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Material · Synthetic

Carbon fibre composite

Carbon fibre in a resin — stiffer than steel for its weight, and nothing like as strong as the fibre's own number suggests.

A carbon fibre composite is carbon fibre held in a resin, usually epoxy, and its reason for existing is stiffness per unit mass. Along the fibres it is several times stiffer than steel for the same weight, which is the property aerospace and racing are actually buying.

The number that gets quoted is almost always the fibre's. A single carbon filament reaches several thousand megapascals along its axis; a laminate contains perhaps 60 per cent fibre by volume, oriented in several directions because a structure is loaded in several directions, and reaches a fraction of that. Quoting the filament figure for the part overstates it by a factor of several, and it is the commonest error made about this material.

Its weaknesses are specific. It is far weaker across the plies than along them, so a laminate delaminates rather than breaking. It is poor in compression relative to tension. It fails suddenly and without deforming first, which removes the warning an engineer relies on in metal. And impact damage can be invisible from the surface while the plies underneath have separated — which is why aerospace composite structure is inspected ultrasonically rather than by eye.

Processing

Pre-impregnated fabric cured under pressure in an autoclave gives the best and most consistent properties and is how aerospace structure is made. Resin transfer moulding and infusion are cheaper and are taking automotive volume. Filament winding for cylinders. Hand lay-up for one-offs and repairs.

The autoclave is the bottleneck: it is a large pressure vessel, cycles take hours, and out-of-autoclave systems have been an industry objective for twenty years.

It is joined by bonding and by bolting, and both are awkward. A drilled hole cuts the fibres that carry the load, so bolted joints need local reinforcement; bonded joints need surface preparation that is hard to verify.

Uses

Aircraft primary structure — the wings and fuselage of the Boeing 787 and Airbus A350 are more composite than metal, which is the change that took the material from secondary parts to the airframe itself. Rocket structures and pressure vessels. Racing car monocoques, where it is a safety structure as much as a light one. Wind turbine blade spar caps.

Bicycles, sporting goods, prosthetics and a great deal of consumer product where the woven surface is bought for what it signifies as much as for what it does.

History

The fibre dates from work at the Royal Aircraft Establishment in the 1960s. The composite moved from secondary structure — fairings, control surfaces — to primary structure over the 1980s and 1990s, and the argument each time was whether a material that fails without warning could be certified for a part whose failure brings the aircraft down. The answer was damage tolerance analysis and ultrasonic inspection rather than a change to the material.

Environmental impact

Energy intensive to make — the fibre alone takes several times the energy per kilogram of aluminium — and not recyclable as itself. Pyrolysis recovers shortened fibres suitable for moulding compounds rather than for structure, which is downcycling by any honest description.

The case for it is entirely in use. A kilogram removed from an airliner saves fuel for twenty-five years, and the arithmetic on that is comfortably favourable. The same argument does not transfer to a bicycle frame or a phone case, and it is routinely made there anyway.

Galvanic corrosion is a practical consequence worth knowing: carbon is electrically conductive and cathodic to aluminium, so the two in contact in a wet environment corrode the aluminium quickly. It is a real reason titanium and PEEK appear where aluminium otherwise would.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

contains

  • Carbon — element · the fibre, and it is electrically conductive — which is why an aluminium fitting bolted to it corrodes

is composed of

  • Carbon fibre — material · and quoting the filament's strength for the laminate overstates it by a factor of several, which is the commonest error made about the material
  • Epoxy resin — material · the matrix, which holds the fibres in place and transfers load between them

is used as

  • Structural engineering — application · aircraft primary structure since the 1990s, where stiffness per unit mass is what is being bought

is used in

  • Aerospace manufacture — industry · primary structure — a modern wide-body wing and fuselage are more composite than metal

is an alternative to

  • Glass fibre composite — material · five times the stiffness at 80 per cent of the weight, and perhaps ten times the price — which is why a hull is glass and a wing is carbon
  • 7000 series aluminium alloy — alloy · stiffer and lighter, and it fails without deforming first — which removes the warning an inspector relies on in metal, and is why composite structure is inspected ultrasonically

is produced by

  • Heat treatment — process · laid up as fibre and resin and cured under heat and pressure, usually in an autoclave — and the cure schedule is as much of the specification as the fibre is

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Carbon fibre composite back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Carbon fibre composite → is composed of (and quoting the filament's strength for the laminate overstates it by a factor of several, which is the commonest error made about the material) → Carbon fibre → is composed of (carbonised from a polymer precursor until little but carbon remains) → Carbon → is produced by (as synthetic graphite, made by driving the reaction past silicon carbide until the silicon boils away and leaves the carbon behind) → Acheson process → takes as input (sand and coke, heated to 2,500 °C, which is how silicon carbide has been made since 1893) → Sand → is composed of (almost everywhere, because quartz is what is left when everything else in a rock has weathered away — though a volcanic beach is basalt grains and a tropical one is broken shell) → Quartz
  • Carbon fibre composite → is composed of (the matrix, which holds the fibres in place and transfers load between them) → Epoxy resin → is produced by (the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Carbon fibre composite → is produced by (laid up as fibre and resin and cured under heat and pressure, usually in an autoclave — and the cure schedule is as much of the specification as the fibre is) → Heat treatment → takes as input (and the same steel becomes a spring, a cutting edge or a machinable bar depending only on the schedule) → Steel → is composed of → Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Carbon fibre composite → is composed of (and quoting the filament's strength for the laminate overstates it by a factor of several, which is the commonest error made about the material) → Carbon fibre → is produced by (polyacrylonitrile filament, first oxidised in air at 200 to 300 °C so it will not melt, then carbonised above 1,000 °C — and the final temperature sets the stiffness, because it sets how ordered the carbon is) → Pyrolysis → takes as input (to make coke, and the coal gas that came off it lit European cities for a century as the by-product) → Coal → is sourced from (every coal seam in the world was a peat bog. Bury it, compress it, heat it, and it passes through lignite and bituminous coal to anthracite, losing water and volatiles at each step) → Peat → is sourced from (waterlogging is the whole mechanism: a bog is anoxic below a few centimetres, so the organisms that would decompose the plant matter cannot work, and it accumulates instead) → Water
  • Carbon fibre composite → is composed of (the matrix, which holds the fibres in place and transfers load between them) → Epoxy resin → is produced by (the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer) → Polymerisation → takes as input (as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is produced by (electrolysis of potassium bifluoride, which is molten and conducts — there is no chemical oxidant strong enough to displace fluorine from a compound, so electricity is the only route and always has been) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
  • Carbon fibre composite → is composed of (the matrix, which holds the fibres in place and transfers load between them) → Epoxy resin → is produced by (the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer) → Polymerisation → takes as input (into polyethylene, and via ethylene dichloride into PVC — the two highest-tonnage plastics between them) → Ethylene → is produced by (the principal product, and the largest-tonnage organic chemical made anywhere) → Steam cracking → takes as input (the European and Asian cracker feed; North American crackers mostly run on ethane from natural gas instead, and make a different product slate as a result) → Naphtha

These are the most distinct paths back. Carbon fibre composite can be traced through others besides.

Downstream — what it becomes