automobilindustrie eine immer größer werdende Rolle. Durch die Umformsimulation kann die Herstellbarkeit einer bestimmten Geometrie virtuell abgesichert und hierfür notwendige Prozessparameter bestimmt werden, wodurch eine zeit- und kostenintensive "Trial and Error" Prozessauslegung vermieden werden kann. In dieser Arbeit werden initial anhand einer experimentellen Umformstudie und Materialcharakterisierungen die Anforderungen an die Umformsimulation von teilkristallinen thermoplastischen UD-Tapes abgeleitet. Hierbei zeigt sich, dass ein thermomechanischer Ansatz, unter Berücksichtigung der raten- und temperaturabhängigen Materialeigenschaften, als auch der Kristallisationskinetik, erstrebenswert ist. Darauf aufbauend wird mit der kommerziellen Finite Elemente (FE) Software Abaqus, in Kombination mit mehreren sogenannten User-Subroutinen, ein entsprechender Simulationsansatz entwickelt. Zunächst werden hypo- und hyperelastischen Materialmodellierungsansätze untersucht, sowie ratenabhängige intra-ply Materialmodellierungsansätze vorgestellt. Dabei liegt ein Schwerpunkt auf dem ratenabhängigen Biegeverhalten, da diese Materialeigenschaft üblicherweise nicht berücksichtigt wird, weshalb hierfür hypoviskoelastische Modellierungsansätze auf Basis eines nichtlinearen Voigt-Kelvin- sowie eines nichtlinearen generalisierten Maxwell-Ansatzes vorgestellt werden. Unter Anwendung dieser Ansätze zeigt sich im Vergleich mit experimentellen Umformergebnissen eine gute Übereinstimmung. Darüber hinaus wird ein Einfluss der ratenabhängigen Biegeeigenschaften auf die Vorhersage der Faltenbildung beobachtet. Im nächsten Schritt wird der Ansatz um eine "Discrete Kirchhoff Triangle" (DKT) Schalenformulierung erweitert, welche in Abaqus als User-Element implementiert ist. Dies ermöglicht im Gegensatz zu dem im vorherigen Kapitel vorgestellten Ansatz die hyperviskoelastische Modellierung des Membran- und des Biegeverhaltens. Darauf aufbauend werden ein nichtlinearer Voigt-Kelvin-, sowie ein nichtlinearer generalisierter Maxwell-Ansatz, welcher auf einer multiplikativen Zerlegung des Deformationsgradienten basiert, vorgestellt. In der Umformsimulation zeigt sich eine gute Übereinstimmung mit experimentellen Umformergebnissen. Darüber hinaus wird beobachtet, dass ein nichtlinearer Voigt-Kelvin-Ansatz für die Modellierung des Membranverhaltens ausreichend ist. Neben intra-ply werden auch inter-ply Modellierungsansätze untersucht. Hierfür wird ein erweiterter Ansatz vorgestellt, der neben den üblicherweise berücksichtigten Zustandsgrößen Abgleitgeschwindigkeit und Transversaldruck auch die Relativorientierung zwischen den abgleitenden Schichten berücksichtigt. Bei der Anwendung dieses Ansatzes in der Umformsimulation werden jedoch nur geringe Unterschiede gegenüber einem herkömmlichen Ansatz beobachtet. Der präsentierte Ansatz für die Umformsimulation von thermoplastischen UD-Tapes wird final zu einem gekoppelten thermomechanischen Ansatz erweitert. Die entsprechende thermische Modellierung berücksichtigt Strahlung, Konvektion und Wärmeleitung, sowie die Kristallisationskinetik, wobei das mechanische Verhalten über die Temperatur und die relative Kristallinität an das thermische Verhalten gekoppelt ist. Hiermit wird der Übergang vom schmelzflüssigen zum Festkörperzustand vorhergesagt und in der Modellierung des Umformverhaltens berücksichtigt. Hierdurch wird eine verbesserte Übereinstimmung mit den experimentellen Umformergebnissen erzielt und auch die lokale Temperaturentwicklung akkurat vorhergesagt. Darüber hinaus zeigt sich, dass bei einer ungünstigen Wahl der Prozessparameter eine starke Kristallisation schon während der Umformung auftritt. Da außerdem nur der thermomechanische Ansatz den Einfluss aller relevanten Prozessparameter berücksichtigen kann, wird geschlussfolgert, dass die Berücksichtigung thermischer Effekte sowie der Kristallisationskinetik vorteilhaft für die virtuelle Prozessauslegung nicht-isothermer Stempelumformverfahren mit teilkristallinen Thermoplasten ist

Rice straw fiber polymer composites: thermal and mechanical performance

Rice straw fiber can be considered as important potential reinforcing filler for thermoplastic composite because of its lignocellulosic characteristics. It is thus of practical significance to understand and predict the thermal decomposition process of rice straw fibers. A method proposed by Málek, Šesták, and co-workers was used to investigate and model thermal decomposition process of common natural fibers with detailed analysis on rice straw system. Assuming a global model occurring within the entire degradation of natural fibers with consideration of fiber as one pseudo-component, model can be used to describe both isothermal and non-isothermal degradation process of most selected fibers within acceptable error limits of 3 and 5%, respectively. The parameters of kinetic model were given in this dissertation. The model obtained has practical significance for introducing straw fiber into some engineering plastics with comparatively lower melting temperature. Influences of different rice straw components, and compatibilizers on various properties of rice-straw based polymer composites were also investigated. Rice straw fibers can work well with both VHDPE and RHDPE as reinforcing filler. Also, different components of rice straw had no significant influence on mechanical properties of composites. The PE-g-MA/EPR ratio affected mechanical properties of composites modified by combined compatibilizers. The optimum PE-g-MA/EPR ratio was considered to be 2:1 and 1:1 for PE-g-MA/uEPR and PE-g-MA/EPR-g-MA modified composites, respectively. At the optimum ratio, composites modified by combined compatibilizers showed better strength and impact toughness, and acceptable modulus compared to those modified by either EPR or EPR-g-MA. It was found that 13% weight loss seemed to be the limit for rice straw to maintain its strength in a composite system. High-temperature one-step extrusion was feasible for manufacturing HDPE/nylon-6/rice-straw composites without significant strength loss caused by thermal degradation of fiber. The two-step method failed to exhibit better performance than the one-step method

A computer simulation of stress transfer in carbon nanotube/polymer nanocomposites

The reinforcing efficiency or stress transfer of carbon nanotubes (CNT) on polymers in polymer/CNT composites mainly is controlled by the polymer-CNT interface. Enhancement of polymer-CNT interactions and interfacial crystallisation is as an important way for improvement of the reinforcement experimentally. However, it is not clear about the crystallisation and orientation of polymer chains on the CNT surface, and how the interfacial crystallisation layer affects the failure of the composite. In this work, poly(vinyl alcohol)/CNT nanocomposites was selected as an example and based on the molecular dynamics simulation, the crystallisation process, failure behaviour and stress transfer in poly(vinyl alcohol)/CNT nanocomposites were analysed. The crystallisation temperature of the polymer chains on the CNT surface is slightly higher than the bulk crystallisation temperature. CNT induced crystallisation can be divided into three stages: chain folding, orientating and growing on the CNT surface. A slower crack growth was observed in the interfacial crystallised polymer/CNT systems, compare to relative amorphous systems. The effect of the interfacial crystalline layer on stress transfer is similar as enhanced polymer-CNT interaction systems. The change of the polymer-CNT surficial energy to strain has been used to analyse the interfacial failure and the stress transfer

Simulation of the thermoforming process of UD fiber-reinforced thermoplastic tape laminates

In this work, initially, the requirements on a simulation model of the non-isothermal stamp forming process of unidirectional fiber-reinforced, and thermoplastic tape laminates are investigated experimentally. On this basis, different isothermal as well as a fully coupled thermomechanical simulation model under consideration of the crystallization kinetics are developed. For validation, a complex shaped geometry is simulated and compared to experimental forming results

Additive manufacturing of carbon fiber-reinforced thermoplastic composites

Additive manufacturing, or 3D printing, encompasses manufacturing processes that construct a geometry by depositing or solidifying material only where it is needed in the absence of a mold. The ability to manufacture complex geometries on demand directly from a digital file, as well as the decreasing equipment costs due to increased competition in the market, have resulted in the AM industry experiencing rapid growth in the past decade. Many companies have emerged with novel technologies well suited to improve products and/or save costs in various industries. Until recently, the applications of polymer additive manufacturing have been mainly limited to prototyping. This can be attributed to multiple factors, namely the high cost of the machines and materials, long print times, and anisotropy of printed parts. In addition, the low unit cost and cycle time of competing processes such as injection molding further skew the economics in favor of other processes. The addition of fiber-reinforcement into polymers used in additive manufacturing processes significantly increases the strength of parts, and also allows larger parts to be manufactured. In 2014, large-scale additive manufacturing of fiber-reinforced polymers was pioneered, and has generated significant attention from both academia and industry. Commercial machines that incorporate high throughput extruders on gantry systems are now available. New applications that require high temperature polymers with low coefficients of thermal expansion and high stiffness are being targeted, for example tooling used in the manufacturing of composite components. The state of the art of this new paradigm in additive manufacturing as well as the target applications will be discussed in detail. Many new challenges arise as AM scales and reinforced polymers are incorporated. One of the most notable challenges is the presence of large temperature gradients induced in parts during the manufacturing process, which lead to residual stresses and sometimes detrimental warpage. The current solution to this problem has been to print faster in order to lessen the temperature gradients, however very high extrusion speeds are likely not ideal for achieving optimal material properties. The high shear rates induce further damage to fibers, and entrapped air during the extrusion process may not escape, leading to high void content. Another significant challenge is overcoming the anisotropy in printed parts, which arises due to the stiff reinforcing fibers orienting primarily in the print direction. This complicates the use in demanding applications such as composite tooling, where high stiffness and low CTE are desirable in all directions. In 2014, a group of graduate students at Purdue University was formed to develop a better understanding of large-scale additive manufacturing processes incorporating high temperature and high fiber content polymer composites. The team spent more than one year designing, developing, and optimizing a lab-scale system that offers full control over all processing parameters, and has begun studying the relevant phenomena and developing models to predict the outcome of printing processes. This thesis will summarizes the system development process, printing process, composite tooling applications, as well as the mechanical, structural, and viscoelastic properties of printed materials, making it one of the most comprehensive documents written in large-scale additive manufacturing of fiber-reinforced polymers to date. The properties of 50 weight percent carbon fiber-reinforced PPS, a material of high interest in the field, will be presented in detail. The viscoelastic properties will be measured and discussed in the context of both stress relaxation during the printing process and the required performance metrics of composite tooling. A summary of the major results and recommendations can be found in chapter 7

Simulation of the thermoforming process of UD fiber-reinforced thermoplastic tape laminates

In this work, initially, the requirements on a simulation model of the non-isothermal stamp forming process of unidirectional fiber-reinforced, and thermoplastic tape laminates are investigated experimentally. On this basis, different isothermal as well as a fully coupled thermomechanical simulation model under consideration of the crystallization kinetics are developed. For validation, a complex shaped geometry is simulated and compared to experimental forming results

Thermoplastic

Composite materials often demand a unique combination of properties, including high thermal and oxidative stability, toughness, solvent resistance and low dielectric constant. This book, "Thermoplastic - Composite Materials", is comprised of seven excellent chapters, written for all specialized scientists and engineers dealing with characterization, thermal, mechanical and technical properties, rheological, morphological and microstructure properties and processing design of composite materials

Process–Structure–Properties in Polymer Additive Manufacturing

Additive manufacturing (AM) methods have grown and evolved rapidly in recent years. AM for polymers is an exciting field and has great potential in transformative and translational research in many fields, such as biomedical, aerospace, and even electronics. Current methods for polymer AM include material extrusion, material jetting, vat polymerisation, and powder bed fusion. With the promise of more applications, detailed understanding of AM—from the processability of the feedstock to the relationship between the process–structure–properties of AM parts—has become more critical. More research work is needed in material development to widen the choice of materials for polymer additive manufacturing. Modelling and simulations of the process will allow the prediction of microstructures and mechanical properties of the fabricated parts while complementing the understanding of the physical phenomena that occurs during the AM processes. In this book, state-of-the-art reviews and current research are collated, which focus on the process–structure–properties relationships in polymer additive manufacturing