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Finite-element model of the impact test.

Methodology developed for safer hood design

Global regulations intended to enhance pedestrian protection in a vehicle collision are presenting significant challenges to vehicle engineers. For example, they are being pushed to use lightweight materials to improve fuel economy. A hood made of a lightweight material would help in that regard but would not be viable if the material did not absorb energy as well as a conventional steel one would.

Typically, the distance between the hood and underhood components is measured in tens of millimeters.

That tiny distance is enough, however, depending on how the hood is designed.

Researchers at General Motors Co. and Stanford University came up with an approach that involves energy absorbers attached to the underside of the hood. The energy absorbers were small, square metal sheets made of different off-the-shelf materials in different thicknesses.

The sheets were bent into a C-shape, and physical impact tests were run on a specially created test apparatus consisting of an Instron Dynatup 9250HV drop tower load frame and stereo system for recording digital images of the impact event.

The researchers developed a finite-element model of the test setup as well, using LS-DYNA from Livermore Software Technology Corp. to determine whether the HIC (head injury criterion) scores from the test could be predicted.

The researchers’ end product is not physical parts that could be fitted to a hood; rather, it is an overall methodology that enables laboratory measurement of component-level HIC scores on C-channel local-impact energy absorbers fabricated from the different materials.

The methodology, according to the researchers, exhibits good repeatability and sensitivity in low HIC value ranges. This was demonstrated by capturing the difference in HIC values, with statistical significance, between the rolling and transverse directions for the magnesium sheet alloys. Despite the low computed HIC values, a qualitative and quantitative difference in response between different materials can be established. This enables identification of materials with the lowest HIC scores from a set of candidate materials and is especially useful in evaluation of new lightweight materials.

High variability in HIC scores in full-vehicle tests often comes from combined variation in dimensions and assembly of all vehicle parts underneath the hood impact location, and therefore it is difficult to establish how much variation comes from the use of different materials in energy-absorbing components. The simple test methodology developed in this research project decouples the absorber material response in impact conditions from the effects of other components in the vehicle and provides more reliable, quicker, and cheaper initial material screening than tests per-formed on vehicle hoods.

For novel materials (such as new types of composites, for example) for which the material models are still under development, the methodology can be used to identify materials with the greatest potential for energy-absorbing applications. Alternatively, it can also be used to validate new material models so that the design development and optimization can be performed virtually for newly developed materials as well.

The methodology enables material selection and design optimization of energy absorbers for pedestrian protection based on a simple laboratory test and FE model, eliminating the need for extensive vehicle testing.

This article is based on SAE International technical paper 2014-01-0513, “Passive Pedestrian Protection Approach for Vehicle Hoods,” by Vesna Savic of General Motors Co.; Matthew Pawlicki of Stanford University; and Paul Krajewski, Mark Voss, Louis Hector, and Keith Snavely of General Motors Co. It will be presented at the SAE 2014 World Congress on April 9 as part of the “Occupant Protection: Pedestrian and Cyclist Safety” technical session planned by the SAE Occupant Protection Committee / Automobile Body Activity and organized by Jason R. Kerrigan of the University of Virginia; Carlos Arregui Dalmases; and Philippe Beillas of Ifsttar-LBMC.

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