Mats 301 Flashcards
What is a composite?
A material that consists of two or more constituent parts. The material is different to its individual components, but they remain separate and distinct.
Dispersed phase (composites)
Particles or fibres.
Matrix phase (composites)
Continuous phase that surrounds the dispersed phase.
Glass fibre production process
- Liquid glass is formed by blending quarry products and heating the mixture in a furnace at very high temp.
- Liquid glass is passed through a platinum bushing with very fine holes.
- The resulting fibres are cooled using water spray, then drawn together using a ‘size’ to provide filament cohesion, and to protect the glass from abrasion.
Three categories of glass fibre
- E-glass (electrical) - lower alkali content, reasonably good tensile and compressive strength.
- C-glass (chemical) - resistant to chemical attack, used in pipes and tanks.
- S-glass - higher tensile strength and modulus than E-glass, achieved by using smaller fibre diameter.
Five advantages of using glass fibres
- Low cost
- Relatively high strength
- Heat resistant
- Insensitive to moisture
- Electrical insulator
Three disadvantages of using glass fibres
- Low stiffness
- Attacked by acids
- Relatively poor fatigue resistance
Aramid fibres (kevlar) production process
- Solid fibres are extruded from a liquid chemical blend using a spinneret.
- Fibres are washed in a neutralising bath, then dried and stretched at 500°C to improve their molecular alignment.
Three advantages of aramid fibres
- High specific tensile strength
- High impact and abrasion resistance
- High fatigue resistance
Three disadvantages of aramid fibres
- Poor in compression
- Attacked by acids and UV light
- Low temperature resistance
Carbon fibre production process
- Produced by the controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors which are already in fibre form, the most common being PAN.
- PAN fibres are greatly stretched to improve molecular alignment, then oxidised in air at 300°C.
- They’re then carbonised at 1500°C to improve crystallinity (nitrogen released) then finally graphitised by heating and stretching at 3000°C
Three advantages of carbon fibres
- High strength and modulus
- High creep and fatigue resistance
- Good energy absorption
Three disadvantages of carbon fibres
- High cost
- Poor impact resistance
- Electrical conductor
Polyester fibres
Low density fibre with good impact resistance, but low modulus.
Polyethylene fibres
Drawing procedure orientates the molecules, giving a very high tensile strength.
Quartz fibres
Very high silica version of glass fibres, with much higher mechanical properties and excellent resistance to high temperatures.
Boron fibres
Carbon or metal fibres coated with a layer of boron. Strong, stiff and light.
Ceramic fibres
Very high temperature resistance, but low impact resistance.
Natural fibres
Fibrous plant material, e.g. coconut.
Five functions of the matrix phase
- Bind fibres together
- Transmit applied load to fibres
- Add ductility/toughness to the composite
- Prevent propagation of cracks
- Protect the fibres from damage
Resin
Refers to a man-made polymer
Resin criteria
The resin must be able to deform to at least the same extent as the fibre, otherwise the full mechanical properties of the fibre component won’t be achieved. Must also have good environmental and stress cycling resistance.
Toughness
The area under a stress-strain curve. It’s a combination of strength and ductility.
Why is high adhesion between resin and fibres important?
To ensure that the loads are transferred efficiently, and to prevent cracking or fibre/resin de-bonding when stressed.
Two types of resin
Thermoplastic and thermoset
Thermoplastic resin
Behave more like metals. Soften when heated and eventually melt, hardening again when cooled. This can be done as often as desired.
Thermoset resin
Formed from a chemical reaction in-situ. Don’t become liquid again when heated, but above a certain temperature the mechanical properties change significantly. This is known as the glass transition temperature.
What happens at the glass transition temperature?
The thermoset resin becomes more flexible, meaning a lower stiffness and reduced strength of the composite. This is reversible on cooling.
Two advantages of thermoset resins
- Liquid at room temp, so easy to work with and get fibres into.
- Can exhibit excellent properties at low raw material cost.
One disadvantage of thermoset resins
Once a thermoset composite has been formed, it cannot be remelted or reshaped, meaning recycling is very difficult.
Two advantages of thermoplastic resins
- Increased impact resistance to comparable thermosets.
- Can be remelted or reshaped, so recycling is possible.
Two disadvantages of thermoplastic resins
- Solid state at room temperature, so much more difficult to introduce reinforcing fibres.
- Resin must be melted, injected around the fibres under pressure, then cooled under pressure to form the composite. This is complex and expensive.
Four common thermosetting resins
Polyester, vinylester, epoxy resin and phenolic resin.
Polyester
A thermoset consisting of a polyester solution in a styrene monomer. The addition of styrene helps to make the resin easier to handle by reducing its viscosity.
Vinylester
Tougher than polyester. Fewer ester groups means better water resistance.
Epoxy resin
Superior mechanical and adhesive properties. Resistant to environmental degradation.
Phenolic resin
Excellent fire resistance - retains properties well at high temperatures. Poor properties overall. Very brittle.
Two types of thermoplastic matrix composites
Glass mat thermoplastics (GMT) and Advanced thermoplastic composites (ATC).
Glass mat thermoplastics
Can use nearly any thermoplastic for the matrix. Short chopped fibres are used.
Advanced thermoplastic composites
Low density and high strength. Good toughness and environmental resistance.
Elastic/ Young’s modulus
Measure of a material’s resistance to elastic deformation when a force is applied to it. It’s the gradient of a stress-strain curve in the elastic region. A stiffer material will have a higher elastic modulus.
Predictions of modulus, strength and Poisson’s ratio can be made from composites based on six assumptions
- Perfect bonding between fibre and matrix
- High adhesion between resin and fibres
- No voids or imperfections
- Consistent fibre size
- Average values of elastic modulus/strength of the components are used
- Uniform strain and stress in the individual components.
Fibre volume fraction + Matrix volume fraction
Vf + Vm = 1
What is the UTS of a composite limited by?
The UTS of the fibres.
Longitudinal force carried by a composite
Fc = Ff + Fm
Longitudinal composite stress (sigma)
sigmac = sigmamVm + sigmafVf
State of isostrain (longitudinal loading)
For a unidirectional composite loaded longitudinally, the strain in the fibres = strain in matrix = strain in composite
Rule of mixtures (longitudinal loading)
Ec = EmVm + EfVf, where E is the modulus.
Major Poisson’s ratio
nu12 = numVm + nufVf
State of isostress (transverse loading)
sigmac = sigmam = sigmaf
Overall strain in composite in transverse direction, epsilonc
epsilonc = epsilonmVm + epsilonfVf
Rule of mixtures (transverse loading)
1/Ec = Vm/Em + Vf/Ef
Ratio of forces carried by matrix and fibres in longitudinal direction
Ff/Fm = EfVf/EmVm
Isotropic material
Material properties are the same in every direction at a point in the body, so the properties are not a function of the orientation.
Orthotropic material
Material properties are different in three mutually perpendicular planes at a point in the body, and have three mutually perpendicular planes of symmetry.
Anisotropic material
Material properties that are different in all directions at a point in the body.
Poisson’s ratio
The negative ratio of lateral strain to axial strain, resulting from a uni-axial stress in the fibre direction.