L2: Context, Fibres and Matrices

Question 1

What are the 4 most common applications for composites and why?

Answer 1

Aerospace, marine/wind energy, sport and automotive

Require combo of lightness, stiffness and strength

Question 2

What is a ‘specific’ property?

Answer 2

Performance per unit weight for fixed volume

Question 3

List 6 key advantages of composites

Answer 3

• high specific stiffness & strength
• Anisotropy (can place fibres very specifically to needs)
• Electrical & thermal insulation
• Corrosion resistance
• Dimensional stability (in temp and humidity changes)
• Easily shaped

Question 4

List 4 key disadvantages of composites

Answer 4

• Cost of materials
• Complexity of design
• Manufacturing cost and tech requirements
• End of life/recycling

Question 5

What assumption is fracture mechanics based on?

Answer 5

All engineering materials contain cracks from which failure starts

Question 6

How does strength change with fibre diameter?

Answer 6

Increases as diameter decreases

Question 7

Give the Griffith relationship

Answer 7

strength = K(IC)/sqrt(pi*c) 
Where K(IC) is the fracture toughness of the fibre, c is the flaw size

Question 8

What are the conditions for very high fibre strength?

Answer 8

Fibres must have very small grain and minimal flaw sizes

Question 9

Why are fibres stronger than bulk material?

Answer 9

There is a lower probability of a critical flaw being present

Question 10

Why do fibres tend to have high levels of axial stiffness?

Answer 10

Molecules are highly oriented during processing -> modulus is significantly enhanced

Question 11

What do carbon fibres consist of?

Answer 11

Graphite basal planes, hexagonal strong covalent bonds between carbon atoms

Question 12

Which word describes the highly differing strength in the transverse and basal plane of graphite?

Answer 12

Anisotropy

Question 13

What is the difference between PAN and Pitch carbon fibres?

Answer 13

PAN: higher strength and failure strain
Pitch: higher modulus, negative or zero thermal expansion along fibre direction

Question 14

What is the strength of carbon fibres determined by?

Answer 14

Artefacts left in them from processing

Question 15

What is the modulus of carbon fibres associated with?

Answer 15

Level of preferred orientation of graphite crystals

Question 16

What are glass fibres based on?

Answer 16

Silica SiO2

Question 17

State whether the following are anisotropic or isotropic:

1. Carbon fibres
2. Glass fibres

Answer 17

1. Anisotropic

2. Isotropic

Question 18

What is glass fibre sizing?

Answer 18

The process of coating the fibres in a thin surface coating of mainly organic materials

Question 19

What are glass fibre sizings all based on?

Answer 19

They are all silane-based

Question 20

What is the main advantage of polymer composites?

Answer 20

Simpler and cheaper manufacturing processes/equipment compared to metal/ceramic reinforced composites

Question 21

What are the disadvantages of polymer composites? (3)

Answer 21

• Low max working temp
• High coefficient of thermal expansion
• Can be sensitive to radiation and moisture

Question 22

Describe thermoset polymers

Answer 22

• Cross-link during curing (heat, pressure and/or hardener)
• Cross-links are strong covalent bonds
• Material degrades with heat

Question 23

Describe thermoplastic polymers

Answer 23

• Linear, not cross-linked
• Weak van der Waals forces between chains
• Chains can flow under stress at high temps

Question 24

Describe thermoset matrices

Answer 24

Mix of 2 or more chemically reactive, low mol weight compounds
Initially a low viscosity liquid which must be heated to initiate polymerisation

Question 25

Describe thermoplastic matrices

Answer 25

Pre-polymerised, high mol weight. Solids which mist be heated to lower viscosity to process

Question 26

What is the main advantage of epoxy?

Answer 26

It is cured in 2 stages, so good for prepreg piles

Question 27

Give advantages of thermosets (4)

Answer 27

- Easy impreg and wet-out of fibres (low mol weight and viscosity) - Crosslinked network - Preferred for processes with continuous/long fibres - History

Question 28

Give issues with thermosets (4)

Answer 28

- Material & artefact formed simultaneously (must get it right first time) - Low fracture toughness - Long cure times - Difficult to recycle

Question 29

Give advantages of thermoplastics (5)

Answer 29

- More ductile - No chemistry involved (clean processing) - More rapid processing - Recyclable - Good hot/wet properties

Question 30

Give issues with thermoplastics (5)

Answer 30

- High melt viscosity for impreg - (Sometimes) high temp required - Control over morphology in crystallising TPs - Lower modulus - Higher creep

Question 31

What are thermoset matrices used for?

Answer 31

- High temp resistance - Chemical resistance - Low viscosity processing requirement

Question 32

What are thermoplastic matrices used for?

Answer 32

- Fast, clean processing - Short fibre for quasi-isotropic material - "green" applications

Question 33

Why is anisotropy an advantage?

Answer 33

Ability to put strong and stiff fibres in the right place, in the right orientation with the right volume fraction

Question 34

What are the important fibre parameters?

Answer 34

- Mechanical properties - Volume fraction - Orientation - Length

Question 35

What are important composite parameters excluding fibre parameters?

Answer 35

- Matrix - Interface - Processing

Question 36

Give an overview of aligned continuous fibres

Answer 36

Higher performance in fibre direction Highly anisotropic Generally slower processing with limited shapes

Question 37

Give an overview of discontinuous fibres

Answer 37

Randomly dispersed | Lower performance, more isotropic, generally faster processing with complex shapes

Question 38

What defines a good interface?

Answer 38

Intimate matrix-fibre contact (good wetting) | High stress transfer capability (good adhesion)

Question 39

What is the interface important for?

Answer 39

- the formation of the material | - determining the performance and lifetime of the part