Fibres

Question 1

Why are fibres important

Answer 1

• statistically fibres have higher strength to weight ratio

Question 2

What does Weibul statistics show

Answer 2

• Smaller materials have statistically less flaws

i. e. smaller volume = stronger material

Question 3

List key fibres

Answer 3

Glass
Carbon
Polymeric

Question 4

Describe process of making glass fibres

Answer 4

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

Question 5

Describe process of making carbon fibres

Answer 5

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

Question 6

Why is sizing important

Answer 6

A coating is put on the fibres which:

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

Question 7

List three subdivisions of polymers

Answer 7

• thermoplastics
• thermosets
• elastomers

Question 8

Property differences between thermoplastics and thermosets

Answer 8

Thermoplastics:

• can flow past each other easily
• can be recycled

Thermosets:

• much harder to recycle
• sets after heating

Question 9

Define amorphous

Answer 9

Have random arrangements

Question 10

Define crystalline

Answer 10

ordered arrangement

These material tend to be opaque

Question 11

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

Answer 11

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

Question 12

What prevent polymer chain motion

Answer 12

chain branches and vinyl groups

always some amorphous nature in polymers

Question 13

What are elastomers

Answer 13

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

Question 14

List some uses of thermoplastics

Answer 14

• wire insulation
• composite materials
• windows + canopy
• polymer films

Question 15

List some uses of thermosets

Answer 15

• composites (epoxy)

- interiors

Question 16

List some uses of elastomers

Answer 16

• seals/damping
• tyres
• seat foams

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

Describe the wet lay-up process for composites

Answer 19

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.)

Question 20

What sort of product can be made using wet lay up

Answer 20

Complex shapes but with low performances

Question 21

Describe the dough/sheet/bulk process and its uses

Answer 21

• 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)

Question 22

Describe RTM and VARTM process for composites

Answer 22

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

Question 23

What is highest volume fraction achievable using RTM/VARTM process

Answer 23

< 50%

Question 24

Considerations for RTM

Answer 24

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

Question 25

Considerations for VARTM and uses

Answer 25

- 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

Question 26

Describe Pultrusion and its uses

Answer 26

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

Question 27

Describe Filament Winding and its uses

Answer 27

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

Question 28

Describe the Braiding process and its uses

Answer 28

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

Question 29

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

Answer 29

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

Question 30

Name 5 intermediate performance processes for composites

Answer 30

``` RTM VARTM Pultrusion Filament Winding Braiding ```

Question 31

Name 3 high performance processes for composites

Answer 31

Resin film infusion Pre-preg Cure processes

Question 32

Describe Resin film infusion

Answer 32

- 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)

Question 33

What is pre-preg

Answer 33

- 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

Question 34

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

Answer 34

- delamination - incomplete cure cause: shelf life condensation cleanliness

Question 35

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

Answer 35

- vacuum bag - shrink wrap (Filament Winding only) - Quickstep - microwave curing

Question 36

What are the advantages and disadvantages of microwave curing

Answer 36

A: rapid heating of the part void removal is easier D: Uniformity Containment issues New types of consumables

Question 37

Describe the properties of sandwich structures

Answer 37

- 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

Question 38

What are the 5 processing routes for thermoplastics

Answer 38

- Autoclave - Automated tape placement - Pultrusion - Hot pressing/stamp forming - Welding is possible

Question 39

Why choose thermoplastics

Answer 39

- High heat resistance - Higher toughness than thermosets - No toxic/messy resins - More chemically resistant - Can be recycled

Question 40

What can be produced through thermoplastic compounding

Answer 40

Convenient, homogeneous material

Question 41

Describe thermoplastic extrusion

Answer 41

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