After more than a century of innovation, advantages over metal are plentiful
Starting with the primitive laminates of the Wright Brothers era, the use of composites in aircraft has evolved over the last century from small amounts on nonstructural components to up to half of some aircraft and use on critical structures, such as wings. A key benefit is reducing weight.
“In general, composites can offer improved performance and reduced weight compared to metal,” Sam Tucker, Boeing research and technology materials, process and physics manager, said. “In addition, composites may be more cost effective than metal and can be tailored to end-use process. Today, carbon-reinforced composites are all over Boeing’s aerospace products.”
Boeing and Lockheed Martin have been using composites for longer than 100 years. Those early natural composites were made of wood, cloth, fiberglass and resins held together with glue and doping compounds and have only a tenuous connection to today’s fiber-reinforced composites, Robert Biggs, a mechanical engineering manager specializing in composites at Lockheed Martin Space Systems, said.
Boeing began using composites made from natural materials on its Model C aircraft in 1916, the same year the company was born, Tucker said. Those first composites were wood laminates and linens held together with glue and doping compounds, he said.
Boeing first used synthetic fiber composites, made with a thermoset resin, on the fiberglass radome of the B-29 Superfortress in 1942, he said. That success led to fiberglass on radomes on the B-47 Stratojet in 1948, the B-52 Stratofortress in 1952 and the 707 in 1957. In 1961, fiberglass was used on the rotor blades of the CH-47 Chinook. McDonnell Douglas, which was not yet part of Boeing, applied carbon fiber composites on the wings of the AV-8B Harrier and F/A-18 Hornet in 1978. In 1981, Boeing’s 767 used carbon, aramid and glass fibers on doors, cowlings and fairlings. The 767 also used composites on leading and trailing edges, including the rudder, elevators, spoilers, flaps and tips, he said.
In the 1980s and 1990s, composites continued to gain traction in fighter jets, bombers, helicopters, space access, satellites, weapons and commercial aircraft, Tucker said. Boeing developed a primary structural composite material system for the 787 Dreamliner in the 1990s.
“Approximately 50% of the 787 Dreamliner is composite structure, including the wing and fuselage,” he said.
Lockheed Martin used composites for reconnaissance aircraft, including the A-12 and SR-71 in the 1960s, Biggs said. The first major composites were used for horizontal and vertical tails on the F-16 in the mid 1970s. These composites used thermoset polymer resins.
More recently, the Orion team at Lockheed Martin’s Space System’s facility near Denver shipped the largest composite heat shield ever built, a company spokesman said. The heat shield is part of the Orion spacecraft, one intended to pave the way for human missions to Mars starting around 2021, according to NASA. The shield is 16.5′ (5 m) in diameter, made of a single seamless piece of Avocoat ablator and will protect the spacecraft during Orion’s Exploratory Mission 1 (EM-1) in 2019, according to NASA and Lockheed Martin. A similar heat shield was used on Orion’s Exploratory Test Flight on Dec. 5, 2014.
Airbus began using composite materials for secondary structural applications in the 1970s, José Sanchez Gómez, Airbus composite materials executive expert, said. Airbus increased its use of composites using a step-by-step approach, with composites making up 5%, including the rudder, of the A310-300 … to 10% on the A320 family … 11% on the A330/A340 family … 25% on the A380 and 35% on the A400M, today.
“Airbus started using glass-fiber composites on secondary structures,” Sanchez said. “Now composite materials are used in most of the structural elements of any commercial aircraft, including primary structures like the wing and fuselage.”
For example, more than two-thirds of the A350’s airframe is made from advanced materials, he said, with 53% of that comprised of composites and the rest titanium and advanced aluminum alloys. The fuselage, stabilizers and wings are built from carbon-fiber-reinforced plastic (CFRP).
One main advantage is reducing scheduled and unscheduled maintenance, Sánchez said.
“A large part of the reduction in maintenance with composites, as compared to metals, is due to the elimination of metallic fatigue and corrosion tasks, as part of the maintenance program for metallic structural parts,” he said. “Indeed with CFRP being corrosion free and fatigue free, this means that for the A350, the traditional ‘intermediate’ check at six years is eliminated and the first ‘heavy’ check tasks do not need to be performed until the 12th year.”
Other advantages over metal
Compared with metal, composites also offer:
- Better strength to weight ratio.
- Better resistance to fatigue and corrosion.
- The ability to tailor properties within a structure to optimize performance.
- The ability to fabricate more complex shapes affordably.
- Better thermal stability.
- The ability to recycle the composites during production or at end of life cycle.
- In many cases, lower fabrication costs.
- In many cases, lower costs overall.
Today’s continuous fiber composites—including thermoset, thermoplastics and even fiberglass—allow manufacturers to orient the fibers in the direction of the load path, again allowing for lighter weight compared with metals, Tucker said.
BASF is developing composite technologies for use in aerospace, Sarah Westerdale, senior manager, aerospace customer development, said. Carbon fiber filled thermoplastic is already replacing metal parts in some seating components, such as armrests.
BASF is using chopped fiber thermoplastics and thermosets to replace wood veneers for skins over finished parts, such as galleys, panel walls and other nonstructural components, Don Cho, technology manager of aerospace BASF, said. Using these composites shortens the labor cycle up to 55% because the composite skins are easier to paint. Plus, the composites save 50% or more on weight, he said.
BASF is working on a line of thermoplastic composites to replace thermoset composites in interior walls, luggage bins and seating components, Westerdale said. “We’ve been proving out their use and functionality, taking the labor and production time out of each part.” These materials are past the proving stage but not quite to mass commercialization, she said.
The ability to recycle the materials at the end of life was important and helped drive the move from thermosets to thermoplastics, Cho said. One part had been made of both nonrecyclable thermoset along with other components that could be recycled, he said.
There’s a lot of scrap built into the quoting process for thermosets, which have to be kept cold, Westerdale said. Manufacturers can only freeze and thaw thermosets a limited number of times.
“Our thinking was: ‘We have the material. We need to develop the technology to make it all out of thermoplastic panels,” Cho said. “It simplifies the tear down to recycle and minimizes landfilling of materials.”
Types of composites in use now include glass, carbon and aramid reinforced thermoset, thermoplastic polymer matrix composites, metal-matrix composites, ceramic-matrix composites and carbon-fiber reinforced carbon matrix composites, Biggs said.
Types of composites
Each type of composite offers different pros and cons, Biggs said.
- Continuous-fiber-reinforced thermoset matrix composites: high strength and stiffness, low thermal expansion, large mass savings, ease of processing, moderate damage tolerance and moderate temperature limit.
- Discontinuous-fiber-reinforced thermoplastic matrix composites: low strength and stiffness, high thermal expansion, small mass savings, capability for rapid processing, moderate damage tolerance, moderate temperature limit.
- Continuous-fiber-reinforced thermoplastic matrix composites: high strength and stiffness, low thermal expansion, large mass savings, high damage tolerance, moderate temperature limit, hard to process.
- Metal-matrix composite: moderate strength and stiffness, high thermal expansion, small mass savings, moderate damage tolerance, high temperature limit, hard to process.
- Ceramic matrix composite: low strength, high stiffness, low thermal expansion, moderate mass savings, very high temperature limit, hard to process, low damage tolerance.
- Carbon-carbon matrix composite: low strength, high stiffness, low thermal expansion, moderate mass savings, very low damage tolerance, extremely high temperature limit, very hard to process.
“These different technologies offer different mechanical performance, manufacturing characteristics and price,” Sánchez said. “After a careful selection process, Airbus goes for the best technology for each application, which is key in getting optimal performance of the structure. At Airbus, we are in a continuing process of monitoring and developing the best composite materials and technologies for the benefit of our operators.”
Although other composites continue to be used, momentum is building around thermosets.
“There is a great deal of excitement and effort ongoing with thermoplastics in aerospace regarding parts manufacturing using unidirectional prepreg and/or fabric based systems,” Michael Buck, product manager for thermoplastic composites at Barrday Composite Solutions, said. “Airbus and Boeing, in particular, are leading this charge. At Barrday, we are working with several companies (original equipment manufacturers Tier 1 suppliers and parts manufacturers) on programs involving thermoplastic composite parts.”
“Thermosets are the industry standard—good mechanical properties, high temperature performance, very low creep, good environmental resistance and a long history of successful use,” Tucker said. “Thermoplastics can absorb more impact energy and offer advantages in the processing of some components. Ceramic-matrix composites are difficult to process, but they offer extreme temperature resistance.”
Companies like BASF and Barrday continue to develop more applications for thermosets in aerospace and other industries.
“Nowadays the degree of development of composite materials for aerospace applications can be considered high,” Sánchez said. “In order to increase and/or even maintain today’s level, our industry has to go a step further developing more cost-efficient processes, including outsourcing of monomers as well as recycling technologies.”