Frontal Impact Simulations

Occupant and Vehicle simulation models have been used since the early days of airbag development to find the optimum parameters for its workings in combination with seat belts, seat, steering column, and dash board. Initially these models were two-dimensional and the results could only be viewed after a series of images were plotted on paper.

Simulation With the introduction of afordable graphical Unix workstations around 1990 the abilities to analyze and view the occupant kinematics vastly improved.
The multi-body and explicit finite element codes such as MADYMO, DYNA3D, Radioss, and Pam-Safe are widely used in the industry for design and optimization of restraint systems. This animation from 1994 shows a MADYMO model of a 30 mph frontal crash with 2 belted adult male dummies. The model used Finite Element Airbags and multi-body descriptions of the other parts. This model started its life known as the TNO Sedan. It was based on the work left behind at TNO by Chuck Bosio. Colleague Roderick Verschut did the seat model and Henk developed the restraint systems and the dummy models. Over the years many incarnations of this model were developed and distributed to interested MADYMO users, who often used it as the basis for their frontal restraints development simulations.

Multi-body techniques can be used when the deformations of the object do not play a significant role or can be represented with a simple force-deflection characteristic. Multi-body techniques have the advantage of relatively fast analysis since they can describe large bodies with a few equations. They also allow accurate modeling of the joints between the body segments.
Finite Element techniques allow a detailed description of the geometry and stiffness characteristics of objects. This is important for objects that are subjected to large deformations, such that the initial geometry is no longer representative. The more detailed description comes with a price of additional computation time, but with the ever increasing speed of computers, this is less of a problem today than it was 15 years ago.
The "Advanced Airbag Rule" enhanced the Federal Motor Vehicle Safety Standard (FMVSS) 208 for vehicles of model year 2003 and newer (with a 4 year phase in through model year 2006). Additional certification of the airbag restraint systems was mandated to demonstrate that they cause no serious harm to closely seated occupants. While the certification procedure requires physical tests to be conducted on the final product, simulation analysis is the desired tool for the development phase.

Developing the models and methods to analyze low-risk-deployments for these out-of-position scenarios has been a long and difficult process. Because of the close proximity of the occupant to the airbag module, high demands are placed on the modeling accuracy of the simulation. In particular the unfolding process of the airbag must be simulated accurately. For that it is necessary that the flow of the gas, which inflates the airbag, is modeled accurately as well as the forces that are transmitted through layers of the folded airbag, contacting each other. This has proven to be a non-trivial issue that has many scientists of various software vendors pull their hairs out.

[ OOP1 - click to animate ] Through years of cooperative development work between the automotive industry and the software developers, reliable methods have been developed to simulate these out-of-position scenarios. The specialty fields of multi-body dynamics, finite element mechanics, computational fluid dynamics, and injury biomechanics are combined to bring solution to this complex issue.

This image shows an analysis of a 5th Percentile Female Hybrid III dummy in the FMVSS 208 Out-of-Position scenario 1 (also known as "chin on the module"). The simulation, which used MSC.Dytran, serves to analyze and mitigate the injury risk to a short statute driver from a deploying airbag. Click the image for an animation (AVI-RLE 1.2 Mb).

Besides child dummies being part of the "Advanced Airbag Rule" they are also featured in certification tests for child safety seats (more formally Child Restraint Systems or CRS). [ ECE R44 simulation ] In Europe this is governed by the ECE R44 regulation and in the USA by FMVSS 213. Slightly different tests, but both with the main objective to assess the integrity of the CRS under vehicle crash conditions.
A more recent development is the inclusion of a CRS with infant or child dummy in new car assessment testing, where the star rating of the vehicle is affected by certain performance measures of the child dummy. This is now part of the Euro-NCAP as well as its derivatives the Latin-NCAP and China-NCAP.
The effectiveness of a CRS in protecting a child is significantly affected by how well it is installed in the vehicle and how well the child is restrained within the CRS. Developments such as ISOFIX and LATCH address the former, while the U.S. Department of Transportation, National Highway Traffic Safety Administration, the Department of Motor Vehicles and various consumer organizations address the latter with informational media.


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