Composites

Composites are comprised of two or more materials working together, where each constituent material retains its unique identity within the composite and contributes its own structural properties, yet upon combination the resulting material has superior properties to those of its constituents. A good example of an everyday composite material is concrete. Concrete is made with select amounts of sand, aggregate, and perhaps even glass fiber mixed with cement to bind it together. If the concrete were broken open, the individual constituents would be visible. The type and quantities of these individual constituents can also be adjusted to give the resulting concrete different properties depending on the application. This textbook is focused on composite laminates, which combine fibers and a matrix material that binds the fibers together. There are many different types of composite materials in use today. One example is fiber-reinforced plastic (FRP) composites made with short glass fibers in a polymer resin or plastic matrix. These materials are used in bath tubs, showers, pools, doors, car fenders, and a variety of construction materials including wall panels, corrugated sheet, profiles, and skylights. 

 

Fibers  • Tensile strength  • Flexural stiffness  • Somewhat brittle

Matrix • Compressive strength • lnterlaminar shear • Controls shape • Low density

Composite • Increased strength • Increased stiffness • Increased toughness • Lightweight

 

Highly loaded composite structures typically use continuous or long-fiber reinforcement that transfers load along bundles or layers (plies) of fibers arranged to run the length and width of the structure, much like the layers in a sheet of plywood. This type of composite laminate is used in the manufacture of boats, bridges, snowboards, bicycle frames, race cars, aircraft and spacecraft structures, to mention a few.

 

Advantages of Composites

Strength and stiffness. Composites typically exhibit high strength and stiffness-to-weight ratios. Composite structures can attain ratios 4 to 10 times better than those made from metals. However, lightweight structures are not automatic. Careful engineering is mandatory, and many tradeoffs are required to achieve truly lightweight structures.

Optimized structures. Fibers are oriented and layers are placed in an engineered stacking sequence to carry specific loads and achieve precise structural performance. Matrix materials are chosen to meet the service environment for which the structures are subjected. (The matrix generally determines the temperature capability of the part.)

Multi-functionality. Composite materials are being developed that not only provide lightweight, load-bearing structures, but also integrate additional functions such as structural health monitoring, thermal and/or electrical conductivity, energy harvesting/storage, impact resistance, acoustic damping, self-healing, and morphing or shape-changing.

Fatigue resistance. Composite structures do not suffer from fatigue like their metallic counterparts. High fatigue life is one reason composites are common in helicopter rotor blade construction; however, composites do exhibit some fatigue behavior, especially around fastener and pin locations. Careful design and good process controls are required to ensure long service-life of both adhesively bonded and mechanically fastened composite joints.

Corrosion issues. Composites themselves do not corrode, hence their popularity within the marine industry. This is also pertinent to chemical plants, fuel storage and piping, and other applications that must withstand chemical attack.

Part geometry. Composite materials are easily molded to shape. Composites can be formed into almost any geometry, usually quite easily and without costly tradeoffs in structural properties. Truly monocoque structures are possible with proper tooling.

Reduced parts count. Integrated composite structures often replace multi-part assemblies, dramatically reducing part and fastener count as well as procurement and manufacturing costs. Sometimes adhesively bonded or welded thermoplastic assemblies can almost completely eliminate fasteners, further reducing part count and production time.

Lower tooling costs. Many composites are manufactured using one-sided tools made from composites or Invar, versus the more expensive multipiece metal closed cavity, machine-tooling or large twosided die sets that are normally required for injection molding of plastics and metal forming processes.

Aerodynamically smooth surfaces. Adhesively bonded structures offer smoother surfaces than riveted structures. Composite skins offer increased aerodynamic efficiency, whereas large, thin-skinned metallic structures may exhibit buckling between frames under load (i.e. "oilcanning").

Low observable (LO) or "stealth" characteristics. Some composite materials can absorb radar and sonar signals and thereby reduce or eliminate "observation" by electronic means. Other materials are "transparent" to radar and work well as a radar "window" in radome applications.

 

Examples of Typical Applications

	Large components of commercial airliners - such as the vertical and horizontal tail plane (stabilizer) on the Airbus A320, A330/340, A380 and Boeing 777, the wing, center wing box and fuselage for the Boeing 787 Dreamliner and Airbus A350, and various structures on many smaller craft such as the wings for the Bombardier C Series airliners.

Large primary structures on military aircraft - such as the wing and cargo doors for the Airbus A400M transport, fuselage/wing for the B-2 Spirit Stealth Bomber, rotor blades and aft fuselage for the V-22 Osprey tilt-rotor, as well as the most of the fuselage and wings for the F-22 Raptor and F-35 Joint Strike Fighter.

	Many other components on modern airliners - such as radomes, control surfaces, spoilers, landing gear doors, wing-to-body fairings, passenger and cargo doors, trailing edges, wingtips and interiors

 

Large marine vessels and structures including hulls, decks and superstructure of military and commercial vessels, as well as composite masts (one of the largest carbon fiber structures in the world is the Ms sailing yacht's 290-foot mast), wing masts and foils, rigging, propellers and propeller shafts.

Primary components on helicopters including rotor blades and rotor hubs have been made from carbon fiber (CF) and glass epoxy composites since the 1980s. Composites can make up 50 to 80% of a rotorcraft's airframe by weight, including radomes, tail cones and large structural assemblies. For example, Bell Helicopter Textron's 429 corporate/EMS/utility helicopter features composite structural sidebody panels, floor panels, bulkheads, nose skins, shroud, doors, fairings, cowlings and stabilizers, most made from CF/epoxy.

Wind turbine rotor blade manufacturers use glass fiber and resin-epoxy, polyester or polyurethane are often used-plus a core material such as balsa wood or foam to create a lightweight yet stiff sandwich structure. The blades are made using resin infusion or prepreg; carbon fiber is increasingly used in spar caps as blades get longer. For example, LM Wind Power makes the 88.4- meter-long blades used on the Adwen 8 MW turbine and is planning a 107-meter-long blade for GE's Haliade-X 12 MW turbine. Blades have also become more slender and may feature aeroelastic tailoring, with airfoils optimized for specific wind conditions.

 

Missiles and space vehicles rely heavily on carbon fiber composite construction due to its high strength- and stiffness-to-weight ratios as well as its negative linear coefficient of thermal expansion (CTE), which gives dimensional stability in the extreme temperatures of space. The Delta family of launch vehicles have used carbon fiber and epoxy construction in more than 950 filament-wound motor cases and are in their sixteenth year of production. The Pegasus rocket is the first allcomposite rocket to enter service. Its payload fairing, which is 4.2 feet/ 1.3 meters in diameter and 14.2 feet/ 4.3 meters in length, is comprised of carbon/epoxy skins and an aluminum honeycomb core. The Chandra X-ray Observatory spacecraft features numerous composite structures, including the optical bench assembly, mirror support sleeves, instrument model structures and telescope thermal enclosure. The next generation James Webb Space Telescope (JWST) uses a carbon fiber reinforced cyanate ester backplane structure, deployable sunshield booms, and other composites in the design of its primary structures.

 

Automotive applications range from carbon fiber reinforced plastic (CFRP) monocoque chassis and driveshafts in race cars and high-end sports cars, to selected structural and aesthetic carbon composite applications in lower-volume luxury models, to applications in higher-volume models such as glass-fiber reinforced polyurethane leaf springs and G F /epoxy coil springs.

Selected applications in production automobiles include roofs for BMW's M3 and M6 models, the hood and fenders for Corvette's LeMans Commemorative Edition Z06, the inner deck lid and seats for the Ford GT, composite coil springs for the 2015 Audi A6 Avant 2.0 TDI ultra, and front and rear axles (actually termed "multifunction suspension blades" because they integrate suspension, steering, anti-vibration/noise and anti-roll) for the Peugeot 208 FE. There is also a significant market in carbon fiber composite parts sold as aftermarket enhancements to production automobiles, including a number of companies mass-producing all-carbon-fiber wheels.

Sporting goods, such as bikes, tennis rackets and golf clubs have taken advantage of carbon fiber for decades. The majority of tennis rackets, golf club shafts and hockey sticks are made using carbon fiber. Composites are also common in bats, arrows, snowboards and skis, and gaining widespread use in helmets and protective shoulder and shin pads. Latest trends in sporting goods include use of spread-tow reinforcements, nanomaterials and biomaterials, as well as added multifunctionality such as vibration damping for increased performance and control.

 

Medical devices continue to spur the advancement of new composites, such as ENDOLIGN™ continuous carbon fiber /PEEK thermoplastic biomaterial from Invibio Biomaterial Solutions, which is being used as an alternative to metals in the development of implantable load-bearing applications in orthopedic, trauma, and spinal implants. Other carbon fiber composites are used in medical imaging tables and accessories because they offer high stiffness and light weight, while helping to minimize imaging issues such as signal attenuation. Composites are also gaining applications in prosthetics, orthotics and surgical tools via 3D printing.

Civil engineering structures are progressing steadily in their use of composites. Carbon fiber wraps for repair and strengthening of columns, beams, concrete slabs and bridge structures have become increasingly common. They offer an alkaline-resistant repair, which is quicker and less costly to install due to its light weight. Carbon fiber in bridge cable stays is also progressing, offering high strength and stiffness at minimal weight as well as excellent resistance to temperature contraction and expansion due to its negative coefficient of thermal expansion.

	Composite structures using a wide range of additive manufacturing processes are offering a cost-effective means to produce structural parts with complex geometry using thermoplastic compounds reinforced with short fibers or continuous filaments.

 

 

Compiled from Dorworth, L. C., Gardiner, G. L., & Mellema, G. M. (2019). Essentials of advanced composite fabrication and repair.