Composite Materials: Types and Classifications

• Composite materials are a combination of 2+ distinct materials achieving improved properties, and can be tailored for specific applications.

Examples of Composite Materials

• Concrete reinforced with steel consists of gravel, cement, and steel.
• Standard concrete comprises gravel and cement.
• Wood is made of lignin and cellulose
• Asphalt concrete is binder and aggregate.

Types of Composites

• Composites can be natural, derived from animals and plants or synthetic, which are man-made.
• Natural composites include wood (cellulose fibers + lignin) and bone (hydroxyapatite + collagen).
• Synthetic composites include fiberglass (plastic + glass), polyester (coal+air+water+petroleum), and plywood.
• Composites are used for their high strength, light weight, and toughness.

Classification of Composite Materials

• Individual materials that form a composite are called constituent materials.
• The primary phase, or matrix, is the continuous phase in which the secondary phase is embedded.
• The secondary phase, or reinforcement, is the discontinuous phase embedded in the matrix, serving to strengthen the composite material.
• Reinforcement adds mechanical properties like strength and stiffness to the composite material.
• The matrix binds the reinforcement together, distributes applied stress among the reinforcement, and protects the reinforcement from damage.
• The matrix holds the reinforcement in place.

Matrix Types

• Matrix can be classified into three types: Metal Matrix Composite (MMC), Ceramic Matrix Composite (CMC) and Polymer Matrix Composite (PMC).

Polymer Matrix Composites (PMCs)

• PMCs consist of organic polymers combined with a fibrous reinforcement.
• They are lightweight with high stiffness and strength.
• Thermosets, a type of PMC, e.g., epoxy and vulcanized rubber.
  • Thermosets are stronger than thermoplastics due to their network transformation.
  • Thermosets cannot be melted and reshaped after being cured.
• Thermoplastics, also a kind of PMC, e.g., nylon and polyethylene.
  • Thermoplastics can shaped when heated
  • Thermoplastics do not show chemical property changes when heated or cooled multiple times.

Metal Matrix Composites (MMCs)

• MMCs consist of low-density metals like aluminum, magnesium, copper, or nickel, reinforced with ceramic materials such as graphite.
• MMCs have properties such as high strength regardless of temperature, high stiffness, low density, and high thermal conductivity.
• MMCs are relatively expensive.
• Applications include fan blades in engines, brake linings, and bicycle frames.

Ceramic Matrix Composites (CMCs)

• CMCs consist of ceramic fibers embedded in a ceramic matrix, forming a ceramic fiber reinforced material.
• CMCs have high melting points, good compressive strength, good corrosion resistance, stability at high temperatures, and excellent mechanical strength.
• Conventional ceramics, such as alumina, aluminum nitride, and silicon nitride are used in CMCs.
• Conventional ceramics fracture easily and crack.
• CMCs are applied aerospace industry (gas turbines) and energy sector (heat exchangers).

Reinforcement Types:

• Types of reinforcement in composites are particles and/or fibers.

Fiber Reinforcement

• Reinforcement materials are fibers that provide composite strength.
• Fiber reinforcement consists of cells glued together by a matrix, commonly epoxy or polyester
• The types of fibers include glass fibers (fiberglass) or advanced fibers (carbon, boron, spectra fiber).
• Glass fibers are the most widely used.
  • Made from either bottle glass fibers or high purity quartz glass.
  • They have good tensile strength, moisture resistance, and thermal properties.
  • They are low cost relative to other composites with moderate strength and weight.
• Advanced fibers, such as Boron, have high costs and high stiffness.
• Spectra fiber is high strength and extremely light weight.
• Carbon, the 2nd most widely used has low density, high stiffness, and strength.

Fiber Geometry and Orientation

• Fiber orientation is a major factor influencing composite strength.
• Aligned fibers, offer best mechanical properties that can be continuous or discontinuous.
  • Continuous aligned fibers uses long fibers enabling load transfer from the matrix to the fibers provide most effective strengthening for fiber composites.
  • Discontinuous aligned fibers uses shorter fibers, is less effective in strengthening, cheaper, with high in tensile strength and are easy to fabricate.
• Random fibers or chopped fibers, have lower strength than aligned fibers, but the material is cheaper.
• Woven fibers are woven into fabric and layered with a matrix material.

Particle Reinforcement

• Particle reinforcement consists of particles from one material dispersed in a matrix of a second material.
• These particles aren't directional and spread randomly throughout the matrix, reinforcing in all directions equally.
• It's added to a liquid matrix that later solidifies.
• Typical uses are situations where high levels of wear resistance are required, such as road surfaces.
• Advantages of particle reinforcements are low cost, high stiffness and strength, wear resistance and a simple manufacturing process.

Reasons for Using Composite Materials

• Some of the reasons to use composites are light weight, corrosion resistance, durability, low cost, heat resistance, design flexibility, and electrical insulation.

Areas for composite application

• In aerospace, strength and light weight capabilities are very favorable.
• In transportation, composites make the vehicles lighter and more fuel efficient, and is a key requirement for applications such as panels and frames.
• Composites in civil infrastructure are useful to construct and repair roads, buildings, and bridges.
• In construction, high strength, light weight and resistance to corrosion properties, make composites favorable for wall panels, doors, and fixtures.
• The uses of composites in marine applications stem from corrosion resistance and light-weighing attributes.