January/February
2001
LS-DYNA:
A Computer Modeling Success Story
by
John D. Reid, Martin W. Hargrave, and
S. Lawrence Paulson
In
September 1998, what had seemed like an open road to federal approval
for the bullnose guardrail system suddenly developed a major barrier.
The bullnose system is one of three guardrail types that have been traditionally
used to protect against median hazards such as a bridge support. The
U-shaped bullnose guardrail wraps around the hazard. State highway departments
like the bullnose guardrail because it is considered an effective median
safety device and because compared to crash cushions (rigid barriers
with cushions on each end) and open guardrail systems, it is relatively
inexpensive.
However, before the bullnose system could be used on federal-aid highways,
it had to meet the crash-test requirements of National Cooperative Highway
Research Program (NCHRP) Report 350, also known as "Recommended
Procedures for the Safety Performance Evaluation of Highway Features."
The report was adopted by the Federal Highway Administration (FHWA)
as a required standard for roadside safety features such as guardrails.
Report 350 recognizes the growing popularity of light trucks and sport
utility vehicles, which are heavier and higher off the ground than cars,
and specifies that crash tests must include light trucks - up to 2,000
kilograms (4,400 pounds) - as well as passenger cars.
In 1997, the Midwest Roadside Safety Facility began a program to develop
a bullnose guardrail system that would meet the requirements of Report
350. To pass the crash tests, the system had to deflect - or in the
case of head-on collisions, "capture" or "trap"
- vehicles hurtling into the barrier at speeds of 100 kilometers per
hour (62 miles per hour).
The first two crash tests conducted by the testing facility, involving
head-on collisions, had mixed results. The barrier captured a small
passenger car, but a small truck plunged right through the rail. A follow-up
test had the same results.
This
sequence of photographs shows the results of a full-scale crash
test in which the barrier failed because it allowed the truck
to "fly" over the barrier system. |
|
|
The project engineers decided to enlist the help of LS-DYNA, a complex
computer analysis system whose predecessor, DYNA3D, was originally developed
in the 1970s at the Lawrence Livermore National Laboratory to simulate
underground nuclear tests and determine the vulnerability of underground
bunkers to strikes by nuclear missiles. LS-DYNA, which uses nonlinear
impact finite element code to simulate vehicle crashes, allowed engineers
at the University of Nebraska-Lincoln (UNL) Center of Excellence, where
the simulations were run, to re-create the head-on collision and analyze
the elements of the crash - about 10,000 of them - in an attempt to
determine what caused the failures. (See "It's a Jungle Out There:
Using the Bullnose Guardrail to Protect the Elephant Traps," Public
Roads, January/February 1999.)
LS-DYNA helped engineers find the culprit in the barrier design: longitudinal
slots cut into the depressions of the three-hump beams, known as thrie
beams, that constitute the guardrail. The simulations showed that the
guardrails ruptured because of stresses in the top two humps of the
thrie beams. The solution was to reinforce the thrie beams with two
cables, a successful design change that was confirmed by a later field
test that showed that the reinforced barrier withstood the collision
and provided protection for the truck's occupants.
Engineers were very optimistic going into the next test - a light truck
hitting the guardrail's critical impact point, which is the point where
it is not known whether the barrier will trap the vehicle or redirect
it. The general feeling was that the cable reinforcement of the guardrail
had solved the barrier system's design problems. This was going to be
a no-brainer; the system would pass.
Because the engineers were under some time constraints and they were
so confident of success, they did not conduct a simulation of the critical
impact-point test, known as Test 6. They ran the crash test and were
surprised when the test was a failure because the truck overrode the
barrier system.
The vehicle was neither redirected nor trapped. Actually, it was launched
by the barrier. In the words of the official report on the test, "Vehicle
trajectory behind the test article was unacceptable as the test vehicle
vaulted and became airborne in the median area behind the bullnose."
And when the vehicle hit the ground, it rolled over.
It was time to go back to the drawing board - or, rather, back to LS-DYNA.
LS-DYNA
to the Rescue
Before running another crash test, the researchers at the UNL Center
of Excellence began to do some simulations. However, simulation of
the critical impact-point test turned out to be extremely difficult
and time-consuming because the nature of the impact was much different
than previously simulated crashes. "There were a lot of things
we hadn't taken into account because it was so much different than
a frontal impact," said one researcher.
Concerned about further delays, the Center of Excellence engineers
decided to forgo a full simulation. They came up with a design for
Test 7 that they thought would work, but because of the simulation
problems that they experienced, they didn't have a detailed simulation
to verify the design. To further hedge their bets, the engineers made
four modifications to the guardrail design - mainly involving the
posts holding up the guardrail - that they thought would strengthen
the system. However, Test 7 was also a failure.
At that point, the UNL engineers knew that the only prudent course
of action was to seek the approval of the project sponsors to allow
more time so that the design could be studied in far more detail.
The sponsors agreed.
The issue seemed relatively straightforward. As the official report
on the test stated, "The lack of tension and lateral resistance
allowed the pickup truck to penetrate into the guardrail with increased
rail deflection and rotation and without the vehicle being captured
or redirected. This combination turned the guardrail into an effective
ramp for the impacting pickup truck to climb up and roll over. As
a result of the failed test, design changes were necessary to allow
the successful containment or redirection of the pickup truck. The
thrie beam rail would need to remain upright and functional long enough
to capture the front of the impacting vehicle, thus preventing vehicle
climbing, vaulting, and rollover. The changes required that the rail
tension and lateral stiffness be increased without adversely affecting
the head-on impact performance of either the pickup truck or small
car impacts."
Finding the solution, however, proved to be a complex and painstaking
process that caused a delay of several months in the project. This
was primarily because the frontal impact simulation that had already
been developed for LS-DYNA had to be converted into a simulation of
the critical impact-point collision. This required a major modeling
effort.
Once the new model was completed, the center ran a simulation of the
failed Test 7. The simulation showed that one tire was hitting the
ground line strut and causing the vehicle to vault. Then, the center's
engineers ran the simulation without a ground line strut, and it made
a big difference in the design. So, they figured out how to make the
bullnose without a ground line strut.
The engineers admit that it is unlikely that they would have singled
out the ground line strut for attention without the computer simulation.
However, they did test and verify the value of other changes.
For example, the research team thought chamfered [grooved] blockouts
would help the rail go under and capture the car better. The blockouts
worked in the simulation, and they were incorporated into the design.
Other changes, all of which were tested in simulations, included a
decrease in the distance between posts for a portion of the guardrail
system to add strength and the addition of double blockouts to reduce
tire snag and hold the rail higher for a longer period of time as
the post rotates during impact.
The simulations weren't fully predictive. Some simulation problems
remained and prohibited a complete simulation run. However, the engineers
were eager to confirm the value of the changes that were substantiated
by the simulations, and so, they moved ahead with Test 8 in September
1999.
The official report summarized Test 8 by noting, "The bullnose
barrier successfully contained and stopped the test vehicle in a controlled
manner. ... The vehicle remained upright during and after collision,
and the vehicle's trajectory did not intrude into adjacent traffic
lanes. Vehicle trajectory behind the test article was acceptable as
the test vehicle was captured in the median area behind the bullnose."
It was, in other words, an absolute success. And the bullnose guardrail
system, having passed all of its Report 350 tests, is now awaiting
final FHWA approval for use on federal-aid highways.
Using
LS-DYNA and DYNA3D Code
As the bullnose guardrail experience makes clear, LS-DYNA simulations
are not perfect; there is still a lot of trial and error involved
in analyzing complex events and identifying causes and effects. However,
without computer modeling, the only way to test possible modifications
after a failed test is by running another actual crash test, and at
between $15,000 and $25,000 per test, that's hardly cost-effective.
It is apparent that performing iterative crash tests without modeling
can easily become a prohibitively expensive exercise.
Another major advantage of LS-DYNA simulations is that specific factors
of a crash - for example, a wheel hitting part of a highway barrier
- can be isolated and examined. The computer system is perfectly suited
for examining "what-if" scenarios that simply cannot be
tested under real-life conditions and for identifying potential problems,
such as the ground line strut, that may not be discovered without
the computer.
Nevertheless, LS-DYNA is definitely not a user-friendly program. The
actual physics itself is so complicated that you cannot expect the
software to be any less complicated. The DYNA3D code is very powerful,
but it is also very complex. An expert user of DYNA3D has spent an
extraordinary amount of time just learning to use the code. It requires
an expertise developed by mastering a number of courses in nonlinear
computational mechanics; solid mechanics; and fluid flow, including
both noncompressible and compressible liquids.
When FHWA got into DYNA3D, FHWA engineers thought that they could
help develop this code and then give it to the state highway departments.
However, over a period of time, they determined that this just was
not possible. Understanding DYNA3D requires a level of dedicated study
(and the subsequent development of expertise) that exceeds the resources
and capabilities of even a very good engineer at a state highway department
or at a private company that manufactures and markets roadside safety
structures.
Centers
of Excellence
This special expertise in DYNA3D is what makes the centers of excellence
such valuable resources.
There are four centers with LS-DYNA capabilities: UNL, Texas A&M,
Worcester Polytechnic Institute, and the University of Cincinnati.
To date, 10 states - Iowa, Kansas, Minnesota, Missouri, Nebraska,
Ohio, Pennsylvania, South Dakota, Texas, and Wisconsin - have worked
with the centers on a pay-as-you-go basis on the design or redesign
of roadway safety structures.
A fifth center, at the Ashburn, Va., campus of The George Washington
University, houses the National Crash Analysis Center.
Besides the states, county and local departments of transportation
and the manufacturers and marketers of roadside safety structures
can also contract with the centers of excellence for tests and analysis.
The relationship between the centers of excellence and the universities
with which they are affiliated is mutually beneficial. In addition
to providing services, the centers also serve an educational function,
and because they are at universities, they involve graduate students
in the process. In that way, the base of LS-DYNA expertise is continually
being replenished and expanded.
Dr.
John D. Reid is an assistant professor of mechanical engineering
at the University of Nebraska-Lincoln (UNL). He is also the director
of the DYNA3D Center of Excellence at UNL. Before joining the faculty
at UNL in 1993, he worked at General Motors Corp. for eight years
-- the last three in safety and crashworthiness. He received his bachelor's
degree, master's degree, and doctorate in mechanical engineering from
Michigan State University.
Martin
W. Hargrave is a research mechanical engineer on the Roadside
Team in FHWA's Office of Safety Research and Development. He conducts
and manages research associated with FHWA's DYNA3D finite element
research program. Before joining FHWA in 1979, he worked for 17 years
in various engineering assignments for private sector companies. He
received a bachelor's degree in mechanical engineering from the University
of Alabama, a master's degree in engineering from Pennsylvania State
University, and a master's degree in civil engineering from The Catholic
University of America.
S.
Lawrence Paulson is a partner in Hoffman Paulson Associates, a
writing/editing and public relations firm in Hyattsville, Md. He has
written and edited numerous studies for the Federal Highway Administration,
Federal Transit Administration, and National Highway Traffic Safety
Administration. He also spent seven years covering Congress as the
Washington bureau chief of a national daily newspaper, The Oil
Daily.
For
additional information about LS-DYNA and its role in supporting a
new bullnose guardrail design, contact Martin Hargrave at (202) 493-3311
(martin.hargrave@fhwa.dot.gov)
or John Reid at (402) 472-3084 (jreid@unl.edu).
Other
Articles in this Issue:
Learning
to Beat Snow and Ice
Safe
Plowing - Applying Intelligent Vehicle Technology
Improving
Roadside Safety by Computer Simulation
Using
the Computer and DYNA3D to save lives
LS-DYNA:
A Computer Modeling Success Story
Preservation
of Wetlands on the Federal-Aid Highway System
Internal
FHWA Partnership Leverages Technology and Innovation
New
Applications Make NDGPS More Pervasive
Center
for Excellence in Advanced Traffic and Logistics Algorithms and Systems
(ATLAS)
National
Work Zone Awareness Week (April 9 to 12) - Enhancing Safety and Mobility
in Work Zones