About a quarter of the Airbus A380 structure will be lightweight carbon-fiber-reinforced plastics (CFRPs). These parts include the central wing box, wing flaps, the entire aft fuselage section (vertical and horizontal stabilizers), and the rear pressure bulkhead which closes the rear end of the cabin. This structure must withstand the entire force of internal cabin pressurization during flight - with considerable safety margin. While keeping weight within specification, A380 designers were also charged with devising low-cost manufacturing processes. Resin-impregnated carbon-fiber tapes (prepregs) are often the choice for relatively large flat components. Complex or tightly curved structures demand other manufacturing concepts. One developed by EADS Corporate Research Center (EADS-CRC) and Airbus for the A380 builds components from CFRP fabrics sewn to the desired shape in the dry state before being impregnated with resin. The use of dry fibers in combination with sewing technologies offers great potential for automation. Moreover, designers can vary stitch density and position to boost composite strength and stiffness. The technique produces 3D-fiber reinforcements with significantly better out-of-plane properties and materials with mechanical properties approaching those of traditional prepreg laminates. A range of these textiles serve in advanced composite structures. Examples include traditional woven fabrics, tubular braids, unidirectional fabrics, and stitchbonded reinforcing fabrics, also known as noncrimp fabrics (NCFs). Recent additions include near-net-shape subpreforms such as fiber placement structures and complex 3D braids. EADS-CRC worked with German-based Herzog Maschinenfabrik GmbH & Co., Oldenburg, and the Institute for Textile Technology of the Aachen University of Technology (ITA), on a braiding machine that builds complex 3D fiber architectures using a 12 by 12 array of horn gears to move a series of bobbins along tracks. Pneumatic switches individually select the bobbins to generate nearly any cross-section shape. The device can produce a wide range of fiber orientations within the braids as well as continuously change the profile cross section. EADS-CRC also partnered with fellow German-based companies ALTIN Nähtechnik GmbH, ITA, and KSL Keilmann Sondermaschinenbau GmbH to devise a number of key technologies that could lead to low-cost production of larger, 3D-reinforced composites with superior damage tolerance and structural integrity. Conventional sewing techniques, including lock stitch and chain stitch, need access to both sides of the material. This limitation restricts these stitching techniques to flat platforms with limited size. Single-side chain stitching from ALTIN uses a sewing needle at 45 deg and a 90 deg hook needle. This setup puts the thread chain only on the upper side of the fabric and produces a simultaneous reinforcement at 90 and 45 deg. Multithread chain stitch from ITA uses two sewing needles at 45 deg and puts the thread chain on the rear side of the fabric. The +/-45 deg threads improve composite shear strength. Unchained open-yarn loops or tufts from KSL can be used to sew up to 1.57-in. (40-mm) thick materials. The open-loop structure of the threads can be kept inside the material or forced to penetrate through. Blind chain stitch also from KSL uses a curved needle (R= 0.98 in. (25 mm)) and puts the thread chain on the upper side of the material. With this head, the needle does not puncture through the material and is an option for sewing inside the mold. For the A380 rear pressure bulkhead, multiaxial carbonfiber composite preform sheets are sewn using an automated blind stitch approach. The assembly of the joined sheets (the 'carpet') is then rolled out over a shaped mold that resembles a big overturned bowl. To get enough strength from the carbon composite, six carpets are laid one on top of the other in alternate directions. The preformed stack of sewn-together, fabric sheets is then placed in an autoclave, where a film of solid resin is melted and pressed between the fibers by vacuum. EADS-CRC also collaborated with the Military Aircraft Dept. of EADS in Augsburg to develop a low-cost approach for resin impregnation without the aid of an autoclave. The key feature of the nonautoclave process called VAP (vacuum assisted process) is the use of a microporous membrane that separates the resin from air. With VAP, designers can build more affordable CFRP structures that meet aerospace specifications in both void content and fiber volume fraction. VAP has been proven by manufacturing several demonstrator components, which include the rear pressure bulkhead in the size of the Airbus A320, a lower-wing skin panel, and the wing attachment box for the EFA fuselage side skin.
Sewing lessons for aerospace engineers
Dreidimensionales Nähen im Flugzeugbau
Machine Design ; 77 , 16 ; 69-72
2005
4 Seiten, 11 Bilder
Aufsatz (Zeitschrift)
Englisch
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