Investigation on the Behaviour of New Type Airbag* Hu Lin,Liu Ping Huang Jing

College of Mechanical Engineering The State Key Laboratory of Advanced Design & Manufacture for Vehicle Body

University of Shanghai for Science and Technology Hunan University Shanghai, China, 200093 Changsha, China ,410082

Hulin888@sohu.com luckycitrine@163.com

* Supported by Shanghai Leading Academic Discipline Project, Project N.J50503

Abstract -This research a new type airbag system (NAB) was developed in this study which consists of two flat layers and a middle layer with tube-type structure. The objective of the new airbag system is to enhance the occupant safety in passenger car collisions with reduction of injury risks for small and OOP occupants. A prototype of the airbag was built up with volume 40 litters. The performance of the prototype airbag was investigated using static deployment test and sled crash test with Hybrid III dummy. A FE model of the airbag was developed and validated using results from the airbag deployment test and sled crash test. In order to determine the potential for occupant protection, computer simulations of the OOP occupant with NAB and the normal driver-side airbag (DAB) are carried out using HYBRID-III FE model. Results indicated that the NAB airbag need less gas than the DAB to get the same load and displacement curve, and the leading velocity of NAB is lower than that of DAB. In the OOP simulation, the acceleration of dummy head using NAB is smaller than that using DAB. Index Terms - Sandwiched airbag, OOP occupant protection, sled crash test, finite element model

I. INTRODUCTION

Airbags have been proven to be very helpful in protecting drivers and passengers in many cases of automotive crashes. However, side-effects may be serious if the system is not carefully designed, manufactured and used[1]. One common side-effect is that it may harm occupants, especially for children and small women, when it deploys improperly on crash (Chritina et al., 1998; John et al., 1993; Alex et al., 1995). Because to achieve occupant protection during a crash using a fully-deployed airbag to dissipate the frontal crash forces experienced by the driver over a larger body area and gradually decelerate the occupant’s head and torso to prevent contact with other interior surfaces, the airbag itself must deploy rapidly in less than 50 milliseconds. Consequently, an occupant positioned extremely close to the airbag module at the time the airbag begins to inflate is exposed to highly localized forces. Two phases of airbag deployment have been associated with high, injury-causing localized forces: the punch-out phase and the membrane-loading phase. According to the findings of biomechanics research, field automotive accident and laboratory test, chest and neck are the most vulnerable objects subjecting injuries during airbag interacting with OOP occupant. It has been found that chest injury often associates with airbag “punch out” force and occurs at very early stage of the airbag deployment, but neck injury is more

likely induced by the “membrane” loading at relative late deployment time[2]. It is believed that the side-effect would be minimized if the early development pattern of the airbag can be controlled. Proper development of the airbag system depends on several factors, including the structure of the airbag, the inflation system, the ignition system and the ignition time etc. Development of the traditional airbag would require a powerful inflation system and quick response of the ignition system, which put high requirements on the systems and thus resulting high costs. To reduce possible side-effect of the airbag system and to have suitable requirement on the inflation system, a sandwiched tube-type airbag system (NAB) [3]was developed (Zhong, 2005) and evaluated in this paper.

II. METHOD AND MATERIALS

A prototype of the NAB was developed, which consists of an upper layer flat airbag, a lower layer flat airbag and a middle layer of tube-type airbags. The upper layer flat airbag is designed to touch the occupant on crash, and the middle layer tube-type airbags are designed to support the upper layer and the lower layer. The whole NAB’s volume is about 40 L. In the event of a crash, the middle layer is first and mostly inflated while the other two layers are inflated later and less rapidly. Two groups of deployment tests using DAB and NAB were performed to support and validate the computational modeling efforts and compare their deployment properties; then the sled test and virtual tests were used to estimate the protection efficiency of the NAB; finally the parametric study was carried out to find the more sensitive and critical design variables to occupant injuries.

Experimental Set-up - An experimental inflation device was designed as shown in Figure 1. The experimental devise can be used to inflate the airbag and provide gas with prescribed pressure and leakage.

The whole deployment system mainly consists of an air compressor, a tank, fast-acting valves and sensors (shown in figure 1). The air compressor 1 provides compressed air to the tank 4. The outlet of fast-acting valve is connected to the inlet of the new structure airbag. Pressure sensor 9 and 6 are separately used to transfer tank pressure signal and airbag inside pressure signal to computer. Safety valve 3 prevented the system from pressure overloading. Solenoid valve 2 functions as a switch between air compressor and tank. A/D board 8 does the conversion from pressure voltage signal to numerical signal. The NAB system and traditional airbag

2009 International Conference on Computer Engineering and Technology

978-0-7695-3521-0/09 $25.00 © 2009 IEEE

DOI 10.1109/ICCET.2009.60

103

system are tested with the device in different loading conditions. The features of the airbag deployments were examined.

Fig.1 Layout of the airbag test system: 1.Air Compressor 2.Solenoid Valve 3.Safety Valve 4.Air Tank 5.Fast Acting Valve 6,9. Pressure Sensor

7.Fixed Airbag 8.A/D Board. Airbag deployment test - The behaviors of the NAB

system are investigated using an experimental device as shown in Figure 1. The NAB system and traditional airbag system are tested with the device in conditions of static and dynamic deployment: (1) static deployment without impact, and (2) dynamic deployment with impact.

Sled crash test - To examine the actual behavior of the NAB system in protecting occupants, sled crash tests are carried out by using a 50% HYBRID-III dummy with standard safety belt at impact speed of 35 km/h. as shown in Figure 4a. Two energy absorbing square metal beam of the size 120×120×500 in mm with thickness of 1.2 mm. The airbag is mounted at an angle of 60 degrees with horizontal plane. The tested NAB system is inflated with gas storage on the sled, providing a prescribed pressure to the system. The acceleration signal of the dummy’s head and the test sled is measured and recorded.

Virtual testing - Simulations of the static and dynamic deployment of the NAB system were carried out by using LS-DYNA program to study the feature of inflation process. Then, computer simulation was also conducted with the HYBRID-III dummy FE models. In response to side-effects of an airbag in low- and moderate-severity crashes, FMVSS 208 issued by NHTSA in May 2000, proposed that static OOP tests should be a mandatory requirement starting in 2003. These tests include performance requirements to ensure that airbags developed in the future do not pose an unreasonable risk of serious injury to OOP occupants[4]. In the FMVSS-208 NPRM, for the 5th percentile female "low risk deployment", the two test positions for OOP situations are as shown in Figure 2.

According to above requirement, the 5th percentile adult female dummy OOP virtual tests were used to do parametric analysis. The driver position 1(~ISO 1) is to obtain maximum neck loading (membrane) and the driver position 2 (~ISO 2) is to obtain maximum chest loading (punch out) from the deploying air bag. Finally, the virtual test results with DAB and NAB were compared.

(a) (b) Fig.2 (a) out of position 1 (b) out of position 2

III. NUMERICAL MODEL AND EXPERIMENTAL VALIDATION

NAB model - The NAB system is modeled with 18408 Belyschko-Tsay membrane elements. Material properties of Nylon 66 are assigned to the membrane elements. Contact interface is defined between the airbag and the dummy’s head, neck,chest and hands. Self-contact interface is also defined within the airbag system.

Static deployment -The high speed films from static airbag deployment are shown in Figure 3a. The dynamic stiffness of the airbag system is estimated through dropping composite blocks onto the inflated airbag. Experimental results show that by adjusting the pressure alone equivalent dynamic stiffness properties may be obtained with different types of airbags. It means that the NAB system has the potential to provide similar load-displacement feature as the traditional airbag system would do although the NAB system requires much less gas. Figure 3b shown the simulation results which are similar with that from deployment tests.

t=0ms t=10ms t=20ms t=30ms t=45ms

(a)

t=0ms t=10ms t=20ms t=30ms t=45ms

(b) Fig.3 (a)The airbag during inflation test; (b) Simulation of the deployment

process of the NAB Sled test - Figure 4a shows the 50th percentile HYBRID III

dummy response and contact with NAB at 50 ms in the sled crash test. The measured acceleration peak value is 16 g at the dummy head center of gravity.

(a) test at 50 ms (b) simulation at 50 ms

104

Fig.4 (a) Sled crash test of the NAB with HIII dummy; (b) Simulation of the interaction of the NAB with the dummy

From virtual testing using FE model, the dummy response and contact with the airbag at 50 ms is shown in the Figure 4b, which is comparable with the crash test result. The time history plot of head accelerations measured from test is presented and compared in Figure 5, and the magnitude and duration of the head acceleration curves are reasonable good. The validated NAB module model will be applied in the OOP occupant simulation as discussed in the next paragraph.

0

40

80

120

160

200

0,000 0,020 0,040 0,060 0,080 0,100 0,120

Time (s)

Hea

d ac

cele

ratio

n (m

/s2) Simulation

Sled test

Fig.5 The time history plots of dummy’s head acceleration from sled crash

test and simulation of the sled test Comparison of deployment property of NAB and DAB -

To compare the NAB and normal DAB’s deployment property and assess the relative potential of different airbag designs to cause injury, two groups of static deployment simulations were conducted. The first group of simulation is to measure the leading-edge speed between the normal DAB and NAB, the normal DAB and NAB have the same volume (40 L), either their inflators’ parameters. Figure 6 shows the different leading-edge velocity results from the static deployment simulation. From the time history plot of the leading-edge velocity, the peak value of the normal DAB is 53.9 m/s at time 0.0225 s, and the NAB is 27.6 m/s at time 0.0025 ms. Reed et al. recorded some ARS 3 (Abrasion Rating System) abrasions to human skin at a contact speed of 234 km/h (65m/s), so the possible abrasion injury caused by NAB is lower than ARS 3, and is lower than the possible abrasion injury caused by DAB also.

Fig.6 Leading-Edge Velocity comparison

In the second group of simulation, the normal DAB’s volume is 60 L and the NAB’s volume is 40 L, their leakage area was set to the same value (1152 mm2), also use the same inflator’s parameters. Figure 7 shows volume history plot of two types of airbag. NAB reached its maximum volume value at time 0.036 s, normal DAB reached its maximum volume value at time 0.039 s. Figure 8 shows the pressure history plot of two types of airbag. NAB reached its peak value (0.214M

Pa) at time 0.035s. DAB reached its peak value (0.1445×MPa) at time 0.037s. So , NAB can use less gas to reach the same airbag inner pressure.

Fig.7 Volume history plot

Fig.8 Pressure history plot

Ⅳ. AIRBAG-DESIGN PARAMETRIC STUDY

Design variables and control levels - Some of the design parameters that might affect the airbag module performance in drivers' OOP conditions are: airbag structure, vent hole, inflator tank pressure characteristics such as the slope and the peak, venting during inflation, cover break out force, fabric material property e.g. density, modulus of elasticity, porous property, etc[5]. The parametric study was carried out in two steps, In the first step, some initial design parametric about NAB’s structure have been carried out, mainly to judge the influence of tether and vent hole to NAB. The collocation of tether, the length of tether, the mass flow rate, the size and location of vent hole are selected as design variables[6]. The design variables and their control levels are showed in Table 1.

Table 1 Design Variables and control levels

Design Variable Control level

1 2 3 4 A=Tether collocation Col 1 Col2 Col 3 Absence

B=Tether Length 100% 90% 80% 70% C=Vent hole Area 1110mm2 80% 70% 110%

D=Vent hole circumferential Position Pos1 Pos2 Pos3 Pos4

E=Vent hole Radial Position Pos1 Pos2

F=Mass flow rate 100% 70% In the second step, based on the research results of the first

step of the parametric study, the influence of inflator characteristics and fabric material properties are discussed, and the design variables and their control levels are showed in Table 2.

Table 2 Design Variables and control levels

105

Design Variable Control level

1 2 3 G= inflator Pressure peak (kpa) 120 160 200 H=inflator Pressure slope (ms) 0 4.5 6.5

I=fabric material Density (kg.m-3) 541 721 901 J=fabric material Porous property 0 50 100

Object function definition - According to the findings of biomechanics research, field automotive accident and laboratory test, head, chest and neck are the most vulnerable objects subjecting injuries during airbag interacting with OOP occupant. In order to valuate NAB’s protection performance synthetically, the minimizing occupant injuries, US NCAP Injury Index—

COMBP and neck injury criteria ijN are chosen as

the object functions. COMBP can presents both HIC and chest

acceleration as only one function. It is formulated as following [7].

( )COMB

Object Function MIN P= (1) Where,

( )COMB HEAD CHEST HEAD CHESTP P P P P= + − × (2)

H E A DP andCHESTP are defined as following:

( )36

1

1 5.02 0.00351HEAD

PEXP HIC

=+ − ×

(3)

( )1

1 5.55 0.00693CHEST

PEXP CHESTG

=+ − ×

(4)

ijN can present both upper neck force and upper neck moment, it is formulated as following:

yzij

critical critical

MFNF M

= + (5)

Simulation matrix - After formulation of design problem, to provide enough representative test data, a design of experiment (DOE) analysis was conducted to design the test matrix; for the first step of parametric study, the orthogonal array is defined as L16 ( 4 34 2× ) matrix. And for the second step of parametric study, the orthogonal array is defined as L9 ( 43 ) matrix.

The traditional test work generally changes one design variable’s value at one time and other variables maintain invariable, so the number of experiments will be very big and can’t get the mutually affecting relations of these variables. The method by using orthogonal array to arrange experiments permits many variables change simultaneously; it can reduce the experiment numbers greatly and can be possible to estimate the effect of these variables more precisely.

Simulations are accomplished in each case of table 3 and table 4 using validated LS-DYNA simulation models, the 5th percentile female dummy seated in three postures: in-position, out-of-position 1 and out-of-position 2. According above orthogonal array experiment matrix, LD-DYNA simulations are not required 43 3× times but only 27.

Ⅴ. RESULTS AND ANALYSIS

Parametric virtual tests results - The virtual tests results for parametric study are shown in Table 3.

Table 3 virtual test results Dummy pos

NO. Chest G HIC36 Fz(N)

My (Nm)

Pcom Nij

IP

1 8.5g 182.0 650 15.0 0.019228 0.264300 2 2.7g 271.1 1200 -35.0 0.021403 0.883049 3 4.3g 191.0 370 17.5 0.017891 0.208264 4 9.0g 243.8 1250 25.0 0.022394 0.483455 5 36g 195.1 1500 31.0 0.057342 0.586598 6 9.0g 1131 3600 28.5 0.264523 1.111706 7 3.0g 244.6 1600 28.0 0.020036 0.593016 8 11.5g 266.4 2250 25.0 0.024956 0.741187 9 13.0g 187.5 800 30.0 0.021954 0.399734

OOP1

1 7.8g 273.1 1500 75.0 0.023451 0.870469 2 7.5g 279.6 950 68.0 0.023699 0.683555 3 27.5g 352.8 1800 90.0 0.047185 1.044563 4 8.2g 318.6 1180 95.0 0.026487 0.917027 5 18.0g 302.3 1400 75.0 0.031829 0.844696 6 18.0g 1061 -2100 81.0 0.225341 1.063818 7 23.0g 297.1 1150 95.0 0.036827 0.909295 8 10.0g 482.7 -1900 96.0 0.042145 1.109046 9 10.5g 333.7 1750 90.0 0.028680 1.031676

OOP2

1 7.0g 217.7 1050 -28.0 0.020170 0.729635 2 8.0g 223.4 1120 -27.0 0.020887 0.731283 3 28.0g 263.4 1300 -29.0 0.042293 0.810461 4 7.8g 220.4 1220 -26.0 0.020650 0.740663 5 10.0g 242.0 1450 -30.0 0.022805 0.865515 6 11.5g 246.8 1300 -32.0 0.023882 0.859642 7 20.0g 216.8 1300 -22.0 0.029033 0.695707 8 11.5g 246.7 1500 -31.0 0.023877 0.894795 9 12.0g 268.7 1400 -38.5 0.025381 0.991972

Analysis of results - The Design Exploration Tools of iSIGHT is used to analyze the previously mentioned virtual test results, the ANOVA, pareto plots and main effects graphs are recorded. ANOVA is the statistical analysis of the contribution of each factor or their interactions to the variance of a response [8]. 2R in ANOVA means the coefficient of determination, in another word it is a measure of the accuracy of the model fit. The pareto plots indicate the relative effect of each design variable on occupant injury. The main effects graphs indicate the desirable direction for each design variable.

In order to make NAB provide good protection for occupant under all conditions, the choice of design variables should be considered synthetically. Based on above three scenes parametric analysis, we can draw a conclusion that ‘the pressure slope of inflator’ and ‘the porous property of fabric’ are sensitive to occupant neck injury, ‘the pressure peak of inflator’ is sensitive to occupant head and chest injuries. And the predicted desirable direction for each design variable are as follows: the pressure peak of inflator should be chosen lower level, the pressure slope of inflator should be chosen lower or medium level, the density and porous property of fabric should be chosen medium or higher level.

Effectiveness of NAB - Adjust the parameters of NAB to the direction of design improvement according to above

106

parametric analysis, and a series of virtual tests were carried out with 5th percentile female dummy and 50th percentile male dummy sitting on position 1 and position 2, 60 L normal DAB and 40 L NAB are used respectively, then total 8 simulations were carried out. The purpose is to verify NAB’s protective efficiency to different stature drivers.

The normalized values with respect to percentages of injuries caused by DAB are shown in figures 9. The recorded injury criteria include: 15ms HIC, 3 ms clip, chest deflection, neck tension, neck compression and Nij.

00.10.20.30.40.5

0.60.70.80.9

1

HIC15 3ms-Clip Chest-D Neck-T Neck-C Nij

STAB injury/DAB injury

5th-OOP1 5th-OOP2 50th-OOP1 50th-OOP2

Fig.9 NAB Injury comparison with DAB

It is seen that all the injury ratios are less than 1, which indicate that the NAB could decrease the risk of OOP occupant’s injury and provide better protection.

Ⅵ. CONCLUSION

Results from computer simulation and sled crash tests show that the NAB system has good potential to provide effective protection to occupants as the traditional airbag system would do. At the same time, the NAB system would require less gas for the airbag to take the same amount of space, the inflation system can be made smaller and less expensive. At least two advantages may be obtained. Firstly, the system can be exploded more rapidly to take as much space as possible between the occupant and the automobile interiors. Secondly, the upper layer which would impact the occupant becomes softer, which is desirable to avoid unexpected injuries to the occupants.

Numerical results also indicate that the NAB system would give less harm to the occupant if the ignition takes place properly. However, the actual performance of the NAB system in commercial product has to be examined with an actual inflation system activated by a matched ignition system.

Nevertheless, the authors believe that the concept and prototype of the NAB system is worthwhile to be exploited for improvement of the airbag for occupant protection. Further investigation will be performed subsequently to develop a new airbag product.

REFERENCES [1] M. U. Khan, M. Moatamedi, A review of airbag test and analysis.

International Journal of Crashworthiness, Volume 13, Issue 1 February 2008 , pages 67 – 76.

[2] Jörg Hoffmann, Michael Freisinger, Mike Blundell, Manoj Mahangare, Peter Ritmeijer, Investigation Into the Effectiveness of Advanced Driver Airbag Modules Designed for OOP Injury Mitigation. In: Proceedings of the 20th International Technical Conference on the Enhanced Safety of Vehicle, paper NO. 07-0319-O

[3] Zhong, Z H and He, W. Sandwiched Tube-Type Airbag, Patent CN200410046609.9, 2005.

[4] Raj Roychoudhury, Dana Sun, Mohamed Hamid , Craig Hanson. 5th Percentile Driver Out of Position Computer Simulation, SAE paper, No 2000-01-1006.

[5] William Mu, Driver out-of-position injuries mitigation and advanced restraint features development. ESV 17th Conference.

[6] J Huang, Z Zhong, L Hu, and D Liu. Modeling and Simulation of Sandwiched Tube-Type Airbag and its Optimization using Design of Experiments, Part D: J. Automobile Engineering, Proc. IMechE Vol. 221.

[7] Seybok Lee, Soongu Hong. A Shape and Tether Study of Mid-Mounted Passenger Airbag Cushion Using Design of Experiments. 9th International MADYMO User's Meeting, Como,Italy . 10-11th,October,2002 , page1-5.

[8] Neter, J., Wasserman, W., Kutner, M., Applied Linear Statistical Models: Regression, Analysis of Variance, and Experimental Designs. Irwin Publishing. Boston, MA, 1990. R. P. Van Zuydam. A driver’s steering aid for an agricultural implement, based on an electronic map and real-time kinematics DGPS, Computer and Electronics in Agriculture, 24(2005), p153-163.

Hu Lin was born in Hunan province P.R.China on September 4,

1978.Hu Lin got doctor degree of mechanical engineering from Hunan University in 2008, and focuses on the research of Automotive Safety and Electronics. He has published 25 articles. (Corresponding author to provide mobile phone: 086-15821431148 ;E-mail: hulin888@sohu.com )

107