Key Engineering Materials Vol. 809

Abstract: To predict the nonlinear mechanical behavior of components made of short fiber-reinforced plastics (SFRP) under long term and cyclic loading, coupled process and component simulations are required. The injection molding process leads to locally varying fiber orientations within the component. This varying microstructure [1] significantly influences the viscoelastic and fatigue behavior. The interaction between the microstructure [2] and the nonlinear macroscopic properties is resolved by a coupled fast Fourier transformation and finite element two-scale method (FFT-FEM), where the fiber orientation tensor is obtained by analyzing μCT images or by the corresponding process simulation. The aim of this work is to reduce the numerical costs of such a multiscale method. In a first step, the highly efficient micro-scale solver FeelMath [3,4] using an FFT-based preconditioner is presented. Afterwards, a numerical scheme based on a precomputed database trained with FeelMath simulations on the microscale and a model order reduction algorithm, is discussed. The combination of these ideas reduces the numerical effort, such that the method is applicable for industrial problems. Comparative studies of the fully coupled and reduced model document the high accuracy of this approach. The overall performance of this methodology is demonstrated by three-dimensional, industrial applications.

Abstract: The growing use of composite materials has generated interest in improving and optimising composite manufacturing processes such as Liquid Composite Moulding (LCM). In LCM, dry preforms are placed in a mould and impregnated with the matrix material. The efficiency of filling the moulds can be improved by using Computational Fluid Dynamics (CFD) filling simulations during the design of the mould. As part of an on-going effort to develop a CFD tool for the simulation of LCM processes, a volume averaged energy balance equation has been derived and implemented in a custom OpenFOAM solver. The energy balance is implemented in a custom OpenFOAM solver with and without the pressure terms for comparison with results from RTM experiments. It is found that the pressure terms do not significantly influence the results for LCM processes.

Abstract: Since years, fiber reinforced polymer composite (FRPC) parts made by Liquid composite molding (LCM) make up a significant share of the composite market. In LCM dry reinforcing structure gets impregnated with a resin system. Permeability, a material parameter with key influence on all LCM processes, quantifies the conductance of technical textiles for resin flow. Today, when a numerical filling simulation is applied for process design, large experimental test programs are required for characterization of permeability, as the permeability has to be measured for all textiles processed and also the dependency e.g. on the fiber volume content has to be considered. Together with Math2Market GmbH (M2M), the IVW currently develops a novel simulative-experimental approach (SEA), using experimental tests to calibrate a simulation model for replacing a significant amount of the experimental tests through “virtual” measurements. In a first step the functionality of the simulation and the most appropriate methods for textile modelling were investigated. For this, three routes were followed: At first a micro-computer tomograph (μCT)-scan of a glass fiber non-crimp fabric was fed into GeoDict, the material simulation software developed by M2M. Second, a digital model (DM) of the textile was created by computer modelling of basic structure and subsequent virtual compaction. μCT-model and DM were then used for computational fluid flow simulation which gives the direction-dependent permeability as an output. The DM calibrated by experiments represents the SEA and results in the digital twin. Third route was the experimental permeability measurement to generate reference values. Comparing the results of all three routes allows statements about the functionality of the simulation and accuracy of modelling. The rather deficient correlation between the results of experiments and μCT-model based simulations revealed that segmentation is a remarkable source of error despite the use of recognized methods. Different modeling approaches were followed to build up the digital twin. The best results were achieved with models undergoing a virtual compaction step, which takes various imperfections such as yarn deformation and varying nesting behavior into account. With this 2.25 - 6.75% deviation from the experimental results at an average standard deviation of 21.9 - 61.2% were achieved. Hence, the digital twin shows a better correlation than the μCT-model and high potential for substitution of experiments. Even better results are expected when in a next step a local anisotropic permeability will be allocated to the yarns.

Abstract: The design process of fiber-reinforced plastics (FRP) is a challenging task, especially concerning passenger vehicles in crashworthiness applications where manufacturing limitations and requirements regarding passive safety have to be considered. Numerical optimization can be a helpful tool during the design process, but most available methods are not applicable because analytical sensitivities are not available in crash simulations. The Graph and Heuristic based Topology Optimization (GHT) can be utilized to optimize the topology of cross-sections of crashworthiness structures while fulfilling a wide range of manufacturing constraints, but it has to be extended for composites. Since the topology changes during optimization runs, the stress state changes as well. This demands high predictive capabilities on the material model. This paper presents the necessary adjustments to describe composite profile structures within the GHT method. A commercial material model for LS-Dyna is parameterized and used for the calculation process.

Abstract: This work presents the analysis of the influence of several material input data to the FE results of the forming process of fiber reinforced thermoplastics within a sensitivity study. The *Fabric material model of Abaqus/Explicit is used for the description of the shear, tensile and compression behavior. It is a test-data based model. The bending behavior will be modeled with beam elements. The interaction between input data will be analyzed and its influence onto the FE forming results and FE analysis of material tests like bias-extension-test, compression and bending test with focus on interactions of input data and test procedure itself.

Abstract: Increasing waste streams of carbon fibers (CF) and carbon fiber reinforced plastics (CFRP) lead to increasing need for recycling and to growing amounts of recycled carbon fibers. A main issue in current research for carbon fiber recycling is the reuse of regained fibers. Carbon staple fibers such as recycled fibers hold big potential for mechanical properties of lightweight parts, if used properly. Applying recycled CF (rCF) as milled reinforcement fibers or as nonwoven in carbon fiber reinforced plastic leads to a poor yield of mechanical proper due to low fiber orientation, limitations in fiber volume content or due to short fiber length. The rC staple fiber tape presents a more efficient approach. Recycled carbon fibers are blended with 50 wt. % thermoplastic nylon 6 fibers and processed through a roller card to a sliver, which is a linear fibrous intermediate. The sliver is continuously drawn, formed, heated and consolidated to the thermoplastic rC staple fiber tape. The tape is similar to common carbon fiber tapes or to continuous tows but has different positive properties, such as high fiber orientation, homogeneous blend of fiber and matrix and suitability for deep drawing.

Abstract: Within the project ‘RecyCarb’ a qualified value-added chain for recycled carbon fibres (rCF) to enable their high-quality and sustainable re-use in sophisticated fibre-reinforced composites was established. A team of four industrial partners and two research institutes closed the technological gap between the actual rCF available on the market and the functional re-use as reinforcing elements in high-quality components. Process optimisation, initiation of a reliable scheme of quality assurance and a process integrated quality monitoring were the main aspects of this project. Besides different kinds of carbon fibre waste, different nonwoven processes (web formation and bonding methods) and an online fibre orientation analysis were investigated. This project focuses a variety of several application markets, e.g. sports equipment, medical technologies or automotive, shown by developed demonstrators.

Abstract: A novel B-stage resin system is developed for the „Resin Transfer Pressing“ (RTP) process. This composite manufacturing technique makes use of nonwoven and recycled carbon fibres (rCF) that are oversaturated with a thermosetting resin. The new resin system is based on two different hardeners, allowing to easily impregnate the rCF, storing the semi-finished parts at room temperature and finally curing them via compression molding. It is shown that a commercially available resin system can be tailored to the required needs by the smart combination of state-of-the art, latent hardener systems.The present study focusses on the route for B-staging of the resin system. The viscosity for impregnation and oversaturation was adjusted to be in the range of 5 to 40 Pa·s at 60°C. The viscosity increases to above 50 Pa·s at room temperature, allowing storage and handling of the semi-finished parts and processing them whenever it is required. Choosing a proper processing temperature is important during the B-staging process to avoid any unintended activation of the second hardener, which is responsible for the final curing stage of the system.

Abstract: Carbon fibre reinforced plastics (CRFP) are high performance materials with an outstanding lightweight potential. Recycling applications for production waste though, are still scarce and not fully established. In the CaroLIn (carbon fibre nonwovens optimised for aircraft interior components) research project a novel aerodynamic textile process is developed, in order to produce highly orientated non-wovens form recycled carbon fibres. In the first stage of the project a laboratory plant for the orientation of fibres has been constructed and implemented. Afterwards a process window has been defined and a number of process parameters identified. The influence of those parameters was then investigated, using the statistical design of experiment (DOE) method.

Abstract: Plate heat exchangers are widely used for heating and cooling activities in domestic and industrial applications. The plates of all these components were made of stainless steel (e.g. type X2 CrNiMo 17-12-2) and they were brazed with pure copper. After cutting and metallographic preparation, the samples were investigated by light optical microscopy (LOM) to measure the depth of copper penetration into the plates (liquid metal embrittlement (LME)). For seven heat exchangers the penetration depth was between 25 and 60 μm, but for one the depth was about 150 μm. At the steel-copper interfaces an additional phase was identified as chromium-rich ferrite by electron beam micro analysis (EBMA). To get information about the sensitivity against corrosion electrochemical tests were performed in two electrolytes (HCO₃^- + SO₄^2- and HCO₃^- + SO₄^2- + Cl^-). It can be summarized that in the Cl^- free electrolyte the copper was attacked mainly. In case of the Cl^- containing electrolyte beside the copper braze, the LME zones in the steel were corroded additionally.