5. Composite Materials

Question 1

What are composite materials?

Answer 1

A composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties are realised.

Artificially made as opposed to one that occurs or forms naturally. In addition, the constituent phases must be chemically dissimilar and separated by a distinct interface. Thus most metallic alloys and many ceramics are not composites because their multi-phases are formed as a consequence of natural phenomena

A mixture of two or more discrete materials - one (reinforcement) dispersed through the volume of the other (matrix)

Materials are separate, clear interface between them

Dispersed phase usually either particles or fibres

Question 2

Example of properties composites may be designed for?

Answer 2

Improvement in

• stiffness
• toughness
• high temp strength

Question 3

Composites have two phases. What are they, define them?

Answer 3

Matrix phase, which is continuous and surrounds the other phase Dispersed phase.

The properties of composites are a function of the properties of the constituent phases, their relative amounts and the geometry of the dispersed phase.

Dispersed phase geometry means the shape of the particles, the particle size, distribution, and orientation. See diagram oh 579 16.1

Question 4

Types of composites?

Answer 4

Three main divisions. Composites that are:

1) particle- reinforced
- large particle
- dispersion strengthened

2) fiber- reinforced
- continuous( aligned)
- discontinuous (short), futher subcategorised into a) aligned b) randomly oriented

3) structural
- laminates
- sandwich panels

Question 5

Applications of particle reinforced polymers?

Answer 5

• carbon black(soot) is vital to reinforce rubber eg in car tyres
• slate dust, talc and other inexpensive materials are also widely used as fillers in polymers
• glass beads/ spheres - increase strength/ stiffness or reduce weight
• nano-composites- ie carbon nanotubes and graphene
• improve various properties and/or reduce cost
• examples: mica(insulators) glass spheres (light weight), silver particles (electrical conductivity), rubber particles (extra toughness)

Question 6

Fibre composite applications?

Answer 6

Used widely in structural applications

Normally glass, carbon or Kevlar fibres in a thermoset polymer matrix

• glass fibres: cheap and strong, but not very stiff and can degrade in moist conditions- boat hulls, covers, moulds, wind turbine blades
• carbon fibres: much more expensive but have much better specific stiffness and durability- automotive, aerospace and sporting goods
• Aramid fibres (kevlar): also expensive and have excellent specific properties- impact resistance often used in combination with glass and carbon, body armour, wing leading edges
• all available in many grades and applications

Question 7

Why use composites?

Answer 7

• To combine the good points of two materials
• to limit the size of defects in brittle materials
• to achieve unusual properties
• to allow cheap fabrications of moulded parts
• synergistic effects can give the composite better properties than either of its constituents eg a composite of two brittle materials can be very tough

Question 8

Disadvantages of composites?

Answer 8

• the mechanics of composite materials are very complex, making their stiffness, strength and fatigue life difficult to predict
• the ingredients can be very expensive
• the fabrication process are often labour intensive and time consuming
• quality control can be difficult
• all these factors often add up to very high development costs