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This report explores the uses of composites in modern aircrafts and investigates the technical effects of applying composites to aircrafts. In addition, this report also evaluates the future usage of composites in aerospace industry.
Composites are defined as engineered materials made from two or more constituent materials with significantly different physical or chemical properties those remain separate and distinct on a macroscopic level within the finished structure. (Definition, 2010)
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The American Heritage Dictionary (Houghton Mifflin, Boston, 1981) comes close, with “a complex material, such as wood or fiber glass, in which two or more complementary substances, especially metals, ceramics, glasses, and polymers, combine to produce some structural or functional properties not present in any individual component.” The problem with even a good definition, of course, is that it is all encompassing, so that every material in the universe could in some sense be defined as a composite. (Composites Manufacturing, 2006)
Composites are first introduced after crash cases of aircrafts due to metal fatigue in the 1950s. And since then the use of composites in aircrafts increases exponentially over time.
Method of Investigation
Information is obtained online in the form of e-books, news articles and industrial reports. Furthermore, relevant book from SP library had been used in this report.
Scope of Investigation
This report explores the history of composites. Besides, this report also explores the types of composites used in aircraft structure, for example carbon fiber, fiberglass and Kevlarâ„¢ aramid, as well as fabrication of the carbon fiber. Furthermore, this report discusses the technical effects and applications of composites in aircraft structure. Composites in the future such as self-repairing composites and shape memory composites are also discussed in this report.
History of Composites
Composites were introduced in the 1950s and 1960s, fiber-reinforced plastic, was introduced to substitute the usage of duralumin in the structure of aircrafts. Before composites were introduced, primary structure of aircrafts was made largely of aluminum alloy.
Duralumin, a type of aluminum alloy, was widely used in aircrafts primary structure due to its high strength to weight ratio and good toughness. Furthermore, good corrosion resistance of duralumin makes it so suitable to be used under harsh condition. However, in 1950s, a fully loaded De Havilland Comet jet airliner crashed due to metal fatigue. This gave rise to demand of materials suitable for aircrafts primary structure with high fatigue resistance.
English scientist, Norman de Bruline, was the first to propose the use of composite materials in aircrafts structures.
Then, more and more composites were introduced to suit certain requirement of properties of aircrafts structure. Modern fibers, such as carbon-reinforced fibers, Kevlarâ„¢ aramid, glass-reinforced fibers and boron fibers, are the stiffest and strongest materials known. (Composites Manufacturing, 2006)
Composites are well known for their favorable properties such as high strength to weight ratio, high stiffness, very high stress level, high fatigue resistance and good corrosion resistance. Some of the widely used composites in aircraft structures will be discussed below.
In common, all composites are made of relatively weak bonding of lamination of materials. Hence, core in composite laminate is introduced to increase the laminate’s stiffness by effectively ‘thickening’ it with a low-density core material. This can provide a dramatic increase in stiffness for very little additional weight, where lightweight implication is so crucial for aircraft structure.
For example, application of honeycomb core provides stronger impact resistance to composite laminates. With its hollow cells, it provides good energy absorption and sound dampening effect to the laminates.
There are many types of composites currently used in aircraft structure. However, in this part only carbon fiber, fiberglass and Kevlarâ„¢ aramid will be discussed.
Carbon or graphite fiber is basically fiber that contains more than 90% of carbon. Carbon fibers are ideal for lightweight reinforcement, as well as high strength, high stiffness application in aircrafts structure, due to its unique combinations of properties. High-performance carbon fibers are available in a range of properties, product forms, and prices. Most of the Carbon Fibers are made from organic polymers, with chains of carbons connected to each other, such as polyacrylonitrile (PAN). (Composites Manufacturing, 2006)
Fiberglass is a reinforcing material that is widely used in aircraft structure. More than 70% of the reinforcement of thermosetting resins is made of fiberglass. In general, the longer the fibers, the greater the strength of the fibers is. And continuous fibers are the strongest amongst all.
Glass does not burn, and at high temperature glass retains its mechanical properties, up to 50 per cent of its strength at 700°F. Furthermore, glass has excellent moisture resistance compared to metals. Thus, it is suitable for application on aircraft structures.
Lime-alumina-borosilicate glass, or E glass, was the first glass developed specifically for continuous fibers production. It is designed for electrical applications, with its good adaptability and high effectiveness in processes and products, ranging from decorative to structural applications in aircrafts. (Composites Manufacturing, 2006)
Kevlarâ„¢ aramid, introduced commercially in the 1970s, is an aromatic long-chain polyamide polymer produced by spinning using standard textile techniques. It was introduced to replace steel in racing tires.
Kevlarâ„¢ aramid is well known for its high tensile strength, low density and good impact resistance with about half the stiffness of graphite structure. These properties offer a better choice for materials used in aircraft structures. For example, Kevlarâ„¢ 49 aramid fiber is widely used as reinforcement for plastic composites in aerospace, marine, automotive, and other industrial applications. (Composites Manufacturing, 2006)
In general, the reinforcing and matrix materials are combined, compacted and processed to undergo a melding event. In this part, fabrication of carbon fiber will be discussed in details. Figure 1 below illustrates the brief process of fabrication of carbon fiber, from its precursor form to the fiber.
Figure 1 – Fabrication Of Carbon Fiber (Flow Chart, 2010)
Firstly, acrylonitrile, CH2CHCN, a chemical compound derived from propylene and ammonia, is mixed with another plastic in powder form, and is then reacted with a catalyst in a solution polymerization process. In results, a polymer is formed. After that, the polymer is put under spinning process to form the internal atomic structure of the fiber. This process is done by heating up the mixture and pump through tiny jets into a chamber where the solvents evaporate. The polymer is then washed and stretched to align the molecules within the fiber. This complete process is called stretching. The final product after this process is known as precursor.
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Next, the fibers will undergo oxidation process. The fibers are heated in air to about 200 to 300°C for 30 to 120 minutes, to convert the linear atomic bonding to ladder bonding. This is to prepare a more thermally stable bonding of fibers.
After the fibers are stabilized, they are heated again to 1000 to 3000°C for several minutes in a furnace. This has to be done in inert environment to prevent fibers from burning with presence of oxygen gas. As the fibers are heated, they start to lose their carbon atoms as well as non-carbon atoms while the remaining carbon atoms will form a tightly bonded carbon crystals that are aligned almost parallel to the longitudinal axis of the fibers. This process is known as carbonization. In some processes, two furnaces are operating in two different temperatures to provide a better control of heating rate.
Lastly, the fibers will undergo surface treatments such as oxidation to provide better bonding properties, coating to protect fibers from external damage. (Composites Manufacturing, 2006)
Generally, composites are superior in fatigue resistance, corrosion resistance, and high strength-to-weight ratio. These properties give composite aircrafts better fuel efficiency as well as better load capacity, as compared to the metal alloy counterparts.
In addition, composites design offers lower maintenance cost compared to that of metal alloy design. Composite parts are manufactured as a whole, compared to metal parts are manufactured in the form of sheet metals and are connected by means of fasteners, the former greatly reduce maintenance cost by simplifying maintenance procedure. Figure 2 below shows the fuselage barrel section being manufactured in one piece, without the needs of fasteners.
Figure 2 – Boeing 787 DreamLiner Fuselage (Composites Material, 2004)
However, composites are not as almighty as they have been told. For instance, most composite materials are poor in electric conductivity and such, causing them to be more susceptible to damage due to lightning strikes. Compared to metal alloy airframes, composite counterparts require extra procedure in order to take care of the electric charge from the lightning as well as static charge from friction when flying.
Furthermore, unlike metals, composites break without sign or physical warning. Compared to aluminum alloy, composites do not bend when subjected to impact but break once they fail to withstand the load.
Composites are currently used widely in aerospace industry. Primary structures, for example fuselage, wings, ailerons, flaps, empennage, are made of composites in modern aircrafts. For example, composites contributed 50 per cent to the weight of Boeing 787 DreamLiner, whilst almost 100 per cent of the fuselage skin and wings surface are composites. Figure 3 below illustrates the usage of composites in Boeing 787 as compared to that in Boeing 777, has increased significantly.
Figure 3 – Goodbye Metal Planes (Goodbye Metal Plane, 2005)
Secondary structures such as pressure bulkheads, floor beams, and landing gear door are also made of composites. Furthermore, composites are also used in interior.
Composites In The Future
Self-repairing composites are composites that are able to repair cracks on the aircrafts as they happen. Figure 4 below illustrates the fundamental of self-repairing composites. By designing a network of glass rods filled with resin, it is possible to stop the crack from propagating and repair the aerodynamics characteristic of the aircraft when crack happen in flight.
Figure 4 – Self Healing Artificial Skin (Self Repairing, 2007)
However, there are limitations in self-repairing composites, for example, the supply of the resin in the network. A system with vascular network has to be developed to resupply the resin so that self-repairing composites serve their purpose from time to time.
Shape Memory Composites
Shape memory composites are composites that retain one or two shapes after being cold-worked. Figure 5 illustrates how shape memory composites retain shapes after being subjected to heat.
Figure 5 – Shape Memory (Shape Memory, 2007)
This is highly useful in aircrafts design as it means the aerodynamics of the aircrafts can now be controlled according to whether maneuverability or stability is needed.
Composites are currently used as a better material used in aircraft primary structures, substituting aluminum alloy that had been used for decades. Due to composites superiority in corrosion resistance, fatigue resistance, and weight saving, it is expected that incoming models of aircrafts will use composites extensively. Application of composites in modern aircrafts once again proves that composites are more reliable than metal alloys in airworthiness. However, further development of composites should be done to overcome weaknesses found currently to further improve the safety of aircrafts.
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