Carbon fiber vs fiberglass vs Kevlar: how to choose

The problem isn't "which is best"

In the workshop, the recurring question is always the same: "Which fiber is best?" The answer —the only honest answer— is that it depends on what you want to achieve. A fiber that excels in one application can be a disaster in another. There is no superior fiber: there are performance profiles that either fit or don't fit the functional and economic requirements of your product.

This guide is written from the perspective of someone who has laminated with all three and seen parts fail because of choosing the wrong material —not through any defect in the material, but through not understanding its limits. Let's get into the detail.

Carbon fiber: the king of specific stiffness

Mechanical properties

Standard-modulus carbon fiber (aerospace grade, Toray T300 equivalent) offers the following typical properties as dry fiber (the final laminate depends on fiber volume fraction and matrix):

• Elastic modulus (E): 230 GPa for standard modulus. Intermediate grades (T800) reach 294 GPa. High-modulus grades (M40J, M55J) reach 377-540 GPa, but with reduced tensile strength and exponentially higher cost.
• Tensile strength (σ_UTS): 3500-5000 MPa for standard modulus. The highest specific strength on the market.
• Density (ρ): ~1.8 g/cm³. About 30% lighter than fiberglass.
• Elongation at break: 1.5-2.0%. Brittle failure with virtually no plastic deformation.

Advantages

The highest specific stiffness on the market (E/ρ ~ 128 GPa·cm³/g), virtually zero creep under sustained load (unlike glass), and exceptional fatigue resistance: a well-designed carbon/epoxy laminate can withstand millions of cycles without significant property degradation. It also has a coefficient of thermal expansion near zero (CTE ~ -0.1 to +1.0 × 10^-6/K), making it ideal for dimensionally stable structures.

Disadvantages

Fracture is brittle and catastrophic: there's no warning before failure. The fiber's electrical conductivity causes galvanic corrosion when in contact with metals like aluminum or steel (a fiberglass isolation layer is needed as a barrier). Cost is high: EUR 20-50/kg for standard-modulus woven fabrics, rising to over EUR 150/kg for high-modulus grades. It's sensitive to impact: a seemingly superficial hit can generate invisible internal delaminations that reduce residual compressive strength (CAI, Compression After Impact) by up to 40-60%.

Fiberglass: the tireless workhorse

Mechanical properties

• Elastic modulus (E): 70-85 GPa for E-glass (the most common type). S-glass (S-2) reaches 85-90 GPa —about 15% higher— but multiplies cost by 5.
• Tensile strength (σ_UTS): 2000-2500 MPa for E-glass, 4500-4800 MPa for S-glass.
• Density (ρ): ~2.5 g/cm³. The densest of the three, roughly 40% heavier than carbon.
• Elongation at break: 3.5-5.4%. Far more ductile than carbon.

Advantages

Cost is unbeatable: EUR 2-5/kg for standard E-glass fabrics. It's a perfect electrical insulator, eliminating galvanic corrosion issues and making it ideal as a barrier between carbon and metals. Its high strain to failure gives it some damage tolerance and energy absorption capacity before failure. It's compatible with polyester and vinylester resins —far cheaper than epoxy— further reducing total laminate cost. Radio wave transparency makes it indispensable in radomes and antenna fairings.

Disadvantages

Low stiffness (E = 70 GPa vs. carbon's 230 GPa) forces thicker laminates or geometric stiffening (ribs, sandwich) to achieve the same structural rigidity. It suffers creep under sustained load, especially in humid or elevated-temperature environments: a fiberglass part under constant tension will progressively deform over time. Its fatigue resistance is inferior to carbon, and its higher density penalizes final weight.

Kevlar / Aramid: the impact specialist

Mechanical properties

• Elastic modulus (E): 70-130 GPa depending on type. Kevlar 29 (ballistic use): 70 GPa. Kevlar 49 (structural use): 130 GPa.
• Tensile strength (σ_UTS): ~3600 MPa. Comparable to standard-modulus carbon fiber.
• Density (ρ): ~1.44 g/cm³. The lightest of the three.
• Elongation at break: 2.5-2.8%. Slightly higher than carbon.

Advantages

Impact resistance is exceptional: Kevlar absorbs energy through plastic deformation of the fibers, unlike carbon which dissipates it through brittle fracture. That's why it's the benchmark material for bulletproof vests, racing helmets, and impact protection. Its excellent vibration damping (tan δ ~ 0.015 vs. ~0.005 for carbon) makes it ideal for dynamically loaded structures. It has a negative coefficient of thermal expansion (~ -2 to -4 × 10^-6/K), enabling near-zero CTE hybrid laminates when combined with positive-CTE fibers. Its low density (1.44 g/cm³) makes it the lightest reinforcement.

Disadvantages

Compressive strength is low: barely 20% of its tensile strength. This means that in a laminate under bending, Kevlar layers on the compressed face contribute almost nothing. It absorbs moisture (up to 3-5% by weight), degrading properties in humid environments and complicating lamination. UV resistance is terrible: Kevlar exposed to sunlight without protection degrades rapidly, yellows, and loses strength; it must always be coated or painted. It's extremely difficult to cut and machine: it dulls any conventional tool and requires specialized carbide-edged scissors or waterjet cutting. Cost is intermediate: EUR 15-30/kg for Kevlar 49 fabrics.

Head-to-head comparison: all three side by side

At equal fiber weight, the property ranking is as follows:

• Stiffness (elastic modulus E): carbon (230 GPa) > Kevlar (70-130 GPa) > glass (70-85 GPa). Carbon doubles high-modulus Kevlar and triples glass.
• Tensile strength (σ_UTS): carbon (~4000 MPa) > Kevlar (~3600 MPa) > glass (~2200 MPa). Carbon and Kevlar are fairly close; glass is roughly half.
• Impact resistance: Kevlar >> glass > carbon. Kevlar is in a different league. A carbon part receiving a moderate impact can have invisible internal damage; a Kevlar part shows external damage but retains structural integrity.
• Cost per kilogram: carbon (EUR 20-50/kg) > Kevlar (EUR 15-30/kg) > glass (EUR 2-5/kg). Fiberglass is an order of magnitude cheaper than the other two.
• Density (lower is better): Kevlar (1.44) < carbon (1.80) < glass (2.50). Kevlar is the lightest.
• Compressive strength: carbon >> glass > Kevlar. Kevlar dramatically underperforms in compression.
• Fatigue resistance: carbon > Kevlar > glass. Glass suffers more under cyclic loading.

Hybrids: when one fiber isn't enough

In practice, the best laminates rarely use a single fiber type. Smart design combines reinforcements to get the best of each:

• Carbon + Kevlar (structural/impact hybrid): very common in motorsport aerodynamic parts (wings, splitters) where you need torsional stiffness (carbon) but also tolerance to stone impacts and vibration (Kevlar). The carbon-Kevlar hybrid fabric is recognizable by its characteristic two-tone appearance (black/yellow). The outer Kevlar layer acts as a "sacrificial skin" that absorbs impacts without transmitting damage to the structural carbon layers beneath.
• Glass + Carbon (cost/stiffness hybrid): typical in marine applications where budget constraints dominate. Fiberglass is placed in the outer layers (for impact resistance and as a galvanic barrier) and carbon in the structural core (for stiffness). A boat hull with this layup can be 30% cheaper than a pure carbon one with only 15% less stiffness.
• Glass + Kevlar (economic anti-impact hybrid): used in motorcycle fairings and industrial guards. Fiberglass provides basic stiffness at low cost, and Kevlar layers in impact zones prevent perforation.

The key to hybrid design is understanding the stacking sequence: fibers are laminated in layers, and each layer contributes according to its orientation and position through the thickness. Outer layers govern impact resistance and appearance; inner layers govern global stiffness. A common beginner's mistake is putting carbon in the outer layers "because it looks nice," exposing it to impact and galvanic corrosion where it's most vulnerable.

The matrix also chooses

Not all fibers work with all resins. Matrix selection dictates which reinforcements you can use:

• Epoxy resin: compatible with all three fibers. The reference matrix for high-demand structural applications. Excellent adhesion, low cure shrinkage, and good mechanical properties. Cost: EUR 10-20/kg.
• Polyester resin: only compatible with fiberglass. Polyester doesn't bond properly to Kevlar (the fiber is too hydrophilic) or to carbon (the sizing is formulated for epoxy). It's the cheapest option (EUR 3-6/kg) and the production marine industry standard. Orthophthalic polyester is the most economical; isophthalic offers better chemical and water resistance.
• Vinylester resin: compatible with fiberglass, with limited adhesion to carbon and Kevlar. It's a midpoint in price (EUR 7-12/kg) and performance between polyester and epoxy, with better chemical resistance than both.
• High-performance thermoplastics (PEEK, PEKK, PEI): compatible with all three fibers, but require radically different manufacturing processes (melt consolidation at 350-400 °C instead of room-temperature curing). Used in aerospace where the toughness and temperature resistance of the thermoplastic matrix justify the cost and complexity.

Decision flowchart: which fiber to use

If you had to reduce it to a mental algorithm, the selection process would follow this order:

1. Budget: if cost is the dominant constraint and stiffness isn't critical, fiberglass. If you can afford EUR 15-50/kg of reinforcement, move to the next criterion.
2. Stiffness: if you need maximum stiffness at minimum weight (aerospace, motorsport, drones), carbon fiber. If stiffness isn't the priority, continue.
3. Impact resistance: if the part will take hits (fairings, guards, helmets), Kevlar dominates. If impact isn't a concern and the budget is tight, fiberglass.
4. Environmental conditions: if there's moisture, Kevlar needs additional protection. If there's UV exposure, Kevlar must be painted or coated. If there's metal contact, carbon needs a glass barrier. Fiberglass is the most tolerant of adverse conditions.
5. Weight: if every gram matters, carbon or Kevlar. If weight is secondary (fixed structures, industrial applications), fiberglass.
6. Consider a hybrid: most real-world applications benefit from a combination. A laminate with a carbon core (stiffness), intermediate Kevlar layers (impact), and a glass outer veil (galvanic barrier + finish) covers all bases.

Property reference in Python

For quick reference, here are the key properties of all three fibers in a Python dictionary you can use in material selection scripts:

materials = {
    "carbon_fiber_T300": {
        "type": "Carbon (Standard Modulus)",
        "E_GPa": 230,
        "sigma_UTS_MPa": 3530,
        "rho_gcm3": 1.80,
        "elongation_pct": 1.5,
        "cost_eur_kg": (20, 50),
        "impact_resistance": "poor",
        "compressive_strength": "excellent",
        "fatigue_resistance": "excellent",
        "CTE_10e6_K": -0.1,
        "compatible_matrices": ["epoxy", "peek", "pekk", "bismaleimide"]
    },
    "fiberglass_E_glass": {
        "type": "Glass (E-Glass)",
        "E_GPa": 72,
        "sigma_UTS_MPa": 2400,
        "rho_gcm3": 2.54,
        "elongation_pct": 4.8,
        "cost_eur_kg": (2, 5),
        "impact_resistance": "moderate",
        "compressive_strength": "good",
        "fatigue_resistance": "moderate",
        "CTE_10e6_K": 5.0,
        "compatible_matrices": ["epoxy", "polyester", "vinylester"]
    },
    "kevlar_49": {
        "type": "Aramid (Kevlar 49)",
        "E_GPa": 130,
        "sigma_UTS_MPa": 3600,
        "rho_gcm3": 1.44,
        "elongation_pct": 2.8,
        "cost_eur_kg": (15, 30),
        "impact_resistance": "excellent",
        "compressive_strength": "poor (~20% of tensile)",
        "fatigue_resistance": "good",
        "CTE_10e6_K": -3.5,
        "compatible_matrices": ["epoxy", "peek", "pekk"]
    }
}

At MaterialTech Labs we use all three fibers depending on the application. For surf fins, high-modulus carbon is the natural choice: you need maximum stiffness at minimum thickness so the fin responds instantly to foot pressure. For additive manufacturing tooling, fiberglass is often the smarter choice: tool stiffness isn't critical, but the cost savings let you iterate faster. The golden rule is simple: don't pay for performance you don't need. The best material isn't the most impressive one on the datasheet. It's the one that meets the functional requirements at the lowest total cost.