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Sometimes one has to leave the beaten path and take a chance. This is called innovation – something that is more talked about than practised. After all, it is more comfortable to keep on doing what one knows how to do well!
(Published on June 2008 – JEC Magazine #41)
MAURICE Reyne INGÉNIEUR-CONSEIL, EXPERT ANVAR, A2I, MINEFI
In aviation, the need for weight reduction has led to replacing aluminium with composite materials to consume less fuel for larger payload. Aircraft manufacturers are proud to inform us that these materials will constitute more than 50% of the structural weight on future airplanes. And yet, we can already note two errors due to lack of knowledge about the possibilities of these materials, their processing techniques and... a reluctance to take a chance, as the two following examples show.
Nose cone and fuselage
The nose cone and fuselage of a metal aircraft (for which the design already was adapted from wooden aircraft) involve a “squirrel-cage” structural design that combines struts of cylindrical section with stringers to form a skeleton, covered with riveted sheet-aluminium cladding. If you use the same type of metal skeleton, but simply introduce composites to replace the aluminium cladding with composite plies, you are still reproducing the same structural design; and even if composite profiles or stringers are used, the basic concept remains the same.
Typical fuselage structure
But composites can make new techniques possible! In particular, you can eliminate the conventional skeleton to create an allcomposite fuselage, using robotized lay-up or winding techniques on a mould. In this case, adhesive bonding replaces most of the riveting for the assembly. This is what Boeing did for its 787. Although it can’t really be considered a true innovation, it does reduce the aircraft’s weight and manufacturing cycle time.
Breakthrough by changing material and process
A real innovation in terms of reducing cost and lead times would be to change the type of composite and manufacturing process used. The aviation industry currently uses a carbon/epoxy combination. Because it is a thermoset composite, carbon/epoxy requires polymerization in autoclave, and this involves heavy investment and a cure time of about one day.
Thermoset resins, which can be supplied in liquid form, facilitate the impregnation of reinforcement. This is a more complicated operation for thermoplastic resins, which generally come in pellet form. For this reason, thermoplastics took longer to develop.
We have known for nearly a decade how to make engineering thermoplastic composites using PPS, PEI, and PEEK plastics reinforced with long carbon fibres. Airbus designed part of the leading edge of its A380 aircraft in carbon/PPS, but declined to pursue this innovatory direction in spite of its promise. Engineering thermoplastic composites have the following advantages:
There are a few disadvantages that can be overcome:
All in all, the economic balance leans in favour of thermoplastic composites – and European prepreg manufacturers know how to make them.
Comparative costs for a fuselage skin panel made in thermoset composite or thermoplastic composite (tooling costs roughly the same for both)
The above example is not necessarily an exact one, but the comparative magnitudes have been respected. This means that one should not make a decision based on the price of the raw materials, but always do a complete economic feasibility study. The savings here are due mainly to processing speed. Here again, we should note that long-fibre-reinforced thermoplastic composites are only just emerging and that their cost will decrease with economies of scale.
Like the thermoplastics that replaced thermoset resins in the 1970s, thermoplastic composites will inevitably become the materials of choice in the near future, for technical and economic reasons, as shown by the table below:
It is time to understand this and prepare for it. It is not enough to talk about innovation – we must practise innovation by daring to change our comfortable, but ultimately inadequate routines.