Composites (fibres/matrices/micromechanics/CLT)

Question 1

What are glass fibres/ advantages and disadvantages of them?

Answer 1

• Mainly silica based (~50-60% si02)
• Contains other oxides( calcium, aluminium) with affects fibre strength
• Low cost, relatively high strength
• Heat resistant, insensitive to moisture and an electrical insulator
• Low stiffness, attacked by acids, poor fatigue resistance.

Question 2

What is the manufacturing process for glass fibres?

Answer 2

1. Liquid glass formed from blending quarry products (sand, limestone)
2. Heat the mixture in a furnace to 1200-1600°C
3. Liquid glass passed through a platinum bushing with very fine holes.
4. Fibres are cooled using a water spray to produce glass filaments 10-20 microns in diameter.

Question 3

What are aramid fibres/ advantages and disadvantages of them?

Answer 3

• man made organic polymer (solid fibre from a liquid chemical bond, 20% PPTA and 80% sulphuric acid)
• High specific tensile strength, good impact/abrasion/fatigue resistance
• poor in compression, attacked by UV/acids, low temperature resistance (becomes brittle)

Question 4

What is the manufacturing process for aramid fibres?

Answer 4

1. “Wet spinning” - polymer powder is dissolved in a solvent and the polymer solution is extruded through spinneret into a mixture (coagulant)
2. Raw materials of aramid react and is isolated by precipitation with water
3. After washing, fibres are dried and stretched at 500°C to improve molecular alignment
4. Chain molecules are highly orientated along the fibre axis, so the strength of the chemical bond can be exploited.

Question 5

What are carbon fibres/ advantages and disadvantages of them?

Answer 5

• Made from thin, strong crystalline filaments of carbon that is used to strengthen material.
• Carbon crystal aligned in long axis (increasing strength in long axis)
• Sheets are long and thin (graphite)
• High strength and modulus, creep and fatigue resistant, good energy absorption
• high cost, poor impact resistance, electrical conductor

Question 6

What is the manufacturing process for carbon fibres?

Answer 6

• Controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors (chemical) which are already in fibre form.

1. PAN fibres stretched by 500-1300% to improve molecular alignment.
2. Stabilised (oxidised) in air at 300°C
3. They are then carbonised at 1500°C to improve crystallinity. Nitrogen released, fibres 90% carbon (high strength and chemical inertness)
4. Graphitised by heating and stretching at temperatures up to 3000°C

Question 7

Explain the structure of graphite

Answer 7

• Basal planes, strong covalent bonds, weak van Der Waals forces
• Contains layers of carbon atoms
• Each carbon atom is only covalently bonded to three other carbon atoms, rather than four as in diamond.

Question 8

Why is platinum used for bushing?

Answer 8

• good properties at high temperatures