Composites

Multiphase material made from 2 or more distinct constituent materials. Designed to combine properties that no single material provides.

Each composite has 2 phases:

• Matrix
  Continuous phase. Holds the reinforcement in place and transfers load.
• Reinforcement
  Dispersed phase. Provides the primary strength or stiffness.

Classification

• Particle-reinforced
  Matrix with dispersed particles (e.g. concrete: cement matrix + aggregate particles).
• Fiber-reinforced
  Matrix with embedded fibers (e.g. GFRP, CFRP). Fibers carry most of the load.
• Structural
  Layers of different materials bonded together (e.g. plywood, honeycomb panels).

Rule of Mixtures

Longitudinal elastic modulus of a fiber-reinforced composite:

Here:

• $V_m$, $V_f$: volume fractions of matrix and fiber
• $E_m$, $E_f$: elastic moduli of matrix and fiber

Examples

• Concrete
  Cement matrix + aggregate. High compressive strength.
• GFRP (glass fiber reinforced polymer)
  Glass fiber + polymer matrix. Not brittle.
• CFRP (carbon fiber reinforced polymer)
  Carbon fiber + epoxy. Very high strength-to-weight ratio.