National Composites Centre (UK) announced that it has achieved a world-first full scale 17-metre integrated wing skin infused in one step.
To meet the growing performance and production demands of the next generation of fuel-efficient, low-emission aircraft, the aerospace industry must simultaneously decrease aircraft weight, lower costs, and increase production rates by a factor of ten beyond today’s state-of-the-art capabilities. To address this challenge and to explore innovative new approaches to manufacture aircraft wings, Airbus created the Wing of Tomorrow programme.
Starting back in 2018, a collaborative team of engineers at the National Composites Centre (NCC) investigated materials, manufacturing, and component sub-assembly techniques for the upper wing cover utilising a pyramid of development which led to the ultimate delivery of three full-scale, 17 metre wing demonstrators.
Composites for aircraft wings
Composites enable wing components to be fully weight optimised and produced with significantly reduced or eliminated sub-assembly and post-manufacturing costs. They also enable faster production cycles.
Today, this is mainly achieved through the use of pre-preg composite materials – and developing the technology to realise the potential rate increase with dry fibre composites is a significant opportunity. To pursue this, the NCC has been working with Airbus to drive dry fibre composites innovation for future wing components and assemblies.
The NCC was tasked with developing technologies and processes to produce three, full-scale, wing cover demonstrators in support of the next generation single-aisle aircraft. In order to decrease the final assembly duration, the target for this program was that the wing cover should also include high levels of component integration, saving assembly time and weight.
Transforming wing cover manufacture
To automate the dry fibre fabric ply lay-up of the skin at such a large scale, the NCC applied the latest digital design tools to simultaneously evaluate different configurations of cutting and layout. This new design philosophy demanded new technical manufacturing capabilities, with the Wing of Tomorrow team driving innovation in automated lay-up and full-scale dry fibre infusion of the wing upper cover.
Automated high-rate deposition
As part of their core composites research and development mandate, utilising the High Value Manufacturing Catapult and Aerospace Technology Institute (ATI) funding, the NCC has developed a world-first high-rate deposition technology – which was designed to specification and supplied by UK manufacturer Loop Technology with collaboration from Güdel and Coriolis. It comprises two bridges, weighing 45 tonnes and 24 tonnes, 7 metres high by 13 metres wide, running along a 26-metre track. These bridges position automated end-effectors to enable cutting and deposition of dry fibre materials to high levels of quality and speed.
The automated process begins at a 20-metre table positioned inside the cell where an ultrasonic cutter profiles the carbon fabric to shape. An algorithm then selects the correct end-effector to pick the material up and then lay it onto the tool. Once lay-up of all the plies, processing and integration is complete, the component can then be infused with resin and cured.
Through Wing of Tomorrow trials, the NCC team has pushed the capabilities of dry fibre deposition technology and successfully demonstrated that automation can be achieved, with the potential to significantly increase production rate. For Wing Cover 3, the complete ply stack of dry fibre piece parts (approximately 170 individual dry fibre pieces) was deposited using the NCC’s Ultra High Rate Composite Deposition, with no manual intervention. During this production process, different rates of deposition were trialled and some of the full 17 metre length plies were deposited at close to maximum rate (0.8m/s). This equates to approximately 30 seconds for an individual 17 metre ply.
Large-scale resin infusion
In parallel, the team has achieved a world first in large-scale, highly complex single shot integrated structure resin infusion. Traditionally, sub-components are supplied separately, which are bonded or mechanically fastened to the upper wing cover during manufacture and/or assembly. For the Wing of Tomorrow demonstrators, the team cut the need for this stage by creating a large-scale infusion capability and using novel methodology. Due to the complex geometry of the wing and the size of the component, NCC and Airbus engineers had to devise a new infusion strategy – ensuring a sophisticated resin delivery system infused each sub-component at exactly the right time, avoiding defects that would affect part integrity from being generated.
The NCC has delivered the final of the three wing covers. This demonstrator was built to show the rate capabilities of automated deposition technology. Wing Cover 1 proved to Airbus they could successfully manufacture and assemble the new integrated wing cover with the other wing components from across their supply chain. The second cover was sent for mechanical testing. As part of the programme trials, the NCC demonstrated that the rates achieved, when extrapolated, will lead to deposition volumes reaching in excess of 350kg per hour, with material being deposited from the head onto a complex double curvature tool.
Fibre friction coefficient determination:
A contribution to increasing precision and efficiency in the manufacture of filament-wound components
The Leibniz Institute for Composite Materials GmbH (IVW), Germany has been active in the research field of fibre composites for more than 30 years, from the concept to the finished part. During this time, it has also been able to acquire profound knowledge in the field of filament winding technology. Fibre winding is an established process that is widely used to manufacture rotationally symmetrical fibre composite components such as pipes, pressure vessels and shafts. Over the years, it has proven to be an extremely efficient method for producing complex structures. Due to the often highly curved surfaces that need to be wound on with fibre material, fibre adhesion on the surface is essential. Too little fibre adhesion can lead to a displacement of the rovings, which are then no longer on the predetermined path, but in an undefined area of the component, without contributing to the overall strength of the component to the calculated amount. Among other things, precise knowledge of the friction coefficient between the roving and the surface is therefore crucial for its planning and control, irrespective of the specific winding process. In the worst case, an incorrect estimation of the frictional properties can significantly change the quality of the wound component. In addition, unwanted slippage of the fibres on the component can lead to disruptions in the process flow.
Fibre displacement on curved surfaces always occurs when the geodesic path is left and the adhesion force of the roving is not sufficient to hold the fibre in position. The geodesic path describes the shortest connection between two points on a surface. Uniaxial stress state prevails here, which means that forces only occur along the longitudinal axis of the fibre. Outside of the geodesic path, restoring forces act on the fibre, which favors slippage. A simplified sketch of the equilibrium of forces acting on the roving as well as the effects of insufficient adhesion force is shown in Figure 1.
The exact determination of the coefficient of friction between fibre and surface is crucial for the entire winding process, however, it is a complex task influenced by many factors (surface condition, temperature, fibre type, etc.). Therefore, various methods have been developed in the past to determine the coefficient of friction. A common determination technique is the capstan method, in which a bundle of fibres is placed over a roller and loaded with a weight. The resulting forces allow conclusions to be drawn about the friction properties. However, due to the specific design of the measurement system, the coefficient of friction can only be determined along the fibre. As a result, the findings of the measurement can only be transferred to the prevailing stress state during the winding process to a limited extent. Another common method uses specially shaped winding mandrels to visually record the slippage behaviour of the fibres. This allows to determine the behaviour very close to the real production process, but the visual evaluation offers only limited possibilities for automating the measurement process. In practice, other methods are used to determine the coefficient of friction, but they all have different limitations and disadvantages.
In order to obtain repeatable and automatically evaluable results for the coefficients of friction of fibres, a system for determining the coefficient of friction transverse to the longitudinal direction of the fibres was developed at IVW. This makes it possible to circumvent some of the major disadvantages of current measurement methods and thus to perform repeatable and automated measurements for determining the coefficient of friction. Both, material parameters such as the fibre type and process parameters such as the surface property for deposition, can be varied and adapted to the desired conditions (fibre-fibre contact, different take-off angles, temperature, aging, etc.). The highly variable setup thus offers extensive potential for investigating the coefficient of friction and its influencing factors. Especially in the manufacture of hydrogen pressure tanks and their specially shaped dome area, knowledge of the exact coefficient of friction is of crucial importance as this has a direct influence on the quality and performance of the composite shell produced. To optimize the layer structure of the pressure tank, it may be necessary to leave the geodesic path. Using an incorrect value for the coefficient of friction can lead to irregular fibre placement, which in turn affects the mechanical properties of the final product. In addition, a correct coefficient of friction can reduce the necessary transfer paths between layers of high and low winding angles to the absolute minimum. Therefore, accurate characterization and control of the coefficient of friction is a critical factor in ensuring that manufactured pressure tanks have the required strength, reliability and safety. In addition, the resulting optimized layer structure and reduction of transfer paths can save cost-intensive fibre material, resulting in a more economically efficient process.
Figure 2 shows the setup of the test rig in a test-ready state with clamped fibre and a characteristic measurement result for a friction value measurement of a carbon fibre towpreg. During the buildup of the fibre adhesion force, a force increase can clearly be seen in the course of measurement. After exceeding the adhesion force, a sharp drop in the measured force follows. This then changes to a sliding friction curve.
The results of the tests can be used to determine the influence of temperature, aging, surface properties, etc. on the adhesion force of the roving on a defined surface. Transferring the results to the practical winding process supports to further optimize with regard to the economy of the overall process and component performance. As a result, the load-path-compliant design of struts, tubes, or pressure vessels such as hydrogen storage tanks, can be precisely adapted to the specific adhesive properties of the fibre material. Slippage of the fibres and the associated reduction in component performance is thus avoided. In addition, precise knowledge of the coefficient of friction means that transfer paths within wound components, which are unnecessary for component strength, can be reduced to the absolute minimum. This saves both fibre material and weight.
For more information:
Figure 1: Fiber displacement due to insufficient adhesion force (left); Simplified force equilibrium on winding surface (right)
Figure 2: Test setup ready for testing with fiber clamped (top) result of a series of four tests, including reference