What is the dispersed (reinforcing) phase in a composite and what are its functions?

In one line

The dispersed phase is the discontinuous phase (fibres, particles, flakes) embedded in the matrix that mainly carries the load and improves strength, stiffness, hardness or other specific properties of the composite.

Explained simply

If the matrix is the body, the dispersed phase is the "skeleton" or "muscle" that gives strength. Steel bars in concrete, glass fibres in fibre-glass, gravel in concrete and carbon black particles in tyres are all dispersed phases. They are usually much stronger and stiffer than the matrix.

How it works

Forms of dispersed phase:

• Fibres: long thin, high length-to-diameter ratio; most effective for strength (glass, carbon, aramid/Kevlar, boron, natural fibres like jute).
• Particles: roughly equal in all dimensions (WC, SiC, Al₂O₃, gravel, carbon black).
• Flakes: flat platelets (mica, glass flakes).
• Whiskers: very thin single crystals, almost defect-free, very strong.

Functions:

1. Bears the major share of the applied load (in fibre composites) and increases tensile strength and stiffness (modulus).
2. Increases hardness and wear resistance (ceramic particles in metal matrix).
3. Improves toughness and impact resistance by stopping or deflecting cracks (fibres bridge cracks).
4. Improves creep and fatigue resistance and dimensional stability; reduces thermal expansion.
5. Raises the service temperature or improves thermal/electrical conductivity (e.g., metal or carbon fillers).
6. In some cases reduces cost (fillers like calcium carbonate in plastics).

Factors controlling its effect: concentration (volume fraction), size, shape, distribution (uniform is better), orientation (aligned fibres give directional strength), and interface bonding with the matrix.

Key formula

For continuous aligned fibres loaded along their length (iso-strain condition):

\( E_c = E_m V_m + E_f V_f \)

• \( E_c, E_m, E_f \) = elastic moduli of composite, matrix and fibre (GPa)
• \( V_m, V_f \) = volume fractions

Quick example

Epoxy (E = 3 GPa, 40%) with aligned glass fibres (E = 70 GPa, 60%): E_c = 3 × 0.4 + 70 × 0.6 = 1.2 + 42 = 43.2 GPa, more than 14 times stiffer than epoxy alone along the fibre direction.

Common mistakes

• Applying the rule of mixtures for modulus in the transverse direction; it holds only for loading along aligned continuous fibres.
• Saying more reinforcement is always better; beyond a limit the matrix cannot wet and bind all fibres.
• Calling the dispersed phase "matrix" (mixing up names).

How MU asks it

"Explain the constituents of composites: matrix phase and dispersed phase, with their functions" (5–6 marks). For full marks: define both phases, give forms of dispersed phase with examples, 4–5 functions, and factors like orientation and volume fraction.