Composites

a combination of materials that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is achieved

what is a composite

• the dispersed component

• the matrix

what are the two main components of composite materials

• this is usually strong, stiff particles (particle reinforced composites), or fibres (fibre reinforced composite)

• functions of the dispersed component:

  • improve strength

  • improve stiffness

  • improve creep resistance

dispersed component

• a continuous phase which surrounds the other component in the composite, the dispersed phase

• typically a polymer or metal

• functions of the matrix:

  • transmit stress to dispersed component

  • protects dispersed component from corrosion and abrasion

  • provide toughness 

    • good for ductility

    • prevents brittle crack propagation

the matrix

• consider two materials A and B with different elastic moduli E_A and E_B respectively and used to form a simple laminate structure

• can be done in two ways:

  • isostress:

    • stress applied to composite = stress applied to each component

    • applied perpendicular to material

  • isostrain

    • strain applied to composite = strain applied to each component

    • applied parallel to material

simple composites

• for oriented continuous fibre composites

• modulus of composite:

  • V = Volume fraction

  • E = Young’s Modulus

  • V_A + V_B = 1

  1. isostrain stiffness (in direction of fibres) → E_C - V_AE_A + V_BE_B = V_AE_A + (1 - V_A)E_B

  2. isostress stiffness (perpendicular to fibres) → 1/E_C = V_A/E_A + V_B/E_B = V_A/E_A + (1-V_A)/E_B

• density of composite:

  • ρ_c = V_Aρ_A+ V_Bρ_{B =} V_Aρ_A + (1 - V_A)ρ_B

• specific modulus of composite:

  • indication of stiffness relative to weight

  • Ê = E_c/ρ_c

rule of mixtures