Occupant protection evaluation for rear-end crash based on integrated simulation

Cheng-yue JIANG, Ke WANG, Kun CHEN, Xiao-yan HU, Wei ZHANG

(College of Vehicle Engineering, Chongqing University of Technology, Chongqing 400054， China)

Abstract: The influences of integrated restraint system, combining with the Automatic Emergency Braking (AEB) system, upon occupant posture and injury were studied in this paper. Based on the rear-end crash test scenarios for NHTSA and vehicle geometry data, both AEB system model and the MADYMO simulation model were built up. After integrating these two models, the integrated model was built up and validated with certain experiment data to ensure its accuracy. The study indicated that the maximum head3ms acceleration could be decreased by 8.6%, the HIC36 could be decreased by 18.3% and chest compression could be decreased by 13.8%, after introducing the AEB system.

Key words: Automatic emergency braking, Rear-end crash, MADYMO simulation, Integrated model

The accident statistics of the Ministry of Public Security Traffic Management Bureau show that there were 212846 road traffic accidents in China, and the number of the victim was as high as 226430 in 2016[1]. Face with the frequent traffic accidents, the passive safety technology has basically solved the problems of vehicle crashworthiness by optimizing the car-body structure and adopting the occupant restraint systems such as airbags and seatbelts. However, with the bad driving behavior such as the fatigue driving, improper operation and illegal parking, a large number of traffic accidents keep occurring[2]. In order to improve the vehicle safety and help the driver to avoid the wrong operations, the active safety technologies have been gradually developed by Advanced Driver Assistance Systems (ADAS). The most representative one is the Automatic Emergency Braking (AEB) system, which has been tested and evaluated among the Euro-NCAP[3]. In accidents that cannot be prevented by active strategies, it may still reduce the occupant injury severity by passive safety elements.

This study focuses on the influences of integrated restraint system, combining with the Automatic Emergency Braking (AEB) system, upon occupant posture and injury during the crash phase, for the purpose of the maximum reduction of accident severity and injury indexes.

1 Integrated simulation

1.1 AEB system model

The key of the AEB system is the collision avoidance algorithm, a good collision avoidance algorithm can not only meet the driving habits, but also avoid the collision in a variety of dangerous conditions. At present, the algorithm of Time to Collision (TTC)[4] is widely used and its effectiveness has been validated. Therefore, the active and passive integrated safety has been analyzed based on the TTC algorithm. The calculation formula of TTC is shown below:

(1)

Dis the relative distance;vris relative speed.

The control logic of AEB system is shown in Table 1. When the TTC value is between the 1.6 s and 0.6 s, the system will send the braking signal to the braking system and adopt the 40% braking force. However, in case the value of TTC is less than 0.6 s, it will adopt the 100% braking force.

Table 1 AEB control logic

1.6 s flag0.6 s flagSystem action00No action1040% braking11100% braking

Based on the rear-end collision accident caused by the lane change as in the NHTSA report[5], the dynamic scene has been established by the Prescan software[6]. The dynamic scene is shown in Fig.1 below. The striking vehicle was overtaking the truck through the lane changing. The radar of the striking vehicle could not detect the stop struck vehicle due to the blind area caused by the truck. After changing the lane, the relative distance of these two car was 8 m and the initial speed of the striking vehicle was 60 km/h.

Fig.1 The dynamic scene

When detecting the struck vehicle, the TTC(0.48 s) was less than 0.6 s and the system sent the braking signal to the braking system and adopted the 100% braking force. The speed was reduced to 50 km/h at the moment of rear-end crash.

1.2 The restraint system model

Based on the vehicle’s CAE model, the front impact model was established by the MADYMO software[7]. The occupant simulation model was built by importing Hybrid-Ⅲ facet dummy from the dummy database, after integrating a correlated driver airbag and knee airbag module.

In addition, adjustment of each substructure according to real vehicle test and definition the contact between the subassembly parts were carried out[8-9]. The front impact model is shown in Fig.2 below.

1.3 The integrated safety model

This paper mainly analyzes integration effects of active and passive safety, including both AEB stage and the collision stage. The start time (0 ms) was set when the AEB started to work, which mainly provided the integrated model with braking deceleration, as shown in Fig.3.

Fig.2 The driver restraint system model

Fig.3 Braking deceleration

The analysis of collision process used the validated finite element vehicle model for rear-end crash which provides the B-pillar acceleration to the integrated model. The pre-crash stage 100% braking strategy was used to reduce the vehicle speed from 60 km/h to 50 km/h at the collision moment. Fig.4 shows the B-pillar acceleration for rear-end crash.

Fig.4 B-pillar acceleration

After integrating the boundary conditions according to these two models, the integrated model was built up, as shown in Fig.5. Then the influences of integrated restraint system upon occupant posture and injury would be studied.

Fig.5 Integrated model

2 Integrated model validation

2.1 Posture of dummy validation

The Pre-crash phase mainly affects the dynamic kinematic of the driver. The model validation was mainly about theXdisplacements of the head and chest. This experiment data was carried out from a series of real vehicle experiment with the volunteer according the research of K Yamada[10]. The simulation animation indicated that theXdisplacement of the head and chest were both in the range of the test, see the Fig.6 below. Thus the simulation model can be used in the Pre-crash phase.

2.2 Restraint system model validation

In the collision phase, the model validation work was mainly about the dummy position, FE-FE and FE-MB contact characteristic definition[11]. The simulation indicated the chest acceleration and chest compression correlated well with that of the front impact in case of 50 km/h frontal impact, see the Fig.7 below. Thus the restraint simulation model can be used for further occupant injury analysis.

Fig.6 Correlation of the displacements

Fig.7 Correlation of occupant injuries

3 AEB parameter analysis

This paper focused on the analysis of AEB system effects on the driver’s dynamic response and injury. After introducing the AEB system, it affected the dynamic response of occupant in Pre-crash phase and the vehicle speed was reduced from 60 km/h to 50 km/h before the rear-end collision in this case. The method of the research is show in the Fig.8.

Fig.8 Different front impact scenarios

3.1 Occupant posture in pre-crash phase

This research selectedXdisplacements of the head and chest as the parameters to compare the occupant posture. When considering the AEB system, it had a great effect on the head and chest displacements, as shown in Table 2.

Table 2 Occupant displacements at the moment of rear-end crash

Head Displace-ment/mmChest Displace-ment/mmWithout AEB00With AEB8547

3.2 Injury index

This paper only selected occupant’s head and chest injury indexes as the main evaluation parameters because the braking scenarios mainly affect the occupant’s up-region postures. The simulation results indicated that the maximum head3msacceleration could be decreased by 8.6%, the HIC36could be decreased by 18.3% and chest compression could be decreased by 13.8% when comparing those without AEB system, as shown in Table 3. The injury indexes could be decreased because the maximum collision deceleration was reduced when introducing the AEB system.

Table 3 Driver injury comparison

Head3msAcceleration(g)HIC36Chest Com-pression/mmWithout AEB24.59329With AEB22.47625

4 Conclusions

In this study, an integrated simulation model including both AEB system and restraint system was built-up and correlated with the experiment data, for the purpose of analyzing the AEB system effects on the dummy displacements and injury index. According to analysis above, the following conclusions can be draw:

(1) The design of an integrated safety system is efficient to analyze the AEB effects on the occupant posture and injury index.

(2) After introducing the 100% braking force from AEB system, the maximum head3msacceleration could be decreased by 8.6%, the HIC36could be decreased by 18.3% and the chest compression could be decreased by 13.8%.

Acknowledgements

This paper is supported by National Natural Science Foundation of China(No.51405050).