Composite Materials

Question 1

What are Particulate composites?

Answer 1

Coarse particles of material dispersed within a matrix

Question 2

What are Fibre composites?

Answer 2

Consist of fibres of the reinforcing material dispersed in the matrix

Question 3

What are Laminar composites?

Answer 3

Composites based on sheets of materials laminated together

can be to provide corrosion resistance, or enhanced fracture toughness alternating layers prevent crack propogation

Question 4

What are Isotropic composites?

Answer 4

same properties in each direction

Question 5

What is the equation for Specific strength?

Answer 5

Tensile stress / density

Question 6

What is the equation for specific modulus?

Answer 6

Elastic modulus / density

Question 7

What are Merit indices?

Answer 7

Describes specific properties

Question 8

Particle composite examples

Answer 8

Concrete
Cemented carbides (cermets)
Tarmac

Question 9

What are Fillers and extenders?

Answer 9

Added to polymers as particles to enhance properties.

e.g. Adhesive glue, carbon tires and clay.

Question 10

What is the ‘Rule of mixtures’?

Answer 10

Helps predict particulate composite properties.

Composite density = ρ1f1 + ρ2f2 + ρ3f3……

Question 11

What are the Properties of fibre reinforced composites?

Answer 11

Usually aim for high strength to weight ratio.
Fibres can be continuous or discontinuous (short or long) through the matrix.
Fibres can be aligned or randomly orientated -determines if properties are anisotropic.

Question 12

What are Continuous fibres?

Answer 12

Can be included in the matrix either as individual fibres, as many fibres bundled together (rovings) or as woven mats.
Typical fibre diameter 7 to 150 µ
Normally give anisotropic properties

Question 13

Fibre reinforce composite examples

Answer 13

Fibre glass, carbon fibre

Question 14

What are the properties of the Matrix?

Answer 14

Usually low cost and relatively poor specific properties
Transfers loads to fibres
Prevents fibres buckling under compressive loads
Protects fibres from corrosion
Protects fibres from mechanical damage

Question 15

What are the types of bonding between fibre and matrix?

Answer 15

Mechanical Bonding – Interlocking of component surfaces or frictional resistance
Adhesive Bonding – Chemical reactions at the interface
Reaction Bonding – the components react together at the interface creating a new chemical compound -this extra component makes the composite properties very complex

Question 16

What is meant by the critical fibre length?

Answer 16

l = σd/2τ
σ is the fibre strength, d is the fibre diameter, τ is the strength of the interface between the matrix and fibre surface
Above 15l fibres behave as though they are continuous
Fibres with length less than l behave like particles composites

Question 17

What are the pros/cons of Fibres?

Answer 17

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

Question 18

What are the pros/cons of Whiskers?

Answer 18

Whiskers are single crystals -very smooth surfaces with 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