Composites for aeroplanes: light as a feather?

Since the birth of manned air travel, the aerospace industry has been on an end­less quest to make planes light­er. From Clé­ment Ader and the Wright broth­ers’ first aero­plane designs to those of today, redu­cing weight much as pos­sible is a determ­in­ing factor. “The aerospace industry spares no expense for each kilo­gram saved, around 100 times more than in the auto­mot­ive industry,” Patri­cia Kraw­czak, a pro­fess­or at engin­eer­ing school Ecole Nationale Supérieure des Mines-Télé­com de Lille-Douai, says. On aver­age, the auto­mot­ive industry is pre­pared to pay €1 per kilo­gram saved, against €100–500/kg for civil avi­ation, and up to €10,000/kg for space travel.

Cur­rently, com­pos­ites are the most prom­in­ent mater­i­als con­trib­ut­ing to this quest. Made from fibres (usu­ally car­bon) con­nec­ted by poly­mer res­in, com­pos­ites have been increas­ingly used by the aerospace sec­tor. In fact, most recent mod­els of air­craft, like the Air­bus A380 or the Boe­ing 787, are made from approx­im­ately half com­pos­ites (along with 20% alu­mini­um, 15% titani­um and 10% steel). “We’ve reached a plat­eau,” Ms. Kraw­czak remarks, “espe­cially since metals like titani­um and alu­mini­um are also get­ting bet­ter.” Com­pos­ites must there­fore evolve.

Engin­eers are work­ing towards sev­er­al object­ives, the first of which is to cut the costs of pro­duc­tion. While com­pos­ites do have the advant­age of being light­weight and guar­an­tee­ing good mech­an­ic­al per­form­ance, the cost of pro­duc­tion is still high­er than that of their met­al coun­ter­parts. To make pro­duc­tion faster and cheap­er, engin­eers are cur­rently look­ing into so-called “ther­mo­plastic” poly­mers. Unlike “ther­moset” poly­mers, which, once hardened, can­not be softened or worked, ther­mo­plastics remain weld­able, work­able and even recyc­lable. They also do not emit volat­ile organ­ic com­pounds, pol­lut­ing gases that are often toxic.

Mak­ing com­pos­ites from ther­mo­plastics is more com­plic­ated, how­ever, because these poly­mers are less flu­id and per­meate fibres less eas­ily. This means that the entire pro­duc­tion chain would have to be redesigned to make these mater­i­als ful­fil the spe­cific­a­tions of the aerospace industry. But, should it work, it would mean few­er assem­blies, less waste from man­u­fac­tur­ing, or waste that could be reused, and bet­ter recyc­ling of parts at end-of-life.

Nowadays, com­pos­ite parts are gen­er­ally made in auto­claves, a sort of huge pres­sure cook­er that “cures” the com­pos­ite. These machines can cost as much as hun­dreds of thou­sands of Euros for the highest-per­form­ing ones, cap­able of pro­du­cing large parts at high tem­per­at­ures and under great pres­sure. The pro­duc­tion cycle takes sev­er­al hours, dur­ing which the equip­ment is locked in place. “We are work­ing on out-of-auto­clave pro­cesses, which are cheap­er and more flex­ible, such as infus­ing or inject­ing liquid res­in dir­ectly into a fibre pre-form, i.e. a ‘skel­et­on’ of fibrous rein­force­ments,” she adds. How­ever, sim­pli­fy­ing man­u­fac­tur­ing pro­cesses should not impact qual­ity. Parts must com­bine safety, reli­ab­il­ity, per­form­ance – all under rel­at­ively high temperatures.

Com­pos­ites also have oth­er qual­it­ies. Not­ably, they are mul­ti­func­tion­al, i.e. they provide more than just mech­an­ic­al power. For example, there are self-repair­ing com­pos­ites that integ­rate cap­sules that poly­mer­ise, and thus “heal,” the dam­aged part. It is also pos­sible to insert sensors and actu­at­ors into com­pos­ites that mon­it­or the parts’ wear or reshape it on com­mand. Oth­er poten­tial func­tions include trans­mit­ting data or pro­du­cing energy through piezo­elec­tri­city to sup­ply power to smart objects.

Finally, thanks to addit­ive man­u­fac­tur­ing (3D print­ing), it is now pos­sible to optim­ise the shape and struc­ture of parts and com­pon­ents, and to design new parts. The poten­tial gains are enorm­ous: no more moulds or cut­ting, res­ult­ing in less raw mater­i­al waste. “We now know how to use addit­ive man­u­fac­tur­ing to depose a poly­mer rein­forced with short or con­tinu­ous fibres,” Ms. Kraw­czak says. “This isn’t always pos­sible, espe­cially for lar­ger parts, but com­pan­ies such as Safran and Stelia Aerospace are work­ing on it.” Some planes already have 3D-prin­ted metal­lic parts that have passed all qual­i­fy­ing checks.

While it’s true that new man­u­fac­tur­ing tech­niques and ther­mo­plastics are mainly aimed at main­tain­ing com­pos­ites’ mar­ket share for the aerospace industry, design innov­a­tions could reduce the weight of parts by 20–30%, which is cer­tainly not neg­li­gible in the con­text of industry efforts to decrease green­house gas emissions.