Carbon fiber

Definition (what it is)

Carbon fiber is a high-strength, high-stiffness filament made predominantly of carbon atoms (>90%) arranged in turbostratic, graphite-like microstructures. It is produced from organic precursors—most commonly polyacrylonitrile (PAN), and less commonly pitch or regenerated cellulose—by stabilization (oxidation), carbonization (inert atmosphere), and, for very high modulus fibers, graphitization. In practice, carbon fibers are almost always used as reinforcement within a matrix to form a composite, most commonly a carbon-fiber-reinforced polymer (CFRP) or carbon-fiber-reinforced thermoplastic (CFRTP).

Key characteristics and performance

High specific stiffness and strength: tensile modulus typically ~230–640 GPa for PAN-based grades (higher for pitch-based, up to ~900 GPa); tensile strength ~3–7 GPa, depending on grade and processing.
Low density: ~1.75–1.95 g/cm³, enabling substantial weight reduction relative to metals at equivalent stiffness/strength targets.
Fatigue and creep: excellent fatigue resistance and very low creep under ambient conditions relative to many metals and polymers.
Thermal behavior: very low (often near-zero or slightly negative) coefficient of thermal expansion along the fiber; good to very high axial thermal conductivity (especially for pitch-based fibers) and poor transverse conductivity; service temperature is limited by the matrix and by oxidation in air.
Electrical properties: electrically conductive along the fiber; composites are anisotropically conductive. Conductivity can provide EMI shielding but introduces galvanic corrosion risk when in contact with certain metals in the presence of electrolytes.
Chemical/environmental resistance: the fiber itself is chemically inert and corrosion resistant; composite durability is governed largely by matrix resin, fiber–matrix interface (sizing), and protective coatings.
Anisotropy: properties are highly directional; laminate design (fiber orientation, stacking sequence, fiber volume fraction) determines component-level behavior.
Damage tolerance: fibers fail in a brittle manner; composite damage modes include matrix cracking, delamination, and fiber–matrix debonding. Through-thickness strength is relatively weak unless engineered.

Manufacture and forms

Precursors and processing: PAN dominates commercial production; pitch fibers offer very high modulus/thermal conductivity; rayon/cellulose is rare. Processing steps include stabilization (200–300°C in air), carbonization (~1000–1500°C in inert gas), optional graphitization (~2000–3000°C), surface treatment, and application of a sizing compatible with the intended matrix.
Fiber formats: continuous tow/roving (e.g., 1k–50k filaments), chopped strands, milled fibers.
Grades: standard modulus (SM), intermediate modulus (IM), high modulus (HM), ultra-high modulus (UHM).
Semi-finished products: unidirectional (UD) tapes, woven fabrics (plain, twill, satin), non-crimp/multiaxial fabrics (NCF), braids, preforms, organosheets (thermoplastic UD/woven laminates), and chopped-fiber compounds or sheet molding compound (SMC).

Common matrices and composite types

Thermosets: epoxy (most common for structural use), vinyl ester, polyester, phenolic, bismaleimide (BMI). Thermosets offer high stiffness, good thermal stability, and well-established processing.
Thermoplastics: PA, PPS, PEEK, PEKK, PP, etc. CFRTP enables faster cycles, weldability, thermoforming, and potential recyclability.
Typical products: prepreg laminates; RTM/HP-RTM parts; compression-molded organosheet parts with overmolding; injection-molded short/long fiber components.

Processing methods (typical)

Prepreg layup with autoclave or out-of-autoclave curing for high-performance parts.
Resin infusion and RTM/HP-RTM for medium- to higher-volume, complex structural parts.
Compression molding of SMC or thermoplastic organosheets for faster cycle times.
Automated tape laying (ATL) and automated fiber placement (AFP) for precise, repeatable laminates.
Filament winding, pultrusion, and braiding for tubes, beams, and profiles.
Injection molding of short/long carbon-fiber-reinforced thermoplastics for brackets, housings, and semi-structural parts.
Post-processing includes trimming/machining (dust control required), drilling, adhesive bonding or co-bond/co-cure joining, surface sealing and coatings.

Applications and relevance

Widely used in aerospace, space, motorsport, high-end automotive, wind energy (blades), pressure vessels (e.g., hydrogen, CNG), civil infrastructure, medical devices, and sporting goods.
In automotive and EVs, CFRP is applied to body-in-white elements, closures (doors, hoods, roofs), battery enclosures, drive shafts, suspension components, crash structures, and interior structural modules to reduce mass, improve stiffness, and tailor crash energy absorption. Electrical conductivity and thermal behavior require careful insulation and fire protection design around battery systems.

Design and engineering considerations

Laminate architecture drives performance: select fiber orientation, stacking sequence, ply thickness, and fiber volume fraction for load paths and stability (buckling, compression).
Interface and sizing: choose fiber surface treatment/sizing compatible with the matrix to maximize adhesion and durability.
Galvanic isolation: isolate CFRP from metals (especially aluminum, magnesium) using coatings, barriers, or insulating fasteners to prevent galvanic corrosion.
Impact and through-thickness reinforcement: consider toughened resins, interleaves, 3D weaving, z-pinning, tufting, and core materials (sandwich construction) to improve damage tolerance.
Joining: prefer adhesive bonding and co-cure/co-bond techniques; when bolting, use inserts, local reinforcements, and proper bearing/bypass design to manage clamp loads and avoid delamination.
Thermal/fire: polymer matrices limit continuous-use temperature; for fire-critical applications (e.g., battery enclosures), incorporate flame-retardant resins, heat barriers, or intumescent/ceramic layers.
Quality and inspection: control void content and fiber volume; use non-destructive inspection (ultrasonics, thermography, CT) where appropriate.
Standards and testing: common standards include ASTM D3039 (tension), D6641 (compression), D7264 (flexure), D2344 (short-beam shear), D7136/D7137 (impact/compression-after-impact), and ISO 527/14125/14126 equivalents. Design data are laminate- and process-specific.

Advantages and limitations

Advantages: exceptional specific stiffness/strength; designable anisotropy; corrosion resistance; dimensional stability; potential part consolidation and complex geometries; excellent fatigue performance.
Limitations: higher material and processing cost; longer cycle times for some methods; brittle failure modes and complex repair; sensitivity to out-of-plane loads and impact; conductive fibers can create ESD or shorting risks; risk of galvanic corrosion with metals.

Related terms

CFRP (carbon-fiber-reinforced polymer/plastic), CFRTP (carbon-fiber-reinforced thermoplastic), carbon laminate, carbon composite.
Graphite fiber (often used for highly graphitized, very high modulus fibers).
Related materials: GFRP (glass-fiber-reinforced polymer), AFRP (aramid-fiber-reinforced polymer, e.g., Kevlar), basalt-fiber composites; hybrid laminates combine fiber types.

Sustainability and end-of-life

Reuse and recycling options include mechanical reclamation, pyrolysis, and solvolysis to recover fibers from thermoset CFRP; recovered fibers are typically shorter and used in non-woven mats or chopped compounds. Thermoplastic CFRTP can be reprocessed more readily.
Carbon fiber production is energy intensive, but life-cycle assessments often show net environmental benefit in transport applications through mass reduction and efficiency gains.

Typical property ranges (indicative, fiber-level)

Density: ~1.75–1.95 g/cm³
Tensile strength: ~3–7 GPa
Tensile modulus: ~230–640 GPa (PAN-based), up to ~900 GPa (pitch-based)
Axial CTE: near zero to slightly negative; transverse CTE positive

Composite properties depend strongly on fiber architecture, matrix, fiber volume fraction, and processing quality.

Safety and handling

Machining and cutting produce fine conductive dust; use appropriate extraction, filtration, and PPE. Prevent contamination of electrical systems and protect skin/eyes from irritation. Ensure proper electrical insulation when CFRP is adjacent to energized components.