How Composite Materials Are Created: A Materials Science Guide

Composite materials are engineered solids made by combining two or more constituents — a reinforcement and a matrix — that keep their separate identities yet act together to deliver properties neither could reach alone. Modern science understands the structure of materials, their behaviour and their "preferences" well enough that new materials are now developed deliberately rather than by luck. This is shown most clearly in how composites are created.

What are composites: definition and basic properties

A composite is a material built from two chemically and physically distinct phases: a reinforcement that carries load and a matrix that binds the reinforcement, transfers stress and protects it. Unlike a metal alloy or a solid solution, where the components dissolve into one another and lose their boundaries, and unlike a simple mixture, the phases in a composite remain distinct and visible at the microscopic scale. That separation is exactly what lets engineers combine, say, the stiffness of a fibre with the lightness of a light-metal matrix.

The defining trait of composites is tailored, often directional strength. By orienting the reinforcement, a composite can be made extremely strong along one axis while behaving normally in others — something homogeneous metals cannot do. Composites also commonly offer high tensile and compressive strength, low density, and good heat and chemical resistance, along with tunable thermal and electrical behaviour depending on the constituents chosen.

Composites fall into families defined by their reinforcement geometry and their matrix. Fibre-reinforced composites use long fibres such as glass, carbon or aramid; particulate composites disperse particles through the matrix; sandwich-structured composites bond thin stiff skins to a light core. Natural composites exist too — wood is cellulose fibre in a lignin matrix, and bone is a mineral-and-collagen composite — proving the principle long before it was named.

The history of composite materials

The history of composite materials stretches from the earliest human construction to today's aerospace laminates, because the idea of reinforcing a weak binder with a strong filler is very old. What changed over time was the sophistication of the constituents, from straw and clay to carbon fibre and epoxy resin.

Ancient composites and their use in construction

Ancient builders created composites long before any theory existed by mixing straw into wet clay to make sun-dried mud bricks. The Egyptians in ancient Egypt reinforced clay bricks with straw so the fibres would carry tension and stop the brick from cracking as it dried — the straw acted as reinforcement and the clay as matrix, exactly as in a modern fibre composite.

The Mesopotamians of the Fertile Crescent went further, gluing thin sheets of wood together with their grain directions crossed to make an early form of plywood that resisted warping and splitting. Around the same era, other cultures learned that layering different materials produced better structural behaviour than any single one on its own.

Concrete is another ancient composite: the Roman engineer Vitruvius described mixing lime with volcanic ash from Pozzuoli to make a binder that set hard even underwater. That tradition eventually led to Portland cement and, in the modern era, to reinforced concrete, in which steel bars carry the tensile loads the brittle concrete cannot.

Medieval composite technology reached a peak in the composite bow perfected by the Mongols, which laminated wood, horn and sinew to store far more energy than a plain wooden bow. Engineered wood products and layered armour show the same principle carried through the centuries.

The chemical revolution and the arrival of synthetic resins

The modern composites industry became possible only when synthetic polymer matrices appeared in the early 1900s. In 1907 Leo Baekeland invented Bakelite, the first fully synthetic plastic — a milestone later commemorated by the American Chemical Society. Bakelite gave engineers a mouldable, heat-resistant matrix that could be combined with fillers.

The distinction between resin types shaped everything that followed. A thermoset — such as epoxy resin, polyester resin or Bakelite — cures irreversibly into a rigid network and cannot be remelted, while a thermoplastic softens on heating and can be reshaped. Thermosetting plastics dominate high-performance composites because their cured matrix resists heat and creep.

Glass fibre turned resin into a genuine structural material. When Owens Corning commercialised glass fibre in the 1930s, the combination of glass fibre and a plastic matrix produced fibreglass — a fibre-reinforced plastic (FRP) that was light, strong and corrosion-proof. World War II accelerated production dramatically as fibreglass found use in aircraft and radar housings, and even improbable experiments like Pykrete, a composite of ice and wood pulp, were trialled.

The development and commercialisation of carbon fibre

Carbon fibre transformed composites from useful to exceptional by offering stiffness far beyond glass at lower weight. Early carbon fibre work drew on figures such as Arthur D. Little's laboratory experiments, and by the 1960s manufacturers were producing continuous high-modulus filament suitable for carbon fiber-reinforced polymer.

Carbon fibre reinforcements give a composite very high specific stiffness and strength, low thermal expansion and excellent fatigue resistance, which is why they replaced metals in demanding structures. Alongside carbon, aramid fibre — sold as Kevlar — brought outstanding toughness and impact resistance, making it central to protective and structural applications.

Advanced armour combined these ideas: Chobham armour layered ceramics and composites to defeat projectiles, showing how multi-material composites could outperform homogeneous steel. By the late twentieth century the composites market had matured from a niche into a mainstream engineering sector.

Materials science as the foundation of composites

Materials science studies the general laws behind creating materials and establishes the links between structure, composition and properties. Using these laws and relationships, developers of new materials mark out where to search and move toward their goal knowingly, rather than groping in the dark as Palissy did while creating French porcelain (more: The history of French porcelain).

The paths that lead to the goal are now well lit — some carry powerful floodlights, others only small lanterns. Brightly lit roads can be travelled quickly, dimly lit ones more slowly, over bumps and through undergrowth, but the direction can still be held clearly.

How composites are created today

These are the roads along which the teams of materials scientists move today. Some of them open up new paths and need the intuition of a Palissy. It is simply wrong to claim that trial and error is no longer used at all — it is still used, but comparatively little.

Trial and error in modern materials science

Trial and error survives in materials science but plays a shrinking role because theory now predicts most outcomes in advance. Where the behaviour of a new constituent is genuinely unknown, experiment still leads; where established models apply, calculation replaces guesswork. The balance has shifted decisively toward prediction, which is why development is far faster and cheaper than in earlier eras.

Reinforced composites and the theory of anisotropic media

Reinforced composites were luckier than most materials. By the time the idea of developing them had matured, the theory of anisotropic media already existed, which made it possible to predict the properties of composites in advance from the known properties of their components.

Where ancient metallurgists added a substance to molten iron without knowing how it would affect the steel (more: The secret of Damascus steel), the creators of reinforced composites know exactly what they will get by introducing a given kind of fibre into a given matrix. They can predict the future material's properties not only qualitatively but quantitatively, because they rest on a firm theoretical foundation.

Carbon fibres and their advantages

Carbon fibres are the reinforcement of choice when a composite must be both stiff and light, because they raise the modulus and strength while actually lowering density. In the worked example below, carbon fibres reduce the density of an aluminium-based composite rather than increasing it — an advantage no dense metallic reinforcement can match. Their high specific stiffness, fatigue resistance and low thermal expansion explain their dominance in aerospace and sports equipment.

Boron, nanotubes and promising reinforcements

Boron fibres offer an even higher modulus than carbon and, like carbon, keep the composite's density close to that of aluminium, which makes them a viable reinforcement for demanding light-metal composites. Looking further ahead, carbon nanotubes, boron nanotubes and graphene promise reinforcements with extraordinary stiffness-to-weight ratios. Research groups including the Research School of Physics and Engineering at the Australian National University have investigated such nanoscale reinforcements as the next step beyond conventional fibres.

Theory and practice: why calculations diverge from reality

This does not mean that in practice you always get what the theory predicts. Unfortunately, full agreement of theory with practice is far from always observed. A theory is usually built for certain specific models that capture the main features of real composites — the main ones, but not all of them.

It simply cannot account for everything, because there are so many factors, and any attempt to include them all leads to a complexity so great that the theory becomes unusable. So each theory holds only within the bounds set when it was built. Real materials know nothing of those bounds.

If their behaviour does not fit within those bounds, the fault lies not with the theory but with those who apply it where it should not be applied. Yet for most reinforced composites the theory of reinforced media gives a reliable estimate of the properties that can be achieved — that is, it provides a target to aim for.

This theory, in its general form, is fairly complex, and to understand it you need special branches of higher mathematics. We will not touch on those; instead, to still get a sense of how the properties of composites can be forecast in advance, we will look at the simplest example, one that a competent fifth-grader could follow.

An example of creating an aluminium-based composite

The task is to create a composite based on aluminium with a strength of 1000 MPa and a Young's modulus of 200 GPa. It is enough to have this strength in one direction only; in the others, strength at the level of aluminium is acceptable. The composite's density must not exceed that of aluminium alloys.

Setting the problem: the strength of steel at the weight of aluminium

Rephrased, the requirement is this: create an aluminium-based material with the strength and stiffness of a good alloy steel while keeping the weight characteristics of aluminium. Traditional metallurgical methods — alloying, heat treatment, plastic deformation — cannot do it. The strength of pure aluminium is 120–150 MPa, and of aluminium alloys up to 500–700 MPa; nothing higher is achievable.

The Young's modulus of aluminium and its alloys is about 70 GPa, and alloying, heat treatment and plastic deformation barely change it — yet we need to almost triple it. Aluminium is a light metal (density 2700 kg/m³), so alloying it with elements heavier than aluminium itself would raise the density and make the requirements impossible to meet.

Follow the reasoning of a composites materials scientist. It is clear the task must be solved by creating a composite material. Since strength is required in only one direction, a reinforced material is needed in which the reinforcing fibres lie parallel to one another; maximum strength and stiffness will be delivered along their axis. The question is which fibres to use and how many to add.

Clearly the reinforcement needs fibres with the highest strength and stiffness, but those figures alone are not enough. Tungsten fibres, though mechanically excellent, are too dense and would break the density requirement; the same rules out steel wires and silicon carbide fibres. Whisker crystals (more: Metallic whiskers) could help, but their use brings large technological difficulties and heavy costs. The most sensible choice is to try boron and carbon fibres, which will not increase the density relative to aluminium — carbon fibres even lower it — while raising the modulus and strength.

Calculating the composite's properties by the rule of mixtures

The calculation uses the additivity principle, the rule of mixtures. This is the old schoolbook problem: a kilogram of caramels costs 1 rouble and a kilogram of toffees 3 roubles; what does a kilogram of a mixture of two kilograms of caramels and four kilograms of toffees cost? Such mixture problems turn out to be genuinely useful in technical calculations, including estimating some properties of composites.

The strength and stiffness of a composite in the reinforcement direction can be computed just like the price of one kilogram of the mixture. To a first approximation a composite can be treated as a mixture of matrix and fibres — not always valid, but acceptable here for estimation.

The price of one kilogram of mixture follows a simple formula: P = P₁ N₁ + P₂ N₂. Here P₁, P₂ and P are the prices of caramels, toffees and the mixture; N₁ is the fraction of caramels and N₂ the fraction of toffees, with N₁ + N₂ = 1. In a mixture of 2 kg of caramels and 4 kg of toffees, N₁ = 2/(2+4) = 1/3 and N₂ = 4/(2+4) = 2/3. By the formula, the price of 1 kg is P = 1 (rub/kg) × 1/3 + 3 (rub/kg) × 2/3 = 2⅓ (rub/kg).

Young's modulus of the composite

By an analogous formula the Young's modulus of the composite in the fibre direction can be calculated: E = E_M V_M + E_B V_B. Here E_M, E_B, E are the Young's moduli of the matrix (more: Reinforced composites), the fibres and the composite; V_M, V_B are the volume fractions of matrix and fibres.

Since V_M + V_B = 1, this can be written as E = E_M (1 − V_B) + E_B V_B. We use this to solve the task. We need a composite with E = 200 GPa. The Young's modulus of the aluminium matrix is E_M = 70 GPa, the average modulus of boron fibres E_B = 400 GPa, and of carbon fibres E_B = 350 GPa.

For the boron-reinforced composite the formula becomes 200 = 70 (1 − V_B) + 400 V_B. Solving for V_B gives V_B = 0.39 = 39 % by volume. For the aluminium–carbon-fibre composite, 200 = 70 (1 − V_B) + 350 V_B, giving V_B = 0.46 = 46 % by volume.

So to meet the stiffness requirement one must add to aluminium either 39 % by volume of boron fibres or 46 % by volume of carbon fibres. The composite's strength can also be calculated by the rule of mixtures.

When the fibres are less ductile than the matrix — exactly our case — the strength of the reinforced material along the fibre direction is estimated approximately by: (σ_B)_c = (σ_B)_f V_B + σ_M (1 − V_B). Here (σ_B)_c and (σ_B)_f are the strength limits of composite and fibres, σ_M is a value close to the matrix yield strength, and V_B is the fibre volume fraction. We need (σ_B)_c = 1000 MPa.

Boron fibres average (σ_B)_f = 3250 MPa and carbon fibres (σ_B)_f = 2500 MPa, while the yield strength of aluminium is about 30 MPa. So for boron-reinforced aluminium: 1000 = 3250 V_B + 30 (1 − V_B), and for carbon-aluminium: 1000 = 2500 V_B + 30 (1 − V_B). Solving gives V_B = 0.30 = 30 % by volume for boron-aluminium and V_B = 0.39 = 39 % by volume for carbon-aluminium.

The results show the task is solved either by making boron-aluminium containing 39 % by volume, or carbon-aluminium reinforced with 46 % by volume of carbon fibres. Composites of this composition will have the required stiffness, and their strength will exceed the target — not ideal economically, but we may not make the material weaker, since reducing fibre concentration also lowers the Young's modulus.

The density of the resulting composites can likewise be found by the rule of mixtures: γ = γ_B V_B + γ_M (1 − V_B), where γ, γ_B, γ_M are the densities of composite, fibres and matrix and V_B is the fibre volume fraction. Taking the densities of boron and carbon fibres (2630 kg/m³ and 1700 kg/m³) and γ_M = 2700 kg/m³, the aluminium – 39 % boron-fibre composite has a density of 2670 kg/m³ and the aluminium – 46 % carbon-fibre composite 2240 kg/m³. The density requirement is met too.

What remains is to decide which of the two composites to choose. That depends on a whole set of conditions — technological, structural and economic. In some cases boron-aluminium is preferable, in others carbon-aluminium. Since no further information is specified, we can stop here and consider the task of how composites are created complete.

Composite manufacturing methods

Composite manufacturing turns loose fibre and liquid resin into a solid part through four fundamental steps: impregnation (wetting the fibre with resin), layup (placing and orienting the fibre), consolidation (pressing out air and excess resin), and solidification (curing a thermoset or cooling a thermoplastic). Every process below is a variation on those same four steps, differing mainly in how pressure and heat are applied.

Open and closed moulding

Open moulding cures the laminate against a single mould surface exposed to the air, while closed moulding traps the part between two tool halves or inside a sealed bag. Open moulding methods are cheap and flexible but slower and less consistent; closed moulding gives better surface finish, tighter dimensional tolerances and lower emissions.

• Hand lay-up — an open method in which resin and reinforcement are placed and rolled into the mould by hand; ideal for large, low-volume parts such as boat hulls.
• Spray-up — chopped fibre and resin are sprayed onto the mould, faster than hand lay-up but with more variable quality.
• Resin Transfer Moulding (RTM) — dry reinforcement is placed in a closed mould and resin is injected under pressure, giving repeatable, high-quality parts.
• Vacuum Infusion — a vacuum draws resin through dry fibre under a sealed bag, improving fibre content and removing voids.
• Compression Moulding — a charge is squeezed between heated mould halves, suited to high-volume automotive parts.
• Injection Moulding — short-fibre compound is injected into a mould for fast, complex, high-rate production.
• Filament Winding — resin-wetted fibre is wound onto a rotating mandrel to make tubes, pipes and pressure vessels.
• Cast Polymer Moulding — filled resin, sometimes a high gravity compound, is poured and cured, used for solid-surface products.

Autoclave curing of composites

Autoclave curing applies simultaneous heat and high pressure inside a pressurised vessel to cure the highest-performance laminates. Controlling the cure kinetics — the temperature ramp and hold — governs how the resin cross-links, while the pressure drives out volatiles and voids to achieve maximum fibre volume fraction. Aerospace prepreg parts are almost universally autoclave-cured because the process yields the tightest tolerances and the strongest, most consistent structures.

Automated fiber placement (AFP)

Automated Fiber Placement (AFP) lays continuous tows of prepreg or dry fibre onto a tool with a robotic head, building the laminate layer by layer with precise fibre orientation. AFP dramatically raises production rate and repeatability compared with hand layup, which is why it is standard on modern airliner structures. Companies such as Addcomposites supply accessible AFP systems, and the wider ecosystem — including Composite HUB and engineering firms like Mar-Bal — reflects how automated manufacturing has spread beyond aerospace.

Braiding and weaving of textile composites

Braiding and weaving interlace fibres into textile preforms that are then impregnated with resin, giving composites strength in multiple directions at once. Woven fabrics and braided sleeves resist delamination and impact better than simple unidirectional stacks, making them valuable for tubular and irregularly shaped parts.

Formation of composite laminates

A composite laminate is built by stacking individual plies — each single layer is called a lamina — with their fibre directions arranged to suit the expected loads. Prepreg workflows use fibre already pre-impregnated with resin, offering precise, clean, repeatable fibre content, while dry-fibre workflows place the reinforcement first and add resin later by infusion or RTM. Choosing between prepreg and dry fibre trades material cost and shelf life against tooling complexity.

Advantages and disadvantages of manufacturing methods

Types of composite materials by matrix

Composites are classified above all by their matrix, which determines the operating temperature, chemical resistance and manufacturing route. Polymer matrix composites, using thermoset resins such as epoxy or polyester or thermoplastics, are the largest group and cover most fibreglass and carbon-fibre parts. Their light weight and easy processing explain their dominance across industry.

Ceramic and metal matrix composites

Ceramic matrix composites and metal matrix composites extend composites into environments where polymers fail. Ceramic matrix composites embed fibres in a ceramic to add toughness to an otherwise brittle material, letting them survive the extreme heat of jet-engine and brake components. Metal matrix composites — such as the boron- and carbon-reinforced aluminium worked out earlier — combine metallic ductility and conductivity with fibre stiffness, serving aerospace and high-performance automotive parts.

Applications of composites

Composites are used wherever high strength at low weight, corrosion resistance or tailored stiffness pays off — from aircraft and cars to boats, sports gear, medical implants and buildings. The same rule-of-mixtures logic that solved the aluminium example above underlies every one of these applications.

Aerospace

The aerospace industry is the largest driver of high-performance composites because every kilogram saved cuts fuel burn. The Boeing 787 Dreamliner and the Airbus A350 are built with airframes that are more than half carbon fibre-reinforced polymer by weight, and NASA has long used composites in spacecraft structures. Engineers including Jim Williams and Al Simison contributed to the mechanics and testing that made such structures trustworthy.

Automotive

The automotive industry uses composites to cut weight, resist corrosion and mould complex shapes in one piece. The Chevrolet Corvette pioneered fibreglass body panels in mass-produced cars, and carbon-fibre parts now appear in performance and electric vehicles where saving weight extends range. Compression and injection moulding make composite production fast enough for volume manufacturing.

Sport and leisure goods

Composite materials dominate sports and recreation because athletes value stiffness and low weight above raw cost. Bicycle frames, tennis rackets, golf shafts, skis, hockey sticks and fishing rods are commonly carbon-fibre or fibreglass composites — the connection with fishing tackle being a familiar everyday example. In the marine industry, fibreglass hulls made composites a household material long before aerospace did.

Emerging technologies and the future of composites

The future of composites lies in stronger nanoscale reinforcements, smarter matrices and greener production. Graphene and carbon nanotubes promise reinforcements far stiffer and lighter than today's fibres, while shape-memory polymer composites can change form on demand for deployable and adaptive structures. Environmental sustainability is now a central research theme, driving recyclable thermoplastic matrices, bio-based resins and lower-energy curing so that the strength-to-weight benefits of composites no longer come at an end-of-life cost.