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Lighter cars are less safe than heavier vehicles

REALITY: New materials can make cars lighter
and as safe as heavier vehicles

The driver of the racecar whizzing around the Indianapolis Motor Speedway had completed only eight laps when his car suddenly crashed into the wall. Flames burst from the car as it ricocheted from the wall, spun around and glanced off another racecar. Hushed spectators followed the accident, first with horror and then relief and amazement, as the driver walked away from the wreck, signaling that he was not hurt, just badly shaken.



Understanding why the driver was not hurt is central to debunking a common myth about the relative safety of large vehicles. The body of his racecar was constructed of carbon-fiber composites, which are one-fifth the weight of steel but just as strong and stiff. Designed to protect the driver in case of a violent collision, the car used 100% ethanol, a high-octane, clean-burning and renewable fuel that reduces air pollution and enhances racing's carbon footprint.

Except for extraordinary speed and limited passenger space, today's racecar provides a glimpse into the future for ordinary cars. The increasing cost of oil suggests that tomorrow's five-passenger vehicles will be smaller and lighter and will get more miles per gallon of fuel, which likely will include ethanol made from biomass. Despite the cost savings, many Americans are concerned that lighter cars represent a compromise of safety now present in heavier vehicles such as pickup trucks and sport utility vehicles.

To encourage a reduction of gasoline consumption nationwide, the U.S. government has mandated that by 2030 about 30% of the gasoline typically used for personal transportation must be replaced by a biofuel such as ethanol. The Energy Independence and Security Act of 2007 calls for an increase in corporate average fuel economy standards for cars, trucks and heavy-duty vehicles, from 27 miles per gallon to 35 mpg by 2020.

A higher percentage of lightweight cars on the road will certainly help the American auto industry achieve the higher fuel economy standard. Less certain is the impact on the 42,000 annual highway deaths if the American fleet shifts to a lower proportion of heavier vehicles made largely of conventional steel.

Since the 1990s the U.S. automobile industry has been a partner with the Department of Energy, Oak Ridge National Laboratory, other national labs, and several universities including the University of Michigan and Stanford University, in studies to determine the safety impact of lighter cars made of advanced materials, such as high-strength steel, aluminum and magnesium. The auto manufacturers have been particularly interested in carbon-fiber composites—if the cost of making carbon fiber drops to under $5 a pound—because of the composites' potential to reduce the weight of a car by more than 40% of a comparable steel vehicle's weight.

Ray Boeman, director of ORNL research at the National Transportation Research Center, is an expert on carbon composites after working for six years with Detroit automakers. He initiated the development of the Test Machine for Automotive Crashworthiness (TMAC) at Oak Ridge. TMAC quantifies the specific energy absorption of a structure in terms of the energy absorbed divided by the mass of the material crushed.

Mike Starbuck, who leads this work for the Laboratory, says that experimental and computational crash results show that steel absorbs energy by bending, folding and deforming plastically like an accordion or a crushed beverage can. In contrast, carbon-fiber composites absorb energy by many mechanisms including delamination (splitting into layers), breakage of fibers bonded to a polymer matrix and fracture of the matrix itself. In many cases composites have been shown to have higher specific energy absorption characteristics than metals.

Starbuck says researchers are working to improve vehicle design and encourage technological innovation aimed at enhancing the safety of lighter vehicles. Boeman notes that perceptions concerning the relative safety of occupants in vehicles of different masses may be right or wrong, depending upon different scenarios.

For example, if an SUV has a head-on collision with a compact car, the occupants of the SUV may be less likely to be injured than are the occupants of the compact car. One law of physics— kinetic energy is proportional to the mass times velocity squared (KE = mv2/2)—dictates that a larger vehicle has more kinetic energy than the smaller one for the same velocity. The resistance of the lighter vehicle to crash damage depends on its ability to absorb the specific energy transferred by the collision, a capability governed more by design than weight.

The situation may be different, Boeman adds, if the two vehicles driving at the same speed crash into a rigid barrier such as a large tree. The lighter vehicle would absorb much less energy than the SUV, potentially making the lightweight car safer for the occupants than is the SUV. If an automaker builds a lighter SUV, then the requirements to absorb energy decrease.

Boeman emphasizes that safer, light-weight vehicles can be designed based on another law of physics: change in kinetic energy equals average impact force multiplied by the distance traveled. The distance in this equation is the crush or crumple length designed into the vehicle. "If you have a specific amount of kinetic energy, and a certain maximum force you must stay below, the principal variable a designer can work with is crush distance," Boeman says.

Since 1978 the most common standardized test has been the frontal crash of a vehicle into a rigid barrier at 35 miles per hour. Results of crash tests and related computer simulations led to design modifications that have improved vehicle crashworthiness.

Starting in 1993 Thomas Zacharia (now an ORNL associate director of computing and computational sciences and a vice president of research at the University of Tennessee) led his group of materials and computer scientists in modifying a materials modeling code so that it could be run on an IBM supercomputer at ORNL.

One computer model used the conventional standards to predict the impacts on a car's structural materials of a frontal crash of the vehicle into a rigid barrier at 35 mph. The results of the collision simulation matched the much more expensive crash tests of vehicles and dummies instrumented with accelerometers. ORNL's simulation results were provided to the auto industry.

From 1993 to 2004 an ORNL research team developed computer models of vehicles with bodies made of composites, regular steel, high-strength steel and aluminum. With funding from DOE and the National Highway Traffic Safety Administration, the team produced detailed computer models of different vehicles after disassembling the actual cars and measuring the parts.

Each finite-element impact model divides the simulated vehicle into hundreds of tiny sections. The model includes a materials model that predicts how much energy will be absorbed and how the car body material will behave after the vehicle traveling at 35 mph collides at different angles with a rigid barrier.

The ORNL group also performed a computational analysis of a concept car made of high-strength steel, which is thinner, lighter and stronger than regular steel. The simulation indicated that lighter, high-strength steel vehicles should hold up in a crash even better than an equivalent vehicle made of regular steel. The group also found that the predicted results of a head-on collision and frontal-side collision involving a heavy vehicle and a light one varied.

As domestic and international automakers race to design and produce fuel-efficient vehicles, ORNL will play an important role in ensuring that a new generation of lightweight vehicles will not come at the expense of safety. —Carolyn Krause


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