Composites
A composite is a material made from two or more constituents with significantly different properties that, when combined, produce a material with characteristics different from the individual components. In engineering, this almost always means strong, stiff fibers embedded in a polymer, metal, or ceramic matrix.
Composites are the fastest-growing structural material class in both aerospace and automotive. The Boeing 787 is 50% composite by weight. The BMW i3 has a full CFRP passenger cell. The reason is simple: composites offer the highest specific strength and specific stiffness of any structural material.
Why Composites?
The fundamental advantage is anisotropy by design — you place strong fibers exactly where the loads go. A unidirectional CFRP tape has a tensile strength of ~2,000 MPa along the fiber direction at a density of 1.55 g/cm³. No metal comes close to this specific strength.
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| Property | CFRP (Quasi-iso) | Al 7075-T6 | Ti-6Al-4V | Steel |
|---|---|---|---|---|
| Density (g/cm³) | 1.55 | 2.81 | 4.43 | 7.85 |
| σu (MPa) | ~800 | ~570 | ~950 | ~500 |
| E (GPa) | ~70–150 | ~70 | ~110 | ~200 |
| Specific strength (kN·m/kg) | ~516 | ~203 | ~214 | ~64 |
| Specific stiffness (MN·m/kg) | ~45–97 | ~25 | ~25 | ~25 |
Fiber Types
Carbon Fiber
The dominant high-performance reinforcement. Made by pyrolysis of polyacrylonitrile (PAN) precursor at 1,000–3,000°C.
| Type | Tensile Strength (GPa) | Modulus (GPa) | Strain to Failure (%) | Cost (USD/kg) | Use |
|---|---|---|---|---|---|
| Standard modulus (T300, T700) | 3.5–5.0 | 230–240 | 1.5–2.0 | ~20–30 | General aerospace & automotive |
| Intermediate modulus (T800, IM7) | 5.5–6.0 | 290–300 | 1.8–2.0 | ~50–80 | Primary aerospace structures |
| High modulus (M55, M60) | 3.5–4.0 | 540–590 | 0.7 | ~100+ | Space structures, satellites |
Glass Fiber
10–20× cheaper than carbon fiber. Lower strength and stiffness but excellent impact resistance and electrical insulation.
| Type | σu (GPa) | E (GPa) | Use |
|---|---|---|---|
| E-glass | 3.4 | 72 | General purpose — boats, wind blades, automotive |
| S-glass | 4.6 | 87 | Higher performance — ballistic armor, aerospace secondary structures |
GFRP dominates by volume: wind turbine blades, boat hulls, truck body panels, automotive leaf springs (Volvo, GM). The Corvette C8 uses GFRP body panels.
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Aramid (Kevlar)
Aramid fibers (Kevlar 29, Kevlar 49) offer exceptional impact and abrasion resistance at low density (1.44 g/cm³). Poor in compression — not used for primary structure. Used in ballistic protection, helicopter blade erosion strips, and honeycomb cores.
Other Fibers
Basalt fiber — Similar to E-glass but better chemical and temperature resistance. Emerging for automotive and infrastructure. Ultra-high-molecular-weight polyethylene (UHMWPE / Dyneema) — Highest specific strength of any fiber. Used in body armor and marine ropes. Poor at elevated temperatures (melts at ~150°C).Matrix Systems
The matrix binds the fibers, transfers loads between them, and protects them from the environment.
Thermoset Matrices
| Resin | Tg (°C) | Pros | Cons | Application |
|---|---|---|---|---|
| Epoxy | 120–180 | High strength, low shrinkage, good adhesion | Brittle, slow cure, moisture absorption | Aerospace prepreg (Hexcel 8552, Cycom 977-3) |
| Vinyl ester | 100–150 | Good toughness, chemical resistance | Lower Tg than epoxy | Marine, chemical tanks, wind blades |
| Polyester | 70–120 | Cheap, fast cure | Lower properties than epoxy/VE | Boats, bathtubs, automotive SMC |
| BMI (bismaleimide) | 250–300 | High-temp capability | Brittle, expensive | Military aerospace (F-22 skin) |
Thermoplastic Matrices
Growing rapidly because they can be welded, are tougher, and have unlimited shelf life (no refrigeration needed).
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| Resin | Tg (°C) | Application |
|---|---|---|
| PEEK | 143 | Aerospace brackets, clips (Airbus A350 uses CF/PEEK clips) |
| PEKK | 160 | Aerospace structural — Gulfstream fuselage panels |
| PPS | 85 | Automotive under-hood, aerospace secondary structure |
| PA6/PA66 | 60–70 | Automotive structural — glass-filled PA for brackets and cross-members |
Layup and Orientation
Unidirectional (UD) Tape
All fibers aligned in one direction. Maximum properties along the fiber axis, minimum perpendicular to it. The building block of aerospace laminates.
Along fiber: σu ~2,000 MPa, E ~140 GPa Perpendicular to fiber: σu ~50 MPa, E ~10 GPaThis extreme anisotropy is why layup design matters so much.
Common Layup Conventions
Plies are stacked at specific angles to distribute load capability:
- 0° — Carries axial (longitudinal) loads
- 90° — Carries transverse loads
- ±45° — Carries shear loads and provides torsional stiffness
Standard Quasi-Isotropic Layup
[0/±45/90]s — Equal proportions of 0°, +45°, -45°, and 90° plies. Approximates isotropic behavior in the plane. This is the conservative baseline — used when loads come from multiple directions or when design is preliminary.Properties are roughly 1/4 to 1/3 of the unidirectional values.
Tailored Layups
Real aerospace structures are optimized: a wing skin might be 60% 0° plies (for bending), 30% ±45° (for torsion), and 10% 90° (minimum for handling transverse loads). This is where composites gain their efficiency advantage over metals — material goes where the loads are.
Ply drop-offs — Thickness is varied by terminating plies where less strength is needed, saving weight. Requires careful design to avoid delamination at ply drops.Sandwich Structures
A thin, stiff face sheet bonded to a thick, lightweight core. This dramatically increases bending stiffness at minimal weight cost — the same principle as an I-beam.
| Core Type | Density (kg/m³) | Shear Strength (MPa) | Application |
|---|---|---|---|
| Nomex honeycomb (aramid) | 32–128 | 1.0–4.5 | Aerospace control surfaces, fairings, radomes |
| Aluminum honeycomb | 50–130 | 2.0–8.0 | Helicopter floors, satellite panels |
| PMI foam (Rohacell) | 32–110 | 0.8–3.5 | Wind blades, racing car bodies |
| PVC foam (Divinycell) | 40–250 | 0.5–3.0 | Marine hulls, automotive roof panels |
Manufacturing Methods
Prepreg + Autoclave
Pre-impregnated fiber sheets (prepreg) are laid up in a mold, vacuum-bagged, and cured in an autoclave at ~180°C and 6–7 bar pressure. The gold standard for aerospace quality — low void content (<1%), consistent properties.
Used for: Boeing 787 fuselage and wings, Airbus A350 wing covers, F-35 skins. Limitation: Autoclaves are expensive (2–20M USD), slow (4–8 hour cycles), and limit part size.Out-of-Autoclave (OOA)
Modified prepregs that achieve low void content under vacuum-only pressure. Eliminates the autoclave bottleneck. Gaining rapid adoption.
Used for: Bombardier CSeries (Airbus A220) wing, business jet structures.Resin Transfer Molding (RTM)
Dry fiber preform placed in a closed mold; resin injected under pressure. Good for complex shapes and medium volumes.
Used for: Automotive CFRP structures (BMW i3 passenger cell), aerospace brackets.Filament Winding
Continuous fiber wound around a mandrel in precise patterns. Excellent for pressure vessels and cylindrical structures.
Used for: Rocket motor cases, CNG/hydrogen storage tanks, drive shafts.Automated Fiber Placement (AFP) / Automated Tape Laying (ATL)
Robotic heads lay down prepreg tape or tow onto a mold surface. Enables large, complex structures with optimized fiber paths.
Used for: Boeing 787 fuselage barrels, Airbus A350 wing skins, F-35 structures.Failure Modes in Composites
Unlike metals, which fail progressively through yielding and necking, composites can fail in multiple modes simultaneously:
- Fiber breakage — Catastrophic in tension. The dominant failure mode for UD laminates loaded along fibers.
- Matrix cracking — Cracks parallel to fibers in off-axis plies. First damage to occur, often at ~40% of ultimate load.
- Delamination — Separation between plies. Driven by out-of-plane stresses, impact damage, or manufacturing defects. The most dangerous failure mode because it can be invisible on the surface.
- Fiber microbuckling — Compressive failure where fibers buckle within the matrix. Composites are typically 40–60% weaker in compression than tension.
Aerospace Composite Applications
Boeing 787 Dreamliner — 50% composite by weight. CFRP fuselage (one-piece barrel sections via AFP), CFRP wings, CFRP empennage. Enabled a wider cabin, larger windows, and higher cabin pressure (6,000 ft equivalent vs. 8,000 ft for aluminum aircraft). Airbus A350 XWB — 53% composite by weight. CFRP wing covers, fuselage panels, empennage. Uses resin-infusion for some wing components. F-35 Lightning II — 35% composite by structure weight. BMI matrix for high-temperature skins near engine. CFRP for wing skins and fuselage panels.Automotive Composite Applications
BMW i3/i7 — CFRP passenger cell (Life Module) manufactured by RTM. First mass-production CFRP car body. Chevrolet Corvette — GFRP body panels since 1953 (the longest-running composite car body). C8 uses SMC and GFRP extensively. Automotive leaf springs — GFRP replacing steel in some applications (Volvo XC90, GM trucks). Single composite spring replaces multi-leaf steel assembly, saving ~60% weight. Carbon fiber wheels — Carbon Revolution supplies CF wheels for Ford GT, Ferrari, Porsche. ~40% lighter than equivalent aluminum wheels.Metal Matrix Composites (MMC)
Metal matrices reinforced with ceramic particles or fibers:
| MMC | Matrix | Reinforcement | Application |
|---|---|---|---|
| Al-SiC | Aluminum | SiC particles | Brake rotors (Porsche PCCB uses SiC in ceramic matrix, but Al-SiC exists for other brake applications) |
| Al-Al₂O₃ | Aluminum | Alumina fibers | Cylinder liner inserts (Honda, Toyota) |
| Ti-SiC | Titanium | SiC fibers | Compressor bling (bladed ring) — research stage |
Ceramic Matrix Composites (CMC)
SiC fibers in a SiC matrix (SiC/SiC). Operating temperature: up to 1,300°C — 200°C higher than nickel superalloys, at 1/3 the density. The next revolution in jet engine materials.
GE LEAP engine — SiC/SiC CMC turbine shrouds. Saved ~120 lb per engine, 1% fuel burn improvement. GE9X engine — CMC combustor liners, HPT shrouds, and nozzle. Most extensive CMC use in any production engine.Key Takeaways
- Composites offer the highest specific strength and stiffness of any structural material class
- Carbon fiber dominates high-performance applications; glass fiber dominates by volume
- Layup orientation is critical — [0/±45/90]s is the quasi-isotropic baseline; tailored layups optimize for specific load paths
- Sandwich structures dramatically increase bending stiffness at minimal weight cost
- Delamination and BVID are the critical failure/damage concerns for composite structures
- CMCs (SiC/SiC) are replacing nickel superalloys in the hottest parts of jet engines
