Reinforcements and the Reinforcement-Matrix Interface

Chapter 2: Reinforcements and the Reinforcement-Matrix Interface

Overview of Reinforcement Types

  • Natural Fibres
  • Synthetic Fibres
    • Synthetic Organic Fibers
    • Synthetic Inorganic Fibers
  • Particulate and Whiskers Reinforcement
  • Reinforcement-Matrix Interface
    • Wettability
    • Interfacial Bonding
    • Bond-strength Measurement

Synthetic Fibres

  • Fibres manufactured for reinforcement in composites.
  • Key aspects:
    • Manufacturing
    • Structure
    • Properties
    • Applications

Manufacturing Processes and Properties

  • Fibre production involves multiple steps.
  • Variability exists even with similar fibres from the same process.
  • Microstructure can differ significantly between processes.

Performance Considerations

  • High tensile strength can decrease due to surface damage during handling/storage.
  • Fibre diameter influences strength; larger diameters typically result in larger flaws, leading to decreased strength.

Synthetic Fibre Characteristics

  • Most synthetic fibres exhibit brittle behavior with limited elastic extension before fracture.
  • Stress-strain curves illustrate performance across various fibre types.
  • Specific strength vs specific modulus plots provide comparative insight into fibre properties.

Organic Synthetic Fibres

  • Aramid and Polyethylene: Key examples of organic fibres with good strength and stiffness.
  • Aramid dominates the organic reinforcement market, with growing interest in polyethylene.
Aramid Fibres (e.g., Kevlar)
  • Aromatic organic compounds made from carbon, hydrogen, oxygen, and nitrogen.
  • Developed in the 1970s as a steel replacement in tyres.
  • Molecular structure incorporates nylon with extra benzene rings for added stiffness.
  • Stiff molecules tend to align during processing, forming a nematic liquid crystal state.
Production of Aramid Fibres
  • Filaments created by extruding a solution into a coagulation bath, improving mechanical properties through alignment in subsequent processes.

Characteristics of Aramid Fibres

  • High tensile strength and modulus.
  • Widely used in automotive, construction, and high-performance applications.
  • Advantages include excellent strength-to-weight ratio and stability over a broad temperature range.
  • Disadvantages: poor compression strength and high cost.

Polyethylene Fibres

  • Simple molecular structure, with a strong C-C bond.
  • Very low density offers superior specific properties compared to Kevlar, but limited to temperatures below 100 °C due to low melting point.
  • Surface treatments can enhance bonding in composites, impacting strength and toughness.