Carbon Fiber vs Steel, Fiberglass and Plastic

"Traditional materials" usually means steel, fiberglass and plastic, and each of them beats carbon fiber at something. This is how they compare on the numbers that decide most parts. Aluminum gets its own article: carbon fiber vs aluminum.

The numbers

Typical room temperature values for common grades. Composite values depend heavily on fiber content and layup, so treat them as a starting point, not design data.

Carbon fiber vs steel

The surprise is stiffness. Divide stiffness by density and steel, aluminum and titanium all land near 25. A quasi-isotropic carbon laminate, with fibers running in several directions, lands near 30. So a carbon copy of a steel bracket, laid up the same way in every direction, isn't much stiffer for its weight.

Carbon pulls ahead in two cases. The first is when the fibers follow the load: unidirectional carbon lands near 85, more than three times steel. The second is panels. Bending stiffness grows with the cube of thickness, so a lighter material can afford to be thicker. A quasi-isotropic carbon panel as stiff as a steel one weighs about a third as much.

Strength is less subtle. Per kilogram, a quasi-isotropic carbon laminate is roughly four to six times as strong in tension as 4130 steel. Compression, bearing strength around bolt holes and strength through the thickness are all weaker, which is why the joints in a carbon part take the most design time.

Steel wins on almost everything else:

• Cost. Steel costs a small fraction of carbon fiber per kilogram, and it doesn't need a mold.
• Fabrication. It can be cut, bent and welded in a day, and repaired the same way.
• Toughness. Steel bends and dents before it breaks, so damage is easy to see. Carbon can hide damage inside the laminate.
• Fatigue. Many steels have an endurance limit, a stress below which they survive an unlimited number of load cycles.
• Heat and wear. There's no resin to soften, and steel can be hardened for threads, bearing seats and sliding surfaces.

Mixing the two works well, but bare carbon touching plain steel in wet conditions corrodes the steel, so seal or isolate the joint.

Carbon fiber vs fiberglass

Glass fiber and carbon fiber use the same resins, molds and processes, so the choice comes down to the fiber.

• Stiffness. E-glass fiber has a modulus of about 72 GPa, and standard carbon fiber about 230 GPa. A glass laminate usually ends up well under half as stiff as a comparable carbon one, and about 20 percent heavier.
• Cost. Glass fiber costs a fraction of carbon fiber per kilogram, which is why boats, tanks and most large covers are glass.
• Impact. E-glass stretches about 4.8 percent before it breaks, against roughly 1.5 to 2 percent for carbon, so glass parts take knocks better.
• Electrical and radio. Glass doesn't conduct and lets radio signals through. Carbon conducts and blocks them, which rules it out for antenna covers and some electrical housings.

The stiffness gap matters more than people expect. A glass laminate rod or tie, loaded in tension, needs roughly two to three times the weight of a steel one to stretch the same amount. Panels are kinder to glass: an equally stiff glass panel weighs about half as much as the steel one. Some parts use glass for the inner plies and carbon for the outer ones, which gets the look and part of the stiffness for less money.

Carbon fiber vs plastic

Injection molded plastic wins on cost per part once volumes are high. A cycle takes seconds to a minute, and clips, bosses and threads come out of the mold finished. A carbon fiber part takes minutes to hours to cure and usually needs trimming and drilling afterwards.

What plastic can't match is stiffness. ABS is about 2 GPa and 30 percent glass-filled nylon about 9, against roughly 50 for a carbon laminate. To be as stiff, a plastic part has to be much thicker or heavily ribbed. Plastics also creep under a steady load, and nylon loses stiffness as it absorbs moisture.

Filled plastics close part of the gap. Nylon with chopped glass or carbon fiber still molds in seconds and can be stiff enough for brackets and housings. If that covers the loads, it's usually the cheaper answer at volume. Our injection molding design rules cover what makes those parts mold well.

Which one?

• Steel: lowest cost, welded structures, heat, wear surfaces and parts that get abused.
• Glass fiber: large composite shapes on a budget, and parts that have to insulate or pass radio signals.
• Plastic: high volumes, molded-in features and moderate loads. Use a filled grade if it needs stiffness.
• Carbon fiber: when weight or stiffness per weight is the point, the loads are well understood and the fibers can follow them.

When to use carbon fiber covers how to decide, and it's the first comparison we make when you send us a carbon fiber project.

Related reading

• Carbon fiber vs aluminum
• 2x2 twill vs spread tow carbon fiber
• Carbon fiber prototypes without a production mold