Lecture 2

Question 1

What are some examples of Thermoset and thermoplastic matrices?

Answer 1

Thermosets: Polyester Vinyl-ester Epoxy

Thermoplastics:
Amorphous PEI, ABS,
Semicrystalline: Polypropylene,Nylon, PEEK

Question 2

Advantages of Thermosets

Answer 2

*Dimensionally stable
* Cost effective
* Low viscosity – easy to wet out fibres

Question 3

Challenges of thermosets

Answer 3

*Long cycle times due to cross-linking
* Generally brittle
* Cannot be remoulded or reshaped
* Difficult to recycle
* Poor Fire, Smoke, Toxicity (FST) rating

Question 4

Advantages of thermoplastics

Answer 4

*Short cycle times
* Easily recycled
* High in-service temps
* Chemically resistant
* High ductility – impact resistant
* Can be welded

Question 5

Challenges of thermoplastics

Answer 5

*High processing temps
* Extremely high viscosity (1000 × TS)
* High cost for engineering materials
* Creep at joining to metals because of thermal degrade
* Poor surface finish – poor paint
* Poor fibre/matrix adhesion

Question 6

State the characteristics of Amorphous thermoplastics

Answer 6

*Randomly ordered molecular structure
* Lack a sharp melting point (Soften gradually with increasing temp)
* Presence of hydrocarbons - sensitive to stress cracking
* Isotropic flow – good dimensional stability
* Translucent

Question 7

State the characteristics of semi-crystalline thermoplastics

Answer 7

*Highly ordered molecular structure

• Sharp melting point –rapidly change into low viscosity fluid
• Good level of toughness
• Anisotropic flow –high shrinkage transverse to flow direction
• Opaque

Question 8

Explain why the mechanical properties of carbon fibres are dominated by heat treatment temperature

Answer 8

• Higher heat treatment temps (with fibre under tension) increases structural order. Stretching of fibres leads to alignment of basal planes.
• Reduction in strength due to large crystal grain formation and increase in internal voids
• Heat treatment temperature influences fibre cost due to affected production times and energy requirements

Graphitization temperatures:

• > 2000oC - High Modulus (Type I)
• ~1500oC - High Strength (Type II)
• 1000oC - Industrial (Type III)

Question 9

Give a typical tensile modulus and strength value for a standard modulus (nonaerospace) carbon fibre.

Answer 9

Tensile stiffness of 240GPa (allow ± 10%) [1]
Tensile strength of 4300 MPa (allow ± 20%) [1]

Question 10

Despite their short cycle times, thermoplastic composites are not widely used for structural automotive applications. Give at least 3 reasons why

Answer 10

• High cost for engineering thermoplastics compared to comparable thermosets
• Joining to metallic structures is problematic due to creep relaxation. Cars see very varied
  temperatures depending on location.
• Poor surface finish due to high shrinkage. ‘Class A’ is difficult to achieve
• Difficult to paint – poor surface adhesion
• Poor interfacial adhesion with standard carbon fibres – expensive bespoke sizings are
  required, which cannot be justified

Question 11

Conversely there is a growing interest in thermoplastic matrix composites in the aerospace sector. Give at least three reasons for this, explaining why they are becoming more attractive than thermosets for some applications.

Answer 11

• High levels of ductility make them impact resistant and more damage tolerant –
  leading edge of a wing to prevent erosion
• Offer a potential route for recycling – large volumes of material used
• Can be welded – enabling large structures to be produced without adhesive
  bonding or fasteners