The competition between metal constructions and innovative fiber-reinforced polymers (FRP) designs in the aerospace industry is going into the next round. For structural components, carbon fiber reinforced thermoplastics are seen as the future ‘material of choice’. Contrary to established process routes of thermoset prepregs with autoclave curing, which is used predominantly for large structures such as fuselage skin, wing shells, and their structural bracing, so-called out-of-Autoclave (ooA) processes of fiber-reinforced thermoplastics are expected to lead to drastic cost savings in the production of FRP components. Unlike thermosets, thermoplastics do not cure under influence of heat by chemical cross-linking processes, but solidify in shape after cooling without permanent cross-linkings. Consequently, thermoplastics can be nearly reshaped as often as required. The formation of adhesive bonds induced by polymer diffusion in the molten state opens up new possibilities in processing and production of FRP.
As a result, fusion bonds, welded joints, and co-consolidations are possible. Ultimately, it is possible to manufacture integral components and simplify assembly work by eliminating the need for form-fit joining elements. Since thermoplastics can be stored at room temperature for an almost unlimited period of time, the complex cooling chain, which is for thermoset PrePregs, is completely eliminated.
So far, thermoplastics have been used mainly for small primary-structure components (i.e. frame and stringer bracing for AIRBUS A350 XWB), manufactured in stamp-forming process. Recent efforts in research aim to produce also large-scale structures like fuselage components and wing shells. The advantages that are to be exploited are manifold. Of great importance is a possible weight saving, justified, inter alia, by the fact that greater toughness of thermplastics offers greater resistance to crack propagation and therefore thinner laminates can be used. In addition, high-performance thermoplastics meet aviation-specific FST (Fire/Smoke/Toxicity) requirements which, in combination with their chemical resistance, predestines them for use in passenger aircrafts. Furthermore, the reversile fusibility of thermoplastic semi-finished products holds great potential for recycling end-of-life components.
In cooperation with the aerospace supplier Premium Aerotec GmbH (PAG) and the German Aerospace Center (DLR) Augsburg, the Institute for Composite Materials (IVW) managed to manufacture a large-scale structural component made of carbon fiber reinforced polyether-ether-ketone (PEEK). Thus, organo sheets were used to mold rear pressure bulkhead segments (AIRBUS A320) on 1:1 scale (please see Fehler! Verweisquelle konnte nicht gefunden werden.). The main challenge was the draping of organo sheets in the size 1.5 m into the doubly curved mold without any wrinkles. The innovative solution is a system developed by IVW and PAG which offers constant tension on the material during molding, and thus prevents wrinkling and distortion of the textile. Compared to the current aluminum design, the CFRP variant has less weight for the same mechanical properties and production is economically more efficient due to shorter cycle times.
In the scope of OSFIT (One-Shot Fully Integrated Thermoplastic-Frame), a publicly funded research project, IVW is developing a process chain to manufacture integral frames made of PEEK in cooperation with PAG, Fraunhofer Research Institution for Casting, Composite and Processing Technology (IGCV), Automotive Center Südwestfalen GmbH ( ACS), and Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM). By combining the processes of automated tape laying (ATL), thermoforming and co-consolidating, the potential of thermoplastics is exploited most effectively. Tapes are processed into 2D-preforms, pre-assembled and then shaped into the final geometry in an innovative one-shot process combining the two steps, molding the tape-preform and joining of reinforcements into one step. The process is shown schematically in figure 2.
The biggest challenge in ooA-processes for the production of large thermoplastic structures is process robustness in the sense of laminate quality. Besides the automation of manufacturing processes, this is the main part of current research.
The research project OSFIT (One-Shot Fully Integrated Thermoplastic Frame) is funded by the BMWi (Federal Ministry of Economics and Energy) within the framework of LuFo-V-3 (funding code 20W1706C).