Application of composite materials in aviation.
Key words: usage of composite materials, lifecycle of aircraft, costs, exhaust gases polution
The increase in air traffic volume has brought to increasing emissions of exhaust gases, fuel consumption and increase in transport charges. In order to stop this process it is necessary to improve the aircraft performances, increase the utilization coefficient and to prolong the lifecycle of the very aircraft.
Aircraft manufacturers have been forced to reduce the costs of development, production and maintenance of the aircraft. In order to achieve these goals, the designers were forced to substitute the classical materials by those that enable faster production, reduce mass, reduce the maintenance requirements and prolong the lifecycle of the aircraft. As a solution, high-quality composite materials have been used.
Composite materials are materials obtained by combining two or more substances which feature different properties, and in which every substance retains its chemical integrity. By rough classification, the composite materials can be divided into basic composites, layered substances (laminates and panels) and interwoven structures.
In order to compare the material properties, the basic composites are usually divided, according to the matrix, into metal, polymer and ceramic. The exception from this rule are the long-fibre composites which are usually qualified according to the substance of which they are made. The latter two groups will be discussed in more detail further in the text because they form the major part of the total volume of composites used in aircraft manufacture.
The advantages of composite materials compared to metals in the aircraft construction are: longer lifecycle due to higher material fatigue resistance, corrosion resistance, easier maintenance, higher resistance to fire, easier material processing, possibility of designing more complex shapes, and lower specific mass of the material.
The drawbacks of the composite materials are indicated in the higher unit price of their manufacture compared to steel, wood or aluminium (2-5 times).
The aircraft fuselage may be made of many different materials such as e.g. wood, fabric, aluminium and other metals, as well as composites. In the construction of today's aircraft several materials are mainly used, mostly aluminium and its alloys, titanium and composites. In selecting the material for a certain part of the structure, the designer makes the choice by selecting the lightest material with adequate properties. However, there are some restrictions here, and in the construction of aircraft, first of all for general and commercial purpose, these usually refer to finances.
Although it may seem so at the first glance, composite materials are no novelty in the construction of aircraft. Already in the 50s of the last century fibreglass was used in the construction of Boeing 707 and it occupied about 2% in the total volume of the used materials.
The first major use of composite materials in the construction of military aircraft occurred in case of aircraft F-14 Tomcat. In 1981 British Aerospace--McDonnell Douglas manufactured AV-8B Harrier in whose structure the composite materials participated with 25 percent. Almost one third of the modern military aircraft such as e.g. F-22 Raptor and Eurofighter is made of composite materials.
Due to the expensiveness of composite material manufacture, their greater use in commercial aviation came at a later date. Thus e.g. in the total volume of materials used to manufacture Boeing 777 composites account for 10%.
The increasing use of composites in the construction of aircraft has resulted also in new composite materials. Thus, in the manufacture of Airbus aircraft A-380, apart from other composites, the upper part of the fuselage will be made of Glare. Glare is aluminium reinforced by glass fibres thus reducing the mass of aluminium, and at the same time improving its mechanical properties. The unit price of manufacturing Glare is somewhat lower than the one in the production of other composites since aluminium-production plants can be extensively used in the process of manufacturing Glare. In the construction of A380 aircraft the composites will account for 16 percent in the total volume of material, whereas their use will reduce the aircraft mass by 15 tonnes compared to completely metal structure. The weight of an entirely empty A380 will be about 170 tonnes.
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Neck and neck with Airbus in the use of composites in the construction of commercial aircraft is Boeing with its 7E7.
In the construction of aircraft today we find solutions which would be almost impossible to realize were it not for the use of composite materials. As example the aircraft Gruman X-29 and Sukhoi S-27 Berkut may be mentioned. They have wings in the form of reverse arrow and in their construction from materials such as aluminium they get deformed due to high loads in single flight phases.
In 2000, research started in the field of using composite materials in the construction of the bottom part of the fuselage. Major European aircraft manufacturers such as BAE systems, Aerospitale Matra Airbus, Daimler Chrysler Aerospace Airbus and Alenia participated in the project together with numerous European research centres. Research was based on theoretical calculations as well as testing of the aircraft structure model in the laboratory. The achieved results are very promising, and a major share of composite materials in the construction of aircraft may be expected in the near future. Aircraft designed in this way will be lighter than today's aircraft of classical construction (made of metals) by about 35 percent.
In the construction of helicopters the things are moving even faster. Thus e.g. today there are helicopters that are almost completely made of composites (NH 90, Tiger Eurocopter EC 135).
Parallel with the integration of composite material application in the construction of the aircraft of today, their greater application appears also in the construction of aircraft engines. Thus, at the end of 2004, the General Electric Company started to test new Genx engines. The novelty in Genx engines consists in the fact that the same single jet engine except for the compressor blades, jet deflectors, etc. even the combustion chamber is made of composite materials. This results in the longer lifecycle of the very engine as well as reduced maintenance costs. The installation of Genx engine is planned on the aircraft Boeing 7E7 Dreamliner. The installation of Genx engine is expected to reduce the total aircraft maintenance costs by about 10 percent. Furthermore, some design innovations in the engine itself reduce fuel consumption by about 40 percent as well as the exhaust emissions.
Although, considering the development, the application of aluminium and its alloys may be traced back to the very beginnings of aviation regarding the share in the total quantity of material used for the construction of aircraft, it still plays a crucial role. However, following the development of composite materials and the improvement of their production process, their declining unit price, it is to be expected that the significance of composites will increase in the near future in the construction of aircraft. Therefore, aircraft will soon be coming out of the workshops, for the production of which the major share in the overall materials used for their construction will be composite materials.
This will consequently mean a significant reduction in fuel consumption, exhaust gases and increase in the aircraft flight range and in the times to come with energy saving playing an important role, this will be of crucial significance. The fact should also be remembered that the use of such aircraft will result in maintenance savings due to the longer lifecycles of composite materials compared to aluminium and their alloys.
K. Milos, I. Milos, D. Curepic: Composite Materials in the Construction of Transport Means, Promet-Traffic-Traffico (1), Zagreb, 2004,
P. K. Mallick and S. Newman: Composite Materials Tehnology-Proces and Properties, Carl Hanser Verlag, Munchen, 1990.
S. Rawal: Metal Matrix composites for Space Aplication, JOM 53(4) 2001,
W. F. Powers: Adwanced Materials and Processes, May 2000.
Table 1. Comparison of properties of some materials Typical properties of structural materials Material density Tensile Tensile strength modulus g/[cm.sup.3] MPa GPa Pinewood 0.5 100 12 Al alloy 2.8 350 75 Ti alloy 4.5 800 110 Steel 7.8 1100 210 GRP 2.1 750 25 CFRP made from HT fibrts with EP matrix 1.5 750 74 CFRP made from HM fibers with EP matrix 1.6 600 100 Material Shear Spec. Spec. modulus strength Young's modulus GPa (MPa)/ (GPa)/ (g/[cm.sup.3]) (g/[cm.sup.3]) Pinewood - 200 24 Al alloy 28 125 27 Ti alloy 42 178 24 Steel 81 140 27 GRP 6 360 12 CFRP made from HT fibrts with EP matrix 19 500 50 CFRP made from HM fibers with EP matrix 25 375 63 Fig. 1 Distribution of the use of single materials in the construction of aircraft Boeing 777 others 1% aluminium 70% composites 11% titanium 7% steel 11% Note: Table made from pie chart.
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|Author:||Frankovic, I.; Lovric, I.; Rados, J.|
|Publication:||Annals of DAAAM & Proceedings|
|Article Type:||Technical report|
|Date:||Jan 1, 2005|
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