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Chapter 1 : Introduction to Composites

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Composites are two inherently different materials that when combined together, produce a material with properties that will exceed the constituent materials.

Phases

  • Matrix (Binder)
    • Functions
      • Transfer load to reinforcement
      • Temperature resistance
      • Chemical resistance
  • Dispersed (Reinforcement)
    • Functions
      • Provides tensile properties
      • Stiffness
      • Impact resistance

Types of Bonding at Interface

  • Mechanical Bonding
  • Chemical Bonding
  • Physical Bonding

Now, the types of composites depend on the type of matrix and the type of dispersed phase. In the next few sections, I will be covering the different types of matrix and dispersed phases.

Types of Matrix

  • Polymers
    • Common Polymer Matrix Composites (PMC)
      • Thermoplastics: PE, Polyesters, (carbon fiber/Polyetheretherketone (PEEK), etc.
      • Thermoset: Polyurethane (PU), Epoxy, etc.
  • Ceramics
    • Common Ceramic Matrix Composites (CMC)
      • Carbon, Special Silicon Carbide (SiC), Alumina (Al203), and mullite (Al2013-SiO2)
  • Metals
    • Common Metal Matrix Composites (MMC)
      • Aluminum Alloys
        • Alumina / Al-Si for Diesel engine piston [lighter, abrasion resistance, and cheaper)
      • Titanium Alloys
      • Magnesium Alloys
        • SiC / Mg to further increase strength of Magnesium
      • Copper
        • Copper / Ceramic composites to increase strength of copper

Types of Dispersed Phases

There are typically 5 basic types of composite materials:

  • Fiber
  • Flake
  • Particle
  • Lamina
  • Filler

Fiber Composites

In fiber composites, the fibers reinforce along the line of their length. Reinforcement may be mainly 1-D, 2-D, or 3-D.

  • 1-D gives maximum strength in one direction.
  • 2-D gives strength in two directions
  • 3-D gives strength equally in all directions

Fiber composite strength depends on following factors:

  • Inherent fiber strength, fiber length
  • Number of flaws
  • Fiber shape
  • Bonding of fiber
  • Flaws
  • Moisture (coupling agents)

Calculating L/D ratio

Before calculating the L/D ratio, it is important to note that the cross-section can be

  • Circular
  • Square
  • Hexagonal

and the diameter ranges from 0.0001" to 0.005".

L/D ratio is important in determining whether the fiber is chopped or continuous (long). If L/D is <= 100, it is the latter. Any value more than 100, it will be the former.

Types of Fibers

  • Glass fiber
  • Aramid fiber
  • Carbon fiber
  • Others (Boron, Ceramics, PE, etc.)

Glass Fibers

The most widely-used fiber, Fiberglass properties vary somewhat according to the type of glass used. It has several well-known properties that contribute to its usefulness as a reinforcing agent:

  • Tensile strength
  • Chemical resistance
  • Moisture resistance
  • Thermal properties
  • Electrical properties

Some of its applications are piping, tanks, boat, sporting goods, etc.

Adv:

  • Low cost
  • Corrosion resistant
  • Low cost relative to other composites

Disadv:

  • Relatively low strength
  • High elongation
  • Moderate strength and weight

Types:

  • C-glass
    • Corrosion resistant
    • More expensive
    • Low strength properties
  • E-glass
    • Good electrical insulation
    • Cheap
    • Commonly-used
    • Good weathering properties
  • S-glass
    • High resistant
    • Temperature resistant
    • More expensive

Aramid Fibers

  • Uses
    • High performance replacement for glass fiber
  • Examples
    • Armor
    • Protective clothing
    • Flame-resistant clothing
    • Ropes and cables
  • Adv
    • Higher strength
    • Lighter than glass
    • More ductile than carbon

Carbon Fibers

Second most widely-used fiber

  • Examples
    • Aerospace
    • Sporting goods
  • Adv
    • High stiffness and strength
    • Low density
    • Intermediate cost
  • Properties
    • High strength
    • Conductive
    • Small diameter (5-8 microns)

Ceramic Fibers

  • Very high temperature applications (engine components)
  • Silicon carbide fiber - in whisker form
  • Ceramic matrix so temperature resistance is not compromised
  • CMC materials overcome the major disadvantages of conventional traditional ceramics, namely brittle failure and low fracture toughness

Particle Composites

Particles usually reinforce a composite equally in all directions (isotropic). Plastics, cermets and metals are examples of particles.

Particles used to strengthen a matrix do not do so in the same way as fibers. For one thing, particles are not directional like fibers. Spread randomly throughout a matrix, particles tend to reinforce in all directions; equally.

Flake Composites

Flakes, because of their shape, usually reinforce in 2-D. A flake composite consists of thin, flat flakes held together by a binder or placed in a matrix. Almost all flake composite matrixes are plastic resins. The most important flake materials are:

  • Aluminum
  • Mica
  • Glass

Laminar Composites

Laminar composites involve 2 or more layers of the same or different materials. The layers can be arranged in different directions to give stregth where needed. One application will be speedboat hulls.

Filled Composites

There are 2 types of filled composites:

  1. Filler materials are added to a normal composite resulting in the strengthening of the composite and reduction in weight.
  2. A skeletal 3-D matrix holding a second material. The most widely-used composites of this kind are sandwich strctures and honeycombs.

This post is an adaption of the Ngee Ann Polytechnic Composite Materials notes