Composites

Question 1

Explain why fibres have higher strength than bulk materials

Answer 1

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

Question 2

Explain why fibres need to be lubricated

Answer 2

Friction between fibres can cause their properties to decay

Question 3

Describe how Glass Fibres are drawn

Answer 3

• 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

Question 4

Advantages and Disadvantages of Glass Fibres

Answer 4

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

Question 5

Applications of Glass Fibre

Answer 5

Radomes (transmit EM radiation)
Landing flaps
Bellypan

Question 6

Describe how carbon fibres are drawn

Answer 6

• 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

Question 7

Advantages and Disadvantages of Carbon Fibre

Answer 7

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

Question 8

Describe how polymeric (Aramid) fibres are drawn

Answer 8

• 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

Question 9

Advantages and Disadvantages of Thermoplastics

Answer 9

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

Question 10

Advantages and Disadvantages of Thermosets

Answer 10

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

Question 11

What is the equation for the modulus of longitudinal composites?

Answer 11

Ec = Vf * Ef + Vm * Em

Question 12

What is the equation for the modulus of transverse composites?

Answer 12

1/Ec = Vf / Ef + Vm / Em

Question 13

Why do composites have anisotropic properties?

Answer 13

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.

Question 14

What are the processing aims of composite manufacture?

Answer 14

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

Question 15

Describe the wet lay up process

Answer 15

• 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

Question 16

Describe bulk/dough moulding

Answer 16

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

Question 17

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

Answer 17

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

Question 18

Describe the RTM process

Answer 18

• 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

Question 19

Describe the VARTM process

Answer 19

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

Question 20

How do you reduce voids in the RTM process

Answer 20

Slow infusion and high pressure/vacuum

Question 21

Advantages and disadvantages of RTM/VARTM process

Answer 21

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

Question 22

Advantages and disadvantages of overbraiding to form a preform for RTM

Answer 22

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

Question 23

Describe the pultrusion process

Answer 23

• 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

Question 24

Describe the Filament Winding process

Answer 24

• 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

Question 25

Advantages and disadvantages of Filament Winding

Answer 25

Adv: quick, higher performance than RTM Disadv: expensive

Question 26

Describe the Resin Film Infusion process

Answer 26

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

Question 27

Describe the Hand lay up process of pre-preg

Answer 27

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

Question 28

What is pre-preg?

Answer 28

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

Question 29

Describe the auto lay up process of pre-preg

Answer 29

- 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

Question 30

How is pre-preg cured?

Answer 30

- 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

Question 31

Advantages and disadvantages of autoclave curing

Answer 31

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

Question 32

Describe the quickstep curing process

Answer 32

- 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

Question 33

Describe the microwave curing process

Answer 33

- 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

Question 34

Describe the direct electric cure process

Answer 34

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

Question 35

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

Answer 35

- Lower density than bulk metal - Easier to machine than billets - Behave like metals for machining and finishing - Dent, don't crack like composites

Question 36

What is a sandwich structure?

Answer 36

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

Question 37

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

Answer 37

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