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Ma 16565: Engineering Composites > L2: Context, Fibres and Matrices > Flashcards

L2: Context, Fibres and Matrices Flashcards

1
Q

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

A

Aerospace, marine/wind energy, sport and automotive

Require combo of lightness, stiffness and strength

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2
Q

What is a ‘specific’ property?

A

Performance per unit weight for fixed volume

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3
Q

List 6 key advantages of composites

A
  • 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
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4
Q

List 4 key disadvantages of composites

A
  • Cost of materials
  • Complexity of design
  • Manufacturing cost and tech requirements
  • End of life/recycling
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5
Q

What assumption is fracture mechanics based on?

A

All engineering materials contain cracks from which failure starts

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6
Q

How does strength change with fibre diameter?

A

Increases as diameter decreases

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7
Q

Give the Griffith relationship

A 
strength = K(IC)/sqrt(pi*c) 
Where K(IC) is the fracture toughness of the fibre, c is the flaw size
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8
Q

What are the conditions for very high fibre strength?

A

Fibres must have very small grain and minimal flaw sizes

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9
Q

Why are fibres stronger than bulk material?

A

There is a lower probability of a critical flaw being present

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10
Q

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

A

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

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11
Q

What do carbon fibres consist of?

A

Graphite basal planes, hexagonal strong covalent bonds between carbon atoms

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12
Q

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

A

Anisotropy

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13
Q

What is the difference between PAN and Pitch carbon fibres?

A

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

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14
Q

What is the strength of carbon fibres determined by?

A

Artefacts left in them from processing

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15
Q

What is the modulus of carbon fibres associated with?

A

Level of preferred orientation of graphite crystals

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16
Q

What are glass fibres based on?

A

Silica SiO2

17
Q

State whether the following are anisotropic or isotropic:

  1. Carbon fibres
  2. Glass fibres
A
  1. Anisotropic

2. Isotropic

18
Q

What is glass fibre sizing?

A

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

19
Q

What are glass fibre sizings all based on?

A

They are all silane-based

20
Q

What is the main advantage of polymer composites?

A

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

21
Q

What are the disadvantages of polymer composites? (3)

A
  • Low max working temp
  • High coefficient of thermal expansion
  • Can be sensitive to radiation and moisture
22
Q

Describe thermoset polymers

A
  • Cross-link during curing (heat, pressure and/or hardener)
  • Cross-links are strong covalent bonds
  • Material degrades with heat
23
Q

Describe thermoplastic polymers

A
  • Linear, not cross-linked
  • Weak van der Waals forces between chains
  • Chains can flow under stress at high temps
24
Q

Describe thermoset matrices

A

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

25
Q

Describe thermoplastic matrices

A

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

26
Q

What is the main advantage of epoxy?

A

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

27
Q

Give advantages of thermosets (4)

A
  • Easy impreg and wet-out of fibres (low mol weight and viscosity)
  • Crosslinked network
  • Preferred for processes with continuous/long fibres
  • History
28
Q

Give issues with thermosets (4)

A
  • Material & artefact formed simultaneously (must get it right first time)
  • Low fracture toughness
  • Long cure times
  • Difficult to recycle
29
Q

Give advantages of thermoplastics (5)

A
  • More ductile
  • No chemistry involved (clean processing)
  • More rapid processing
  • Recyclable
  • Good hot/wet properties
30
Q

Give issues with thermoplastics (5)

A
  • High melt viscosity for impreg
  • (Sometimes) high temp required
  • Control over morphology in crystallising TPs
  • Lower modulus
  • Higher creep
31
Q

What are thermoset matrices used for?

A
  • High temp resistance
  • Chemical resistance
  • Low viscosity processing requirement
32
Q

What are thermoplastic matrices used for?

A
  • Fast, clean processing
  • Short fibre for quasi-isotropic material
  • “green” applications
33
Q

Why is anisotropy an advantage?

A

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

34
Q

What are the important fibre parameters?

A
  • Mechanical properties
  • Volume fraction
  • Orientation
  • Length
35
Q

What are important composite parameters excluding fibre parameters?

A
  • Matrix
  • Interface
  • Processing
36
Q

Give an overview of aligned continuous fibres

A

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

37
Q

Give an overview of discontinuous fibres

A

Randomly dispersed

Lower performance, more isotropic, generally faster processing with complex shapes

38
Q

What defines a good interface?

A

Intimate matrix-fibre contact (good wetting)

High stress transfer capability (good adhesion)

39
Q

What is the interface important for?

A
  • the formation of the material

- determining the performance and lifetime of the part

Ma 16565: Engineering Composites (2 decks)

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