Behaviour of CFRP during flights verified precisely

As part of the Clean Sky research initiative and with the help of a measurement configuration based on fibre optics, Fraunhofer researchers have accurately verified the degree to which carbon fibre reinforced plastic (CFRP) parts deform during flight.

“CFRP structures behave differently during a flight to those made of aluminium,” said Conchin Contell Asins, scientist at the Fraunhofer Institute for Structural Durability and System Reliability LBF. She and her colleague Oliver Schwarzhaupt determined how this happens by using a special setup during the test flights. They used optical measuring fibres to detect the minimal deformations. “That‘s not possible with conventional metallic strain gauges. These are based on changes in their electrical resistance by deformation, but their resolution is not as fine,” Contell Asins explained.

The test aircraft was a medium-range model that holds 70 passengers. For flights, a CFRP component which was about 5 x 3m long was used on the upper fuselage between the cockpit and wing. This area is one of the most heavily stressed components during flight. The researchers applied the optical measuring fibres on the side facing the aircraft interior. “The thin, elongated glass fibres are well suited to display even very weak changes of larger components,” said Contell Asins.

The aim of the measurement flights was to obtain solid data that can be compared with the theoretical calculations of the flight behaviour of CFRP, which has so far only been possible by approximation. Therefore, aircraft manufacturers are integrating ‘overdimensioned’ CFRP in their models as a precaution.

Due to the lower air pressure, the fuselage expands during the flight by a few centimetres. If this happens too much, cracks can occur. CFRP tolerates stretching of up to 1.5% of its original length before breaking. In order for CFRP to stretch to the same extent as aluminium, the forces have to be much higher. Optical measuring fibres are able to show changes as small as a few nanometres.

Schwarzhaupt explained: “The test flights have shown that our test setup works: We have been able to assign a unique CFRP deformation to each flight manoeuvre. The values were so accurate that conclusions could have also been reached about the flight profile based upon the strain signals.”

Tom Austin-Morgan

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