Composites properties

Introduction to Composites

Key Concepts

Matrix and Reinforcement

  • Matrix:

    • Continuous phase that binds the reinforcement materials.

    • Typically softer, weaker, and more ductile than reinforcement, which helps in absorbing stress and distributing loads across the structure.

    • Commonly made from polymers, metals, or ceramics.

  • Reinforcement:

    • Discontinuous phase that adds strength, stiffness, and toughness to the composite material.

    • Fibres:

      • Can be continuous or discontinuous.

      • Continuous fibres (aligned): Provide superior properties such as high strength-to-weight ratios and improved load distribution. Examples include carbon fibres and glass fibres.

      • Discontinuous fibres (aligned, random): Facilitate easier manufacturing processes but generally yield lower performance compared to continuous fibres.

    • Particles:

      • Can be used for reinforcement through dispersion strengthening mechanisms.

      • Particle size and distribution greatly influence mechanical properties; smaller particles can enhance toughness and resistance to crack propagation.

      • Examples include silica, alumina, and various metal particles which enhance wear resistance and thermal stability.

Composite Properties

  • Influenced by:

    • Properties of matrix and reinforcement

    • Percentage reinforcement

    • Shape and orientation of reinforcement

    • Interfacial bond strength

Classification of Composites

  • Based on particles:

    • Large particle

    • Dispersion strengthening: Use of fine particles to prevent dislocation movement and enhance strength.

  • Based on fibres:

    • Continuous (aligned): Superior mechanical properties and structural integrity.

    • Discontinuous (aligned, random): Easier processing but variable properties based on orientation.

  • Based on structural design:

    • Laminates

    • Sandwich panels

    • Foams (voids)

Applications and Examples

  • Particle Reinforced:

    • Generally less effective than fibre reinforcement in terms of strength enhancement but beneficial for specific applications like turbine blades for improved wear resistance due to hard surface properties.

    • Important in construction materials, including concrete, which combines gravel, sand, cement, and water as particulate components.

Mechanical Properties

  • Impact of fibre length and alignment on stress distribution and load transfer effectiveness.

  • Critical fibre length is essential for ensuring effective stress transfer and composite integrity, particularly under tensile loads.

  • Aspect ratio (length to diameter) significantly affects composite properties; higher aspect ratios generally improve performance.

Rule of Mixtures

  • Predicts properties like density, strength, and elastic modulus based on constituent properties Identifies how varying the volume fractions of fibres and particles affects overall composite behavior.

Limitations of Models

  • Basic models may overlook:

    • Local geometry

    • Component arrangement

    • Bonding imperfections which can lead to misestimation of mechanical properties.

Calculations

  • Young’s modulus calculation examples demonstrate the variations depending on reinforcement type and proportion.

  • Elasticity estimation in composites requires knowledge of individual constituent properties for accurate design.

  • Design tasks for targeted composite properties through volume fraction validation ensure optimal performance for specified applications.