Composite future draws ever closer

 - July 7, 2008, 10:03 AM

Ongoing research into new composite materials is expected to yield major enhancements in performance, weight and cost for the aerospace industry in the coming years. New ways of laying up carbon fiber, such as weaving, are already enabling more complex shapes. Thermoplastic resins are making manufacturing easier, and the practice of integrating several functions into one part is reducing part counts.

“Woven composites account for a large part of the composites’ future,” said Nicolas Carrère, a research engineer with French agency Onera. Instead of being aligned, fibers are spun together into strands. The diameter of one fiber measures close to 10 microns, while that of a strand is roughly one millimeter. “You can then weave the strands in three dimensions, which improves impact resistance,” Carrère emphasized, adding that, after molding, the part is very close to its final shape so little further machining is needed. Snecma has tested woven composites in engine fan blades.

Another way to optimize the use of carbon fibers would be to lay up the plies in only one direction, rather than in multiple directions. Currently, multiple layers of unidirectional plies result in an almost isotropic material. “Having fibers in every direction reassures airframers, but it is not optimal. Instead, plies should be laid up only in the direction that has to withstand forces,” Carrère said. This could immediately yield major weight savings. Structures could be better modeled, making safety margins more accurate, which would trigger further weight savings.

Thermoplastic, as opposed to thermoset, resins are now close to finding an application in airframes. Benoît Sagot-Duvauroux, operations coordinator at the EMC2 competitive hub in France, sees the Airbus A350XWB as the first application for such resins. The new resins have a number of benefits over thermoset resins.
“They have a better impact resistance,” Carrère said. Two parts can be bonded together, whereas as thermosets can only be bolted. Moreover, these resins are recyclable.

Sagot-Duvauroux added that, after they are cured, thermoplastics can have their shape changed. For example, they can be twisted. In addition, they can be used to create a number of different parts.

But the new materials are not free from challenges. For instance, manufacturing thermoplastics with a conventional resin transfer molding (RTM) process is tricky. “These resins are difficult to inject,” Carrère explained. In addition, production costs may be higher with thermoplastics. However, Carrère deemed this partly an issue of economy of scale, meaning that the expense associated with them might decrease as their use in production increases.

Sagot-Duvauroux insisted there is a huge potential in making composite parts multifunctional, to decrease the parts count. The idea is to manufacture parts that can perform several functions. Designers have to change the way they think, Sagot-Duvauroux said. So far there has been one part per function. Now, one part can integrate more than one function. For example, a piece of furniture can also be a stiffener. In addition, one cabin component could have a water duct integrated. Or wires could be sunk into the matrix.

Nonetheless, composites remain more expensive than metal alloys. Even so, a multifunction composite part can be cheaper than a conventional subassembly. “The parts count can be reduced from 25 to three or four, for example,” Sagot-Duvauroux claimed.

The resulting cost reduction is very difficult to assess, however. “The processes are so different that airframers may only have a rough idea of multifunction composite part’s contribution to cost cutting,” Sagot-Duvauroux explained.

Finally, in terms of multifunctionality, adding properties to the material could save weight. For example, electric conductivity relies on mesh or expanded metallic foil embedded in composite panels. “Having conductive particulates or carbon nanotubes in the matrix would be lighter and cheaper,” Carrère said.