It’s thermo - dynamic!

Winning an Innovation Award at JEC Composites was the icing on the cake for Fokker Aerostructures' development team. As Dr Neil Calder discovers, the roots of this innovation go much deeper into the development of the portfolio of design and manufacturing capabilities for the new Gulfstream G650.
Fokker manufactures the body panels, floor and the tail structure for the Gulfstream G650. Its first flight was in November 2009 and is expected to achieve certification within two years. At Mach 0.925 it will be the fastest civil aircraft in the sky, but the development of the manufacturing technology that makes its structure special has been a bit slower to mature.

With the G650 vertical tail rudder and horizontal tail control surfaces, Fokker has replaced assemblies of thermoset composites and aluminium with more monolithic structures using induction welded thermoplastics. The novel material component of this is Ticona's Fortron polyphenylene sulphide (PPS) resin, which is pre-impregnated into carbon fibres by TenCate and has now been qualified for use in aircraft primary structures – the first time thermoplastics have been used in this way. Fokker reports 10% weight reduction and 20% cost reduction for these tail structures compared with alternative processing routes.

There is a strong current trend towards a greater use of thermoplastics in high performance composites structures, driven by considerations of mass reduction as well as tackling issues of sustainability and recyclability. Thermoplastic composites are already well established within aircraft interiors but airframe components in primary structure from this class of material are just emerging. This progression of thermoplastic resins from secondary to primary structure is opening up the design and manufacturing envelope with a new set of production characteristics.

There are numerous engineering reasons why thermoplastic composites are attractive as aerostructures, such as increased toughness compared with thermosetting alternatives and inherent flame retardancy. Because the processes of material consolidation and forming don't involve exothermic curing reactions they can use shorter autoclave cycle times, although the temperatures involved are generally higher than those for thermosets. The meltability of the material also provides better potential for recycling.

The composite materials used by the Fokker-led team are a new generation of carbon/PPS semipregs. Instead of being impregnated with resin films as is more usual, these fibre-reinforced thermoplastic materials are made using a powder coating process. PPS powder is directly applied and fused to the carbon fabric which cuts out a number of processing steps and brings the material to a price level comparable to that of traditional thermosetting composites. It also makes the subsequent processing simpler as the material is easier to form and the end result is stronger.

Fokker worked extensively with TenCate on the previous development of Cetex glass/PPS resin as a semi-impregnated material to overcome manufacturing challenges for earlier Airbus products and this relationship has been furthered in the current context of the carbon reinforced Fortron material system.

Design and manufacturing technology has been advanced through incremental phases over the last decade. The production of Gulfstream's tail demonstration items is the tip of this iceberg with most of the critical steps along the way in this particular development hidden below the water line. It is always easier to see where things came from – much harder to navigate the course for the first time.

Fokker has manufactured the tail sections for the Gulfstream 5 series aircraft since 1993, and has been moulding thermoplastic leading edge components with glass fibre reinforcements for Airbus since 2002. Previous work in welding the A340 and A380 J-nose leading edge structures used the resistance heating effect of electrical current passed though an embedded metal mesh about 0.2mm thick. The electrical conductivity of carbon fibres makes this method impossible and so joule heating in the joint area is induced through a strong electromagnetic field. An induction welding assembly technique for carbon fibre-bearing thermoplastic resin was patented by KVE Composites Group in 2007 and has now been licensed to Fokker who has industrialised this process and taken it into series production.

Once the process had been established by KVE, the main challenge in productionisation was the creation of the induction welding jig and the establishment of the design rules for this. The weld tooling has to be made from an electrically non-conducting material and must provide the restraint forces sufficient to ensure that the joint faces have a positive pressure throughout the joining cycle. It had to satisfy the conflicting requirements of no metallic materials within at least 150mm from the weld zone, reliable holding and clamping forces generated on the details parts during welding and sufficient access for the welding head. It also has to provide effective and controllable heat sinking.

Development work used thermocouples located in the joint for process control, but series production is without them. This presented one of the major scale-up challenges. The material must have a particular time and temperature profile to obtain good entanglement of the PPS polymer chains. Research has shown that the welding process is tolerant of larger gaps than would be possible in mechanical joining. PPS has dimensional stability at elevated temperatures which is a critical factor in welding without causing consequential problems of distortion or residual stressing of components. The structure displayed at JEC still includes the occasional chicken rivet to prevent peel failure of the joint, but the next step may be to trust in structural integrity without these. By successfully manufacturing and testing pre-production components, the induction weld process was qualified and released for series production by September 2009.

The whole project has demonstrated the fact that the engineering of aerospace composites is a partnership-rich environment. It has been driven by Fokker, but has involved all parts of the engineering and manufacturing value chain from raw material to customer. The effective collaboration between partners has been well demonstrated: Ticona produce the resin, TenCate pre-impregnate carbon fabric with this and Fokker process this using welding technology developed with and licensed from KVE. Gulfstream has been the end customer and has enabled the certification of this combination of materials and processes.

The team is now set for more and bigger challenges, and as TenCate is now transferring the materials technology to unidirectional tape, this means that larger structures such as a whole tail could be achievable.

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