Automotive Steel Industry Announces New Steel Research
4 October 2000
American Iron and Steel Institute: Automotive Steel Industry Announces New Steel Research
DETROIT, Oct. 3 With the increasing use of high-strength
steels in auto structures, it is becoming more important to accurately predict
steel behavior in a vehicle crash, according to a paper presented at the
International Body Engineering Conference by Jody Shaw, manager, Technical
Marketing, U.S. Steel Group.
With a better understanding of strain-rate effects in steel, along with
new strain-rate data available from the Auto/Steel Partnership and other
sources, North American steel and auto industry engineers are advancing the
predictability of steel behavior in crash-management structures.
Vehicle makers prefer steel for crush-zone and safety-cage structures
(i.e., for crash management) because of its inherent strength, plus its
ability to work harden as it deforms. Another benefit of steel is that as it
deforms rapidly in a crash, it becomes stronger and absorbs more energy. This
effect is called strain-rate sensitivity. The more rapid the deformation, the
greater the strength levels.
With availability of more powerful computers, auto manufacturers are
beginning to use strain-rate data in modeling vehicle structures, enabling
them to more closely predict crash behavior. The paper by Shaw summarizes
research on this topic conducted by Oak Ridge National Laboratory (ORNL) in
collaboration with American Iron and Steel Institute (AISI). The study is
co-funded by the Department of Energy.
"Work performed by the Auto/Steel Partnership and AISI, as well as other
published research around the world, shows that there is a measurable increase
in the strength of the steel products used in the structural elements of
vehicles when they are deformed at high strain rates," said Jim Fekete, Senior
Advisor, Metal Fabricating Division, General Motors.
"The ability of finite element models to capture this effect, through the
use of advanced materials models, will improve the correlation of Finite
Element Analysis (FEA) to physical testing. This will assist the development
of more efficient structures in less time and with lower cost."
The Evolution in Steel Materials
Compared to automotive steels available just a few years ago, today's
high-strength steels exhibit very steep and sustained hardening rates. Rapid
strain hardening has the benefit of increasing forming limits and buckling
resistance, as well as the capacity to dissipate energy upon impact. However,
the increase in the apparent material yield and flow stresses has been
difficult to predict by computer simulations. Now, with the advent of new
high-speed computers and newly available strain-rate data, engineers can
replicate, with a high degree of confidence, quantitative, real-life crash
behavior using high-strength steels.
"When conducting crash simulations, we can't neglect the fact that high-
strength steels are strain-rate sensitive," said Marcel van Schaik, manager,
Advanced Materials Technology, AISI, and project coordinator of the ORNL
program. "The closer the crash models come to real-life, the more information
they provide the engineer as to whether the design is safe, or how it might be
optimized for better performance. This opens the door to further weight
reductions with the use of advanced high-strength steels."
This benefit also has led Porsche Engineering Services, Inc. to use strain
rate sensitivity during the design phase of the ULSAB-AVC (Advanced Vehicle
Concepts) program. ULSAB-AVC will be one of the first vehicle design concepts
to demonstrate the combination of high- and ultra high-strength steels with a
full spectrum of the latest steel technologies, such as tailor welded blanks,
tailored tubes, and tube and sheet hydroforming processes. It will allow
Porsche's engineers to develop steel parts and structures that are fully
optimized for structural performance and crash management at the lowest
possible mass.
Study Reveals High-Strength Steel Contributes to Crash Management
For the paper being presented, the UltraLight Steel Auto Body (ULSAB) LS-
DYNA3D crash model, developed by Porsche Engineering as part of the ULSAB
program (completed in 1998), provided a starting point for the material
modeling evaluations.
Members from the steel industry and ORNL then performed research on the
effect that strain-rate sensitivity has on high-strength steel-intensive
vehicles.
Only the results from the NCAP frontal impact crash simulations are
presented because they suffice for illustration of the main trends that are
observed across different impact scenarios. The researchers performed NCAP
frontal impact crash simulations for vehicle models with and without material
strain-rate sensitivity. They compared several characteristics with each
other and drew conclusions.
One important conclusion is that additional studies will be useful in
developing highly accurate, predictive models for the applications of high-
strength steels. Further work with ORNL is anticipated for next year.
The paper, "Material Modeling Effects on Impact Deformation of UltraLight
Steel Auto Body," (authors Jody Shaw, U.S. Steel Group, and Gustavo A. Aramayo
and Srdan Simunovic of ORNL) available at SAE under #2000-01-2715, reports on
one element of a three-year project with ORNL.
Additionally, at IBEC, Srdan Simunovic reports on vehicle-to-vehicle crash
modeling using ULSAB as the baseline. At SAE 2001, Jody Shaw will report on
the effect of using actual part data in crash simulations.