Carbon fiber composite

Carbon fiber composite (CFRP, carbon fiber reinforced polymer, spelled fibre in British English and often just called carbon) is a material made of carbon fibers embedded in a polymer matrix, most commonly epoxy resin. The fibers carry the load, while the matrix holds them in place, transfers forces between them and protects them. Most parts are made as a composite laminate of several plies, so their properties depend on the fiber type, the fiber volume fraction and the fiber orientation.

The fibers are supplied as tows designated by the number of filaments in thousands (for example 3K or 12K), as unidirectional (UD) plies or as fabrics in plain or twill weave, the latter giving visible cosmetic parts their typical look. Standard modulus fibers have a modulus of elasticity of around 230 GPa, high-modulus fibers 350 GPa or more. The reinforcement is either laid up dry with separately applied resin (hand layup with vacuum bagging, vacuum infusion, RTM) or used as prepreg, that is reinforcement pre-impregnated with resin and cured with heat, often in an autoclave. Tubes and rods are also made by filament winding and pultrusion.

Carbon fiber composite has a density of about 1.5 to 1.6 g/cm³. A unidirectional laminate of standard modulus fibers typically has a modulus of 120 to 150 GPa and a tensile strength of 1.5 to 2.5 GPa along the fibers, but only a fraction of these values across them. A quasi-isotropic laminate, which behaves roughly the same in every in-plane direction, has a modulus of around 45 to 60 GPa, less than aluminum, and with a density about 40 % lower it offers only slightly higher specific stiffness. Large weight savings therefore come mainly where the fibers run in the direction of the main load. Thermal expansion along the fibers is close to zero.

Carbon fiber composite makes sense where weight or stiffness-to-weight ratio is decisive: drone and robot frames, arms and end effectors of fast handling robots (lower inertia allows higher dynamics), race car and aircraft parts, sporting goods, prostheses and orthoses, drive shafts and rollers, or dimensionally stable frames for measuring and optical instruments. Prototypes and small batches often use ready-made stock: plates cut by CNC milling or waterjet cutting, and pultruded or filament-wound tubes joined by bonding.

For covers and parts without demanding stiffness requirements, cheaper fiberglass is usually the better option, and for geometrically complex parts in small batches, machined aluminum. Base the decision on a calculation: if the fibers cannot be oriented along the loads, the savings over aluminum tend to be smaller than expected.

Carbon fiber composite is brittle: it does not yield plastically but fails suddenly, and an impact can cause delamination inside the laminate even when the surface looks intact. Temperature resistance is set by the matrix. Room-temperature-cure resins without a post-cure often have a glass transition temperature (Tg) of only around 50 to 70 °C, and epoxy degrades under UV radiation unless protected by a clear coat. Carbon fibers are electrically conductive, block radio signals and cause galvanic corrosion of aluminum or steel in contact with them, so isolate the mating surfaces, for example with a layer of glass fabric.

Holes and bolted joints need inserts or local reinforcement and a limited tightening torque. Machining requires diamond or diamond-coated tools and effective extraction of the conductive, hazardous dust. Do not confuse a structural carbon fiber composite with 3D printing filaments filled with chopped carbon fibers or with a decorative "carbon look", whose properties come nowhere close. Check the fiber type, layup, fiber content and manufacturing method with your supplier.