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27 Cards in this Set
- Front
- Back
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What makes composite fibres so strong? |
- Low atomic number elements (C, B, Al, Si) have strong directional inter-atomic bonds - Probability of a flaw is inversely proportional to the volume for a given length - Very sensitive to flaws if they are present however - Minimise flaws by proper coating procedures - Crystal structures is fine (fast cooling) and crystal structure is well aligned (strained when it's thinned out) Page 3-2, Page 3-3 |
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Properties of Glass Fibres |
- High strength and low cost - Low stiffness limits large structural use - Used in radomes and aerial covers, because they are EM transparent - Formed by cooling a viscous liquid at a very high rate to prevent crystalline formation -Additives significantly affect mechanical properties Page 3-8, 3-9 |
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!! What is the manufacturing process for glass fibre? !! Draw a diagram illustrating the process as well. |
- First melted between 1250 and 1400 C - Drawn into spinneret with holes in the base - Drops out of holes are draw in to fibres at approx. 50m/s - Cooled by a water spray and coated with a size - Combined into a strand (of 52, 102, 204 fibres) and wound on a spool Page 3-11, 3-12 |
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What determines the diameter of the glass fibres? How do their properties differ from bulk glass? |
- Diameter is a function of the spinneret hole size and viscosity of the melt - Cooling rate is greater than 10,000 degrees/sec - Because of high cooling rate, higher tensile strength but lower elastic modulus and chem. resistance than bulk glass Page 3-12 |
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What are the types of glass fibres and their uses? |
E Glass - for electrical applications because of higher electrical resistivity and low dielectric constant S Glass - for structural applications because of higher strength and low cost Page 3-14 |
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Purpose of the glass Fibre coating? |
- Reduce friction damage (which can be significant) - prevent absorption of moisture - Can be used to help adhere fibres together Page 3-15 |
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What is the difference between carbon and graphite? |
- Graphite is a form of carbon, not all carbon is graphite - There is easy slip between basal planes, therefore graphite is often used as a lubricant Page 3-18 |
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What is the difference between wet and dry spinning for carbon fibres made from PAN? What effect does stretching the fibres have? |
- Stretching the fibres reduces diameter, aligns molecular chains and increases stiffness - Dry Spinning (produces smooth fibres) - Wet spinning (in a bath) provides different cross-sections and a greater bonding surface area - Fibre tows typically contain up to 10,000 fibres (fibres between 5 - 10 micro metres) Page 3-19 Page 3 - 19 |
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!! How are carbon fibres produced from PolyAcryloNitrile (PAN)? !! |
Stage 1: PAN is stabilised at 250 degrees C by oxidation. It has a high transition temp. Stage 2: Removal of N, O and H in inert H-atmosphere at 1200-1600C. Basal planes align along the fibre axis. (increasing strength) Stage 3: Final heat treatment at 1500-2500C in an inert atmosphere, where basal layers then grow along the fibre direction to provide directional properties - NO MELTING Page 3-20, 3-21 |
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!! Describe the characteristics of carbon fibres made from pitch !! |
- If produced at low cost, have poor mechanical properties - Can produce ultra-stiff fibres, but it has elaborate and expensive procedures - Does not need tension to develop molecular orientation, because of the anisotropic crystal nature of pitch Page 3-22 |
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Describe the manufacture of carbon fibre from pitch |
1. Isotropic to Mesophase - Prolonged heating in an inert atmosphere to form a liquid crystal phase, then melt-spun into a fibre form 2. Cross Linking - Fibres are cross-linked (to reduce relaxation) by heating for a short time in an O2 atmosphere 3. Pre-Carbonisation - Heating at 1000 C to reduce rate of gas evolution and creation of surface flaws 4. Carbonisation 5. Graphitisation Page 3-23 |
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Compare pitch and PAN based carbon fibres |
- Pitch fibres have a higher tensile modulus, because they are more highly graphetisable - Pitch fibres have lower compression and shear properties - Pitch based fibres are more porous, therefore lower tensile strength - Pitch fibres have a higher carbon content (80%) compared to PAN fibres (50%) - Pitch based fibres electrical and thermal conductivity properties - used in space applications Page 3-24 |
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Describe boron fibres |
- Large monfilaments around 125-140 micro metres in diameter - Almost as hard as diamond - hard to machine - Boron is largely replaced by carbon fibres, which are cheaper, more machinable and formable - Used for composite patch repair on aircraft |
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!! Describe what CVD is, in the context of producing boron fibres !! |
- CVD - Chemical Vapour Deposition - Made by CVD of boron onto a fine tungsten or pitch-based carbon fibre core of about 10 micro metres - Deposition occurs by hydrogen reduction of Boron trichloride over 1000C - If T > 1200 C some crystalline Boron is formed, which reduced mech. properties Page 3-26, Page 3-27 |
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Why are boron fibres so expensive? |
- The CVD grows the boron about about 3-4 micrometres/min, which is very slow - Tungsten core is expensive, so to help reduce cost, the fibres are made thick (140 micro metres) Page 3-26 |
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Describe the properties of Aramid fibres |
- Trade name is 'Kevlar' - Good tensile properties up to 400 C - Poor compressive strength - Can absorb large amounts of energy during fracture, because of: high strain-to-failure, can undergo plastic deformation in compression, can defibrilate during fracture Page 3-30 |
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Describe the structure of aramid fibres |
- made up of bunches of fibrils, that are weakly bonded together Page 3-31 |
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How are aramid fibres made? |
- Based off an aromatic polyamide (Pol para-phenylene terphalamide PPD-T) - The PPD-T is dissolved in acid and then extruded through a spinneret at 100C - The fibre precipitates after emerging from the spinneret and then coagulates after passing through a water bath, which removes the acid Page 3-31 |
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Describe mechanical properties of Aramid fibres |
- Under tension, they fail by defibrillation - Strength reduces by about 20% at T = 180 C - Prone to short-term creep, long term creep is negligible - Non-linear in compression due to formation of kink bands - Can be damaged by UV exposure, but can be protected by a matrix Page 3-32 |
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Describe polyethylene (PE) fibres |
- Specific gravity less than one - Low compressive strength - Susceptible to creep deformation under long term loading, even at room temps - Not suitable for prolonged static loading due to poor creep behaviour - Servivce temp < 100 C - Good chemical resistance and low moisture absorption - Very high toughness, hence used for ballistic protection Page 3-33, Page 3-34 |
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How are PE fibres made? |
- Drawing melt-crystallised PE to very high draw ratios - Other methods of manufacture are solution and gel spinning Page 3-33 |
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What are rovings, tows and yarns? |
- Rovings consist of an untwisted bundle of strands (which contain 102, 204 filaments) -> mainly glass fibres - Tows are untwisted carbon fibres (1000 - 48,000) produced directly from PAN precursor - Yarns are a twisted collection of strands or filaments - the twist holds the fibres in place and maintains an even tension Page 3-36 |
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What is the difference between a mat and a woven fabric? |
- Mats (have near-isotropic) properties and are made of randomly oriented strands held together by a binder (usually glass) - Woven fabrics - made with conventional weaving looms in a number of patterns Page 3-37 |
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What is crimping? |
Crimping is a bending of fibres due to them being placed in a woven fabric - results in a loss of reinforcing efficiency. I.e. the fibres need to be straightened before they can take load - Compressive strength is particularly reduced by crimping Page 3-38 |
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What forms of woven fabric are available? |
- Hybrid form (mixture of weaves) - Hybrid materials (carbon fibres and glass fibres) - Comingled - reinforcing fibres combined with thermoplastic fibres, which are later melted Page 3-38 |
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What are some dry fibre forms apart from woven fabrics? |
- Braided fabrics - more expensive than woven, but stronger. 2D braiding is used to make flat or tubular pre-forms - Non Crimp fabrics - warp-knit fabrics with fibres held in plane by a stitched or knitted thermoplastic polymer fibre. Good behaviour, since it is non-crimped - Tapes - woven or UD fabrics less than 100mm wide - 3D textile preforms: used for a complete complex shaped component |
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What properties make Carbon fibre useful? |
- High strength - High stiffness - Good at high temperatures - Fibres are very strong, because there are very few defects, and the molecules are aligned Page 3-16 |
