What is a composite material?

Composite materials are formed by merging two components that have different chemical and physical properties. The merged output is termed as a composite, whose properties are completely different from the individual components. Composite materials also have better strength than the individual components.
Composite materials usually have two main parts, reinforcement material, and matrix material. The matrix material acts as a binder and surrounds the fiber of another material that bears a much higher strength. This material is known as the reinforcement material. The matrix materials are usually high-strength additives resins, while the fibers of the reinforced material can be random, oriented, and in a mat format.
Composite materials are also known as fiber-reinforced composite polymers because they are composed of the polymer matrix which is reinforced to laboratory-created fibers or naturally occurring fibers, such as glass fibers, carbon fibers, or aramid fibers. Under the application of external loads, the matrix material absorbs and transfers the loads to the fibers, these fibers in return provide good compressive strength, tensile strength, and stiffness to the fiber-reinforced composite materials.
Depending upon the matrix and the reinforcement material, different properties can be induced to the fiber-reinforced composite materials, including good corrosion resistance properties. The most common matrix material is the resin while the glass fibers act as the reinforcement material. Strong fibers provide strength and stiffness to the composite materials while, a soft and flexible resin provides shape to the composite materials.

Composite types

The classification of composite materials generally falls under two common types, naturally occurring composite materials as well as synthetically prepared composite materials. The perfect example of natural composite material is wood. Wood is a combination of wood fibers with a matrix material known as lignin. Lignin is a natural glue of adhesive which binds the wood fibers and strengthens them. Under the composite types, the plywood is an artificial composite. Here the wood veneer is combined with a suitable adhesive to form flat sheets. It provides the plywood with more strength than the original wood material.
There are different classifications of composite materials.

Metal matrix composites

The metal matrix composite (MMCs) usually consists of two parts, in which at least one part is a metallic part which the other part may be a metal or other material. Metal matrix composites are widely used in structural engineering applications. In these composites, the matrix used is of a lighter metal like, titanium, magnesium, or aluminum, while carbon, silicon carbide, or ceramics fibers form the major constituent of the reinforcement material. Metal matrix composites are used in the automotive industries and are widely used in the aerospace industries because of their zero coefficient of thermal expansion and high specific strength. There are different advantages of using MMCs over polymer composites, MMCs can be used where long term applications are need which needs to withstand tough environmental conditions, the yield strength and modulus of MMCs are better than the polymer composites, most importantly the properties of the MMCs can be further improved by many thermal treatments like heat treatment and mechanical treatments like cold forming. Some of the other remarkable properties of the MMCs that distinguish them from the polymer composites are fire resistance, good electrical thermal conductivity, moisture resistant, better damping properties, and compressive strength, and good resistance to harmful radiation.

Ceramic matrix composites

In the ceramic matrix composites (CMC), both the binder material and the reinforcement material are ceramics. The common reinforcing fiber materials used are carbon, silicon carbide, alumina, and, alumina-silica. These fibers are usually of polycrystalline forms which can take different orientations like, short, whiskers, long and continuous, particles, and nanofibers. In common applications, the short, whiskers, and continuous fibers are used. Whiskers and short fibers induce good crack propagation resistance and toughness to the composite. While long and continuous fibers induce good strength to the composite material. The long and continuous fibers can hold the CMC together which imparts better resistance to crack propagation and strength-to-weight ratio. The matrix material used is usually boron nitride, which surrounds the fiber material and is passed through a slurry bath. The mixture then undergoes pyrolysis which converts it into a ceramic.

Polymer matrix composite

Polymer matrix composites (PMCs) are those composites whose fibers are surrounded by an organic polymer matrix. Based on the level of strength and stiffness the matrix provides, it is classified based on two categories, reinforced plastics, and advanced composites. A high degree of strength is imparted to plastics by additional fibrous materials. Advanced composites contain continuous fibers such as high stiffness glass fibers, aramid fibers, graphite fibers, and other organic fibers, they impart high stiffness strength to the material.

Wood-plastic composites

Wood-plastic composites are the composites created by blending wood materials with fibers of plastics. Materials like sawdust, pulp, bamboo, peanut hulls, and unused woodworking materials are usually combined with plastic powders to yield different qualities and plastics to the composite material. Wood-plastics composites have improved life and longevity than the wood itself.

Mechanical properties of a composite material

• High tensile strength: The tensile strength of composite material is about four to six times more than conventional material, like steel and aluminum.
• Better torsional properties: A composite material has good torsional stiffness which allows it to be widely used in the automotive industry.
• High fatigue endurance limit: The composite materials can withstand high magnitudes of cyclic loadings and induced cyclic stresses. This property of composite material allows it to be used widely for aerospace applications.
• Good vibration damping property: While under operation, the composite materials make low noise and vibrations. The fibers of the composite material absorb the maximum amplitude of vibrations and result in quieter operation.
• Light in weight: The composite materials are 30 to 45% lighter in weight than the conventional materials. Due to this, composite materials are widely used for the production of aircraft.

Some other commonly used composites

Apart from the composite materials discussed above, there are some other conventionally and widely used common composite materials.

Mud bricks

The mud bricks are generally air-dried. They are also a type of composite material. In these kinds of bricks, the matrix materials are rice husks and straw, it binds with a mixture of loam, mud, sand, and water.

Pykrete

These are composites that contain 14 to 15% sawdust or wood pulp mixed with 86% ice. It is also termed bulletproof ice. Ice is usually a brittle material, it gets fractured under the application of forces, however, the cellulose content of the wood allows it to absorb the high magnitude of forces, and can even deflect bullets.

Papier-mache

These are usually household-made materials where the pulp of the paper is generally mixed with glue and other binding materials. Sometimes different layers of paper are stacked, glued together, and pressed together. They turn hard when dried.

Context and Applications

This topic well finds its application in the following areas:

• Bachelors in Technology (Mechanical engineering)
• Bachelors in Technology (Civil engineering)
• Masters in Science (Mechanical engineering)
• Bachelors in Science (Physics)
• Bachelors in Science (Chemistry)

Practice Problems

1. Which of the following is the mechanical property of a composite material?

1. Good tensile strength
2. Good vibration damping property
3. Good corrosion resistance property
4. Both a and b

Correct option: d

Explanation: Good tensile strength and good vibration damping properties are two of the important mechanical properties of a composite material.

2. Which of the following is true for Pykrete?

1. It contains 80% ice and 14 to 15% sawdust.
2. It contains 86% ice and 20% sawdust.
3. It contains 86% ice and 14 to 15% sawdust.
4. It contains 86% ice and 14 to 15% sawdust.

Correct option: d

Explanation: Pykrete contains 86% ice combined with 14 to 15% sawdust or wood pulp.

3. Which of the following is a property of a wood-plastic composite?

1. Good moisture resistant property
2. Good self-life and longevity
3. Good tensile property
4. None of these

Correct option: b

Explanation: A wood-plastic composite is a composite material made by combining woodworking materials with plastic powders. The composite has an increased self-life and longevity than the wood itself.

4. Which of the following properties of composite materials makes it suitable for aerospace applications?

1. Lightweight
2. Good endurance and fatigue resistance
3. High tensile strength
4. All of these

Correct option: d

Explanation: Enhanced properties such as lightweight, excellent endurance limit and high tensile strength has permitted the composite material to be used for aerospace applications.

5. In which of the following classification of composite material, both the binder material and the reinforcement material are ceramics?

1. Polymer reinforced composites
2. Metal matrix composites
3. Ceramic matrix composites
4. None of these

Correct option: c

Explanation: In ceramic matrix composites, both the binder material and the reinforced material are ceramics.