The CFK-Valley hosted, June 24-25, the 2014 edition of the CFK-Valley Stade Convention, focusing on some of the latest innovations in material and process development in composites fabrication. An advanced composites networking organization based in Stade, Germany, CFK-Valley comprises more than 100 member companies as well as the Composites Technology Centre (CTC GmbH), an Airbus (Toulouse, France) research organization.
This year’s event attracted 350 participants from 15 countries. Most papers came from Germany, but two had their sources in France and one came from distant South Korea.
The conference was opened by Dr. Axel Herrmann, CFK-Valley’s chief technology officer, who told the audience that the network has seen much change and growth in its first 10 years. He noted that “more power” is needed now. To that end, he announced that Dr. Gunnar Merz, previously with Dow Deutschland AG (Scwalbach, Germany), would be engaged full time as chief executive officer and chair of the board. Herrmann will continue in a part-time role.
Dr. Michael Jischa of the Institute of Applied Mechanics (Clausthal-Zellerfeld, Germany) gave the keynote address, “Limits to Growth.” Jischa highlighted the accuracy of the world predictions made by the Club of Rome in the 1970s. A global think tank, the Club of Rome was founded in 1968 at Accademia dei Lincei in Rome, Italy. Describing itself as “a group of world citizens, sharing a common concern for the future of humanity,” the group consists of current and former heads of state, United Nations bureaucrats, high-level politicians and government officials, diplomats, scientists, economists and business leaders from around the globe. Unfortunately, Jischa noted, not enough attention was paid to these predictions until recently. It’s clear, he added, that scientists and engineers must now think beyond technical and economic reasoning and consider the environment and the impact of new technology on the future development of the world. In conclusion, Jischa pointed out that technology offers answers to many challenging questions, but society must be very clear about the questions.
Henri Girardy, business development manager at Hexcel Composites (Stamford, Conn., and Duxford, U.K.), presented “Hexcel’s Development Status of Advanced Carbon Reinforcements for Primary Aircraft Structures Made by Using Cost Effective OOA Technologies.” Girardy explained that although high-performance prepreg is the benchmark material for primary aircraft structure, the drive now is to reduce cost and weight and — as manufacturing rates increase to unprecedented levels — to make ever-larger, integrated parts. He noted that out-of-autoclave manufacturing via vacuum-assisted resin transfer molding (VARTM) is recognized as a cost-effective process, but he said improvements in material performance are required if the process is to enable fabrication of primary aircraft structure. This, he reported, has been the focus of Hexcel’s effort. Two types of dry fiber material have been developed by Hexcel to match unidirectional (UD) prepreg performance. The first is a noncrimp fabric (NCF) that exhibits improved performance. The second, a dry UD tape called HiTape, is designed for use with standard automated tape laying (ATL) and automated fiber placement (AFP) machines.
Hexcel’s approach with HiTape involves the application of a thin veil of thermoplastic material to the dry reinforcement. The veil forms a toughened interlayer in the final cured composite. Girardy says that Hexcel has demonstrated, in laminates built up with widely qualified HexFlow RTM6 and HiTape, at a fiber volume fraction of 60 percent after infusion, almost equivalent properties to prepreg (see Figs. 1 & 2, at left).
Tassilo Witte, project manager at Airbus CTC, gave a paper titled, “Recycled Carbon Fibres and the Development of Semi-Finished Products for Aircraft Interiors.” Witte reviewed CFK-Valley’s work on an industrial process for recycling composites by a pyrolysis method, using the gases emitted by the heated resin matrix as the fuel to drive the process. The goal now of a CTC project that will run through 2016 is to develop a recycled carbon fiber veil prepreg to replace current glass fiber products now used in aircraft interior applications. Reportedly, this would be the first application of recycled composite material on a civil aircraft. Witte’s work, thus far, has shown that the recycled fiber is comparable to the original virgin fiber in terms of strength. However, the fiber length is short, and when it is reprocessed into a veil of random orientation, the strength of the original textile cannot be expected. In aircraft interiors, however, this is of less importance than the flammability, heat release and smoke-generation properties of the material.
Michael Schumann, an R&D engineer from Fraunhofer Institute for Manufacturing Engineering and Automation (Rostock, Germany), gave a paper titled, “Development of Strand-Laying-CFRP Bridge Systems With Automated Manufacturing Processes.” This project’s goal was to manufacture a bridge support structure by an automated strand-layering technology that applies wet-impregnated carbon fiber roving of a defined, pre-stressed CFRP design to carry pedestrians and cyclists over a 50m/164-ft clear span. The possibility to carry motor vehicles already has been calculated but has yet to be approved. The work at Fraunhofer has developed a strand-laying machine that precoats rovings in a resin bath and then applies them, pre-tensioned, over pre-positioned GFRP ribs (see Fig. 3, at left).
Dr. Hyun Kyul Shin, director of the Korea Institute of Carbon Convergence (Seoul, South Korea), gave a compelling talk on his work to develop and manufacture a geometrically complex CFRP automotive engine cover, using microwave curing (see Fig. 4). Shin noted a 30 percent reduction in cure time and energy consumption, using this process. Although mechanical properties fall short of those achieved by oven or autoclave cure (by less than 10 percent), they are still consistent with acceptable part quality. He believes that properties will improve further as the process is refined.
Roland Lichtinger, research associate at the Technical University of Munich (Germany), presented an impressive selection of simulation videos showing AFP in process. They demonstrated clearly that, due to the nature of the process and because of an accumulation of overlapping areas within the infrared preheating lamp’s footprint, the result is a nonlinear thermal map across a layup. Depending on the part thickness and considering that large, thick parts see a long outtime before the part is finally cured, it is possible that some areas of the part may be beyond their outlife or even partially precured before final cure.
A paper by Dr. Steffan Czichon, stress engineer at Elan-Ausy GmbH (Hamburg, Germany), described “Numerical Modeling of Fiber Undulation Induced by Voids and SHM Sensors.” Fiber undulations, waviness and voids are known to negatively affect the properties of composites. Czichon showed that the effect of voids can be predicted by simulation. He went on to say that embedded structural health monitoring (SHM) sensors have similar effects. Czichon pointed out that with careful design of the sensors’ shape, their effect on composite properties can be minimized.
Fabian-Timmo Seebo, an engineer at Hamburg-based Spitzner Engineers HAW, impressed the audience with his paper, “Transferring Knowledge From Aerospace to Wind Energy.” He began by noting that it is common to see modern civil aircraft with winglets. These devices serve to reduce turbulence at the wingtips in flight, thus improving aircraft fuel economy. Seebo’s work has adopted this principle by applying a winglet device onto the end of wind turbine blades. Spitzner Engineers also has redesigned the leading edge of the blade root, including holes that allow air to flow into the blade then exit through corresponding holes at the blade’s far end. The result is an increase of almost 14 percent in output on a wind turbine fitted with such blades. (see Figs. 5 & 6, at left).
Dr. Claus Bremer, president at BCT GmbH (Dortmund, Germany), gave an account of the sophisticated composites repair machining equipment that BCT has developed and manufactured with several partners, in cooperation with Airbus. The machines, both mobile and static, automatically adapt themselves to the surface to be machined in five axes via laser scanning. Using a high-speed ultrasonic milling spindle, they mill out damaged areas and scarf the laminations to standards that are reportedly impossible to meet by manual preparation. The machines also apply atmospheric plasma to the scarfed area to activate it for subsequent bonding of the repair patch (see images online).
Dr. York Roth, head of components methods and processes at Airbus Operations GmbH (Stade, Germany), presented “High Quality Components Require Intelligent and Robust Tooling.” Roth explained that the tooling for the production of earlier Airbus aircraft families (e.g., the A320) was designed and manufactured years ago and was not robust enough to keep up with ever-increasing Airbus production rates (up to 50 aircraft per month). Tooling, therefore, has had to evolve to maintain the exacting quality standards. Roth gave an interesting explanation of the methods necessary to achieve this goal. For example, he explained that extensive use had been made of Ishikawa diagrams to highlight all of the causes that affect the overall issue of tool robustness, molded part tolerance and manufacturing repeatability. Each cause has been addressed systematically, and that has led to the development of improved tooling that can withstand the rigors of very demanding production rates. He noted, further, that much of the tooling originally developed in metal is now fabricated from composites, avoiding many of the issues created by CTE mismatches and part springback.