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EASi Crash Dyna software from ESI Group is used to determine instrument panel head-impact points.
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Federal Motor Vehicle Safety Standard (FMVSS) 201 defines strict requirements for instrument panel head impact. FMVSS 201 regulations state that when the area of the instrument panel (IP) that is within the head-impact area is impacted by a 6.8 kg (15 lb), 165 mm (6.5 in) diameter head form at 19 km/h (12 mph), the deceleration of the head form shall not exceed 80 g continuously for more than 3 ms. The regulations do not define the locations of all the impact points, so it is understood that the manufacturer is responsible for all points within the impact zones.
Ultimately, automobile manufacturers must prove that their IP designs meet these regulations by subjecting them to physical testing. The IP is held in a test buck. An impact device accelerates a head form into the IP while accelerometers in the head form provide a time history of the resulting deceleration. While this approach provides the gold standard for evaluating IP head impact performance, it is also very expensive. Physical testing requires the construction of a prototype IP and also occupies a considerable amount of time of skilled personnel and expensive testing equipment. In addition, this test can also only be performed in the latter stages of the development process when prototypes are available.
During the earlier stages of the development process, automobile manufacturers use crash simulation to evaluate the performance of alternative IP designs. They build prototypes and perform physical testing only after they have confirmed through simulation that the design will pass the tests. This ensures that the expensive physical tests only need to be performed once in most cases.
The manual process for simulating the head impact of an IP design begins with developing the head-impact zones and identifying the worst-case points within the zones. Analysts normally perform this process while working in software designed to create a model for crash analysis, such as EASi-CRASH DYNA (ECD) from ESI Group. A line is constructed to pass through the driver's upper body and a sphere is placed at one end of the line to represent the position of the head. Then the line along with the sphere is rotated towards the IP until the sphere makes contact while the bottom of the line is held fixed.
The ball is swung from side to side to sweep out a contact zone. Next, the worst-case points within the contact zone are identified. The assumption is made that if these points meet the standards then the entire IP has acceptable head impact performance. Generally, the worst-case points are chosen based on previous history and experience, their shape and structure with sharp edges and points that stick out from the panel generally presenting the greatest potential for injury.
The next step is determining the angle at which the head form would strike the IP at the identified points. A line is positioned vertically to represent the initial position of the driver's upper body and then rotated toward the IP until contact occurs. The perpendicular of the line at the point at which it intersects with the IP is considered to represent the direction of impact. In the past, an analyst was responsible for manually setting up the impact angle, and configuring the contacts between the impact point and the sphere of the head impact device. At this point the model is ready for crash analysis. ECD generates an output file that is solved by LS-DYNA crash simulation software, which runs on various high performance computing platforms.
An instrument panel typically has 12 to 15 points and the analysis is usually re-run 10 to 15 times during the vehicle development process due to design changes, so well over 100 hours were devoted to this process during the development of a single IP. Another weakness of the manual approach is that different analysts usually obtain different results on the same instrument panel design.
Visteon decided to automate head impact modeling and selected EASi-PROCESS as their automation tool. "The key advantage of EASi-PROCESS and the reason that we selected it for this project is its customizability and the ability to interface with popular CAE tools," said Srikanth Krishnaraj of Visteon's Interior Systems CAE & Safety group. "In this application, we leveraged its interfaces with ECD and LS-DYNA." EASi-PROCESS includes a process builder for constructing CAD process flow templates, an extensive library of process building blocks to create components for CAE process automation, and a process executor that runs processes in an intuitive, graphical environment. EASi-PROCESS has been adopted by numerous original equipment manufacturers for automating complex processes, extending CAE technology to a broader audience and ensuring that the best CAE practices are incorporated into future vehicle development.
Visteon engineers developed a flow chart that describes the steps required to model the body mounts in considerable detail. Visteon and ESI Group developers worked together to develop the custom routine that automates the instrument panel head impact simulation. The developers used building blocks to capture the manual process in an automated routine that closely follows the manual process. The analyst inputs the LS-DYNA finite element model of the instrument panel, the line representing the initial position of the driver, the spherical head form and a table of impact points in ASCII format. The routine determines whether the chosen impact point is within the head impact zone. It then computes the impact angle, positions the head form at the impact point, assigns an initial velocity to the head form, and creates LS-DYNA contacts between the head form and instrument panel interior and among the instrument panel interior components. The routine then creates LS-DYNA input-output control cards, exports LS-DYNA input data and submits it to the solver. The routine reads the LS-DYNA results and generates an electronic report of the analysis.
"In this application we were able to reduce the time required to generate a model for head impact crash simulation to a small fraction of the time required in the past," Krishnaraj said. "This substantial time savings allows us to focus highly-skilled resources on addressing design challenges as opposed to repetitive modeling tasks. Besides saving time, the accuracy of the modeling process has been substantially improved by harnessing a computer to perform repetitive tasks and ensuring that every model is created exactly the same way."
Olivier Morisot, Marketing Manager, ESI North America, wrote this article for AEI.