Lecture 2 Flashcards
(14 cards)
What are the main disadvantages of fibre reinforced plastics?
Low maximum working temperature
High coefficients of thermal expansion
Can be sensitive to radiation and moisture
What typical polymers are used as matrices?
Thermoset
Thermoplastic
Rubbers
Describe thermoset polymers?
There is a cross-link during curing which provides a strong covalent cross-link bond. This keeps the chain together and restricts movements. When reheated they cannot be de-bonded, thus the material only degrades with heat and are non-recyclable.
Describe thermoplastic polymers?
They do not cross-link and chains can flow under stress at a high temperature. They can be reheated and thus reshaped making them recyclable.
Give 4 advantages and 4 disadvantages of thermoset polymers?
Advantages:
- Low molecular weight and low viscosity making them easier to impregnate and wet-out fibres.
- Crosslinked permanent molecular network
- Preferred matrix for many manufacturing process using long or continuous fibres
- They have a longer history
Disadvantages:
- Must get it right first time (cure is important)
- Low fracture toughness
- Long curing time
- Difficult to recycle
What are amorphous and semicrystalline polymers?
Thermoplastic polymers
Amorphous has interpenetrating random coiled molecules.
Semicrystalline has regions of molecular alignment and increased intramolecular interaction.
Why are crystalline regions good in thermoplastics?
Crystalline regions act as physical crosslinks improving many mechanical and thermal properties.
In the process of crystallisation, polymer chains arrange themselves to fit the crystal lattice.
Give 5 advantages and 5 disadvantages of thermoplastics?
Advantages:
- More ductile
- No chemistry involved (clean process)
- More rapid processing
- Recyclable
- Good hot/wet properties
Disadvantages
- High melt viscosity for impregnation
- High temperatures required
- Control over morphology in crystallising
- Lower modulus
- Higher creep
What are some of the important parameters affecting the properties of composite materials?
Fibres:
- Mechanical properties
- Volume fraction
- Orientation
- Length
Matrix:
- Mechanical properties
- Processability
Interface:
- Good wetting
- Good adhesion
What is important about the interface?
When an external load is being applied, the load transfers across the interface and thus a good interface is critical to nearly all the composite performance criteria and lifetime of the part.
What gives a “good” interface?
An intimate matrix/fibre contact (good wetting).
High stress transfer capability (good adhesion).
Give 2 different ways that fibres be arranged and explain them?
Aligned continuous fibres: These have the highest performance in the fibre direction, but are highly anisotropic and are generally slow to process.
Discontinuous fibres: These are random in orientation and can be more complex with lower performance, they are however more isotropic and are generally quicker to process.
What does the processing and production of composites depend upon? (4 to get)
Overall Cost
Shape to be made
Number to be made
Required performance
Name 7 different fabrication methods and the typical applications in which they are used?
Hand Lay-up: Manual method of resin impregnation of dry fibre with a brush or roller (GRP boat hull).
Spray Lay-up: Direct application of chopped rovings and resin to the desired shape (GRP boat hull).
Autoclave: Forming of stacked prepreg in a vacuum bag (Aircraft wing or rotor blade).
Filament Winding: Wet resin carried by fibres onto a mandrel at predetermined angles (Pipes or pressure vessel).
Pultrusion: Wet resin-impregnated roving or cloth formed through die (A beam or rod).
Compression moulding: Pressed to shape in a closed mould between matched dyes (Lorry body panel).
Resin Injection Moulding or Transfer Moulding: Wet resin injected into woven or CSM preform in a closed mould (GRP car body panels).
