07 Composite Materials

How composites are built

A composite material combines two or more distinct materials that remain identifiable as separate phases. The combination is designed to provide useful properties that its constituents alone may not offer. In a common fiber-reinforced composite, strong fibers carry much of the load while a surrounding matrix holds them in position and transfers load between them. Composites can also use particles, layers, or combinations of different materials.

Constituents and their interface

The reinforcement provides much of a composite's stiffness or strength. Examples include glass, carbon, and aramid fibers; ceramic fibers; and particles such as mineral or ceramic powders. The matrix surrounds and supports the reinforcement, transfers load, and helps protect it from the environment. Matrices may be polymers, metals, or ceramics.

The interface is the boundary between reinforcement and matrix. Its bonding and chemistry affect load transfer, damage, and failure. If the interface is too weak, reinforcement may detach before carrying its share of the load. Controlled debonding can sometimes help deflect cracks and improve toughness, but the best interface depends on the material system and intended performance.

Layers and reinforcement architectures

A lamina is one reinforced layer, often with fibers aligned in a particular direction. A laminate is made by bonding multiple layers together. Different layers can use different fiber directions to tailor the response of the overall part.

Choosing a reinforcement architecture

The arrangement of reinforcement—its architecture—is a major design choice. Unidirectional fibers run mainly in one direction, providing high stiffness and strength along the fibers but much less across them.

Cross-ply or multidirectional laminates stack layers at selected angles, such as $0^\circ$, $90^\circ$, and $\pm45^\circ$. The layup balances directional requirements: a $0^\circ$ layer supports lengthwise loading, while a $\pm45^\circ$ layer can help carry in-plane shear.

Woven and braided fabrics place fibers along multiple directions and can drape over shaped molds. Three-dimensional preforms also include reinforcement through the thickness, which can help resist separation between layers. Short-fiber or particle-reinforced composites distribute discontinuous reinforcement through a matrix. They can be shaped efficiently, though their properties may be less directional than those of continuous-fiber composites.

Sandwich structures bond thin, stiff face sheets to a lightweight core, such as foam or honeycomb. The separated faces provide bending stiffness while the core supports them and carries shear.

Tailored properties and limitations

Because many composites are anisotropic, their properties depend on the direction of the applied load. Stiffness, strength, and thermal expansion can be tailored by choosing the constituents, reinforcement fraction, architecture, and layer orientations. Some composites offer high strength or stiffness for their weight, corrosion resistance, or performance at elevated temperatures.

These advantages are not universal. Performance depends on the specific system, and parts may be sensitive to impact damage, defects, temperature, moisture, or manufacturing quality. Design choices therefore need to account for directional behavior and the needs of the application.

Applications across industries

Composite use depends on the properties needed in the application:

• Transportation and aerospace: Carbon- or glass-fiber polymer composites are used in aircraft structures, spacecraft components, pressure vessels, and vehicle parts where low mass and tailored stiffness are valuable.

• Construction and infrastructure: Glass-fiber polymer products are used in structural profiles and reinforcement. Composite sandwich panels provide lightweight panels with bending stiffness.

• Energy and marine equipment: Fiber-reinforced polymers are used in wind-turbine blades, boats, and other large structures that benefit from lightweight, corrosion-resistant construction.

• High-temperature equipment: Ceramic-matrix composites, such as silicon-carbide fibers in a silicon-carbide matrix, are developed for hot sections of gas turbines and other demanding thermal environments.