Fibres Flashcards
Why are fibres important
- statistically fibres have higher strength to weight ratio
What does Weibul statistics show
- Smaller materials have statistically less flaws
i. e. smaller volume = stronger material
List key fibres
Glass
Carbon
Polymeric
Describe process of making glass fibres
- melt all constituents
- refine and complete homogenisation
- forehearth - conditions the glass for drawing and ensures that the bushings are fed with molten glass
- cooling spray on fibres as they are drawn
- fibres drawn onto spools
Describe process of making carbon fibres
- PAN/pitch added
- Under tension: fibres stabilised
- carbonisation
- graphitisation
- Not under tension: electrolytic bath
- wash
- sizing
- drier
- spool
Why is sizing important
A coating is put on the fibres which:
- lubricates
- compatibilists it with resin
- adhesion (sticking) promotion
List three subdivisions of polymers
- thermoplastics
- thermosets
- elastomers
Property differences between thermoplastics and thermosets
Thermoplastics:
- can flow past each other easily
- can be recycled
Thermosets:
- much harder to recycle
- sets after heating
Define amorphous
Have random arrangements
Define crystalline
ordered arrangement
These material tend to be opaque
What is special about the glass transition temperature (Tg) of polymers
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
What prevent polymer chain motion
chain branches and vinyl groups
always some amorphous nature in polymers
What are elastomers
Thermoplastics or Thermosets
flexible backbone chains that uncoil and recoil with applied load.
Range greatly
List some uses of thermoplastics
- wire insulation
- composite materials
- windows + canopy
- polymer films
List some uses of thermosets
- composites (epoxy)
- interiors
List some uses of elastomers
- seals/damping
- tyres
- seat foams
What is the longitudinal rule of mixtures and what does it mean
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
What is the transverse rule of mixtures and what does it mean
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
Describe the wet lay-up process for composites
- Lay fibres into mould (often strand mat)
- Mix resin and curing agent
- Apply resin to fibres and work in
- Place in vacuum
- Leave to cure (often in ambient temp.)
What sort of product can be made using wet lay up
Complex shapes but with low performances
Describe the dough/sheet/bulk process and its uses
- 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)
Describe RTM and VARTM process for composites
- Lay dry fibre into mould (1 part RTM, 2 parts VARTM)
- Cover with necessary consumables
- Close mould (RTM), cover with vaccum bag
- Pump (RTM), Drawn in (VARTM) low viscosity resin
- Remove pressure and cure
What is highest volume fraction achievable using RTM/VARTM process
< 50%
Considerations for RTM
- Resin viscosity:
lowest possible although this shrinks
injection pressure minimises voids - Mould:
2 part rigid (metal)
closed mould so less exposure to resin
Considerations for VARTM and uses
- 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
Describe Pultrusion and its uses
- Continuous process
- Assemble bundles of fibres
- Draw into die and inject resin
- Heated to cure
- Draw out and cut to length
- 50% max volume fraction
uses:
floor beams
stringers and spars
Describe Filament Winding and its uses
- Fibre tow impregnated with liquid resin
- Excess resin squeezed out from tows that are under tension
- Wetted tow/filaments wound onto mandrel
- Vary winding direction + angle by alerting speed of rotation of mandrel and of the fibre delivery head
- Cure part
uses:
making tubes
rail way tanks
Describe the Braiding process and its uses
- the process of producing a preform for RTM
- complex shape production
- very high equipment cost
- a 3D complex weave
- combination of fibre types
What is the advantage in using RTM/VARTM/P/FW/B
All quick processing techniques using liquid resin
Create higher performance parts than Wet lay-up
Increasingly used in aerospace applications
Name 5 intermediate performance processes for composites
RTM VARTM Pultrusion Filament Winding Braiding
Name 3 high performance processes for composites
Resin film infusion
Pre-preg
Cure processes
Describe Resin film infusion
- 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)
What is pre-preg
- 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
List the problems seen when laying up pre-preg and the causes
- delamination
- incomplete cure
cause:
shelf life
condensation
cleanliness
For high performance composites, list 4 other ways than autoclave, to cure
- vacuum bag
- shrink wrap (Filament Winding only)
- Quickstep
- microwave curing
What are the advantages and disadvantages of microwave curing
A:
rapid heating of the part
void removal is easier
D:
Uniformity
Containment issues
New types of consumables
Describe the properties of sandwich structures
- 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
What are the 5 processing routes for thermoplastics
- Autoclave
- Automated tape placement
- Pultrusion
- Hot pressing/stamp forming
- Welding is possible
Why choose thermoplastics
- High heat resistance
- Higher toughness than thermosets
- No toxic/messy resins
- More chemically resistant
- Can be recycled
What can be produced through thermoplastic compounding
Convenient, homogeneous material
Describe thermoplastic extrusion
- similar to composite Pultrusion
- continuous section in large volume
- can have film extrusion to create plastic bags
- can have fibre drawing
