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Breaking down the cost structure of state-of-the-art CFRP part shows that a major share of the costs is caused by labor and equipment as well as process energy consumption.
Therefore, the main goal of the EU funded FP7 project LOWFLIP (Low Cost Flexible Integrated Composite Process) has been the reduction of these costs by introducing new technologies into CFRP production processes.
The LOWFLIP concept focuses on three main aspects:
• Development of a new out-of-autoclave (OOA) prepreg system with snap cure capabilities.
• Development of a direct 3D placement technology for plies and tapes.
• Development of energy efficient and fast heating toolings.
The main content of this paper is detailed information on a novel direct 3D prepreg layup process for automated production of large-scale fiber reinforced parts of small and medium lot sizes. The advanced ply placement process, which is able to drape and compact unidirectional prepreg tapes with currently up to 300 mm ply width directly into a double curved tooling, is being introduced. Two large-scale demonstrator parts from the transport and aerospace sector will be presented. Experiences gained during prototype manufacturing will be reflected and benchmarks of the equipment are presented.
Carbon fiber reinforced plastics (CFRP) allow to redesign well established metallic structures like manipulators in order to decrease their weight and to increase their level of performance. This task can only be successful, when the CFRP-design process does not copy the metallic structure, but solves the underlying functions in a new way. The present study shows, how an I-beam steel manipulator was transferred to a braided design with hollow cross sections and locally adapted material properties. Step by step we show, how to define the manipulator’s functions and mechanics and how they can be considered with a braided CFRP-structure. This includes analytical pre-design as well as detailed finite element method (FEM) design. A mechanical testing program was performed in order to measure input-values for the numerical calculations on coupon-level and validate the simulation on system level. Static and dynamic tests of the CFRP-manipulator showed that it fulfils all requirements to be certified for sale. Therefore, the braiding process for commercial production was investigated for two machine setups and the state of serial production was reached. The braided CFRP-manipulator is 70% lighter than the steel reference and has increased handling properties because of locally adapted material properties.
The effect of the cover factor on the in-plane mechanical properties of biaxial and triaxial braided carbon fiber composites is investigated. Low braid coverage occurs if the mandrel circumference exceeds a specific length, so that gaps occur in between the different yarn systems.
Braids are manufactured on different mandrel sizes with a braid angle of ±45°. The cover factor is calculated analytically and measured by means of gray scale analysis of scanned samples. A new analytical model is presented which allows for calculating the cover factor of triaxial braids in good approximation with real results, taking into account the off-centered alignment of the axial yarns. Tensile and compression test results are presented for the thermoset resin systems EPIKOTE MGS® RIM 235 and HexFlow® RTM6. The results show that there is a significant influence of decreased cover factors on the mechanical properties, especially in case of triaxial braids tested in axial direction. Micro computed tomography images based on synchrotron radiation reveal that low coverage leads to an increased fiber undulation of the axial yarns, which mainly determine the mechanical properties of triaxial braids. An influence of the different resin systems is observed for matrix-dominated properties of biaxial braids, where the composite failure strain is in the range of neat resin failure strain.
Scope of the presented work is a detailed comparison of a macroscopic draping model with real fibre architecture on a complex non-crimp-fabric preform using a new robot-based optical measurement system. By means of a preliminary analytical process design approach, a preforming test centre is set up to manufacture dry non-crimp-fabric preforms. A variable blank holder setup is used to investigate the effect of different process parameters on the fibre architecture. The real fibre architecture of those preforms is captured by the optical measurement system, which generates a three-dimensional model containing information about the fibre orientation along the entire surface of the preform. The measured and calculated fiber orientations are then compared with the simulation results in a three-dimensional overlay file. The results show that the analytical approach is able to predict local hot spots with high shear angles on the preform. Macroscopic simulations show a higher sensitivity towards changes in blank holder pressure than reality and limit the approach to precisely predict fibre architecture parameters on complex geometries.
The data represents tensile tests of 2D braided biaxial and triaxial composite laminates. Braids were produced on a Herzog radial braiding machine RF 1-176-100 with carbon fiber yarns (Toho Tenax F13 HTS40 12K 800 tex). Test laminates are infused with a VARI process with the 2-component epoxy resin system Hexion RIM235/RIMH237. The infusion is performed at vacuum pressure leves of 20-30 mbar and the resin is cured at room temperature for 24 hours at a pressure level of 250 mbar. A subsequent post-cure step maximizes matrix properties at 70°C for 10 hours.
The braid setups have been chosen to achieve four different braids with a fiber angle of 45° on braid mandrels with a diameter of 120 mm, two biaxial ones and two triaxial ones. By using different sizes of braiding ring diameters and yarn forces, homogeneous and inhomogeneous fiber architectures are generated.
Braiding setups 1 (biaxial) and 2 (triaxial) with 130 mm braiding ring diameter and 350 g yarn force result in braids with homogeneous fiber architecture. 3 (biaxial, 400 mm braiding ring, 600 g yarn force) and 4 (triaxial, 250 mm braiding ring, 600 g yarn force) in braids with inhomogeneous scattering of visual characteristics. Single layers are braided on mandrels with a length of 1500 mm and laminates with 6 layers of biaxial and 4 layers of triaxial braids are subsequently infused.
Tensile tests are performed on a Schenck-Trebel RM250 universal testing machine according to AITM 1-0007. For each specimen, the individual fiber orientation in each layer is measured prior to infusion by image analysis performed on scans of the braided layers. The results show a strong dependence of the individual fiber orientation of single test specimens on their mechanical properties in case of the presented tensile tests
Tensile properties of 2D braid reinforced composites are analyzed with regards to process-induced fiber architecture variations. First, a parametric study concerning the effect of radial braiding parameters on the braid characteristics is performed and a quantitative quality index based on visual attributes is introduced. Second, test laminates are produced with vacuum resin infusion to conduct tensile tests with biaxial and triaxial braids of varying qualities. To correlate fiber architecture variations with mechanical properties, each braided layer is scanned and analyzed by means of gray scale based image processing. This allows for determining specimen-specific fiber orientations and correlating them with mechanical properties. The results show strong correlations of measured fiber architectures and mechanical properties. Thus, the study suggests that detailed information about the (local) fiber architecture should allow for significantly improving the prediction of 2D braided composite properties.