Composite Materials
A composite material is a new material created by combining, at a macroscopic level, at least two materials with different physical and chemical properties, in order to obtain superior properties that neither of them has on its own. Even after the components have been brought together, they do not dissolve into one another or mix completely. A distinct interface remains between them, and that interface plays a very important role in the behaviour of the composite.
Composites generally consist of two main components. The first is the reinforcement (fibre, filament or particle), which carries the load and provides the strength. The second is the matrix (resin, metal or ceramic), which holds these elements together, transfers the load to the fibres and protects them from external effects.
The greatest advantage of composites is that they offer a combination of properties that none of their components has on its own. A carbon fibre reinforced composite, for example, can be as strong as steel while being far lighter. This is largely the answer to the question “why are composite materials so important?” They make it possible to produce a part that does the same job while being lighter, more durable and usually longer lasting.
A Short History of Composite Materials
The idea of the composite is in fact very old. The ancient Egyptians and Mesopotamians added straw to mud bricks to increase their resistance to cracking. Wood is also a composite that formed naturally: cellulose fibres sit within a lignin matrix. Reinforced concrete, meanwhile, is one of the most widely used structural composites today.
Modern composites took shape in the twentieth century. Glass fibre went into commercial production in the 1930s, carbon fibre developed in the 1960s, and aramid fibre (Kevlar) was developed in the DuPont laboratories in the mid-1960s. After that, composites moved out of the aerospace and defence industries and spread into the automotive, sports and construction sectors.
The Components of a Composite Material
The reinforcement. This carries most of the load. Glass, carbon, aramid, basalt and natural fibres are the most common. The fibre can be used unidirectionally, as a woven fabric, as a mat, or as short chopped pieces.
The matrix. This holds the fibres in place, transfers the load to them and protects them from moisture, chemicals and impact. There are polymer, metal and ceramic matrices.
Fillers and additives. These are used to lower cost, increase fire resistance, add colour or provide UV resistance.
The interface. This is the bonding region between the fibre and the matrix. If this bond is weak, even the best fibre cannot deliver its performance. Special coatings called “sizing” are therefore applied to the fibre surfaces.
Types of Composite Material
The question “what are the types of composite material?” can be answered from two different angles: classification by matrix material, and classification by the form of the reinforcement.
Composites by Matrix Material
1. Polymer Matrix Composites (PMC)
This is the most common type of composite. Reinforcements such as glass fibre, carbon fibre or aramid (Kevlar) are combined with resins such as epoxy, polyester or vinyl ester. The resins can be thermosetting (they harden with heat and cannot be reshaped afterwards) or thermoplastic (they can be reshaped when heated). They are widely used in aircraft fuselages, wind turbine blades, boat hulls and sports equipment (bicycles, tennis rackets), where light weight and high strength are required.
2. Metal Matrix Composites (MMC)
These are produced by placing ceramic particles or fibres (silicon carbide, for example) into a metal matrix such as aluminium, titanium or magnesium. They are preferred in aerospace and defence components that require resistance to high temperature and wear. Some aluminium matrix brake discs, pistons and cylinder liners are examples of this group.
3. Ceramic Matrix Composites (CMC)
These are produced by placing ceramic or carbon fibres into a ceramic matrix. Ceramics on their own are brittle; fibre reinforcement reduces that brittleness. Because they can withstand extremely high temperatures, they are used in rocket engine components, in brake discs (particularly on high performance cars and aircraft) and in gas turbine components.
4. Natural and Hybrid Composites
These are produced by combining natural fibres such as hemp or flax with a polymer matrix. Because they are environmentally friendly and low in cost, their use is increasing in automotive interior trim parts and in the packaging sector. Hybrid composites, in which different fibres are used together, are preferred for striking a balance between cost and performance.
Composites by Form of Reinforcement
Fibre reinforced composites. These are structures strengthened with continuous or short fibres. They are used where high strength and stiffness are required.
Particle reinforced composites. Spherical or irregular particles are added into the matrix. Concrete is the classic example of this.
Laminated composites. These are formed by bonding many layers, arranged in different directions, on top of one another. The layer directions are chosen according to the design, and this is how it is decided in which direction the part will be stronger.
Sandwich structures. A light core (honeycomb, foam or balsa) sits between two thin, strong face sheets. Very light but very stiff panels are obtained. Aircraft floor panels and boat decks are typical examples.
Comparing Reinforcing Fibres: Glass, Carbon and Aramid
“Which is better, carbon fibre or glass fibre?” is a much-asked question. The answer depends on the application. The values below are approximate and vary with the manufacturer and the quality.
| Property | Glass fibre (E-glass) | Carbon fibre | Aramid (Kevlar) |
|---|---|---|---|
| Density (g/cm³) | about 2.5 | about 1.75 | about 1.44 |
| Stiffness | Medium | Very high | Medium to high |
| Impact resistance | Good | Low to medium | Very good |
| Cost | Low | High | High |
| Typical use | Boats, pipes, panels | Aircraft, F1, bicycles | Ballistic vests, ropes |
To sum up briefly: glass fibre is economical and versatile, carbon fibre offers the highest stiffness and the lowest weight, and aramid stands out for resistance to impact and cutting.
How Are Composite Materials Produced?
The answer to “what are the composite manufacturing methods?” varies with the production quantity, the size of the part and the expected quality.
Hand lay-up. Fibre fabric is laid into the mould and impregnated with resin using a brush or a roller. It is simple and cheap, and is used for boats and small production runs. The quality depends largely on the workmanship.
Vacuum infusion. Dry fibre is laid into the mould, closed off with a vacuum bag, and resin is drawn in by vacuum. More homogeneous and lighter parts are obtained. It is common in wind turbine blades.
Prepreg and autoclave. Fibre layers pre-impregnated with resin (prepreg) are laid into the mould and cured in an autoclave under pressure and heat. The highest quality is obtained with this method. Aerospace and Formula 1 parts are produced this way.
RTM (Resin Transfer Moulding). Resin is injected under pressure into fibre placed in a closed mould. It is suitable for medium-volume automotive parts.
Filament winding. Continuous fibre dipped in resin is wound onto a rotating mandrel. Pressure vessels, pipes and shafts are produced with this method.
Pultrusion. Fibres are passed through a resin bath and pulled through a heated die. Profiles of constant cross-section (rods, I-beams, tubes) are produced continuously.
SMC / BMC moulding. A mixture of short fibre and resin is pressed in a hot mould. It is preferred for mass-produced body panels and bodywork in the automotive industry.
Advantages of Composite Materials
- High strength-to-weight ratio: For the same strength, they are far lighter than metal. That means fuel savings and a higher load capacity.
- Corrosion resistance: Unlike metals, they do not suffer from rust or chemical attack. This is a great advantage in the marine and chemical industries.
- Design flexibility: They can be produced in complex geometries and as large single-piece structures. This reduces the number of joints and bolts.
- Fatigue resistance: They last longer than many metals under repeated loading.
- Directional design: By laying the fibres along the direction of the load, the material can be strengthened only where it is needed.
- Vibration damping: Many composites damp vibration better than metals, which brings benefits in comfort and noise.
- Thermal and electrical insulation: Glass fibre composites are good electrical insulators.
Disadvantages of Composite Materials
- High production cost: Advanced composites, carbon fibre in particular, are expensive.
- Difficulty of recycling: Unlike metals, composites are technically harder to recycle. Thermosetting resins cannot be melted down again.
- Difficulty of repair: Repairing a damaged composite part requires more specialised knowledge and equipment than repairing metal.
- Anisotropic behaviour: Their strength varies with direction. If they are not designed correctly, they can show weakness at unexpected points.
- Risk of hidden damage: Even when no mark is visible on the surface after an impact, separation between the inner layers (delamination) can occur. Non-destructive inspection methods such as ultrasonic testing are therefore required.
- Effect of environment and moisture: Prolonged UV, heat and moisture can degrade the properties of some resin systems over time.
Where Are Composite Materials Used?
The answer to “where are composite materials used?” is far broader than people assume.
Aerospace. The share of composites in the fuselages and wings of modern passenger aircraft is very high. In aircraft such as the Boeing 787 and the Airbus A350, roughly half the structure by weight consists of composite material. They are also widespread in satellites and launch vehicles.
Automotive. In Formula 1 cars, the carbon fibre monocoque forms the driver’s safety cell. Carbon ceramic brake discs are used on high performance cars. In mass production, glass fibre reinforced panels, bumper inner structures, springs and natural fibre interior trim parts are seen.
Energy. Wind turbine blades are made from glass and carbon fibre reinforced epoxy composites. Some blades are over 80 metres long, and at those sizes using metal would not be practical.
Marine. Boat and yacht hulls are produced with glass fibre reinforced polyester or vinyl ester. Freedom from rust and freedom of design are major advantages.
Sports equipment. In products such as bicycle frames, tennis rackets, golf clubs, skis, fishing rods and helmets, light weight translates directly into performance.
Medicine. Prostheses and orthopaedic supports, carbon fibre prosthetic feet, X-ray tables (carbon fibre is transparent to X-rays) and some implant applications.
Construction and infrastructure. Carbon fibre strips for strengthening reinforced concrete structures, corrosion-resistant glass fibre reinforcing bars, bridge decks and structural profile elements.
Defence. Ballistic vests and armour panels (aramid, polyethylene fibre), light vehicle armour and radar-transparent domes.
In the aerospace and automotive sectors in particular, where weight is critical, composites continue to replace metals rapidly.
The Future of Composite Materials
These are the subjects the industry is talking about most.
Recycling. Methods such as pyrolysis, solvolysis and mechanical grinding are being developed, especially for wind turbine blades and aircraft parts that have reached the end of their life. Recyclable resin systems are also being researched.
Thermoplastic matrices. These are attracting interest because they can be reshaped with heat, welded, and produced more quickly.
Bio-based composites. Natural fibres and plant-derived resins are being used in an effort to lower the carbon footprint.
Additive manufacturing (3D printing). Printing composite parts containing continuous carbon or glass fibre is becoming more widespread.
Nanocomposites. The aim here is to improve mechanical and electrical properties with nano-scale additives such as carbon nanotubes or graphene.
Conclusion
Composite materials bring together the best properties of different materials and provide a balance of light weight and strength that conventional metals and plastics cannot offer. Despite disadvantages such as high cost and the difficulty of recycling, their use continues to increase rapidly, particularly in the aerospace, automotive and energy sectors. It is therefore important for anyone working or studying in the field of materials science to have a good understanding of composites, which are seen as the material of the future.
Frequently Asked Questions
A composite material is a material obtained by combining two or more different materials at a macroscopic level, offering properties that none of the components has on its own. It generally consists of a reinforcement and a matrix.
Concrete and reinforced concrete, glass fibre reinforced polyester (boat hulls), carbon fibre reinforced epoxy (aircraft and F1 parts), wood, plywood and aramid reinforced armour panels are the best known examples.
No. Carbon fibre is a reinforcing fibre. When it is combined with resin, the resulting material is called a carbon fibre reinforced composite (CFRP). In everyday speech the two are often confused.
Per unit weight, some composites are stronger than steel. In absolute terms steel can show higher strength in many situations, but because composites are far lighter they come out ahead on strength-to-weight ratio. The strength of a composite also depends on direction.
Raw material (carbon fibre in particular), long and precise production processes, special equipment such as autoclaves, skilled labour and quality control costs all push the price up. Glass fibre composites, on the other hand, are far more economical.
They do not rust the way metals do. However, UV, moisture, heat and chemicals can affect the resin matrix over time. In addition, galvanic corrosion can occur if carbon fibre comes into contact with certain metals, which is why an insulating layer is applied between them.
It is technically possible, but harder and more costly than with metals. Methods such as pyrolysis and mechanical recovery are used. Recycling is one of the subjects the industry is working on most.
Courses on composite materials are found in mechanical engineering, materials science and engineering, metallurgical and materials engineering, aerospace engineering and polymer engineering programmes.
Yes, but the method depends on the type of damage. For small surface damage, resin and a patch are applied. For structural parts, controlled methods such as stepped sanding and patching (scarf repair) are required. In aviation these operations are carried out by certified personnel.
Related Posts
Vickers Hardness Testing Method 
The Relationship Between Laser Cutting Quality and Material Composition 
Welding of Stainless Steels 
Classification of Steels 
Cast Irons and Types of Cast Iron 
Forces Acting on the Crankshaft. Crankshaft Material 
Material Types and Their Properties (per DIN Standard) 
The Effect of Alloying Elements in Cast Iron