Composite Materials: Types, Advantages, and Uses

Posted on Mar 24, 2025 in Materials Engineering

Chapter 1: Introduction to Composite Materials

What are Composites?

Composites are materials consisting of two or more constituents. The constituents are combined in such a way that they keep their individual physical phases and are not soluble in each other or do not form a new chemical compound.

One constituent is called the reinforcing phase, and the one in which the reinforcing phase is embedded is called the matrix. Historical or natural examples of composites are abundant: brick made of clay reinforced with straw, mud walls with bamboo shoots, concrete, concrete reinforced with steel rebar, granite consisting of quartz, mica, and feldspar, and wood (cellulose fibers in a lignin matrix).

What are Advanced Composites?

Advanced composite materials refer to those composite materials developed and used in the aerospace industries. They usually consist of high-performance fibers as reinforcing phases and polymers or metals as matrices.

Classification of Composite Materials

(Based on the geometry of the reinforcing phase)

  1. Particulate reinforced composites (PRC)
  2. Flake reinforced composite
  3. Fiber reinforced composites (FRC) (continuous fibers, short fibers, whiskers)

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Advantages of Composite Materials

  1. High specific stiffness (E/ρ) and high specific strength (σult/ρ) – leading to weight reduction, used in aerospace and sporting goods.
  2. High corrosion resistance – Acid and alkali resistance of polymers, chemical and marine applications, and infrastructure applications.
  3. High impact resistance – High internal damping of Kevlar fiber/epoxy composites, ballistics protection.
  4. High wear resistance – Ceramic particle reinforced metal matrix composites, ceramic matrix composites.
  5. Tailor-able properties – design both materials and structures. Choose the appropriate combination of reinforcements and matrices. Choose optional fiber orientation and lay-up sequences.

Disadvantages of Composite Materials

  1. Higher cost
  2. Complexity in mechanical characterization and difficulty in analysis
  3. Weak in the transverse direction and low toughness
  4. Difficulty in attaching (joining)
  5. Environmental degradation (Polymer matrix absorbs moisture)

Fiber Reinforced Polymeric Composites (FRPC)

The most common advanced composites are fiber-reinforced polymeric composites. These composites consist of a polymer matrix reinforced with high-performance, thin-diameter fibers. The reasons why they are the most common composites include low cost, high strength, and simple manufacturing principles.

  • Glass fiber is the most common fiber used in FRPCs because of its low cost, high strength, high chemical resistance, and good insulating properties.
  • Carbon/graphite fibers have very high specific stiffness, specific ultimate strength, and high fatigue strength.
  • Aramid fiber (Kevlar) is an aromatic organic compound (a kind of polymer) made of carbon, hydrogen, oxygen, and nitrogen. Its advantages are low density, high tensile strength, low cost, and high impact resistance.

Main Manufacturing Methods

Main manufacturing methods for composite components include Wet Lay-up, Dry Lay-up, Spray-up, Resin Transfer Molding (RTM), Pultrusion, Filament Winding, and Tape Winding.

Why are Fibers of a Thin Diameter?

The diameters of fibers are generally very small (from a few to a hundred microns). Using thinner fibers in composites has the following advantages:

  1. Thinner fiber has higher ultimate strength because there is less chance for inherent flaws. A similar phenomenon in metals and alloys (the strength of a thin wire can be higher than its bulk material).
  2. For the same volume of fibers, thinner fibers have a larger surface area, thus having a stronger bond with the matrix. (The total surface area of fibers is inversely proportional to the diameter of fibers)
  3. Thinner fiber has larger flexibility (∝ 1/(EI)) and therefore is able to be bent without breaking (Woven fabric performs can be made before impregnated with the polymer matrix).

Basic Terminology of Composites

  1. Isotropic material — An isotropic material has properties that are the same in all directions. The measured properties of an isotropic material are independent of the axis of testing. An anisotropic material has properties that are not the same in all directions.
  2. Homogeneous body — A homogeneous body has properties that are the same at all points in the body. An inhomogeneous (nonhomogeneous, heterogeneous) material has different properties at different points.
  3. Orthotropic material — An orthotropic material has three mutually perpendicular planes of material symmetry, i.e., for each plane of symmetry, properties in the two directions perpendicular to the plane are the same.
  4. Lamina – A lamina is a single thin ply or layer of unidirectional fibers arranged in a matrix.
  5. Laminate – A laminate is a stack of laminae. Each layer can be laid at various orientations and can be of different material systems.