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CDM > D2: Constituents > Flashcards

D2: Constituents Flashcards

1
Q

Reinforcement types? What do we use in aerospace?

A
  • Aligned or random orientation.
  • Short, long, or continuous.
  • Aerospace use aligned, continuous fibres.
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2
Q

What type of bonding is present in carbon fibres?

A
  • Strong covalent bonds between molecules in fibre direction, forming a graphite crystalline structure.
  • Weak electrostatic bonds transverse to fibre direction, between the crystal planes.
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3
Q

What causes the difference between HM and HS carbon fibres?

A
  • Alignment of the crystals with the fibre, HM are more aligned resulting in a stiffer fibre.
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4
Q

What processes are used in the manufacture of carbon fibres?

A
  • Elements other than carbon are removed though graphitisation (or pyrolysis), where the fibres are heated to 2000 deg in an inert atmosphere.
  • Molecular alignment is achieved using tension or spinning.
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5
Q

Up to what temperature are carbon fibres thermally stable?

A

2000 deg in the absence of oxidising agents.

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6
Q

What are the advantages and disadvantages of kevlar fibres?

A
  • High strength and high modulus.
  • Bad in compression due to fibrillation (collapse of the fibrils that make up a fibre).
  • Low operating temperature, up to 250 deg.
  • Susceptible to moisture ingress.
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7
Q

What process is used to create kevlar fibres?

A

Solvent spinning.

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8
Q

What structure do glass fibres have? What are the material properties like?

A
  • Amorphous structure.
  • High strength, moderate stiffness.
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9
Q

How are glass fibres made?

A

Drawing molten glass. Coated with a sizing agent (typically silanes) immediately afterwards, to improve handling and promote bonding with the matrix material.

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10
Q

What temperature are glass fibres thermally stable to?

A

850 deg, but modulus degrades after about 250 deg depending on the composition.

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11
Q

What are the 3 groups of glass fibre and what are their benefits?

A
  • E and R glass fibres are cheap with reasonable properties.
  • S glass fibres have better properties for increased cost.
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12
Q

What are the stress strain curves of the different fibre materials like?

A
  • Practically linear-elastic up to failure, exhibiting virtually no plastic deformation. i.e they’re brittle.
  • Kevlar fibres tend to exhibit some ductile necking, increasing their toughness.
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13
Q

Why are hybrids used? What are the 2 types?

A
  • To negate the drawbacks of a certain fibre material with the benefits of a different one.
  • Intermingled and layered.
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14
Q

What fibre material should be used for high specific strength and stiffness?

A

Carbon

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15
Q

What fibre material should be used for high impact resistance?

A

Kevlar or kevlar hybrids

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16
Q

What fibre material should be used for non-critical components, or lower cost?

A

Glass

17
Q

What fibre material should be used for microwave/electrical components, and why?

A

Glass because they are transparent to EM radiation.

18
Q

What are the 3 main fibre materials, and what are their typical strains-to-failure?

A
  • Carbon, 1-1.5%
  • Kevlar, 2.5%
  • Glass, 2%
19
Q

What are the 4 typical matrix materials, and what temperature ranges can they operate at?

A
  • Epoxies, up to 150 deg
  • Phenolics and polyimides, up to 300 deg
  • Metals, up to 400 deg
  • Ceramics, higher than 1000 deg
20
Q

What is different about TS and TP’s molecule structures? What does this result in?

A
  • TS molecules are cross linked, whereas TP molecules are held together with weak Van der Vaal forces and entanglement.
  • Means that TSs can’t be softened once they have hardened, but TPs just need enough heat to allow molecules to slide past each other.
21
Q

What is needed to harden a TS?

A

A catalyst, heat (125-175 deg), and pressure (0.75-1.5MPa).

22
Q

What 3 things limit TS performance?

A
  • Temperature
  • Toughness (brittle)
  • Moisture - they can absorb up to 2% of their weight in water, which tends to plasticise the material (soften and degrade). Worsened in higher temperatures.
23
Q

What is the TP molecular structure called? What are the implications and benefits?

A
  • Semi-crystalline.
  • Cooling rates must be within a certain range to achieve required crystallinity.
  • Able to melt them, meaning they can be healed in service, and scraps can be reused.
  • Infinite shelf life.
24
Q

What are the disadvantages of TPs?

A
  • High temperatures required for processing (300-400 deg). More energy, and ancillary materials capable of withstanding the temps required (more expensive, and often worse handling quality than their normal temp counterparts).
  • High pressures (2-3 MPa) required to overcome high viscosity in curing process.
25
Q

What are the advantages of TPs?

A
  • While they tend to have worse material properties compared to TS, they are often inherently tougher (high impact resistance).
  • Do not suffer from moisture absorption (less than 0.2%, with no adverse effects).
26
Q

What temperature is used as the operating temp for TPs? Why?

A

Glass transition temperature, because material becomes rubbery above this.

27
Q

How does PEEK (TP) compare with TSs? (6 things)

A
  • As good general mechanical properties
  • As good temperature performance
  • Better toughness
  • Better at dealing with moisture
  • Better in space applications (because of outgassing and radiation)
  • Better cryogenic performance (micro cracking at low temperatures)
28
Q

What are the limiting factors of TPs?

A
  • Difficult to process (high temps and pressures)
  • Expensive material and ancillaries
  • Difficult to bond due to low surface energy
29
Q

What matrix materials are best for: 1) general mechanical performance, 2) high temperature environments, and 3) toughness, chemical resistance, or fire resistance?

A

1) Epoxy
2) Phenolic/polyimide
3) Toughened epoxy

30
Q

Why are sizing agents used?

A
  • To protect fibre surfaces
  • To aid wetting by increasing fibre surface energy
  • To act as a coupling agent for chemical bonding
  • Bond strength control
31
Q

What are the 3 possibilities when a fibre cracks? Which is desirable and why? How can we achieve this?

A
  • Brittle matrix cracking, ductile matrix yielding, and interface disbonding.
  • Interface disbonding is desirable because it effectively blunts crack tips, allowing the fibres to act as crack stoppers.
  • Careful selection of interface strength.
32
Q

What are the 4 components of composite fracture energy, from largest to smallest?

A
  • Fibre pull out energy
  • Fibre disbond energy
  • Matrix fracture energy
  • Fibre fracture energy
33
Q

What are the 3 types of sandwich core?

A

Honeycomb, foam, and syntactic

34
Q

What core should be used for:
1) High strength
2) Corrosion resistance
3) lightweight/closed cell/moderate loading
4) thin sandwich

A

1) Honeycomb (metal or nomex)
2) Nomex honeycomb
3) Foam
4) Syntactic

35
Q

What adhesives are generally used?

A
  • Epoxies with higher ductility than the matrix, to cope with shear and peeling stress concentrations present at the ends of bonded joints.
  • Polyimides, phenolids, and urethanes are also used.
36
Q

What are the forms of adhesives, with examples of their usage?

A
  • Films - sandwich face skins
  • Paste - joints
  • Potting compounds - fasteners/fittings in sandwich cores
  • Foaming adhesives - sandwich core splicing/edge filling

CDM (10 decks)

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