Designers at Automotive Systems Laboratory are finding that virtual product development using simulation is the preferred tool for testing airbag restraint systems. While physical tests are required for product certification, simulation is more cost-effective and repeatable than physical tests. And analyzing more product configurations on the computer, without the need for costly prototypes, is resulting in better products and reduced testing requirements.
The cost of testing airbags has been growing because government and industry regulatory groups are phasing in testing requirements for a wide group of drivers and occupants, including large males, small females, and children. Some of these tests involve out-of-position (OOP) testing without safety belts.
Increased testing demands
"The increased demands, if it weren't for computer analysis, would have resulted in a tremendous increase in testing," says Henk Helleman, manager of Restraint Systems Research for ASL (http://rbi.ims.ca/4385-561).
Helleman adds that computer simulations have also taken the guesswork out of product development, especially in the early stages of projects.
"With testing, you can really only verify what you have assumed about your product—the infamous 'make it, test it, break it, and make it again' cycle," he says. "With simulations you can break that cycle because you can analyze simulation results more thoroughly than test results. You can understand better what is going on, and give the product design better direction, which results in a better, more sophisticated product with fewer testing requirements."
By establishing a robust virtual product development (VPD) environment for scientists and product engineers using MSC.Dytran (http://rbi.ims.ca/4385-562), ASL has been able to reduce testing needs and to bring product concepts to fruition more quickly.
Combined structural/fluid analysis
Helleman says a unique feature of the simulation environment is that it provides coupled Euler-Lagrange analysis, allowing combined structural and gaseous fluid simulation. The Lagrangian method is used for structural components that may undergo large deformations and for which the dimensions, deformed geometry, and residual stress state are of major importance.
The Eulerian method enables complex material flow to be modeled without limiting the amount of deformation. With greater deformation, boundaries between the materials may become less precise. This method is used for bodies of fluids and gas that may experience extremely large deformations.
With structural simulation, the mesh deforms and material follows the mesh while the gas/fluid analysis keeps the mesh stationary as the material flows through. Performing both simulations at the same time allows the flow of gas inside a deploying airbag to be modeled.
More testing requirements
Helpful Tools: Analyzing the
split-second deployment of an airbag using computer simulations allows
analytical tools to dissect the crash process, improve product designs
without building costly prototypes, and augment physical
About 1996, Helleman says it became evident that the force of airbag
deployment could cause injuries to children as well as short-stature females
sitting very close to the steering wheel. The U.S. Government changed the
regulatory certification requirements, allowing the industry to reduce the power
of the inflators. Most airbags now have a dual stage inflator with a sensor
determining whether zero, one, or two stages are ignited.
Helleman says a typical analysis performed at ASL might include a vehicle driver side geometry with a 5th percentile female Hybrid III dummy under various loading conditions, including OOP1, OOP2, 25 mph unbelted rigid barrier impact and 35 mph belted rigid barrier impact.
OOP1 and OOP2 are specified in Federal Vehicle Motor Safety Standard 208, which is applicable to frontal impact restraint systems. OOP1 puts the 5th percentile female in close proximity to the steering wheel with the chin resting on the airbag module before the airbag is set off. The airbag is deployed against the head and neck of the dummy, which tests the loads in the neck to make sure they are not so high as to cause serious injury.
Helleman says that OOP2 is similar, but the chin rests on the rim of the steering wheel. The airbag deploys against the chest to make sure the rib cage is not traumatized to the extent it would be life threatening. The analysis may set guidelines for the product design to help it go from the concept stage to a working prototype, or from a general design to an optimized design.
He adds that the mesh for the airbag model is made of approximately 16,000 Lagrangian elements with a nominal size of 10 mm and some 16,000 variable-size Euler elements. Additional elements are used for the airbag module, dummy, and vehicle environment. Total number of degrees of freedom is approximately 250,000.
Beginning with a phased introduction for model year 2004, 14 tests, with four different size test dummies, are required. These include driver side tests with the average size male dummy and 5th percentile female dummy, and passenger side tests with three- and six-year old child dummies.
Other organizations, such as the Insurance Institute for Highway Safety, have designed crash tests. Because these test results are published, every auto maker wants to participate and show good performance. In Europe, there are additional tests required. With a growing number of expensive physical tests required, VPD can be used to assure the restraint system will function as desired for all those test conditions ahead of time.
"At ASL, we really try to concentrate on the needs that are five years out and make sure we have the tools and methods in place to tackle them," Helleman says. He adds that software simulation tools are becoming both more sophisticated and easier to use at the same time.
Helleman asserts that one of the exciting long-term developments in simulation analysis is modeling being done to get an accurate simulation model of a human body. The idea would be to start looking at the effects of a crash on a human being rather than a crash test dummy.
"That would be a sophistication level where the simulation would rise above what you can do with testing," Helleman says. He adds that there's a tremendous amount of development in that area and, although it's a very exciting prospect, it will still take a decade or so as well as a concerted effort to accomplish those goals.