Sewing lessons for aerospace engineers

Aug. 18, 2005
Designers of high-performance composite components rediscover the art of stitching.

Jrgen Brandt
Head of department Composite Technologies (retired)

Jrgen Filsinger
Manager Preforming and Process Engineering

Andreas Gessler
Manager Textile Technologies and Testing
EADS Deutschland GmbH
Corporate Research Center
Ottobrunn near Munich, Germany

The A380 takes the innovative application of composites a step further, with greater use of them than ever before, including the center wing box for the first time and the rear fuselage. Sewing is used in the manufacture of the CFRP rear-pressure bulkhead.

The composite rear pressure bulkhead of the Airbus A380 weighs only 551 lb (250 kg).

Test specimen for a multispar beam.

The blind-stitching technique uses a curved needle.

The advanced-resin-infusion technique or VAP (vacuum-assisted process) creates complex aerospace composites without the aid of an autoclave. This new approach suits especially large components particularly well. It is more economical than traditional compositefabrication techniques and is being used in the series production of panels for the flap tracks of the Airbus A380 and in the manufacture of components for the wind-power and shipbuilding industries.

Composites increasingly replace metals in aerospace components because they are much lighter. In aerospace, weight certainly matters. Every pound shaved away brings higher payload capacities or lower fuel costs. About a quarter of the Airbus A380 structure, for example, will be lightweight carbon-fiber-reinforced plastics (CFRPs). The parts include the central wing box, wing flaps, the entire aft fuselage section (vertical and horizontal stabilizers), and the rear pressure bulkhead. The latter is the "wall" that closes the rear end of the cabin. This structure must withstand the entire force of internal cabin pressurization during flight — with considerable safety margin.

Although one key objective for A380 designers was keeping weight within spec, they were also charged with devising manufacturing processes that would offer the best economics. Resin-impregnated carbon-fiber tapes (prepregs) are often the choice for relatively flat components with large surface area. The outer skin of the tail unit is one example.

The use of carbon-fiber tapes in aerospace composites is well established and the in-plane material properties of carbon-tape laminates are unbeatable. The prepregs take their ultimate shape via a computer-controlled tape-laying machine before being hardened in an autoclave by heat and pressure. Prepreg laminates are relatively expensive, however, and not particularly well suited for mass production because the process is labor intensive.

Structures that are more complex and tightly curved 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 "sewing machine" used to construct these components bears little resemblance to the familiar domestic appliance. The build starts with several lengths of carbon-fiber fabric laid side by side on a table measuring over 26 ft (8 m) in length and width. A metal crossbeam carries the sewing head and travels back and forth across the table. A needle pulls the thread through the material at a rate of up to 100 stitches/min.

Use of dry fibers in combination with sewing technologies offers great potential for automation. Designers can vary stitch density and position to boost composite strength and stiffness. This sort of tailoring lets component sections meet the highstress concentration gradients they will see during use. The technique produces 3D-fiber reinforcements with significantly better out-of-plane properties and materials with mechanical attributes approaching those of traditional prepreg laminates.

A range of 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.

Basic (low-cost) textiles: Compared to woven fabrics, which have been available for years, carbon-fiber NCFs are relatively new and continue to improve. They sport straight fibers that let them reach about 98% of conventional prepreg laminate properties. They come in many stacking sequences, different area weights, and fiber types.

Additionally, with the NCF process fibers can be laid diagonally for even more cost-effective manufacturing of high-performance composite structures.

NCFs are most frequently used in the construction of flat or slightly curved subpreforms. But they can also work for complex 3D shapes with careful folding techniques. Plain lock-stitch sewing or binders frequently stabilize these structures.

Three-dimensional braids and fabrics: New generation braiding machines build continuous 3D structures with good intrinsic through-thethickness reinforcement as well as variable cross-section structures. Typical applications include filler "noodles" without straight fibers and smaller frame profiles. The 3D weaves are also textiles with inherent through-the-thickness reinforcement. Fibers can be oriented in a 90 or 45° direction to the surface depending on performance requirements.

Overbraiding: Overbraiding processes use contoured mandrels to build net-shape preforms. The mandrels can be either removable or lightweight foam and PET (polyethylene terephthalate) structures that stay in the part. There are many ways of designing removable mandrel cores. For simple profiles they easily dismount and can be designed to expand when forming the preform and contract for easy removal.

Options for more complex geometries include soluble salts and sand-binder compounds, meltable materials ( highperformance waxes and low-melt metals), and granulate-filled contoured bags. Removable mandrels can build hollow fiber preforms. With subsequent folding processes they provide an option for more complex geometries. Overbraiding can process a high number of yarns simultaneously with good economics. And the process can be highly automated.

Tailored fiber placement: TFP uses embroidery machine hardware that is more or less conventional. TFP stitches carbon rovings to thin substrates. The technique can orient fibers to exactly accommodate the load path. So, in theory, it's possible to maximize mechanical properties while minimizing structural weight.

TFPs have been applied successfully to the reinforcement of holes and load introductions, and recently to large complex structures such as robot arms and I-beams.

EADS-CRC partnered with fellow German-based companies ALTIN Nhtechnik GmbH, the Institute for Textile Technology of the Aachen University of Technology (ITA), and KSL Keilmann Sondermaschinenbau GmbH to devise a number of key technologies that could lead to economical production of larger, 3D-reinforced composites with superior damage tolerance and structural integrity.

Conventional lock-stitch machines are well established for preform stabilization, attaching stiffeners to flat panels, and to get fast, through-thethickness reinforcement of wide areas.

Preform stabilization makes it easier to handle and store composites before they go into an autoclave. At first glance this may appear to be a relatively simple task. But in practice, the technique is much trickier. Stitching must only prevent the subpreforms from relative movement. Unsuitable sewing parameters can easily destroy material properties, misalign the fibers, or ultimately cause the resin to pool in the seams.

Manufacturers use special sewing yarns with high stiffness and strength to improve structural performance. Today, only threads of aramide, glass, and carbon meet the requirements. Parameters such as stitch density, seam position, and penetration angle greatly influence material properties.

Conventional sewing techniques, including lock stitch and chain stitch, need access to both sides of the material. So this limitation restricts these stitching techniques to flat platforms with limited size.

Single-side chain stitching from ALTIN uses a sewing needle at 45° and a 90° hook needle. This setup puts the thread chain on the upper side of the fabric and produces a simultaneous reinforcement at 90 and 45°.

Multithread chain stitch from ITA uses two sewing needles at 45° and puts the thread chain on the rear side of the fabric. The ±45° 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. This process uses thick yarns.

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, designers specified a multiaxial carbonfiber preform from an outside vendor. The composite sheets are sewn using an automated blind stitch approach. The curved needle works the seam from one side and lets almost any length of material be joined. The complete assembly of joined sheets emerges from a robot sewing machine in the form of a large "carpet." The main advantages of a robot-guided, singleside sewing machine is that preform size is virtually unlimited and the sewing heads can produce spatial curved seams on complex shaped preforms.

The carpet is then rolled up and repositioned over a shaped mold that resembles a giant overturned bowl. It is then rolled out. 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. The highly automated process is extremely cost effective and reliable.

EADS-CRC collaborated with the Military Aircraft Dept. of EADS in Augsburg to develop an alternative, low-cost, approach for resin impregnation. The nonautoclave manufacturing process called VAP (vacuum-assisted process) is well suited for complex, high-performance CFRP structures. The key feature of VAP 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 specs in both void content and fiber volume fraction.

VAP has been proven by manufacturing several demonstrator components. These include the rear pressure bulkhead in the size of the Airbus A320, a 39.4 8.2 ft (12 2.5 m) lower-wing skin panel that weighs 882 lb (400 kg), and the wing attachment box (WAB) for the EFA fuselage side skin.

The WAB, for example, has an NCF skin and eight NCF J-frames. The overall dimensions of the part are 7.5 5.9 3.6 ft (2.3 1.8 1.1 m). Four of the J-frames are full-size (5.9 3.6 ft), the others are cut out for manholes. Skin and frames were stitch-bonded with two different sewing systems, the KSL blind stitch and the ALTIN OSS RN810 chain-stitch system.

Because of seam geometry, four blindstitch seams are sewn onto the frame. With the ALTIN chain stitch, two seams are sufficient. A lightweight, aluminum/foam-sandwich fixture is laser cut to exacting tolerances to lay-up the preform. The fixture fits into slots on a comblike device for accurate positioning from one frame to the next. Aftersewing, the fixture and preform are inverted and placed in a female mold. This lets the fixture be easily removed so the preform can undergo resin infusion via VAP.

With the highly automated process, a textile J-shaped fuselage frame slashed manufacturing costs by 80% compared to its prepreg counterpart. In addition, designers also realized a significant drop in the number of fabric precuts. About 300 individual precuts were needed for the prepreg frame. The textile version only needed 36 pieces of NCF, unidirectional woven fabrics and one 3D braided filler noodle.

Stitching Yarns

Dry carbon-fiber fabrics are sewn together and draped over a mold.

Manufacturers typically pick a sewing yarn based on the seam's function. Polyester yarns, for example, are sufficient for assembling preforms without the need for structural improvement. These seams just prevent the relative movement of the preform layers or subpreforms.

However, the seams must not disturb the textile structure or degrade the material in-plane properties. Typically, designers can expect about a 2% drop in static compression strength. Stitch type, needle and yarn thickness, yarn tension, and fiber undulation or curvature, are the most critical parameters needing tight control.

Threads for structural seams must meet more-stringent criteria. Their main function is to carry and distribute out-of-plane and/or shear stress. Obviously, therefore, they must be thicker, but many other properties play key roles as well. The most important is the yarn modulus in combination with the resin interface. Carbon sewing threads are the optimum choice. They provide the highest combination of stiffness and ultimate strength compared to glass and aramide yarns.

Compression after impact (CAI) tests at EADS-CRC show that the delamination area drops significantly as stitch density rises. The compression strength after impact rises 80% from 172 to 312 MPa.

A 3D Braiding Machine

A 3D braiding machine builds almost any sized cross-section profile up to 93 in. 2 (600 cm2) with a wide range of fiber orientations within the braid.

EADS-CRC worked jointly with German-based Herzog Maschinenfabrik GmbH & Co., Oldenburg, and ITA, Aachen, on a braiding machine that builds complex profiles with a 3D fiber architecture. The system uses 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 also produce a wide range of fiber orientations within the braids as well as continuously change the profile cross section.

ALTIN Nähtechnik GmbH, (+49) 3447/595-406,

European Aeronautic Defence and Space Co., Corporate Research Center Germany, +49 89 607 22713,

Herzog Maschinenfabrik GmbH & Co.,+49 (0)441-3008300,

Institute for Textile Technology of the Aachen University of Technology (ITA),+49 (0)241/80 95621,

KSL Keilmann Sondermaschinenbau GmbH,(+49) 062 51/96 20-0,

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