Carbon fiber pultruded structural profiles—channels, angle bars, flat strips, and rectangular tubes—fill a specific niche between standard composite tubes and custom-cured prepreg assemblies. They deliver 1,200–1,800 MPa tensile strength in the fiber direction at 1.55–1.60 g/cm³, roughly one-fifth the weight of steel with comparable specific stiffness. These shapes are not drop-in replacements for every metal section, but for applications where corrosion resistance, dielectric properties, or weight savings drive the design, pultruded carbon profiles offer a factory-consistent solution that requires no post-cure machining beyond cut-to-length.
Pultrusion Process for Structural Profiles
Pultrusion produces continuous lengths of constant cross-section by pulling fiber tows through a resin bath and then through a heated steel die that cures the thermoset matrix. For structural carbon profiles, the typical line speed is 100–300 mm/min with a die temperature profile of 130–180 °C for epoxy systems. Fiber volume fraction reaches 60–70%, compared to 30–45% in hand lay-up or spray-up. The resulting cross-section has zero voids visible at 10× magnification when the process is under control, and the surface finish replicates the die cavity surface to within ±0.05 mm.
The reinforcement architecture is uniaxial—all fibers run parallel to the length of the profile. This gives maximum strength along the pultrusion axis but creates an inherent weakness in the transverse direction. Unidirectional pultruded carbon profiles typically show transverse tensile strength of only 30–60 MPa, roughly 3–5% of the longitudinal value. For applications requiring multi-directional loading, designers must either increase the section thickness to compensate for transverse weakness, or specify a hybrid architecture that incorporates continuous filament mat (CFM) between unidirectional layers.
Common Structural Shapes and Their Applications
| Shape | Typical Dimensions | Common Applications | Bending Stiffness vs Steel (Same Weight) |
|---|---|---|---|
| Angle bar | 25 × 25 × 3 mm to 100 × 100 × 10 mm | Bracing, support brackets, frame corners | 4.5–5.5× |
| Channel (U-section) | 30 × 15 × 3 mm to 150 × 50 × 8 mm | Rail systems, equipment frames, solar supports | 3.8–5.0× |
| Flat strip | 10 × 2 mm to 200 × 12 mm | Stiffeners, splice plates, reinforcement | 4.0–5.2× |
| Rectangular tube | 20 × 10 × 2 mm to 100 × 50 × 5 mm | Structural framing, robotics, conveyor rails | 5.5–7.0× |
Each shape serves a different load path. Channels and rectangular tubes carry bending primarily through the flanges, while angle bars work well in tension or as diagonal bracing members. Flat strips are the simplest to produce and are often used as doublers or reinforcement where the section thickness of the host profile is insufficient.
Gallery: Pultruded Carbon Fiber Profiles
Mechanical Properties by Profile Geometry
Longitudinal tensile modulus for standard-modulus carbon pultrusion is 120–140 GPa, compared to 70 GPa for aluminum and 200 GPa for steel. The specific modulus (modulus divided by density) reaches 75–90 GPa·cm³/g, outperforming steel (25 GPa·cm³/g) and aluminum (26 GPa·cm³/g). This is the fundamental advantage: a carbon profile can achieve the same axial stiffness at 30–40% of the weight of an aluminum section of the same geometry.
Interlaminar shear strength (ILSS) is the limiting property for pultruded carbon profiles under transverse or torsional loading. Typical ILSS values range from 40–65 MPa for epoxy-based systems. Below 40 MPa, the profile is likely to delaminate at joints or under cyclic loading. Designers should specify a minimum ILSS of 50 MPa for structural applications and verify it per ASTM D2344 short-beam shear testing.
Compression strength along the fiber axis reaches 700–1,000 MPa, roughly 60–70% of the tensile value. This asymmetry matters in buckling-dominated designs: a carbon channel used as a column has a compression-limited load capacity that is lower than its tension-limited capacity, so the compression property is the design constraint.
Connection and Joining Methods
Pultruded carbon profiles cannot be welded. All connections rely on mechanical fastening, adhesive bonding, or a hybrid combination. Bolted joints require care: carbon’s low ductility means stress concentrations at the bolt hole are not redistributed through plastic deformation as they would be in steel. A 6 mm bolted connection in a carbon angle bar should have a minimum edge distance of 3× the hole diameter to reduce the risk of shear-out failure.
Adhesive bonding provides more uniform stress transfer and avoids drilling holes that interrupt the fibers. Epoxy adhesives with 25–35 MPa lap shear strength, combined with a 0.1–0.2 mm bond line thickness, deliver joint efficiencies of 80–90% of the base material strength. Surface preparation—light abrasion followed by solvent wipe—increases bond strength by 40–60% compared to unprepared surfaces.
Cost Considerations
The raw material cost for pultruded carbon profiles is dominated by the carbon fiber: 12K standard-modulus tow at $20–30/kg, with epoxy resin adding $4–8/kg. For a 50 mm × 50 mm × 5 mm carbon angle bar weighing approximately 0.4 kg/m, the material cost is roughly $10–14 per linear meter. Tooling for a custom pultrusion die is $5,000–15,000 per shape. Minimum production runs of 500–1,000 linear meters are typical before the die cost becomes negligible per meter.
For comparison, a steel angle of equivalent bending stiffness weighs 2.0–2.5 kg/m and costs $3–5/m. The carbon profile costs 3–4× more per meter but saves 75–80% in weight. In transportation or aerospace applications where weight carries a cost penalty of $50–200 per kg saved, the carbon premium is recovered in the first year of service.
FAQ
Can pultruded carbon fiber profiles replace aluminum extrusions directly?
Not as a direct drop-in replacement. The machining behavior, thermal expansion (CTE of −0.4 to +0.2 ppm/°C for carbon vs. 23 ppm/°C for aluminum), and connection methods are fundamentally different. Aluminum can be welded, bent, and re-drilled. Carbon profiles require adhesive bonding or bolted connections with larger edge distances. A proper substitution requires redesigning the joints and tolerancing for the different CTE.
What is the maximum length available for pultruded carbon profiles?
Continuous lengths are limited only by transportation constraints. Standard shipping lengths are 6 m (containers) or 12 m (flatbed). Custom lengths up to 18 m are available with special logistics. The pultrusion process itself can run continuously for 8–24 hours, producing hundreds of meters per run.
Do pultruded carbon profiles require UV protection for outdoor use?
Epoxy matrices degrade under direct UV exposure—surface erosion of 10–30 µm per year is typical in unprotected outdoor exposure. A UV-resistant topcoat (polyurethane or acrylic, 30–50 µm) extends the service life to 10–15 years. Carbon fibers themselves are UV-stable, so the degradation is limited to the resin surface layer.
Can pultruded carbon profiles be machined with standard tools?
Yes, with tungsten carbide or PCD (polycrystalline diamond) tooling. Standard HSS tools dull rapidly—carbon fibers have 3–5× the abrasiveness of fiberglass. Drilling requires 2,000–4,000 rpm with controlled feed to avoid delamination. Water mist cooling extends tool life by 2–3× and prevents dust.
For process optimization details, see our carbon fiber pultrusion speed and temperature guide and our pultrusion die design guide. Industry references: CompositesWorld, ACMA, and ASTM D2344 short-beam shear test standard.
