Universität Stuttgart

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Author: search.filters.author.Middendorf, PeterSubject: search.filters.subject.620

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Now showing 1 - 8 of 8
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    Rheology, dispersion, and cure kinetics of epoxy filled with amine‐ and non‐functionalized reduced graphene oxide for composite manufacturing
    (2021) Ackermann, Annika C.; Carosella, Stefan; Rettenmayr, Markus; Fox, Bronwyn L.; Middendorf, Peter
    This study evaluates the effect of plasma surface functionalization of reduced graphene oxide particles on the processing characteristics and homogeneity of dispersion of a bisphenol A‐(epichlorhydrin) epoxy matrix and amine‐based hardener with varying weight fractions from 0.00 to 1.50 wt%. It was observed that amine‐functionalized reduced graphene oxide leads to a more drastic viscosity increase of up to 18‐fold of the uncured suspensions and that its presence influences the conversion rates of the curing reaction. Optical microscopy of thin sections and transmission electron microscopy analysis showed that a more homogeneous dispersion of the particles could be achieved especially at higher weight fractions by using an appropriate surface functionalization. This knowledge can be used to define suitable processing conditions for epoxies with amine‐based hardeners depending on the loading and functionalization of graphene‐related particles.
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    Thermomechanical analysis of thermoplastic mono-material sandwich structures with honeycomb core
    (2024) Latsuzbaya, Temuri; Middendorf, Peter; Voelkle, Dietmar; Weber, Christoph
    The application of fiber-reinforced thermoplastic mono-material sandwich panels has many advantages, such as recyclability, reduction in processing cycle times, integration of additional elements by means of welding, and a great potential for in-line production. The most efficient way to produce a curved thermoplastic sandwich panel is thermoforming, which has several challenges. One of them is to achieve a higher thermal gradient in the panel. On the one hand, the temperature at the skin-core interface must exceed the softening point of the polymer to reach a sufficient bonding degree. On the other hand, the core should not be overheated and overloaded to avoid its collapse. Furthermore, several fiber distortions, such as wrinkles or buckles, can be developed during thermoforming. All these flaws have a negative impact on the mechanical performance of the sandwich structure. The objective of this study is the development of a simulation tool for the thermoforming process, which can replace the time-consuming trial-and-error-based method. Therefore, a coupled thermomechanical model was developed for a novel thermoplastic sandwich structure, which is able to predict the temperature distribution and its influence on the mechanical properties of the panel. Experimental trials were conducted to validate the thermomechanical forming model, which demonstrated a good agreement with numerical results.
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    Mechanical, thermal and electrical properties of epoxy nanocomposites with amine-functionalized reduced graphene oxide via plasma treatment
    (2022) Ackermann, Annika C.; Fischer, Michael; Wick, Alexander; Carosella, Stefan; Fox, Bronwyn L.; Middendorf, Peter
    A suitable functionalization of graphene and its derivatives can further enhance the material properties of nanocomposites. In contrast to chemical functionalization methods that have been extensively researched, functionalization by plasma treatment is relatively unexplored. In this work, we compare the mechanical, thermal and electrical characteristics of an epoxy matrix incorporating loadings from 0.00 to 1.50 wt% of non-functionalized (rGO) and amine-functionalized reduced graphene oxide (frGO) for which the functionalization is realized by plasma processing. No significant difference between the rGO- and frGO-including nanocomposites was observed with respect to the stiffness, strength, specific heat capacity, coefficient of thermal expansion and electrical conductivity. Yet, the composites with 1.50 wt% frGO (rGO) exhibited a thermal conductivity that was 27% (20%) higher than the neat polymer due to the enhanced interface, which enabled a better transfer of heat. In addition, a considerable increase in the specific heat capacity and thermal conductivity was established with rising temperatures. This information will facilitate the choice of materials depending on the loading and functionalization of graphene materials for composite applications with an epoxy matrix.
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    Topological design using multivariate laminate stackings for tailored fiber placement
    (2022) Schwingel, Johannes; Middendorf, Peter
    In structural optimization of fiber-reinforced composites, unidirectional design material is normally applied due to its high anisotropy character. Using volume constraints to save weight often results in truss-like frameworks with defined tension and shear loaded areas, while the latter one is often neglected or improperly described with unidirectional material. Here, a method is proposed to extend the material design space to a continuously variable domain between UD and cross-ply laminates which is simultaneously optimized with topology. This allowed us to increase the design improvement and create smoother material distributions which is more beneficial for fiber placement technologies such as Tailored Fiber Placement. Parameter studies have been performed to investigate the effects of variable shear properties and different design spaces on the structural performance of the final designs, concluding with classical benchmarks to validate the proposed method.
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    Improving the thermal properties of aircraft cabin interiors with the integration of vacuum insulation panels
    (2022) Latsuzbaya, Vakhtang; Middendorf, Peter; Völkle, Dietmar; Weber, Christoph
    Commercial aircrafts require insulation to protect passengers in the cabin from thermal and acoustic loads. The conventional insulation in aircrafts consists of blankets made from layers of glass wool wrapped in foil that keeps the glass wool from being adversely affected by the environment. There is a potential to improve the thermal and acoustic properties of the cabin by replacing the interior panels with conventional secondary insulation by new panels combined with vacuum insulation panels (VIP). This article is focusing on the study of the VIP integration into the interior panels. First, the new structure solutions are defined on the basis of a requirement analysis for interior panels and VIP and theoretical analysis. Second, the manufacturing feasibility study for the new solutions is performed. The results show that the new structures can be manufactured. Third, the thermal properties of the new structure solutions are measured. The test results show a decrease of thermal conductivity of the new panels by a factor of 3-6 compared to the conventional solutions. Finally, the impact of the hot molding press on the vacuum maintaining inside the VIP is investigated. The trials demonstrate that the high barrier films can withstand high-temperature and pressure conditions and that the thermal conductivity of the test specimens didn’t worsen after 1 year.
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    Experimental and numerical studies of process variabilities in biaxial carbon fiber braids
    (2020) Czichos, Ruben; Bareiro, Oscar; Pickett, Anthony K.; Middendorf, Peter; Gries, Thomas
    This paper investigates the manufacture of biaxial carbon fiber braids and the influence that different machine settings have on variability of the textile architecture produced. In parallel, numerical simulations of the braiding process with these different machine settings have been conducted. For these studies yarn tension and process speed are varied to generate cylindrical biaxial braids with an average braid angle of±45°. The overall preform quality is characterized by means of variability in braid angle, yarn width, cover factor and fiber damage, using a variety of experimental techniques. Furthermore, from the final infused composite variations in yarn cross-section dimensions have been measured. A method is presented to transfer braid process simulation results to a detailed three dimensional finite element model of the architecture using a technique based on thermal expansion and compaction simulation. This method also allows the possibility to introduce experimentally observed variability in yarn cross-section dimensions. Such a model provides a valuable starting point for mesoscopic infusion or mechanical analysis of the textile composite. A comparison between experimental and numerical results shows that the process simulation can well reproduce the real braid angles in terms of mean value and scatter under different machine configurations and that the meso-scale textile model gives a good reproduction of the true textile architecture.
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    Vacuum chamber infusion for fiber-reinforced composites
    (2024) Grisin, Benjamin; Carosella, Stefan; Middendorf, Peter
    A new approach to an automatable fiber impregnation and consolidation process for the manufacturing of fiber-reinforced composite parts is presented in this article. Therefore, a vacuum chamber sealing machine classically used in food packaging is modified for this approach-Vacuum Chamber Infusion (VCI). Dry fiber placement (DFP) preforms, made from 30 k carbon fiber tape, with different layer amounts and fiber orientations, are infused with the VCI and with the state-of-the-art process-Vacuum Assisted Process (VAP)-as the reference. VCI uses a closed system that is evacuated once, while VAP uses a permanently evacuated open system. Since process management greatly influences material properties, the mechanical properties, void content, and fiber volume fraction (FVF) are analyzed. In addition, the study aims to identify how the complexity of a resin infusion process can be reduced, the automation potential can be increased, and the number of consumables can be reduced. Comparable material characteristics and a reduction in consumables, setup complexity, and manufacturing time by a factor of four could be approved for VCI. A void content of less than 2% is measured for both processes and an FVF of 39% for VCI and 45% for VAP is achieved.