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.