Composite Material In The Aviation Industry

Published Date: 02 Nov 2017

Trevor Frounfelter

Professor Canipe

English Comp. II

4 February 2014

Composite Material in the Aviation Industry

Composite have been used in airplanes since the Wright Brother’s started their quest for flight back in 1903. We use composites every day; however, we probably do not think about it or do not know exactly what they are. Composites are in the showers and bath tubs as well as the circuit boards that run our computers and TV’s. Many of the inventions of the past 60 years in plastics have been spurred by war. Initially it was World War II and the use of nylon in parachutes caused an increase in development in plastics. The natural sources that provide us with rubber were in such a high demand that we needed to come up with another source. In more recent years the Cold War between Russia caused the government to develop new planes that could reach Russia without the need for refueling that could not be detected by surface to air missiles. This has lead to the development of the stealth bombers and fighters. The use of composite materials provides these planes with an ability to scatter the majority of the radar that is sent from ground stations. The new F22 Raptor is one of the latest developments of the military that is on the leading edge of composite technology. As the needs and demands increase for more efficient, lighter, stronger and faster aircraft, the need for composite materials in many aspects of the aircraft increases in both the military and civil aircraft.

Composite material systems and structures

Composites combine the properties of two or more materials (constituents). Any two materials, such as metals, ceramics, polymers, elastomers, and glasses, can be combined to make a composite. They may be mixed in many geometries (particulate, chopped-fiber, woven, unidirectional fibrous, and laminate composites) to create a system with a property profile not offered by any monolithic material. In mechanical design, this is often done to improve the stiffness-to-weight ratio, strength-to-weight ratio, or toughness, while in thermomechanical design, it is to reduce thermal expansion, maximize heat transfer, or minimize thermal distortion. Composites have gained popularity in high-performance products that need to be lightweight yet strong enough to take high loads, such as aerospace structures, space launchers, satellites, and racing cars. Their growing use has arisen from their high specific strength and stiffness when compared to metals, and the ability to shape and tailor their structure to produce more aerodynamically efficient configurations.

Fiber-reinforced polymers, especially carbon-fiber-reinforced plastics (CFRP), can and will in the near future contribute more than 50% of the structural mass of an aircraft. The main advantages provided by CFRP include mass and part reduction, complex shape manufacture, reduced scrap, improved fatigue life, design optimization, and generally improved corrosion resistance. The main challenges restricting their use are material and processing costs, damage tolerance, repair and inspection, dimensional tolerance, and conservatism associated with uncertainties about relatively new and sometimes variable materials. For secondary structures, weight savings approaching 40% are feasible by using composites, while for primary structures, such as wings and fuselages, 20% is more realistic.

Fiber-reinforced plastics

The adoption of composite materials as a major contributor to aircraft structures followed the discovery of carbon fiber in 1964. In the late 1960s, these materials began to be applied, on a demonstration basis, to military aircraft in such components as trim tabs, spoilers, rudders, and doors. With increasing application and experience with their use came improved fibers and resins resulting in composites with improved mechanical properties, allowing them to displace aluminum and titanium alloys for primary structures.

Modern carbon fibers

High-strength, high-modulus carbon fibers are about 6 micrometers in diameter and consist of small crystallites of "turbostratic" graphite, one of the allotropic forms of carbon. Refinements in fiber process technology have led to considerable improvements in tensile strength (4.5 gigapascals) and in strain to fracture (more than 2%) for PAN-based (polyacrylonitrile) fibers. These come in three basic forms: high modulus (HM, 380 GPa); intermediate modulus (IM, 290 GPa); and high strength (HS), with a modulus of around 230 GPa, tensile strength of 4.5 GPa, and strain of 2% before fracture. The selection of the appropriate fiber depends very much on the application. For military aircraft, both high modulus and high strength are desirable. Satellite applications, in contrast, benefit from the use of high fiber modulus, improving stability and stiffness for reflector dishes, antennas, and their supporting structures.

Rovings are the basic forms in which fibers are supplied, a roving being a number of strands or bundles of filaments wound into a package or reel. Rovings or tows can be woven into fabrics, and a range of fabric constructions are available commercially, such as plain weave, twills, and various satin weave styles, woven with a choice of roving or tow size depending on the weight or areal density of fabric required. Fabrics can be woven with different kinds of fiber, for example, carbon in the weft (fiber running perpendicular to the lengthwise fibers) and glass or Kevlar® in the warp (lengthwise) direction, and these options increase the range of available properties. One advantage of fabrics for reinforcing purposes is their ability to drape or conform to curved surfaces without wrinkling. It is possible, with certain types of knitting machines, to produce fiber preforms tailored to the shape of the eventual component.

Fiber-matrix interface

The fibers are surface treated during manufacture to prepare adhesion with the polymer matrix, whether thermosetting or thermoplastic. The fiber surface is roughened by chemical etching and then coated with an appropriate size to aid bonding to the specified resin. Whereas composite strength is primarily a function of fiber properties, the ability of the matrix to both support the fibers and provide out-of-plane strength is, in many situations, equally important. The aim of the material supplier is to provide a system with a balanced set of properties. While improvements in fiber and matrix properties can lead to improved lamina (single ply or layer) or laminate (layered) properties, the fiber-matrix interface is all-important. The load acting on the matrix has to be transferred to the reinforcement via the interface. Thus, fibers must be strongly bonded to the matrix if their high strength and stiffness are to be imparted to the composite. The fracture behavior is also dependent on the strength of the interface. A weak interface results in low stiffness and strength but high resistance to fracture, whereas a strong interface produces high stiffness and strength but often a brittle fracture. Conflict therefore exists, and the designer must select the most suitable material. Resistance to creep, fatigue life, and environmental degradation are also affected by the characteristics of the interface.

Matrix materials

The matrix is the weak point of the composite system and limits the fiber from exhibiting its full potential in terms of laminate properties. The matrix performs several functions, among which are stabilizing the fiber in compression, translating the fiber properties into the laminate, minimizing damage due to impact, and providing out-of-plane properties to the laminate.

Conventional epoxy aerospace resins (thermosets) are designed to cure at 120–135°C (248–275°F) or 180°C (356°F), usually in an autoclave at pressures up to 8 bar (0.8 megapascal), occasionally with a postcure at higher temperature. The resins must have a room-temperature life beyond the time it takes to lay-up (placing individual layers, or plies, on top of each other to create the laminate, or layered composite plate) a part and have time, temperature, and viscosity properties suitable for handling. The resultant resin characteristics are normally a compromise. For example, improved damage tolerance performance usually causes a reduction in hot-wet compression properties, and if this is attained by an increased thermoplastic content, then the resin viscosity can increase significantly. Increased viscosity is especially undesirable for a resin transfer molding (RTM) process.

The first generation of composites introduced to aircraft construction in the 1960s and 1970s employed brittle epoxy resins, leading to structures with a poor tolerance to low-energy impact caused by runway debris thrown up by aircraft wheels or the impacts occurring during manufacture and subsequent servicing operation. Although the newer toughened epoxies provide improvements in this respect, they are still not as damage tolerant as thermoplastic materials. Polyetheretherketone (PEEK) is a relatively costly thermoplastic with good mechanical properties. Carbon fiber/PEEK is a competitor with carbon fiber/epoxies and aluminum alloys in the aircraft industry. On impact at relatively low energies (5–10 joules), carbon fiber-PEEK laminates show only an indentation on the impact site, while in carbon/epoxy systems, ultrasonic C scans (a nondestructive inspection technique utilizing short pulses of ultrasonic energy) show that delamination (layer separation) extends a considerable distance, affecting more dramatically the strength and stiffness properties of the composite. In the effort to improve the through-the-thickness strength properties, the composites industry has moved away from brittle resins and progressed to thermoplastic resins, toughened epoxies, Z-fiber (carbon, steel, or titanium pins driven through the z-direction to improve the through-thickness properties), stitched fabrics, and stitched preforms. The focus is now on affordability, particularly affordable processing methods such as RTM processing, nonthermal electron beam curing by radiation, and cost-effective fabrication.

Design and analysis

Aircraft design since the 1940s has been based primarily on the use of aluminum alloys. With the introduction of laminated composites that exhibit anisotropic properties (properties varying with the direction of applied load) the methodology of design had to be reviewed and in many cases replaced. It is accepted that designs in composites should not merely replace the metallic alloy but should take advantage of exceptional composite properties if the most efficient designs are to evolve. Of course, the design should account for through-thickness effects that are not encountered in the analysis of isotropic materials. For instance, in a laminated structure, since the layers (laminae) are elastically connected through their faces, shear stresses are developed on the faces of each lamina. These stresses can be large near a free boundary (free edge, cut-out, or open hole) and may influence the failure of the laminate.

The lay-up geometry and stacking sequence of a composite strongly affects not only crack initiation but also crack propagation, with the result that some laminates appear highly notch sensitive, whereas others are totally insensitive to the presence of open holes. The selection of fibers and resins, the manner in which they are combined in the lay-up, and the quality of the manufactured composite must all be carefully controlled if optimum toughness is to be achieved. Compared with fracture in metals, research into the fracture behavior of composites is in its infancy. Much of the necessary theoretical framework is not yet fully developed, and there is no simple recipe for predicting with certainty the toughness of all composites.

Fracture in composite materials seldom occurs catastrophically without warning, but tends to be progressive, with substantial damage widely dispersed through the material. Tensile loading can produce matrix cracking, fiber bridging, fiber pull-out, fiber/matrix debonding, and fiber rupture, which provide extra toughness (energy sinks) and delay failure. Compression failures can occur either at the macroscale or at each individual reinforcing fiber, as occurs in compression buckling (fiber microbuckling). The fracture behavior of the composite can be reasonably well explained in terms of some summation of the contributions from these mechanisms, but as noted above, it is not yet possible to design a laminated composite to have a given toughness.

Other important modeling issues relate to shock, impact, or repeated cyclic stresses (fatigue) that cause the laminate to separate (delaminate) at the interface between two layers. In contrast to metals, in which fatigue failure generally occurs by the initiation and propagation of a single crack, the fatigue process in composites is complex and involves several damage modes, including fiber/matrix debonding, matrix cracking, delamination, and fiber fracture. By a combination of these processes, widespread damage develops throughout the bulk of the composite and leads to a permanent degradation in laminate stiffness and strength.

Although these complexities lengthen the design process, they are more than compensated for by the mass savings and improvements in aerodynamic efficiency that result. Also, the introduction of virtual manufacturing will play an enormous role in further reducing overall cost. The use of virtual reality in design prior to manufacture to identify potential problems is relatively new but has already demonstrated great potential. Virtual manufacturing validates the product definition and optimizes the product cost; it reduces rework and improves learning.

Manufacturing techniques

A number of techniques have been developed for the accurate placement of the material, ranging from labor-intensive hand lay-up techniques to those requiring high capital investments in automatic tape-laying and fiber-placement machines (Fig. 1). Once the component is laid-up on, the mold is enclosed in a flexible bag tailored to the desired shape, and the assembly is enclosed, usually in an autoclave and fitted with a means of raising the internal temperature to that required to cure the resin. The flexible bag is first evacuated, thereby removing trapped air and organic vapors from the composite, after which the chamber is pressurized (to 15 bar or 1.5 MPa) to provide additional consolidation during cure. The process produces structures of low porosity (less than 1%) and high mechanical integrity.

Fig. 1  V22-Osprey tilt-rotor plane. Fiber-placement technologies are being used to build this aircraft.

Alternatively, inexpensive nonautoclave processing methods can be used, such as vacuum molding, resin transfer molding (RTM), vacuum-assisted RTM, and resin film infusion (RFI). Vacuum molding makes use of atmospheric pressure to consolidate the material while curing (at 60–120°C or 140–248°F), thereby obviating the need for an autoclave or a hydraulic press. Vacuum-assisted RTM, a liquid-resin infusion process, is considered by the aircraft industry to be the favored low-cost manufacturing process for the future. Further cost reduction can be achieved by reducing the assembly cost, by moving away from fastening (drilling of thousands of holes followed by fastener insertion and sealing) toward bonding. Of course, certification challenges must be addressed with an adhesively bonded joint for a primary aircraft structure application.

Applications

Composites have gained popularity, especially in high-performance products that need to be lightweight but strong enough to sustain harsh loading conditions, such as aircraft components (tails, wings, fuselages), ship hulls, bicycle frames, and racing car bodies. Other uses include fishing rods and sports equipment such as tennis rackets and golf clubs. Carbon composite is a key material in launch vehicles and spacecraft. It is widely used in satellite applications for reflector dishes, antennas, and their supporting structures.

Current civil aircraft applications have concentrated on replacing the secondary structure with fiber-reinforced epoxies whose reinforcement media have been carbon, glass, Kevlar, or hybrids of these materials. Typical examples of the extensive application of composites in this manner are the Boeing 757, 767, and 777 and the Airbus A310, A320, A330, and A340 airliners. The A310 carries a vertical stabilizer 8.3 m (27 ft) high by 7.8 m (26 ft) wide at the base, fabricated in its entirety from carbon composite with a total weight saving of almost 400 kg (880 lb) when compared with the aluminum unit previously used. The A320 has extended the use of composites to the horizontal stabilizer in addition to secondary control surfaces leading to a weight saving of 800 kg (1760 lb) over aluminum alloy skin construction. It has been estimated that 1 kg (2.2 lb) weight reduction saves over 2900 liters (766 gal) of fuel per year. Larger amounts of composites are used in the larger A330 and A340 models, and in the A380 super jumbo jet.

The Boeing 787 Dreamliner structure is made of over 50% composites (80% by volume). The all-composite fuselage makes it the first composite airliner in production. Each fuselage barrel is manufactured in one piece about 13.5 m (45 ft) long, eliminating the need of more than 50,000 fasteners used in conventional aircraft building. However, there are major assembly issues with the composite fuselage sections and there are problems with what is coming out of the autoclave. There is also concern about electromagnetic hazards such as lightning strikes, since the material does not conduct away electric energy.

Composites are also extensively applied in the A400M military cargo plane and the tail of the C17 strategic airlifter. Agile fighter aircraft currently being designed or built in the United States and Europe contain roughly 40% of composites in their structural mass, covering 70% of the surface area of the aircraft. The essential agility of the aircraft would be lost if this amount of composite material were not used because of the consequential mass increase.

Advanced composite materials include some of the lightest, strongest, stiffest, and most corrosion-resistant materials available to the engineering community. Lightweight, high-strength, high-stiffness structures fabricated with advanced polymeric composite materials may result in a weight reduction as great as 50%. In addition, there is a striking contrast of the magnitudes of the stiffness-to-weight ratio or the strength-to-weight ratio of the commercial metals relative to those of the advanced composite materials (see illustration). While the specific stiffness of aluminum can be increased threefold by the addition of silicon carbide fibers to create a metal matrix composite, the specific stiffness of a graphite-epoxy, fiber-reinforced, polymeric material can be over four times greater than the specific strength of steel. The increased strength of composites compared to metals, however, comes at a high cost because the raw materials used in composites are expensive.

Polymer composites have become prevalent in automotive and aircraft design and manufacture. For example, newer commercial aircraft can have up to 80% of their structure made of polymer composites, including the whole fuselage. The automotive industry introduced composites in their body panel design in the early 1950s. Today, many automobiles not only have body panels that are compression- or injection-molded out of fiber-reinforced polymers, but also polymer-composite structural components as well as many under-the-hood applications such as valve covers and oil pans. Other applications of polymer composites include filament-wound pressure vessels and pipes for the chemical industry, braided fiber–reinforced epoxy tubes used in the construction of composite bicycles, and resin transfer–molded boats, skies, and snowboards, to name a few.

Self-healing polymers

Materials that have the ability to heal automatically have been a dream of engineers and scientists for a long time. After long periods of use cracks will form and propagate in materials, eventually leading to mechanical failures, some of which (such as airplane crashes) could be catastrophic. In addition, micro-cracks are always hidden deep within materials. For decades, scientists worldwide have been looking for materials that can heal themselves after crack formation, especially polymers.

When separate molecules or polymer chains are connected by covalent bonds, they are called cross-linked. Highly cross-linked polymers have been widely studied and used as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, and so on. Their densely cross-linked structure provides superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, highly cross-linked polymers are susceptible to irreversible damage by high stresses due to the formation and propagation of cracks, which may lead to a dangerous loss in their load-carrying capacity.

There has been an intense search to find methods to heal cracks of linear polymers, also known as thermoplastics. Hotplate welding and crack healing of thermoplastics is well established, where intermolecular noncovalent interactions (chain entanglements) at the crack interface are responsible for mending. Small-molecule-induced crack healing has been studied for thermoplastics; in this mechanism low-molecular-weight solvents (such as ethanol) are supplied to the interfaces, followed by heating to heal the cracks.

Research for re-mending and self-healing of cross-linked polymeric materials has been increasingly exciting in recent years. However, because it is impossible to cause chain entanglement in highly cross-linked polymers, scientists did not obtain significant results until two new design concepts for polymeric materials recently were revealed. In 2001, a method was introduced for making polymer composites embedded with encapsulated "healing" chemical reagents. And in 2002, a concept was introduced for a re-mendable polymeric matrix that can be healed multiple times by simple thermal treatment.

Self-healing polymer composites

With some specific additional ingredients, self-healing polymer composites can be prepared. To the best of our knowledge, the concept of self-repair was first introduced to heal cracks in composites by embedding in the composites hollow fibers that release repair chemicals when a crack propagates. No significant progress was made for this type of composite until 2001, when S. R. White and coworkers reported a polymeric composite with an epoxy resin matrix that contained a catalyst and encapsulated add-monomer (healing agent).

Figure 1 shows the autonomic healing concept. Dicyclopentadiene (DCPD) is microencapsulated as the healing agent, and a catalytic chemical trigger, called Grubbs' (ruthenium-based) catalyst, is embedded within the epoxy matrix. A propagating crack will rupture the embedded microcapsules, releasing the healing agent into the crack plane through capillary action. Polymerization of the dicyclopentadiene is then triggered by contact with the embedded catalyst to bond the crack faces. This damage-induced triggering mechanism provides site-specific repair in polymer composites, with a reported typical healing efficiency of 75% and an average healing efficiency of 60%.

Fig. 1  Self-healing composite concept. Microcapsules containing DCPD (the healing agent) are embedded in an epoxy resin matrix containing a catalyst capable of polymerizing the healing agent. (a) A crack forms in the matrix wherever damage occurs. (b) The crack ruptures the microcapsules to release the healing agent into the crack plane through capillary action. (c) The healing agent contacts the catalyst to trigger polymerization that bonds the crack faces closed. (Reproduced with permission from Nature, 409:794–797; copyright 2001 Nature Publishing Group. http://www.nature.com)

While autonomic healing was clearly established by White and coworkers, they are currently addressing a few remaining problems with this approach, such as crack-healing kinetics and the stability of the catalyst to environmental conditions.

Multi-re-mendable polymer matrices

Polymers, especially highly cross-linked polymers, are widely used as matrices for the majority of modern structural composite materials. Carbon and glass fibers and other fillers are embedded in the polymer to obtain better mechanical properties. In all these composite materials, the polymer matrices are usually the weakest component and tend to produce microcracks after long-term use. A thermally re-mendable polymer matrix could provide a much longer service time and dramatically increased reliability.

During the past 20 years, thermally reversible reactions have been pioneered and studied for linear and cross-linked polymers, particularly the Diels-Alder reaction—the cycloaddition (unsaturated molecules combine to form a ring) of a diene and an alkene (dienophile). In 2002, X. Chen and coworkers reported a macromolecular network formed in its entirety by reversible cross-linking covalent bonds. The polymer exhibits multiple cycles of autogenous crack mending with simple and uncatalyzed thermal treatments and without additional ingredients. A special feature of the material is that a series of covalent bonds, which are chemically and physically the same as the original material, are formed at the interface of the mended areas. In hotplate welding and crack healing of thermoplastics, by contrast, only intermolecular, noncovalent interactions (for example, chain entanglements) are responsible for the mend and no new covalent bonds are formed between the mended parts.

As shown in the reaction below, a thermally reversible Diels-Alder cycloaddition of a multidiene (multifuran, nF) and multidienophile (multimaleimide, mM) was used to prepare a polymeric material. Monomer 1 (4F) contains four furan moieties on each molecule, and monomer 2 (3M) includes three maleimide moieties on each molecule. A highly cross-linked network (polymer 3M4F) can be formed via the Diels-Alder reaction of furan and maleimide moieties, while thermal reversibility can be accomplished by the retro Diels-Alder reaction.

The mechanical properties of 3M4F and 2M4F were found to be well within the range of widely used engineering materials such as epoxy resins and unsaturated polyesters (see table).

*Data from Instron® floor model (TT) testing system.

Upon heating, the retro Diels-Alder reaction is preferred over (nonreversible) bond-breaking degradation reactions in the polymer network. The thermal reversibility of cross-linking was studied, and it was found that the Diels-Alder connections can be disconnected at 120°C (248°F) and above. About 30% of the cross-linking bonds are observed to disconnect when the material is heated to 150°C (302°F), and upon cooling to room temperature a new cross-linked network forms.

Because the bond strength between the diene and dienophile of the Diels-Alder adduct is much weaker than all the other covalent bonds, the retro Diels-Alder reaction should be the major reason for crack propagation. In principle, when the sample is reheated and cooled, the furan and maleimide moieties connect again, and the cracks heal and fractures mend.

Using scanning electron microscopy, Chen and coworkers showed that the fracture healed almost completely to produce a homogeneous material with a few minor defects, suggesting a remarkable mending efficiency.

This mending/healing efficiency has been tested using compact tension specimens, as shown in Fig. 2. Fracture tests were used to quantify the healing efficiency. Representative load-displacement curves for a polymer specimen are plotted in Fig. 2, showing recovery of about 57% of the original fracture load. The second mending efficiency was determined to be about 45% of the original load, which strongly suggests that the material can be healed multiple times.

Fig. 2  Mending efficiency obtained by fracture-toughness testing of compact tension test specimens. The original and healed fracture toughness were determined by the propagation of the starter crack along the middle plane of the specimen at the critical load.

Because of its more flexible polymer chains, polymer 2M4F shows much better healing/mending efficiency, up to 81%. The second mending efficiency achieved was, on average, 78% of the original load, which is much higher than that of 3M4F, and clearly shows that the material can be effectively healed multiple times without any additional ingredients.

Polymers consisting of cross-linked furan-maleimide have also been developed by J. R. McElhanon and coworkers for solvent-removable epoxy resins and other applications. Although healing properties were not reported, in principle, these materials might heal cracks. Besides furan-maleimide linkage, other reversible linkages reviewed by L. P. Engle and K. B. Wagener might be used to prepare self-healing polymers. In addition, noncovalent reversible linkages, such as hydrogen bonding, may also be applied to prepare self-healing polymers.

The multi-re-mendable polymers are being used as matrices for composite structures by various research groups. Potential applications involve, but are not limited to, transparent (from visible light to microwave) structural materials, optical lenses, and plastic insulators.

 See also: Diels-Alder reaction; Epoxide; Polyether resins; Polymer; Polymer composite; Polymerization

Xiangxu Chen

Fred Wudl