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Materials Selection and Processing > Composites > Flashcards

Composites Flashcards

1
Q

Explain why fibres have higher strength than bulk materials

A

The maximum flaw size in a fibrous material is the diameter of the fibre, which is much smaller than the max flaw size in a bulk material

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2
Q

Explain why fibres need to be lubricated

A

Friction between fibres can cause their properties to decay

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3
Q

Describe how Glass Fibres are drawn

A
  • Constituents are melted and homogenised
  • Bubbles are removed from the molten material
  • Viscosity of molten material is increased to prepare for drawing
  • Material is fed to bushings and drawn onto spools
  • As material is drawn it is cooled and lubricated
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4
Q

Advantages and Disadvantages of Glass Fibres

A

Adv: Transmit EM radiation, high max temp
Disadv: Low modulus, high density

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5
Q

Applications of Glass Fibre

A

Radomes (transmit EM radiation)
Landing flaps
Bellypan

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6
Q

Describe how carbon fibres are drawn

A
  • PAN or pitch is drawn into graphite fibres
  • The outer layer of graphite is removed to improve the strength of the fibre
  • The fibres are washed, lubricated and coated in resin
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7
Q

Advantages and Disadvantages of Carbon Fibre

A

Adv: High modulus, high max temperature, low density
Disadv: expensive

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8
Q

Describe how polymeric (Aramid) fibres are drawn

A
  • The polymer is synthesised, filtered and washed
  • Polymer is solvated in H2SO4
  • The polymer is sent to spinnerets to form fibres and fed though a spinning bath to remove H2SO4
  • Polymer fibres are washed and lubricated
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9
Q

Advantages and Disadvantages of Thermoplastics

A

Adv: Can be recycled, can be welded, chemically and thermally resistant, don’t involve toxic resins
Disadv: difficult to produce high performance thermoplastics

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10
Q

Advantages and Disadvantages of Thermosets

A

Adv: Thermally stable, easy to process, can chemically bond to a surface
Disadv: cannot be recycled

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11
Q

What is the equation for the modulus of longitudinal composites?

A

Ec = Vf * Ef + Vm * Em

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12
Q

What is the equation for the modulus of transverse composites?

A

1/Ec = Vf / Ef + Vm / Em

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13
Q

Why do composites have anisotropic properties?

A

Fibres have a much higher modulus than the matrix, so when strained longitudinally, the matrix transmits the stress to the fibres, but when strained transversely, the matrix takes as much stress as the fibres.

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14
Q

What are the processing aims of composite manufacture?

A
  • Form net shape product
  • High fibre volume fraction
  • Good bond between fibres and matrix
  • No secondary bondings/fixings
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15
Q

Describe the wet lay up process

A
  • Fibre mat laid into mould
  • Resin and curing agent applied and rolled in
  • Repeat to build up thickness
  • Can use pressure/vacuum to consolidate layers
  • Heat to activate cure
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16
Q

Describe bulk/dough moulding

A
  • Dough (high viscosity mix of fibres and resin) is placed into the mould
  • Dough is hot pressed to form shape and activate cure
17
Q

Advantages and disadvantages of wet lay up and bulk/dough moulding

A

Adv: Easy, cheap, quick
Disadv: low performance, exposure to resin

18
Q

Describe the RTM process

A
  • Lay dry fibre mat into one half of the mould and seal with other half
  • Pump low viscosity resin though the fibre mat
  • Remove pressure and cure
19
Q

Describe the VARTM process

A
  • Lay fibre mat into mould and cover with bagging material
  • Draw low viscosity resin in with vacuum
  • Remove vacuum and cure
20
Q

How do you reduce voids in the RTM process

A

Slow infusion and high pressure/vacuum

21
Q

Advantages and disadvantages of RTM/VARTM process

A

Adv: low skill requirement, low exposure to resin, cheap, easily reproducible
Disadv: low performance, 50% fibre volume fraction

22
Q

Advantages and disadvantages of overbraiding to form a preform for RTM

A

Adv: reduces risk of interlaminar failure (delamination), can produce complex shapes
Disadv: very high cost

23
Q

Describe the pultrusion process

A
  • Continuous process of assembling fibres, drawing through die and injecting resin
  • End of die is heated to cure composite as it is produces
  • Can produce lengths with continuous cross section
24
Q

Describe the Filament Winding process

A
  • Fibres pulled though resin bath and laid onto mandrel
  • Excess resin squeezed out under tension
  • Winding speed and direction is altered by changing rotation speed of mandrel
  • Part is covered in vacuum bag to cure
25
Q

Advantages and disadvantages of Filament Winding

A

Adv: quick, higher performance than RTM
Disadv: expensive

26
Q

Describe the Resin Film Infusion process

A
  • Lay dry reinforcement (woven fibres) and resin film in alternating layers into mould
  • Apply heat and pressure (autoclave) to make rein flow into fibres
27
Q

Describe the Hand lay up process of pre-preg

A
  • Plies of pre-preg are cut at required angles
  • Plies are manually placed and pressed together
  • Part is cured in a vacuum chamber
28
Q

What is pre-preg?

A
  • Mats of unidirectional fibres pre impregnated with resin
  • Resin is partially cured and frozen
  • Pre-preg is defrosted immediately before use
29
Q

Describe the auto lay up process of pre-preg

A
  • Tapes of pre-preg laid onto mould by robotic arm
  • Heated as it is laid to ensure adhesion and pressed for consolidation
  • Can lay tapes at complex angles with better accuracy and faster than hand lay up, but cannot form complex shapes
30
Q

How is pre-preg cured?

A
  • Part is covered with bagging material and exposed to vacuum to remove surface volatiles and voids
  • External pressure and high temperature (autoclave) compresses voids, improves volume fraction and cures part
31
Q

Advantages and disadvantages of autoclave curing

A

Adv: High performance part (high vol fraction)
Disadv: Expensive, size of part is limited, pre-preg can delaminate

32
Q

Describe the quickstep curing process

A
  • Flexible mould surfaces apply pressure and heat to the part
  • Temperature and pressure are high to reduce curing time and ensure consolidation
  • Cheaper than autoclave
33
Q

Describe the microwave curing process

A
  • Part is surrounded in microwave arrays and heated due to microwave radiation
  • Heat develops in centre of part first so voids are removed
  • Size restrictions as microwaves must be contained
34
Q

Describe the direct electric cure process

A
  • Electricity conducted though carbon fibres and they heat up
  • Heat develops in centre of part first so voids are removed
  • Quick and efficient
35
Q

What are the advantages of Fibre-Metal hybrid structures (GLARE, TiGr)?

A
  • Lower density than bulk metal
  • Easier to machine than billets
  • Behave like metals for machining and finishing
  • Dent, don’t crack like composites
36
Q

What is a sandwich structure?

A
  • Hollow core between two skins, usually used in large composite panels
  • Increases 2nd moment of area and reduces mass
37
Q

What processing routes are possible with thermoplastic composites, and why is the processing restricted?

A

Autoclave, auto tape placement, pultrusion, film infusion, pre-preg
Processing is more difficult as thermoplastics are much more viscous than the constituents of thermosets, so require higher temperature and pressure during processing

Materials Selection and Processing (4 decks)

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