Throughout the last four decades the exploitation of fibre-reinforced plastics (FRP) in engineering structures has been steadily diversifying from sports equipment and high performance racing cars, to helicopters and most recently commercial aeroplanes. Composite materials are essentially a combination of two or more dissimilar materials that are used together in order to combine best properties, or impart a new set of characteristics that neither of the constituent materials could achieve on their own. Engineering composites are typically built-up from individual plies that take the form of continuous, straight fibres (eg. carbon, glass, aramid etc.) embedded in a host polymer matrix (eg. phenolic, polyester, epoxy etc.), which are laminated layer-by-layer in order to built up the final material/structure.
An example of a composite laminate (1)
Cross-Section of Composite Laminate. The individual fibres and surrounding matrix are clearly discernible (2)
In the aerospace industry the benefits of exploiting the excellent specific strength and stiffness properties (strength and stiffness per unit weight) of composites in terms of lightweight structural design are immediately apparent. Furthermore, the laminated nature of high performance composite materials enables the designer to tailor optimum mechanical properties by orientating the fibre direction with the primary load paths. As a result, the first generation of commercial aircraft that contain large proportions of composite parts, such as the Boeing 787 Dreamliner and Airbus A350 XWB, are planned to enter service in the near future. Other advantages of fibre reinforced plastics, such as the relative ease to manufacture complex shapes, and their excellent fatigue and corrosion resistance, have made FRP composites increasingly attractive in the renewable energy sector.
Composite materials have actually been around for quite a long time. As early as 3000 B.C. the ancient Egyptians embedded straw in their mud bricks in order to control shrinkage cracks and improve the tensile strength. Furthermore, papyrus based cartonage and paper maché were used to make mummy cases. In fact, manufacturing tubular shells using metals is quite difficult such that this ancient approach remains an important exploitation of composites today. Of course none of these materials would be suitable for the high performance requirements of the aerospace industry.
Mud and Straw Brick (3)
It was not until the invention of phenolic resin in 1909 that composites took-off in aircraft. The most famous example was the deHavilland Albatross transport aircraft manufactured from a ply-balsa-ply sandwich fuselage construction, which was later developed into the deHavilland Mosquito multi-role combat aircraft for WWII. The large-scale wooden construction made the Mosquito extremely light, fast and agile. Furthermore, the Mosquito was cheaper than its metallic counterparts and allowed highly skilled carpenters from all over the UK to be contracted to help with the war effort. One disadvantage of early phenolic resins was their inability to cope with hot-wet conditions such that the Mosquito became notorious for disintegrating in mid-air in the Pacific War arena.
DeHavilland Mosquito Timber Fuselage (4)
Since the development of carbon and glass fibres in the 1950’s the aerospace industry is steadily moving towards “all-composite” civil aircraft. The most common fibre and resin types used today are:
Aramid – Kevlar™
Diameter ≈ 10 mm
Diameter ≈ 8 mm
Stiffness ≈ 125GPa in tension
Strength > 3GPa due to lack of defects on small diameter fibre
Strength > 5GPa due to highly aligned planes of graphite
Strength > 3GPa because of highly aligned linear polymer chains
Stiffness ≈ 70 GPa for cheaper E-glass and 85 GPa for more expensive R- or S – Glass
Stiffness ≈ 160-700 GPa but 230-400 GPa is the usual
Much weaker and less stiff in compression as linear polymer chains come apart
Susceptible to environmental attack and fatigue
Not susceptible to degradation by chemicals and good in fatigue
Susceptible to degradation by UV light and moisture
Fibres need silane treatment to bond well to matrix
Fibres bond well with surface treatment
Fibres do not bond well at all leading to a weak fibre/matrix interface
Used in boats, wind turbine blades and other cost critical applications
Expensive material cost limits use to high performance applications were the higher mechanical properties are justified i.e. Racecars, aerospace etc.
Weak interface gives excellent energy absorption. Thus used for bullet-proof vests, helmets and impact protection on aircraft
First modern resin
Most commonly used matrix
Most common in aerospace
Tends to be brittle
Resin can be quite tough
Can be made quite tough
Wets out fibres badly
Wets out reinforcement very well
Wets out reinforcements very well
Good chemical, heat and fire resistance and don’t produce toxic gases in a fire
Poor chemical resistance and burns very easily
Good chemical resistance but will burn
Thus used in aircraft interiors
Very cheap resin used alongside glass fibres in boat hulls, wind turbine blades and other cost critical applications
Generally used in combination with carbon fibre for high performance, lightweight applications
The shift from metallic to composite construction has naturally induced a change in the design methodology of aircraft components. It has to be borne in mind that not only the mechanical properties of composites differ from those of metals, but that a whole range of physical and chemical properties are different.
– All composites have relatively low through-thickness thermal conductivities and thermal expansion coefficients in and out of plane may be widely different. Therefore thermal expansion mismatch stresses at attachment points can be a problem.
– Composites can be made with very high translucency to electromagnetic radiation eg. X-Ray.
– Electrical conductivity of composites is generally fairly low. Consequently, a copper mesh is often integrated in aerospace laminates to protect against lightning strike damage. However, this compromises a lot of the potential weight savings.
– Direct contact between carbon fibre reinforced plastics and aluminium components will corrode the aluminium over time. Therefore contact between carbon and aluminium at lug attachments and joints has to be prevented.
– All resins pick up water and their properties change as a result of this.
– Composites are not very resistant to mechanical wear effects. External surfaces may need treatment prior to painting.
– Composites tend to have relatively low stiffnesses on an absolute basis, from <10% to about 60% of steel.
– The failure modes in composites are very diverse and include fibre failure, resin failure, fibre/matrix debonding, delaminations etc., which generally increases the analytical workload. Often these failure modes are related such that it can be difficult to exactly predict the failure load.
– Composites will absorb impact energy by damage modes rather than local plastic deformation. This means failure is typically sudden and catastrophic without any prior warning that the structure has been overloaded.
– Fatigue, stress rupture and creep resistance varies from rather poor for glass FRP in wet conditions to excellent for many carbon FRP layups.
Especially due the uncertainty of correctly modelling the complicated failure modes, engineers have tended to revert to a “black” aluminium approach that has inhibited the full exploitation of composite materials in terms of potential weight savings. However, the ongoing research activities into advanced composites and increasing teaching in higher education will hope to resolve these issues in the near future.
Potter, Kevin (1996). An Introduction to Composite Products: Design, Development and Manufacture. Springer, 5th Ed. Chapman & Hall, London.
Composites and Advanced Materials
For many years, aircraft designers could propose theoretical designs that they could not build because the materials needed to construct them did not exist. (The term "unobtainium" is sometimes used to identify materials that are desired but not yet available.) For instance, large spaceplanes like the Space Shuttle would have proven extremely difficult, if not impossible, to build without heat-resistant ceramic tiles to protect them during reentry. And high-speed forward-swept-wing airplanes like Grumman's experimental X-29 or the Russian Sukhoi S-27 Berkut would not have been possible without the development of composite materials to keep their wings from bending out of shape.
Composites are the most important materials to be adapted for aviation since the use of aluminum in the 1920s. Composites are materials that are combinations of two or more organic or inorganic components. One material serves as a "matrix," which is the material that holds everything together, while the other material serves as a reinforcement, in the form of fibers embedded in the matrix. Until recently, the most common matrix materials were "thermosetting" materials such as epoxy, bismaleimide, or polyimide. The reinforcing materials can be glass fiber, boron fiber, carbon fiber, or other more exotic mixtures.
Fiberglass is the most common composite material, and consists of glass fibers embedded in a resin matrix. Fiberglass was first used widely in the 1950s for boats and automobiles, and today most cars have fiberglass bumpers covering a steel frame. Fiberglass was first used in the Boeing 707 passenger jet in the 1950s, where it comprised about two percent of the structure. By the 1960s, other composite materials became available, in particular boron fiber and graphite, embedded in epoxy resins. The U.S. Air Force and U.S. Navy began research into using these materials for aircraft control surfaces like ailerons and rudders. The first major military production use of boron fiber was for the horizontal stabilizers on the Navy's F-14 Tomcat interceptor. By 1981, the British Aerospace-McDonnell Douglas AV-8B Harrier flew with over 25 percent of its structure made of composite materials.
Making composite structures is more complex than manufacturing most metal structures. To make a composite structure, the composite material, in tape or fabric form, is laid out and put in a mold under heat and pressure. The resin matrix material flows and when the heat is removed, it solidifies. It can be formed into various shapes. In some cases, the fibers are wound tightly to increase strength. One useful feature of composites is that they can be layered, with the fibers in each layer running in a different direction. This allows materials engineers to design structures that behave in certain ways. For instance, they can design a structure that will bend in one direction, but not another. The designers of the Grumman X-29 experimental plane used this attribute of composite materials to design forward-swept wings that did not bend up at the tips like metal wings of the same shape would have bent in flight.
The greatest value of composite materials is that they can be both lightweight and strong. The heavier an aircraft weighs, the more fuel it burns, so reducing weight is important to aeronautical engineers.
Despite their strength and low weight, composites have not been a miracle solution for aircraft structures. Composites are hard to inspect for flaws. Some of them absorb moisture. Most importantly, they can be expensive, primarily because they are labor intensive and often require complex and expensive fabrication machines. Aluminum, by contrast, is easy to manufacture and repair. Anyone who has ever gotten into a minor car accident has learned that dented metal can be hammered back into shape, but a crunched fiberglass bumper has to be completely replaced. The same is true for many composite materials used in aviation.
Modern airliners use significant amounts of composites to achieve lighter weight. About ten percent of the structural weight of the Boeing 777, for instance, is composite material. Modern military aircraft, such as the F-22, use composites for at least a third of their structures, and some experts have predicted that future military aircraft will be more than two-thirds composite materials. But for now, military aircraft use substantially greater percentages of composite materials than commercial passenger aircraft primarily because of the different ways that commercial and military aircraft are maintained.
Aluminum is a very tolerant material and can take a great deal of punishment before it fails. It can be dented or punctured and still hold together. Composites are not like this. If they are damaged, they require immediate repair, which is difficult and expensive. An airplane made entirely from aluminum can be repaired almost anywhere. This is not the case for composite materials, particularly as they use different and more exotic materials. Because of this, composites will probably always be used more in military aircraft, which are constantly being maintained, than in commercial aircraft, which have to require less maintenance.
Thermoplastics are a relatively new material that is replacing thermosets as the matrix material for composites. They hold much promise for aviation applications. One of their big advantages is that they are easy to produce. They are also more durable and tougher than thermosets, particularly for light impacts, such as when a wrench dropped on a wing accidentally. The wrench could easily crack a thermoset material but would bounce off a thermoplastic composite material.
In addition to composites, other advanced materials are under development for aviation. During the 1980s, many aircraft designers became enthusiastic about ceramics, which seemed particularly promising for lightweight jet engines, because they could tolerate hotter temperatures than conventional metals. But their brittleness and difficulty to manufacture were major drawbacks, and research on ceramics for many aviation applications decreased by the 1990s.
Aluminum still remains a remarkably useful material for aircraft structures and metallurgists have worked hard to develop better aluminum alloys (a mixture of aluminum and other materials). In particular, aluminum-lithium is the most successful of these alloys. It is approximately ten percent lighter than standard aluminum. Beginning in the later 1990s it was used for the Space Shuttle's large External Tank in order to reduce weight and enable the shuttle to carry more payload. Its adoption by commercial aircraft manufacturers has been slower, however, due to the expense of lithium and the greater difficulty of using aluminum-lithium (in particular, it requires much care during welding). But it is likely that aluminum-lithium will eventually become a widely used material for both commercial and military aircraft.
--Dwayne A. Day
Sources and further reading:
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