The lighter an aircraft is, the less fuel it consumes. Given the need to cut carbon dioxide emissions, this is a key aspect of materials research. Aircraft manufacturers are therefore pinning their hopes on particularly lightweight construction materials. These include not only lightweight metals, but also fiber composite plastics, particularly carbon-fiber reinforced plastics (CFRPs). Whenever two CFRP components have to be joined together, this has so far been accomplished primarily by riveting.
Researchers at the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM in Bremen are experts in adhesive techniques and plan to enlarge their expertise to include mechanical joining. At the Composites Europe trade fair in Essen, they show a state-of-the-art C-clamp riveting machine (Hall 10-11, Stand 150). This device enables the necessary rivet holes, complete with one- or two-part riveted bolts, to be installed accurately and automatically in compliance with aviation standards.
The IFAM researchers now intend to go a step further. "Rivet holes are a problem, particularly in CFRP structures," explains Dr. Oliver Klapp of the IFAM. "They disturb the flow of forces in the CFRP structures and reduce the load-bearing capacity of the material." The researchers are therefore planning to make use of adhesive bonding processes that are already employed for CFRP materials. "But the aviation industry is not yet ready to rely exclusively on bonded components and assemblies," says Klapp. This is why the engineers are exploring the potential of hybrid joining – a combination of riveting and a special bonding process. The advantages of hybrid joining are obvious: the CFRP materials are not riddled with so many rivet holes. The particularly high load-bearing capacity of these materials is more effectively brought to bear in the truest sense of the word, because bonding results in a more effective, all-over distribution of forces. The researchers are currently optimizing the parameters of the joining process.
"It's true that riveting will not be eliminated from aircraft construction in the next several years," says Klapp. But the aviation industry will soon be unable to manage without structural bonding of primary structures such as the airframe, the wings and the tail units.