Due to the lower air pressure, the fuselage expands during the flight. In part 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. That doesn‘t sound like much, but in order for CFRP to stretch so much, the forces have to be much higher than in the case of aluminium. Optical measuring fibres are able to show changes as small as a few nanometers.
Very little is known about the behaviour of carbon fibre reinforced plastics or CFRP for short during the course of an aircrafts flight. Researchers at Fraunhofer have now accurately verified the degree to which CFRP parts deform during flight.
In order to test the stresses on CFRP the researchers stripped a medium-range test aircraft of seats, interior walls and passengers with cables, sensors, and measurement equipment. A component made of carbon fibre reinforced plastic forms the upper fuselage from the cockpit to the wings. Lightweight yet stable, It is certain that it will withstand the stress of the test flight, but the question is to what degree will it deform during different flight manoeuvres? Exact values had not yet been known.
Conchin Contell Asins, scientist at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt and her colleague Oliver Schwarzhaupt determined exactly how this happens by using a special test setup during the test flights. With the help of fibre optic technology, optical measuring fibres detected even minimal deformations.
The goal is to build even lighter CFRP components and to increase the operating time of the components and saving on unnecessary material means needing less fuel.
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. The real data requires aircraft manufacturers to build components so precisely that they can withstand the stresses that occur in the respective aircraft model. This has so far only been possible by approximation. Therefore, aircraft manufacturers are integrating CFRP over-dimensioned in new models, as a precaution.
The test flights have shown that the researchers testing setup works and they 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. It is also possible with this system to monitor the structure for its condition during the flight. A change in the deformation behaviour might indicate damage, with such monitoring of the structure, components could remain in use for much longer.
In the test flight, the team of scientists from Darmstadt installed the complete measurement hardware in the plane and evaluated the data. The aircraft manufacturer analyses the results in JTI Clean Sky Green Regional Aircraft Platform of the 7th Framework Program, a research initiative of the European Commission and the European aviation industry. The goal: to build even lighter CFRP components and to increase the operating time of those components.
The CFRP component which was about five-by-three-meters long and 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.
An optical-electrical evaluation unit recorded the signals of the measuring fibres. The black box provided additional information about the altitude, airspeed and flight manoeuvres, all of this data was then analysed with the help of special software.
To attach the strain sensors to the right places, the researchers had to know where stress typically occurs during flight manoeuvres. They were able to bring in expertise about the behaviour of CFRP using previous test data. In CFRP structures in aircraft, the attached stiffeners primarily bear the stress. These are located on the inside of the hull in the longitudinal and circumferential direction of the fuselage.
The researchers are currently working with partners on a new project: In this case, the test setup is mounted on an aircraft fuselage, which is made with new production processes. However, this is tested on the ground. The falling air pressure at high altitudes is simulated by air which is filled into the body and which increases the internal pressure in the cabin.