JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
Thermoplastic composites have inherent properties that make them ideally suited for low-weight, low-cost aerospace structures. These composites boast high toughness, reprocessability and excellent fire safety performance.
ARNT OFFRINGA R&D DIRECTOR FOKKER AEROSTRUCTURES(published on June-July 2010 - JEC Magazine #58)
Steps are currently being taken to move towards large primary structures, e.g. integrally-stiffened skin panels. New materials, innovative design concepts and aeronautics are key innovation factors.
Thermoplastic composites in aerospace to date
Thermoplastic composites are of growing interest in the aerospace sector because of their inherent properties. Their high toughness results in low-weight designs. The physical process only involves melting without a curing cycle, which results in fast and robust processing. Their excellent FST (fire, smoke and toxicity) properties and recyclability are also advantageous. Airbus recently teamed up with a Dutch cluster on the TAPAS project. An international research centre for thermoplastic composites, called TPRC, was also founded in the Netherlands in 2009.
Currently, a growing number of thermoplastic composite parts and assemblies can be found in state-of-the-art aircraft. Examples include multi-ribs concepts in many aircraft programmes and welded fixed wing leading edges for the A380 (Fig. 1) made from Ten Cate’s glass/PPS.
Thermoplastic composite primary structures are an area where inroads have been made in the past. The pressure bulkhead floors of the Gulfstream 550 and G650 aircraft (Fig. 2) are a series of structurally-bonded carbon/PEI sandwich structures with pressformed stiffeners.
Airbus produces the cockpit floor of the A400M in thermoplastic composite.Recently, steps have been made to move towards welded primary control surfaces. The rudder and elevators of the new Gulfstream G650 business jet (JEC 2010 Innovation Award winner) are induction-welded multi-rib torsion box structures (Fig. 3).
Induction welding, a technology researched by Dutch specialist KVE Composites Group, was industrialized by Fokker Aerostructures (Fig. 4).
The carbon/PPS (Ten Cate Advanced Composites) multi-rib design is 10% lighter and 20% cheaper than its carbon/epoxy sandwich design predecessor. Replacing adhesive bonding and bolts by welding is an important cost reduction factor, as are the pressforming of ribs and the ease of layup/consolidation for skins and spars.
Development of an integrally-stiffened thermoplastic skin concept
Development of an integrally-stiffened thermoplastic skin concept
In 2003, an airliner floor beam (Fig. 5) was developed in carbon/PEKK UD tape (Cytec). This high-build-rate product is suited for aeronautics, so that pick & place was chosen as the layup method for web and cap preforms. These are then melted together in a process called co-consolidation. During the development of the I-beam shaped component, a method was sought to simplify the manufacture of large numbers of stiffening ribs. The solution was to ‘butt joint’ a flat laminate by co-consolidating it with the basic I-beam (Fig. 6). This proved an effective solution, much simpler to manufacture than the initial design with press-formed stiffeners. The butt joint strength was subsequently optimized by adding a radius using an injection-moulded filler (Fig. 7).
The development of the butt joint system opened a whole new range of opportunities for innovative design, such as a sine wave beam (Fig. 8), which normally is not easy to achieve in composites. Another design that sprung from the butt joint concept was an integrally-stiffened skin panel.
T-stiffeners are made from flat preforms and co-consolidated with a skin. The panel concept was developed and tested successfully in shear and compression together with the Dutch National Aerospace Laboratory in 2008 (Fig. 9).
Large thermoplastic primary structures
The Dutch industry, institutes and Airbus have been cooperating in the area of thermoplastics since 2005. This cooperation was intensified in 2009. Approached by Airbus, the Dutch Ministry of Economic Affairs initiated a cluster of Dutch companies and institutes. They joined forces with Airbus and launched a 4-year project called TAPAS: Thermoplastic Affordable Primary Aircraft Structure.
The goal of the TAPAS project is to create the thermoplastic composite technology necessary to produce large primary structures for future aircraft programmes. Materials, manufacturing processes, design concepts and tools must be developed to a technology-readiness level (TRL) 6. Full-scale demonstrator products are being developed as part of this effort. The technological challenges include: development and qualification of suitable materials, the butt joint, and manufacturing technologies such as fibre welding, press forming and fibre placement.
Partners in the TAPAS project are Airbus, Fokker Aerostructures, Ten Cate Advanced Composites, Airborne Technology Center, KVE Composites Group, DTC, Technobis Fibre Technologies, Technical University Delft and the University of Twente. The Dutch National Aerospace Laboratory carries out related research projects and testing programmes in parallel.
Ultrasonic fibre placement
Fibre placement is the lay-up technology of choice for large surface area products with a double curvature. The currently available fibre placement equipment uses different heating systems, such as hot gas and laser. In 2008, Fokker Aerostructures started to look for suitable fibre placement equipment. Unfortunately, a chicken-and-egg situation arose in which serious investment was needed upfront whereas the production volume was still some time away for customers. In this context, ultrasonics appeared to be a good option for the heating source at a relatively low-cost.
Fokker Aerostructures has a long history of ultrasonic spot welding experience, such as welding injection moulded parts to thermoplastic plate components. Moreover, the layup of A380 fixed wing leading edge skins is done by ultrasonic spot welding (Fig. 10). Ultrasonic welding is a wellestablished, low-cost welding process.
Although continuous ultrasonic welding of plates onto plates had proved difficult in the past, it seemed feasible to ultrasonically weld a thin carbon/UD tape onto a stack of UD tapes. To this end, ultrasonic welding units were integrated into a fibre placement head mounted on a robot (Fig. 11).
The initial trials – welding 10-mm-wide carbon/PEKK UD strips by hand – were successful. The next step was to weld on a linear rail. By mid-2009, a 2x1x1 metre 3D R&D fibre placement cell with a single 10 mm-wide strip was built by Dutch aeronautics specialist Boikon. The UD strips were melted over a small portion of strip width, enough to affix the tape. With ultrasonic fibre placement, the challenge was to succeed in tacking a tape onto an underlying thick stack of plies at high speed, without slowing down the process and thus making it ineffective.
A new fixed wing leading edge concept was laid up with the fibre placement cell (Fig. 12).
The product development process, which started as an internal Fokker product improvement exercise, is now a part of a European Framework project called COALESCE (Cost Efficient Advanced Leading Edge Structure).
The new leading edge is a multi-rib thin skin design. The ribs are flat plate preforms (Fig. 13) which are butt joined to the skin during melting (co-consolidation) of the product.