Carbon-fiber reinforced composites are popular in the aerospace and marine industries as a lightweight alternative to aluminum, steel and other metals. The material is also used to produce railcars, wind turbine blades and sporting goods.
Now, the auto industry is jumping on the bandwagon. According to many industry observers, automakers and suppliers will become heavy users of carbon-fiber composites during the second half of this decade. However, engineers still need to address several challenges, such as cost-effective assembly processes.
Carbon-fiber composites offer lighter weight, better corrosion resistance and higher impact strength than steel and aluminum. They also offer engineers greater design flexibility and allow them to significantly reduce the number of parts in assemblies.
Composite materials weigh about one-fifth as much as steel, but are comparable in terms of stiffness and strength, depending on fiber grade and orientation.
They have the potential to reduce vehicle weight by more than 50 percent.
However, the drawback to composites is their steep price tag; they currently cost much more than steel. Carbon-fiber parts also take longer to produce than traditional steel stampings, which can be punched out in seconds.
To make composite parts, layers of carbon-fiber monofilaments must be wound and woven into fabric sheets. Then, the sheets are laid into molds that are baked at high temperatures in large ovens.
Several different processing techniques are available, including sheet molding compounds (SMCs), long-fiber reinforced thermoplastics and high-pressure resin transfer molding. Each technology has pros and cons.
Conventional carbon-fiber composites use thermosetting resins and require a long time for molding, which has limited their use in the auto industry.
Unfortunately, cycle times of 90 minutes are not conducive to producing parts for 30,000 or more vehicles at a time.
“The most dominant carbon-fiber composite material for automotive applications are SMCs, because they’re cost-competitive,” says Cliff Eberle, technology manager for composites at Oak Ridge National Laboratory, which is leading a consortium in the United States to develop low-cost sustainable materials and rapid manufacturing processes for use in the auto industry.
“The goal is to get cycle times under a minute,” adds Eberle. “The fastest manufacturing processes today are in the five- to 10-minute range. Composite manufacturing processes are currently optimized for aerospace volumes. But, producing eight aircraft per month is a whole lot different than making eight vehicles per minute.”
Engineers around the world are on a crusade to develop materials and equipment that enable fast processing speeds. For instance, a European consortium called ACOMPLICE (Affordable Composites for Lightweight Car Structures) recently embarked on a two-year program to develop low-cost materials for mainstream automotive applications.
“Aluminum and high-strength, steel-based alloys have been employed in response to [growing pressure to manufacture lighter weight, fuel-efficient vehicles],” says Elaine Arnold, collaborative research and technology project manager at Cytec Industrial Materials, which is leading the consortium along with ABB Robotics, Aston Martin Lagonda and several other partners. “But, the physical limitations of these materials have to a large extent, already been reached, so other options are being pursued.”
Arnold and her colleagues hope to reduce the cost of composite body-in-white vehicle structures by developing “robotic lamination and fast-cure technologies . . . that facilitate the [production] of complex-shaped geometries via automated pro-cesses.”
Until recently, most composite use in the auto industry has been confined to use in motorsports and exotic supercars. Indeed, one high-profile example is the Lamborghini Sesto Elemento, a V10-powered vehicle that weighs only 2,202 pounds. Engineers at the Italian car company, which is a subsidiary of Audi AG, worked with the Advanced Composite Structures Laboratory at the University of Washington to develop the carbon-fiber intensive vehicle.