Hybrid joints on the way for aircraft

German engineers are working on a combined riveting and bonding process for aircraft parts made of composite materials.

Aircraft manufacturers are increasingly turning towards the use of lightweight construction materials. These include not only lightweight metals, but also fibre-composite plastics, particularly carbon-fibre reinforced plastics (CFRPs). Whenever two CFRP components have to be joined together, this has so far been accomplished primarily by riveting.

An aircraft is held together by hundreds of thousands of rivets. Fully automatic machines install rivet holes and rivets with precision in numerous materials. A new hybrid technology combines this mechanical joining technique with adhesive bonding.

Researchers at the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM) are experts in adhesive techniques and plan to enlarge their expertise to include mechanical joining. They will be presenting a state-of-the-art C-clamp riveting machine at the Composites Europe trade fair in Essen this month. 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," explained 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 these materials.

"The aviation industry is not yet ready to rely exclusively on bonded components and assemblies," said Klapp, so 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. Bonding gives a better all-over distribution of forces, so the high load-bearing capacity of the materials can therefore be exploited more effectively.The researchers are currently optimising the parameters of the joining process.

"It's true that riveting will not be eliminated from aircraft construction in the next several years," said Klapp, but he believes 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.

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