Testing Pedestrian Head Impact on Automotive Closure Parts

Author(s):
Dipl.-Ing. Lutz Berger - ika - Institut für Kraftfahrwesen Aachen

Industry:
Automotive

Products:
DIAdem

The Challenge:
Comparison between the pedestrian safety levels of aluminum and steel, used as hood materials. The draft that was submitted to the European Commission early in 2005, involving special structural requirements relating to the deformation properties for the front of vehicles when they impact on pedestrians, is having a substantial influence in this area.

The Solution:
Evaluation of the head impact using an analysis routine that was created in NI DIAdem. The task was automated in DIAdem using the integrated scripting language, VisualBasic.

Abstract

Various factors such as cost, weight, and structural performance influence the selection of materials used for automotive body parts. For automotive hoods, the weight factor for front-wheel driven cars with a front engine is particularly important, because of the total mass distribution. Unlike other automotive closure parts, hoods must also meet increasing requirements for pedestrian safety. The draft that was submitted to the European Commission early in 2005, involving special structural requirements relating to the deformation properties for the front of vehicles when they impact on pedestrians, is having a substantial influence in this area. According to the draft, all vehicles must have an optimized, energy-absorbent front structure by 2005.

In this case study, the test procedure specified in the planned guidelines is executed on a vehicle. The vehicle is available on the market with an aluminum engine hood or with a steel engine hood, and the design is otherwise identical. To measure the energy-absorbing behavior of hoods in head impact accidents, two different head impactors are used, one child head impactor and one adult head impactor.

The main aim of the study was to compare the general pedestrian friendliness of steel and aluminum, used as hood material. The hood of the test vehicle was not designed to meet pedestrian safety requirements, which means that the results compare the application of both steel and aluminum to assess which hood material is better suited for pedestrian protection.

A pedestrian protection testing facility for head impacts is available and was used at the Institut für Kraftfahrwesen in Aachen. An evaluation routine that was created in DIAdem was used to execute the head impact tests. This routine processes acceleration and optoelectronic barrier signals, calculates the injury criteria, and automatically generates diagrams. The test focuses on the analysis of the head impact. In this context, functions for determining impactor velocity, Head Injury Criterion (HIC), and maximum/average acceleration are implemented. The results can be provided in various formats, such as diagrams and PDF or Excel-compatible files.

Introduction

To develop vehicle fronts under pedestrian safety aspects, a wide range of accident types and impact positions must be tested. Experiments with full-body pedestrian dummies involve so much time and expense, that a European test procedure has been developed based on subsystems such as the head, pelvis, or legs of pedestrians. The conditions, such as velocity and impact angle, are clearly defined, which means that the test routine is easy to run and the results can easily be analyzed.

In this study, the protection potential of automotive hoods is analyzed in relation to the materials, using a production vehicle on which the manufacturer changed the hood material from steel to aluminum in 2002 without changing any other aspects of the exterior design.

The vehicles made between 1999 and 2001 have the same engine capacity and component structure as the vehicles manufactured in 2002 and later, allowing for a comparison based solely on the materials.

DIAdem was used as the measurement and evaluation software. The test tasks were automated using the integrated scripting language VisualBasic.

Structure Tests on the Automotive Hoods

Before the pedestrian tests are executed, the structural performance of the aluminum and steel hoods are compared. Therefore, spare part hoods were used, and reused later in the pedestrian safety tests. DIAdem was used to evaluate the results. These were compared with the results from original hoods that had already been tested in an automotive hood benchmarking project. Therefore, the results of these tests were intended to provide information about the differences between the aluminum hood and the steel hood and about the differences in structure stiffness between the original and the spare parts, (bake-hardening effect). The results for the lateral and the torsional stiffness are shown in Figures 1 and 2 for the serial production hoods made of steel and of aluminum and for the associated spare parts.

The comparison of the hood materials shows that the structure of the steel hood is stiffer than that of the aluminum hood. The advantages of the steel hoods compared to aluminum hoods results in the following percentages in the tested load case:

• Lateral stiffness + 46 %
• Vertical stiffness + 53 %
• Torsional stiffness + 42 %

In this context it must be considered that the aluminum hood is 47% lighter than the steel hood.

The comparison between the original parts and the spare parts showed no significant difference in structural stiffness. Although the tests in the elastic deformation area do not provide any conclusive information about hardening effects in dynamic tests that have a high degree of plastic deformation, such as a head impact, the pedestrian impact tests were executed on spare part automotive hoods and fenders.

Head Impact Tests

The test setup is shown in Figure 3. A servo hydraulic testig facility is used in the test. During the test the pedestrian protection testing facility is positioned at the required angle over the impact points. The impactor is mounted at the end of the piston. The piston accelerates the head to the required velocity of 40 km/h and releases the head, so it impacts on the bonnet in free flight.

For both hood types, nine tests each were executed with an adult head impactor and with a child head impactor. This made a total of 36 tests. The test results were documented using the HIC value, the time interval used to calculate the HIC value, a_3ms, a_max, high-speed video recordings, and digital photos of the physical hood deformation. The video sequences allow a detailed analysis of the deformation behavior of the hood, for example, or of the times of the primary (head to hood) and secondary (hood to underlying structure) impacts.

A triaxial acceleration sensor is positioned at the center of each impactor. Figure 4 shows an example of a test evaluation that was automated and evaluated using DIAdem.

DIAdem can evaluate the measured values from the tests separately or as a series. Figure 5 shows an example of how the user can enter the measurement file that is to be evaluated and the variables that are required for the calculation, in user dialog boxes. The dialog boxes are defined in scripts that are created in DIAdem.

DIAdem was also used to verify the specified impactor velocity. A optoelectronic barrier is used to record the acceleration of the impactor on the piston. A beam laser is reflected by a reflection film that is mounted on the piston. The film has a specified grid of light-dark transitions. A displacement-time calculation analyzes the recorded rectangle signal. This is done with another VisualBasic script. The calculation determines the peaks of the rectangle signal and the time interval between two consecutive equal values. Using the specified distance between the light-dark transitions, the velocity can be determined.

The user enters the grid of light-dark transitions and other variables in user dialog boxes at the beginning of the routine. This provides flexibility in the use of the various test types, although basically the same calculation is always executed. This saves time and guarantees a high level of consistency, because the user does not need to adjust the variables within the scripts.

The evaluation of the 36 tests executed on the automotive hoods showed that in 13 of 18 cases, the steel bonnet produced a lower impact force on the head. When analyzing the results, it must be taken into account that most of the results for both bonnet materials greatly exceeded the biomechanical limit (HIC 1000).

The HIC values that greatly exceed the limit resulted from the secondary impact on the underlying vehicle structure. The deformation displacement (5-20 mm) between the internal hood metal and the underlying structure is particularly short around the spring strut and the hood hinges on the tested vehicle. The high degree of stiffness of the affected components resulted in enormous acceleration peaks and HIC values.

To analyze the effects of the secondary impact on the acceleration curve and the HIC in Figure 6, two points were selected and compared as examples for the steel and aluminum versions. During the impact at point Ch-M-2, no significant impact was recorded on the underlying structure. The resulting acceleration curve led to:

• a greater window for the HIC calculation in the aluminum version,
• a higher acceleration peak for the steel version,
• 15-20 % more deformation displacement for the aluminum version and
• a higher HIC value for the steel version.

During the impact at point Ch-M-3, a stronger secondary impact was recorded on the underlying structure. This led to:

• almost equal windows for the HIC calculation, because the acceleration curve was substantially affected by the impact on the underlying structure,
• a slightly higher first acceleration peak for the steel version,
• a higher secondary acceleration peak for the aluminum version and
• a higher HIC value for the aluminum version.

Summary

The results of this study refute the widely-accepted view that aluminum hoods generally provide better protection for pedestrians. The results of the head impact test show that in 13 of 18 cases, the steel hood produced a lower degree of head impact.

Using DIAdem to perform the measurement tasks, evaluation tasks, and automation tasks allowed a highly efficient study. The user-friendly dialog boxes prevent errors and have a time-saving effect that is reinforced by the option to analyze the results of entire test series. The evaluated data can be presented as a diagram, a PDF, or in Excel format, facilitating any follow-up work that may be required.

Author Information:
For more information on this Case Study, contact:
Dipl.-Ing. Lutz Berger
ika - Institut für Kraftfahrwesen Aachen
Steinbachstrasse 7
52074 Aachen
Germany
berger@ika.rwth-aachen.de

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