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This report shows the history of composites used in aircraft structure. Duralumin was once the leading material for the usage in aircraft primary structure, but was replaced due to metal fatigue causing airplane crashes. Hence, composites were introduced to replace duralumin as the suitable choice for making aircraft structures. This report will include some of the types of composites used in airplane like carbon fiber and fibreglass.
In addition, the fabrication process behind carbon fiber will be discussed here and the technical effects and the applications of these composites will be included. Composites are known for their lightweight and strength to weight ratio but they tend to break when the stress acting on them exceeded the ultimate strength of the composites.
Lastly, some of the possible composites in the future will be included in this report suh as self-repairing composites and shape memory composites are introduced in this report to overcome their disadvantage of the current composites.
List Of Illustrations iv
Method Of Investigation
Scope Of Investigation
History Of Composites 1
Composites Today 2
Carbon Fiber 2
Fabrication Of Carbon Fiber 3
Technical Effects 4
Composites In The Future 6
Shape Memory Composites
List Of Illustrations Page
List Of Figures
Figure 1 - Fabrication Of Carbon Fiber 4
Figure 2 - Boeing 787 DreamLiner Fuselage Barrel Section 5
Figure 3 - Application Of Composites In Boeing 787 Dreamliner 6
Figure 4 - Shape Memory 6
Figure 5 - Self-Repairing Skin 7
List Of Tables
Table 1 - Comparison Of Composites To Other Material 2
Table 2 - Properties From Different Cores 2
This report emphases the uses of composites in modern aircrafts and investigates the technical effects of using composites in aircrafts. This report also evaluates the future of composites in aerospace industry.
Composites are materials fabricated from two or more constituent materials with different physical or chemical properties that can remain separate and distinct on a microscopic level within the finished structure. (Wikipedia, 2010) There were reports of many aircraft crashes due to metal fatigue in the 1950s. This lead to a search of stronger and better materials then metal alloys which resulted with the introduction of composites.
Method Of Investigation
Data for the report was obtained from the internet such as reports and articles as well as books from the SP library.
Scope Of Investigation
This report would cover the history of composites, the composites used today and the composites uses in the future. In this report, the different types of composites used today, carbon fiber and ibreglass, will be introduce, with the fabrication process of making carbon fiber, the technical effects of using composites rather than metal alloys in modern aircraft and the different applications of using composites in modern aircraft. While for the future of composites, the possible types of composites will be introduce such as the shape memory composites and the self-repair composites.
History Of Composites
There are natural composites exists in the early days such as wood. The first ever man-made composite materials were combination of straw and mud to form bricks for building purpose. These can still be found in parts of Egypt. However as time pass by, the ancient bricks were improved to form reinforced cement or asphalt concrete that is used to pave roadways in our everyday life. Metal alloys is then found and is widely used in all industries. In Aerospace industry, duralumin was the primary material used for the structures of aircrafts.
Composites are known for their properties like high strength to weight ratio, high tensile strength and low thermal expansion which are important for aerospace industry is shown below in Table 1. These properties can be improved by adding a core in between the laminated composites. The core is to increase the stiffness by thickening it with a low-density core material for a very little additional weight. Table 2 below illustrates the different properties that can be obtained by using different cores.
Table 1 - Comparison Of Composites To Other Material
Table 2 - Properties From Different Cores
Carbon fiber consists of extremely thin fibers which contains at least 90% carbon in the fibers. The carbon atoms are bonded together that are aligned parallel to the long axis of the fiber. These made carbon fiber to be the suitable material for applications where strength, stiffness, lower weight are critical requirements. It is widely used on aircraft structure.
About 90% of the carbon fiber is made from polyacrylonitrile (PAN), an organic polymer with chains of carbons atom connected to one another in long chains while the remaining 10% are made from rayon or pitch-based precursor.
These strains of carbon fibers are twisted together to form a yarn that will be woven into a fabric. There are many ways to weave the fibers to provide different properties and is usually combined with a plastic resin such as epoxy, polyamide and vinyl ester to contribute strength, durability and chemical resistance. They provide high performance at elevated temperature such as in the engine or areas around the engine.These will be mould to form composite materials, Carbon Fiber Reinforced Plastic, to provide high strength-to-weight ratio material, low weight and low thermal expansion.
Fiberglass consists of extremely fine fibers of glass that is used for reinforce many product which results in composites material, Fiber-Reinforced Polymer (FRP). It is most suitable for applications where lower cost, weight, electrical insulator and corrosion resistance. It is widely used in aircraft structure.
The basis of fiberglass is silica that has the properties of retaining its mechanical properties at high temperature and does not burn.
Fabrication Of Carbon Fiber
The process to produce carbon fiber begins with stretching process where acrylonitrile plastic powder is mixed with another plastic such as methyl acrylate or methyl methacrylate. It is then reacted with a catalyst in a solution polymerization process to form a polyarcylonitrile plastic. After forming polyarcylonitrile, it undergoes spinning process where plastic mixture is heated and pumped throught tiny jets into a chamber where solvents vaporate, leaving a solid fiber. This formed the internal atomic structure of the fiber. It will be washed and stretched to align the molecules to provide the basis for the formation of the tightly bonded crystals after carbonization.
It will undergoes oxidization there the fibers are heated in air to about 200-300°C for 30 - 120 minutes where their linear atomic bonding will be converted to a more thermally stable ladder bonding. Carbonization will ensue. They will be heated again in an inert environment such as nitrogen to about 1000-3000°C for several minutes in a furnace, to prevent the fibers from burning with the present of oxygen. This makes the fibers to lose their non-carbon atoms and some carbon atoms and at the same time, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less to the long axis of the fibers. In some processes, 2 furnaces operating at 2 different temperatures are used to better control the rate of heating during carbonization.
After carbonization, the surface is slightly oxidized to provide better bonding properties.The fibers are coated to protect from damage during winding or weaving with epoxy, polyester or nylon. After coating, the firers are wounded onto cylinders which are loaded into a spinning machine and twisted into yarns of various size. Figure below shows the
Figure 1 - Fabrication Of Carbon Fiber
Composites have excellent properties such as fatigue and corrosion resistance, and high strength to weight ratio, to provide the aircrafts better load capacity and fuel efficiency compared to metal alloy.
Furthermore, it requires lower maintenance cost than the metal alloy design. The parts made from composites are manufactured as a whole, compared to the metal parts where it is manufactured in sheet metals and joined together with fasteners, to reduce the cost of maintenance by reducing the amount of parts required.
However, composites does have its weakness. For example, they are poor in electrical conductivity which are more likely to be damaged by lightning strikes. Also, composites does not show any sign before breaking. Compared to metals alloy, they do not bend when force is subjected but breaks once they fail to withstand the load.
Figure 2 - Boeing 787 DreamLiner Fuselage Barrel Section
Currently, composites are commonly used in aerospace and automotive industry. In aerospace industry, modern aircraft's primary structure are made from composites such as fuselage, wings, ailerons, flaps, slats, and empennage. For instance, composites in the latest Boeing commercial jet, Boeing 787 DreamLiner, contributes about 50% to the weight of the aircraft, including almost 100% of the fuselage skin and wings surfaces.
Composites are not only used in primary structure of an aircraft, but uses in secondary structure such as bulkheads, floor beams, doors and landing gears and the interior of the plane such as overhead storage, and cabinet. Figure below shows the amount of composites and the parts used in Boeing 787 Dreamliner.
Figure 3 - Application Of Composites In Boeing 787 Dreamliner
Composites In The Future
Shape Memory Composites
Shape memory composites are composites which retain one or 2 shapes after cold-worked. This can be used on the aerofoil structure of the aircraft that the aerodynamics can be controlled by the temperature of the air at ground level and cruising attitude.
Figure 4 - Shape Memory
Self-repairing composites comsist of a material, which includes inorganic as well as organic matrices, as well as reinforcements disposed within the matrix, hollow fibers having a selectively releasable modifying agent contained therein. (Natural, 2008)
This technology does allow the airplanes to be safe even when it is being damage during flight as internal damage is very common in composites. This works by the chemicals within the hollow fibers flows into the crack and these crack faces will be rebonded. (Natural, 2008)
Figure 5 - Self-Repairing Skin
As technology improved over time, the recognition of composites greatly improved over time too. Composites are now the leading material to use in manufacturing aircraft parts. It does have better properties than duralumin such as corrosion, fatigue resistance and light weight. This will greater increase the uses of this composites for the parts in future aircrafts. Currently the aircraft parts made from composites are proved more reliable than those made from duralumin. Even though there are some disadvantages of composites comparing to duralumin, further development on composites will be done and this will greatly improve the safety of the aircrafts.