L2: Context, Fibres and Matrices Flashcards
What are the 4 most common applications for composites and why?
Aerospace, marine/wind energy, sport and automotive
Require combo of lightness, stiffness and strength
What is a ‘specific’ property?
Performance per unit weight for fixed volume
List 6 key advantages of composites
- high specific stiffness & strength
- Anisotropy (can place fibres very specifically to needs)
- Electrical & thermal insulation
- Corrosion resistance
- Dimensional stability (in temp and humidity changes)
- Easily shaped
List 4 key disadvantages of composites
- Cost of materials
- Complexity of design
- Manufacturing cost and tech requirements
- End of life/recycling
What assumption is fracture mechanics based on?
All engineering materials contain cracks from which failure starts
How does strength change with fibre diameter?
Increases as diameter decreases
Give the Griffith relationship
strength = K(IC)/sqrt(pi*c) Where K(IC) is the fracture toughness of the fibre, c is the flaw size
What are the conditions for very high fibre strength?
Fibres must have very small grain and minimal flaw sizes
Why are fibres stronger than bulk material?
There is a lower probability of a critical flaw being present
Why do fibres tend to have high levels of axial stiffness?
Molecules are highly oriented during processing -> modulus is significantly enhanced
What do carbon fibres consist of?
Graphite basal planes, hexagonal strong covalent bonds between carbon atoms
Which word describes the highly differing strength in the transverse and basal plane of graphite?
Anisotropy
What is the difference between PAN and Pitch carbon fibres?
PAN: higher strength and failure strain
Pitch: higher modulus, negative or zero thermal expansion along fibre direction
What is the strength of carbon fibres determined by?
Artefacts left in them from processing
What is the modulus of carbon fibres associated with?
Level of preferred orientation of graphite crystals
What are glass fibres based on?
Silica SiO2
State whether the following are anisotropic or isotropic:
- Carbon fibres
- Glass fibres
- Anisotropic
2. Isotropic
What is glass fibre sizing?
The process of coating the fibres in a thin surface coating of mainly organic materials
What are glass fibre sizings all based on?
They are all silane-based
What is the main advantage of polymer composites?
Simpler and cheaper manufacturing processes/equipment compared to metal/ceramic reinforced composites
What are the disadvantages of polymer composites? (3)
- Low max working temp
- High coefficient of thermal expansion
- Can be sensitive to radiation and moisture
Describe thermoset polymers
- Cross-link during curing (heat, pressure and/or hardener)
- Cross-links are strong covalent bonds
- Material degrades with heat
Describe thermoplastic polymers
- Linear, not cross-linked
- Weak van der Waals forces between chains
- Chains can flow under stress at high temps
Describe thermoset matrices
Mix of 2 or more chemically reactive, low mol weight compounds
Initially a low viscosity liquid which must be heated to initiate polymerisation
Describe thermoplastic matrices
Pre-polymerised, high mol weight. Solids which mist be heated to lower viscosity to process
What is the main advantage of epoxy?
It is cured in 2 stages, so good for prepreg piles
Give advantages of thermosets (4)
- Easy impreg and wet-out of fibres (low mol weight and viscosity)
- Crosslinked network
- Preferred for processes with continuous/long fibres
- History
Give issues with thermosets (4)
- Material & artefact formed simultaneously (must get it right first time)
- Low fracture toughness
- Long cure times
- Difficult to recycle
Give advantages of thermoplastics (5)
- More ductile
- No chemistry involved (clean processing)
- More rapid processing
- Recyclable
- Good hot/wet properties
Give issues with thermoplastics (5)
- High melt viscosity for impreg
- (Sometimes) high temp required
- Control over morphology in crystallising TPs
- Lower modulus
- Higher creep
What are thermoset matrices used for?
- High temp resistance
- Chemical resistance
- Low viscosity processing requirement
What are thermoplastic matrices used for?
- Fast, clean processing
- Short fibre for quasi-isotropic material
- “green” applications
Why is anisotropy an advantage?
Ability to put strong and stiff fibres in the right place, in the right orientation with the right volume fraction
What are the important fibre parameters?
- Mechanical properties
- Volume fraction
- Orientation
- Length
What are important composite parameters excluding fibre parameters?
- Matrix
- Interface
- Processing
Give an overview of aligned continuous fibres
Higher performance in fibre direction
Highly anisotropic
Generally slower processing with limited shapes
Give an overview of discontinuous fibres
Randomly dispersed
Lower performance, more isotropic, generally faster processing with complex shapes
What defines a good interface?
Intimate matrix-fibre contact (good wetting)
High stress transfer capability (good adhesion)
What is the interface important for?
- the formation of the material
- determining the performance and lifetime of the part
