Survey on Advanced Aircraft Materials - Literature review Example

Survey on Advanced Aircraft Materials 2006 This is the picture of one of the aircraft incidents. It is particularly interesting to our study due to its direct relevance to the aircraft materials failure. This is Aloha Airlines Boeing 737-297, which during its flight on April 28, 1988 underwent the accident due to the fuselage break. It’s a wonder that during this accident there were not many victims. The cause of the accidents was rapid decompression, which happened because 24,000 feet section of the fuselage tore from the airplane /Miller, 1990/. In spite of not numerous victims this air crash raised much noise and investigation. The latter showed that the causes of the accident were metal fatigue. Experts detected multiple fatigue cracks in aircraft structure /Fildey, 1990/. Numerous site fatigue damage, which resulted in structural failure, were detected in the process of investigation. The investigation, conducted by the National Transportation Safety Board explained the accident by the inability of the operators maintenance program to detect corrosion damage /Miller, 1990 /. One of the possible interpretations of the problem was the function of lap joints to bond large panels of skin to one another. These lap points used to run longitudinally along the fuselage. It was supposed that there should be no problem of fatigue cracking due to the strong connection among the overlapping panels. Similar aircraft, which underwent thorough inspection, also showed disbonding, corrosion and cracking problems in the lap joints /Miller, 1990 /. Faying surfaces were torn separately in the course of the “pillowing”, which happened in the result of corrosion processes combined with the subsequent build-up of voluminous corrosion products inside the lap joints / Komorowski, 1996/. So, the main cause of the Boeing 737 air crush was defined structural failure: fatigue. This type pf failure is sometimes called “metal fatigue”, however, this is not precise as even plastic parts can undergo cyclic loading. Cyclic loading of a piece of metal or plastic first of all results in cracks, which further grow and expand. This in its turn leads to fracture /Pizzo, 1995 /. The next problem of main aircraft materials is corrosion, which happens in steel, aluminum and titanium. Except of the gross corrosion, which is particularly common to steel equipment, there also exists another type of corrosion, which is more subtle but not less dangerous /Pizzo, 1995 /. One of the most dangerous parts of corrosion is the one occurring in mated, moving parts. This results in difficulties in moving one part past the other. Even subtle corrosion can have a great negative influence on the aircraft parts. Thus, due to the increased friction between moving parts the performance of the necessary functions will demand higher operational loads /Pizzo, 1995 /. There exists one more type of localized corrosion, which particularly often happens in high-strength aluminum alloys. It is called “exfoliation corrosion”, which was the main cause of the Aloha Flight 243 air disaster. Corrosion appeared in fastener holes in the aluminum sheets, which construct the “skin” of the fuselage. This corrosion gradually went through numerous rivet holes and along the mid-thickness of the skin in different directions. This results in the weakness of the ability of the fuselage skin to carry. This type of corrosion -exfoliation corrosion is quite difficult to determine as it happens not on the surface of the material. /Pizzo, 1995/. One possible way out to eliminate these problems is the use of the new advantaged materials, which are now being widely investigated and produced. These materials are composite materials and advanced composite materials, the advantages and weak points of which will be discussed further. This paper present an insight into the nature of the advanced composite aircraft materials, which made it possible to realize the projects of numerous designers, who could not could not actualize their plans because of the lack of the appropriate materials. I will present an overview of the new materials used for the commercial aircrafts. The greatest advantage of these materials is that they combine solidity with a light weight and open new objectives for aircraft industry. In this segment, we will take a closer look at the matrix. I will dwell with more detail on the nature, characteristics and application of advanced composite materials. Advanced composite materials are not just a dream. They are extensively used nowadays and make it possible to fulfil such projects as the "Seawind", the Airbus A-380-550, the B-2 stealth bomber, or the F-22 /Renneboog, 2005/ Composite materials can be defined as the ones, received from the combination of two or more separate materials for the sake of creation of a single construct with more desirable characteristics /Renneboog, 2005/ In aircraft, composite materials are presented as a solid matrix, composed of the tight connections of fibers, or other types of linear structures. Although the matrix material also possesses its strength and structural properties, it is intended primarily for the sake of better connection of the fibers or reinforcing structures /Renneboog, 2005/ However, in spite of the main advantages of the composite materials they still are not perfect in the sphere of aviation. The main disadvantage is that composite materials are not so easy to inspect for flaws. One more weak point is that some composites can absorb moisture. Besides, the majority of composite materials are rather expensive due to their labour-extensive nature and complex and high-cost production This is the point where composite materials yield to aluminum, which is cheap produce manufacture and easy to repair / Christensen, 1979/. Composite materials are extensively used in present-day airliners to achieve use lighter weight. For example, composite materials constitute over ten percent of the structural weight of the Boeing 777, while the F-22 consists of more than 30% of composites. Many researchers consider that soon there will be such military pains, which will include over two-thirds of composite materials. In the present day situation military sphere more often uses composites than commercial one mostly due to the different ways of maintenance / Hoskin, 1984/. Commercial aircraft still mostly prefer aluminium, which is more tolerant and can be easily repaired. This is not the case of composites, which require immediate and quite expensive repair and require special training and equipment / Noton, 1974 /. However, not all composites are the same in usage and production. Thermoplastics, which gradually replace thermosets as the matrix material for composites, are much easier to produce and are much stronger. They appear to be much more reliable in cases when a wrench dropped on a wing accidentally / Tien & Caulfield,1989 /. Alongside with the researches in the sphere of composites aviation investigates the application of other advanced materials. For example, in the 1980s some aircraft designers believed in successful application of ceramics for lightweight jet engines, partially to their greater tolerance of high temperatures. However, because of its vulnerability and difficulties in production, the usage of ceramics decreased sharply in 90s / Noton, 1974 /. In spite of all innovations, aluminum is even nowadays the most popular material in aircraft sphere and researchers constantly seek new ways of making aluminum alloys. The most popular of them is aluminum-lithium, which is nearly 10% lighter than standard aluminum. Since 1990s aluminium-lithium was extensively applied for the Space Shuttle’s large External Tank due to its quality of being lighter. In commercial aircraft this alloy was introduced much later and was not such popular partially due to its high cost and complications of usage aluminum-lithium. However, in the course of time it became apparent that aluminum-lithium will soon become one of the most popular materials as in military as in commercial aircraft / Tien & Caulfield,1989 /. However, in the course of time new and new materials were applied for the aircraft sphere. Advanced composite materials are among the most interesting and most popular “new-comers” in the sphere of aircraft materials. The best-known example of ACMs is fiberglass, which is the bound of a fine glass fibers into a sheet of polyester resin. Due to its light weight and strength made it one of the most widely-used ACMs. Glass fiber has several types, each of which can be applied in different cases provided they are compatible with the matrix material /Renneboog, 2005/ We can see that fibers are not so numerous in their scope and the ones, which are most commonly used include glass fiber, aramid, carbon, and boron. Besides, basalt fibers are also gaining their place /Renneboog, 2005/ Polymeric materials secure great versatility in the nature of the matrix materials. Matrix materials can include even metals. For example, the Airbus A-380 gradually increases its use of an ACM called “GLARE”, which is the combination of a glass fibers with aluminium /Renneboog, 2005/ The choice of a certain type of matrix materials depends on the required ability either to be formed into the necessary shape with heating (thermoplastic polymers) or on the basis of their ability to loose their formability with heating (thermosetting plastics). Thermoplastics are most commonly used in the situations, where the retention of shape and strength with the change of the temperature is not critical. These types of materials are represented by polyethylene, polystyrene, polypropylene, rubber, etc. /Renneboog, 2005/ The other type - thermosets, are characterized by the loss of the mobile nature and transformation from a liquid resin to a strong, unbending and highly cross-linked polymer. Such materials are more readily to decompose than melt after great heating followed the set /Renneboog, 2005/ The main properties of advance composite materials are actualised due to their nature. ACMs have their strong and weak points due to their structure as a combination of layers of fibers within the matrix. Thus, the fibers are weak against forces, which cut across their length. Therefore, the matrix material is intended to preserve the fibers against these forces. In order to increase the strength of the ACM in all directions different layers of fibers are placed in different orientations. So, in the process of the actual production of layers of fibers are arranged according to specific pattern, and then united with the help of the matrix material /Renneboog, 2005/ In this paper I’d like to take a closer look at a particular type of ACM – at specific matrix materials. I will briefly overview its main characteristics and practical application of thesematerials. Although the most important part o ACM is the fibre but it is actually useless if not supported by a solid matrix. On the one hand matrix is used primarily for holding the fibers securely together in order to secure the transmission of forces along the length of the fibers /Renneboog, 2005/ On the other hand the matrix serves as a protection for the fibers against the contact with outside influences. In order to resist forces affecting the ACM across the length, a matrix is to be resilient, strong, thermally and chemically stable /Renneboog, 2005/ Matrix should not come into reaction with the fiber materials or any other outside materials. It should be thermally stable to have consistent properties in spite of the temperature changes /Renneboog, 2005/ Matrix materials for advanced composite materials vary greatly in their range and characteristics. Matrix can be simple plastics or very complex ones or even include metals. For example, the Airbus A-380, which is the passenger plane, applies Glare, which is an advanced composite material consisting of glass fibers supported by an aluminum matrix /Renneboog, 2005/ The main types of matrix plastics are either “thermoplastic” or “thermosetting”. Each of these types is applied in different situations but both are simple compounds, which are polymerized and become a matrix material /Renneboog, 2005/ Thermoplastics change their properties with the change of the temperature. With the increase of the temperature thermoplastics become softer and more flexible and if the temperature further increases they can even melt to the state of the liquid. Vice versa with the decrease of the temperature, these materials become more rigid and vulnerable /Renneboog, 2005/ The “glass transition temperature” (Tg) is a certain temperature, at which thermoplastic can become either plastic or glass-like. Due to these characteristics of thermoplastics their application should be limited to certain cases. Therefore, thermoplastics in advanced composite materials can be used only in cases if the temperature changes are narrow and thus, the properties of the matrix materials under these small changes are predictable and consistent /Renneboog, 2005/ The other type of matrix materials is thermosetting ones, which are thermally stable and are created as simple liquid or semi-liquids, which are polymerized into strong materials. After the thermosetting polymers are completely set they are insensitive to temperature changes. These materials do not have a Tg and can endure considerable changes in temperature. However, if the temperature is too high the chemical bonds within the material are destroyed and the matrix material undergoes irreversible damage /Renneboog, 2005/ Polymers used for matrix materials are not only thermally stable but also dimensionally stable, which is important for the retention of the necessary shape and dimensions even with considerable environmental changes. Thermoplastic matrix materials for the most part fail to correspond to this requirement due to their property of being easily deformed with the change of the temperature /Renneboog, 2005/ The other component of advanced composite materials is fiber materials, which have many different forms from sheets of parallel fibers up to plain-woven and satin- or harness-woven fabrics. The most popular contemporary fiber materials are glass fibers, aramid fiber, carbon fiber, boron fiber and basalt fiber /Renneboog, 2005/ The main requirement to the fibers used in aviation is their strength along the length, great resistance to compressive and tensile loads and chemical and thermal stability. These requirements are met by several fiber materials: glass, aramid, carbon, boron and basalt /Renneboog, 2005/ Glass fibers are based on the glass, which is either softened with heat and than pulled into thin and flexible fibers or extruded through fine dies. Extrusion has also one advantage of being applicable for the production of fibers of infinite length, while the first method can only be used to produce fibers of limited length /Renneboog, 2005/ The other type of fiber used for the advanced composite materials is aramid, which is also known under the name “Kevlar” and which is based on terephthallic acid and para-phenylenediamine /Renneboog, 2005/ Aramid has a quite rigid structure, which is highly resistant to stretching. Due to the particular qualities of aramid fiber advanced composite structures of are able to absorb and dissipate the majority of energy resulting from the outside impact. Therefore, aramid fiber is readily used in aircraft to protect against the impact of birds and other things, which can strike aircraft /Renneboog, 2005/ One more type of fiber material is carbon fiber, manufactured from the pyrolysis of polyacrylonitrile (PAN) /Renneboog, 2005/ Carbon fiber advance composite materials found their application in aircraft and aerospace due to its easy manufacture and the capacities of the carbon. The greatest advantage of carbon fiber is its great strength and a negative coefficient of thermal expansion /Renneboog, 2005/ Not so long ago the researchers became greatly interested in boron fiber as the basis of the advanced composite materials. If compared with carbon fiber, this one has the advantage of being lighter and stiffer at one and the same time. Due to the peculiarities of production boron fibers remain quite expensive technology, which is seldom applicable to commercial passenger aircraft. Because of the characteristics of boron fiber as being both rigid and hard it is most commonly used to cover the fuselage of jet fighter aircraft and to hold the wings on /Renneboog, 2005/ Basalt fiber is still a novelty and is under the research, but is already considered to be one of the best materials for fiber in ACM. Basalt fiber materials are now used as an alternative to glass fiber. They gained much populatiry due to their peculiar characteristics of being extremely heat resistant and high chemical stability /Renneboog, 2005/ Although basalt fiber is only a developing sphere of knowledge but it can be said with confidence that its characteristics will ensure its popularity. However, with each coming year still new and new researches are taking place and new types of fiber are being discovered, which posses the qualities of being the lightest, the strongest and the cheapest in production, which will make it possible to use advanced composite materials in the sphere of commercial aircraft /Renneboog, 2005/ The other materials, which can be used in the process of production of advanced materials, are thin structures, which are both rigid and light. These are “core” materials, like balsa wood or specially manufactured “honeycomb” fillers. Such cores are supposed to be exceptionally resistant to various influences like flexing, twisting, compression and others, which can deform the panel. The main purpose of these core materials is to extend the strength properties of the matrix into the fiber /Renneboog, 2005/ So, we can say that an advanced composite structure is the one composed of core materials, fiber fabrics, and matrix materials. Each ACM structure is unique and possesses its particular qualities due to the combination of these components. Still new advanced composite materials can be created on the basis of various combinations of these components. References Christensen, R.M. (1979). Mechanics of Composite Materials. New York, John Wiley & Sons. Wildey II, J.F. (1990). Aging Aircraft, Materials Performance. Hoskin, B.C., and Baker, A.A., (1984) Composite Materials for Aircraft Structures, New York: American Institute of Aeronautics and Astronautics, Inc. Komorowski, J.P. (1996). Quantification of Corrosion in Aircraft Structures with Double Pass Retroreflection, Canadian Aeronautics and Space Journal, Vol.42, No.2. Miller, D. (1990). Corrosion Control on Aging Aircraft: What is being done ?, Materials Performance. Noton, B. R. (1974). Engineering Applications of Composites. New York, Academic Press. Pizzo, P.P. (1995). Investigation Aircraft Accident. Materials Engineering, San Jose State University Renneboog, R. M. J. (2005). Advanced Composite Materials: An Overview. Renaissance Aeronautics Associates Incorporated. Tien, J. K., and Caulfield, T. (1989). Superalloys, Supercomposites and Superceramics. New York: Academic Press. Read More