Materials and Manufacturing Processes - Composite Materials

Composite Materials

Introduction

  • Composites combine materials to achieve properties difficult to obtain otherwise.
    • Improved strength-to-weight ratio
    • Improved Young’s modulus to weight ratio
    • Increased hardness and reduced wear
    • Electrically and thermally conductive polymers
    • Improved corrosion resistance
  • Composites are classified by structure:
    • Particulate composites
    • Fibre composites
    • Laminar composites

Classification of Composites

  • Matrix: The surrounding/continuous material.
  • Reinforcement: The added material intended to increase mechanical properties.
  • Particulate Composites: Particles dispersed within the matrix.
    • Example: Tungsten Carbide (WC) particles in cobalt for cutting tools.
  • Fibre Composites: Fibres of reinforcing material dispersed in the matrix.
    • Example: Glass fibre in epoxy for printed circuit boards.
  • Laminar Composites: Sheets of materials laminated together.
    • Example: Plywood.
  • Composite properties can be isotropic or anisotropic depending on the structure.

Particulate Composites

  • True Particulate Composites: Coarse particles dispersed within the matrix.
  • Used to gain combinations of material properties.
    • Cemented carbides (cermets): tungsten carbide particles in a cobalt matrix for cutting tools.
    • Grinding wheels: SiC, BN, or Al2O3 in glass or polymer matrix.
    • Concrete: stones and sand dispersed in a cement matrix.
    • Tarmac: stones dispersed in tar.

Particulate Composite Examples

  • Fillers and Extenders: Added to polymers to enhance properties and reduce the amount of polymer required.
    • Carbon black (carbon particles) in rubber for car tires.
    • Calcium carbonate, glass spheres, or clay as extenders in polymers.
  • Thermally and Electrically Conductive Adhesives: Epoxy with metal particles added.

Particulate Composite Properties

  • Many properties can be predicted using the rule of mixtures.
  • Composite density: \rhoc = \rho1f1 + \rho2f2 + \rho3f_3 + …
    • \rho_i is the density of component i.
    • f_i is the volume fraction of component i in the composite.
  • The strength and stiffness of particulate composites vary with the amount of particle added.

Fibre Reinforced Composites

  • Aim for high strength or modulus for low weight.
  • Fibres can be continuous or discontinuous (short or long) through the matrix.
  • Fibres can be aligned or randomly oriented, determining anisotropic properties.
  • Continuous fibres can be included as individual fibres, bundled together (rovings), or as woven mats.
    • Typical fibre diameter: 7 to 150 µm.
  • Density can be predicted using the rule of mixtures, but not strength or modulus.

Specific Properties of Materials

  • Specific properties are defined as:
    • Specific strength = tensile strength / density (or specific gravity).
    • Specific modulus = elastic modulus / density (or specific gravity).
      • Specific gravity = material density / density of water.
  • Specific quantities provide a method of comparing performance.
  • Steel has superior strength compared to bulk glass, but glass fibres have superior specific properties due to their lower density.
  • Glass is brittle, but using it as fibres in a composite allows exploiting specific material properties, assuming loads can be effectively transferred to the fibres.
  • As the cross-sectional area of a material decreases, the maximum tensile stress it can carry increases; hence, using small diameter fibres, enhanced properties can be achieved.

Fracture Mechanics

  • Fracture Mechanics concerns the understanding of fractures in materials (particularly brittle materials).
  • A.A. Griffith proposed the theory of flaws and defects.
    • Measured the tensile strength of glass fibres with different diameters.
    • Narrow fibres able to support a higher tensile stress (have higher tensile strength) than thick fibres.
    • The number of defects in the glass was proportional to the volume and surface area.
    • Fewer defects in the narrow fibres required a higher stress to fracture.

Fibre Reinforced Composites Examples

  • Glass fibre reinforced polymer (GFRP): fibreglass.
  • Carbon fibre reinforced polymer (CFRP): aircraft components and sports equipment.
  • Carbon nanotube and graphene reinforcement also investigated.
  • Aramid fibre reinforced polymer: aramid fibres in an epoxy or polyester matrix – bullet proof vests, car brake components.
  • Metal matrix composites: metals strengthened by the addition of ceramic material fibres e.g. borsic fibre reinforced aluminum.

Continuous and Discontinuous Fibres

  • Continuous fibres oriented in one direction – anisotropic properties.
  • Continuous fibres oriented in two directions – properties are more uniform.
  • Discontinuous fibres oriented in one direction – anisotropic properties; overall properties depend on fibre length.
  • Discontinuous fibres oriented in random directions – isotropic properties; overall properties depend on fibre length.

Fibre Orientation Effects

  • Effect of fibre orientation on the strength of composites.

Fibre Matrix Properties

  • The matrix material is usually low cost and has relatively poor specific properties.
  • The fibres are often expensive but have superior specific properties.
  • The matrix material performs a number of roles:
    • Transfers loads to the fibres.
    • Holds the fibres in place.
    • Prevents fibres buckling under compressive loads.
    • Protects fibres from corrosion and mechanical damage.
  • The matrix transmits load to the stiffer fibres.

Adhesion Between Fibre and Matrix

  • Bonding at the fibre - matrix interface has a very strong influence on performance.
  • Mechanical Bonding:
    • Interlocking of component surfaces or frictional resistance.
  • Adhesive Bonding:
    • Chemical interactions at the interface (fibres often have a coating known as a “size” added to them).
  • Reaction Bonding:
    • The components react together at the interface creating a new chemical compound.
  • Good adhesion – load carried by fibre and matrix.
  • Poor adhesion – load carried by matrix only.

Discontinuous vs Continuous Fibre Composites

  • Fibres can be continuous and extend throughout the length of the composite structure.
  • Fibres can be discontinuous, but their reinforcing properties depend on their length.
  • A critical fibre length, l_c, can be determined:
    • lc = \frac{\sigmaf d}{2 \tau_i}
      • \sigma_f is the fibre strength.
      • d is the fibre diameter.
      • \tau_i is the strength of the interface between the matrix and fibre surface.

Effect of Critical Fibre Length on Behaviour

  • Above 15l_c fibres behave as though they are continuous through the material (maximum strengthening).
  • Discontinuous fibres with length less than 15lc and greater than lc
    • Lower reinforcement efficiency than continuous fibres (50 - 90%).
    • Composite properties depend on volume fraction of fibre and length.
    • Aligned discontinuous fibres: anisotropic properties.
    • Random discontinuous fibres: isotropic properties.
  • Fibres with length less than l_c behave like particulate composites.

Stress-Strain Behaviour of Continuous Fibre Composites

  • Fibre material is usually strong, but brittle. Matrix is usually weaker, but ductile.
  • A composite stressed in the direction of the fibre orientation shows behaviour intermediate between the fibre and matrix

Fibres vs Whiskers

  • Fibres show higher strength than bulk materials due to reduced probability of surface flaws and defects (stress raisers).
  • Fibre surfaces have to be carefully treated to ensure a minimum number of defects – increase fracture strength.
  • Whiskers are single crystals - very smooth surfaces, no grain boundaries for defects to start from.
  • Whiskers are much stronger than fibres of the same material.
  • Whiskers are much more expensive than fibres.

Laminar Composites

  • Sheets of materials laminated together.
  • Can be simply to avoid anisotropic material properties e.g. plywood.
  • Or can be to provide corrosion resistance or enhanced fracture toughness; alternating layers prevent crack propagation.
  • Can be coatings on materials, claddings, or laminates.
  • Laminates: e.g., safety glass (glass sheets bonded by an adhesive layer), Arall (aramid and aluminium laminate).
  • Clad metals: e.g. Alclad - pure aluminum (corrosion resistant) bonded to aluminum alloy (strength).
  • Can also have sandwich structures e.g. corrugated cardboard or honeycomb structures for aerospace applications.

Conclusion / Key Points

  • Composites can be particulate, fibre, or laminar.
  • Composites enable properties to be achieved that would be difficult to obtain otherwise.
  • Composites enable the superior properties of small pieces of strong material to be exploited by embedding them in a matrix.
    • e.g. a higher strength or modulus for a given weight or volume.
    • The superior properties of the reinforcing material can be exploited without the negative properties e.g. brittleness.
  • The length of the fibre reinforcement is very important for determining the mechanical properties.
  • The adhesion between the matrix and fibre (or particles) is critical to achieve good properties and reliability.
  • Composite properties are often anisotropic.