TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 09/18 pp290-295 Volume 12, Number 3, June 2007

Side Structure Sensitivity to Passenger Car Crashworthiness During Pole Side Impact

DONG Guang (董 光), WANG Dazhi (王大志), ZHANG Jinhuan (张金换)**, HUANG Shilin (黄世霖)

State Key Laboratory of Automotive Safety and Energy, Department of Automotive Engineering,

Tsinghua University, Beijing 100084, China

Abstract: Side impact accidents of passenger cars with fixed poles may result in severe injuries to the vehi-

cle occupants. In this paper, side structure intrusion was considered as the criterion for passenger cars

crashworthiness during side impact with a pole. The relationship between side intrusions and the side struc-

ture stiffness was analyzed. The acceleration of the unstruck side vehicle body was selected as the criterion

for studying the influence of different side structure components on the side structure stiffness during pas-

senger car pole side impacts. The behavior was analyzed using finite element simulations. The results show

that the rocker and the lower part of the B-pillar are the key parts of the side structures in determining the

passenger car side stiffness. Passenger car pole side impact crashworthiness is, therefore, most sensitive

to these two components.

Key words: passenger car; pole side impact; crashworthiness; finite element simulation

Introduction

Side impacts are frequent and often result in extremely harmful crashes[1]. Global accident statistics show that side impacts account for approximately 30% of all im-pacts and 35% of total fatalities (source: German In Depth Accident Study-GIDAS, National Automotive Sampling System-NASS & BMW accident database)[2]. According to the Chinese road traffic accident statistics for 2002, more than 33% of all traffic accidents were side impacts[3]. Research focus in China is therefore being switched from frontal impact safety to side im-pact vehicle safety[4]. Side impacts also require more attention in that there is considerably less crash zone for absorbing energy in the side of passenger cars compared to the front and rear structures[5], and

consequently the occupants sit almost within the crash zone, which often results in severe injuries[6]. Research and development into minimizing structure intrusions during side impacts of passenger cars is essential to re-duce the effects of side impacts.

Most side impacts can be classified into two types, car-to-broad-object and car-to-narrow-object. The lar-ger intrusion caused by the latter type of side impact is generally more dangerous to the occupants. Examples of narrow objects involved in side impacts are poles, lamp posts, and barrier tubes. In this paper, we report an investigation of the characteristics of passenger car side structure crashworthiness and provide some in-sights into the improvement of occupant safety during pole side impact.

1 Analysis of Passenger Car Side Structure Under Pole Side Impact

Figure 1 shows the typical side structure of a passenger

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Received: 2006-06-02; revised: 2006-10-31 To whom correspondence should be addressed. E-mail: zhjh@tsinghua.edu.cn; Tel: 86-10-62781628

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car, which includes the A-pillar, the B-pillar, the C-pillar, the front and rear doors, the crash beam, the side panel, the rocker, and the roof cross member.

Fig. 1 Typical side structures of passenger cars

During a pole side impact, the side structure of a passenger car is struck by a narrow object. The stiff-ness of a vehicle’s side structure is much lower than that of the front structure[7] and there are not enough components or space to absorb the impact energy. Consequently, there will be a substantial intrusion con-centrated in the impact zone.

In order to investigate passenger car protection sys-tems in pole side impact, EuroNCAP (European New Car Assessment Program)[8] and FMVSS214 (Federal Motor Vehicle Safe Standard 214)[9] prescribe a pole side impact test procedure, in which a moving vehicle comes into contact with a stationary pole.

2 A Simplified Dynamic Model of a Pole Side Impact

In order to investigate vehicle crashworthiness in pole side impacts a simplified dynamic model is introduced to analyze the dynamic response of the passenger car. This dynamic model is a modification of Liu et al.[10]. In this model the struck side of the vehicle is set as one part, M1 and other parts of the vehicle body are set as the rigid part, M2. M1 is connected with the vehicle body by linear elastic springs K1, representing the stiffness of the side structure of the vehicle. The spring K2 is equivalent to the stiffness of the door trim.

The simplified dynamic model for vehicle pole side impact is illustrated in Fig. 2.

Fig. 2 The simplified dynamic model of passenger cars in pole side impact

Let the impact force be Fimpact and the side structure intrusion be Xintrusion. Kequivalent is introduced to repre-sent the combined spring constant of K1 and K2. The approximate relationship between Fimpact and Xintrusion can be described according to Hooke’s law:

Fimpact = Kequivalent Xintrusion. It has been shown that the magnitude of test vehicle

acceleration during pole side impacts is smaller than that during side moving deformable barrier (MDB) impacts, though the resulting structural deformation or intrusion is much larger than that during side MDB impacts[11]. Consequently, in the analysis of pole side impact vehicle crashworthiness, the intrusion of the vehicle side structure is an important and reasonable criterion for the study of vehicle crashworthiness.

In order to improve pole side impact vehicle crash-worthiness it is necessary to increase Kequivalent, the side structure stiffness. A higher value of Kequivalent will for a given impact force Fimpact result in a smaller intrusion and larger survival space.

According to Newton’s second law, Fimpact = m2 a2 = Kequivalent Xintrusion.

The side structure stiffness Kequivalent can be investi-gated by studying the vehicle body acceleration a2. m2 is the mass of the vehicle body M2. This can be con-veniently achieved using finite element simulations. The vehicle body acceleration can also be used to in-vestigate the side structure sensitivity to crashworthi-ness during pole side impact.

During vehicle side impacts with broader objects, the crash zone covers a relatively larger set of struc-tures with a different overall stiffness. The different types and amount of structures involved in an impact

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results in a wide variation in the passenger car side stiffness[12]. In this paper we consider only impacts with fixed narrow poles and examine the influence of side stiffness on such impacts.

3 Side Structure Sensitivity of Passenger Cars to Pole Side Impact Crashworthiness

The vehicle structure sensitivity to pole side impact crashworthiness is defined as the influence of the change in strength of the side structure components on the vehicle crashworthiness, which can be represented by the vehicle body acceleration during pole side im-pact. The finite element vehicle model used in this pa-per has been validated by Wang et al.[11] and can be used as the basic model[12] for further research on vehi-cle side structure sensitivity. The finite element (FE) simulations were run with LS-DYNA massively paral-lel processors (MPP) code version 970 on an 8-node PC cluster[13,14].

In the model, the strengths of some key components of the side structure in the basic model have been changed by increasing their yield stress values to up to two times the original value. The key components of the vehicle side structure are divided into four areas. These are the B-pillar including the lower and upper part of the B-pillar and the B-pillar inner panel, the floor and rocker, the doors and side panel, and the roof and roof cross member.

All the pole side impact FE simulations were con-ducted according to the European Pole Side Impact Testing Protocol (Version 4.1)[8]. All the vehicle body acceleration data was measured by an accelerometer placed at the lower point of the B-pillar on the un-struck side of the vehicle model.

3.1 Strengthening the B-pillar

It is clear that strengthening the B-pillar reinforcement panel helps to increase the vehicle acceleration (Fig. 3). The stiffness of the side structure is therefore increased by strengthening the B-pillar of the vehicle, according to the dynamic model introduced previously. As the stiffness of the side structure is increased, side intru-sions will be decreased resulting in an increase in pas-senger car crashworthiness during pole side impacts.

In the pole side impact test, the centerline of the

Fig. 3 Effect of strengthening the B-pillar on the ve-hicle body acceleration

impact pole is aligned with the gravity center of the head of the test dummy[8,9] so that the impact location is in the front of the upper part of the B-pillar. The de-formation of the side structure is concentrated at the impact zone, so that the upper part of the B-pillar re-mains almost undeformed (Fig. 4). However, the lower part of the B-pillar is located in the impact zone and deforms substantially. The deformation mode of the lower part of the B-pillar determines the configuration of the entire B-pillar after impact. Strengthening the lower part of the B-pillar will therefore be an effective way to increase the side stiffness of a passenger car.

Fig. 4 Deformation of B-pillar

3.2 Strengthening the floor and the rocker of the vehicle

As a coverage component, strengthening the floor has almost no effect on the first peak of the vehicle accel-eration, but does lead to an increase in the later peak (Fig. 5a) as a consequence of the deformation sequence. The door’s side panel deforms first, then the rocker, and then finally the floor.

A significant increase of the vehicle body accelera-tion results from strengthening the rocker, especially for the first peak of the vehicle body acceleration

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(Fig. 5b). The rocker is a key component in the frame structure of the vehicle. The rocker plays an important role in determining the stiffness of the side structure and pole side impact crashworthiness.

Fig. 5 Vehicle body acceleration after strengthening the floor and the rocker

3.3 Strengthening the door inner panel, door outer panel, and side panel

Strengthening of the door inner panel, door outer panel, and vehicle side panel has almost no effect on the ve-hicle acceleration, and consequently there is no need to change the stiffness of the side structure (Fig. 6). This is because of the planar structure of these three com-ponents and because of their bending deformation mode (Fig. 7). Only a small amount of energy is ab-sorbed by these components, so the effect of strength-ening them is quite small.

3.4 Strengthening the roof and roof cross member

A strengthened roof and roof cross member also do not result in an increase of the acceleration (Fig. 8), again due to the special planar shape of these components and their bending deformation mode (Fig. 9). These components do not, therefore, have much influence on the side structure stiffness.

Fig. 6 Vehicle body acceleration

Fig. 7 Deformation of side panel (Top view)

From a consideration of the change in the accelera-tion caused by strengthening different side components, it is seen that the B-pillar and the floor/rocker affect the vehicle side stiffness the most. The components in these two areas are therefore most sensitive to the pole side impact crashworthiness of a passenger car. This is especially true for the lower part of the B-pillar and the

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rocker as shown in the following analysis.

Fig. 8 Vehicle body acceleration of strengthening the roof and roof cross member

Fig. 9 Deformation of roof components

4 Validation by Strengthening the Lower Part of B-pillar and Rocker

As shown in Fig. 10, the strengthening of the lower part of the B-pillar and the rocker results in an in-creased acceleration of the vehicle body and therefore to a large increase in the side structure stiffness, and to a substantial improvement in the vehicle crashworthiness during side impacts with fixed poles.

Fig. 10 Vehicle body acceleration of strengthening the lower part of B-pillar and rocker

Consequently, the conclusions derived from an analysis of the different side vehicle components are validated.

A comparison of the change in vehicle body accel-eration as a result of either strengthening the lower part of the B-pillar or strengthening the rocker (Figs. 3 and 5b) shows that strengthening of the rocker has a larger effect on the side stiffness than strengthening of the lower part of the B-pillar of the vehicle.

5 Conclusions

The crash mode of the side structure of a passenger car takes a particular form in pole side impact and is dif-ferent from common side impacts. Consequently, the parts of the side structure controlling the passenger car impact crashworthiness are different from those during other types of impact. The results of an investigation into the factors controlling the crashworthiness during pole side impact yield the following conclusions:

(1) Intrusion of the vehicle side structure is a mean-ingful and reasonable criterion for assessing vehicle pole side impact crashworthiness.

(2) A feasible and effective method to study the side structure stiffness and crashworthiness of passenger cars in pole side impacts is to follow the acceleration of the vehicle body.

(3) The rocker is the most important component in determining the passenger car crashworthiness during pole side impact, and consequently improving this component is the most effective way to increase the stiffness of the vehicle side structure. The next most important component is the lower part of the B-pillar.

(4) The stiffness of the side structure can be in-creased by strengthening the vehicle components that are relevant to the passenger car crashworthiness in pole side impacts. Consequently, side intrusions can be reduced and the vehicle pole side impact crashworthi-ness can be improved.

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