Composites in the Aerospace and Aviation Sector

In aerospace, the performance criteria applied to materials can be considerably more stringent than in other sectors, with lightweight characteristics, high strength, high stiffness and good fatigue resistance being among the leading requirements. Today, composites are widely used in the aerospace and space sectors. As confidence in FRPs (fibre-reinforced plastics) has grown, integrated composite designs have been developed and have begun to replace metal components. Aerospace and space industry's relentless drive to enhance the performance of commercial and military aircraft has continuously directed the development of increasingly improved high-performance structural materials. Composite materials are particularly attractive for aerospace and space applications due to their exceptional strength, stiffness-to-weight ratios and superior physical properties. A composite material typically consists of relatively strong, stiff fibres in a rigid resin matrix. For example, wood and bone are natural composite materials; wood consists of cellulose fibres in a lignin matrix and bone consists of hydroxyapatite particles in a collagen matrix. Synthetic composite materials used in aerospace and other industries and more commonly known are reinforced plastics made from carbon and glass fibres, both of which are hard and strong (CFRP - carbon fibre reinforced plastic and GFRP - glass fibre reinforced plastic). In very simple terms, what is being done here is combining materials with complementary properties to eliminate the weaknesses of individual component materials and obtain a composite material that possesses many or all of the benefits (high strength, stiffness, toughness and low density). CFRP and GFRP are fibre composites; another category of composite material is particulate composites. Metal matrix composites (MMC - Metal Matrix Composites), which are currently being developed and used by the aerospace and space industry, are examples of particulate composites and typically consist of non-metallic particles within a metallic matrix; for example, silicon carbide particles combined with aluminium alloy. Probably the most significant difference between fibre and particulate composites and indeed between fibre composites and conventional metallic materials relates to the directionality of properties. Particulate composites and conventional metallic materials are nominally at least isotropic, meaning their properties (strength, stiffness, etc.) are the same in all directions, whereas fibre composites are anisotropic, meaning their properties vary depending on the direction of loading relative to fibre orientation. Consider a small balsa wood plank; it is much easier to bend (and break) it along a line parallel to the fibres rather than perpendicular to them. This anisotropy is overcome by stacking layers, each typically only a millimetre thick, with fibres oriented at different angles to form a laminate. Except in very special cases, the laminate will still be anisotropic, but the variation in properties with respect to direction will be less extreme. In most aerospace applications, this approach is taken one step further and different oriented layers (ranging from very few to several hundred in number) are stacked in a specific sequence to tailor the laminate's properties to best withstand the loads it will experience. In this way, material and therefore weight savings can be achieved, a factor of paramount importance in the aerospace and space industry. Another advantage of composite materials is that, when viewed broadly, they can be formed into more complex shapes than their metallic counterparts. This not only reduces the number of parts that make up a particular component, but also reduces the need for fasteners and joints. Fasteners and joints can be weak points in a component; for example, a rivet creates a stress concentration and thus a potential crack initiation site. Therefore, fewer fasteners and joints can mean shorter assembly time. However, the shorter assembly times must be weighed against the potentially greater time required to manufacture the component in the first place. To produce a composite component, individual layers typically pre-impregnated with resin matrix are generally cut to the required shapes, all of which are likely to differ to some extent, and then stacked onto a mould (a solid or framed structure used to hold uncured layers in the required configuration before and during curing) in the specified sequence. This assembly is then subjected to a series of temperatures and pressures to 'cure' the material. The product is then inspected throughout to ensure both that dimensional tolerances have been met and that the curing process has been successful (for example, bubbles or voids may have formed in the laminate as a result of contamination of the raw materials).  

Advantages of Composite Use in Aerospace

Some of the key benefits of using composites for aerospace applications are:

• Weight reduction: savings in the range of 20-50% are typically achieved. • Assembly of complex components is straightforward using automatic fibre placement machines and rotational moulding processes. • Monocoque ('single shell') moulded structures provide higher strength at considerably lower weight. • Mechanical properties can be customised through 'lay-up' design with reinforcement fabric thickness and fabric orientation. • Thermal stability of composites means they do not expand or contract excessively with temperature changes. • High impact resistance: Kevlar (aramid) armour also protects aircraft. • High damage tolerance, increasing crash survival. • 'Galvanic' - prevents electrical corrosion problems that can occur when two dissimilar metals come into contact (particularly in damp marine environments). (Non-conductive glass fibre plays an important role here.) • Combined fatigue/corrosion problems are virtually eliminated.

Composite Use in Aircraft Design

Composites were used militarily before this technology was applied to commercial aircraft. Early military applications were in radomes and later in secondary structures and internal components. However, the modulus of glass is lower when compared to metals and therefore the transition to primary composite structures began with the development of carbon reinforcements. Today composites are widely used and this is a result of the gradual direct replacement of metal components and subsequently, as confidence in FRPs has grown, the development of integrated composite designs. Fibre composite materials were first used in small quantities in military aircraft in the 1960s and in civil aviation from the 1970s onwards. Since the 1980s composites have been used primarily for secondary wing and tail components such as wing trailing edge panels and rudders. Each new generation of aircraft developed by Boeing had an increasing percentage of composite material, with the highest proportion being 50% with the Boeing 787 Dreamliner. The main structural elements of Boeing's 787 Dreamliner are made of advanced carbon laminates, moving away more from carbon 'sandwich' composites and archaic glass fibre composites. The use of composite materials in commercial transport aircraft is attractive because reduced fuselage weight provides better fuel economy and therefore lowers operating costs. The first significant uses of composite material in commercial aircraft were in 1983 by Airbus in the rudder of the A300 and A310 and subsequently in 1985 in the vertical tail fin. In the latter case, 2,000 parts of the metal wing (excluding fasteners) were reduced to fewer than 100 for the composite wing, reducing weight and manufacturing costs. Subsequently, a CFRP faced honeycomb core was used for the A310's elevator. Following these successes, composite materials were used for the entire tail structure of the A320; these included composite fuselage hub fairings, wing/fuselage fairings, fixed leading and trailing edge lower access panels and deflectors, trailing edge flaps, radomes, stringers, winglets, wheel doors, main landing gear door doors and engine nacelles. Additionally, floor panels were manufactured from GFRP. The A380 is approximately 20-22 per cent composite by weight and also extensively utilises GLARE (glass fibre reinforced aluminium alloy) in the upper nacelle, upper fuselage shells, crown and side panels, upper forward and aft upper fuselage. The upper and lower cladding panels and forward, middle and aft pillars of the A380, aft pressure bulkhead, upper deck floor beams and stringers, stiffeners and external wings also use CFRP. The hub cladding consists of approximately 100 composite honeycomb panels. Approximately 20 per cent of the Boeing 777's weight is composite and the wing's fixed leading edge, trailing edge panels, winglets and stiffeners, stringers and externally mounted engines use composite materials. These are also used for floor beams, wing-to-fuselage fairings and landing gear doors. The use of composite materials for the tail provides approximately 1,500 lb weight savings. Looking at the Boeing 787, composite materials comprise nearly 50 per cent of the aircraft with an average 20 per cent weight saving. The excellent strength-to-weight ratio of composites is also used in helicopters to generally maximise useful loads and performance. Boeing Vertol used composites for rotorcraft transmissions in the 1950s and built the first composite rotor blades in the 1970s. Composites are used in the main structural elements of many modern helicopters, including V22 tilt-rotor aircraft, which are approximately 50 per cent composite by weight. The formability of composites has been utilised to provide particular advantages in helicopter manufacturing by reducing the number of component parts and therefore cost.

Space Applications

The successful application of composites in rockets and missiles has led to the development of primary structures for spacecraft. In fact, space applications enable the use of new materials in many respects. For example, for satellites, time schedules from concept to production can be as short as two years and typically involve short production runs, with the material element in final cost usually being relatively low. Furthermore, in many applications there is no other suitable material for technical reasons. When a spacecraft or rocket enters orbit, mechanical loads are relatively low. Environmental conditions can be extreme and severe thermal cycling can occur, in addition to effects of erosion through high vacuum and atomic oxygen or micrometeorite impacts. Glass fibre composite (GRP), for example, is used in local corner applications and in applications where heat insulation is important. The material is also used in some antenna reflectors. However, carbon fibre composite (CFRP) is most commonly associated with space applications. Its very high stiffness over a wide temperature range and excellent potential for thermal stability make CFRPs ideal. Application examples include coatings, manipulator arms, antenna reflectors, solar panel panels and optical platforms. In the past, metals were preferred due to the need for a combination of stiffness and strength with thermal and electrical conductivity, but recently composite applications for primary structures have been preferred. Continuous demand for weight reduction has made it attractive for some satellites to be constructed with predominantly composite structural subsystems.  

Data to Develop Engineering Design, Performance and Methodology

ESDU (Engineering Sciences Data Unit) Composites Series provides a collection of "Data Items" and programmes for use in the design of fibre-reinforced laminate composite materials. The information is provided primarily for use in the aerospace industry, but has wide application in other engineering disciplines where composite materials offer similar design benefits. The ESDU Composites Series contains solutions to many strength analysis problems encountered in the design of fibre-reinforced laminate composite structures. These applications include, in addition to the calculation of basic rigidities, stresses and embedded thermal stresses, fracture criteria, plate vibration and buckling, analysis of bonded joints and stress concentrations.  

Demand for Composite Technology Will Increase

Continuously rising fuel costs and environmental sensitivities necessitate continuous improvement in the performance of commercial aviation. Weight reduction is a very important factor in this equation. Beyond day-to-day operating costs, aircraft maintenance programmes can also be simplified by reducing the number of components and reducing corrosion. The competitive nature of aircraft manufacturing ensures that every opportunity to reduce operating costs wherever possible is explored and utilised. It is possible to foresee that the use of composites in the form of basalt and carbon nanotubes will accelerate and expand composite use. When it comes to aviation, we can say that the use of composite materials will continue to increase.       Sources • https://ihsmarkit.com/pdf/Composites-Aerospace-Applicationswhitepaper_264558110913046532.pdf • https://www.azom.com/article.aspx?ArticleID=8152 • https://www.thoughtco.com/composites-in-aerospace-820418 • https://compositesuk.co.uk/composite-materials/applications/aerospace • Images: pixabay.com     Compilation and Translation B. Serhat Cengiz