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26 Cards in this Set
- Front
- Back
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What are the most important factors in determining the properties of a unidirectional continuous composite? (Gives a helpful context to isolating the effects of different factors? |
- Properties of the Fibres - Properties of the matrix - Volume fraction of the fibres (matrix) - The fibre/matrix bond strength (interfacial strength) - alignment, flaws and strength variation in the fibres - Temperature and moisture in operating environment Page 8-2 |
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What is one of the main factors limiting the widespread use of Boron fibres in aerospace applications? |
The very high cost (about 60 times more expensive than E-Glass) Page 8-10 |
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Name two of the three types of glass fibres and their characteristics |
E-Glass -> moderate strength and stiffness, low cost, non-structural applications S-Glass -> higher strength/stiffness than E-Glass, cost comparable to that of Carbon Epoxy D-Glass -> better dielectric properties than E-glass, protection against lightning strikes Page 8-11 |
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What are the key points when comparing Glass Fibre Composites to other PMC's? |
- Cheaper for non (or less) load bearing structures - Lower specific stiffness than 'A' (Aramid/epoxy), 'B' (Boron/epoxy) and 'C' (carbon/epoxy) - Better dielectric properties than other PMC's - Poor fatigue performance compared to other PMC's (accumulates load damage over time) Page 8-14, Page 8-15 |
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Are glass fibre composites suitable for use in weight critical load bearing structures? |
No, the low specific stiffness means that glass fibre composites are no good for weight critical load bearing structures. Page 8-14 |
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What makes Glass Fibre Reinforced Polymers (GFRP) susceptible to thermal fatigue? |
GFRP's have a low thermal conductivity. Therefore, when heat builds up in the material from high frequency cyclic loading, it cannot diffuse the heat quick enough to prevent damage Page 8-15 |
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List the advantages and disadvantages of GFRP |
Advantages - Low cost - Superior impact resistance - Better dielectric properties (a good insulator) Disadvantages - lowest specific stiffness of all PMC's - Poorer fatigue performance - Susceptible to stress rupture - Drop in properties due to moisture Page 8-21 |
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Briefly discuss the performance of Aramid fibre composites in tension and compression, compared to other fibre composites |
- Very high tensile strength, high enough to be used for ballistic protection in composites. - Can absorb lots of energy in impacts, because of: - High strain-to-failure - Complex failure modes absorb lots of energy - Good vibration damping characteristics - Very poor compressive strength, due to the micro-buckling of fibres. Page 8-22, Page 8-29 |
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How do aramid fibres compare to other GFRP and CFRP? |
- Elastic moduli is greater than GFRP, less then CFRP - Compressive strength is very low - Interlaminar shear strength is low - Better fatigue resistance than GFRP - Less susceptible to stress rupture than GFRP - High damping characteristics Page 8-24 |
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What are some importants points to consider when producing a PMC with an Aramid fibre? |
- Poor fibre-matrix adhesion - Difficult to cut and machine because fibres defibrillate under high compressive and shear stresses - Fibres absorb water (hygroscopic) so they need to be stored at low humidity Page 8-27 |
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What makes carbon fibre reinforced composites suitable for aerospace structural applications? |
- Best specific strength and stiffness properties - Low density compared to aramid fibres Page 8-35 |
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What are the two production methods for carbon fibres and what is a summary of their characteristics? |
- PAN based fibres have a low cost, higher strain-to-failure and good overall properties - Pitch based fibres are more expensive, but have a higher stiffness, high electrical conductivity and low coefficient of thermal expansion Page 8-36 |
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Why is it important to get 'just the right amount' of fibre-matrix adhesion for carbon fibres? |
- If the bonding is too strong, the fibres cannot disbond to alleviate local stress concentrations (eg. at microcracks) and so the stress builds up and causes a larger failure - If the bonding is too weak, it will not be able to support the fibres against micro-buckling, which is the main mode of compression failure Page 8-40 |
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How does the impact damage resistance of CFRP's compare with other composites? |
- Impact resistance of CFRP is lower than that of GFRP and Kevlar - Low velocity impacts due to dropped tools, runaway stones, etc. can cause barely visible impact damage (causes delamination, which significantly reduces the bending stiffness) Page 8-41 |
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How do CFRP's perform in fatigue? |
- Carbon fibres are generally fatigue resistant, so the STRESS - NUMBER OF CYCLES Curve is generally flat at high strain rates - Important to maintain high fibre matrix-to-stiffness ratio so matrix strains are low to avoid cracking - In the presence of BVID, the strength drops to about 50% of that for undamaged CFRPs Page 8-45 |
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How do glass fibres perform in fatigue? |
- Glass fibres lose strength under fatigue, due to stress rupture from extended time at peak values. - Low modulus of glass fibres, causes high strains (and hence cracking) the matrix, which causes further strain concentration by exposing fibres to the environment Page 8-46 |
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Is it better to have moderate or high toughness and fibre-matrix adhesion in a GFRP? |
In general, composites with moderately tough matrices and moderate fibre-matrix adhesion do better than those with high toughness and high bond strength - This is because the moderately strong combination allows damaged fibres to be isolated from the good fibres, and thus not weakening them Page 8-46 |
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What is the effect of notches (stress concentrations) on fatigue? |
Minimal, due to formation of micro-cracking and delaminations Page 8-52 |
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What is the difference in how CFRP, AFRP and GFRP perform in fatigue, with and without damage? |
- Fatigue of undamaged GFRP and AFRP is a concern - Fatigue of CFRP is only a concern if BVID is present - If CFRP's have BVID, there is an approximate 50% loss in strength, but only a small impact on fatigue life Page 8-55 |
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What common materials are the most susceptible to moisture damage? |
- Epoxies (generally used in a matrix) and Aramid fibres are vulnerable to moisture absorption - Phenolics are the least vulnerable matrix material Page 8-59 |
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What does the moisture absorbed vs. time graph generally look like? |
The moisture absorbed asymptotes to a fixed value. Note, the image is for constant T, humidity. It is graphed against the square root of time. Page 8-60 |
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What effect does matrix thickness temperature have on moisture absorption? |
- A thicker laminate will take longer for saturation (max absorption) to be achieved - The moisture absorption rate increases with temperature Page 8-63 |
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What effects does moisture absorption have at room temperature? |
- Water in fibre-resin interface can cause debonding (hence loss of strength) - Water trapped in voids will expand/shrink with temperature and cause cracking - Water acts as a plasticiser (promote plasticity and reduce brittleness) Page 8-65 |
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What effect does moisture absorption have at elevated temperatures? |
- Water reduces the glass transition temperature GTT (the point where the polymer goes soft)- A moisture content of about 1.2% can reduce the GTT by about 40-50 C - Particularly relevant for aircraft that go from a (very cold -50 C) high-altitude cruise to supersonic heating (over 100 C) in a matter of seconds Page 8-66 |
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What does the value 'R' stand for, when discussing fatigue? |
R = stress_minimum/stress_maximum |
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Should fibres have a higher or lower stiffness than their matrix? |
Fibres should have a higher stiffness, to avoid tensile loading of the matrix. |
