5 - Composites Flashcards
What 3 things define a composite?
- Each constituent must be present in reasonable quantities (5%)
- The properties of the composite are notably different than those of the constituents
- Usually produced by immediately mixing and combing the constituents
Composite definition
Combination of 2 or more materials differing in composition or form, when combined produce a material of different properties while retaining their individual identities
What is the matrix? what is it typically made of? what is its purpose?
- Constituent that is continuous and often present in greatest quantities
- Typically a resin
- May be ceramic, metallic or polymeric (typically in liquid form)
- Glues the other constituents together
- Maintains fibre orientation/transfers loads
- Protects other constituents from damage
- Provides composite heat resistance, shear and compressive strength
What matrix is most common?
Polymeric
What is a thermoset?
Type of matrix
Become hard, infusible solids after a cure cycle (heat and pressure application), or the addition of a catalyst (curing agent or hardener
What are the types of thermosets? (6)
Used for a/c:
Epoxies - most widely used
Bismaleimides
Polyimides
Not used for a/c:
Polyesters
Vinyl esters
Phenolics
What are the issues with using epoxies?
Relatively poor impact resistance
Unsuitable for use above 120 celsius
Affinity for moisture
Where are bismaleimides useful? What are the disadvantages?
- Higher temp applications where epoxies do not suffice
- More difficult to process and require longer cure times
What are the disadvantages of polyimides?
- Prone to microcracking
- More expensive
- Longer, more complex curing
- Mechanical properties lower than epoxies at room temp but significantly better at higher temps
What are thermoplastics?
Type of matrix
Can be softened with heating/harden with cooling, can be moulded and remoulded, remain chemically unaltered during the process
What are the properties of thermoplastics?
- Derive strength from inherent structure of monomer units (high molecular weight)
- Not as prone to moisture ingress
- Mechanical properties highly dependant on temperature and applied strain
- Useful weight saving for non-critical components
What is typically used for reinforcement? what is its purpose?
- Fibres (either fibrous or particulate)
- Reinforces the mechanical properties of the matrix
- Provides composites tensile strength and stiffness
What are the advantages of using composites compared to other traditional materials?
Light (up to 25% weight savings)
Reduced number of parts
Non-corrosive in salt environments
Good fatigue resistance
More integrated design/manufacture
Dimensional stability
What are the disadvantages of using composites?
High material cost
High labour cost
Susceptibility to operational impact
Long term environmental degradation
High localised damage from lightening strikes
Uncertainties on repair technique
Initial high capital investment (plant, equip, etc)
Certifying new materials (how will they perform in 20-30 years)
What are the predominant types of fibres used? What components are they predominantly used for?
Carbon - primary
Glass - secondary
Aramid (Kevlar) - where impact resistance is required
