The lighter the structural materials that make up a car, the less fuel it requires per mile of travel. But, if a car is lighter than the steel cars most of us drive, will it hold up as well in a crash?
To help answer this question, the Computational Material Sciences Group in ORNL’s Computer Science and Mathematics Division has completed a computational simulation of an all-aluminum Audi A8 car crashing against a rigid barrier at 35 miles per hour (mph). The group, led by Srdan Simunovic, built the computer model after disassembling an Audi A8 car and scanning its structural components into a computer. The model contains equations and numbers representing approximately 290,000 finite elements and 200 different material components. The model has been run on the IBM RS/6000 SP supercomputer at DOE’s Center for Computational Sciences at ORNL.
While the ORNL supercomputer was crunching the numbers, an actual crash test of an Audi A8 car was performed by the National Highway Traffic Safety Administration (NHTSA). The test results were used to tune the computer model and determine whether its predictions about the ex-tent of deformation throughout the car were correct.
“The deformation predicted by our model was verified against the crash test data,” Simunovic says. “That suggests that this model can be used as a low-cost method for testing new design concepts and materials-processing technologies without the need for building and crashing expensive prototypes. The model can be used to test crashworthiness and analyze stiffness of structural components.”
The ORNL group also has developed computer models of vehicles whose bodies are made of regular steel and high-strength steel. With funding from NHTSA, the group recently developed detailed computer models of the Ford Explorer. One material model predicts how the Explorer’s body material will behave as the vehicle collides from different angles with a rigid barrier at 35 mph.
“We are also working on the computational analysis of a concept car made of high-strength steel,” Simunovic says. The high-strength-steel, UltraLight Steel Auto Body (ULSAB) design and the computational crash models were developed by Porsche Engineering Services, Inc., for the ULSAB Consortium. “We use the models to predict the effects on new advanced materials of various collisions, such as two cars colliding with each other,” he says. “Because these new steel alloys have such high strength, less steel is needed for the body of the car, making it lighter. We found that the ability of the high-strength steel vehicles to hold up in a crash can be even better than that of today’s heavier steel vehicles.”
To explain what happens when cars made of metal crash, Simunovic squeezes an empty beverage can until it folds and collapses like an accordion. “This is what you want to happen to a car during a collision with a rigid barrier or another car,” he says. “Metals tend to bend and deform as they absorb the energy of the impact. It is this simple plasticity of metals in response to sudden impacts that we can simulate using our materials modeling codes.”
For Simunovic an even bigger challenge is modeling fiber-reinforced polymer composites, a project he has been working on since 1993. These composites, which are lighter than steel and aluminum, consist of glass or carbon fibers embedded in a polymer matrix.
“We are developing constitutive models to predict how the material will behave during an impact at 35 mph,” Simunovic says. “Composites don’t act like metals and dissipate energy by bending and deforming plastically in response to a blow. Although composites have higher specific strength, they tend to be brittle, making them less likely to give as easily. They are more likely to shatter like glass.
“The impact could cause fibers to break away, or de-bond, from the polymer matrix. The goal is to develop a composite that exhibits controlled progressive fracture during impact. Such a material could dissipate a large amount of impact energy and gradually decelerate the vehicle. We must learn how to model these effects and accurately predict how they improve the ability of the material to resist breaking catastrophically in a crash.”
For computer simulations of crashes involving cars made of carbon-fiber composites, the ORNL group will use data from the intermediate strain rate crush test station, which will be installed in 2002 at the National Transportation Research Center, where ORNL and University of Tennessee researchers work. The station will compress samples at speeds up to 15 mph, providing information on changes in the number of small and long cracks produced as the impact velocity varies.
“Our goal,” Simunovic says, “is to provide the material models and computational tools that designers need to develop highly efficient, low-emission, lightweight vehicles that have improved safety features.”
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