What makes a composite “advanced”
A composite is a material made of two or more distinct constituents that stay separate on a fine scale but work together. Plywood, reinforced concrete and fiberglass boats are all composites in a broad sense. The term advanced composites usually refers to high-performance types, in which strong, stiff fibers such as carbon, aramid or high-grade glass are embedded in a carefully chosen matrix to build structures where weight and stiffness are priorities, such as aircraft, high-performance vehicles and sporting equipment.
The central idea is a division of labor. The fibers carry most of the load, and the matrix holds them in place, transfers load between them, and protects them from damage and the environment. The general principles are described in composite materials, and this article focuses on the ingredients and the production route.
The reinforcement: fibers
Carbon fiber is the best-known advanced reinforcement. It is very stiff and strong for its weight, does not corrode, and holds up well under repeated loading. It is produced by heating organic precursor fibers, most commonly polyacrylonitrile, in controlled stages until almost everything but carbon is driven off, leaving thin, carbon-rich filaments bundled into “tows.” The details and uses are covered in carbon fiber.
Glass fiber is the most widely used reinforcement overall because it is relatively inexpensive. It is less stiff than carbon fiber but tough and electrically insulating.
Aramid fiber, best known by trade names such as Kevlar, is valued for toughness and impact or ballistic resistance, though it is weaker in compression.
The relative merits of these three are compared in the article on glass, aramid and carbon fiber reinforcements.
Fibers can be used in several forms: continuous unidirectional tapes, woven or stitched fabrics, and chopped fibers. Continuous fibers give the best performance because the load can be carried along their length. Chopped fibers are easier to process but give lower strength.
The matrix
Polymer matrices dominate. Thermoset resins, mainly epoxies, start as liquids and then cure through a chemical reaction into a rigid, cross-linked solid that cannot be remelted. Epoxies bond well to fibers and give good strength and low shrinkage. Polyesters and vinyl esters are cheaper and common in glass-fiber products. Thermoplastic matrices, which soften when heated, are of growing interest because they can be reshaped and welded, and may ease recycling. See polymer matrix for more.
Metal matrices, such as aluminum reinforced with ceramic particles or fibers, are used where higher temperature, conductivity or wear resistance is needed, as discussed in metal matrix composites.
Ceramic and carbon matrices serve at the highest temperatures. Ceramic matrix composites and carbon-carbon composites are examples, both aimed at heat-intensive uses.
Sizing and the interface
Fibers are normally coated with a thin “sizing” during manufacture. It protects them from handling damage and helps the matrix stick to them. This matters because the boundary between fiber and matrix controls how loads pass between them and how cracks travel. A weak bond lets fibers pull out easily, and a bond that is too strong can make the composite fail in a brittle way. The subject has its own coverage under the fiber-matrix interface.
Making the part
Most advanced composite parts are built up in layers, called plies, with fiber directions chosen to match the expected loads. Although specifics vary, the production route usually includes the following stages.
1. Preparing the material. Fibers may be bought as dry fabric or as prepreg, which is fiber already impregnated with partially cured resin. Prepreg is stored cold to prevent premature curing.
2. Layup. Plies are cut and placed on a mold, by hand or by machines that lay tape or place fiber automatically. Orientation and ply order are carefully specified, since they set the part’s stiffness and strength in each direction. Layup techniques are described further in composite layup.
3. Consolidation. The stack is covered with materials that allow air to be drawn out under a vacuum bag. Removing trapped air and extra resin helps avoid voids, which weaken the part.
4. Curing. Heat, often with pressure, drives the resin chemistry. Aerospace parts are frequently cured in an autoclave, a heated pressure vessel, while other methods use ovens, presses or resin infusion into dry fabric under vacuum. Time and temperature follow a defined schedule.
5. Finishing. After demolding, parts are trimmed, drilled and inspected. Machining carbon composites is tricky because the fibers are abrasive and the layers can separate, so hard tooling and careful technique are used.
6. Inspection. Voids, delaminations and fiber misalignment can hide inside a part, so methods such as ultrasonic scanning are common, in line with nondestructive evaluation.
Processes for lower-cost or higher-volume parts include filament winding, where fiber is wound around a rotating form to make pipes and pressure vessels, and pultrusion, which pulls fiber through a resin bath and heated die to make constant-section profiles.
What affects quality
Several factors repeatedly decide whether a composite part performs as designed:
- Fiber volume fraction. The share of fiber compared with resin influences stiffness and strength.
- Fiber alignment. Waviness or misorientation reduces performance.
- Void content. Trapped air or volatiles create weak spots.
- Cure state. Under-cured or overheated resin has altered properties.
- Moisture and contamination. Water or dirt on fibers or tools can affect bonding.
Because quality depends so much on process control, composite makers usually test sample panels alongside production parts.
Limits and trade-offs
Advanced composites are not always the best choice. Raw materials and labor can be costly, curing takes time, and repair is more complicated than for metals. Failure can also be different in character, with delamination and damage that is not obvious from the surface, which is why composite failure modes get close study. End-of-life handling is an open concern, since thermoset parts are hard to recycle, a topic covered in composite recycling. Even so, where weight savings and stiffness are valuable, the economics often favor composites.