Composite materials may someday have big advantages over steel in automobile manufacturing. Composites are being considered to make lighter, safer and more fuel-efficient vehicles. A composite is composed of a high-performance fiber (such as carbon or glass) in a matrix material (epoxy polymer) that when combined provides enhanced properties compared with the individual materials by themselves. Carbon-fiber composites weigh about one-fifth as much as steel, but are as good or better in terms of stiffness and strength. They also do not rust or corrode like steel or aluminum, and they could significantly increase vehicle fuel economy by reducing vehicle weight by as much as 60 percent, according to the Oak Ridge National Laboratory (ORNL). “With composite materials, we get high strength-to-weight and stiffness-to-weight ratios, as well as excellent energy-absorbing capability per mass,” says Dan Adams, professor of mechanical engineering at the University of Utah who is collaborating with ORNL on the development of test methods for automotive composites. “Steel is strong and inexpensive, which is why it’s the material of choice today. But composites can be designed tobe strong and light to provide better safety and fuel efficiency.” Adams says that the strength and stiffness factors are why composites are currently used in aerospace applications, which also require a material that is extremely light. And compared to single-layered steel in cars, multiple-layer composite laminates can be designed to absorb more energy in a crash. “However, the use of these materials in the automotive industry has been very limited partly because of the costs associated with the materials and manufacturing,” he says. Adams and his associates are addressing these issues, along with design safety, as they develop test methods and assess candidate composites for automotive applications.
Engineering for Crashworthiness
Adams’ research focuses on the mechanics of sandwich composites, which are a special class of composite structures made by attaching two thin composite facesheets to a thick, low-density core of balsa wood or foam. Sandwich composites are of interest for automotive floor and roof applications. Nearly two decades ago, Adams began studying how to make sandwich composites more “damage tolerant,” where the structure can still meet its load requirements after it has sustained some type of initial damage. “Investigating the mechanics of damage tolerance required understanding the failure progression (the types and locations of failure),” says Adams. “It required that we look carefully at what was happening after there was already an initial failure.” Eventually Adams was selected to lead the U.S. Automotive Composite Consortium’s first research investigation into the “crashworthiness” (the ability to protect occupants in a crash) of sandwich composites. “For me, crashworthiness was a natural extension of damage tolerance,” he says. “We just kept applying load to an already failed sandwich panel and examined how it crushed. Conventional sandwich composites would quickly come apart with the facesheets debonding and the remaining core buckling or being pushed aside with little energy being absorbed.” With the proper design, Adams says that composites can meet or even exceed safety requirements in a crash because they can be designed to absorb significantly more energy than traditional metallic metals when crushed. “Metals absorb energy in a crush by yielding, whereas composites typically crush in a more brittle manner,” he says. “Energy is absorbed through repeated failure of the material. The ideal case would be the composite structure being broken into tiny pieces where the crush occurs, but everything else is intact. The materials have to be designed to fail in a controlled manner.” Already the initial sandwich design Adams and his students worked on a few years ago has made its way into the Chevrolet Corvette. Adams is continuing to develop test methods for assessing crashworthiness of composites with organizations such as ORNL and Engenuity Limited, an engineering consulting company based in the U.K. “No accepted test methods exist and we need them to screen materials for crashworthiness as well as to provide the required experimental results for validating computational modeling methods,” he says. In 2010, Adams was selected by the U.S. Department of Transportation to write a 100-page white paper on the current status and research needs for plastic and composite intensive vehicles. The government is hoping to facilitate their safe deployment by 2020. “It was really exciting to have the opportunity to identify what needs to be done in the next ten years to make automotive composites a reality,” Adams says.
Lowering Costs for Automotive Composites
Affordability is an important issue in vehicle manufacturing, which includes factoring in the costs associated with a car’s complete life-cycle—including manufacturing, operating and disposal costs. “The issue with today’s composites is that they have been developed for aerospace applications where cost is not as critical,” says Dan Adams, professor of mechanical engineering. Pound for pound, material costs of carbon fiber composites are at least 20 times as much as steel, and the automotive industry is unlikely to use them until the price of carbon fiber drops significantly. The processing of carbon fibers is too expensive and slow, says Adams. The raw carbon material is converted to carbon fibers using thermal pyrolysis, a slow, energy-consuming process that is combined with stressing to achieve a high percentage of carbon with the proper fiber tension. The raw material, the energy needed to heat it to make fibers, and the required equipment all contribute to the high cost. As a result, carbon-fiber composites cannot yet compete economically with steel in the auto industry. “The development of low-cost carbon fiber is an active research area with great promise,” says Adams. “Also the development of low-cost manufacturing methods for automotive composites is receiving a lot of attention. There are less expensive ways of manufacturing composite automobile parts that also reduce the number of joints and fasteners. We could make these materials very affordable.”