Lecture sections that are some 20 % lighter
Lecture based on Materials (Book: The science of strong materials, Penguin Publishers. ) The aeronautical journal (Library can order for you), one article written in 1996, by Peal, I. J. Mocolm. Ceramic Science, pg 123-131. (Relevant to exam) There are two key reasons for using thermoplastic composites: * save time & handling costs in production * eliminate labour-intensive riveting & bonding * can produce large profile sections that are some 20 % lighter than metal & alloys.
* materials are exceptionally stiff and can tolerate vibrationsA380 (largest passenger plane) – 25 % of the structure & components are composites. And in the next generation of aircraft, the content of composites will double. A350 is expected to take off for the first time in 2011 with over 50 % of its total weight made up of fibre-reinforced composite materials. The Dreamliner, Boeing 787 – 50% of the components are produced from composite materials.
Airbus itself manufacture ribs, stiffeners and other elements from the PPS composites. The individual components are welded together to form a strong, inseparable unit.This process eliminates the need for costly drilling & riveting operations & achieves higher strength and safety – reducing weight & saves time and money. Components made from PPS composites remain hard, impact-resistant, stiff & stable, even when exposed to high temperatures. PPS is also resistant to aviation fuel, engine and hydraulic oils, solvents and antifreezes. A very precise welding method in which the components are only welded at the points where it is necessary, meeting very high safety standards.
Wings One of the most important part is the wing, they have to be RIGID to help lift.If they are not rigid they will bend increasingly across the wing. The aeroplane moving forward causes the air to move over & under the wing, giving a lift, the lift is greatest at the tips of the wings. As aircraft starts to move, the wing bends upwards and one of the most important aspects of wing materials is that it should resist the bending.
(Resistance to bending) Youngs Modulus E (measured in Giga Nutons per square meter) Any material used to build a wing should have the highest E value. The highest value known is 400 (diamond) but we cannot use this as its too expensive and not realistic.However a few years ago the Russians perfected a method of making diamond at a low temperature of 600-800 degrees centigrade (usually you need 3000 degrees & massive pressure). So perhaps in the future we may get a diamond coating as it may become cheap in the future as it will make it safe and stable. Recently the Americans have done this on several wings so it is possible.
Metals Are very poor, have average E value of 40 so not very good. You need strength, Tensile (pulling/tearing strength). There are 3 common ways of defining strength: Tensile (pulling/tearing strength)Sheer (pulling one way & pushing the other way) Compressive (squeezing a material till it breaks). | Compressive strength value is higher than sheering & tensile. Tensile strength should be as high as possible as its carrying the load (250 tonnes).
Also wings need a high Fatigue Resistance, and nowhere on the wing do we allow more than 20 mega nutons per sq meter. Until recently wings were made from metals but the regular movement of the wing metals causes the material to lose fatigue strength. So we need to ask how long it takes before it loses the strength?Usually this is very short time in metals. We didn’t know that until after the 1970’s after air accidents were examined.
The job of an aeroplane is to carry as much as possible for as long as it can. The lighter the aircraft, the less mass we need to make it to have the high strengths, the lighter the better – Low Density. So the key properties of materials we need to focus on are Young’s Modulus, Tensile Strength, Fatigue Resistance and Low Density. Testing aircrafts, so you need materials that allow easy and accurate non-destructive testing.
If you find a problem, you want to simply repair it rather than replace the whole part. Cost of materials dominates everything and there reduced the rate of progress and adds limitations. 50 years ago we could have had better aircrafts if cost was not an issue. Fuselarge Is there any difference? We don’t want the fuselarge to bend, high strength, high fatigue resistance.
The fuselarge is a large tube but it is just as bad, because when the aircraft is up in the sky, the inside of the fuselarge has high pressure and the outside has low pressure, this causes the fuselarge to expand as the aircraft changes altitude.However the top part of the tube expands different to the bottom half as the bottom is full of all the baggage, the main mass is below so it does not breathe as much as the top part, and therefore the top is in greater danger of having fatigue and failing. The bottom is made from different material to the top part, then it is joined together as the top half needs to be very resistant to fatigue.
Using two types of alloys is very expensive. We still want low density materials, as light as possible, easy & accurate non-destructive testing, easy to repair, cheap and also one of the dominant property is the ‘Resistance to Fire’.According to international standards you must evacuate aircrafts within 90 seconds but if you pour kerosene on the fuselarge and set fire to it (which is what happens in a crash), it will burn through within 30-40 seconds. The fuselarge is not capable of resisting flames for more than 30-40 seconds! So the international standards is well beyond what the materials will do. Ideally we want materials to be more than 1 minute and 30 seconds, which is what is beginning to happen now. Engine Need to be light, strong, non fatigue, easily tested, however it needs materials that resist high temperature.The engine is the critical thing, at the moment the hottest part on the jet engines is 1350 degrees centigrade, at that temperature its not efficient.
The only way is to get that temp up, the current engines RolesRoyce is making is 1850 degrees, there is no other metal other than tungsten that can resist high temp. So the next generation of engines need this problem solved. However the density for the engine is too high, heavy. So essentially engineers are looking for materials that can resist with low density.
Fittings The fittings have to be as light as possible. FuelThere is not enough oil in the world to provide kerosene or what they call jet A fuel, so it is a big problem and we need a new fuel. The American air force have already solved that for themselves by introducing hydrogen fuel for their aircrafts, and at the moment there is not a better replacement for it. There will probably be a shortage a fuel and the prices will raise. So this would mean commercial aircrafts will eventually adopt hydrogen fuel too.
Now cost seems to have dominated the thinking behind airline industry, claiming they cannot afford the materials suggested.However 40 years ago they could have built the same aircrafts that are being built today. How much does it cost to run an aeroplane for the lifetime on the fuel it will use? e. g. Boeing 747, what is the weight of the journey from London to New York? We look at the half way point, which is 285 tonnes.
(285,000 kg). When it takes off its over 100 tonnes heavier. The average cruising use of fuel is 11 tonnes per hour (11,000 kg per hour), so every hour the aeroplane is 11 tonnes lighter. How long would a reputable airline last? 20 Years at 4000 hours per year So how much fuel does it cost? 1000 KG X 4000 HOURS X 20 YEARS = 880 million kg How much does 1KG of JetA fuel cost? 60p per/kg Total Cost = ? 528m (880m x 0. 60) So one aeroplane cost ? 528m in its lifetime. So what is the cost to carry one kg? 528m X 10,000000 / 285kg X 10,000 = ? 1850 Typical alloys to make the 747 * Aluminium Alloy = ? 7 per kg * Titanium alloy = ? 56 per kg * Carbon Fibre = ? 90 per kg So although some materials are expensive, compared to the fuel cost (? 1850 per kg) it is not expensive and the materials should be incorporated in to the aircraft.
The only aircraft that has much 90% titanium & aluminium is McDonald Douglas F15 Fighter, around the engines & wings. In 1960, Rolls Royce jet engines were made from: * 60% steel * 15% nickel * 5% titanium By 1990: * Steel reduced to less than 20% * Aluminium less than 5% * Nickel increased 50% * Titanium increased * Resin-based composites (plastic with carbon fibre) started increasing * Ceramic Matrix composites (stones & glass, carbon mixed with clay based materials) started increasing By 2010: 35% of all components should be made of ceramic glass clay materials reinforced with fibres. You can make an engine out of Glass nowadays, by melting and pouring the material in to a mould. However if you include Carbon Fibre and heat treat it, then it turns it into a glass ceramic and it will withstand temperature of about 2000 degrees with enormous strength. This is probably how they will make wings in the future. (Ceramic means glass and stone etc) Super Alloy – an alloy of Aluminium & Titanium used on the best fighter aircrafts costs ? 600k per tonne Glass Ceramic would cost ? 000 per tonne Materials being researched for Wings, Engine Fan Blades, Fuselarge: * Aluminium Nitrite (a hard stone material) * Silicon Carbide (occurs naturally, for example Africa) * Silicon Nitrite (silicon is sand and nitrogen is air) if you mix that with plastic it becomes so rigid, it doesn’t burn. * Carbon (available everywhere) * Aluminium Oxide (available everywhere) In a jet engine, there are about 20 blades, so to be efficient the blades have to almost touching the edge.
Normally if you warm materials up they expand, so the blades would get stuck.However you cannot have any gap as it would be inefficient. So to solve the problem, you can have materials with no thermal expansion and stay the same shape, such as carbon, glass, silicon nitrite and aluminium oxide. So the next generation aeroplanes will probably be made from the above materials in the future. The new European Fighter has now a majority of carbon fibre body and wings. Metals & Alloys * Cost Medium to High * There are 72 pure Metals and ? million Alloys * ALL LOW Modulus (E) – Low * Strength – Low * Fatigue – Poor * Density – HighCeramics & Glass * Cost Low, unless you want special shapes, e. g.
very long fibres * ALL HIGH Modulus (E) – High * Strength – High * Fatigue – Excellent * DONT BURN * The reason why this was not adopted years ago as engineers classify them as BRITTLE (smash). Plastic & Polymers * Cost Very High * Modulus (E) – Low (between 0-4 , we should be looking for 200) * ALL LOW Strength – Low * Fatigue – Poor * Density – low (not heavy) Composite Materials * Cost Very High * Modulus (E) – Low (between 0-4 , we should be looking for 200) * ALL LOWStrength – Low * Fatigue – Poor * Density – low (not heavy) The Brittleness problem was overcome by the following: Wouldn’t it be good if we could fill in an equation and theoretically predict how strong something should be, not what it is. So Dr I. J.
Mocolm researched and came up with the Theoretical Strength Equation. DHsub = Heat Required to make 1g of material boil E – Young’s ModulusM= Molecular Weight = Density Theoretical Strength Equation Therefore, according to the above equation, if you want high strength then M should be a smaller number.This means, the future of engineering material is the ones with the lowest molecular weight. E. g.
Carbon has got 12, hence the reason why it is becoming popular. To get a high value for E, it has to be a covalent material, with a high boiling point and low molecular weight. All known Metals All Ceramics However brittle & difficult to shape them So in the future aeroplanes will be made from materials like: * Boron * Carbon * Nitrogen * Boron Nitrite * Silicon Nitrite * Aluminium Carbide The only way to sort this problem was to use composites.
A composite is not a mixture, its more than one material. If you mixed 2, it will simply be half of each. But in a composite, 2+2=6. You need a specials shape for one of the material. You need an interface between the Reinforcement and Matrix.
Reinforcement has to have the special shape (like the woman), very long, very thin fibres MATRIX REINFORCEMENT INTERFACE Matrix has to be soft, yielding, protecting (like the man) However only some combinations work, lots of materials cannot form an interface. However any plastic can form the interface.Glass Fibre will form an interface with any metal, Carbon Fibre with 3. interface makes 2 + 2 = 6 and removes the brittleness so if you want to make a composite, they should be long and thin fibres. If you have something very long and thing, the chances of it being flawed is less. The fibres are spun into millions of fibres and then shaped, then put into a press and after that it is solid.
So aeroplane parts can be made from this, and you can make the fuselarge in one go as the composite does not suffer from fatigue, the interface will remove that.Silica glass and carbon fibre, as it is much better than current material in the Boeings carbon fibre of plastic, because the matrix plastic is destroyed as the matrix glass. You got to make a second break through as brittleness is still important. If you want to stop a fibre from coming out of the matrix, then you need to make the fibres short and fat. If you did this to carbon fibre then the strength would drop but it would not snap. Short and fat fibres are tough, they bring anti brittleness.
Ideally you want 90% of the fibres long and thin, but 10% into short and fat. If you mix that you get super stretch and strong.