Universität Stuttgart

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Author: search.filters.author.Middendorf, Peter

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    Data processing, analysis, and evaluation methods for co-design of coreless filament-wound building systems
    (2023) Gil Pérez, Marta; Mindermann, Pascal; Zechmeister, Christoph; Forster, David; Guo, Yanan; Hügle, Sebastian; Kannenberg, Fabian; Balangé, Laura; Schwieger, Volker; Middendorf, Peter; Bischoff, Manfred; Menges, Achim; Gresser, Götz T.; Knippers, Jan
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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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    Method of manufacturing structural, optically transparent glass fiber-reinforced polymers (tGFRP) using infusion techniques with epoxy resin systems and E-glass fabrics
    (2023) Heudorfer, Klaus; Bauer, Johannes; Caydamli, Yavuz; Gompf, Bruno; Take, Jens; Buchmeiser, Michael R.; Middendorf, Peter
    Recently, fiber-reinforced, epoxy-based, optically transparent composites were successfully produced using resin transfer molding (RTM) techniques. Generally, the production of structural, optically transparent composites is challenging since it requires the combination of a very smooth mold surface with a sufficient control of resin flow that leads to no visible voids. Furthermore, it requires a minimum deviation of the refractive indices (RIs) of the matrix polymer and the reinforcement fibers. Here, a new mold design is described and three plates of optically transparent glass fiber-reinforced polymers (tGFRP) with reproducible properties as well as high fiber volume fractions were produced using the RTM process and in situ polymerization of an epoxy resin system enclosing E-glass fiber textiles. Their mechanical (flexural), microstructural (fiber volume fraction, surface roughness, etc.), thermal (DSC, TGA, etc.), and optical (dispersion curves of glass fibers and polymer as well as transmission over visible spectra curves of the tGFRP at varying tempering states) properties were evaluated. The research showed improved surface quality and good transmission data for samples manufactured by a new Optical-RTM setup compared to a standard RTM mold. The maximum transmission was reported to be ≈74%. In addition, no detectable voids were found in these samples. Furthermore, a flexural modulus of 23.49 ± 0.64 GPa was achieved for the Optical-RTM samples having a fiber volume fraction of ≈42%.
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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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    Structural optimization through biomimetic-inspired material-specific application of plant-based natural fiber-reinforced polymer composites (NFRP) for future sustainable lightweight architecture
    (2020) Sippach, Timo; Dahy, Hanaa; Uhlig, Kai; Grisin, Benjamin; Carosella, Stefan; Middendorf, Peter
    Under normal conditions, the cross-sections of reinforced concrete in classic skeleton construction systems are often only partially loaded. This contributes to non-sustainable construction solutions due to an excess of material use. Novel cross-disciplinary workflows linking architects, engineers, material scientists and manufacturers could offer alternative means for more sustainable architectural applications with extra lightweight solutions. Through material-specific use of plant-based Natural Fiber-Reinforced Polymer Composites (NFRP), also named Biocomposites, a high-performance lightweight structure with topology optimized cross-sections has been here developed. The closed life cycle of NFRPs promotes sustainability in construction through energy recovery of the quickly generative biomass-based materials. The cooperative design resulted in a development that were verified through a 1:10 demonstrator, whose fibrous morphology was defined by biomimetically-inspired orthotropic tectonics, generated with by the fiber path optimization software tools, namely EdoStructure and EdoPath in combination with the appliance of the digital additive manufacturing technique: Tailored Fiber Placement (TFP).
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    Dry fibre placement : the influence of process parameters on mechanical laminate properties and infusion behaviour
    (2021) Grisin, Benjamin; Carosella, Stefan; Middendorf, Peter
    Within the dry fibre placement (DFP) process, spread and pre-bindered carbon fibre rovings are automatically processed into dry textile preforms using 2-D and 3-D laying systems. The aim was to automate existing hand lay-up processes, reducing the complexity, increasing robustness, and facilitating the handling of the DFP technology. Process reliability, low waste rates, and flexible production are demonstrated. In this publication, the influences of the process parameters, 2 mm wide gaps and the percentage of 90° plies in the laminate, are investigated with regard to the mechanical properties, the permeability, and the infusion times in the preform z-direction (thickness). The effects on stiffness and strength are compared for several use cases. An approach to determine the infusion times as a function of the laminate thickness, the ply structure, and 2 mm wide gaps is demonstrated and analysed using vacuum-assisted process (VAP) infusion tests. The investigations are performed with carbon fibre tows (24 k), a reactive epoxy-based binder system, and a thermoset infusion resin system.
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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.