09 Composite Materials
Learn how composite materials combine constituents, how reinforcement arrangements shape their properties, and how to select them for particular applications.
What makes a composite
A combines two or more distinct constituents that remain identifiable within the material. Usually, one constituent is a continuous and another is distributed through it. Their combination can provide a useful balance of properties that neither constituent provides alone.
Common engineered classes include polymer- composites (PMCs), metal- composites (MMCs), and ceramic- composites (CMCs).
How the constituents work together
The provides much of a composite’s strength and stiffness, especially along its own direction. It can take the form of continuous or chopped fibers, whiskers, particles, or platelets; carbon, glass, and aramid fibers are common examples.
The binds and supports the , transfers load between it and neighboring regions, and helps protect it from damage and the environment. In fiber composites, it also supports fibers under compression. choice influences processing, temperature capability, and other properties.
The is the boundary where and meet. Effective bonding and compatibility at this boundary are important for transferring load, and the can strongly affect shear and compressive performance.
For example, in a carbon-fiber/epoxy composite, stiff carbon fibers carry much of the tensile load, while the epoxy holds them in position and transfers load between fibers.
arrangements
describes how the is shaped, oriented, and arranged. It is a design choice that determines which directions are strongest and stiffest.
Unidirectional fibers: Fibers run mainly in one direction, providing high performance along that direction but less across it.
Woven or multidirectional fabric: Fibers cross in two or more directions. This can provide strength in multiple directions and help the material conform to shapes.
Laminates: Layers, or plies, are stacked with selected fiber directions. A might use plies at , , and to respond to loads in several directions. The order and angles of the plies affect the finished part’s behavior.
Chopped-fiber or particle composites: Shorter fibers or particles are dispersed through the . These can be formed into complex shapes, but typically do not provide the same directional as continuous, aligned fibers.
Sandwich structures: Thin, strong face sheets are separated by a lightweight core. This arrangement can provide high bending stiffness at low weight.
Properties and design trade-offs
Composite properties depend on the constituents, their proportions, the , and the . Fiber composites are often , meaning their properties vary with direction. For instance, a unidirectional composite is generally much stiffer along the fibers than across them. Arranging plies in different directions allows designers to tailor a to expected loads.
Composites can offer high strength or stiffness relative to their weight, corrosion resistance, or performance in demanding environments. These benefits must be weighed against limitations: properties may depend on direction, manufacturing and inspection can be demanding, and can be difficult to detect.
Selection should account for the part’s loading, service environment, manufacturing process, and cost rather than rely on a single property. Relevant considerations include form, fiber content, , ply arrangement, processing, cost, and required end-item properties.
Applications
Fiber-reinforced polymer composites are used in lightweight structural components for aircraft and spacecraft. The chosen fiber, , and ply directions depend on the design requirements.
Carbon-fiber composites are used in sporting goods and bicycles to provide stiff, lightweight structures. Their specific properties depend on the fiber arrangement and part design.
SiC-fiber/SiC- ceramic- composites are developed for demanding applications such as turbine components, thermal-protection systems, and furnace parts.