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MAT2730 - Non-Metals > Fibres > Flashcards

Fibres Flashcards

1
Q

Why are fibres important

A
  • statistically fibres have higher strength to weight ratio
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2
Q

What does Weibul statistics show

A
  • Smaller materials have statistically less flaws

i. e. smaller volume = stronger material

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3
Q

List key fibres

A

Glass
Carbon
Polymeric

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4
Q

Describe process of making glass fibres

A
  1. melt all constituents
  2. refine and complete homogenisation
  3. forehearth - conditions the glass for drawing and ensures that the bushings are fed with molten glass
  4. cooling spray on fibres as they are drawn
  5. fibres drawn onto spools
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5
Q

Describe process of making carbon fibres

A
  1. PAN/pitch added
  2. Under tension: fibres stabilised
  3. carbonisation
  4. graphitisation
  5. Not under tension: electrolytic bath
  6. wash
  7. sizing
  8. drier
  9. spool
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6
Q

Why is sizing important

A

A coating is put on the fibres which:

  • lubricates
  • compatibilists it with resin
  • adhesion (sticking) promotion
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7
Q

List three subdivisions of polymers

A
  • thermoplastics
  • thermosets
  • elastomers
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8
Q

Property differences between thermoplastics and thermosets

A

Thermoplastics:

  • can flow past each other easily
  • can be recycled

Thermosets:

  • much harder to recycle
  • sets after heating
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9
Q

Define amorphous

A

Have random arrangements

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10
Q

Define crystalline

A

ordered arrangement

These material tend to be opaque

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11
Q

What is special about the glass transition temperature (Tg) of polymers

A

Below glass transition temp:
polymer is glassy in nature/brittle

Above glass transition temp:
chains can move in relation to each other, can deform easily
Bonds will want to coil up as energetically more favourable

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12
Q

What prevent polymer chain motion

A

chain branches and vinyl groups

always some amorphous nature in polymers

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13
Q

What are elastomers

A

Thermoplastics or Thermosets
flexible backbone chains that uncoil and recoil with applied load.
Range greatly

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14
Q

List some uses of thermoplastics

A
  • wire insulation
  • composite materials
  • windows + canopy
  • polymer films
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15
Q

List some uses of thermosets

A
  • composites (epoxy)

- interiors

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16
Q

List some uses of elastomers

A
  • seals/damping
  • tyres
  • seat foams
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17
Q

What is the longitudinal rule of mixtures and what does it mean

A

Ec = EfVf + EmVm

Ef/m = fibre/matrix mod.
Vf/m = f/m volume fraction
Means that with correct proportions and direction, then properties are the same

18
Q

What is the transverse rule of mixtures and what does it mean

A

Ec = 1/ (Vf/Ef + (1-Vf)/Em)

Ef/m = fibre/matrix mod.
Vf/m = f/m volume fraction
Means that fibres almost act as defects and will initiate failure

19
Q

Describe the wet lay-up process for composites

A
  1. Lay fibres into mould (often strand mat)
  2. Mix resin and curing agent
  3. Apply resin to fibres and work in
  4. Place in vacuum
  5. Leave to cure (often in ambient temp.)
20
Q

What sort of product can be made using wet lay up

A

Complex shapes but with low performances

21
Q

Describe the dough/sheet/bulk process and its uses

A
  • short fibre reinforcement
  • low volume fraction
  • very high viscosity
  • curing agent already incorporated
  • easy to handle

uses:

  • interior panels
  • seat bases
  • electrical housing (material doesn’t burn)
22
Q

Describe RTM and VARTM process for composites

A
  1. Lay dry fibre into mould (1 part RTM, 2 parts VARTM)
  2. Cover with necessary consumables
  3. Close mould (RTM), cover with vaccum bag
  4. Pump (RTM), Drawn in (VARTM) low viscosity resin
  5. Remove pressure and cure
23
Q

What is highest volume fraction achievable using RTM/VARTM process

A

< 50%

24
Q

Considerations for RTM

A
  • Resin viscosity:
    lowest possible although this shrinks
    injection pressure minimises voids
  • Mould:
    2 part rigid (metal)
    closed mould so less exposure to resin
25
Q

Considerations for VARTM and uses

A
  • Infusion considerations:
    resin viscosity
    preform must be porous enough to allow resin flow
    slow infusion minimises risk of voids
  • Mould
    1 side meaning only 1 good surface finish

uses: nose cone on aerospace, not for structural parts

26
Q

Describe Pultrusion and its uses

A
  1. Continuous process
  2. Assemble bundles of fibres
  3. Draw into die and inject resin
  4. Heated to cure
  5. Draw out and cut to length
  6. 50% max volume fraction

uses:
floor beams
stringers and spars

27
Q

Describe Filament Winding and its uses

A
  1. Fibre tow impregnated with liquid resin
  2. Excess resin squeezed out from tows that are under tension
  3. Wetted tow/filaments wound onto mandrel
  4. Vary winding direction + angle by alerting speed of rotation of mandrel and of the fibre delivery head
  5. Cure part

uses:
making tubes
rail way tanks

28
Q

Describe the Braiding process and its uses

A
  • the process of producing a preform for RTM
  • complex shape production
  • very high equipment cost
  • a 3D complex weave
  • combination of fibre types
29
Q

What is the advantage in using RTM/VARTM/P/FW/B

A

All quick processing techniques using liquid resin
Create higher performance parts than Wet lay-up
Increasingly used in aerospace applications

30
Q

Name 5 intermediate performance processes for composites

A 
RTM
VARTM
Pultrusion
Filament Winding
Braiding
31
Q

Name 3 high performance processes for composites

A

Resin film infusion
Pre-preg
Cure processes

32
Q

Describe Resin film infusion

A
  • Dry reinforcement (woven fabric)
    1. stack reinforcements in to a mould
    2. interleave layers of the fabric with a resin film
    3. Heat and apply pressure/vacuum to make the resin flow into the fibre stack

(similar to VARTM but can monitor resin more precisely - makes sure resin doesn’t pool, also uses heated not liquid resin)

33
Q

What is pre-preg

A
  • continuous uni-directional fibre, or fabric
  • impregnated with resin in the factory
  • controlled thickness & volume fraction
  • resin partially cured and then frozen
  • defrost immediately before use (has sell by date)
  • lay-up on mould
  • cure
34
Q

List the problems seen when laying up pre-preg and the causes

A
  • delamination
  • incomplete cure
    cause:
    shelf life
    condensation
    cleanliness
35
Q

For high performance composites, list 4 other ways than autoclave, to cure

A
  • vacuum bag
  • shrink wrap (Filament Winding only)
  • Quickstep
  • microwave curing
36
Q

What are the advantages and disadvantages of microwave curing

A

A:
rapid heating of the part
void removal is easier

D:
Uniformity
Containment issues
New types of consumables

37
Q

Describe the properties of sandwich structures

A
  • A core is fixed between two skins
  • This increases SMoA
  • Reduces mass and increases resistance to vibrations and panel stiffness
  • Need a seal around edges as middle can fill up with water in high humidity
38
Q

What are the 5 processing routes for thermoplastics

A
  • Autoclave
  • Automated tape placement
  • Pultrusion
  • Hot pressing/stamp forming
  • Welding is possible
39
Q

Why choose thermoplastics

A
  • High heat resistance
  • Higher toughness than thermosets
  • No toxic/messy resins
  • More chemically resistant
  • Can be recycled
40
Q

What can be produced through thermoplastic compounding

A

Convenient, homogeneous material

41
Q

Describe thermoplastic extrusion

A
  • similar to composite Pultrusion
  • continuous section in large volume
  • can have film extrusion to create plastic bags
  • can have fibre drawing

MAT2730 - Non-Metals (5 decks)

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