Srdan Simunovic
takes an empty beverage can and squeezes it 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 rather than
fragment into numerous pieces, as fiber-reinforced composites do. It
is this simple plasticity of metals in response to sudden impacts that
we can simulate using our materials modeling codes." The codes are run
on the IBM RS/6000 SP supercomputer at ORNL.
Simunovic and
the six researchers in the group he leads—the Computational Material
Sciences Group in ORNL's Computer Science and Mathematics Division—have
developed computer models of vehicles whose bodies are made of regular
steel, high-strength steel, and aluminum. With funding from the National
Highway Traffic Safety Administration (NHTSA), the group recently developed
detailed computer models of the Ford Explorer. These models have been
included on ORNL's
external Web site, which has received a number of hits because of
growing interest in the causes of rollover accidents involving this
sport utility vehicle.
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Computer
simulation (courtesy of Srdan Simunovic) of a Ford Explorer crashing
against a rigid barrier.
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The models were
built after disassembling a Ford Explorer and scanning in the parts.
Each finite-element impact model, which divides the vehicle into hundreds
of small sections, includes a materials model that predicts how the
car body material will behave as the vehicle collides from different
directions, with a rigid barrier, at 35 miles per hour (mph).
"We are also working on
the computational analysis of a concept car made of high-strength steel,"
Simunovic says. "We use the models to predict the effects on new advanced
materials of various collisions, such as two cars colliding with each
other. 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."
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Computational
visualization of a high-strength steel automobile body after crashing
into a rigid barrier. An NTRC computer will be used for crash
modeling.
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In addition, the
group has modeled an all-aluminum Audi A8 car crashing against a rigid
barrier at 35 mph. The data used to improve the model came from NHTSA
tests of both an Audi A8 car and its space frame (without the interior,
doors, and fender) crashing against a barrier. "We needed crash data
on the space frame," Simunovic says, "so we could predict the extent
of deformation more accurately."
For Simunovic an even
bigger challenge is modeling fiber-reinforced polymer composites, a
project he has been working on since 1993. These composites consist
of glass or carbon fibers embedded in a polymer matrix. Advanced carbon-fiber
composites are made by the P4 process. P4 stands for the programmable
powder preform process, in which robots spray fibers of different lengths
in a desired orientation on a wire mesh before the fibers are glued
together in a mold by means of an activated resin.
"We are developing
constitutive models to predict how the material will behave during an
impact at 35 mph," Simunovic says. Referring to the crushed beverage
can on his desk, he notes that metals tend to dissipate energy by bending
and deforming plastically in response to a blow, whereas composites,
which are lighter than steel and have higher specific strength, tend
to be stiff, making them less likely to give as easily.
"The impact could
cause fibers to break away, or debond, from the polymer matrix," Simunovic
says. "The goal is to develop a composite that exhibits controlled progressive
fracture during impact. Such a material can 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 change 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 intermediates strain rate crush test station,
which will be installed in 2001 at the National Transportation Research
Center (NTRC). 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.
Members of the
Computational Material Sciences Group include J. D. Allen, Jr.; G. A.
Aramayo; C. O. Beasley, Jr.; H. K. Lee; B. Radhakrishnan, and G. B.
Sarma. A collaborator is A. Bobrek of the University of Tennessee at
Knoxville.
"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."
Beginning
of Article
Related Web
sites
Computational Material Sciences Group
ORNL's Computer
Science and Mathematics Division
National
Highway Traffic Safety Administration
