Overview
Reinforcing fibers can be classified by length (continuous vs. discontinuous) and by origin (synthetic vs. natural). Continuous fibers provide higher mechanical performance due to their ability to transfer and retain loads within unbroken strands, while natural fibers offer environmental benefits including biodegradability and recyclability.
Continuous vs. Discontinuous Fibers
| Property | Discontinuous (Short) Fibers | Continuous Fibers |
|---|---|---|
| Aspect ratio | Short (<1000) | Long (>1000) |
| Orientation | Random | Preferred/aligned |
| Max volume fraction | ~50% (limited by viscosity) | Higher possible |
| Load transfer | Matrix-dependent | Direct fiber-to-fiber |
| Strength improvement | Limited | Significant |
| Fiber pull-out | Primary cause of failure | Reduced occurrence |
Short fibers rely on the matrix material for load transfer between fibers, while continuous fibers can transfer loads within unbroken strands, reducing matrix stress and improving load-bearing capacity.
Synthetic Fibers
Synthetic fibers are derived from petroleum-based raw materials and constitute approximately 50% of all fiber used globally.
Carbon Fibers
The most widely used reinforcing element for high-performance composites.
- Typical diameter: 5-7 μm
- Tensile strength: Up to 5.3 GPa
- Optimal aspect ratio for FDM: ≥1000
- Short fiber categories: nano (<1 μm), micro (50-400 μm), milli (mm range)
Applications: Aerospace, automotive, biomedical, electronics
| Matrix | Volume % | Results | Reference |
|---|---|---|---|
| ABS | 6.5% | Flexural strength 127 MPa, UTS 147 MPa | Yang et al., 2017 |
| PLA | 6.6% | In-nozzle impregnation demonstration | Matsuzaki et al., 2016 |
| PLA | 27% | Bending strength 335 MPa, modulus 30 GPa | Tian et al., 2016 |
| PLA | 34% | 14% tensile, 164% bending strength increase | Li et al., 2016 |
| Nylon | 26.8-72.4% | Highest shear strength among tested fibers | Caminero et al., 2018 |
Glass Fibers
High performance-to-cost ratio fiber widely used across industries.
- Lower stiffness than carbon fiber
- High strength combined with low density
- Least expensive reinforcement option
- Excellent resistance to chemical damage
Applications: Electronics, aviation, civil engineering, defense technology
| Matrix | Volume % | Results | Reference |
|---|---|---|---|
| TPU | 34.8% | >700% increase in tensile strength and modulus | Akhoundi et al., 2020 |
| PLA | 30.5% | >700% increase in tensile strength and modulus | Akhoundi et al., 2020 |
| Nylon | 27.5-73.8% | High shear strength | Caminero et al., 2018 |
| Nylon | Various | Highest impact strength (250-300 MPa) | Caminero et al., 2018 |
Aramid / Kevlar Fibers
First organic fiber used as reinforcement in advanced composites.
- Strength: 5-6× higher than steel wire (same diameter)
- Modulus: 2-3× higher than steel wire (same diameter)
- Weight: 1/5 of steel wire
- Naturally heat- and flame-resistant
- Poor UV resistance (color change when exposed)
- Excellent corrosion resistance
Applications: Bulletproof vests, blast protection, cooling systems, ship hulls, spacecraft, sporting goods
| Matrix | Volume % | Results | Reference |
|---|---|---|---|
| Nylon | 4.04-10.1% | Elastic modulus 1767-9001 MPa | Melenka et al., 2016 |
| Nylon | 27.2-73.4% | Impact strength 80-200 MPa | Caminero et al., 2018 |
| PLA | 8.6% | Comprehensive mechanical investigation | Bettini et al., 2017 |
| PETG | 45% | +1550% modulus, +1150% strength vs unreinforced | Rijckaert et al., 2022 |
Natural Fibers
Natural fibers offer environmental advantages including biodegradability, recyclability, and reduced environmental impact compared to synthetic fibers.
European Automotive Industry Fiber Usage (2012)
| Wood | 38% |
| Cotton | 25% |
| Flax | 19% |
| Kenaf | 8% |
| Hemp | 5% |
| Others (jute, coir, sisal, abaca) | 7% |
Source: Pecas et al., 2018
Flax
One of the strongest natural cellulosic fibers; first plant stem fiber used for textiles. Extracted from flax plant stem skin, soft, lustrous, and flexible. Stronger than cotton but less elastic, with high stiffness-to-weight ratio.
Applications: Textiles, composite reinforcement, food production, personal care, animal feed
| Matrix | Results | Reference |
|---|---|---|
| PLA | Tensile properties comparable to glass fiber/PA composites | Le Duigou et al., 2019 |
| PLA | 211% flexural strength increase, 224% modulus increase | Zhang et al., 2020 |
| PLA | 325% tensile strength increase | Kuschmitz et al., 2021 |
Hemp
Among the strongest members of the bast natural fibers family. Derived from Cannabis species, biodegradable and low density, with inherent mechanical, thermal, and acoustic properties.
| Matrix | Results | Reference |
|---|---|---|
| PBS | 63% improvement in Young's modulus with overlap | Donitz et al., 2023 |
| PP | 5% hemp: highest tensile strength; 20% hemp: highest modulus | Sultan et al., 2024 |
Kenaf
Well-known natural fiber for polymer matrix composites. Sourced from kenaf plant bast with mechanical properties comparable to glass fiber, lower density than synthetics, and reduces wear rate of polymer composites.
| Matrix | Fiber Content | Results | Reference |
|---|---|---|---|
| ABS | 0-5% | Tensile strength decreased from 23.20 to 11.48 MPa | Han et al., 2022 |
| ABS | 5-10% | Tensile strength increased from 11.48 to 18.59 MPa | Han et al., 2022 |
Cotton
Natural hollow fibers; purest form of cellulose (~90% cellulose content). Most widely used fiber in textile industry with water absorption 24-27× own weight. Strong, dye-absorbent, abrasion-resistant.
| Matrix | Results | Reference |
|---|---|---|
| PLA | Exceptional tensile strength and stiffness rivaling glass composites | Kelch et al., 2018 |
Basalt
Created by melting crushed basalt rocks at 1400°C and drawing the molten material. Superior mechanical and physical properties vs glass fibers, fire resistant, chemical resistant, vibration and acoustic insulation. More costly than E-glass but cheaper than carbon fiber.
| Matrix | Results | Reference |
|---|---|---|
| PLA | Comparable tensile, superior flexural properties vs PLA/CF | Sang et al., 2019 |
Wood
Cellulosic elements extracted from trees with high total porosity. Combined with thermoplastics produces waterproof outdoor products. Wood-plastic composites (WPCs) used in automotive and building products.
| Matrix | Results | Reference |
|---|---|---|
| PLA | Aligned wood fibers enhanced mechanical performance | Billings et al., 2023 |
Jute
Produced from plants in the genus Corchorus (Malvaceous family). Lignocellulosic bast fiber, completely biodegradable and recyclable. Good thermal and acoustic insulation with moderate moisture regain and no skin irritations.
Fiber Comparison
| Fiber | Type | Strength | Cost | Environmental Impact |
|---|---|---|---|---|
| Carbon | Synthetic | Highest | High | Non-biodegradable |
| Glass | Synthetic | High | Low | Non-biodegradable |
| Aramid/Kevlar | Synthetic | High | Moderate-High | Non-biodegradable |
| Flax | Natural | Moderate-High | Low | Biodegradable |
| Hemp | Natural | Moderate | Low | Biodegradable |
| Kenaf | Natural | Moderate | Low | Biodegradable |
| Basalt | Natural (mineral) | High | Moderate | Inert |
| Cotton | Natural | Moderate | Low | Biodegradable |