The CFK-Valley in the town of Stade, northern Germany, hosts an annual conference where participants present the latest developments in carbon-fiber reinforced plastic (CFRP). In all, 360 composites professionals attended and 34 companies exhibited this year during the 7th annual event, held June 11-12. Although most participants came from northern Germany, others hailed from as far away as the United Arab Emirates.
The area, considered the Carbon Composite Valley of Germany, a name that calls to mind America’s Silicon Valley, is curiously set in one of the flattest flood plains in Germany. The event was supported by local industry, including the Composites Technology Centre (CTC GmbH), which is an Airbus (Toulouse, France) composites research organization located in the outskirts of Stade. The conference took place over one-and-a-half days, at the new CFK NORD facility, and included an awards dinner.
Technical presenters and inventors
The keynote address was given by Dr. Amer Affan (see Fig. 1), whose company, Affan Innovative Structures (Dubai, U.A.E.), has designed and installed CFRP panels and beams for use in futuristic-looking buildings around the Middle East. Using carbon fiber, Affan was able to design very slim structures that were simply not possible with steel. He explained that doing so with composites brought benefits that included a large reduction in building weight. As a result, the cost of building foundations could be reduced and, thus, help offset the extra cost of the carbon fiber materials.
About half the papers were focused on aerospace, with the remainder split between those aimed at the automotive industry and others on a variety of topics for general application. The conference offered a wide range of new innovations in areas that ranged from robot end-effectors for picking and forming fabrics to assemble resin transfer molding preforms, to nondestructive testing (NDT), using eddy current techniques on dry textiles, and ways and means of using composite structures to protect electric car batteries during a crash. Speakers also covered sustainability issues, such as recycling, and prevention of health risks when machining cured CFRP, including newer alternatives to current practices. The conference highlights included papers presented by the following standout speakers:
Dr.-Ing. Christian Hűhne, from the German Aerospace Center (DLR, Braunschweig, Germany), explained the requirements that must be met to achieve laminar flow over the upper surface of an aircraft wing. They include not only obvious measures, such as reducing or eliminating gaps and steps after assembly, but also the need to avoid waviness that develops in the region of stiffeners during panel manufacturing. Reportedly, the use of cocured U-shaped stiffeners can eliminate rivets so the skin itself incorporates rib caps in the so-called shoe box design. A demonstration structure was shown at the exhibition.
Dr. Holger Purol, head of production at Xperion Aerospace (Immenstaad, Germany), described the work done thus far by that company on solar panels for the BepiColombo mission, which is slated to put a satellite in orbit around Mercury. The craft will be launched in 2015, perform fly-by maneuvers near Venus, Earth and Mercury, then achieve orbit around Mercury in 2022. One of the design drivers for the composite structures is the temperature extremes that will be encountered (150°C/302°F on the sunny side of Mercury and -250°C/-418°F in its shadow). To meet these requirements, the sandwich panels will be made from ultrahigh-modulus carbon fibers surrounding either aluminum or CFRP cores. Each skin will be made from a single layer of thin fabric impregnated with cyanate ester resin and then cured in an autoclave. Xperion will provide 13 panels for the three arrays that will make the flight, but it will have to provide a total of 30 panels, including those used in the test program — a massive number for any space project.
Alexander Kerner from Eurocopter (Donauworth, Germany) gave a refreshingly candid account of the problems that can be encountered when an automated fiber placement (AFP) machine is used to lay up complex sandwich structures like those typically found on helicopters. These problems include tack issues with adhesives, gaps due to tolerances in machine movement or tape width, wrinkles due to tape steerage, and bridging on honeycomb ramps.
Alexander Gillessen, from the CTC at Stade, presented a paper on the underfloor structure (see Fig. 2) for an electric car; 20,000 units will be produced per year. The challenges include protecting passengers in a crash and housing the car’s heavy batteries in a crashworthy but weight-efficient way. The demonstrated solution is an integrated structural lower floor pan that incorporates sandwich and cross-beam components, made using resin transfer molding. The upper cover is made from sheet molding compound (SMC). Gillessen said models showed that the final structures will protect the integrity of the passenger compartment and the underfloor battery cell during severe side-impact collisions.
Dr.-Ing Annika Raatz of Technische Universität (Braunschweig, Germany) described an innovative method of picking up precut plies and assembling them to make complex RTM preforms. The process describes a robot end-effector that can pick up a 2-D ply, lay it into a complex mold, form it into a shape and then apply heat to activate the binder. The device is called FormHand (see Fig. 3). It can best be described as a cushion, filled with granules, whose formability can be altered by adjusting the vacuum in the cushion. Heat can be applied to set the shape of the ply either by resistance heaters in the cushion surface or by induction heating.
Dr. Henning Heuer from Fraunhofer (Dresden, Germany) said he has determined, through testing, that none of the current NDT techniques used for cured composites can be used to inspect CFRP preforms before they are infused or injected with resin. He has, therefore, developed a technique using high-frequency eddy currents (see Fig. 4). The method reportedly can detect inconsistencies in the way a preform is assembled so it can be corrected or scrapped before infusion. He revealed, however, that there are some limitations to the depth at which this process is effective).
Dr. Heike Illing-Gűnther, from Sächsisches Textilforschungsinstitut (Chemnitz, Germany), reported on efforts to make nonwoven carbon fiber material from scrap carbon fibers or scrapped CFRP parts. The process uses dry fibers obtained either directly or after pyrolysis of CFRP structures. The process uses a guillotine to cut the fibers, a tearing machine, then a carding machine plus a cross lapper. (It was curious to see carding machines, invented in the 18th Century for processing wool, used to process modern materials.) The material is then fed through a stitch-bonding process to make a material that is 1m/3.28-ft wide and weighs between 140 and 1,400 g/m2.
Simon-Markus Kothe, from the Fraunhofer Institute facility in Stade, said he has developed a flexible tooling system for assembly of large composite structures. It reportedly avoids the need for traditional tooling systems used to hold both metal and composite panels so they can be first machined and then assembled with other panels and their stiffeners and fittings. Traditional tooling is part-specific and consists of heavy, expensive steel structures that require expert design and manufacture, plus regular calibration. Fraunhofer has developed a flexible system for fuselage shells, which holds the skin in its correct shape and in a known position prior to machining. This is achieved by six robotic actuators, each fitted with vacuum suction cups that align with the panel’s contours. The shape of the panel is measured with a Laser Radar, and any deviations from the defined dimensions are corrected by the robotic actuators prior to machining.
The most topical paper, by Jan Christoph Kako, a manufacturing engineer at Airbus Stade, was a detailed account of the manufacturing methods used to make the wing stringers for the A350 XWB and how they were cobonded to the wingskin. One skin requires 18 stringers, with a total length of more than 300m/1,000 ft. Each stringer differs both in shape and ply layup. Initially an automated tape layer (ATL) was used to lay carpets of materials for one particular stringer. These carpets were slit and laminates were stored for future use. However, Airbus then realized that if an AFP machine were used, it would be possible to tailor carpets to make blanks for one set of individual stringers, which would result in less storage space and simplified logistics. Kako also correctly predicted the day the A350 XWB would make its first flight — just two days after his presentation.
Exhibits that excelled
There were many interesting processes and parts on display at the CFK-Valley Stade 2013 exhibition. Most, but not all, were there to support topics discussed in during the conference. A sampling can be viewed in Figs. 5 though 9, at left.
The 8th annual CFK-Valley Stade Convention will be held June 24-25, 2014.