Research of effective construction of aluminum hood for pedestrian protection and low hood styling

Author: 
Osamu Ito (Honda R&D Co. Ltd, Automobile R&D Center)

CHALLENGE - According to Japanese traffic accident statistics, the number of overall casualties on a downward trend but still more than a 1000 fatalities of pedestrian still occurred every year. Therefore, it is important that automobile manufacturers research the safety of vehicles for pedestrians. Especially, it is necessary to study bonnet hood construction that the pedestrian’s head impacts reduce head injury as cause of fatality. On the other hand, body construction of vehicle must consider weight reduction so as to reduce the CO2 emissions. For that reason, automobile manufacturers have increased the use of aluminum for bonnet hood. It is known that longer crash strokes are needed for pedestrian protection if aluminum hood is used compared with steel hood. It is because that energy absorption characteristic is inferior in aluminum on account of low inertia weight and low stiffness. Accordingly, longer clearances under the hood are needed and restrictions of layout increase if the aluminum hood is adopted. To combine pedestrian protection and weight reduction has high design difficulty for automobile manufacture.

SOLUTION - The authors studied aluminum hood construction that can reduce pedestrian crash stroke convent conserving pedestrian protection performance in order to reduce restrictions of layout. Since the pedestrian's head may impact to any location on the hood, several evaluation points are needed for validating the pedestrian protection performance. In order to design validated point easily, independent construction is desirable. In this research, emboss construction was adopted for the hood, in order to lower the stiffness response for each evaluation point. Furthermore, a CAD parameter model of this emboss hood was created, and optimization CAE was combined. Injury values and strokes were evaluated by the optimization CAE. Convergence of solution takes much time from the several evaluation points and variables if optimization is performed. Then, at the beginning, 30 designs were created with uniform random number. Secondly, they were calculated so that it could look down at overall performance in order to find the most severe evaluation point, which it is difficult to meet the HIC requirement. Thirdly, for the most severe evaluation point, optimization CAE was performed so that stroke could be minimized. Lastly, the optimized shape in the severe evaluation point was validated whether performance of other validation points were also improved.

BENEFITS - Because of optimization CAE, the head impactor stroke was reduced 6% compared to the conventional aluminum hood. Moreover, HIC value of all validation points was below target value. As a result of analyzing design variable contribution, various factor existed for HIC and stroke. Sheet thickness and gap between hood frame and skin had influence on both stroke and HIC. In addition, the array pattern had influence on only HIC. After deceleration of the impactor was determined by hood gap and sheet thickness from 0msec to 4msec, it was determined by emboss pattern after 4msec. However, in order to apply this research to vehicle development, it is necessary to review the stamping manufacturability of aluminum and to re-set design variable as probable realistic range. The authors studied aluminum hood construction where both low HIC and low stroke were realizable. This research could contribute to improvement in both pedestrian protection performance and weight reduction.

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