D2: Constituents Flashcards
Reinforcement types? What do we use in aerospace?
- Aligned or random orientation.
- Short, long, or continuous.
- Aerospace use aligned, continuous fibres.
What type of bonding is present in carbon fibres?
- Strong covalent bonds between molecules in fibre direction, forming a graphite crystalline structure.
- Weak electrostatic bonds transverse to fibre direction, between the crystal planes.
What causes the difference between HM and HS carbon fibres?
- Alignment of the crystals with the fibre, HM are more aligned resulting in a stiffer fibre.
What processes are used in the manufacture of carbon fibres?
- Elements other than carbon are removed though graphitisation (or pyrolysis), where the fibres are heated to 2000 deg in an inert atmosphere.
- Molecular alignment is achieved using tension or spinning.
Up to what temperature are carbon fibres thermally stable?
2000 deg in the absence of oxidising agents.
What are the advantages and disadvantages of kevlar fibres?
- High strength and high modulus.
- Bad in compression due to fibrillation (collapse of the fibrils that make up a fibre).
- Low operating temperature, up to 250 deg.
- Susceptible to moisture ingress.
What process is used to create kevlar fibres?
Solvent spinning.
What structure do glass fibres have? What are the material properties like?
- Amorphous structure.
- High strength, moderate stiffness.
How are glass fibres made?
Drawing molten glass. Coated with a sizing agent (typically silanes) immediately afterwards, to improve handling and promote bonding with the matrix material.
What temperature are glass fibres thermally stable to?
850 deg, but modulus degrades after about 250 deg depending on the composition.
What are the 3 groups of glass fibre and what are their benefits?
- E and R glass fibres are cheap with reasonable properties.
- S glass fibres have better properties for increased cost.
What are the stress strain curves of the different fibre materials like?
- Practically linear-elastic up to failure, exhibiting virtually no plastic deformation. i.e they’re brittle.
- Kevlar fibres tend to exhibit some ductile necking, increasing their toughness.
Why are hybrids used? What are the 2 types?
- To negate the drawbacks of a certain fibre material with the benefits of a different one.
- Intermingled and layered.
What fibre material should be used for high specific strength and stiffness?
Carbon
What fibre material should be used for high impact resistance?
Kevlar or kevlar hybrids
What fibre material should be used for non-critical components, or lower cost?
Glass
What fibre material should be used for microwave/electrical components, and why?
Glass because they are transparent to EM radiation.
What are the 3 main fibre materials, and what are their typical strains-to-failure?
- Carbon, 1-1.5%
- Kevlar, 2.5%
- Glass, 2%
What are the 4 typical matrix materials, and what temperature ranges can they operate at?
- Epoxies, up to 150 deg
- Phenolics and polyimides, up to 300 deg
- Metals, up to 400 deg
- Ceramics, higher than 1000 deg
What is different about TS and TP’s molecule structures? What does this result in?
- TS molecules are cross linked, whereas TP molecules are held together with weak Van der Vaal forces and entanglement.
- Means that TSs can’t be softened once they have hardened, but TPs just need enough heat to allow molecules to slide past each other.
What is needed to harden a TS?
A catalyst, heat (125-175 deg), and pressure (0.75-1.5MPa).
What 3 things limit TS performance?
- Temperature
- Toughness (brittle)
- Moisture - they can absorb up to 2% of their weight in water, which tends to plasticise the material (soften and degrade). Worsened in higher temperatures.
What is the TP molecular structure called? What are the implications and benefits?
- Semi-crystalline.
- Cooling rates must be within a certain range to achieve required crystallinity.
- Able to melt them, meaning they can be healed in service, and scraps can be reused.
- Infinite shelf life.
What are the disadvantages of TPs?
- High temperatures required for processing (300-400 deg). More energy, and ancillary materials capable of withstanding the temps required (more expensive, and often worse handling quality than their normal temp counterparts).
- High pressures (2-3 MPa) required to overcome high viscosity in curing process.
What are the advantages of TPs?
- While they tend to have worse material properties compared to TS, they are often inherently tougher (high impact resistance).
- Do not suffer from moisture absorption (less than 0.2%, with no adverse effects).
What temperature is used as the operating temp for TPs? Why?
Glass transition temperature, because material becomes rubbery above this.
How does PEEK (TP) compare with TSs? (6 things)
- As good general mechanical properties
- As good temperature performance
- Better toughness
- Better at dealing with moisture
- Better in space applications (because of outgassing and radiation)
- Better cryogenic performance (micro cracking at low temperatures)
What are the limiting factors of TPs?
- Difficult to process (high temps and pressures)
- Expensive material and ancillaries
- Difficult to bond due to low surface energy
What matrix materials are best for: 1) general mechanical performance, 2) high temperature environments, and 3) toughness, chemical resistance, or fire resistance?
1) Epoxy
2) Phenolic/polyimide
3) Toughened epoxy
Why are sizing agents used?
- To protect fibre surfaces
- To aid wetting by increasing fibre surface energy
- To act as a coupling agent for chemical bonding
- Bond strength control
What are the 3 possibilities when a fibre cracks? Which is desirable and why? How can we achieve this?
- Brittle matrix cracking, ductile matrix yielding, and interface disbonding.
- Interface disbonding is desirable because it effectively blunts crack tips, allowing the fibres to act as crack stoppers.
- Careful selection of interface strength.
What are the 4 components of composite fracture energy, from largest to smallest?
- Fibre pull out energy
- Fibre disbond energy
- Matrix fracture energy
- Fibre fracture energy
What are the 3 types of sandwich core?
Honeycomb, foam, and syntactic
What core should be used for:
1) High strength
2) Corrosion resistance
3) lightweight/closed cell/moderate loading
4) thin sandwich
1) Honeycomb (metal or nomex)
2) Nomex honeycomb
3) Foam
4) Syntactic
What adhesives are generally used?
- Epoxies with higher ductility than the matrix, to cope with shear and peeling stress concentrations present at the ends of bonded joints.
- Polyimides, phenolids, and urethanes are also used.
What are the forms of adhesives, with examples of their usage?
- Films - sandwich face skins
- Paste - joints
- Potting compounds - fasteners/fittings in sandwich cores
- Foaming adhesives - sandwich core splicing/edge filling
