Definition of a Composite
- Combination of two materials in which one of the materials is called the reinforcing phase, which is embedded in the other material called the matrix phase.
- Materials made from two or more constituent materials with significantly different physical or chemical properties
General Advantages of Composites?
- Higher specific Strength
- Design Flexibility
- Corrosion Resistance
What types of Reinforcing fibres are there?
Glass fibres, Carbon fibres, Boron fibres and Aramid(Kevlar) fibres.
Properties of Glass Fibres?
- Most common and least expensive
2. High Strength, low stiffness and high density
Properties of Carbon Fibres?
- More expensive than glass fibres
2. Compared to glass fibres, lower in density, higher in stiffness and strength
Properties of Boron Fibres?
- Consists of boron deposited on tungsten fibres
- High strength and stiffness in tension and compression
- Resistance to high temperature
- Heavy & expensive
Properties of Aramid (Kevlar) fibres
- Highest specific strength
- Toughest fibre
- Undergoes plastic deformation before fracture
- Absorbs moisture and is expensive af
Role of matrix?
- Binds fibres together
- Protects fibres
- Transfers loads to and from fibres
- Provides rigidity and shape to structure
- Isolate fibres to slow crack propagation
- Surface quality, corrosion and wear protection for fibres
Demands on matrix?
- Interlaminar shear strength
- Toughness
- Moisture/Environmental resistance
- Processing Temperature
- Service Temperature
- Processing and cost
Role of fibres?
- Carry the load
- Provide structural properties to composite, (strength and stiffness)
- Improve thermal stability
- Provide electrical conductivity or insulation
- Influences formability and machinability of composites
Do long or short fibres provide higher effective stiffness & strengthening?
Long fibres
Definition of rule of mixtures?
Predicts the density of a composite as well as other properties,
Which loading (longitudinal/transverse) undergoes isostrain condition? With the other undergoing isostress condition
Longitudinal
Is a composite weaker under longitudinal or transverse loading (with respect to fibre direction)?
Transverse
In what orientation is maximum strength of a long fibre obtained?
0 degrees, force parallel to fibre direction
Advantages of carbon fibres?
- High Stiffness and strength
- Low density
- Intermediate cost
- Non-reactive
Disadvantages of carbon fibres?
Low toughness, Brittle
Which carbon fibre type is dominating the market?
Polyacrylonitrile (PAN)
What is the process of processing carbon fibres?
- Precursor fibres stabilize at about 200-400C by an oxidation process
- Subject to high temperature treatment at around 1000C to remove hydrogen, oxygen, nitrogen and other carbon elements (This is called carbonization).
- Carbonized fibres can be further graphitized at even higher temperature (around 3000C) to achieve higher carbon content
What are the three types of carbon fibre layer structures?
- Graphitic
- Turbostatic
- Hybrid Structure
Which types of plastic can provide good wettability and why?
Why is it better than the other type?
Thermosetting polymer provides good wettability due to it’s low molecular weight of uncured resin.
This allows for high molecular mobility during processing.
Enables resin to quickly wet surface.
Thermoplastic has high molecular weight hence high viscosity at molten state, hence slower wetting.
Which type of plastic has lower processing temperature?
Thermosets which can be cured from room temperature to 200C.
Thermoplastics typically has a range of 300-400.
What are the difference in properties of a thermoplastic and a thermoset?
Thermoset - Hard, higher usage temperature
Thermoplastic - Soft, lower usage temperature
What are the three most common thermosetting polymers?
- Phenolics
- Epoxies
- Polyester
