LS-DYNA A Computer Modeling Success Story

Public Roads, Jan, 2001 by John D. Reid, Martin W. Hargrave, 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-tip test had the same results.

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.